Autoconf

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Autoconf

This manual (28 January 2021) is for GNU Autoconf (version 2.71), a package for creating scripts to configure source code packages using templates and an M4 macro package.

Copyright © 1992–1996, 1998–2017, 2020–2021 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

Table of Contents


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1 Introduction

A physicist, an engineer, and a computer scientist were discussing the nature of God. “Surely a Physicist,” said the physicist, “because early in the Creation, God made Light; and you know, Maxwell’s equations, the dual nature of electromagnetic waves, the relativistic consequences...” “An Engineer!,” said the engineer, “because before making Light, God split the Chaos into Land and Water; it takes a hell of an engineer to handle that big amount of mud, and orderly separation of solids from liquids...” The computer scientist shouted: “And the Chaos, where do you think it was coming from, hmm?”

—Anonymous

Autoconf is a tool for producing shell scripts that automatically configure software source code packages to adapt to many kinds of Posix-like systems. The configuration scripts produced by Autoconf are independent of Autoconf when they are run, so their users do not need to have Autoconf.

The configuration scripts produced by Autoconf require no manual user intervention when run; they do not normally even need an argument specifying the system type. Instead, they individually test for the presence of each feature that the software package they are for might need. (Before each check, they print a one-line message stating what they are checking for, so the user doesn’t get too bored while waiting for the script to finish.) As a result, they deal well with systems that are hybrids or customized from the more common Posix variants. There is no need to maintain files that list the features supported by each release of each variant of Posix.

For each software package that Autoconf is used with, it creates a configuration script from a template file that lists the system features that the package needs or can use. After the shell code to recognize and respond to a system feature has been written, Autoconf allows it to be shared by many software packages that can use (or need) that feature. If it later turns out that the shell code needs adjustment for some reason, it needs to be changed in only one place; all of the configuration scripts can be regenerated automatically to take advantage of the updated code.

Those who do not understand Autoconf are condemned to reinvent it, poorly. The primary goal of Autoconf is making the user’s life easier; making the maintainer’s life easier is only a secondary goal. Put another way, the primary goal is not to make the generation of configure automatic for package maintainers (although patches along that front are welcome, since package maintainers form the user base of Autoconf); rather, the goal is to make configure painless, portable, and predictable for the end user of each autoconfiscated package. And to this degree, Autoconf is highly successful at its goal—most complaints to the Autoconf list are about difficulties in writing Autoconf input, and not in the behavior of the resulting configure. Even packages that don’t use Autoconf will generally provide a configure script, and the most common complaint about these alternative home-grown scripts is that they fail to meet one or more of the GNU Coding Standards (see Configuration in The GNU Coding Standards) that users have come to expect from Autoconf-generated configure scripts.

The Metaconfig package is similar in purpose to Autoconf, but the scripts it produces require manual user intervention, which is quite inconvenient when configuring large source trees. Unlike Metaconfig scripts, Autoconf scripts can support cross-compiling, if some care is taken in writing them.

Autoconf does not solve all problems related to making portable software packages—for a more complete solution, it should be used in concert with other GNU build tools like Automake and Libtool. These other tools take on jobs like the creation of a portable, recursive makefile with all of the standard targets, linking of shared libraries, and so on. See The GNU Build System, for more information.

Autoconf imposes some restrictions on the names of macros used with #if in C programs (see Preprocessor Symbol Index).

Autoconf requires GNU M4 version 1.4.6 or later in order to generate the scripts. It uses features that some versions of M4, including GNU M4 1.3, do not have. Autoconf works better with GNU M4 version 1.4.14 or later, though this is not required.

See Upgrading From Version 1, for information about upgrading from version 1. See History of Autoconf, for the story of Autoconf’s development. See Frequent Autoconf Questions, with answers, for answers to some common questions about Autoconf.

See the Autoconf web page for up-to-date information, details on the mailing lists, pointers to a list of known bugs, etc.

Mail suggestions to the Autoconf mailing list. Past suggestions are archived.

Mail bug reports to the Autoconf Bugs mailing list. Past bug reports are archived.

If possible, first check that your bug is not already solved in current development versions, and that it has not been reported yet. Be sure to include all the needed information and a short configure.ac that demonstrates the problem.

Autoconf’s development tree is accessible via git; see the Autoconf Summary for details, or view the actual repository. Patches relative to the current git version can be sent for review to the Autoconf Patches mailing list, with discussion on prior patches archived; and all commits are posted in the read-only Autoconf Commit mailing list, which is also archived.

Because of its mission, the Autoconf package itself includes only a set of often-used macros that have already demonstrated their usefulness. Nevertheless, if you wish to share your macros, or find existing ones, see the Autoconf Macro Archive, which is kindly run by Peter Simons.


2 The GNU Build System

Autoconf solves an important problem—reliable discovery of system-specific build and runtime information—but this is only one piece of the puzzle for the development of portable software. To this end, the GNU project has developed a suite of integrated utilities to finish the job Autoconf started: the GNU build system, whose most important components are Autoconf, Automake, and Libtool. In this chapter, we introduce you to those tools, point you to sources of more information, and try to convince you to use the entire GNU build system for your software.


2.1 Automake

The ubiquity of make means that a makefile is almost the only viable way to distribute automatic build rules for software, but one quickly runs into its numerous limitations. Its lack of support for automatic dependency tracking, recursive builds in subdirectories, reliable timestamps (e.g., for network file systems), and so on, mean that developers must painfully (and often incorrectly) reinvent the wheel for each project. Portability is non-trivial, thanks to the quirks of make on many systems. On top of all this is the manual labor required to implement the many standard targets that users have come to expect (make install, make distclean, make uninstall, etc.). Since you are, of course, using Autoconf, you also have to insert repetitive code in your Makefile.in to recognize @CC@, @CFLAGS@, and other substitutions provided by configure. Into this mess steps Automake.

Automake allows you to specify your build needs in a Makefile.am file with a vastly simpler and more powerful syntax than that of a plain makefile, and then generates a portable Makefile.in for use with Autoconf. For example, the Makefile.am to build and install a simple “Hello world” program might look like:

bin_PROGRAMS = hello
hello_SOURCES = hello.c

The resulting Makefile.in (~400 lines) automatically supports all the standard targets, the substitutions provided by Autoconf, automatic dependency tracking, VPATH building, and so on. make builds the hello program, and make install installs it in /usr/local/bin (or whatever prefix was given to configure, if not /usr/local).

The benefits of Automake increase for larger packages (especially ones with subdirectories), but even for small programs the added convenience and portability can be substantial. And that’s not all...


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2.2 Gnulib

GNU software has a well-deserved reputation for running on many different types of systems. While our primary goal is to write software for the GNU system, many users and developers have been introduced to us through the systems that they were already using.

Gnulib is a central location for common GNU code, intended to be shared among free software packages. Its components are typically shared at the source level, rather than being a library that gets built, installed, and linked against. The idea is to copy files from Gnulib into your own source tree. There is no distribution tarball; developers should just grab source modules from the repository. The source files are available online, under various licenses, mostly GNU GPL or GNU LGPL.

Gnulib modules typically contain C source code along with Autoconf macros used to configure the source code. For example, the Gnulib stdalign module implements a stdalign.h header that nearly conforms to C11, even on old-fashioned hosts that lack stdalign.h. This module contains a source file for the replacement header, along with an Autoconf macro that arranges to use the replacement header on old-fashioned systems.

For more information, consult the Gnulib website, https://www.gnu.org/software/gnulib/.


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2.3 Libtool

Often, one wants to build not only programs, but libraries, so that other programs can benefit from the fruits of your labor. Ideally, one would like to produce shared (dynamically linked) libraries, which can be used by multiple programs without duplication on disk or in memory and can be updated independently of the linked programs. Producing shared libraries portably, however, is the stuff of nightmares—each system has its own incompatible tools, compiler flags, and magic incantations. Fortunately, GNU provides a solution: Libtool.

Libtool handles all the requirements of building shared libraries for you, and at this time seems to be the only way to do so with any portability. It also handles many other headaches, such as: the interaction of Make rules with the variable suffixes of shared libraries, linking reliably with shared libraries before they are installed by the superuser, and supplying a consistent versioning system (so that different versions of a library can be installed or upgraded without breaking binary compatibility). Although Libtool, like Autoconf, can be used without Automake, it is most simply utilized in conjunction with Automake—there, Libtool is used automatically whenever shared libraries are needed, and you need not know its syntax.


2.4 Pointers

Developers who are used to the simplicity of make for small projects on a single system might be daunted at the prospect of learning to use Automake and Autoconf. As your software is distributed to more and more users, however, you otherwise quickly find yourself putting lots of effort into reinventing the services that the GNU build tools provide, and making the same mistakes that they once made and overcame. (Besides, since you’re already learning Autoconf, Automake is a piece of cake.)

There are a number of places that you can go to for more information on the GNU build tools.


3 Making configure Scripts

The configuration scripts that Autoconf produces are by convention called configure. When run, configure creates several files, replacing configuration parameters in them with appropriate values. The files that configure creates are:

To create a configure script with Autoconf, you need to write an Autoconf input file configure.ac and run autoconf on it. If you write your own feature tests to supplement those that come with Autoconf, you might also write files called aclocal.m4 and acsite.m4. If you use a C header file to contain #define directives, you might also run autoheader, and you can distribute the generated file config.h.in with the package.

Here is a diagram showing how the files that can be used in configuration are produced. Programs that are executed are suffixed by ‘*’. Optional files are enclosed in square brackets (‘[]’). autoconf and autoheader also read the installed Autoconf macro files (by reading autoconf.m4).

Files used in preparing a software package for distribution, when using just Autoconf:

your source files --> [autoscan*] --> [configure.scan] --> configure.ac

configure.ac --.
               |   .------> autoconf* -----> configure
[aclocal.m4] --+---+
               |   `-----> [autoheader*] --> [config.h.in]
[acsite.m4] ---'

Makefile.in

Additionally, if you use Automake, the following additional productions come into play:

[acinclude.m4] --.
                 |
[local macros] --+--> aclocal* --> aclocal.m4
                 |
configure.ac ----'

configure.ac --.
               +--> automake* --> Makefile.in
Makefile.am ---'

Files used in configuring a software package:

                       .-------------> [config.cache]
configure* ------------+-------------> config.log
                       |
[config.h.in] -.       v            .-> [config.h] -.
               +--> config.status* -+               +--> make*
Makefile.in ---'                    `-> Makefile ---'

3.1 Writing configure.ac

To produce a configure script for a software package, create a file called configure.ac that contains invocations of the Autoconf macros that test the system features your package needs or can use. Autoconf macros already exist to check for many features; see Existing Tests, for their descriptions. For most other features, you can use Autoconf template macros to produce custom checks; see Writing Tests, for information about them. For especially tricky or specialized features, configure.ac might need to contain some hand-crafted shell commands; see Portable Shell Programming. The autoscan program can give you a good start in writing configure.ac (see Using autoscan to Create configure.ac, for more information).

Previous versions of Autoconf promoted the name configure.in, which is somewhat ambiguous (the tool needed to process this file is not described by its extension), and introduces a slight confusion with config.h.in and so on (for which ‘.in’ means “to be processed by configure”). Using configure.ac is now preferred, while the use of configure.in will cause warnings from autoconf.


3.1.1 A Shell Script Compiler

Just as for any other computer language, in order to properly program configure.ac in Autoconf you must understand what problem the language tries to address and how it does so.

The problem Autoconf addresses is that the world is a mess. After all, you are using Autoconf in order to have your package compile easily on all sorts of different systems, some of them being extremely hostile. Autoconf itself bears the price for these differences: configure must run on all those systems, and thus configure must limit itself to their lowest common denominator of features.

Naturally, you might then think of shell scripts; who needs autoconf? A set of properly written shell functions is enough to make it easy to write configure scripts by hand. Sigh! Unfortunately, even in 2008, where shells without any function support are far and few between, there are pitfalls to avoid when making use of them. Also, finding a Bourne shell that accepts shell functions is not trivial, even though there is almost always one on interesting porting targets.

So, what is really needed is some kind of compiler, autoconf, that takes an Autoconf program, configure.ac, and transforms it into a portable shell script, configure.

How does autoconf perform this task?

There are two obvious possibilities: creating a brand new language or extending an existing one. The former option is attractive: all sorts of optimizations could easily be implemented in the compiler and many rigorous checks could be performed on the Autoconf program (e.g., rejecting any non-portable construct). Alternatively, you can extend an existing language, such as the sh (Bourne shell) language.

Autoconf does the latter: it is a layer on top of sh. It was therefore most convenient to implement autoconf as a macro expander: a program that repeatedly performs macro expansions on text input, replacing macro calls with macro bodies and producing a pure sh script in the end. Instead of implementing a dedicated Autoconf macro expander, it is natural to use an existing general-purpose macro language, such as M4, and implement the extensions as a set of M4 macros.


3.1.2 The Autoconf Language

The Autoconf language differs from many other computer languages because it treats actual code the same as plain text. Whereas in C, for instance, data and instructions have different syntactic status, in Autoconf their status is rigorously the same. Therefore, we need a means to distinguish literal strings from text to be expanded: quotation.

When calling macros that take arguments, there must not be any white space between the macro name and the open parenthesis.

AC_INIT ([oops], [1.0]) # incorrect
AC_INIT([hello], [1.0]) # good

Arguments should be enclosed within the quote characters ‘[’ and ‘]’, and be separated by commas. Any leading blanks or newlines in arguments are ignored, unless they are quoted. You should always quote an argument that might contain a macro name, comma, parenthesis, or a leading blank or newline. This rule applies recursively for every macro call, including macros called from other macros. For more details on quoting rules, see Programming in M4.

For instance:

AC_CHECK_HEADER([stdio.h],
                [AC_DEFINE([HAVE_STDIO_H], [1],
                   [Define to 1 if you have <stdio.h>.])],
                [AC_MSG_ERROR([sorry, can't do anything for you])])

is quoted properly. You may safely simplify its quotation to:

AC_CHECK_HEADER([stdio.h],
                [AC_DEFINE([HAVE_STDIO_H], 1,
                   [Define to 1 if you have <stdio.h>.])],
                [AC_MSG_ERROR([sorry, can't do anything for you])])

because ‘1’ cannot contain a macro call. Here, the argument of AC_MSG_ERROR must be quoted; otherwise, its comma would be interpreted as an argument separator. Also, the second and third arguments of ‘AC_CHECK_HEADER’ must be quoted, since they contain macro calls. The three arguments ‘HAVE_STDIO_H’, ‘stdio.h’, and ‘Define to 1 if you have <stdio.h>.’ do not need quoting, but if you unwisely defined a macro with a name like ‘Define’ or ‘stdio’ then they would need quoting. Cautious Autoconf users would keep the quotes, but many Autoconf users find such precautions annoying, and would rewrite the example as follows:

AC_CHECK_HEADER(stdio.h,
                [AC_DEFINE(HAVE_STDIO_H, 1,
                   [Define to 1 if you have <stdio.h>.])],
                [AC_MSG_ERROR([sorry, can't do anything for you])])

This is safe, so long as you adopt good naming conventions and do not define macros with names like ‘HAVE_STDIO_H’, ‘stdio’, or ‘h’. Though it is also safe here to omit the quotes around ‘Define to 1 if you have <stdio.h>.’ this is not recommended, as message strings are more likely to inadvertently contain commas.

The following example is wrong and dangerous, as it is underquoted:

AC_CHECK_HEADER(stdio.h,
                AC_DEFINE(HAVE_STDIO_H, 1,
                   Define to 1 if you have <stdio.h>.),
                AC_MSG_ERROR([sorry, can't do anything for you]))

In other cases, you may want to use text that also resembles a macro call. You must quote that text (whether just the potential problem, or the entire line) even when it is not passed as a macro argument; and you may also have to use m4_pattern_allow (see Forbidden Patterns), to declare your intention that the resulting configure file will have a literal that resembles what would otherwise be reserved for a macro name. For example:

dnl Simulate a possible future autoconf macro
m4_define([AC_DC], [oops])
dnl Underquoted:
echo "Hard rock was here!  --AC_DC"
dnl Correctly quoted:
m4_pattern_allow([AC_DC])
echo "Hard rock was here!  --[AC_DC]"
[echo "Hard rock was here!  --AC_DC"]

which results in this text in configure:

echo "Hard rock was here!  --oops"
echo "Hard rock was here!  --AC_DC"
echo "Hard rock was here!  --AC_DC"

When you use the same text in a macro argument, you must therefore have an extra quotation level (since one is stripped away by the macro substitution). In general, then, it is a good idea to use double quoting for all literal string arguments, either around just the problematic portions, or over the entire argument:

m4_pattern_allow([AC_DC])
AC_MSG_WARN([[AC_DC] stinks  --Iron Maiden])
AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])

It is also possible to avoid the problematic patterns in the first place, by the use of additional escaping (either a quadrigraph, or creative shell constructs), in which case it is no longer necessary to use m4_pattern_allow:

echo "Hard rock was here!  --AC""_DC"
AC_MSG_WARN([[AC@&t@_DC stinks  --Iron Maiden]])

You are now able to understand one of the constructs of Autoconf that has been continually misunderstood... The rule of thumb is that whenever you expect macro expansion, expect quote expansion; i.e., expect one level of quotes to be lost. For instance:

AC_COMPILE_IFELSE(AC_LANG_SOURCE([char b[10];]), [],
 [AC_MSG_ERROR([you lose])])

is incorrect: here, the first argument of AC_LANG_SOURCE is ‘char b[10];’ and is expanded once, which results in ‘char b10;’; and the AC_LANG_SOURCE is also expanded prior to being passed to AC_COMPILE_IFELSE. (There was an idiom common in Autoconf’s past to address this issue via the M4 changequote primitive, but do not use it!) Let’s take a closer look: the author meant the first argument to be understood as a literal, and therefore it must be quoted twice; likewise, the intermediate AC_LANG_SOURCE macro should be quoted once so that it is only expanded after the rest of the body of AC_COMPILE_IFELSE is in place:

AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char b[10];]])], [],
  [AC_MSG_ERROR([you lose])])

Voilà, you actually produce ‘char b[10];’ this time!

On the other hand, descriptions (e.g., the last parameter of AC_DEFINE or AS_HELP_STRING) are not literals—they are subject to line breaking, for example—and should not be double quoted. Even if these descriptions are short and are not actually broken, double quoting them yields weird results.

Some macros take optional arguments, which this documentation represents as [arg] (not to be confused with the quote characters). You may just leave them empty, or use ‘[]’ to make the emptiness of the argument explicit, or you may simply omit the trailing commas. The three lines below are equivalent:

AC_CHECK_HEADERS([stdio.h], [], [], [])
AC_CHECK_HEADERS([stdio.h],,,)
AC_CHECK_HEADERS([stdio.h])

It is best to put each macro call on its own line in configure.ac. Most of the macros don’t add extra newlines; they rely on the newline after the macro call to terminate the commands. This approach makes the generated configure script a little easier to read by not inserting lots of blank lines. It is generally safe to set shell variables on the same line as a macro call, because the shell allows assignments without intervening newlines.

You can include comments in configure.ac files by starting them with the ‘#’. For example, it is helpful to begin configure.ac files with a line like this:

# Process this file with autoconf to produce a configure script.

3.1.3 Standard configure.ac Layout

The order in which configure.ac calls the Autoconf macros is not important, with a few exceptions. Every configure.ac must contain a call to AC_INIT before the checks, and a call to AC_OUTPUT at the end (see Outputting Files). Additionally, some macros rely on other macros having been called first, because they check previously set values of some variables to decide what to do. These macros are noted in the individual descriptions (see Existing Tests), and they also warn you when configure is created if they are called out of order.

To encourage consistency, here is a suggested order for calling the Autoconf macros. Generally speaking, the things near the end of this list are those that could depend on things earlier in it. For example, library functions could be affected by types and libraries.

Autoconf requirements
AC_INIT(package, version, bug-report-address)
information on the package
checks for programs
checks for libraries
checks for header files
checks for types
checks for structures
checks for compiler characteristics
checks for library functions
checks for system services
AC_CONFIG_FILES([file…])
AC_OUTPUT

3.2 Using autoscan to Create configure.ac

The autoscan program can help you create and/or maintain a configure.ac file for a software package. autoscan examines source files in the directory tree rooted at a directory given as a command line argument, or the current directory if none is given. It searches the source files for common portability problems and creates a file configure.scan which is a preliminary configure.ac for that package, and checks a possibly existing configure.ac for completeness.

When using autoscan to create a configure.ac, you should manually examine configure.scan before renaming it to configure.ac; it probably needs some adjustments. Occasionally, autoscan outputs a macro in the wrong order relative to another macro, so that autoconf produces a warning; you need to move such macros manually. Also, if you want the package to use a configuration header file, you must add a call to AC_CONFIG_HEADERS (see Configuration Header Files). You might also have to change or add some #if directives to your program in order to make it work with Autoconf (see Using ifnames to List Conditionals, for information about a program that can help with that job).

When using autoscan to maintain a configure.ac, simply consider adding its suggestions. The file autoscan.log contains detailed information on why a macro is requested.

autoscan uses several data files (installed along with Autoconf) to determine which macros to output when it finds particular symbols in a package’s source files. These data files all have the same format: each line consists of a symbol, one or more blanks, and the Autoconf macro to output if that symbol is encountered. Lines starting with ‘#’ are comments.

autoscan accepts the following options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.

--verbose
-v

Print the names of the files it examines and the potentially interesting symbols it finds in them. This output can be voluminous.

--debug
-d

Don’t remove temporary files.

--include=dir
-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir
-B dir

Prepend dir to the include path. Multiple invocations accumulate.


3.3 Using ifnames to List Conditionals

ifnames can help you write configure.ac for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, ifnames can thus help you figure out what its configure needs to check for. It may help fill in some gaps in a configure.ac generated by autoscan (see Using autoscan to Create configure.ac).

ifnames scans all of the C source files named on the command line (or the standard input, if none are given) and writes to the standard output a sorted list of all the identifiers that appear in those files in #if, #elif, #ifdef, or #ifndef directives. It prints each identifier on a line, followed by a space-separated list of the files in which that identifier occurs.

ifnames accepts the following options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.


3.4 Using autoconf to Create configure

To create configure from configure.ac, run the autoconf program with no arguments. autoconf processes configure.ac with the M4 macro processor, using the Autoconf macros. If you give autoconf an argument, it reads that file instead of configure.ac and writes the configuration script to the standard output instead of to configure. If you give autoconf the argument -, it reads from the standard input instead of configure.ac and writes the configuration script to the standard output.

The Autoconf macros are defined in several files. Some of the files are distributed with Autoconf; autoconf reads them first. Then it looks for the optional file acsite.m4 in the directory that contains the distributed Autoconf macro files, and for the optional file aclocal.m4 in the current directory. Those files can contain your site’s or the package’s own Autoconf macro definitions (see Writing Autoconf Macros, for more information). If a macro is defined in more than one of the files that autoconf reads, the last definition it reads overrides the earlier ones.

autoconf accepts the following options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.

--verbose
-v

Report processing steps.

--debug
-d

Don’t remove the temporary files.

--force
-f

Remake configure even if newer than its input files.

--include=dir
-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir
-B dir

Prepend dir to the include path. Multiple invocations accumulate.

--output=file
-o file

Save output (script or trace) to file. The file - stands for the standard output.

--warnings=category[,category...]
-Wcategory[,category...]

Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

all

Enable all categories of warnings.

none

Disable all categories of warnings.

error

Treat all warnings as errors.

no-category

Disable warnings falling into category.

The enviroment variable WARNINGS may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of --warnings, but unknown categories are silently ignored. The command line takes precedence; for instance, if WARNINGS is set to obsolete, but -Wnone is given on the command line, no warnings will be issued.

Some categories of warnings are on by default. Again, for details see m4_warn.

--trace=macro[:format]
-t macro[:format]

Do not create the configure script, but list the calls to macro according to the format. Multiple --trace arguments can be used to list several macros. Multiple --trace arguments for a single macro are not cumulative; instead, you should just make format as long as needed.

The format is a regular string, with newlines if desired, and several special escape codes. It defaults to ‘$f:$l:$n:$%’; see Invoking autom4te, for details on the format.

--initialization
-i

By default, --trace does not trace the initialization of the Autoconf macros (typically the AC_DEFUN definitions). This results in a noticeable speedup, but can be disabled by this option.

It is often necessary to check the content of a configure.ac file, but parsing it yourself is extremely fragile and error-prone. It is suggested that you rely upon --trace to scan configure.ac. For instance, to find the list of variables that are substituted, use:

$ autoconf -t AC_SUBST
configure.ac:2:AC_SUBST:ECHO_C
configure.ac:2:AC_SUBST:ECHO_N
configure.ac:2:AC_SUBST:ECHO_T
More traces deleted

The example below highlights the difference between ‘$@’, ‘$*’, and ‘$%’.

$ cat configure.ac
AC_DEFINE(This, is, [an
[example]])
$ autoconf -t 'AC_DEFINE:@: $@
*: $*
%: $%'
@: [This],[is],[an
[example]]
*: This,is,an
[example]
%: This:is:an [example]

The format gives you a lot of freedom:

$ autoconf -t 'AC_SUBST:$$ac_subst{"$1"} = "$f:$l";'
$ac_subst{"ECHO_C"} = "configure.ac:2";
$ac_subst{"ECHO_N"} = "configure.ac:2";
$ac_subst{"ECHO_T"} = "configure.ac:2";
More traces deleted

A long separator can be used to improve the readability of complex structures, and to ease their parsing (for instance when no single character is suitable as a separator):

$ autoconf -t 'AM_MISSING_PROG:${|:::::|}*'
ACLOCAL|:::::|aclocal|:::::|$missing_dir
AUTOCONF|:::::|autoconf|:::::|$missing_dir
AUTOMAKE|:::::|automake|:::::|$missing_dir
More traces deleted

3.5 Using autoreconf to Update configure Scripts

Installing the various components of the GNU Build System can be tedious: running autopoint for Gettext, automake for Makefile.in etc. in each directory. It may be needed either because some tools such as automake have been updated on your system, or because some of the sources such as configure.ac have been updated, or finally, simply in order to install the GNU Build System in a fresh tree.

autoreconf runs autoconf, autoheader, aclocal, automake, libtoolize, intltoolize, gtkdocize, and autopoint (when appropriate) repeatedly to update the GNU Build System in the specified directories and their subdirectories (see Configuring Other Packages in Subdirectories). By default, it only remakes those files that are older than their sources. The environment variables AUTOM4TE, AUTOCONF, AUTOHEADER, AUTOMAKE, ACLOCAL, AUTOPOINT, LIBTOOLIZE, INTLTOOLIZE, GTKDOCIZE, M4, and MAKE may be used to override the invocation of the respective tools.

If you install a new version of some tool, you can make autoreconf remake all of the files by giving it the --force option.

See Automatic Remaking, for Make rules to automatically rebuild configure scripts when their source files change. That method handles the timestamps of configuration header templates properly, but does not pass --autoconf-dir=dir or --localdir=dir.

Gettext supplies the autopoint command to add translation infrastructure to a source package. If you use autopoint, your configure.ac should invoke AM_GNU_GETTEXT and one of AM_GNU_GETTEXT_VERSION(gettext-version) or AM_GNU_GETTEXT_REQUIRE_VERSION(min-gettext-version). See Invoking the autopoint Program in GNU gettext utilities, for further details.

autoreconf accepts the following options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.

--verbose
-v

Print the name of each directory autoreconf examines and the commands it runs. If given two or more times, pass --verbose to subordinate tools that support it.

--debug
-d

Don’t remove the temporary files.

--force
-f

Consider all generated and standard auxiliary files to be obsolete. This remakes even configure scripts and configuration headers that are newer than their input files (configure.ac and, if present, aclocal.m4).

If deemed appropriate, this option triggers calls to ‘automake --force-missing’. Passing both --force and --install to autoreconf will in turn undo any customizations to standard files. Note that the macro AM_INIT_AUTOMAKE has some options which change the set of files considered to be standard.

--install
-i

Install any missing standard auxiliary files in the package. By default, files are copied; this can be changed with --symlink.

If deemed appropriate, this option triggers calls to ‘automake --add-missing’, ‘libtoolize’, ‘autopoint’, etc.

--no-recursive

Do not rebuild files in subdirectories to configure (see Configuring Other Packages in Subdirectories, macro AC_CONFIG_SUBDIRS).

--symlink
-s

When used with --install, install symbolic links to the missing auxiliary files instead of copying them.

--make
-m

When the directories were configured, update the configuration by running ‘./config.status --recheck && ./config.status’, and then run ‘make’.

--include=dir
-I dir

Append dir to the include path. Multiple invocations accumulate. Passed on to aclocal, autoconf and autoheader internally.

--prepend-include=dir
-B dir

Prepend dir to the include path. Multiple invocations accumulate. Passed on to autoconf and autoheader internally.

--warnings=category[,category...]
-Wcategory[,category...]

Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

all

Enable all categories of warnings.

none

Disable all categories of warnings.

error

Treat all warnings as errors.

no-category

Disable warnings falling into category.

The enviroment variable WARNINGS may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of --warnings, but unknown categories are silently ignored. The command line takes precedence; for instance, if WARNINGS is set to obsolete, but -Wnone is given on the command line, no warnings will be issued.

Some categories of warnings are on by default. Again, for details see m4_warn.

If you want autoreconf to pass flags that are not listed here on to aclocal, set ACLOCAL_AMFLAGS in your Makefile.am. Due to a limitation in the Autoconf implementation these flags currently must be set on a single line in Makefile.am, without any backslash-newlines. Also, be aware that future Automake releases might start flagging ACLOCAL_AMFLAGS as obsolescent, or even remove support for it.


4 Initialization and Output Files

Autoconf-generated configure scripts need some information about how to initialize, such as how to find the package’s source files and about the output files to produce. The following sections describe the initialization and the creation of output files.


4.1 Initializing configure

Every configure script must call AC_INIT before doing anything else that produces output. Calls to silent macros, such as AC_DEFUN, may also occur prior to AC_INIT, although these are generally used via aclocal.m4, since that is implicitly included before the start of configure.ac. The only other required macro is AC_OUTPUT (see Outputting Files).

Macro: AC_INIT (package, version, [bug-report], [tarname], [url])

Process any command-line arguments and perform initialization and verification.

Set the name of the package and its version. These are typically used in --version support, including that of configure. The optional argument bug-report should be the email to which users should send bug reports. The package tarname differs from package: the latter designates the full package name (e.g., ‘GNU Autoconf’), while the former is meant for distribution tar ball names (e.g., ‘autoconf’). It defaults to package with ‘GNU ’ stripped, lower-cased, and all characters other than alphanumerics and underscores are changed to ‘-’. If provided, url should be the home page for the package.

Leading and trailing whitespace is stripped from all the arguments to AC_INIT, and interior whitespace is collapsed to a single space. This means that, for instance, if you want to put several email addresses in bug-report, you can put each one on its own line:

# We keep having problems with the mail hosting for
# gnomovision.example, so give people an alternative.
AC_INIT([Gnomovision], [17.0.1], [
    bugs@gnomovision.example
    or gnomo-bugs@reliable-email.example
])

The arguments to AC_INIT may be computed by M4, when autoconf is run. For instance, if you want to include the package’s version number in the tarname, but you don’t want to repeat it, you can use a helper macro:

m4_define([gnomo_VERSION], [17.0.1])
AC_INIT([Gnomovision],
        m4_defn([gnomo_VERSION]),
        [bugs@gnomovision.example],
        [gnomo-]m4_defn([gnomo_VERSION]))

This uses m4_defn to produce the expansion of gnomo_VERSION as a quoted string, so that if there happen to be any more M4 macro names in gnomo_VERSION, they will not be expanded. See Renaming Macros in GNU m4 macro processor.

Continuing this example, if you don’t want to embed the version number in configure.ac at all, you can use m4_esyscmd to look it up somewhere else when autoconf is run:

m4_define([gnomo_VERSION],
  m4_esyscmd([build-aux/git-version-gen .tarball-version]))
AC_INIT([Gnomovision],
        m4_defn([gnomo_VERSION]),
        [bugs@gnomovision.example],
        [gnomo-]m4_defn([gnomo_VERSION]))

This uses the utility script git-version-gen to look up the package’s version in its version control metadata. This script is part of Gnulib (see Gnulib).

The arguments to AC_INIT are written into configure in several different places. Therefore, we strongly recommend that you write any M4 logic in AC_INIT arguments to be evaluated before AC_INIT itself is evaluated. For instance, in the above example, the second argument to m4_define is not quoted, so the m4_esyscmd is evaluated only once, and gnomo_VERSION is defined to the output of the command. If the second argument to m4_define were quoted, m4_esyscmd would be evaluated each time the version or tarname arguments were written to configure, and the command would be run repeatedly.

In some of the places where the arguments to AC_INIT are used, within configure, shell evaluation cannot happen. Therefore, the arguments to AC_INIT may not be computed when configure is run. If they contain any construct that isn’t always treated as literal by the shell (e.g. variable expansions), autoconf will issue an error.

The tarname argument is used to construct filenames. It should not contain wildcard characters, white space, or anything else that could be troublesome as part of a file or directory name.

Some of M4’s active characters (notably parentheses, square brackets, ‘,’ and ‘#’) commonly appear in URLs and lists of email addresses. If any of these characters appear in an argument to AC_INIT, that argument will probably need to be double-quoted to avoid errors and mistranscriptions. See M4 Quotation.

The following M4 macros (e.g., AC_PACKAGE_NAME), output variables (e.g., PACKAGE_NAME), and preprocessor symbols (e.g., PACKAGE_NAME), are defined by AC_INIT:

AC_PACKAGE_NAME, PACKAGE_NAME

Exactly package.

AC_PACKAGE_TARNAME, PACKAGE_TARNAME

Exactly tarname, possibly generated from package.

AC_PACKAGE_VERSION, PACKAGE_VERSION

Exactly version.

AC_PACKAGE_STRING, PACKAGE_STRING

Exactly ‘package version’.

AC_PACKAGE_BUGREPORT, PACKAGE_BUGREPORT

Exactly bug-report, if one was provided. Typically an email address, or URL to a bug management web page.

AC_PACKAGE_URL, PACKAGE_URL

Exactly url, if one was provided. If url was empty, but package begins with ‘GNU ’, then this defaults to ‘https://www.gnu.org/software/tarname/’, otherwise, no URL is assumed.

If your configure script does its own option processing, it should inspect ‘$@’ or ‘$*’ immediately after calling AC_INIT, because other Autoconf macros liberally use the set command to process strings, and this has the side effect of updating ‘$@’ and ‘$*’. However, we suggest that you use standard macros like AC_ARG_ENABLE instead of attempting to implement your own option processing. See Site Configuration.


4.2 Dealing with Autoconf versions

The following optional macros can be used to help choose the minimum version of Autoconf that can successfully compile a given configure.ac.

Macro: AC_PREREQ (version)

Ensure that a recent enough version of Autoconf is being used. If the version of Autoconf being used to create configure is earlier than version, print an error message to the standard error output and exit with failure (exit status is 63). For example:

AC_PREREQ([2.71])

This macro may be used before AC_INIT.

Macro: AC_AUTOCONF_VERSION

This macro was introduced in Autoconf 2.62. It identifies the version of Autoconf that is currently parsing the input file, in a format suitable for m4_version_compare (see m4_version_compare); in other words, for this release of Autoconf, its value is ‘2.71’. One potential use of this macro is for writing conditional fallbacks based on when a feature was added to Autoconf, rather than using AC_PREREQ to require the newer version of Autoconf. However, remember that the Autoconf philosophy favors feature checks over version checks.

You should not expand this macro directly; use ‘m4_defn([AC_AUTOCONF_VERSION])’ instead. This is because some users might have a beta version of Autoconf installed, with arbitrary letters included in its version string. This means it is possible for the version string to contain the name of a defined macro, such that expanding AC_AUTOCONF_VERSION would trigger the expansion of that macro during rescanning, and change the version string to be different than what you intended to check.


4.3 Notices in configure

The following macros manage version numbers for configure scripts. Using them is optional.

Macro: AC_COPYRIGHT (copyright-notice)

State that, in addition to the Free Software Foundation’s copyright on the Autoconf macros, parts of your configure are covered by the copyright-notice.

The copyright-notice shows up in both the head of configure and in ‘configure --version’.

Macro: AC_REVISION (revision-info)

Copy revision stamp revision-info into the configure script, with any dollar signs or double-quotes removed. This macro lets you put a revision stamp from configure.ac into configure without RCS or CVS changing it when you check in configure. That way, you can determine easily which revision of configure.ac a particular configure corresponds to.

For example, this line in configure.ac:

AC_REVISION([$Revision: 1.30 $])

produces this in configure:

#!/bin/sh
# From configure.ac Revision: 1.30

4.4 Configure Input: Source Code, Macros, and Auxiliary Files

The following macros help you manage the contents of your source tree.

Macro: AC_CONFIG_SRCDIR (unique-file-in-source-dir)

Distinguish this package’s source directory from other source directories that might happen to exist in the file system. unique-file-in-source-dir should name a file that is unique to this package. configure will verify that this file exists in srcdir, before it runs any other checks.

Use of this macro is strongly recommended. It protects against people accidentally specifying the wrong directory with --srcdir. See configure Invocation, for more information.

Packages that use aclocal to generate aclocal.m4 should declare where local macros can be found using AC_CONFIG_MACRO_DIRS.

Macro: AC_CONFIG_MACRO_DIRS (dir1 [dir2 ... dirN])
Macro: AC_CONFIG_MACRO_DIR (dir)

Specify the given directories as the location of additional local Autoconf macros. These macros are intended for use by commands like autoreconf or aclocal that trace macro calls; they should be called directly from configure.ac so that tools that install macros for aclocal can find the macros’ declarations. Tools that want to learn which directories have been selected should trace AC_CONFIG_MACRO_DIR_TRACE, which will be called once per directory.

AC_CONFIG_MACRO_DIRS is the preferred form, and can be called multiple times and with multiple arguments; in such cases, directories in earlier calls are expected to be searched before directories in later calls, and directories appearing in the same call are expected to be searched in the order in which they appear in the call. For historical reasons, the macro AC_CONFIG_MACRO_DIR can also be used once, if it appears first, for tools such as older libtool that weren’t prepared to handle multiple directories. For example, a usage like

AC_CONFIG_MACRO_DIR([dir1])
AC_CONFIG_MACRO_DIRS([dir2])
AC_CONFIG_MACRO_DIRS([dir3 dir4])

will cause the trace of AC_CONFIG_MACRO_DIR_TRACE to appear four times, and should cause the directories to be searched in this order: ‘dir1 dir2 dir3 dir4’.

Note that if you use aclocal from an Automake release prior to 1.13 to generate aclocal.m4, you must also set ACLOCAL_AMFLAGS = -I dir1 [-I dir2 ... -I dirN] in your top-level Makefile.am. Due to a limitation in the Autoconf implementation of autoreconf, these include directives currently must be set on a single line in Makefile.am, without any backslash-newlines.

Some Autoconf macros require auxiliary scripts. AC_PROG_INSTALL and AC_PROG_MKDIR_P (see Particular Program Checks) require a fallback implementation of install called install-sh, and the AC_CANONICAL macros (see Manual Configuration) require the system-identification scripts config.sub and config.guess. Third-party tools, such as Automake and Libtool, may require additional auxiliary scripts.

By default, configure looks for these scripts next to itself, in srcdir. For convenience when working with subdirectories with their own configure scripts (see Configuring Other Packages in Subdirectories), if the scripts are not in srcdir it will also look in srcdir/.. and srcdir/../... All of the scripts must be found in the same directory.

If these default locations are not adequate, or simply to reduce clutter at the top level of the source tree, packages can use AC_CONFIG_AUX_DIR to declare where to look for auxiliary scripts.

Macro: AC_CONFIG_AUX_DIR (dir)

Look for auxiliary scripts in dir. Normally, dir should be a relative path, which is taken as relative to srcdir. If dir is an absolute path or contains shell variables, however, it is used as-is.

When the goal of using AC_CONFIG_AUX_DIR is to reduce clutter at the top level of the source tree, the conventional name for dir is build-aux. If you need portability to DOS variants, do not name the auxiliary directory aux. See File System Conventions.

Macro: AC_REQUIRE_AUX_FILE (file)

Declare that file is an auxiliary script needed by this configure script, and set the shell variable ac_aux_dir to the directory where it can be found. The value of ac_aux_dir is guaranteed to end with a ‘/’.

Macros that need auxiliary scripts must use this macro to register each script they need.

configure checks for all the auxiliary scripts it needs on startup, and exits with an error if any are missing.

autoreconf also detects missing auxiliary scripts. When used with the --install option, autoreconf will try to add missing scripts to the directory specified by AC_CONFIG_AUX_DIR, or to the top level of the source tree if AC_CONFIG_AUX_DIR was not used. It can always do this for the scripts needed by Autoconf core macros: install-sh, config.sub, and config.guess. Many other commonly-needed scripts are installed by the third-party tools that autoreconf knows how to run, such as missing for Automake and ltmain.sh for Libtool.

If you are using Automake, auxiliary scripts will automatically be included in the tarball created by make dist. If you are not using Automake you will need to arrange for auxiliary scripts to be included in tarballs yourself. Auxiliary scripts should normally not be checked into a version control system, for the same reasons that configure shouldn’t be.

The scripts needed by Autoconf core macros can be found in $(datadir)/autoconf/build-aux of the Autoconf installation (see Installation Directory Variables). install-sh can be downloaded from https://git.savannah.gnu.org/cgit/automake.git/plain/lib/install-sh. config.sub and config.guess can be downloaded from https://git.savannah.gnu.org/cgit/config.git/tree/.


4.5 Outputting Files

Every Autoconf script, e.g., configure.ac, should finish by calling AC_OUTPUT. That is the macro that generates and runs config.status, which in turn creates the makefiles and any other files resulting from configuration. This is the only required macro besides AC_INIT (see Configure Input: Source Code, Macros, and Auxiliary Files).

Macro: AC_OUTPUT

Generate config.status and launch it. Call this macro once, at the end of configure.ac.

config.status performs all the configuration actions: all the output files (see Creating Configuration Files, macro AC_CONFIG_FILES), header files (see Configuration Header Files, macro AC_CONFIG_HEADERS), commands (see Running Arbitrary Configuration Commands, macro AC_CONFIG_COMMANDS), links (see Creating Configuration Links, macro AC_CONFIG_LINKS), subdirectories to configure (see Configuring Other Packages in Subdirectories, macro AC_CONFIG_SUBDIRS) are honored.

The location of your AC_OUTPUT invocation is the exact point where configuration actions are taken: any code afterwards is executed by configure once config.status was run. If you want to bind actions to config.status itself (independently of whether configure is being run), see Running Arbitrary Configuration Commands.

Historically, the usage of AC_OUTPUT was somewhat different. See Obsolete Macros, for a description of the arguments that AC_OUTPUT used to support.

If you run make in subdirectories, you should run it using the make variable MAKE. Most versions of make set MAKE to the name of the make program plus any options it was given. (But many do not include in it the values of any variables set on the command line, so those are not passed on automatically.) Some old versions of make do not set this variable. The following macro allows you to use it even with those versions.

Macro: AC_PROG_MAKE_SET

If the Make command, $MAKE if set or else ‘make’, predefines $(MAKE), define output variable SET_MAKE to be empty. Otherwise, define SET_MAKE to a macro definition that sets $(MAKE), such as ‘MAKE=make’. Calls AC_SUBST for SET_MAKE.

If you use this macro, place a line like this in each Makefile.in that runs MAKE on other directories:

@SET_MAKE@

4.6 Performing Configuration Actions

configure is designed so that it appears to do everything itself, but there is actually a hidden slave: config.status. configure is in charge of examining your system, but it is config.status that actually takes the proper actions based on the results of configure. The most typical task of config.status is to instantiate files.

This section describes the common behavior of the four standard instantiating macros: AC_CONFIG_FILES, AC_CONFIG_HEADERS, AC_CONFIG_COMMANDS and AC_CONFIG_LINKS. They all have this prototype:

AC_CONFIG_ITEMS(tag…, [commands], [init-cmds])

where the arguments are:

tag…

A blank-or-newline-separated list of tags, which are typically the names of the files to instantiate.

You are encouraged to use literals as tags. In particular, you should avoid

… && my_foos="$my_foos fooo"
… && my_foos="$my_foos foooo"
AC_CONFIG_ITEMS([$my_foos])

and use this instead:

… && AC_CONFIG_ITEMS([fooo])
… && AC_CONFIG_ITEMS([foooo])

The macros AC_CONFIG_FILES and AC_CONFIG_HEADERS use special tag values: they may have the form ‘output’ or ‘output:inputs’. The file output is instantiated from its templates, inputs (defaulting to ‘output.in’).

AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk])’, for example, asks for the creation of the file Makefile that contains the expansion of the output variables in the concatenation of boiler/top.mk and boiler/bot.mk.

The special value ‘-’ might be used to denote the standard output when used in output, or the standard input when used in the inputs. You most probably don’t need to use this in configure.ac, but it is convenient when using the command line interface of ./config.status, see config.status Invocation, for more details.

The inputs may be absolute or relative file names. In the latter case they are first looked for in the build tree, and then in the source tree. Input files should be text files, and a line length below 2000 bytes should be safe.

commands

Shell commands output literally into config.status, and associated with a tag that the user can use to tell config.status which commands to run. The commands are run each time a tag request is given to config.status, typically each time the file tag is created.

The variables set during the execution of configure are not available here: you first need to set them via the init-cmds. Nonetheless the following variables are pre-computed:

srcdir

The name of the top source directory, assuming that the working directory is the top build directory. This is what configure’s --srcdir option sets.

ac_top_srcdir

The name of the top source directory, assuming that the working directory is the current build directory.

ac_top_build_prefix

The name of the top build directory, assuming that the working directory is the current build directory. It can be empty, or else ends with a slash, so that you may concatenate it.

ac_srcdir

The name of the corresponding source directory, assuming that the working directory is the current build directory.

tmp

The name of a temporary directory within the build tree, which you can use if you need to create additional temporary files. The directory is cleaned up when config.status is done or interrupted. Please use package-specific file name prefixes to avoid clashing with files that config.status may use internally.

The current directory refers to the directory (or pseudo-directory) containing the input part of tags. For instance, running

AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], […], […])

with --srcdir=../package produces the following values:

# Argument of --srcdir
srcdir='../package'
# Reversing deep/dir
ac_top_build_prefix='../../'
# Concatenation of $ac_top_build_prefix and srcdir
ac_top_srcdir='../../../package'
# Concatenation of $ac_top_srcdir and deep/dir
ac_srcdir='../../../package/deep/dir'

independently of ‘in/in.in’.

init-cmds

Shell commands output unquoted near the beginning of config.status, and executed each time config.status runs (regardless of the tag). Because they are unquoted, for example, ‘$var’ is output as the value of var. init-cmds is typically used by configure to give config.status some variables it needs to run the commands.

You should be extremely cautious in your variable names: all the init-cmds share the same name space and may overwrite each other in unpredictable ways. Sorry...

All these macros can be called multiple times, with different tag values, of course!


4.7 Creating Configuration Files

Be sure to read the previous section, Performing Configuration Actions.

Macro: AC_CONFIG_FILES (file…, [cmds], [init-cmds])

Make AC_OUTPUT create each file by copying an input file (by default file.in), substituting the output variable values. This macro is one of the instantiating macros; see Performing Configuration Actions. See Substitutions in Makefiles, for more information on using output variables. See Setting Output Variables, for more information on creating them. This macro creates the directory that the file is in if it doesn’t exist. Usually, makefiles are created this way, but other files, such as .gdbinit, can be specified as well.

Typical calls to AC_CONFIG_FILES look like this:

AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile])
AC_CONFIG_FILES([autoconf], [chmod +x autoconf])

You can override an input file name by appending to file a colon-separated list of input files. Examples:

AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk]
                [lib/Makefile:boiler/lib.mk])

Doing this allows you to keep your file names acceptable to DOS variants, or to prepend and/or append boilerplate to the file.

The file names should not contain shell metacharacters. See Special Characters in Output Variables.


4.8 Substitutions in Makefiles

Each subdirectory in a distribution that contains something to be compiled or installed should come with a file Makefile.in, from which configure creates a file Makefile in that directory. To create Makefile, configure performs a simple variable substitution, replacing occurrences of ‘@variable@’ in Makefile.in with the value that configure has determined for that variable. Variables that are substituted into output files in this way are called output variables. They are ordinary shell variables that are set in configure. To make configure substitute a particular variable into the output files, the macro AC_SUBST must be called with that variable name as an argument. Any occurrences of ‘@variable@’ for other variables are left unchanged. See Setting Output Variables, for more information on creating output variables with AC_SUBST.

A software package that uses a configure script should be distributed with a file Makefile.in, but no makefile; that way, the user has to properly configure the package for the local system before compiling it.

See Makefile Conventions in The GNU Coding Standards, for more information on what to put in makefiles.


4.8.1 Preset Output Variables

Some output variables are preset by the Autoconf macros. Some of the Autoconf macros set additional output variables, which are mentioned in the descriptions for those macros. See Output Variable Index, for a complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation directories. Below are listed the other preset ones, many of which are precious variables (see Setting Output Variables, AC_ARG_VAR).

The preset variables which are available during config.status (see Performing Configuration Actions) may also be used during configure tests. For example, it is permissible to reference ‘$srcdir’ when constructing a list of directories to pass via the -I option during a compiler feature check. When used in this manner, coupled with the fact that configure is always run from the top build directory, it is sufficient to use just ‘$srcdir’ instead of ‘$top_srcdir’.

Variable: CFLAGS

Debugging and optimization options for the C compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_CC (or empty if you don’t). configure uses this variable when compiling or linking programs to test for C features.

If a compiler option affects only the behavior of the preprocessor (e.g., -Dname), it should be put into CPPFLAGS instead. If it affects only the linker (e.g., -Ldirectory), it should be put into LDFLAGS instead. If it affects only the compiler proper, CFLAGS is the natural home for it. If an option affects multiple phases of the compiler, though, matters get tricky:

  • If an option selects a 32-bit or 64-bit build on a bi-arch system, it must be put direcly into CC, e.g., CC='gcc -m64'. This is necessary for config.guess to work right.
  • Otherwise one approach is to put the option into CC. Another is to put it into both CPPFLAGS and LDFLAGS, but not into CFLAGS.

However, remember that some Makefile variables are reserved by the GNU Coding Standards for the use of the “user”—the person building the package. For instance, CFLAGS is one such variable.

Sometimes package developers are tempted to set user variables such as CFLAGS because it appears to make their job easier. However, the package itself should never set a user variable, particularly not to include switches that are required for proper compilation of the package. Since these variables are documented as being for the package builder, that person rightfully expects to be able to override any of these variables at build time. If the package developer needs to add switches without interfering with the user, the proper way to do that is to introduce an additional variable. Automake makes this easy by introducing AM_CFLAGS (see Flag Variables Ordering in GNU Automake), but the concept is the same even if Automake is not used.

Variable: configure_input

A comment saying that the file was generated automatically by configure and giving the name of the input file. AC_OUTPUT adds a comment line containing this variable to the top of every makefile it creates. For other files, you should reference this variable in a comment at the top of each input file. For example, an input shell script should begin like this:

#!/bin/sh
# @configure_input@

The presence of that line also reminds people editing the file that it needs to be processed by configure in order to be used.

Variable: CPPFLAGS

Preprocessor options for the C, C++, Objective C, and Objective C++ preprocessors and compilers. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when preprocessing or compiling programs to test for C, C++, Objective C, and Objective C++ features.

This variable’s contents should contain options like -I, -D, and -U that affect only the behavior of the preprocessor. Please see the explanation of CFLAGS for what you can do if an option affects other phases of the compiler as well.

Currently, configure always links as part of a single invocation of the compiler that also preprocesses and compiles, so it uses this variable also when linking programs. However, it is unwise to depend on this behavior because the GNU Coding Standards do not require it and many packages do not use CPPFLAGS when linking programs.

See Special Characters in Output Variables, for limitations that CPPFLAGS might run into.

Variable: CXXFLAGS

Debugging and optimization options for the C++ compiler. It acts like CFLAGS, but for C++ instead of C.

Variable: DEFS

-D options to pass to the C compiler. If AC_CONFIG_HEADERS is called, configure replaces ‘@DEFS@’ with -DHAVE_CONFIG_H instead (see Configuration Header Files). This variable is not defined while configure is performing its tests, only when creating the output files. See Setting Output Variables, for how to check the results of previous tests.

Variable: ECHO_C
Variable: ECHO_N
Variable: ECHO_T

How does one suppress the trailing newline from echo for question-answer message pairs? These variables provide a way:

echo $ECHO_N "And the winner is... $ECHO_C"
sleep 100000000000
echo "${ECHO_T}dead."

Some old and uncommon echo implementations offer no means to achieve this, in which case ECHO_T is set to tab. You might not want to use it.

Variable: ERLCFLAGS

Debugging and optimization options for the Erlang compiler. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when compiling programs to test for Erlang features.

Variable: FCFLAGS

Debugging and optimization options for the Fortran compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_FC (or empty if you don’t). configure uses this variable when compiling or linking programs to test for Fortran features.

Variable: FFLAGS

Debugging and optimization options for the Fortran 77 compiler. If it is not set in the environment when configure runs, the default value is set when you call AC_PROG_F77 (or empty if you don’t). configure uses this variable when compiling or linking programs to test for Fortran 77 features.

Variable: LDFLAGS

Options for the linker. If it is not set in the environment when configure runs, the default value is empty. configure uses this variable when linking programs to test for C, C++, Objective C, Objective C++, Fortran, and Go features.

This variable’s contents should contain options like -s and -L that affect only the behavior of the linker. Please see the explanation of CFLAGS for what you can do if an option also affects other phases of the compiler.

Don’t use this variable to pass library names (-l) to the linker; use LIBS instead.

Variable: LIBS

-l options to pass to the linker. The default value is empty, but some Autoconf macros may prepend extra libraries to this variable if those libraries are found and provide necessary functions, see Library Files. configure uses this variable when linking programs to test for C, C++, Objective C, Objective C++, Fortran, and Go features.

Variable: OBJCFLAGS

Debugging and optimization options for the Objective C compiler. It acts like CFLAGS, but for Objective C instead of C.

Variable: OBJCXXFLAGS

Debugging and optimization options for the Objective C++ compiler. It acts like CXXFLAGS, but for Objective C++ instead of C++.

Variable: GOFLAGS

Debugging and optimization options for the Go compiler. It acts like CFLAGS, but for Go instead of C.

Variable: builddir

Rigorously equal to ‘.’. Added for symmetry only.

Variable: abs_builddir

Absolute name of builddir.

Variable: top_builddir

The relative name of the top level of the current build tree. In the top-level directory, this is the same as builddir.

Variable: top_build_prefix

The relative name of the top level of the current build tree with final slash if nonempty. This is the same as top_builddir, except that it contains zero or more runs of ../, so it should not be appended with a slash for concatenation. This helps for make implementations that otherwise do not treat ./file and file as equal in the top-level build directory.

Variable: abs_top_builddir

Absolute name of top_builddir.

Variable: srcdir

The name of the directory that contains the source code for that makefile.

Variable: abs_srcdir

Absolute name of srcdir.

Variable: top_srcdir

The name of the top-level source code directory for the package. In the top-level directory, this is the same as srcdir.

Variable: abs_top_srcdir

Absolute name of top_srcdir.


4.8.2 Installation Directory Variables

The following variables specify the directories for package installation, see Variables for Installation Directories in The GNU Coding Standards, for more information. Each variable corresponds to an argument of configure; trailing slashes are stripped so that expressions such as ‘${prefix}/lib’ expand with only one slash between directory names. See the end of this section for details on when and how to use these variables.

Variable: bindir

The directory for installing executables that users run.

Variable: datadir

The directory for installing idiosyncratic read-only architecture-independent data.

Variable: datarootdir

The root of the directory tree for read-only architecture-independent data files.

Variable: docdir

The directory for installing documentation files (other than Info and man).

Variable: dvidir

The directory for installing documentation files in DVI format.

Variable: exec_prefix

The installation prefix for architecture-dependent files. By default it’s the same as prefix. You should avoid installing anything directly to exec_prefix. However, the default value for directories containing architecture-dependent files should be relative to exec_prefix.

Variable: htmldir

The directory for installing HTML documentation.

Variable: includedir

The directory for installing C header files.

Variable: infodir

The directory for installing documentation in Info format.

Variable: libdir

The directory for installing object code libraries.

Variable: libexecdir

The directory for installing executables that other programs run.

Variable: localedir

The directory for installing locale-dependent but architecture-independent data, such as message catalogs. This directory usually has a subdirectory per locale.

Variable: localstatedir

The directory for installing modifiable single-machine data. Content in this directory typically survives a reboot.

Variable: runstatedir

The directory for installing temporary modifiable single-machine data. Content in this directory survives as long as the process is running (such as pid files), as contrasted with /tmp that may be periodically cleaned. Conversely, this directory is typically cleaned on a reboot. By default, this is a subdirectory of localstatedir.

Variable: mandir

The top-level directory for installing documentation in man format.

Variable: oldincludedir

The directory for installing C header files for non-GCC compilers.

Variable: pdfdir

The directory for installing PDF documentation.

Variable: prefix

The common installation prefix for all files. If exec_prefix is defined to a different value, prefix is used only for architecture-independent files.

Variable: psdir

The directory for installing PostScript documentation.

Variable: sbindir

The directory for installing executables that system administrators run.

Variable: sharedstatedir

The directory for installing modifiable architecture-independent data.

Variable: sysconfdir

The directory for installing read-only single-machine data.

Most of these variables have values that rely on prefix or exec_prefix. It is deliberate that the directory output variables keep them unexpanded: typically ‘@datarootdir@’ is replaced by ‘${prefix}/share’, not ‘/usr/local/share’, and ‘@datadir@’ is replaced by ‘${datarootdir}’.

This behavior is mandated by the GNU Coding Standards, so that when the user runs:

make

she can still specify a different prefix from the one specified to configure, in which case, if needed, the package should hard code dependencies corresponding to the make-specified prefix.

make install

she can specify a different installation location, in which case the package must still depend on the location which was compiled in (i.e., never recompile when ‘make install’ is run). This is an extremely important feature, as many people may decide to install all the files of a package grouped together, and then install links from the final locations to there.

In order to support these features, it is essential that datarootdir remains defined as ‘${prefix}/share’, so that its value can be expanded based on the current value of prefix.

A corollary is that you should not use these variables except in makefiles. For instance, instead of trying to evaluate datadir in configure and hard-coding it in makefiles using e.g., ‘AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])’, you should add -DDATADIR='$(datadir)' to your makefile’s definition of CPPFLAGS (AM_CPPFLAGS if you are also using Automake).

Similarly, you should not rely on AC_CONFIG_FILES to replace bindir and friends in your shell scripts and other files; instead, let make manage their replacement. For instance Autoconf ships templates of its shell scripts ending with ‘.in’, and uses a makefile snippet similar to the following to build scripts like autoheader and autom4te:

edit = sed \
        -e 's|@bindir[@]|$(bindir)|g' \
        -e 's|@pkgdatadir[@]|$(pkgdatadir)|g' \
        -e 's|@prefix[@]|$(prefix)|g'

autoheader autom4te: Makefile
        rm -f $@ $@.tmp
        srcdir=''; \
          test -f ./$@.in || srcdir=$(srcdir)/; \
          $(edit) $${srcdir}$@.in >$@.tmp
        chmod +x $@.tmp
        chmod a-w $@.tmp
        mv $@.tmp $@

autoheader: $(srcdir)/autoheader.in
autom4te: $(srcdir)/autom4te.in

Some details are noteworthy:

@bindir[@]

The brackets prevent configure from replacing ‘@bindir@’ in the Sed expression itself. Brackets are preferable to a backslash here, since Posix says ‘\@’ is not portable.

$(bindir)

Don’t use ‘@bindir@’! Use the matching makefile variable instead.

$(pkgdatadir)

The example takes advantage of the variable ‘$(pkgdatadir)’ provided by Automake; it is equivalent to ‘$(datadir)/$(PACKAGE)’.

/

Don’t use ‘/’ in the Sed expressions that replace file names since most likely the variables you use, such as ‘$(bindir)’, contain ‘/’. Use a shell metacharacter instead, such as ‘|’.

special characters

File names, file name components, and the value of VPATH should not contain shell metacharacters or white space. See Special Characters in Output Variables.

dependency on Makefile

Since edit uses values that depend on the configuration specific values (prefix, etc.) and not only on VERSION and so forth, the output depends on Makefile, not configure.ac.

$@

The main rule is generic, and uses ‘$@’ extensively to avoid the need for multiple copies of the rule.

Separated dependencies and single suffix rules

You can’t use them! The above snippet cannot be (portably) rewritten as:

autoconf autoheader: Makefile
.in:
        rm -f $@ $@.tmp
        $(edit) $< >$@.tmp
        chmod +x $@.tmp
        mv $@.tmp $@

See Single Suffix Rules and Separated Dependencies, for details.

$(srcdir)

Be sure to specify the name of the source directory, otherwise the package won’t support separated builds.

For the more specific installation of Erlang libraries, the following variables are defined:

Variable: ERLANG_INSTALL_LIB_DIR

The common parent directory of Erlang library installation directories. This variable is set by calling the AC_ERLANG_SUBST_INSTALL_LIB_DIR macro in configure.ac.

Variable: ERLANG_INSTALL_LIB_DIR_library

The installation directory for Erlang library library. This variable is set by using the ‘AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR’ macro in configure.ac.

See Erlang Libraries, for details.


4.8.3 Changed Directory Variables

In Autoconf 2.60, the set of directory variables has changed, and the defaults of some variables have been adjusted (see Installation Directory Variables) to changes in the GNU Coding Standards. Notably, datadir, infodir, and mandir are now expressed in terms of datarootdir. If you are upgrading from an earlier Autoconf version, you may need to adjust your files to ensure that the directory variables are substituted correctly (see How Do I #define Installation Directories?), and that a definition of datarootdir is in place. For example, in a Makefile.in, adding

datarootdir = @datarootdir@

is usually sufficient. If you use Automake to create Makefile.in, it will add this for you.

To help with the transition, Autoconf warns about files that seem to use datarootdir without defining it. In some cases, it then expands the value of $datarootdir in substitutions of the directory variables. The following example shows such a warning:

$ cat configure.ac
AC_INIT
AC_CONFIG_FILES([Makefile])
AC_OUTPUT
$ cat Makefile.in
prefix = @prefix@
datadir = @datadir@
$ autoconf
$ configure
configure: creating ./config.status
config.status: creating Makefile
config.status: WARNING:
               Makefile.in seems to ignore the --datarootdir setting
$ cat Makefile
prefix = /usr/local
datadir = ${prefix}/share

Usually one can easily change the file to accommodate both older and newer Autoconf releases:

$ cat Makefile.in
prefix = @prefix@
datarootdir = @datarootdir@
datadir = @datadir@
$ configure
configure: creating ./config.status
config.status: creating Makefile
$ cat Makefile
prefix = /usr/local
datarootdir = ${prefix}/share
datadir = ${datarootdir}

In some cases, however, the checks may not be able to detect that a suitable initialization of datarootdir is in place, or they may fail to detect that such an initialization is necessary in the output file. If, after auditing your package, there are still spurious configure warnings about datarootdir, you may add the line

AC_DEFUN([AC_DATAROOTDIR_CHECKED])

to your configure.ac to disable the warnings. This is an exception to the usual rule that you should not define a macro whose name begins with AC_ (see Macro Names).


4.8.4 Build Directories

You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory.

To support doing this, make uses the VPATH variable to find the files that are in the source directory. GNU Make can do this. Most other recent make programs can do this as well, though they may have difficulties and it is often simpler to recommend GNU make (see VPATH and Make). Older make programs do not support VPATH; when using them, the source code must be in the same directory as the object files.

If you are using GNU Automake, the remaining details in this section are already covered for you, based on the contents of your Makefile.am. But if you are using Autoconf in isolation, then supporting VPATH requires the following in your Makefile.in:

srcdir = @srcdir@
VPATH = @srcdir@

Do not set VPATH to the value of another variable (see Variables listed in VPATH.

configure substitutes the correct value for srcdir when it produces Makefile.

Do not use the make variable $<, which expands to the file name of the file in the source directory (found with VPATH), except in implicit rules. (An implicit rule is one such as ‘.c.o’, which tells how to create a .o file from a .c file.) Some versions of make do not set $< in explicit rules; they expand it to an empty value.

Instead, Make command lines should always refer to source files by prefixing them with ‘$(srcdir)/’. It’s safer to quote the source directory name, in case it contains characters that are special to the shell. Because ‘$(srcdir)’ is expanded by Make, single-quoting works and is safer than double-quoting. For example:

time.info: time.texinfo
        $(MAKEINFO) '$(srcdir)/time.texinfo'

4.8.5 Automatic Remaking

You can put rules like the following in the top-level Makefile.in for a package to automatically update the configuration information when you change the configuration files. This example includes all of the optional files, such as aclocal.m4 and those related to configuration header files. Omit from the Makefile.in rules for any of these files that your package does not use.

The ‘$(srcdir)/’ prefix is included because of limitations in the VPATH mechanism.

The stamp- files are necessary because the timestamps of config.h.in and config.h are not changed if remaking them does not change their contents. This feature avoids unnecessary recompilation. You should include the file stamp-h.in in your package’s distribution, so that make considers config.h.in up to date. Don’t use touch (see Limitations of Usual Tools); instead, use echo (using date would cause needless differences, hence CVS conflicts, etc.).

$(srcdir)/configure: configure.ac aclocal.m4
        cd '$(srcdir)' && autoconf

# autoheader might not change config.h.in, so touch a stamp file.
$(srcdir)/config.h.in: stamp-h.in ;
$(srcdir)/stamp-h.in: configure.ac aclocal.m4
        cd '$(srcdir)' && autoheader
        echo timestamp > '$(srcdir)/stamp-h.in'

config.h: stamp-h ;
stamp-h: config.h.in config.status
        ./config.status

Makefile: Makefile.in config.status
        ./config.status

config.status: configure
        ./config.status --recheck

(Be careful if you copy these lines directly into your makefile, as you need to convert the indented lines to start with the tab character.)

In addition, you should use

AC_CONFIG_FILES([stamp-h], [echo timestamp > stamp-h])

so config.status ensures that config.h is considered up to date. See Outputting Files, for more information about AC_OUTPUT.

See config.status Invocation, for more examples of handling configuration-related dependencies.


4.9 Configuration Header Files

When a package contains more than a few tests that define C preprocessor symbols, the command lines to pass -D options to the compiler can get quite long. This causes two problems. One is that the make output is hard to visually scan for errors. More seriously, the command lines can exceed the length limits of some operating systems. As an alternative to passing -D options to the compiler, configure scripts can create a C header file containing ‘#define’ directives. The AC_CONFIG_HEADERS macro selects this kind of output. Though it can be called anywhere between AC_INIT and AC_OUTPUT, it is customary to call it right after AC_INIT.

The package should ‘#include’ the configuration header file before any other header files, to prevent inconsistencies in declarations (for example, if it redefines const, or if it defines a macro like _FILE_OFFSET_BITS that affects the behavior of system headers). Note that it is okay to only include config.h from .c files; the project’s .h files can rely on config.h already being included first by the corresponding .c file.

To provide for VPATH builds, remember to pass the C compiler a -I. option (or -I..; whichever directory contains config.h). Even if you use ‘#include "config.h"’, the preprocessor searches only the directory of the currently read file, i.e., the source directory, not the build directory.

With the appropriate -I option, you can use ‘#include <config.h>’. Actually, it’s a good habit to use it, because in the rare case when the source directory contains another config.h, the build directory should be searched first.

Macro: AC_CONFIG_HEADERS (header …, [cmds], [init-cmds])

This macro is one of the instantiating macros; see Performing Configuration Actions. Make AC_OUTPUT create the file(s) in the blank-or-newline-separated list header containing C preprocessor #define statements, and replace ‘@DEFS@’ in generated files with -DHAVE_CONFIG_H instead of the value of DEFS. The usual name for header is config.h; header should not contain shell metacharacters. See Special Characters in Output Variables.

If header already exists and its contents are identical to what AC_OUTPUT would put in it, it is left alone. Doing this allows making some changes in the configuration without needlessly causing object files that depend on the header file to be recompiled.

Usually the input file is named header.in; however, you can override the input file name by appending to header a colon-separated list of input files. For example, you might need to make the input file name acceptable to DOS variants:

AC_CONFIG_HEADERS([config.h:config.hin])
Macro: AH_HEADER

This macro is defined as the name of the first declared config header and undefined if no config headers have been declared up to this point. A third-party macro may, for example, require use of a config header without invoking AC_CONFIG_HEADERS twice, like this:

AC_CONFIG_COMMANDS_PRE(
        [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])

See Performing Configuration Actions, for more details on header.


4.9.1 Configuration Header Templates

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with #undef statements which are used as hooks. For example, suppose your configure.ac makes these calls:

AC_CONFIG_HEADERS([conf.h])
AC_CHECK_HEADERS([unistd.h])

Then you could have code like the following in conf.h.in. The conf.h created by configure defines ‘HAVE_UNISTD_H’ to 1, if and only if the system has unistd.h.

/* Define as 1 if you have unistd.h.  */
#undef HAVE_UNISTD_H

The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, ‘#undef’, and ‘HAVE_UNISTD_H’. The use of ‘#define’ instead of ‘#undef’, or of comments on the same line as ‘#undef’, is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as ‘#ifdef’ or ‘#include’, are copied verbatim from the template into the generated header.

Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see Using autoheader to Create config.h.in.

During the instantiation of the header, each ‘#undef’ line in the template file for each symbol defined by ‘AC_DEFINE’ is changed to an appropriate ‘#define’. If the corresponding ‘AC_DEFINE’ has not been executed during the configure run, the ‘#undef’ line is commented out. (This is important, e.g., for ‘_POSIX_SOURCE’: on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.)

Currently, all remaining ‘#undef’ lines in the header template are commented out, whether or not there was a corresponding ‘AC_DEFINE’ for the macro name; but this behavior is not guaranteed for future releases of Autoconf.

Generally speaking, since you should not use ‘#define’, and you cannot guarantee whether a ‘#undef’ directive in the header template will be converted to a ‘#define’ or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct

#ifdef THIS
# define THAT
#endif

you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and ‘#include’ that file from the config header template. If you are using autoheader, you would probably use ‘AH_BOTTOM’ to append the ‘#include’ directive.


4.9.2 Using autoheader to Create config.h.in

The autoheader program can create a template file of C ‘#define’ statements for configure to use. It searches for the first invocation of AC_CONFIG_HEADERS in configure sources to determine the name of the template. (If the first call of AC_CONFIG_HEADERS specifies more than one input file name, autoheader uses the first one.)

It is recommended that only one input file is used. If you want to append a boilerplate code, it is preferable to use ‘AH_BOTTOM([#include <conf_post.h>])’. File conf_post.h is not processed during the configuration then, which make things clearer. Analogically, AH_TOP can be used to prepend a boilerplate code.

In order to do its job, autoheader needs you to document all of the symbols that you might use. Typically this is done via an AC_DEFINE or AC_DEFINE_UNQUOTED call whose first argument is a literal symbol and whose third argument describes the symbol (see Defining C Preprocessor Symbols). Alternatively, you can use AH_TEMPLATE (see Autoheader Macros), or you can supply a suitable input file for a subsequent configuration header file. Symbols defined by Autoconf’s builtin tests are already documented properly; you need to document only those that you define yourself.

You might wonder why autoheader is needed: after all, why would configure need to “patch” a config.h.in to produce a config.h instead of just creating config.h from scratch? Well, when everything rocks, the answer is just that we are wasting our time maintaining autoheader: generating config.h directly is all that is needed. When things go wrong, however, you’ll be thankful for the existence of autoheader.

The fact that the symbols are documented is important in order to check that config.h makes sense. The fact that there is a well-defined list of symbols that should be defined (or not) is also important for people who are porting packages to environments where configure cannot be run: they just have to fill in the blanks.

But let’s come back to the point: the invocation of autoheader

If you give autoheader an argument, it uses that file instead of configure.ac and writes the header file to the standard output instead of to config.h.in. If you give autoheader an argument of -, it reads the standard input instead of configure.ac and writes the header file to the standard output.

autoheader accepts the following options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.

--verbose
-v

Report processing steps.

--debug
-d

Don’t remove the temporary files.

--force
-f

Remake the template file even if newer than its input files.

--include=dir
-I dir

Append dir to the include path. Multiple invocations accumulate.

--prepend-include=dir
-B dir

Prepend dir to the include path. Multiple invocations accumulate.

--warnings=category[,category...]
-Wcategory[,category...]

Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

all

Enable all categories of warnings.

none

Disable all categories of warnings.

error

Treat all warnings as errors.

no-category

Disable warnings falling into category.

The enviroment variable WARNINGS may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of --warnings, but unknown categories are silently ignored. The command line takes precedence; for instance, if WARNINGS is set to obsolete, but -Wnone is given on the command line, no warnings will be issued.

Some categories of warnings are on by default. Again, for details see m4_warn.


4.9.3 Autoheader Macros

autoheader scans configure.ac and figures out which C preprocessor symbols it might define. It knows how to generate templates for symbols defined by AC_CHECK_HEADERS, AC_CHECK_FUNCS etc., but if you AC_DEFINE any additional symbol, you must define a template for it. If there are missing templates, autoheader fails with an error message.

The template for a symbol is created by autoheader from the description argument to an AC_DEFINE; see Defining C Preprocessor Symbols.

For special needs, you can use the following macros.

Macro: AH_TEMPLATE (key, description)

Tell autoheader to generate a template for key. This macro generates standard templates just like AC_DEFINE when a description is given.

For example:

AH_TEMPLATE([NULL_DEVICE],
  [Name of the file to open to get
   a null file, or a data sink.])

generates the following template, with the description properly justified.

/* Name of the file to open to get a null file, or a data sink. */
#undef NULL_DEVICE
Macro: AH_VERBATIM (key, template)

Tell autoheader to include the template as-is in the header template file. This template is associated with the key, which is used to sort all the different templates and guarantee their uniqueness. It should be a symbol that can be defined via AC_DEFINE.

Macro: AH_TOP (text)

Include text at the top of the header template file.

Macro: AH_BOTTOM (text)

Include text at the bottom of the header template file.

Please note that text gets included “verbatim” to the template file, not to the resulting config header, so it can easily get mangled when the template is processed. There is rarely a need for something other than

AH_BOTTOM([#include <custom.h>])

4.10 Running Arbitrary Configuration Commands

You can execute arbitrary commands before, during, and after config.status is run. The three following macros accumulate the commands to run when they are called multiple times. AC_CONFIG_COMMANDS replaces the obsolete macro AC_OUTPUT_COMMANDS; see Obsolete Macros, for details.

Macro: AC_CONFIG_COMMANDS (tag…, [cmds], [init-cmds])

Specify additional shell commands to run at the end of config.status, and shell commands to initialize any variables from configure. Associate the commands with tag. Since typically the cmds create a file, tag should naturally be the name of that file. If needed, the directory hosting tag is created. The tag should not contain shell metacharacters. See Special Characters in Output Variables. This macro is one of the instantiating macros; see Performing Configuration Actions.

Here is an unrealistic example:

fubar=42
AC_CONFIG_COMMANDS([fubar],
                   [echo this is extra $fubar, and so on.],
                   [fubar=$fubar])

Here is a better one:

AC_CONFIG_COMMANDS([timestamp], [date >timestamp])

The following two macros look similar, but in fact they are not of the same breed: they are executed directly by configure, so you cannot use config.status to rerun them.

Macro: AC_CONFIG_COMMANDS_PRE (cmds)

Execute the cmds right before creating config.status.

This macro presents the last opportunity to call AC_SUBST, AC_DEFINE, or AC_CONFIG_ITEMS macros.

Macro: AC_CONFIG_COMMANDS_POST (cmds)

Execute the cmds right after creating config.status.


4.12 Configuring Other Packages in Subdirectories

In most situations, calling AC_OUTPUT is sufficient to produce makefiles in subdirectories. However, configure scripts that control more than one independent package can use AC_CONFIG_SUBDIRS to run configure scripts for other packages in subdirectories.

Macro: AC_CONFIG_SUBDIRS (dir …)

Make AC_OUTPUT run configure in each subdirectory dir in the given blank-or-newline-separated list. Each dir should be a literal, i.e., please do not use:

if test "x$package_foo_enabled" = xyes; then
  my_subdirs="$my_subdirs foo"
fi
AC_CONFIG_SUBDIRS([$my_subdirs])

because this prevents ‘./configure --help=recursive’ from displaying the options of the package foo. Instead, you should write:

if test "x$package_foo_enabled" = xyes; then
  AC_CONFIG_SUBDIRS([foo])
fi

If a given dir is not found at configure run time, a warning is reported; if the subdirectory is optional, write:

if test -d "$srcdir/foo"; then
  AC_CONFIG_SUBDIRS([foo])
fi

If a given dir contains configure.gnu, it is run instead of configure. This is for packages that might use a non-Autoconf script Configure, which can’t be called through a wrapper configure since it would be the same file on case-insensitive file systems.

The subdirectory configure scripts are given the same command line options that were given to this configure script, with minor changes if needed, which include:

  • - adjusting a relative name for the cache file;
  • - adjusting a relative name for the source directory;
  • - propagating the current value of $prefix, including if it was defaulted, and if the default values of the top level and of the subdirectory configure differ.

This macro also sets the output variable subdirs to the list of directories ‘dir’. Make rules can use this variable to determine which subdirectories to recurse into.

This macro may be called multiple times.


4.13 Default Prefix

By default, configure sets the prefix for files it installs to /usr/local. The user of configure can select a different prefix using the --prefix and --exec-prefix options. There are two ways to change the default: when creating configure, and when running it.

Some software packages might want to install in a directory other than /usr/local by default. To accomplish that, use the AC_PREFIX_DEFAULT macro.

Macro: AC_PREFIX_DEFAULT (prefix)

Set the default installation prefix to prefix instead of /usr/local.

It may be convenient for users to have configure guess the installation prefix from the location of a related program that they have already installed. If you wish to do that, you can call AC_PREFIX_PROGRAM.

Macro: AC_PREFIX_PROGRAM (program)

If the user did not specify an installation prefix (using the --prefix option), guess a value for it by looking for program in PATH, the way the shell does. If program is found, set the prefix to the parent of the directory containing program, else default the prefix as described above (/usr/local or AC_PREFIX_DEFAULT). For example, if program is gcc and the PATH contains /usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.


5 Existing Tests

These macros test for particular system features that packages might need or want to use. If you need to test for a kind of feature that none of these macros check for, you can probably do it by calling primitive test macros with appropriate arguments (see Writing Tests).

These tests print messages telling the user which feature they’re checking for, and what they find. They cache their results for future configure runs (see Caching Results).

Some of these macros set output variables. See Substitutions in Makefiles, for how to get their values. The phrase “define name” is used below as a shorthand to mean “define the C preprocessor symbol name to the value 1”. See Defining C Preprocessor Symbols, for how to get those symbol definitions into your program.


5.1 Common Behavior

Much effort has been expended to make Autoconf easy to learn. The most obvious way to reach this goal is simply to enforce standard interfaces and behaviors, avoiding exceptions as much as possible. Because of history and inertia, unfortunately, there are still too many exceptions in Autoconf; nevertheless, this section describes some of the common rules.


5.1.1 Standard Symbols

All the generic macros that AC_DEFINE a symbol as a result of their test transform their argument values to a standard alphabet. First, argument is converted to upper case and any asterisks (‘*’) are each converted to ‘P’. Any remaining characters that are not alphanumeric are converted to underscores.

For instance,

AC_CHECK_TYPES([struct $Expensive*])

defines the symbol ‘HAVE_STRUCT__EXPENSIVEP’ if the check succeeds.


5.1.2 Default Includes

Test programs frequently need to include headers that may or may not be available on the system whose features are being tested. Each test can use all the preprocessor macros that have been AC_DEFINEd by previous tests, so for example one may write

#include <time.h>
#ifdef HAVE_SYS_TIME_H
# include <sys/time.h>
#endif

if sys/time.h has already been tested for.

All hosted environments that are still of interest for portable code provide all of the headers specified in ISO C90 (as amended in 1995): assert.h, ctype.h, errno.h, float.h, iso646.h, limits.h, locale.h, math.h, setjmp.h, signal.h, stdarg.h, stddef.h, stdio.h, stdlib.h, string.h, time.h, wchar.h, and wctype.h. Most programs can safely include these headers unconditionally. All other headers, including all headers from later revisions of the C standard, need to be tested for (see Header Files).

If your program needs to be portable to a freestanding environment, such as an embedded OS that doesn’t provide all of the facilities of the C90 standard library, you may need to test for some of the above headers as well. Note that many Autoconf macros internally assume that the complete set of C90 headers are available.

Most generic macros use the following macro to provide a default set of includes:

Macro: AC_INCLUDES_DEFAULT ([include-directives])

Expand to include-directives if present and nonempty, otherwise to:

#include <stddef.h>
#ifdef HAVE_STDIO_H
# include <stdio.h>
#endif
#ifdef HAVE_STDLIB_H
# include <stdlib.h>
#endif
#ifdef HAVE_STRING_H
# include <string.h>
#endif
#ifdef HAVE_INTTYPES_H
# include <inttypes.h>
#endif
#ifdef HAVE_STDINT_H
# include <stdint.h>
#endif
#ifdef HAVE_STRINGS_H
# include <strings.h>
#endif
#ifdef HAVE_SYS_TYPES_H
# include <sys/types.h>
#endif
#ifdef HAVE_SYS_STAT_H
# include <sys/stat.h>
#endif
#ifdef HAVE_UNISTD_H
# include <unistd.h>
#endif

Using this macro without include-directives has the side effect of checking for stdio.h, stdlib.h, string.h, inttypes.h, stdint.h, strings.h, sys/types.h, sys/stat.h, and unistd.h, as if by AC_CHECK_HEADERS_ONCE. For backward compatibility, the macro STDC_HEADERS will be defined when both stdlib.h and string.h are available.

Portability Note: It is safe for most programs to assume the presence of all of the headers required by the original 1990 C standard. AC_INCLUDES_DEFAULT checks for stdio.h, stdlib.h, and string.h, even though they are in that list, because they might not be available when compiling for a “freestanding environment” (in which most of the features of the C library are optional). You probably do not need to write ‘#ifdef HAVE_STDIO_H’ in your own code.

inttypes.h and stdint.h were added to C in the 1999 revision of the standard, and strings.h, sys/types.h, sys/stat.h, and unistd.h are POSIX extensions. You should guard uses of these headers with appropriate conditionals.

Macro: AC_CHECK_INCLUDES_DEFAULT

Check for all the headers that AC_INCLUDES_DEFAULT would check for as a side-effect, if this has not already happened.

This macro mainly exists so that autoupdate can replace certain obsolete constructs with it. You should not need to use it yourself; in fact, it is likely to be safe to delete it from any script in which it appears. (autoupdate does not know whether preprocessor macros such as HAVE_STDINT_H are used in the program, nor whether they would get defined as a side-effect of other checks.)


Next: , Previous: , Up: Existing Tests   [Contents][Index]

5.2 Alternative Programs

These macros check for the presence or behavior of particular programs. They are used to choose between several alternative programs and to decide what to do once one has been chosen. If there is no macro specifically defined to check for a program you need, and you don’t need to check for any special properties of it, then you can use one of the general program-check macros.


5.2.1 Particular Program Checks

These macros check for particular programs—whether they exist, and in some cases whether they support certain features.

Macro: AC_PROG_AWK

Check for gawk, mawk, nawk, and awk, in that order, and set output variable AWK to the first one that is found. It tries gawk first because that is reported to be the best implementation. The result can be overridden by setting the variable AWK or the cache variable ac_cv_prog_AWK.

Using this macro is sufficient to avoid the pitfalls of traditional awk (see Limitations of Usual Tools).

Macro: AC_PROG_GREP

Look for the best available grep or ggrep that accepts the longest input lines possible, and that supports multiple -e options. Set the output variable GREP to whatever is chosen. See Limitations of Usual Tools, for more information about portability problems with the grep command family. The result can be overridden by setting the GREP variable and is cached in the ac_cv_path_GREP variable.

Macro: AC_PROG_EGREP

Check whether $GREP -E works, or else look for the best available egrep or gegrep that accepts the longest input lines possible. Set the output variable EGREP to whatever is chosen. The result can be overridden by setting the EGREP variable and is cached in the ac_cv_path_EGREP variable.

Macro: AC_PROG_FGREP

Check whether $GREP -F works, or else look for the best available fgrep or gfgrep that accepts the longest input lines possible. Set the output variable FGREP to whatever is chosen. The result can be overridden by setting the FGREP variable and is cached in the ac_cv_path_FGREP variable.

Macro: AC_PROG_INSTALL

Set output variable INSTALL to the name of a BSD-compatible install program, if one is found in the current PATH. Otherwise, set INSTALL to ‘dir/install-sh -c’, checking the directories specified to AC_CONFIG_AUX_DIR (or its default directories) to determine dir (see Outputting Files). Also set the variables INSTALL_PROGRAM and INSTALL_SCRIPT to ‘${INSTALL}’ and INSTALL_DATA to ‘${INSTALL} -m 644’.

@INSTALL@’ is special, as its value may vary for different configuration files.

This macro screens out various instances of install known not to work. It prefers to find a C program rather than a shell script, for speed. Instead of install-sh, it can also use install.sh, but that name is obsolete because some make programs have a rule that creates install from it if there is no makefile. Further, this macro requires install to be able to install multiple files into a target directory in a single invocation.

Autoconf comes with a copy of install-sh that you can use. If you use AC_PROG_INSTALL, you must include install-sh in your distribution; otherwise autoreconf and configure will produce an error message saying they can’t find it—even if the system you’re on has a good install program. This check is a safety measure to prevent you from accidentally leaving that file out, which would prevent your package from installing on systems that don’t have a BSD-compatible install program.

If you need to use your own installation program because it has features not found in standard install programs, there is no reason to use AC_PROG_INSTALL; just put the file name of your program into your Makefile.in files.

The result of the test can be overridden by setting the variable INSTALL or the cache variable ac_cv_path_install.

Macro: AC_PROG_MKDIR_P

Set output variable MKDIR_P to a program that ensures that for each argument, a directory named by this argument exists, creating it and its parent directories if needed, and without race conditions when two instances of the program attempt to make the same directory at nearly the same time.

This macro uses the ‘mkdir -p’ command if possible. Otherwise, it falls back on invoking install-sh with the -d option, so your package should contain install-sh as described under AC_PROG_INSTALL. An install-sh file that predates Autoconf 2.60 or Automake 1.10 is vulnerable to race conditions, so if you want to support parallel installs from different packages into the same directory you need to make sure you have an up-to-date install-sh. In particular, be careful about using ‘autoreconf -if’ if your Automake predates Automake 1.10.

This macro is related to the AS_MKDIR_P macro (see Programming in M4sh), but it sets an output variable intended for use in other files, whereas AS_MKDIR_P is intended for use in scripts like configure. Also, AS_MKDIR_P does not accept options, but MKDIR_P supports the -m option, e.g., a makefile might invoke $(MKDIR_P) -m 0 dir to create an inaccessible directory, and conversely a makefile should use $(MKDIR_P) -- $(FOO) if FOO might yield a value that begins with ‘-’. Finally, AS_MKDIR_P does not check for race condition vulnerability, whereas AC_PROG_MKDIR_P does.

@MKDIR_P@’ is special, as its value may vary for different configuration files.

The result of the test can be overridden by setting the variable MKDIR_P or the cache variable ac_cv_path_mkdir.

Macro: AC_PROG_LEX (options)

Search for a lexical analyzer generator, preferring flex to plain lex. Output variable LEX is set to whichever program is available. If neither program is available, LEX is set to ‘:’; for packages that ship the generated file.yy.c alongside the source file.l, this default allows users without a lexer generator to still build the package even if the timestamp for file.l is inadvertently changed.

The name of the program to use can be overridden by setting the output variable LEX or the cache variable ac_cv_prog_LEX when running configure.

If a lexical analyzer generator is found, this macro performs additional checks for common portability pitfalls. If these additional checks fail, LEX is reset to ‘:’; otherwise the following additional macros and variables are provided.

Preprocessor macro YYTEXT_POINTER is defined if the lexer skeleton, by default, declares yytext as a ‘char *’ rather than a ‘char []’.

Output variable LEX_OUTPUT_ROOT is set to the base of the file name that the lexer generates; this is usually either lex.yy or lexyy.

If generated lexers need a library to work, output variable LEXLIB is set to a link option for that library (e.g., -ll), otherwise it is set to empty.

The options argument modifies the behavior of AC_PROG_LEX. It should be a whitespace-separated list of options. Currently there are only two options, and they are mutually exclusive:

yywrap

Indicate that the library in LEXLIB needs to define the function yywrap. If a library that defines this function cannot be found, LEX will be reset to ‘:’.

noyywrap

Indicate that the library in LEXLIB does not need to define the function yywrap. configure will not search for it at all.

Prior to Autoconf 2.70, AC_PROG_LEX did not take any arguments, and its behavior was different from either of the above possibilities: it would search for a library that defines yywrap, and would set LEXLIB to that library if it finds one. However, if a library that defines this function could not be found, LEXLIB would be left empty and LEX would not be reset. This behavior was due to a bug, but several packages came to depend on it, so AC_PROG_LEX still does this if neither the yywrap nor the noyywrap option is given.

Usage of AC_PROG_LEX without choosing one of the yywrap or noyywrap options is deprecated. It is usually better to use noyywrap and define the yywrap function yourself, as this almost always renders the LEXLIB unnecessary.

Caution: As a side-effect of the test, this macro may delete any file in the configure script’s current working directory named lex.yy.c or lexyy.c.

Caution: Packages that ship a generated lex.yy.c cannot assume that the definition of YYTEXT_POINTER matches the code in that file. They also cannot assume that LEXLIB provides the library routines required by the code in that file.

If you use Flex to generate lex.yy.c, you can work around these limitations by defining yywrap and main yourself (rendering -lfl unnecessary), and by using either the --array or --pointer options to control how yytext is declared. The code generated by Flex is also more portable than the code generated by historical versions of Lex.

If you have used Flex to generate lex.yy.c, and especially if your scanner depends on Flex features, we recommend you use this Autoconf snippet to prevent the scanner being regenerated with historical Lex:

AC_PROG_LEX
if test "x$LEX" != xflex; then
  LEX="$SHELL $missing_dir/missing flex"
  AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy])
  AC_SUBST([LEXLIB], [''])
fi

The shell script missing can be found in the Automake distribution.

Remember that the user may have supplied an alternate location in LEX, so if Flex is required, it is better to check that the user provided something sufficient by parsing the output of ‘$LEX --version’ than by simply relying on test "x$LEX" = xflex.

Macro: AC_PROG_LN_S

If ‘ln -s’ works on the current file system (the operating system and file system support symbolic links), set the output variable LN_S to ‘ln -s’; otherwise, if ‘ln’ works, set LN_S to ‘ln’, and otherwise set it to ‘cp -pR’.

If you make a link in a directory other than the current directory, its meaning depends on whether ‘ln’ or ‘ln -s’ is used. To safely create links using ‘$(LN_S)’, either find out which form is used and adjust the arguments, or always invoke ln in the directory where the link is to be created.

In other words, it does not work to do:

$(LN_S) foo /x/bar

Instead, do:

(cd /x && $(LN_S) foo bar)
Macro: AC_PROG_RANLIB

Set output variable RANLIB to ‘ranlib’ if ranlib is found, and otherwise to ‘:’ (do nothing).

Macro: AC_PROG_SED

Set output variable SED to a Sed implementation that conforms to Posix and does not have arbitrary length limits. Report an error if no acceptable Sed is found. See Limitations of Usual Tools, for more information about portability problems with Sed.

The result of this test can be overridden by setting the SED variable and is cached in the ac_cv_path_SED variable.

Macro: AC_PROG_YACC

If bison is found, set output variable YACC to ‘bison -y’. Otherwise, if byacc is found, set YACC to ‘byacc’. Otherwise set YACC to ‘yacc’. The result of this test can be influenced by setting the variable YACC or the cache variable ac_cv_prog_YACC.


5.2.2 Generic Program and File Checks

These macros are used to find programs not covered by the “particular” test macros. If you need to check the behavior of a program as well as find out whether it is present, you have to write your own test for it (see Writing Tests). By default, these macros use the environment variable PATH. If you need to check for a program that might not be in the user’s PATH, you can pass a modified path to use instead, like this:

AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd],
             [$PATH$PATH_SEPARATOR/usr/libexec$PATH_SEPARATOR]dnl
[/usr/sbin$PATH_SEPARATOR/usr/etc$PATH_SEPARATOR/etc])

You are strongly encouraged to declare the variable passed to AC_CHECK_PROG etc. as precious. See Setting Output Variables, AC_ARG_VAR, for more details.

Macro: AC_CHECK_PROG (variable, prog-to-check-for, value-if-found, [value-if-not-found], [path = ‘$PATH], [reject])

Check whether program prog-to-check-for exists in path. If it is found, set variable to value-if-found, otherwise to value-if-not-found, if given. Always pass over reject (an absolute file name) even if it is the first found in the search path; in that case, set variable using the absolute file name of the prog-to-check-for found that is not reject. If variable was already set, do nothing. Calls AC_SUBST for variable. The result of this test can be overridden by setting the variable variable or the cache variable ac_cv_prog_variable.

Macro: AC_CHECK_PROGS (variable, progs-to-check-for, [value-if-not-found], [path = ‘$PATH])

Check for each program in the blank-separated list progs-to-check-for existing in the path. If one is found, set variable to the name of that program. Otherwise, continue checking the next program in the list. If none of the programs in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls AC_SUBST for variable. The result of this test can be overridden by setting the variable variable or the cache variable ac_cv_prog_variable.

Macro: AC_CHECK_TARGET_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the target type as determined by AC_CANONICAL_TARGET, followed by a dash (see Getting the Canonical System Type). If the tool cannot be found with a prefix, and if the build and target types are equal, then it is also searched for without a prefix.

As noted in Specifying target triplets, the target is rarely specified, because most of the time it is the same as the host: it is the type of system for which any compiler tool in the package produces code. What this macro looks for is, for example, a tool (assembler, linker, etc.) that the compiler driver (gcc for the GNU C Compiler) uses to produce objects, archives or executables.

Macro: AC_CHECK_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_PROG, but first looks for prog-to-check-for with a prefix of the host type as specified by --host, followed by a dash. For example, if the user runs ‘configure --build=x86_64-gnu --host=aarch64-linux-gnu’, then this call:

AC_CHECK_TOOL([RANLIB], [ranlib], [:])

sets RANLIB to aarch64-linux-gnu-ranlib if that program exists in path, or otherwise to ‘ranlib’ if that program exists in path, or to ‘:’ if neither program exists.

When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.

Macro: AC_CHECK_TARGET_TOOLS (variable, progs-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_TARGET_TOOL, each of the tools in the list progs-to-check-for are checked with a prefix of the target type as determined by AC_CANONICAL_TARGET, followed by a dash (see Getting the Canonical System Type). If none of the tools can be found with a prefix, and if the build and target types are equal, then the first one without a prefix is used. If a tool is found, set variable to the name of that program. If none of the tools in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls AC_SUBST for variable.

Macro: AC_CHECK_TOOLS (variable, progs-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_TOOL, each of the tools in the list progs-to-check-for are checked with a prefix of the host type as determined by AC_CANONICAL_HOST, followed by a dash (see Getting the Canonical System Type). If none of the tools can be found with a prefix, then the first one without a prefix is used. If a tool is found, set variable to the name of that program. If none of the tools in the list are found, set variable to value-if-not-found; if value-if-not-found is not specified, the value of variable is not changed. Calls AC_SUBST for variable.

When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.

Macro: AC_PATH_PROG (variable, prog-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_PROG, but set variable to the absolute name of prog-to-check-for if found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in the ac_cv_path_variable variable.

Macro: AC_PATH_PROGS (variable, progs-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_PROGS, but if any of progs-to-check-for are found, set variable to the absolute name of the program found. The result of this test can be overridden by setting the variable variable. A positive result of this test is cached in the ac_cv_path_variable variable.

Macro: AC_PATH_PROGS_FEATURE_CHECK (variable, progs-to-check-for, feature-test, [action-if-not-found], [path = ‘$PATH])

This macro was introduced in Autoconf 2.62. If variable is not empty, then set the cache variable ac_cv_path_variable to its value. Otherwise, check for each program in the blank-separated list progs-to-check-for existing in path. For each program found, execute feature-test with ac_path_variable set to the absolute name of the candidate program. If no invocation of feature-test sets the shell variable ac_cv_path_variable, then action-if-not-found is executed. feature-test will be run even when ac_cv_path_variable is set, to provide the ability to choose a better candidate found later in path; to accept the current setting and bypass all further checks, feature-test can execute ac_path_variable_found=:.

Note that this macro has some subtle differences from AC_CHECK_PROGS. It is designed to be run inside AC_CACHE_VAL, therefore, it should have no side effects. In particular, variable is not set to the final value of ac_cv_path_variable, nor is AC_SUBST automatically run. Also, on failure, any action can be performed, whereas AC_CHECK_PROGS only performs variable=value-if-not-found.

Here is an example, similar to what Autoconf uses in its own configure script. It will search for an implementation of m4 that supports the indir builtin, even if it goes by the name gm4 or is not the first implementation on PATH.

AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4],
  [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4],
    [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4`
      test "x$m4out" = x0 \
      && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]],
    [AC_MSG_ERROR([could not find m4 that supports indir])])])
AC_SUBST([M4], [$ac_cv_path_M4])
Macro: AC_PATH_TARGET_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_TARGET_TOOL, but set variable to the absolute name of the program if it is found.

Macro: AC_PATH_TOOL (variable, prog-to-check-for, [value-if-not-found], [path = ‘$PATH])

Like AC_CHECK_TOOL, but set variable to the absolute name of the program if it is found.

When cross-compiling, this macro will issue a warning if no program prefixed with the host type could be found. For more information, see Specifying target triplets.


5.3 Files

You might also need to check for the existence of files. Before using these macros, ask yourself whether a runtime test might not be a better solution. Be aware that, like most Autoconf macros, they test a feature of the host machine, and therefore, they die when cross-compiling.

Macro: AC_CHECK_FILE (file, [action-if-found], [action-if-not-found])

Check whether file file exists on the native system. If it is found, execute action-if-found, otherwise do action-if-not-found, if given. Cache the result of this test in the ac_cv_file_file variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_FILES (files, [action-if-found], [action-if-not-found])

For each file listed in files, execute AC_CHECK_FILE and perform either action-if-found or action-if-not-found. Like AC_CHECK_FILE, this defines ‘HAVE_file’ (see Standard Symbols) for each file found and caches the results of each test in the ac_cv_file_file variable, with characters not suitable for a variable name mapped to underscores.


Next: , Previous: , Up: Existing Tests   [Contents][Index]

5.4 Library Files

The following macros check for the presence of certain C, C++, Fortran, or Go library archive files.

Macro: AC_CHECK_LIB (library, function, [action-if-found], [action-if-not-found], [other-libraries])

Test whether the library library is available by trying to link a test program that calls function function with the library. function should be a function provided by the library. Use the base name of the library; e.g., to check for -lmp, use ‘mp’ as the library argument.

action-if-found is a list of shell commands to run if the link with the library succeeds; action-if-not-found is a list of shell commands to run if the link fails. If action-if-found is not specified, the default action prepends -llibrary to LIBS and defines ‘HAVE_LIBlibrary’ (in all capitals). This macro is intended to support building LIBS in a right-to-left (least-dependent to most-dependent) fashion such that library dependencies are satisfied as a natural side effect of consecutive tests. Linkers are sensitive to library ordering so the order in which LIBS is generated is important to reliable detection of libraries.

If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., -lXt -lX11. Otherwise, this macro may fail to detect that library is present, because linking the test program can fail with unresolved symbols. The other-libraries argument should be limited to cases where it is desirable to test for one library in the presence of another that is not already in LIBS.

AC_CHECK_LIB requires some care in usage, and should be avoided in some common cases. Many standard functions like gethostbyname appear in the standard C library on some hosts, and in special libraries like nsl on other hosts. On some hosts the special libraries contain variant implementations that you may not want to use. These days it is normally better to use AC_SEARCH_LIBS([gethostbyname], [nsl]) instead of AC_CHECK_LIB([nsl], [gethostbyname]).

The result of this test is cached in the ac_cv_lib_library_function variable.

Macro: AC_SEARCH_LIBS (function, search-libs, [action-if-found], [action-if-not-found], [other-libraries])

Search for a library defining function if it’s not already available. This equates to calling ‘AC_LINK_IFELSE([AC_LANG_CALL([], [function])])’ first with no libraries, then for each library listed in search-libs.

Prepend -llibrary to LIBS for the first library found to contain function, and run action-if-found. If the function is not found, run action-if-not-found.

If linking with library results in unresolved symbols that would be resolved by linking with additional libraries, give those libraries as the other-libraries argument, separated by spaces: e.g., -lXt -lX11. Otherwise, this macro fails to detect that function is present, because linking the test program always fails with unresolved symbols.

The result of this test is cached in the ac_cv_search_function variable as ‘none required’ if function is already available, as ‘no’ if no library containing function was found, otherwise as the -llibrary option that needs to be prepended to LIBS.


5.5 Library Functions

The following macros check for particular C library functions. If there is no macro specifically defined to check for a function you need, and you don’t need to check for any special properties of it, then you can use one of the general function-check macros.


5.5.1 Portability of C Functions

Most usual functions can either be missing, or be buggy, or be limited on some architectures. This section tries to make an inventory of these portability issues. By definition, this list always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering Current Posix Functions in Gnulib, Legacy Functions in Gnulib, and Glibc Functions in Gnulib. Please help us keep the Gnulib list as complete as possible.

exit

On ancient hosts, exit returned int. This is because exit predates void, and there was a long tradition of it returning int.

On current hosts, the problem more likely is that exit is not declared, due to C++ problems of some sort or another. For this reason we suggest that test programs not invoke exit, but return from main instead.

free

The C standard says a call free (NULL) does nothing, but some old systems don’t support this (e.g., NextStep).

isinf
isnan

In C99 and later, isinf and isnan are macros. On some systems just macros are available (e.g., HP-UX and Solaris 10), on some systems both macros and functions (e.g., glibc 2.3.2), and on some systems only functions (e.g., IRIX 6 and Solaris 9). In some cases these functions are declared in nonstandard headers like <sunmath.h> and defined in non-default libraries like -lm or -lsunmath.

In C99 and later, isinf and isnan macros work correctly with long double arguments, but pre-C99 systems that use functions typically assume double arguments. On such a system, isinf incorrectly returns true for a finite long double argument that is outside the range of double.

The best workaround for these issues is to use Gnulib modules isinf and isnan (see Gnulib). But a lighter weight solution involves code like the following.

#include <math.h>

#ifndef isnan
# define isnan(x) \
    (sizeof (x) == sizeof (long double) ? isnan_ld (x) \
     : sizeof (x) == sizeof (double) ? isnan_d (x) \
     : isnan_f (x))
static int isnan_f  (float       x) { return x != x; }
static int isnan_d  (double      x) { return x != x; }
static int isnan_ld (long double x) { return x != x; }
#endif

#ifndef isinf
# define isinf(x) \
    (sizeof (x) == sizeof (long double) ? isinf_ld (x) \
     : sizeof (x) == sizeof (double) ? isinf_d (x) \
     : isinf_f (x))
static int isinf_f  (float       x)
{ return !isnan (x) && isnan (x - x); }
static int isinf_d  (double      x)
{ return !isnan (x) && isnan (x - x); }
static int isinf_ld (long double x)
{ return !isnan (x) && isnan (x - x); }
#endif

Some optimizing compilers mishandle these definitions, but systems with that bug typically have many other floating point corner-case compliance problems anyway, so it’s probably not worth worrying about.

malloc

The C standard says a call malloc (0) is implementation dependent. It can return either NULL or a new non-null pointer. The latter is more common (e.g., the GNU C Library) but is by no means universal. AC_FUNC_MALLOC can be used to insist on non-NULL (see Particular Function Checks).

putenv

Posix prefers setenv to putenv; among other things, putenv is not required of all Posix implementations, but setenv is.

Posix specifies that putenv puts the given string directly in environ, but some systems make a copy of it instead (e.g., glibc 2.0, or BSD). And when a copy is made, unsetenv might not free it, causing a memory leak (e.g., FreeBSD 4).

On some systems putenv ("FOO") removes ‘FOO’ from the environment, but this is not standard usage and it dumps core on some systems (e.g., AIX).

On MinGW, a call putenv ("FOO=") removes ‘FOO’ from the environment, rather than inserting it with an empty value.

realloc

The C standard says a call realloc (NULL, size) is equivalent to malloc (size), but some old systems don’t support this (e.g., NextStep).

signal handler

Normally signal takes a handler function with a return type of void, but some old systems required int instead. Any actual int value returned is not used; this is only a difference in the function prototype demanded.

All systems we know of in current use return void. The int was to support K&R C, where of course void is not available. The obsolete macro AC_TYPE_SIGNAL (see AC_TYPE_SIGNAL) can be used to establish the correct type in all cases.

In most cases, it is more robust to use sigaction when it is available, rather than signal.

snprintf

In C99 and later, if the output array isn’t big enough and if no other errors occur, snprintf and vsnprintf truncate the output and return the number of bytes that ought to have been produced. Some older systems return the truncated length (e.g., GNU C Library 2.0.x or IRIX 6.5), some a negative value (e.g., earlier GNU C Library versions), and some the buffer length without truncation (e.g., 32-bit Solaris 7). Also, some buggy older systems ignore the length and overrun the buffer (e.g., 64-bit Solaris 7).

sprintf

The C standard says sprintf and vsprintf return the number of bytes written. On some ancient systems (SunOS 4 for instance) they return the buffer pointer instead, but these no longer need to be worried about.

sscanf

On various old systems, e.g., HP-UX 9, sscanf requires that its input string be writable (though it doesn’t actually change it). This can be a problem when using gcc since it normally puts constant strings in read-only memory (see Incompatibilities of GCC in Using and Porting the GNU Compiler Collection). Apparently in some cases even having format strings read-only can be a problem.

strerror_r

Posix specifies that strerror_r returns an int, but many systems (e.g., GNU C Library version 2.2.4) provide a different version returning a char *. AC_FUNC_STRERROR_R can detect which is in use (see Particular Function Checks).

strnlen

AIX 4.3 provides a broken version which produces the following results:

strnlen ("foobar", 0) = 0
strnlen ("foobar", 1) = 3
strnlen ("foobar", 2) = 2
strnlen ("foobar", 3) = 1
strnlen ("foobar", 4) = 0
strnlen ("foobar", 5) = 6
strnlen ("foobar", 6) = 6
strnlen ("foobar", 7) = 6
strnlen ("foobar", 8) = 6
strnlen ("foobar", 9) = 6
sysconf

_SC_PAGESIZE is standard, but some older systems (e.g., HP-UX 9) have _SC_PAGE_SIZE instead. This can be tested with #ifdef.

unlink

The Posix spec says that unlink causes the given file to be removed only after there are no more open file handles for it. Some non-Posix hosts have trouble with this requirement, though, and some DOS variants even corrupt the file system.

unsetenv

On MinGW, unsetenv is not available, but a variable ‘FOO’ can be removed with a call putenv ("FOO="), as described under putenv above.

va_copy

C99 and later provide va_copy for copying va_list variables. It may be available in older environments too, though possibly as __va_copy (e.g., gcc in strict pre-C99 mode). These can be tested with #ifdef. A fallback to memcpy (&dst, &src, sizeof (va_list)) gives maximum portability.

va_list

va_list is not necessarily just a pointer. It can be a struct (e.g., gcc on Alpha), which means NULL is not portable. Or it can be an array (e.g., gcc in some PowerPC configurations), which means as a function parameter it can be effectively call-by-reference and library routines might modify the value back in the caller (e.g., vsnprintf in the GNU C Library 2.1).

Signed >>

Normally the C >> right shift of a signed type replicates the high bit, giving a so-called “arithmetic” shift. But care should be taken since Standard C doesn’t require that behavior. On those few processors without a native arithmetic shift (for instance Cray vector systems) zero bits may be shifted in, the same as a shift of an unsigned type.

Integer /

C divides signed integers by truncating their quotient toward zero, yielding the same result as Fortran. However, before C99 the standard allowed C implementations to take the floor or ceiling of the quotient in some cases. Hardly any implementations took advantage of this freedom, though, and it’s probably not worth worrying about this issue nowadays.


5.5.2 Particular Function Checks

These macros check for particular C functions—whether they exist, and in some cases how they respond when given certain arguments.

Macro: AC_FUNC_ALLOCA

Check for the alloca function. Define HAVE_ALLOCA_H if alloca.h defines a working alloca. If not, look for a builtin alternative. If either method succeeds, define HAVE_ALLOCA. Otherwise, set the output variable ALLOCA to ‘${LIBOBJDIR}alloca.o’ and define C_ALLOCA (so programs can periodically call ‘alloca (0)’ to garbage collect). This variable is separate from LIBOBJS so multiple programs can share the value of ALLOCA without needing to create an actual library, in case only some of them use the code in LIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same purpose as in LIBOBJS (see AC_LIBOBJ vs. LIBOBJS).

Source files that use alloca should start with a piece of code like the following, to declare it properly.

#include <stdlib.h>
#include <stddef.h>
#ifdef HAVE_ALLOCA_H
# include <alloca.h>
#elif !defined alloca
# ifdef __GNUC__
#  define alloca __builtin_alloca
# elif defined _MSC_VER
#  include <malloc.h>
#  define alloca _alloca
# elif !defined HAVE_ALLOCA
#  ifdef  __cplusplus
extern "C"
#  endif
void *alloca (size_t);
# endif
#endif

If you don’t want to maintain this piece of code in your package manually, you can instead use the Gnulib module alloca-opt or alloca. See Gnulib.

Macro: AC_FUNC_CHOWN

If the chown function is available and works (in particular, it should accept -1 for uid and gid), define HAVE_CHOWN. The result of this macro is cached in the ac_cv_func_chown_works variable.

If you want a workaround, that is, a chown function that is available and works, you can use the Gnulib module chown. See Gnulib.

Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value, define CLOSEDIR_VOID. Otherwise, callers ought to check its return value for an error indicator.

Currently this test is implemented by running a test program. When cross compiling the pessimistic assumption that closedir does not return a meaningful value is made.

The result of this macro is cached in the ac_cv_func_closedir_void variable.

This macro is obsolescent, as closedir returns a meaningful value on current systems. New programs need not use this macro.

Macro: AC_FUNC_ERROR_AT_LINE

If the error_at_line function is not found, require an AC_LIBOBJ replacement of ‘error’.

The result of this macro is cached in the ac_cv_lib_error_at_line variable.

The AC_FUNC_ERROR_AT_LINE macro is obsolescent. New programs should use Gnulib’s error module. See Gnulib.

Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define HAVE_FNMATCH. Detect common implementation bugs, for example, the bugs in Solaris 2.4.

Unlike the other specific AC_FUNC macros, AC_FUNC_FNMATCH does not replace a broken/missing fnmatch. This is for historical reasons. See AC_REPLACE_FNMATCH below.

The result of this macro is cached in the ac_cv_func_fnmatch_works variable.

This macro is obsolescent. New programs should use Gnulib’s fnmatch-posix module. See Gnulib.

Macro: AC_FUNC_FNMATCH_GNU

Behave like AC_REPLACE_FNMATCH (replace) but also test whether fnmatch supports GNU extensions. Detect common implementation bugs, for example, the bugs in the GNU C Library 2.1.

The result of this macro is cached in the ac_cv_func_fnmatch_gnu variable.

This macro is obsolescent. New programs should use Gnulib’s fnmatch-gnu module. See Gnulib.

Macro: AC_FUNC_FORK

This macro checks for the fork and vfork functions. If a working fork is found, define HAVE_WORKING_FORK. This macro checks whether fork is just a stub by trying to run it.

If vfork.h is found, define HAVE_VFORK_H. If a working vfork is found, define HAVE_WORKING_VFORK. Otherwise, define vfork to be fork for backward compatibility with previous versions of autoconf. This macro checks for several known errors in implementations of vfork and considers the system to not have a working vfork if it detects any of them.

Since this macro defines vfork only for backward compatibility with previous versions of autoconf you’re encouraged to define it yourself in new code:

#ifndef HAVE_WORKING_VFORK
# define vfork fork
#endif

The results of this macro are cached in the ac_cv_func_fork_works and ac_cv_func_vfork_works variables. In order to override the test, you also need to set the ac_cv_func_fork and ac_cv_func_vfork variables.

Macro: AC_FUNC_FSEEKO

If the fseeko function is available, define HAVE_FSEEKO. Define _LARGEFILE_SOURCE if necessary to make the prototype visible on some systems (e.g., glibc 2.2). Otherwise linkage problems may occur when compiling with AC_SYS_LARGEFILE on largefile-sensitive systems where off_t does not default to a 64bit entity. All systems with fseeko also supply ftello.

The Gnulib module fseeko invokes AC_FUNC_FSEEKO and also contains workarounds for other portability problems of fseeko. See Gnulib.

Macro: AC_FUNC_GETGROUPS

If the getgroups function is available and works (unlike on Ultrix 4.3 and NeXTstep 3.2, where ‘getgroups (0, 0)’ always fails), define HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries needed to get that function. This macro runs AC_TYPE_GETGROUPS.

This macro is obsolescent. New programs need not use this macro. But they may want to use the Gnulib module getgroups, which provides workarounds to other portability problems of this function.

Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. To perform its tests properly, this macro needs the file getloadavg.c; therefore, be sure to set the AC_LIBOBJ replacement directory properly (see Generic Function Checks, AC_CONFIG_LIBOBJ_DIR).

If the system has the getloadavg function, define HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries necessary to get that function. Also add GETLOADAVG_LIBS to LIBS. Otherwise, require an AC_LIBOBJ replacement for ‘getloadavg’ and possibly define several other C preprocessor macros and output variables:

  1. Define C_GETLOADAVG.
  2. Define SVR4, DGUX, UMAX, or UMAX4_3 if on those systems.
  3. If nlist.h is found, define HAVE_NLIST_H.
  4. If ‘struct nlist’ has an ‘n_un.n_name’ member, define HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbol NLIST_NAME_UNION is still defined, but do not depend upon it.
  5. Programs may need to be installed set-group-ID (or set-user-ID) for getloadavg to work. In this case, define GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID to ‘true’ (and otherwise to ‘false’), and set KMEM_GROUP to the name of the group that should own the installed program.

The AC_FUNC_GETLOADAVG macro is obsolescent. New programs should use Gnulib’s getloadavg module. See Gnulib.

Macro: AC_FUNC_GETMNTENT

Check for getmntent in the standard C library, and then in the sun, seq, and gen libraries, for UNICOS, IRIX 4, PTX, and UnixWare, respectively. Then, if getmntent is available, define HAVE_GETMNTENT and set ac_cv_func_getmntent to yes. Otherwise set ac_cv_func_getmntent to no.

The result of this macro can be overridden by setting the cache variable ac_cv_search_getmntent.

The AC_FUNC_GETMNTENT macro is obsolescent. New programs should use Gnulib’s mountlist module. See Gnulib.

Macro: AC_FUNC_GETPGRP

Define GETPGRP_VOID if it is an error to pass 0 to getpgrp; this is the Posix behavior. On older BSD systems, you must pass 0 to getpgrp, as it takes an argument and behaves like Posix’s getpgid.

#ifdef GETPGRP_VOID
  pid = getpgrp ();
#else
  pid = getpgrp (0);
#endif

This macro does not check whether getpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for getpgrp.

The result of this macro is cached in the ac_cv_func_getpgrp_void variable.

This macro is obsolescent, as current systems have a getpgrp whose signature conforms to Posix. New programs need not use this macro.

If link is a symbolic link, then lstat should treat link/ the same as link/.. However, many older lstat implementations incorrectly ignore trailing slashes.

It is safe to assume that if lstat incorrectly ignores trailing slashes, then other symbolic-link-aware functions like unlink also incorrectly ignore trailing slashes.

If lstat behaves properly, define LSTAT_FOLLOWS_SLASHED_SYMLINK, otherwise require an AC_LIBOBJ replacement of lstat.

The result of this macro is cached in the ac_cv_func_lstat_dereferences_slashed_symlink variable.

The AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK macro is obsolescent. New programs should use Gnulib’s lstat module. See Gnulib.

Macro: AC_FUNC_MALLOC

If the malloc function is compatible with the GNU C library malloc (i.e., ‘malloc (0)’ returns a valid pointer), define HAVE_MALLOC to 1. Otherwise define HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for ‘malloc’, and define malloc to rpl_malloc so that the native malloc is not used in the main project.

Typically, the replacement file malloc.c should look like (note the ‘#undef malloc’):

#include <config.h>
#undef malloc

#include <sys/types.h>

void *malloc ();

/* Allocate an N-byte block of memory from the heap.
   If N is zero, allocate a 1-byte block.  */

void *
rpl_malloc (size_t n)
{
  if (n == 0)
    n = 1;
  return malloc (n);
}

The result of this macro is cached in the ac_cv_func_malloc_0_nonnull variable.

If you don’t want to maintain a malloc.c file in your package manually, you can instead use the Gnulib module malloc-gnu.

Macro: AC_FUNC_MBRTOWC

Define HAVE_MBRTOWC to 1 if the function mbrtowc and the type mbstate_t are properly declared.

The result of this macro is cached in the ac_cv_func_mbrtowc variable.

The Gnulib module mbrtowc not only ensures that the function is declared, but also works around other portability problems of this function.

Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on 8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 bytes or more and with at least one buffer not starting on a 4-byte boundary (such as the one on NeXT x86 OpenStep), require an AC_LIBOBJ replacement for ‘memcmp’.

The result of this macro is cached in the ac_cv_func_memcmp_working variable.

This macro is obsolescent, as current systems have a working memcmp. New programs need not use this macro.

Macro: AC_FUNC_MKTIME

If the mktime function is not available, or does not work correctly, require an AC_LIBOBJ replacement for ‘mktime’. For the purposes of this test, mktime should conform to the Posix standard and should be the inverse of localtime.

The result of this macro is cached in the ac_cv_func_working_mktime variable.

The AC_FUNC_MKTIME macro is obsolescent. New programs should use Gnulib’s mktime module. See Gnulib.

Macro: AC_FUNC_MMAP

If the mmap function exists and works correctly, define HAVE_MMAP. This checks only private fixed mapping of already-mapped memory.

The result of this macro is cached in the ac_cv_func_mmap_fixed_mapped variable.

Note: This macro asks for more than what an average program needs from mmap. In particular, the use of MAP_FIXED fails on HP-UX 11, whereas mmap otherwise works fine on this platform.

Macro: AC_FUNC_OBSTACK

If the obstacks are found, define HAVE_OBSTACK, else require an AC_LIBOBJ replacement for ‘obstack’.

The result of this macro is cached in the ac_cv_func_obstack variable.

The AC_FUNC_OBSTACK macro is obsolescent. New programs should use Gnulib’s obstack module. See Gnulib.

Macro: AC_FUNC_REALLOC

If the realloc function is compatible with the GNU C library realloc (i.e., ‘realloc (NULL, 0)’ returns a valid pointer), define HAVE_REALLOC to 1. Otherwise define HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for ‘realloc’, and define realloc to rpl_realloc so that the native realloc is not used in the main project. See AC_FUNC_MALLOC for details.

The result of this macro is cached in the ac_cv_func_realloc_0_nonnull variable.

If you don’t want to maintain a realloc.c file in your package manually, you can instead use the Gnulib module realloc-gnu.

Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed for each of the select function’s arguments, and defines those types in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults to ‘int’, SELECT_TYPE_ARG234 defaults to ‘int *’, and SELECT_TYPE_ARG5 defaults to ‘struct timeval *’.

This macro is obsolescent, as current systems have a select whose signature conforms to Posix. New programs need not use this macro.

Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the Posix version), define SETPGRP_VOID. Otherwise, it is the BSD version, which takes two process IDs as arguments. This macro does not check whether setpgrp exists at all; if you need to work in that situation, first call AC_CHECK_FUNC for setpgrp. This macro also does not check for the Solaris variant of setpgrp, which returns a pid_t instead of an int; portable code should only use the return value by comparing it against -1 to check for errors.

The result of this macro is cached in the ac_cv_func_setpgrp_void variable.

This macro is obsolescent, as all forms of setpgrp are also obsolescent. New programs should use the Posix function setpgid, which takes two process IDs as arguments (like the BSD setpgrp).

Macro: AC_FUNC_STAT
Macro: AC_FUNC_LSTAT

Determine whether stat or lstat have the bug that it succeeds when given the zero-length file name as argument. The stat and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do this.

If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ replacement of it.

The results of these macros are cached in the ac_cv_func_stat_empty_string_bug and the ac_cv_func_lstat_empty_string_bug variables, respectively.

These macros are obsolescent, as no current systems have the bug. New programs need not use these macros.

Macro: AC_FUNC_STRCOLL

If the strcoll function exists and works correctly, define HAVE_STRCOLL. This does a bit more than ‘AC_CHECK_FUNCS(strcoll)’, because some systems have incorrect definitions of strcoll that should not be used. But it does not check against a known bug of this function on Solaris 10.

The result of this macro is cached in the ac_cv_func_strcoll_works variable.

Macro: AC_FUNC_STRERROR_R

If strerror_r is available, define HAVE_STRERROR_R, and if it is declared, define HAVE_DECL_STRERROR_R. If it returns a char * message, define STRERROR_R_CHAR_P; otherwise it returns an int error number. The Thread-Safe Functions option of Posix requires strerror_r to return int, but many systems (including, for example, version 2.2.4 of the GNU C Library) return a char * value that is not necessarily equal to the buffer argument.

The result of this macro is cached in the ac_cv_func_strerror_r_char_p variable.

The Gnulib module strerror_r not only ensures that the function has the return type specified by Posix, but also works around other portability problems of this function.

Macro: AC_FUNC_STRFTIME

Check for strftime in the intl library, for SCO Unix. Then, if strftime is available, define HAVE_STRFTIME.

This macro is obsolescent, as no current systems require the intl library for strftime. New programs need not use this macro.

Macro: AC_FUNC_STRTOD

If the strtod function does not exist or doesn’t work correctly, ask for an AC_LIBOBJ replacement of ‘strtod’. In this case, because strtod.c is likely to need ‘pow’, set the output variable POW_LIB to the extra library needed.

This macro caches its result in the ac_cv_func_strtod variable and depends upon the result in the ac_cv_func_pow variable.

The AC_FUNC_STRTOD macro is obsolescent. New programs should use Gnulib’s strtod module. See Gnulib.

Macro: AC_FUNC_STRTOLD

If the strtold function exists and conforms to C99 or later, define HAVE_STRTOLD.

This macro caches its result in the ac_cv_func_strtold variable.

The Gnulib module strtold not only ensures that the function exists, but also works around other portability problems of this function.

Macro: AC_FUNC_STRNLEN

If the strnlen function is not available, or is buggy (like the one from AIX 4.3), require an AC_LIBOBJ replacement for it.

This macro caches its result in the ac_cv_func_strnlen_working variable.

The AC_FUNC_STRNLEN macro is obsolescent. New programs should use Gnulib’s strnlen module. See Gnulib.

Macro: AC_FUNC_UTIME_NULL

If ‘utime (file, NULL)’ sets file’s timestamp to the present, define HAVE_UTIME_NULL.

This macro caches its result in the ac_cv_func_utime_null variable.

This macro is obsolescent, as all current systems have a utime that behaves this way. New programs need not use this macro.

Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if _doprnt is found, define HAVE_DOPRNT. (If vprintf is available, you may assume that vfprintf and vsprintf are also available.)

This macro is obsolescent, as all current systems have vprintf. New programs need not use this macro.

Macro: AC_REPLACE_FNMATCH

If the fnmatch function does not conform to Posix (see AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.

The files fnmatch.c, fnmatch_loop.c, and fnmatch_.h in the AC_LIBOBJ replacement directory are assumed to contain a copy of the source code of GNU fnmatch. If necessary, this source code is compiled as an AC_LIBOBJ replacement, and the fnmatch_.h file is linked to fnmatch.h so that it can be included in place of the system <fnmatch.h>.

This macro caches its result in the ac_cv_func_fnmatch_works variable.

This macro is obsolescent, as it assumes the use of particular source files. New programs should use Gnulib’s fnmatch-posix module, which provides this macro along with the source files. See Gnulib.


5.5.3 Generic Function Checks

These macros are used to find functions not covered by the “particular” test macros. If the functions might be in libraries other than the default C library, first call AC_CHECK_LIB for those libraries. If you need to check the behavior of a function as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

Macro: AC_CHECK_FUNC (function, [action-if-found], [action-if-not-found])

If C function function is available, run shell commands action-if-found, otherwise action-if-not-found. If you just want to define a symbol if the function is available, consider using AC_CHECK_FUNCS instead. This macro checks for functions with C linkage even when AC_LANG(C++) has been called, since C is more standardized than C++. (see Language Choice, for more information about selecting the language for checks.)

This macro caches its result in the ac_cv_func_function variable.

Macro: AC_CHECK_FUNCS (function…, [action-if-found], [action-if-not-found])

For each function enumerated in the blank-or-newline-separated argument list, define HAVE_function (in all capitals) if it is available. If action-if-found is given, it is additional shell code to execute when one of the functions is found. You can give it a value of ‘break’ to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the functions is not found.

Results are cached for each function as in AC_CHECK_FUNC.

Macro: AC_CHECK_FUNCS_ONCE (function…)

For each function enumerated in the blank-or-newline-separated argument list, define HAVE_function (in all capitals) if it is available. This is a once-only variant of AC_CHECK_FUNCS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.


Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a Posix environment. Some functions may be missing or unfixable, and your package must be ready to replace them.

Suitable replacements for many such problem functions are available from Gnulib (see Gnulib).

Macro: AC_LIBOBJ (function)

Specify that ‘function.c’ must be included in the executables to replace a missing or broken implementation of function.

Technically, it adds ‘function.$ac_objext’ to the output variable LIBOBJS if it is not already in, and calls AC_LIBSOURCE for ‘function.c’. You should not directly change LIBOBJS, since this is not traceable.

Macro: AC_LIBSOURCE (file)

Specify that file might be needed to compile the project. If you need to know what files might be needed by a configure.ac, you should trace AC_LIBSOURCE. file must be a literal.

This macro is called automatically from AC_LIBOBJ, but you must call it explicitly if you pass a shell variable to AC_LIBOBJ. In that case, since shell variables cannot be traced statically, you must pass to AC_LIBSOURCE any possible files that the shell variable might cause AC_LIBOBJ to need. For example, if you want to pass a variable $foo_or_bar to AC_LIBOBJ that holds either "foo" or "bar", you should do:

AC_LIBSOURCE([foo.c])
AC_LIBSOURCE([bar.c])
AC_LIBOBJ([$foo_or_bar])

There is usually a way to avoid this, however, and you are encouraged to simply call AC_LIBOBJ with literal arguments.

Note that this macro replaces the obsolete AC_LIBOBJ_DECL, with slightly different semantics: the old macro took the function name, e.g., foo, as its argument rather than the file name.

Macro: AC_LIBSOURCES (files)

Like AC_LIBSOURCE, but accepts one or more files in a comma-separated M4 list. Thus, the above example might be rewritten:

AC_LIBSOURCES([foo.c, bar.c])
AC_LIBOBJ([$foo_or_bar])
Macro: AC_CONFIG_LIBOBJ_DIR (directory)

Specify that AC_LIBOBJ replacement files are to be found in directory, a name relative to the top level of the source tree. The replacement directory defaults to ., the top level directory, and the most typical value is lib, corresponding to ‘AC_CONFIG_LIBOBJ_DIR([lib])’.

configure might need to know the replacement directory for the following reasons: (i) some checks use the replacement files, (ii) some macros bypass broken system headers by installing links to the replacement headers (iii) when used in conjunction with Automake, within each makefile, directory is used as a relative path from $(top_srcdir) to each object named in LIBOBJS and LTLIBOBJS, etc.


It is common to merely check for the existence of a function, and ask for its AC_LIBOBJ replacement if missing. The following macro is a convenient shorthand.

Macro: AC_REPLACE_FUNCS (function…)

Like AC_CHECK_FUNCS, but uses ‘AC_LIBOBJ(function)’ as action-if-not-found. You can declare your replacement function by enclosing the prototype in ‘#ifndef HAVE_function’. If the system has the function, it probably declares it in a header file you should be including, so you shouldn’t redeclare it lest your declaration conflict.


5.6 Header Files

The following macros check for the presence of certain C header files. If there is no macro specifically defined to check for a header file you need, and you don’t need to check for any special properties of it, then you can use one of the general header-file check macros.


5.6.1 Portability of Headers

This section documents some collected knowledge about common headers, and the problems they cause. By definition, this list always requires additions. A much more complete list is maintained by the Gnulib project (see Gnulib), covering Posix Headers in Gnulib and Glibc Headers in Gnulib. Please help us keep the Gnulib list as complete as possible.

When we say that a header “may require” some set of other headers, we mean that it may be necessary for you to manually include those other headers first, or the contents of the header under test will fail to compile. When checking for these headers, you must provide the potentially-required headers in the includes argument to AC_CHECK_HEADER or AC_CHECK_HEADERS, or the check will fail spuriously. AC_INCLUDES_DEFAULT (see Default Includes) arranges to include a number of common requirements and should normally come first in your includes. For example, net/if.h may require sys/types.h, sys/socket.h, or both, and AC_INCLUDES_DEFAULT handles sys/types.h but not sys/socket.h, so you should check for it like this:

AC_CHECK_HEADERS([sys/socket.h])
AC_CHECK_HEADERS([net/if.h], [], [],
[AC_INCLUDES_DEFAULT[
#ifdef HAVE_SYS_SOCKET_H
# include <sys/socket.h>
#endif
]])

Note that the example mixes single quoting (forAC_INCLUDES_DEFAULT, so that it gets expanded) and double quoting (to ensure that each preprocessor # gets treated as a literal string rather than a comment).

limits.h

In C99 and later, limits.h defines LLONG_MIN, LLONG_MAX, and ULLONG_MAX, but many almost-C99 environments (e.g., default GCC 4.0.2 + glibc 2.4) do not define them.

memory.h

This header file is obsolete; use string.h instead.

strings.h

On some systems, this is the only header that declares strcasecmp, strncasecmp, and ffs.

This header may or may not include string.h for you. However, on all recent systems it is safe to include both string.h and strings.h, in either order, in the same source file.

inttypes.h vs. stdint.h

C99 specifies that inttypes.h includes stdint.h, so there’s no need to include stdint.h separately in a standard environment. However, some implementations have inttypes.h but not stdint.h (e.g., Solaris 7), and some have stdint.h but not inttypes.h (e.g. MSVC 2012). Therefore, it is necessary to check for each and include each only if available.

linux/irda.h

This header may require linux/types.h and/or sys/socket.h.

linux/random.h

This header may require linux/types.h.

net/if.h

This header may require sys/types.h and/or sys/socket.h.

netinet/if_ether.h

This header may require some combination of sys/types.h, sys/socket.h, netinet/in.h, and net/if.h.

sys/mount.h

This header may require sys/params.h.

sys/ptem.h

This header may require sys/stream.h.

sys/socket.h

This header may require sys/types.h.

sys/ucred.h

This header may require sys/types.h.

X11/extensions/scrnsaver.h

Using XFree86, this header requires X11/Xlib.h, which is probably so required that you might not even consider looking for it.


5.6.2 Particular Header Checks

These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.

Macro: AC_CHECK_HEADER_STDBOOL

Check whether stdbool.h exists and conforms to C99 or later, and cache the result in the ac_cv_header_stdbool_h variable. If the type _Bool is defined, define HAVE__BOOL to 1.

This macro is intended for use by Gnulib (see Gnulib) and other packages that supply a substitute stdbool.h on platforms lacking a conforming one. The AC_HEADER_STDBOOL macro is better for code that explicitly checks for stdbool.h.

Macro: AC_HEADER_ASSERT

Check whether to enable assertions in the style of assert.h. Assertions are enabled by default, but the user can override this by invoking configure with the --disable-assert option.

Macro: AC_HEADER_DIRENT

Check for the following header files. For the first one that is found and defines ‘DIR’, define the listed C preprocessor macro:

dirent.hHAVE_DIRENT_H
sys/ndir.hHAVE_SYS_NDIR_H
sys/dir.hHAVE_SYS_DIR_H
ndir.hHAVE_NDIR_H

The directory-library declarations in your source code should look something like the following:

#include <sys/types.h>
#ifdef HAVE_DIRENT_H
# include <dirent.h>
# define NAMLEN(dirent) strlen ((dirent)->d_name)
#else
# define dirent direct
# define NAMLEN(dirent) ((dirent)->d_namlen)
# ifdef HAVE_SYS_NDIR_H
#  include <sys/ndir.h>
# endif
# ifdef HAVE_SYS_DIR_H
#  include <sys/dir.h>
# endif
# ifdef HAVE_NDIR_H
#  include <ndir.h>
# endif
#endif

Using the above declarations, the program would declare variables to be of type struct dirent, not struct direct, and would access the length of a directory entry name by passing a pointer to a struct dirent to the NAMLEN macro.

This macro also checks for the SCO Xenix dir and x libraries.

This macro is obsolescent, as all current systems with directory libraries have <dirent.h>. New programs need not use this macro.

Also see AC_STRUCT_DIRENT_D_INO and AC_STRUCT_DIRENT_D_TYPE (see Particular Structure Checks).

Macro: AC_HEADER_MAJOR

Detect the headers required to use makedev, major, and minor. These functions may be defined by sys/mkdev.h, sys/sysmacros.h, or sys/types.h.

AC_HEADER_MAJOR defines MAJOR_IN_MKDEV if they are in sys/mkdev.h, or MAJOR_IN_SYSMACROS if they are in sys/sysmacros.h. If neither macro is defined, they are either in sys/types.h or unavailable.

To properly use these functions, your code should contain something like:

#include <sys/types.h>
#ifdef MAJOR_IN_MKDEV
# include <sys/mkdev.h>
#elif defined MAJOR_IN_SYSMACROS
# include <sys/sysmacros.h>
#endif

Note: Configure scripts built with Autoconf 2.69 or earlier will not detect a problem if sys/types.h contains definitions of major, minor, and/or makedev that trigger compiler warnings upon use. This is known to occur with GNU libc 2.25, where those definitions are being deprecated to reduce namespace pollution. If it is not practical to use Autoconf 2.70 to regenerate the configure script of affected software, you can work around the problem by setting ‘ac_cv_header_sys_types_h_makedev=no’, as an argument to configure or as part of a config.site site default file (see Setting Site Defaults).

Macro: AC_HEADER_RESOLV

Checks for header resolv.h, checking for prerequisites first. To properly use resolv.h, your code should contain something like the following:

#ifdef HAVE_SYS_TYPES_H
#  include <sys/types.h>
#endif
#ifdef HAVE_NETINET_IN_H
#  include <netinet/in.h>   /* inet_ functions / structs */
#endif
#ifdef HAVE_ARPA_NAMESER_H
#  include <arpa/nameser.h> /* DNS HEADER struct */
#endif
#ifdef HAVE_NETDB_H
#  include <netdb.h>
#endif
#include <resolv.h>
Macro: AC_HEADER_STAT

If the macros S_ISDIR, S_ISREG, etc. defined in sys/stat.h do not work properly (returning false positives), define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.

This macro is obsolescent, as no current systems have the bug. New programs need not use this macro.

Macro: AC_HEADER_STDBOOL

If stdbool.h exists and conforms to C99 or later, define HAVE_STDBOOL_H to 1; if the type _Bool is defined, define HAVE__BOOL to 1. To fulfill the standard’s requirements, your program could contain the following code:

#ifdef HAVE_STDBOOL_H
# include <stdbool.h>
#else
# ifndef HAVE__BOOL
#  ifdef __cplusplus
typedef bool _Bool;
#  else
#   define _Bool signed char
#  endif
# endif
# define bool _Bool
# define false 0
# define true 1
# define __bool_true_false_are_defined 1
#endif

Alternatively you can use the ‘stdbool’ package of Gnulib (see Gnulib). It simplifies your code so that it can say just #include <stdbool.h>, and it adds support for less-common platforms.

This macro caches its result in the ac_cv_header_stdbool_h variable.

This macro differs from AC_CHECK_HEADER_STDBOOL only in that it defines HAVE_STDBOOL_H whereas AC_CHECK_HEADER_STDBOOL does not.

Macro: AC_HEADER_STDC

This macro is obsolescent. Its sole effect is to make sure that all the headers that are included by AC_INCLUDES_DEFAULT (see Default Includes), but not part of ISO C90, have been checked for.

All hosted environments that are still of interest for portable code provide all of the headers specified in ISO C90 (as amended in 1995).

Macro: AC_HEADER_SYS_WAIT

If sys/wait.h exists and is compatible with Posix, define HAVE_SYS_WAIT_H. Incompatibility can occur if sys/wait.h does not exist, or if it uses the old BSD union wait instead of int to store a status value. If sys/wait.h is not Posix compatible, then instead of including it, define the Posix macros with their usual interpretations. Here is an example:

#include <sys/types.h>
#ifdef HAVE_SYS_WAIT_H
# include <sys/wait.h>
#endif
#ifndef WEXITSTATUS
# define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8)
#endif
#ifndef WIFEXITED
# define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
#endif

This macro caches its result in the ac_cv_header_sys_wait_h variable.

This macro is obsolescent, as current systems are compatible with Posix. New programs need not use this macro.

_POSIX_VERSION is defined when unistd.h is included on Posix systems. If there is no unistd.h, it is definitely not a Posix system. However, some non-Posix systems do have unistd.h.

The way to check whether the system supports Posix is:

#ifdef HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif

#ifdef _POSIX_VERSION
/* Code for Posix systems.  */
#endif
Macro: AC_HEADER_TIOCGWINSZ

If the use of TIOCGWINSZ requires <sys/ioctl.h>, then define GWINSZ_IN_SYS_IOCTL. Otherwise TIOCGWINSZ can be found in <termios.h>.

Use:

#ifdef HAVE_TERMIOS_H
# include <termios.h>
#endif

#ifdef GWINSZ_IN_SYS_IOCTL
# include <sys/ioctl.h>
#endif

5.6.3 Generic Header Checks

These macros are used to find system header files not covered by the “particular” test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see Writing Tests).

Macro: AC_CHECK_HEADER (header-file, [action-if-found], [action-if-not-found], [includes])

If the system header file header-file is compilable, execute shell commands action-if-found, otherwise execute action-if-not-found. If you just want to define a symbol if the header file is available, consider using AC_CHECK_HEADERS instead.

includes should be the appropriate prerequisite code, i.e. whatever might be required to appear above ‘#include <header-file>’ for it to compile without error. This can be anything, but will normally be additional ‘#include’ directives. If includes is omitted or empty, configure will use the contents of the macro AC_INCLUDES_DEFAULT. See Default Includes.

This macro used to check only for the presence of a header, not whether its contents were acceptable to the compiler. Some older configure scripts rely on this behavior, so it is still available by specifying ‘-’ as includes. This mechanism is deprecated as of Autoconf 2.70; situations where a preprocessor-only check is required should use AC_PREPROC_IFELSE. See Running the Preprocessor.

This macro caches its result in the ac_cv_header_header-file variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_HEADERS (header-file…, [action-if-found], [action-if-not-found], [includes])

For each given system header file header-file in the blank-separated argument list that exists, define HAVE_header-file (in all capitals). If action-if-found is given, it is additional shell code to execute when one of the header files is found. You can give it a value of ‘break’ to break out of the loop on the first match. If action-if-not-found is given, it is executed when one of the header files is not found.

includes is interpreted as in AC_CHECK_HEADER, in order to choose the set of preprocessor directives supplied before the header under test.

This macro caches its result in the ac_cv_header_header-file variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_HEADERS_ONCE (header-file…)

For each given system header file header-file in the blank-separated argument list that exists, define HAVE_header-file (in all capitals).

If you do not need the full power of AC_CHECK_HEADERS, this variant generates smaller, faster configure files. All headers passed to AC_CHECK_HEADERS_ONCE are checked for in one pass, early during the configure run. The checks cannot be conditionalized, you cannot specify an action-if-found or action-if-not-found, and AC_INCLUDES_DEFAULT is always used for the prerequisites.

In previous versions of Autoconf, these macros merely checked whether the header was accepted by the preprocessor. This was changed because the old test was inappropriate for typical uses. Headers are typically used to compile, not merely to preprocess, and the old behavior sometimes accepted headers that clashed at compile-time (see Header Present But Cannot Be Compiled). If for some reason it is inappropriate to check whether a header is compilable, you should use AC_PREPROC_IFELSE (see Running the Preprocessor) instead of these macros.

Requiring each header to compile improves the robustness of the test, but it also requires you to make sure that the includes are correct. Most system headers nowadays make sure to #include whatever they require, or else have their dependencies satisfied by AC_INCLUDES_DEFAULT (see Default Includes), but see Portability of Headers, for known exceptions. In general, if you are looking for bar.h, which requires that foo.h be included first if it exists, you should do something like this:

AC_CHECK_HEADERS([foo.h])
AC_CHECK_HEADERS([bar.h], [], [],
[#ifdef HAVE_FOO_H
# include <foo.h>
#endif
])

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5.7 Declarations

The following macros check for the declaration of variables and functions. If there is no macro specifically defined to check for a symbol you need, then you can use the general macros (see Generic Declaration Checks) or, for more complex tests, you may use AC_COMPILE_IFELSE (see Running the Compiler).


5.7.1 Particular Declaration Checks

There are no specific macros for declarations.


5.7.2 Generic Declaration Checks

These macros are used to find declarations not covered by the “particular” test macros.

Macro: AC_CHECK_DECL (symbol, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

If symbol (a function, variable, or constant) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declaration under test.

This macro actually tests whether symbol is defined as a macro or can be used as an r-value, not whether it is really declared, because it is much safer to avoid introducing extra declarations when they are not needed. In order to facilitate use of C++ and overloaded function declarations, it is possible to specify function argument types in parentheses for types which can be zero-initialized:

AC_CHECK_DECL([basename(char *)])

This macro caches its result in the ac_cv_have_decl_symbol variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_DECLS (symbols, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

For each of the symbols (comma-separated list with optional function argument types for C++ overloads), define HAVE_DECL_symbol (in all capitals) to ‘1’ if symbol is declared, otherwise to ‘0’. If action-if-not-found is given, it is additional shell code to execute when one of the function declarations is needed, otherwise action-if-found is executed.

includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declarations under test.

This macro uses an M4 list as first argument:

AC_CHECK_DECLS([strdup])
AC_CHECK_DECLS([strlen])
AC_CHECK_DECLS([malloc, realloc, calloc, free])
AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]])
AC_CHECK_DECLS([[basename(char *)], [dirname(char *)]])

Unlike the other ‘AC_CHECK_*S’ macros, when a symbol is not declared, HAVE_DECL_symbol is defined to ‘0’ instead of leaving HAVE_DECL_symbol undeclared. When you are sure that the check was performed, use HAVE_DECL_symbol in #if:

#if !HAVE_DECL_SYMBOL
extern char *symbol;
#endif

If the test may have not been performed, however, because it is safer not to declare a symbol than to use a declaration that conflicts with the system’s one, you should use:

#if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC
void *malloc (size_t *s);
#endif

You fall into the second category only in extreme situations: either your files may be used without being configured, or they are used during the configuration. In most cases the traditional approach is enough.

This macro caches its results in ac_cv_have_decl_symbol variables, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_DECLS_ONCE (symbols)

For each of the symbols (comma-separated list), define HAVE_DECL_symbol (in all capitals) to ‘1’ if symbol is declared in the default include files, otherwise to ‘0’. This is a once-only variant of AC_CHECK_DECLS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.


Next: , Previous: , Up: Existing Tests   [Contents][Index]

5.8 Structures

The following macros check for the presence of certain members in C structures. If there is no macro specifically defined to check for a member you need, then you can use the general structure-member macros (see Generic Structure Checks) or, for more complex tests, you may use AC_COMPILE_IFELSE (see Running the Compiler).


5.8.1 Particular Structure Checks

The following macros check for certain structures or structure members.

Macro: AC_STRUCT_DIRENT_D_INO

Perform all the actions of AC_HEADER_DIRENT (see Particular Header Checks). Then, if struct dirent contains a d_ino member, define HAVE_STRUCT_DIRENT_D_INO.

HAVE_STRUCT_DIRENT_D_INO indicates only the presence of d_ino, not whether its contents are always reliable. Traditionally, a zero d_ino indicated a deleted directory entry, though current systems hide this detail from the user and never return zero d_ino values. Many current systems report an incorrect d_ino for a directory entry that is a mount point.

Macro: AC_STRUCT_DIRENT_D_TYPE

Perform all the actions of AC_HEADER_DIRENT (see Particular Header Checks). Then, if struct dirent contains a d_type member, define HAVE_STRUCT_DIRENT_D_TYPE.

Macro: AC_STRUCT_ST_BLOCKS

If struct stat contains an st_blocks member, define HAVE_STRUCT_STAT_ST_BLOCKS. Otherwise, require an AC_LIBOBJ replacement of ‘fileblocks’. The former name, HAVE_ST_BLOCKS is to be avoided, as its support will cease in the future.

This macro caches its result in the ac_cv_member_struct_stat_st_blocks variable.

Macro: AC_STRUCT_TM

If time.h does not define struct tm, define TM_IN_SYS_TIME, which means that including sys/time.h had better define struct tm.

This macro is obsolescent, as time.h defines struct tm in current systems. New programs need not use this macro.

Macro: AC_STRUCT_TIMEZONE

Figure out how to get the current timezone. If struct tm has a tm_zone member, define HAVE_STRUCT_TM_TM_ZONE (and the obsoleted HAVE_TM_ZONE). Otherwise, if the external array tzname is found, define HAVE_TZNAME; if it is declared, define HAVE_DECL_TZNAME.


5.8.2 Generic Structure Checks

These macros are used to find structure members not covered by the “particular” test macros.

Macro: AC_CHECK_MEMBER (aggregate.member, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

Check whether member is a member of the aggregate aggregate. If no includes are specified, the default includes are used (see Default Includes).

AC_CHECK_MEMBER([struct passwd.pw_gecos], [],
                [AC_MSG_ERROR([we need 'passwd.pw_gecos'])],
                [[#include <pwd.h>]])

You can use this macro for submembers:

AC_CHECK_MEMBER(struct top.middle.bot)

This macro caches its result in the ac_cv_member_aggregate_member variable, with characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_MEMBERS (members, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

Check for the existence of each ‘aggregate.member’ of members using the previous macro. When member belongs to aggregate, define HAVE_aggregate_member (in all capitals, with spaces and dots replaced by underscores). If action-if-found is given, it is executed for each of the found members. If action-if-not-found is given, it is executed for each of the members that could not be found.

includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the members under test.

This macro uses M4 lists:

AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])

5.9 Types

The following macros check for C types, either builtin or typedefs. If there is no macro specifically defined to check for a type you need, and you don’t need to check for any special properties of it, then you can use a general type-check macro.


5.9.1 Particular Type Checks

These macros check for particular C types in sys/types.h, stdlib.h, stdint.h, inttypes.h and others, if they exist.

The Gnulib stdint module is an alternate way to define many of these symbols; it is useful if you prefer your code to assume a C99-or-better environment. See Gnulib.

Macro: AC_TYPE_GETGROUPS

Define GETGROUPS_T to be whichever of gid_t or int is the base type of the array argument to getgroups.

This macro caches the base type in the ac_cv_type_getgroups variable.

Macro: AC_TYPE_INT8_T

If stdint.h or inttypes.h does not define the type int8_t, define int8_t to a signed integer type that is exactly 8 bits wide and that uses two’s complement representation, if such a type exists. If you are worried about porting to hosts that lack such a type, you can use the results of this macro in C89-or-later code as follows:

#if HAVE_STDINT_H
# include <stdint.h>
#endif
#if defined INT8_MAX || defined int8_t
 code using int8_t
#else
 complicated alternative using >8-bit 'signed char'
#endif

This macro caches the type in the ac_cv_c_int8_t variable.

Macro: AC_TYPE_INT16_T

This is like AC_TYPE_INT8_T, except for 16-bit integers.

Macro: AC_TYPE_INT32_T

This is like AC_TYPE_INT8_T, except for 32-bit integers.

Macro: AC_TYPE_INT64_T

This is like AC_TYPE_INT8_T, except for 64-bit integers.

Macro: AC_TYPE_INTMAX_T

If stdint.h or inttypes.h defines the type intmax_t, define HAVE_INTMAX_T. Otherwise, define intmax_t to the widest signed integer type.

Macro: AC_TYPE_INTPTR_T

If stdint.h or inttypes.h defines the type intptr_t, define HAVE_INTPTR_T. Otherwise, define intptr_t to a signed integer type wide enough to hold a pointer, if such a type exists.

Macro: AC_TYPE_LONG_DOUBLE

If the C compiler supports a working long double type, define HAVE_LONG_DOUBLE. The long double type might have the same range and precision as double.

This macro caches its result in the ac_cv_type_long_double variable.

This macro is obsolescent, as current C compilers support long double. New programs need not use this macro.

Macro: AC_TYPE_LONG_DOUBLE_WIDER

If the C compiler supports a working long double type with more range or precision than the double type, define HAVE_LONG_DOUBLE_WIDER.

This macro caches its result in the ac_cv_type_long_double_wider variable.

Macro: AC_TYPE_LONG_LONG_INT

If the C compiler supports a working long long int type, define HAVE_LONG_LONG_INT. However, this test does not test long long int values in preprocessor #if expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

This macro caches its result in the ac_cv_type_long_long_int variable.

Macro: AC_TYPE_MBSTATE_T

Define HAVE_MBSTATE_T if <wchar.h> declares the mbstate_t type. Also, define mbstate_t to be a type if <wchar.h> does not declare it.

This macro caches its result in the ac_cv_type_mbstate_t variable.

Macro: AC_TYPE_MODE_T

Define mode_t to a suitable type, if standard headers do not define it.

This macro caches its result in the ac_cv_type_mode_t variable.

Macro: AC_TYPE_OFF_T

Define off_t to a suitable type, if standard headers do not define it.

This macro caches its result in the ac_cv_type_off_t variable.

Macro: AC_TYPE_PID_T

Define pid_t to a suitable type, if standard headers do not define it.

This macro caches its result in the ac_cv_type_pid_t variable.

Macro: AC_TYPE_SIZE_T

Define size_t to a suitable type, if standard headers do not define it.

This macro caches its result in the ac_cv_type_size_t variable.

Macro: AC_TYPE_SSIZE_T

Define ssize_t to a suitable type, if standard headers do not define it.

This macro caches its result in the ac_cv_type_ssize_t variable.

Macro: AC_TYPE_UID_T

Define uid_t and gid_t to suitable types, if standard headers do not define them.

This macro caches its result in the ac_cv_type_uid_t variable.

Macro: AC_TYPE_UINT8_T

If stdint.h or inttypes.h does not define the type uint8_t, define uint8_t to an unsigned integer type that is exactly 8 bits wide, if such a type exists. This is like AC_TYPE_INT8_T, except for unsigned integers.

Macro: AC_TYPE_UINT16_T

This is like AC_TYPE_UINT8_T, except for 16-bit integers.

Macro: AC_TYPE_UINT32_T

This is like AC_TYPE_UINT8_T, except for 32-bit integers.

Macro: AC_TYPE_UINT64_T

This is like AC_TYPE_UINT8_T, except for 64-bit integers.

Macro: AC_TYPE_UINTMAX_T

If stdint.h or inttypes.h defines the type uintmax_t, define HAVE_UINTMAX_T. Otherwise, define uintmax_t to the widest unsigned integer type.

Macro: AC_TYPE_UINTPTR_T

If stdint.h or inttypes.h defines the type uintptr_t, define HAVE_UINTPTR_T. Otherwise, define uintptr_t to an unsigned integer type wide enough to hold a pointer, if such a type exists.

Macro: AC_TYPE_UNSIGNED_LONG_LONG_INT

If the C compiler supports a working unsigned long long int type, define HAVE_UNSIGNED_LONG_LONG_INT. However, this test does not test unsigned long long int values in preprocessor #if expressions, because too many compilers mishandle such expressions. See Preprocessor Arithmetic.

This macro caches its result in the ac_cv_type_unsigned_long_long_int variable.


5.9.2 Generic Type Checks

These macros are used to check for types not covered by the “particular” test macros.

Macro: AC_CHECK_TYPE (type, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

Check whether type is defined. It may be a compiler builtin type or defined by the includes. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the type under test.

In C, type must be a type-name, so that the expression ‘sizeof (type)’ is valid (but ‘sizeof ((type))’ is not). The same test is applied when compiling for C++, which means that in C++ type should be a type-id and should not be an anonymous ‘struct’ or ‘union’.

This macro caches its result in the ac_cv_type_type variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_TYPES (types, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT])

For each type of the types that is defined, define HAVE_type (in all capitals). Each type must follow the rules of AC_CHECK_TYPE. If no includes are specified, the default includes are used (see Default Includes). If action-if-found is given, it is additional shell code to execute when one of the types is found. If action-if-not-found is given, it is executed when one of the types is not found.

This macro uses M4 lists:

AC_CHECK_TYPES([ptrdiff_t])
AC_CHECK_TYPES([unsigned long long int, uintmax_t])
AC_CHECK_TYPES([float_t], [], [], [[#include <math.h>]])

Autoconf, up to 2.13, used to provide to another version of AC_CHECK_TYPE, broken by design. In order to keep backward compatibility, a simple heuristic, quite safe but not totally, is implemented. In case of doubt, read the documentation of the former AC_CHECK_TYPE, see Obsolete Macros.


Next: , Previous: , Up: Existing Tests   [Contents][Index]

5.10 Compilers and Preprocessors

All the tests for compilers (AC_PROG_CC, AC_PROG_CXX, AC_PROG_F77) define the output variable EXEEXT based on the output of the compiler, typically to the empty string if Posix and ‘.exe’ if a DOS variant.

They also define the output variable OBJEXT based on the output of the compiler, after .c files have been excluded, typically to ‘o’ if Posix, ‘obj’ if a DOS variant.

If the compiler being used does not produce executables, the tests fail. If the executables can’t be run, and cross-compilation is not enabled, they fail too. See Manual Configuration, for more on support for cross compiling.


5.10.1 Specific Compiler Characteristics

Some compilers exhibit different behaviors.

Static/Dynamic Expressions

Autoconf relies on a trick to extract one bit of information from the C compiler: using negative array sizes. For instance the following excerpt of a C source demonstrates how to test whether ‘int’ objects are 4 bytes wide:

static int test_array[sizeof (int) == 4 ? 1 : -1];

To our knowledge, there is a single compiler that does not support this trick: the HP C compilers (the real ones, not only the “bundled”) on HP-UX 11.00. They incorrectly reject the above program with the diagnostic “Variable-length arrays cannot have static storage.” This bug comes from HP compilers’ mishandling of sizeof (int), not from the ? 1 : -1, and Autoconf works around this problem by casting sizeof (int) to long int before comparing it.


5.10.2 Generic Compiler Characteristics

Macro: AC_CHECK_SIZEOF (type-or-expr, [unused], [includes = ‘AC_INCLUDES_DEFAULT])

Define SIZEOF_type-or-expr (see Standard Symbols) to be the size in bytes of type-or-expr, which may be either a type or an expression returning a value that has a size. If the expression ‘sizeof (type-or-expr)’ is invalid, the result is 0. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the expression under test.

This macro now works even when cross-compiling. The unused argument was used when cross-compiling.

For example, the call

AC_CHECK_SIZEOF([int *])

defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.

This macro caches its result in the ac_cv_sizeof_type-or-expr variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_CHECK_ALIGNOF (type, [includes = ‘AC_INCLUDES_DEFAULT])

Define ALIGNOF_type (see Standard Symbols) to be the alignment in bytes of type. ‘type y;’ must be valid as a structure member declaration. If ‘type’ is unknown, the result is 0. If no includes are specified, the default includes are used (see Default Includes).

This macro caches its result in the ac_cv_alignof_type-or-expr variable, with ‘*’ mapped to ‘p’ and other characters not suitable for a variable name mapped to underscores.

Macro: AC_COMPUTE_INT (var, expression, [includes = ‘AC_INCLUDES_DEFAULT], [action-if-fails])

Store into the shell variable var the value of the integer expression. The value should fit in an initializer in a C variable of type signed long. To support cross compilation (in which case, the macro only works on hosts that use twos-complement arithmetic), it should be possible to evaluate the expression at compile-time. If no includes are specified, the default includes are used (see Default Includes).

Execute action-if-fails if the value cannot be determined correctly.

Macro: AC_LANG_WERROR

Normally Autoconf ignores warnings generated by the compiler, linker, and preprocessor. If this macro is used, warnings count as fatal errors for the current language. This macro is useful when the results of configuration are used where warnings are unacceptable; for instance, if parts of a program are built with the GCC -Werror option. If the whole program is built using -Werror it is often simpler to put -Werror in the compiler flags (CFLAGS, etc.).

Macro: AC_OPENMP

OpenMP specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multi-core CPUs.

If the current language is C, the macro AC_OPENMP sets the variable OPENMP_CFLAGS to the C compiler flags needed for supporting OpenMP. OPENMP_CFLAGS is set to empty if the compiler already supports OpenMP, if it has no way to activate OpenMP support, or if the user rejects OpenMP support by invoking ‘configure’ with the ‘--disable-openmp’ option.

OPENMP_CFLAGS needs to be used when compiling programs, when preprocessing program source, and when linking programs. Therefore you need to add $(OPENMP_CFLAGS) to the CFLAGS of C programs that use OpenMP. If you preprocess OpenMP-specific C code, you also need to add $(OPENMP_CFLAGS) to CPPFLAGS. The presence of OpenMP support is revealed at compile time by the preprocessor macro _OPENMP.

Linking a program with OPENMP_CFLAGS typically adds one more shared library to the program’s dependencies, so its use is recommended only on programs that actually require OpenMP.

If the current language is C++, AC_OPENMP sets the variable OPENMP_CXXFLAGS, suitably for the C++ compiler. The same remarks hold as for C.

If the current language is Fortran 77 or Fortran, AC_OPENMP sets the variable OPENMP_FFLAGS or OPENMP_FCFLAGS, respectively. Similar remarks as for C hold, except that CPPFLAGS is not used for Fortran, and no preprocessor macro signals OpenMP support.

For portability, it is best to avoid spaces between ‘#’ and ‘pragma omp’. That is, write ‘#pragma omp’, not ‘# pragma omp’. The Sun WorkShop 6.2 C compiler chokes on the latter.

This macro caches its result in the ac_cv_prog_c_openmp, ac_cv_prog_cxx_openmp, ac_cv_prog_f77_openmp, or ac_cv_prog_fc_openmp variable, depending on the current language.

Caution: Some of the compiler options that AC_OPENMP tests, mean “enable OpenMP” to one compiler, but “write output to a file named mp or penmp” to other compilers. We cannot guarantee that the implementation of AC_OPENMP will not overwrite an existing file with either of these names.

Therefore, as a defensive measure, a configure script that uses AC_OPENMP will issue an error and stop (before doing any of the operations that might overwrite these files) upon encountering either of these files in its working directory. autoconf will also issue an error if it finds either of these files in the same directory as a configure.ac that uses AC_OPENMP.

If you have files with either of these names at the top level of your source tree, and you need to use AC_OPENMP, we recommend you either change their names or move them into a subdirectory.


5.10.3 C Compiler Characteristics

The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around.

Don’t use lines containing solitary backslashes

They tickle a bug in the HP-UX C compiler (checked on HP-UX 10.20, 11.00, and 11i). When given the following source:

#ifdef __STDC__
/\
* A comment with backslash-newlines in it.  %{ %} *\
\
/
char str[] = "\\
" A string with backslash-newlines in it %{ %} \\
"";
char apostrophe = '\\
\
'\
';
#endif

the compiler incorrectly fails with the diagnostics “Non-terminating comment at end of file” and “Missing ‘#endif’ at end of file.” Removing the lines with solitary backslashes solves the problem.

Don’t compile several files at once if output matters to you

Some compilers, such as HP’s, report names of files being compiled when given more than one file operand. For instance:

$ cc a.c b.c
a.c:
b.c:

This can cause problems if you observe the output of the compiler to detect failures. Invoking ‘cc -c a.c && cc -c b.c && cc -o c a.o b.o’ solves the issue.

Don’t rely on #error failing

The IRIX C compiler does not fail when #error is preprocessed; it simply emits a diagnostic and continues, exiting successfully. So, instead of an error directive like #error "Unsupported word size" it is more portable to use an invalid directive like #Unsupported word size in Autoconf tests. In ordinary source code, #error is OK, since installers with inadequate compilers like IRIX can simply examine these compilers’ diagnostic output.

Don’t rely on correct #line support

On Solaris, c89 (at least Sun C 5.3 through 5.8) diagnoses #line directives whose line numbers are greater than 32767. Nothing in Posix makes this invalid. That is why Autoconf stopped issuing #line directives.

Macro: AC_PROG_CC ([compiler-search-list])

Determine a C compiler to use.

If the environment variable CC is set, its value will be taken as the name of the C compiler to use. Otherwise, search for a C compiler under a series of likely names, trying gcc and cc first. Regardless, the output variable CC is set to the chosen compiler.

If the optional first argument to the macro is used, it must be a whitespace-separated list of potential names for a C compiler, which overrides the built-in list.

If no C compiler can be found, configure will error out.

If the selected C compiler is found to be GNU C (regardless of its name), the shell variable GCC will be set to ‘yes’. If the shell variable CFLAGS was not already set, it is set to -g -O2 for the GNU C compiler (-O2 on systems where GCC does not accept -g), or -g for other compilers. CFLAGS is then made an output variable. You can override the default for CFLAGS by inserting a shell default assignment between AC_INIT and AC_PROG_CC:

: ${CFLAGS="options"}

where options are the appropriate set of options to use by default. (It is important to use this construct rather than a normal assignment, so that CFLAGS can still be overridden by the person building the package. See Preset Output Variables.)

If necessary, options are added to CC to enable support for ISO Standard C features with extensions, preferring the newest edition of the C standard that is supported. Currently the newest edition Autoconf knows how to detect support for is ISO C 2011. After calling this macro you can check whether the C compiler has been set to accept standard C by inspecting the shell variable ac_prog_cc_stdc. Its value will be ‘c11’, ‘c99’, or ‘c89’, respectively, if the C compiler has been set to use the 2011, 1999, or 1990 edition of the C standard, and ‘no’ if the compiler does not support compiling standard C at all.

The tests for standard conformance are not comprehensive. They test the values of __STDC__ and __STDC_VERSION__, and a representative sample of the language features added in each version of the C standard. They do not test the C standard library, because the C compiler might be generating code for a “freestanding environment” (in which most of the standard library is optional). If you need to know whether a particular C standard header exists, use AC_CHECK_HEADER.

None of the options that may be added to CC by this macro enable strict conformance to the C standard. In particular, system-specific extensions are not disabled. (For example, for GNU C, the -std=gnunn options may be used, but not the -std=cnn options.)

Many Autoconf macros use a compiler, and thus call ‘AC_REQUIRE([AC_PROG_CC])’ to ensure that the compiler has been determined before the body of the outermost AC_DEFUN macro. Although AC_PROG_CC is safe to directly expand multiple times, it performs certain checks (such as the proper value of EXEEXT) only on the first invocation. Therefore, care must be used when invoking this macro from within another macro rather than at the top level (see Expanded Before Required).

Macro: AC_PROG_CC_C_O

If the C compiler does not accept the -c and -o options simultaneously, define NO_MINUS_C_MINUS_O. This macro actually tests both the compiler found by AC_PROG_CC, and, if different, the first cc in the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.

For the compiler compiler, this macro caches its result in the ac_cv_prog_cc_compiler_c_o variable.

Macro: AC_PROG_CPP

Set output variable CPP to a command that runs the C preprocessor. If ‘$CC -E’ doesn’t work, tries cpp and /lib/cpp, in that order.

It is only portable to run CPP on files with a .c extension.

Some preprocessors don’t indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. For most preprocessors, though, warnings do not cause include-file tests to fail unless AC_PROG_CPP_WERROR is also specified.

Macro: AC_PROG_CPP_WERROR

This acts like AC_PROG_CPP, except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices.

The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see Running the Compiler) or AC_RUN_IFELSE (see Checking Runtime Behavior).

Macro: AC_C_BACKSLASH_A

Define ‘HAVE_C_BACKSLASH_A’ to 1 if the C compiler understands ‘\a’.

This macro is obsolescent, as current C compilers understand ‘\a’. New programs need not use this macro.

Macro: AC_C_BIGENDIAN ([action-if-true], [action-if-false], [action-if-unknown], [action-if-universal])

If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute action-if-true. If words are stored with the least significant byte first (like Intel and VAX CPUs), execute action-if-false.

This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep’ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system.

In some cases a single run of a compiler can generate code for multiple architectures. This can happen, for example, when generating Mac OS X universal binary files, which work on both PowerPC and Intel architectures. In this case, the different variants might be for architectures with differing endianness. If configure detects this, it executes action-if-universal instead of action-if-unknown.

The default for action-if-true is to define ‘WORDS_BIGENDIAN’. The default for action-if-false is to do nothing. The default for action-if-unknown is to abort configure and tell the installer how to bypass this test. And finally, the default for action-if-universal is to ensure that ‘WORDS_BIGENDIAN’ is defined if and only if a universal build is detected and the current code is big-endian; this default works only if autoheader is used (see Using autoheader to Create config.h.in).

If you use this macro without specifying action-if-universal, you should also use AC_CONFIG_HEADERS; otherwise ‘WORDS_BIGENDIAN’ may be set incorrectly for Mac OS X universal binary files.

Macro: AC_C_CONST

If the C compiler does not fully support the const keyword, define const to be empty. Some C compilers that do not define __STDC__ do support const; some compilers that define __STDC__ do not completely support const. Programs can simply use const as if every C compiler supported it; for those that don’t, the makefile or configuration header file defines it as empty.

Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler. This causes problems with const, because C and C++ treat const differently. For example:

const int foo;

is valid in C but not in C++. These differences unfortunately cannot be papered over by defining const to be empty.

If autoconf detects this situation, it leaves const alone, as this generally yields better results in practice. However, using a C++ compiler to compile C code is not recommended or supported, and installers who run into trouble in this area should get a C compiler like GCC to compile their C code.

This macro caches its result in the ac_cv_c_const variable.

This macro is obsolescent, as current C compilers support const. New programs need not use this macro.

Macro: AC_C__GENERIC

If the C compiler supports C11-style generic selection using the _Generic keyword, define HAVE_C__GENERIC.

Macro: AC_C_RESTRICT

If the C compiler recognizes a variant spelling for the restrict keyword (__restrict, __restrict__, or _Restrict), then define restrict to that; this is more likely to do the right thing with compilers that support language variants where plain restrict is not a keyword. Otherwise, if the C compiler recognizes the restrict keyword, don’t do anything. Otherwise, define restrict to be empty. Thus, programs may simply use restrict as if every C compiler supported it; for those that do not, the makefile or configuration header defines it away.

Although support in C++ for the restrict keyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.

This macro caches ‘no’ in the ac_cv_c_restrict variable if restrict is not supported, and a supported spelling otherwise.

Macro: AC_C_VOLATILE

If the C compiler does not understand the keyword volatile, define volatile to be empty. Programs can simply use volatile as if every C compiler supported it; for those that do not, the makefile or configuration header defines it as empty.

If the correctness of your program depends on the semantics of volatile, simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support volatile, you are at its mercy anyway. At least your program compiles, when it wouldn’t before. See Volatile Objects, for more about volatile.

In general, the volatile keyword is a standard C feature, so you might expect that volatile is available only when __STDC__ is defined. However, Ultrix 4.3’s native compiler does support volatile, but does not define __STDC__.

This macro is obsolescent, as current C compilers support volatile. New programs need not use this macro.

Macro: AC_C_INLINE

If the C compiler supports the keyword inline, do nothing. Otherwise define inline to __inline__ or __inline if it accepts one of those, otherwise define inline to be empty.

Macro: AC_C_CHAR_UNSIGNED

If the C type char is unsigned, define __CHAR_UNSIGNED__, unless the C compiler predefines it.

These days, using this macro is not necessary. The same information can be determined by this portable alternative, thus avoiding the use of preprocessor macros in the namespace reserved for the implementation.

#include <limits.h>
#if CHAR_MIN == 0
# define CHAR_UNSIGNED 1
#endif
Macro: AC_C_STRINGIZE

If the C preprocessor supports the stringizing operator, define HAVE_STRINGIZE. The stringizing operator is ‘#’ and is found in macros such as this:

#define x(y) #y

This macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro.

Macro: AC_C_FLEXIBLE_ARRAY_MEMBER

If the C compiler supports flexible array members, define FLEXIBLE_ARRAY_MEMBER to nothing; otherwise define it to 1. That way, a declaration like this:

struct s
  {
    size_t n_vals;
    double val[FLEXIBLE_ARRAY_MEMBER];
  };

will let applications use the “struct hack” even with compilers that do not support flexible array members. To allocate and use such an object, you can use code like this:

size_t i;
size_t n = compute_value_count ();
struct s *p =
   malloc (offsetof (struct s, val)
           + n * sizeof (double));
p->n_vals = n;
for (i = 0; i < n; i++)
  p->val[i] = compute_value (i);
Macro: AC_C_VARARRAYS

If the C compiler does not support variable-length arrays, define the macro __STDC_NO_VLA__ to be 1 if it is not already defined. A variable-length array is an array of automatic storage duration whose length is determined at run time, when the array is declared. For backward compatibility this macro also defines HAVE_C_VARARRAYS if the C compiler supports variable-length arrays, but this usage is obsolescent and new programs should use __STDC_NO_VLA__.

Macro: AC_C_TYPEOF

If the C compiler supports GNU C’s typeof syntax either directly or through a different spelling of the keyword (e.g., __typeof__), define HAVE_TYPEOF. If the support is available only through a different spelling, define typeof to that spelling.

Macro: AC_C_PROTOTYPES

If function prototypes are understood by the compiler (as determined by AC_PROG_CC), define PROTOTYPES and __PROTOTYPES. Defining __PROTOTYPES is for the benefit of header files that cannot use macros that infringe on user name space.

This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro.

Macro: AC_PROG_GCC_TRADITIONAL

Add -traditional to output variable CC if using a GNU C compiler and ioctl does not work properly without -traditional. That usually happens when the fixed header files have not been installed on an old system.

This macro is obsolescent, since current versions of the GNU C compiler fix the header files automatically when installed.


5.10.4 C++ Compiler Characteristics

Macro: AC_PROG_CXX ([compiler-search-list])

Determine a C++ compiler to use.

If either the environment variable CXX or the environment variable CCC is set, its value will be taken as the name of a C++ compiler. If both are set, CXX is preferred. If neither are set, search for a C++ compiler under a series of likely names, trying g++ and c++ first. Regardless, the output variable CXX is set to the chosen compiler.

If the optional first argument to the macro is used, it must be a whitespace-separated list of potential names for a C++ compiler, which overrides the built-in list.

If no C++ compiler can be found, as a last resort CXX is set to g++ (and subsequent tests will probably fail).

If the selected C++ compiler is found to be GNU C++ (regardless of its name), the shell variable GXX will be set to ‘yes’. If the shell variable CXXFLAGS was not already set, it is set to -g -O2 for the GNU C++ compiler (-O2 on systems where G++ does not accept -g), or -g for other compilers. CXXFLAGS is then made an output variable. You can override the default for CXXFLAGS by inserting a shell default assignment between AC_INIT and AC_PROG_CXX:

: ${CXXFLAGS="options"}

where options are the appropriate set of options to use by default. (It is important to use this construct rather than a normal assignment, so that CXXFLAGS can still be overridden by the person building the package. See Preset Output Variables.)

If necessary, options are added to CXX to enable support for ISO Standard C++ features with extensions, preferring the newest edition of the C++ standard that is supported. Currently the newest edition Autoconf knows how to detect support for is ISO C++ 2011. After calling this macro, you can check whether the C++ compiler has been set to accept standard C++ by inspecting the shell variable ac_prog_cc_stdc. Its value will be ‘cxx11’ or ‘cxx98’, respectively, if the C++ compiler has been set to use the 2011 or 1990 edition of the C++ standard, and ‘no’ if the compiler does not support compiling standard C++ at all.

The tests for standard conformance are not comprehensive. They test the value of __cplusplus and a representative sample of the language features added in each version of the C++ standard. They do not test the C++ standard library, because this can be extremely slow, and because the C++ compiler might be generating code for a “freestanding environment” (in which most of the C++ standard library is optional). If you need to know whether a particular C++ standard header exists, use AC_CHECK_HEADER.

None of the options that may be added to CXX by this macro enable strict conformance to the C++ standard. In particular, system-specific extensions are not disabled. (For example, for GNU C++, the -std=gnu++nn options may be used, but not the -std=c++nn options.)

Macro: AC_PROG_CXXCPP

Set output variable CXXCPP to a command that runs the C++ preprocessor. If ‘$CXX -E’ doesn’t work, tries cpp and /lib/cpp, in that order. Because of this fallback, CXXCPP may or may not set C++-specific predefined macros (such as __cplusplus).

It is portable to run CXXCPP only on files with a .c, .C, .cc, or .cpp extension.

Some preprocessors don’t indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. However, it is not known whether such broken preprocessors exist for C++.

Macro: AC_PROG_CXX_C_O

Test whether the C++ compiler accepts the options -c and -o simultaneously, and define CXX_NO_MINUS_C_MINUS_O, if it does not.


5.10.5 Objective C Compiler Characteristics

Macro: AC_PROG_OBJC ([compiler-search-list])

Determine an Objective C compiler to use. If OBJC is not already set in the environment, check for Objective C compilers. Set output variable OBJC to the name of the compiler found.

This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C compiler. For example, if you didn’t like the default order, then you could invoke AC_PROG_OBJC like this:

AC_PROG_OBJC([gcc objcc objc])

If using a compiler that supports GNU Objective C, set shell variable GOBJC to ‘yes’. If output variable OBJCFLAGS was not already set, set it to -g -O2 for a GNU Objective C compiler (-O2 on systems where the compiler does not accept -g), or -g for other compilers.

Macro: AC_PROG_OBJCPP

Set output variable OBJCPP to a command that runs the Objective C preprocessor. If ‘$OBJC -E’ doesn’t work, tries cpp and /lib/cpp, in that order. Because of this fallback, CXXCPP may or may not set Objective-C-specific predefined macros (such as __OBJC__).


5.10.6 Objective C++ Compiler Characteristics

Macro: AC_PROG_OBJCXX ([compiler-search-list])

Determine an Objective C++ compiler to use. If OBJCXX is not already set in the environment, check for Objective C++ compilers. Set output variable OBJCXX to the name of the compiler found.

This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Objective C++ compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Objective C++ compiler. For example, if you didn’t like the default order, then you could invoke AC_PROG_OBJCXX like this:

AC_PROG_OBJCXX([gcc g++ objcc++ objcxx])

If using a compiler that supports GNU Objective C++, set shell variable GOBJCXX to ‘yes’. If output variable OBJCXXFLAGS was not already set, set it to -g -O2 for a GNU Objective C++ compiler (-O2 on systems where the compiler does not accept -g), or -g for other compilers.

Macro: AC_PROG_OBJCXXCPP

Set output variable OBJCXXCPP to a command that runs the Objective C++ preprocessor. If ‘$OBJCXX -E’ doesn’t work, tries cpp and /lib/cpp, in that order. Because of this fallback, CXXCPP may or may not set Objective-C++-specific predefined macros (such as __cplusplus and __OBJC__).


5.10.7 Erlang Compiler and Interpreter Characteristics

Autoconf defines the following macros for determining paths to the essential Erlang/OTP programs:

Macro: AC_ERLANG_PATH_ERLC ([value-if-not-found], [path = ‘$PATH])

Determine an Erlang compiler to use. If ERLC is not already set in the environment, check for erlc. Set output variable ERLC to the complete path of the compiler command found. In addition, if ERLCFLAGS is not set in the environment, set it to an empty value.

The two optional arguments have the same meaning as the two last arguments of macro AC_PATH_PROG for looking for the erlc program. For example, to look for erlc only in the /usr/lib/erlang/bin directory:

AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin])
Macro: AC_ERLANG_NEED_ERLC ([path = ‘$PATH])

A simplified variant of the AC_ERLANG_PATH_ERLC macro, that prints an error message and exits the configure script if the erlc program is not found.

Macro: AC_ERLANG_PATH_ERL ([value-if-not-found], [path = ‘$PATH])

Determine an Erlang interpreter to use. If ERL is not already set in the environment, check for erl. Set output variable ERL to the complete path of the interpreter command found.

The two optional arguments have the same meaning as the two last arguments of macro AC_PATH_PROG for looking for the erl program. For example, to look for erl only in the /usr/lib/erlang/bin directory:

AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin])
Macro: AC_ERLANG_NEED_ERL ([path = ‘$PATH])

A simplified variant of the AC_ERLANG_PATH_ERL macro, that prints an error message and exits the configure script if the erl program is not found.


5.10.8 Fortran Compiler Characteristics

The Autoconf Fortran support is divided into two categories: legacy Fortran 77 macros (F77), and modern Fortran macros (FC). The former are intended for traditional Fortran 77 code, and have output variables like F77, FFLAGS, and FLIBS. The latter are for newer programs that can (or must) compile under the newer Fortran standards, and have output variables like FC, FCFLAGS, and FCLIBS.

Except for the macros AC_FC_SRCEXT, AC_FC_FREEFORM, AC_FC_FIXEDFORM, and AC_FC_LINE_LENGTH (see below), the FC and F77 macros behave almost identically, and so they are documented together in this section.

Macro: AC_PROG_F77 ([compiler-search-list])

Determine a Fortran 77 compiler to use. If F77 is not already set in the environment, then check for g77 and f77, and then some other names. Set the output variable F77 to the name of the compiler found.

This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn’t like the default order, then you could invoke AC_PROG_F77 like this:

AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])

If using a compiler that supports GNU Fortran 77, set the shell variable G77 to ‘yes’. If the output variable FFLAGS was not already set in the environment, set it to -g -02 for g77 (or -O2 where the GNU Fortran 77 compiler does not accept -g), or -g for other compilers.

The result of the GNU test is cached in the ac_cv_f77_compiler_gnu variable, acceptance of -g in the ac_cv_prog_f77_g variable.

Macro: AC_PROG_FC ([compiler-search-list], [dialect])

Determine a Fortran compiler to use. If FC is not already set in the environment, then dialect is a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variable FC to the name of the compiler found.

By default, newer dialects are preferred over older dialects, but if dialect is specified then older dialects are preferred starting with the specified dialect. dialect can currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler name to prefer (e.g., f90 or f95), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid the dialect option, and use AC_PROG_FC only for code compatible with the latest Fortran standard.

This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in AC_PROG_F77.

If using a compiler that supports GNU Fortran, set the shell variable GFC to ‘yes’. If the output variable FCFLAGS was not already set in the environment, then set it to -g -02 for a GNU Fortran compiler (or -O2 where the compiler does not accept -g), or -g for other compilers.

The result of the GNU test is cached in the ac_cv_fc_compiler_gnu variable, acceptance of -g in the ac_cv_prog_fc_g variable.

Macro: AC_PROG_F77_C_O
Macro: AC_PROG_FC_C_O

Test whether the Fortran compiler accepts the options -c and -o simultaneously, and define F77_NO_MINUS_C_MINUS_O or FC_NO_MINUS_C_MINUS_O, respectively, if it does not.

The result of the test is cached in the ac_cv_prog_f77_c_o or ac_cv_prog_fc_c_o variable, respectively.

The following macros check for Fortran compiler characteristics. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see Running the Compiler) or AC_RUN_IFELSE (see Checking Runtime Behavior), making sure to first set the current language to Fortran 77 or Fortran via AC_LANG([Fortran 77]) or AC_LANG(Fortran) (see Language Choice).

Macro: AC_F77_LIBRARY_LDFLAGS
Macro: AC_FC_LIBRARY_LDFLAGS

Determine the linker flags (e.g., -L and -l) for the Fortran intrinsic and runtime libraries that are required to successfully link a Fortran program or shared library. The output variable FLIBS or FCLIBS is set to these flags (which should be included after LIBS when linking).

This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (see Mixing Fortran 77 With C and C++ in GNU Automake).

For example, if object files from a C++ and Fortran compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).

However, the Fortran intrinsic and runtime libraries must be linked in as well, but the C++ compiler/linker doesn’t know by default how to add these Fortran 77 libraries. Hence, this macro was created to determine these Fortran libraries.

The macros AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN are probably also necessary to link C/C++ with Fortran; see below. Further, it is highly recommended that you use AC_CONFIG_HEADERS (see Configuration Header Files) because the complex defines that the function wrapper macros create may not work with C/C++ compiler drivers.

These macros internally compute the flag needed to verbose linking output and cache it in ac_cv_prog_f77_v or ac_cv_prog_fc_v variables, respectively. The computed linker flags are cached in ac_cv_f77_libs or ac_cv_fc_libs, respectively.

Macro: AC_F77_DUMMY_MAIN ([action-if-found], [action-if-not-found = ‘AC_MSG_FAILURE])
Macro: AC_FC_DUMMY_MAIN ([action-if-found], [action-if-not-found = ‘AC_MSG_FAILURE])

With many compilers, the Fortran libraries detected by AC_F77_LIBRARY_LDFLAGS or AC_FC_LIBRARY_LDFLAGS provide their own main entry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say) MAIN__ to run the user’s program. The AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN macros figure out how to deal with this interaction.

When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one’s own main and skip the Fortran library initializations. In this case, however, one may still need to provide a dummy MAIN__ routine in order to prevent linking errors on some systems. AC_F77_DUMMY_MAIN or AC_FC_DUMMY_MAIN detects whether any such routine is required for linking, and what its name is; the shell variable F77_DUMMY_MAIN or FC_DUMMY_MAIN holds this name, unknown when no solution was found, and none when no such dummy main is needed.

By default, action-if-found defines F77_DUMMY_MAIN or FC_DUMMY_MAIN to the name of this routine (e.g., MAIN__) if it is required. action-if-not-found defaults to exiting with an error.

In order to link with Fortran routines, the user’s C/C++ program should then include the following code to define the dummy main if it is needed:

#ifdef F77_DUMMY_MAIN
#  ifdef __cplusplus
     extern "C"
#  endif
   int F77_DUMMY_MAIN () { return 1; }
#endif

(Replace F77 with FC for Fortran instead of Fortran 77.)

Note that this macro is called automatically from AC_F77_WRAPPERS or AC_FC_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.

The result of this macro is cached in the ac_cv_f77_dummy_main or ac_cv_fc_dummy_main variable, respectively.

Macro: AC_F77_MAIN
Macro: AC_FC_MAIN

As discussed above, many Fortran libraries allow you to provide an entry point called (say) MAIN__ instead of the usual main, which is then called by a main function in the Fortran libraries that initializes things like Fortran I/O. The AC_F77_MAIN and AC_FC_MAIN macros detect whether it is possible to utilize such an alternate main function, and defines F77_MAIN and FC_MAIN to the name of the function. (If no alternate main function name is found, F77_MAIN and FC_MAIN are simply defined to main.)

Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so:

#ifdef __cplusplus
  extern "C"
#endif
int F77_MAIN (int argc, char *argv[]);

(Again, replace F77 with FC for Fortran instead of Fortran 77.)

The result of this macro is cached in the ac_cv_f77_main or ac_cv_fc_main variable, respectively.

Macro: AC_F77_WRAPPERS
Macro: AC_FC_WRAPPERS

Defines C macros F77_FUNC (name, NAME), FC_FUNC (name, NAME), F77_FUNC_(name, NAME), and FC_FUNC_(name, NAME) to properly mangle the names of C/C++ identifiers, and identifiers with underscores, respectively, so that they match the name-mangling scheme used by the Fortran compiler.

Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by AC_F77_WRAPPERS or AC_FC_WRAPPERS. For example, suppose you have the following Fortran 77 subroutine:

      subroutine foobar (x, y)
      double precision x, y
      y = 3.14159 * x
      return
      end

You would then declare its prototype in C or C++ as:

#define FOOBAR_F77 F77_FUNC (foobar, FOOBAR)
#ifdef __cplusplus
extern "C"  /* prevent C++ name mangling */
#endif
void FOOBAR_F77 (double *x, double *y);

Note that we pass both the lowercase and uppercase versions of the function name to F77_FUNC so that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (see Mixing Fortran 77 With C and C++ in GNU Automake).

(Replace F77 with FC for Fortran instead of Fortran 77.)

Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn’t support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether F77_FUNC or FC_FUNC is defined.

Now, to call that routine from a C program, we would do something like:

{
    double x = 2.7183, y;
    FOOBAR_F77 (&x, &y);
}

If the Fortran identifier contains an underscore (e.g., foo_bar), you should use F77_FUNC_ or FC_FUNC_ instead of F77_FUNC or FC_FUNC (with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.

The name mangling scheme is encoded in the ac_cv_f77_mangling or ac_cv_fc_mangling cache variable, respectively, and also used for the AC_F77_FUNC and AC_FC_FUNC macros described below.

Macro: AC_F77_FUNC (name, [shellvar])
Macro: AC_FC_FUNC (name, [shellvar])

Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran linker (see also AC_F77_WRAPPERS or AC_FC_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable is simply name. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above, for example, to call Fortran routines from some language other than C/C++.

Macro: AC_FC_SRCEXT (ext, [action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])
Macro: AC_FC_PP_SRCEXT (ext, [action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

By default, the FC macros perform their tests using a .f extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for .f90 files, or preprocessing only with .F files or maybe other upper-case extensions. On the other hand, some other compilers expect all source files to end in .f and require special flags to support other file name extensions. The AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros deal with these issues.

The AC_FC_SRCEXT macro tries to get the FC compiler to accept files ending with the extension .ext (i.e., ext does not contain the dot). If any special compiler flags are needed for this, it stores them in the output variable FCFLAGS_ext. This extension and these flags are then used for all subsequent FC tests (until AC_FC_SRCEXT or AC_FC_PP_SRCEXT is called another time).

For example, you would use AC_FC_SRCEXT(f90) to employ the .f90 extension in future tests, and it would set the FCFLAGS_f90 output variable with any extra flags that are needed to compile such files.

Similarly, the AC_FC_PP_SRCEXT macro tries to get the FC compiler to preprocess and compile files with the extension .ext. When both fpp and cpp style preprocessing are provided, the former is preferred, as the latter may treat continuation lines, // tokens, and white space differently from what some Fortran dialects expect. Conversely, if you do not want files to be preprocessed, use only lower-case characters in the file name extension. Like with AC_FC_SRCEXT(f90), any needed flags are stored in the FCFLAGS_ext variable.

The FCFLAGS_ext flags can not be simply absorbed into FCFLAGS, for two reasons based on the limitations of some compilers. First, only one FCFLAGS_ext can be used at a time, so files with different extensions must be compiled separately. Second, FCFLAGS_ext must appear immediately before the source-code file name when compiling. So, continuing the example above, you might compile a foo.f90 file in your makefile with the command:

foo.o: foo.f90
       $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'

If AC_FC_SRCEXT or AC_FC_PP_SRCEXT succeeds in compiling files with the ext extension, it calls action-if-success (defaults to nothing). If it fails, and cannot find a way to make the FC compiler accept such files, it calls action-if-failure (defaults to exiting with an error message).

The AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros cache their results in ac_cv_fc_srcext_ext and ac_cv_fc_pp_srcext_ext variables, respectively.

Macro: AC_FC_PP_DEFINE ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Find a flag to specify defines for preprocessed Fortran. Not all Fortran compilers use -D. Substitute FC_DEFINE with the result and call action-if-success (defaults to nothing) if successful, and action-if-failure (defaults to failing with an error message) if not.

This macro calls AC_FC_PP_SRCEXT([F]) in order to learn how to preprocess a conftest.F file, but restores a previously used Fortran source file extension afterwards again.

The result of this test is cached in the ac_cv_fc_pp_define variable.

Macro: AC_FC_FREEFORM ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Try to ensure that the Fortran compiler ($FC) allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags to FCFLAGS.

This macro is most important if you are using the default .f extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with AC_FC_SRCEXT, such as .f90, then AC_FC_FREEFORM ordinarily succeeds without modifying FCFLAGS. For extensions which the compiler does not know about, the flag set by the AC_FC_SRCEXT macro might let the compiler assume Fortran 77 by default, however.

If AC_FC_FREEFORM succeeds in compiling free-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_freeform variable.

Macro: AC_FC_FIXEDFORM ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Try to ensure that the Fortran compiler ($FC) allows the old fixed-format source code (as opposed to free-format style). If necessary, it may add some additional flags to FCFLAGS.

This macro is needed for some compilers alias names like xlf95 which assume free-form source code by default, and in case you want to use fixed-form source with an extension like .f90 which many compilers interpret as free-form by default. If you specify a different extension with AC_FC_SRCEXT, such as .f, then AC_FC_FIXEDFORM ordinarily succeeds without modifying FCFLAGS.

If AC_FC_FIXEDFORM succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_fixedform variable.

Macro: AC_FC_LINE_LENGTH ([length], [action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Try to ensure that the Fortran compiler ($FC) accepts long source code lines. The length argument may be given as 80, 132, or unlimited, and defaults to 132. Note that line lengths above 250 columns are not portable, and some compilers do not accept more than 132 columns at least for fixed format source. If necessary, it may add some additional flags to FCFLAGS.

If AC_FC_LINE_LENGTH succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_line_length variable.

Macro: AC_FC_CHECK_BOUNDS ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

The AC_FC_CHECK_BOUNDS macro tries to enable array bounds checking in the Fortran compiler. If successful, the action-if-success is called and any needed flags are added to FCFLAGS. Otherwise, action-if-failure is called, which defaults to failing with an error message. The macro currently requires Fortran 90 or a newer dialect.

The result of the macro is cached in the ac_cv_fc_check_bounds variable.

Macro: AC_F77_IMPLICIT_NONE ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])
Macro: AC_FC_IMPLICIT_NONE ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Try to disallow implicit declarations in the Fortran compiler. If successful, action-if-success is called and any needed flags are added to FFLAGS or FCFLAGS, respectively. Otherwise, action-if-failure is called, which defaults to failing with an error message.

The result of these macros are cached in the ac_cv_f77_implicit_none and ac_cv_fc_implicit_none variables, respectively.

Macro: AC_FC_MODULE_EXTENSION

Find the Fortran 90 module file name extension. Most Fortran 90 compilers store module information in files separate from the object files. The module files are usually named after the name of the module rather than the source file name, with characters possibly turned to upper case, plus an extension, often .mod.

Not all compilers use module files at all, or by default. The Cray Fortran compiler requires -e m in order to store and search module information in .mod files rather than in object files. Likewise, the Fujitsu Fortran compilers uses the -Am option to indicate how module information is stored.

The AC_FC_MODULE_EXTENSION macro computes the module extension without the leading dot, and stores that in the FC_MODEXT variable. If the compiler does not produce module files, or the extension cannot be determined, FC_MODEXT is empty. Typically, the result of this macro may be used in cleanup make rules as follows:

clean-modules:
        -test -z "$(FC_MODEXT)" || rm -f *.$(FC_MODEXT)

The extension, or ‘unknown’, is cached in the ac_cv_fc_module_ext variable.

Macro: AC_FC_MODULE_FLAG ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Find the compiler flag to include Fortran 90 module information from another directory, and store that in the FC_MODINC variable. Call action-if-success (defaults to nothing) if successful, and set FC_MODINC to empty and call action-if-failure (defaults to exiting with an error message) if not.

Most Fortran 90 compilers provide a way to specify module directories. Some have separate flags for the directory to write module files to, and directories to search them in, whereas others only allow writing to the current directory or to the first directory specified in the include path. Further, with some compilers, the module search path and the preprocessor search path can only be modified with the same flag. Thus, for portability, write module files to the current directory only and list that as first directory in the search path.

There may be no whitespace between FC_MODINC and the following directory name, but FC_MODINC may contain trailing white space. For example, if you use Automake and would like to search ../lib for module files, you can use the following:

AM_FCFLAGS = $(FC_MODINC). $(FC_MODINC)../lib

Inside configure tests, you can use:

if test -n "$FC_MODINC"; then
  FCFLAGS="$FCFLAGS $FC_MODINC. $FC_MODINC../lib"
fi

The flag is cached in the ac_cv_fc_module_flag variable. The substituted value of FC_MODINC may refer to the ac_empty dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a Makefile.

Macro: AC_FC_MODULE_OUTPUT_FLAG ([action-if-success], [action-if-failure = ‘AC_MSG_FAILURE])

Find the compiler flag to write Fortran 90 module information to another directory, and store that in the FC_MODOUT variable. Call action-if-success (defaults to nothing) if successful, and set FC_MODOUT to empty and call action-if-failure (defaults to exiting with an error message) if not.

Not all Fortran 90 compilers write module files, and of those that do, not all allow writing to a directory other than the current one, nor do all have separate flags for writing and reading; see the description of AC_FC_MODULE_FLAG above. If you need to be able to write to another directory, for maximum portability use FC_MODOUT before any FC_MODINC and include both the current directory and the one you write to in the search path:

AM_FCFLAGS = $(FC_MODOUT)../mod $(FC_MODINC)../mod $(FC_MODINC). …

The flag is cached in the ac_cv_fc_module_output_flag variable. The substituted value of FC_MODOUT may refer to the ac_empty dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a Makefile.


5.10.9 Go Compiler Characteristics

Autoconf provides basic support for the Go programming language when using the gccgo compiler (there is currently no support for the 6g and 8g compilers).

Macro: AC_PROG_GO ([compiler-search-list])

Find the Go compiler to use. Check whether the environment variable GOC is set; if so, then set output variable GOC to its value.

Otherwise, if the macro is invoked without an argument, then search for a Go compiler named gccgo. If it is not found, then as a last resort set GOC to gccgo.

This macro may be invoked with an optional first argument which, if specified, must be a blank-separated list of Go compilers to search for.

If output variable GOFLAGS was not already set, set it to -g -O2. If your package does not like this default, GOFLAGS may be set before AC_PROG_GO.


5.11 System Services

The following macros check for operating system services or capabilities.

Macro: AC_PATH_X

Try to locate the X Window System include files and libraries. If the user gave the command line options --x-includes=dir and --x-libraries=dir, use those directories.

If either or both were not given, get the missing values by running xmkmf (or an executable pointed to by the XMKMF environment variable) on a trivial Imakefile and examining the makefile that it produces. Setting XMKMF to ‘false’ disables this method.

If this method fails to find the X Window System, configure looks for the files in several directories where they often reside. If either method is successful, set the shell variables x_includes and x_libraries to their locations, unless they are in directories the compiler searches by default.

If both methods fail, or the user gave the command line option --without-x, set the shell variable no_x to ‘yes’; otherwise set it to the empty string.

Macro: AC_PATH_XTRA

An enhanced version of AC_PATH_X. It adds the C compiler flags that X needs to output variable X_CFLAGS, and the X linker flags to X_LIBS. Define X_DISPLAY_MISSING if X is not available.

This macro also checks for special libraries that some systems need in order to compile X programs. It adds any that the system needs to output variable X_EXTRA_LIBS. And it checks for special X11R6 libraries that need to be linked with before -lX11, and adds any found to the output variable X_PRE_LIBS.

Macro: AC_SYS_INTERPRETER

Check whether the system supports starting scripts with a line of the form ‘#!/bin/sh’ to select the interpreter to use for the script. After running this macro, shell code in configure.ac can check the shell variable interpval; it is set to ‘yes’ if the system supports ‘#!’, ‘no’ if not.

Macro: AC_SYS_LARGEFILE

Arrange for 64-bit file offsets, known as large-file support. On some hosts, one must use special compiler options to build programs that can access large files. Append any such options to the output variable CC. Define _FILE_OFFSET_BITS and _LARGE_FILES if necessary.

Large-file support can be disabled by configuring with the --disable-largefile option.

If you use this macro, check that your program works even when off_t is wider than long int, since this is common when large-file support is enabled. For example, it is not correct to print an arbitrary off_t value X with printf ("%ld", (long int) X). Also, when using this macro in concert with AC_CONFIG_HEADERS, be sure that config.h is included before any system header.

The LFS introduced the fseeko and ftello functions to replace their C counterparts fseek and ftell that do not use off_t. Take care to use AC_FUNC_FSEEKO to make their prototypes available when using them and large-file support is enabled.

Macro: AC_SYS_LONG_FILE_NAMES

If the system supports file names longer than 14 characters, define HAVE_LONG_FILE_NAMES.

Macro: AC_SYS_POSIX_TERMIOS

Check to see if the Posix termios headers and functions are available on the system. If so, set the shell variable ac_cv_sys_posix_termios to ‘yes’. If not, set the variable to ‘no’.


5.12 C and Posix Variants

The following macro makes it possible to use C language and library extensions defined by the C standards committee, features of Posix that are extensions to C, and platform extensions not defined by Posix.

Macro: AC_USE_SYSTEM_EXTENSIONS

If possible, enable extensions to C or Posix on hosts that normally disable the extensions, typically due to standards-conformance namespace issues. This should be called before any macros that run the C compiler. Also, when using this macro in concert with AC_CONFIG_HEADERS, be sure that config.h is included before any system header.

The following preprocessor macros are defined unconditionally:

_ALL_SOURCE

Enable extensions on AIX 3 and Interix.

_DARWIN_C_SOURCE

Enable extensions on macOS.

_GNU_SOURCE

Enable extensions on GNU systems.

_NETBSD_SOURCE

Enable general extensions on NetBSD. Enable NetBSD compatibility extensions on Minix.

_OPENBSD_SOURCE

Enable OpenBSD compatibility extensions on NetBSD. Oddly enough, this does nothing on OpenBSD.

_POSIX_PTHREAD_SEMANTICS

Enable Posix-compatible threading on Solaris.

__STDC_WANT_IEC_60559_ATTRIBS_EXT__

Enable extensions specified by ISO/IEC TS 18661-5:2014.

__STDC_WANT_IEC_60559_BFP_EXT__

Enable extensions specified by ISO/IEC TS 18661-1:2014.

__STDC_WANT_IEC_60559_DFP_EXT__

Enable extensions specified by ISO/IEC TS 18661-2:2015.

__STDC_WANT_IEC_60559_FUNCS_EXT__

Enable extensions specified by ISO/IEC TS 18661-4:2015.

__STDC_WANT_IEC_60559_TYPES_EXT__

Enable extensions specified by ISO/IEC TS 18661-3:2015.

__STDC_WANT_LIB_EXT2__

Enable extensions specified by ISO/IEC TR 24731-2:2010.

__STDC_WANT_MATH_SPEC_FUNCS__

Enable extensions specified by ISO/IEC 24747:2009.

_TANDEM_SOURCE

Enable extensions on HP NonStop systems.

The following preprocessor macros are defined only when necessary; they enable access to extensions on some operating systems but disable extensions on other operating systems.

__EXTENSIONS__

Enable general extensions on Solaris. This macro is defined only if the headers included by AC_INCLUDES_DEFAULT (see Default Includes) work correctly with it defined.

_MINIX
_POSIX_SOURCE
_POSIX_1_SOURCE

Defined only on MINIX. _POSIX_SOURCE and _POSIX_1_SOURCE are needed to enable a number of POSIX features on this OS. _MINIX does not affect the system headers’ behavior; future versions of Autoconf may stop defining it. Programs that need to recognize Minix should use AC_CANONICAL_HOST.

_XOPEN_SOURCE

Defined (with value 500) only if needed to make wchar.h declare mbstate_t. This is known to be necessary on some versions of HP/UX.

The C preprocessor macro __STDC_WANT_DEC_FP__ is not defined. ISO/IEC TR 24732:2009 was superseded by ISO/IEC TS 18661-2:2015.

The C preprocessor macro __STDC_WANT_LIB_EXT1__ is not defined, as C11 Annex K is problematic. See: O’Donell C, Sebor M. Field Experience With Annex K—Bounds Checking Interfaces.

The Autoconf macro AC_USE_SYSTEM_EXTENSIONS was introduced in Autoconf 2.60.


5.13 Erlang Libraries

The following macros check for an installation of Erlang/OTP, and for the presence of certain Erlang libraries. All those macros require the configuration of an Erlang interpreter and an Erlang compiler (see Erlang Compiler and Interpreter Characteristics).

Macro: AC_ERLANG_SUBST_ERTS_VER

Set the output variable ERLANG_ERTS_VER to the version of the Erlang runtime system (as returned by Erlang’s erlang:system_info(version) function). The result of this test is cached if caching is enabled when running configure. The ERLANG_ERTS_VER variable is not intended to be used for testing for features of specific ERTS versions, but to be used for substituting the ERTS version in Erlang/OTP release resource files (.rel files), as shown below.

Macro: AC_ERLANG_SUBST_ROOT_DIR

Set the output variable ERLANG_ROOT_DIR to the path to the base directory in which Erlang/OTP is installed (as returned by Erlang’s code:root_dir/0 function). The result of this test is cached if caching is enabled when running configure.

Macro: AC_ERLANG_SUBST_LIB_DIR

Set the output variable ERLANG_LIB_DIR to the path of the library directory of Erlang/OTP (as returned by Erlang’s code:lib_dir/0 function), which subdirectories each contain an installed Erlang/OTP library. The result of this test is cached if caching is enabled when running configure.

Macro: AC_ERLANG_CHECK_LIB (library, [action-if-found], [action-if-not-found])

Test whether the Erlang/OTP library library is installed by calling Erlang’s code:lib_dir/1 function. The result of this test is cached if caching is enabled when running configure. action-if-found is a list of shell commands to run if the library is installed; action-if-not-found is a list of shell commands to run if it is not. Additionally, if the library is installed, the output variable ‘ERLANG_LIB_DIR_library’ is set to the path to the library installation directory, and the output variable ‘ERLANG_LIB_VER_library’ is set to the version number that is part of the subdirectory name, if it is in the standard form (library-version). If the directory name does not have a version part, ‘ERLANG_LIB_VER_library’ is set to the empty string. If the library is not installed, ‘ERLANG_LIB_DIR_library’ and ‘ERLANG_LIB_VER_library’ are set to "not found". For example, to check if library stdlib is installed:

AC_ERLANG_CHECK_LIB([stdlib],
  [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\""
   echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""],
  [AC_MSG_ERROR([stdlib was not found!])])

The ‘ERLANG_LIB_VER_library’ variables (set by AC_ERLANG_CHECK_LIB) and the ERLANG_ERTS_VER variable (set by AC_ERLANG_SUBST_ERTS_VER) are not intended to be used for testing for features of specific versions of libraries or of the Erlang runtime system. Those variables are intended to be substituted in Erlang release resource files (.rel files). For instance, to generate a example.rel file for an application depending on the stdlib library, configure.ac could contain:

AC_ERLANG_SUBST_ERTS_VER
AC_ERLANG_CHECK_LIB([stdlib],
  [],
  [AC_MSG_ERROR([stdlib was not found!])])
AC_CONFIG_FILES([example.rel])

The example.rel.in file used to generate example.rel should contain:

{release,
    {"@PACKAGE@", "@VERSION@"},
    {erts, "@ERLANG_ERTS_VER@"},
    [{stdlib, "@ERLANG_LIB_VER_stdlib@"},
     {@PACKAGE@, "@VERSION@"}]}.

In addition to the above macros, which test installed Erlang libraries, the following macros determine the paths to the directories into which newly built Erlang libraries are to be installed:

Macro: AC_ERLANG_SUBST_INSTALL_LIB_DIR

Set the ERLANG_INSTALL_LIB_DIR output variable to the directory into which every built Erlang library should be installed in a separate subdirectory. If this variable is not set in the environment when configure runs, its default value is ${libdir}/erlang/lib.

Macro: AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR (library, version)

Set the ‘ERLANG_INSTALL_LIB_DIR_library’ output variable to the directory into which the built Erlang library library version version should be installed. If this variable is not set in the environment when configure runs, its default value is ‘$ERLANG_INSTALL_LIB_DIR/library-version’, the value of the ERLANG_INSTALL_LIB_DIR variable being set by the AC_ERLANG_SUBST_INSTALL_LIB_DIR macro.


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6 Writing Tests

If the existing feature tests don’t do something you need, you have to write new ones. These macros are the building blocks. They provide ways for other macros to check whether various kinds of features are available and report the results.

This chapter contains some suggestions and some of the reasons why the existing tests are written the way they are. You can also learn a lot about how to write Autoconf tests by looking at the existing ones. If something goes wrong in one or more of the Autoconf tests, this information can help you understand the assumptions behind them, which might help you figure out how to best solve the problem.

These macros check the output of the compiler system of the current language (see Language Choice). They do not cache the results of their tests for future use (see Caching Results), because they don’t know enough about the information they are checking for to generate a cache variable name. They also do not print any messages, for the same reason. The checks for particular kinds of features call these macros and do cache their results and print messages about what they’re checking for.

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. See Writing Autoconf Macros, for how to do that.


6.1 Language Choice

Autoconf-generated configure scripts check for the C compiler and its features by default. Packages that use other programming languages (maybe more than one, e.g., C and C++) need to test features of the compilers for the respective languages. The following macros determine which programming language is used in the subsequent tests in configure.ac.

Macro: AC_LANG (language)

Do compilation tests using the compiler, preprocessor, and file extensions for the specified language.

Supported languages are:

C

Do compilation tests using CC and CPP and use extension .c for test programs. Use compilation flags: CPPFLAGS with CPP, and both CPPFLAGS and CFLAGS with CC.

C++

Do compilation tests using CXX and CXXCPP and use extension .C for test programs. Use compilation flags: CPPFLAGS with CXXCPP, and both CPPFLAGS and CXXFLAGS with CXX.

Fortran 77

Do compilation tests using F77 and use extension .f for test programs. Use compilation flags: FFLAGS.

Fortran

Do compilation tests using FC and use extension .f (or whatever has been set by AC_FC_SRCEXT) for test programs. Use compilation flags: FCFLAGS.

Erlang

Compile and execute tests using ERLC and ERL and use extension .erl for test Erlang modules. Use compilation flags: ERLCFLAGS.

Objective C

Do compilation tests using OBJC and OBJCPP and use extension .m for test programs. Use compilation flags: CPPFLAGS with OBJCPP, and both CPPFLAGS and OBJCFLAGS with OBJC.

Objective C++

Do compilation tests using OBJCXX and OBJCXXCPP and use extension .mm for test programs. Use compilation flags: CPPFLAGS with OBJCXXCPP, and both CPPFLAGS and OBJCXXFLAGS with OBJCXX.

Go

Do compilation tests using GOC and use extension .go for test programs. Use compilation flags GOFLAGS.

Macro: AC_LANG_PUSH (language)

Remember the current language (as set by AC_LANG) on a stack, and then select the language. Use this macro and AC_LANG_POP in macros that need to temporarily switch to a particular language.

Macro: AC_LANG_POP ([language])

Select the language that is saved on the top of the stack, as set by AC_LANG_PUSH, and remove it from the stack.

If given, language specifies the language we just quit. It is a good idea to specify it when it’s known (which should be the case…), since Autoconf detects inconsistencies.

AC_LANG_PUSH([Fortran 77])
# Perform some tests on Fortran 77.
# …
AC_LANG_POP([Fortran 77])
Macro: AC_LANG_ASSERT (language)

Check statically that the current language is language. You should use this in your language specific macros to avoid that they be called with an inappropriate language.

This macro runs only at autoconf time, and incurs no cost at configure time. Sadly enough and because Autoconf is a two layer language 2, the macros AC_LANG_PUSH and AC_LANG_POP cannot be “optimizing”, therefore as much as possible you ought to avoid using them to wrap your code, rather, require from the user to run the macro with a correct current language, and check it with AC_LANG_ASSERT. And anyway, that may help the user understand she is running a Fortran macro while expecting a result about her Fortran 77 compiler...

Macro: AC_REQUIRE_CPP

Ensure that whichever preprocessor would currently be used for tests has been found. Calls AC_REQUIRE (see Prerequisite Macros) with an argument of either AC_PROG_CPP or AC_PROG_CXXCPP, depending on which language is current.


6.2 Writing Test Programs

Autoconf tests follow a common scheme: feed some program with some input, and most of the time, feed a compiler with some source file. This section is dedicated to these source samples.


6.2.1 Guidelines for Test Programs

The most important rule to follow when writing testing samples is:

Look for realism.

This motto means that testing samples must be written with the same strictness as real programs are written. In particular, you should avoid “shortcuts” and simplifications.

Don’t just play with the preprocessor if you want to prepare a compilation. For instance, using cpp to check whether a header is functional might let your configure accept a header which causes some compiler error. Do not hesitate to check a header with other headers included before, especially required headers.

Make sure the symbols you use are properly defined, i.e., refrain from simply declaring a function yourself instead of including the proper header.

Test programs should not write to standard output. They should exit with status 0 if the test succeeds, and with status 1 otherwise, so that success can be distinguished easily from a core dump or other failure; segmentation violations and other failures produce a nonzero exit status. Unless you arrange for exit to be declared, test programs should return, not exit, from main, because on many systems exit is not declared by default.

Test programs can use #if or #ifdef to check the values of preprocessor macros defined by tests that have already run. For example, if you call AC_HEADER_STDBOOL, then later on in configure.ac you can have a test program that includes stdbool.h conditionally:

#ifdef HAVE_STDBOOL_H
# include <stdbool.h>
#endif

Both #if HAVE_STDBOOL_H and #ifdef HAVE_STDBOOL_H will work with any standard C compiler. Some developers prefer #if because it is easier to read, while others prefer #ifdef because it avoids diagnostics with picky compilers like GCC with the -Wundef option.

If a test program needs to use or create a data file, give it a name that starts with conftest, such as conftest.data. The configure script cleans up by running ‘rm -f -r conftest*’ after running test programs and if the script is interrupted.


6.2.2 Test Functions

These days it’s safe to assume support for function prototypes (introduced in C89).

Functions that test programs declare should also be conditionalized for C++, which requires ‘extern "C"’ prototypes. Make sure to not include any header files containing clashing prototypes.

#ifdef __cplusplus
extern "C"
#endif
void *valloc (size_t);

If a test program calls a function with invalid parameters (just to see whether it exists), organize the program to ensure that it never invokes that function. You can do this by calling it in another function that is never invoked. You can’t do it by putting it after a call to exit, because GCC version 2 knows that exit never returns and optimizes out any code that follows it in the same block.

If you include any header files, be sure to call the functions relevant to them with the correct number of arguments, even if they are just 0, to avoid compilation errors due to prototypes. GCC version 2 has internal prototypes for several functions that it automatically inlines; for example, memcpy. To avoid errors when checking for them, either pass them the correct number of arguments or redeclare them with a different return type (such as char).


6.2.3 Generating Sources

Autoconf provides a set of macros that can be used to generate test source files. They are written to be language generic, i.e., they actually depend on the current language (see Language Choice) to “format” the output properly.

Macro: AC_LANG_CONFTEST (source)

Save the source text in the current test source file: conftest.extension where the extension depends on the current language. As of Autoconf 2.63b, the source file also contains the results of all of the AC_DEFINE performed so far.

Note that the source is evaluated exactly once, like regular Autoconf macro arguments, and therefore (i) you may pass a macro invocation, (ii) if not, be sure to double quote if needed.

This macro issues a warning during autoconf processing if source does not include an expansion of the macro AC_LANG_DEFINES_PROVIDED (note that both AC_LANG_SOURCE and AC_LANG_PROGRAM call this macro, and thus avoid the warning).

This macro is seldom called directly, but is used under the hood by more common macros such as AC_COMPILE_IFELSE and AC_RUN_IFELSE.

Macro: AC_LANG_DEFINES_PROVIDED

This macro is called as a witness that the file conftest.extension appropriate for the current language is complete, including all previously determined results from AC_DEFINE. This macro is seldom called directly, but exists if you have a compelling reason to write a conftest file without using AC_LANG_SOURCE, yet still want to avoid a syntax warning from AC_LANG_CONFTEST.

Macro: AC_LANG_SOURCE (source)

Expands into the source, with the definition of all the AC_DEFINE performed so far. This macro includes an expansion of AC_LANG_DEFINES_PROVIDED.

In many cases, you may find it more convenient to use the wrapper AC_LANG_PROGRAM.

For instance, executing (observe the double quotation!):

AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
        [https://www.example.org/])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
  [Greetings string.])
AC_LANG([C])
AC_LANG_CONFTEST(
   [AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])])
gcc -E -dD conftest.c

on a system with gcc installed, results in:

…
# 1 "conftest.c"

#define PACKAGE_NAME "Hello"
#define PACKAGE_TARNAME "hello"
#define PACKAGE_VERSION "1.0"
#define PACKAGE_STRING "Hello 1.0"
#define PACKAGE_BUGREPORT "bug-hello@example.org"
#define PACKAGE_URL "https://www.example.org/"
#define HELLO_WORLD "Hello, World\n"

const char hw[] = "Hello, World\n";

When the test language is Fortran, Erlang, or Go, the AC_DEFINE definitions are not automatically translated into constants in the source code by this macro.

Macro: AC_LANG_PROGRAM (prologue, body)

Expands into a source file which consists of the prologue, and then body as body of the main function (e.g., main in C). Since it uses AC_LANG_SOURCE, the features of the latter are available.

For instance:

AC_INIT([Hello], [1.0], [bug-hello@example.org], [],
        [https://www.example.org/])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
  [Greetings string.])
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                 [[fputs (hw, stdout);]])])
gcc -E -dD conftest.c

on a system with gcc installed, results in:

…
# 1 "conftest.c"

#define PACKAGE_NAME "Hello"
#define PACKAGE_TARNAME "hello"
#define PACKAGE_VERSION "1.0"
#define PACKAGE_STRING "Hello 1.0"
#define PACKAGE_BUGREPORT "bug-hello@example.org"
#define PACKAGE_URL "https://www.example.org/"
#define HELLO_WORLD "Hello, World\n"

const char hw[] = "Hello, World\n";
int
main (void)
{
fputs (hw, stdout);
  ;
  return 0;
}

In Erlang tests, the created source file is that of an Erlang module called conftest (conftest.erl). This module defines and exports at least one start/0 function, which is called to perform the test. The prologue is optional code that is inserted between the module header and the start/0 function definition. body is the body of the start/0 function without the final period (see Checking Runtime Behavior, about constraints on this function’s behavior).

For instance:

AC_INIT([Hello], [1.0], [bug-hello@example.org])
AC_LANG(Erlang)
AC_LANG_CONFTEST(
[AC_LANG_PROGRAM([[-define(HELLO_WORLD, "Hello, world!").]],
                 [[io:format("~s~n", [?HELLO_WORLD])]])])
cat conftest.erl

results in:

-module(conftest).
-export([start/0]).
-define(HELLO_WORLD, "Hello, world!").
start() ->
io:format("~s~n", [?HELLO_WORLD])
.
Macro: AC_LANG_CALL (prologue, function)

Expands into a source file which consists of the prologue, and then a call to the function as body of the main function (e.g., main in C). Since it uses AC_LANG_PROGRAM, the feature of the latter are available.

This function will probably be replaced in the future by a version which would enable specifying the arguments. The use of this macro is not encouraged, as it violates strongly the typing system.

This macro cannot be used for Erlang tests.

Expands into a source file which uses the function in the body of the main function (e.g., main in C). Since it uses AC_LANG_PROGRAM, the features of the latter are available.

As AC_LANG_CALL, this macro is documented only for completeness. It is considered to be severely broken, and in the future will be removed in favor of actual function calls (with properly typed arguments).

This macro cannot be used for Erlang tests.


6.3 Running the Preprocessor

Sometimes one might need to run the preprocessor on some source file. Usually it is a bad idea, as you typically need to compile your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist the temptation of following the easiest path.

Nevertheless, if you need to run the preprocessor, then use AC_PREPROC_IFELSE.

The macros described in this section cannot be used for tests in Erlang, Fortran, or Go, since those languages require no preprocessor.

Macro: AC_PREPROC_IFELSE (input, [action-if-true], [action-if-false])

Run the preprocessor of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by AC_LANG_PROGRAM and friends.

This macro uses CPPFLAGS, but not CFLAGS, because -g, -O, etc. are not valid options to many C preprocessors.

It is customary to report unexpected failures with AC_MSG_FAILURE. If needed, action-if-true can further access the preprocessed output in the file conftest.i.

For instance:

AC_INIT([Hello], [1.0], [bug-hello@example.org])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
  [Greetings string.])
AC_PREPROC_IFELSE(
   [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                    [[fputs (hw, stdout);]])],
   [AC_MSG_RESULT([OK])],
   [AC_MSG_FAILURE([unexpected preprocessor failure])])

might result in:

checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C... yes
checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... -std=gnu11
checking how to run the C preprocessor... gcc -std=gnu11 -E
OK

The macro AC_TRY_CPP (see Obsolete Macros) used to play the role of AC_PREPROC_IFELSE, but double quotes its argument, making it impossible to use it to elaborate sources. You are encouraged to get rid of your old use of the macro AC_TRY_CPP in favor of AC_PREPROC_IFELSE, but, in the first place, are you sure you need to run the preprocessor and not the compiler?

Macro: AC_EGREP_HEADER (pattern, header-file, action-if-found, [action-if-not-found])

If the output of running the preprocessor on the system header file header-file matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

See below for some problems involving this macro.

Macro: AC_EGREP_CPP (pattern, program, [action-if-found], [action-if-not-found])

program is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on program matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

See below for some problems involving this macro.

AC_EGREP_CPP and AC_EGREP_HEADER should be used with care, as preprocessors can insert line breaks between output tokens. For example, the preprocessor might transform this:

#define MAJOR 2
#define MINOR 23
Version MAJOR . MINOR

into this:

Version
       2
                 .
                   23

Because preprocessors are allowed to insert white space, change escapes in string contants, insert backlash-newline pairs, or do any of a number of things that do not change the meaning of the preprocessed program, it is better to rely on AC_PREPROC_IFELSE than to resort to AC_EGREP_CPP or AC_EGREP_HEADER.


6.4 Running the Compiler

To check for a syntax feature of the current language’s (see Language Choice) compiler, such as whether it recognizes a certain keyword, or simply to try some library feature, use AC_COMPILE_IFELSE to try to compile a small program that uses that feature.

Macro: AC_COMPILE_IFELSE (input, [action-if-true], [action-if-false])

Run the compiler and compilation flags of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by AC_LANG_PROGRAM and friends.

It is customary to report unexpected failures with AC_MSG_FAILURE. This macro does not try to link; use AC_LINK_IFELSE if you need to do that (see Running the Linker). If needed, action-if-true can further access the just-compiled object file conftest.$OBJEXT.

This macro uses AC_REQUIRE for the compiler associated with the current language, which means that if the compiler has not yet been determined, the compiler determination will be made prior to the body of the outermost AC_DEFUN macro that triggered this macro to expand (see Expanded Before Required).

For tests in Erlang, the input must be the source code of a module named conftest. AC_COMPILE_IFELSE generates a conftest.beam file that can be interpreted by the Erlang virtual machine (ERL). It is recommended to use AC_LANG_PROGRAM to specify the test program, to ensure that the Erlang module has the right name.


6.5 Running the Linker

To check for a library, a function, or a global variable, Autoconf configure scripts try to compile and link a small program that uses it. This is unlike Metaconfig, which by default uses nm or ar on the C library to try to figure out which functions are available. Trying to link with the function is usually a more reliable approach because it avoids dealing with the variations in the options and output formats of nm and ar and in the location of the standard libraries. It also allows configuring for cross-compilation or checking a function’s runtime behavior if needed. On the other hand, it can be slower than scanning the libraries once, but accuracy is more important than speed.

AC_LINK_IFELSE is used to compile test programs to test for functions and global variables. It is also used by AC_CHECK_LIB to check for libraries (see Library Files), by adding the library being checked for to LIBS temporarily and trying to link a small program.

Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by AC_LANG_PROGRAM and friends. If needed, action-if-true can further access the just-linked program file conftest$EXEEXT.

LDFLAGS and LIBS are used for linking, in addition to the current compilation flags.

It is customary to report unexpected failures with AC_MSG_FAILURE. This macro does not try to execute the program; use AC_RUN_IFELSE if you need to do that (see Checking Runtime Behavior).

The AC_LINK_IFELSE macro cannot be used for Erlang tests, since Erlang programs are interpreted and do not require linking.


6.6 Checking Runtime Behavior

Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine’s endianness when your program initializes itself.

If you really need to test for a runtime behavior while configuring, you can write a test program to determine the result, and compile and run it using AC_RUN_IFELSE. Avoid running test programs if possible, because this prevents people from configuring your package for cross-compiling.

Macro: AC_RUN_IFELSE (input, [action-if-true], [action-if-false], [action-if-cross-compiling = ‘AC_MSG_FAILURE])

Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, then execute the resulting program. If the program returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.

The input can be made by AC_LANG_PROGRAM and friends. LDFLAGS and LIBS are used for linking, in addition to the compilation flags of the current language (see Language Choice). Additionally, action-if-true can run ./conftest$EXEEXT for further testing.

In the action-if-false section, the failing exit status is available in the shell variable ‘$?’. This exit status might be that of a failed compilation, or it might be that of a failed program execution.

If cross-compilation mode is enabled (this is the case if either the compiler being used does not produce executables that run on the system where configure is being run, or if the options --build and --host were both specified and their values are different), then the test program is not run. If the optional shell commands action-if-cross-compiling are given, those commands are run instead; typically these commands provide pessimistic defaults that allow cross-compilation to work even if the guess was wrong. If the fourth argument is empty or omitted, but cross-compilation is detected, then configure prints an error message and exits. If you want your package to be useful in a cross-compilation scenario, you should provide a non-empty action-if-cross-compiling clause, as well as wrap the AC_RUN_IFELSE compilation inside an AC_CACHE_CHECK (see Caching Results) which allows the user to override the pessimistic default if needed.

It is customary to report unexpected failures with AC_MSG_FAILURE.

autoconf prints a warning message when creating configure each time it encounters a call to AC_RUN_IFELSE with no action-if-cross-compiling argument given. If you are not concerned about users configuring your package for cross-compilation, you may ignore the warning. A few of the macros distributed with Autoconf produce this warning message; but if this is a problem for you, please report it as a bug, along with an appropriate pessimistic guess to use instead.

To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).

To provide a default for calls of AC_RUN_IFELSE that are embedded in other macros, including a few of the ones that come with Autoconf, you can test whether the shell variable cross_compiling is set to ‘yes’, and then use an alternate method to get the results instead of calling the macros.

It is also permissible to temporarily assign to cross_compiling in order to force tests to behave as though they are in a cross-compilation environment, particularly since this provides a way to test your action-if-cross-compiling even when you are not using a cross-compiler.

# We temporarily set cross-compile mode to force AC_COMPUTE_INT
# to use the slow link-only method
save_cross_compiling=$cross_compiling
cross_compiling=yes
AC_COMPUTE_INT([…])
cross_compiling=$save_cross_compiling

A C or C++ runtime test should be portable. See Portable C and C++ Programming.

Erlang tests must exit themselves the Erlang VM by calling the halt/1 function: the given status code is used to determine the success of the test (status is 0) or its failure (status is different than 0), as explained above. It must be noted that data output through the standard output (e.g., using io:format/2) may be truncated when halting the VM. Therefore, if a test must output configuration information, it is recommended to create and to output data into the temporary file named conftest.out, using the functions of module file. The conftest.out file is automatically deleted by the AC_RUN_IFELSE macro. For instance, a simplified implementation of Autoconf’s AC_ERLANG_SUBST_LIB_DIR macro is:

AC_INIT([LibdirTest], [1.0], [bug-libdirtest@example.org])
AC_ERLANG_NEED_ERL
AC_LANG(Erlang)
AC_RUN_IFELSE(
  [AC_LANG_PROGRAM([], [dnl
    file:write_file("conftest.out", code:lib_dir()),
    halt(0)])],
  [echo "code:lib_dir() returned: `cat conftest.out`"],
  [AC_MSG_FAILURE([test Erlang program execution failed])])

6.7 Systemology

This section aims at presenting some systems and pointers to documentation. It may help you addressing particular problems reported by users.

Posix-conforming systems are derived from the Unix operating system.

The Rosetta Stone for Unix contains a table correlating the features of various Posix-conforming systems. Unix History is a simplified diagram of how many Unix systems were derived from each other.

The Heirloom Project provides some variants of traditional implementations of Unix utilities.

Darwin

Darwin is also known as Mac OS X. Beware that the file system can be case-preserving, but case insensitive. This can cause nasty problems, since for instance the installation attempt for a package having an INSTALL file can result in ‘make install’ report that nothing was to be done!

That’s all dependent on whether the file system is a UFS (case sensitive) or HFS+ (case preserving). By default Apple wants you to install the OS on HFS+. Unfortunately, there are some pieces of software which really need to be built on UFS. We may want to rebuild Darwin to have both UFS and HFS+ available (and put the /local/build tree on the UFS).

QNX 4.25

QNX is a realtime operating system running on Intel architecture meant to be scalable from the small embedded systems to the hundred processor super-computer. It claims to be Posix certified. More information is available on the QNX home page.

Unix version 7

Officially this was called the “Seventh Edition” of “the UNIX time-sharing system” but we use the more-common name “Unix version 7”. Documentation is available in the Unix Seventh Edition Manual. Previous versions of Unix are called “Unix version 6”, etc., but they were not as widely used.


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6.8 Multiple Cases

Some operations are accomplished in several possible ways, depending on the OS variant. Checking for them essentially requires a “case statement”. Autoconf does not directly provide one; however, it is easy to simulate by using a shell variable to keep track of whether a way to perform the operation has been found yet.

Here is an example that uses the shell variable fstype to keep track of whether the remaining cases need to be checked. Note that since the value of fstype is under our control, we don’t have to use the longer ‘test "x$fstype" = xno’.

AC_MSG_CHECKING([how to get file system type])
fstype=no
# The order of these tests is important.
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statvfs.h>
#include <sys/fstyp.h>]])],
                  [AC_DEFINE([FSTYPE_STATVFS], [1],
                     [Define if statvfs exists.])
                   fstype=SVR4])
if test $fstype = no; then
  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/fstyp.h>]])],
                  [AC_DEFINE([FSTYPE_USG_STATFS], [1],
                     [Define if USG statfs.])
                   fstype=SVR3])
fi
if test $fstype = no; then
  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h>
#include <sys/vmount.h>]])]),
                  [AC_DEFINE([FSTYPE_AIX_STATFS], [1],
                     [Define if AIX statfs.])
                   fstype=AIX])
fi
# (more cases omitted here)
AC_MSG_RESULT([$fstype])

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7 Results of Tests

Once configure has determined whether a feature exists, what can it do to record that information? There are four sorts of things it can do: define a C preprocessor symbol, set a variable in the output files, save the result in a cache file for future configure runs, and print a message letting the user know the result of the test.


7.1 Defining C Preprocessor Symbols

A common action to take in response to a feature test is to define a C preprocessor symbol indicating the results of the test. That is done by calling AC_DEFINE or AC_DEFINE_UNQUOTED.

By default, AC_OUTPUT places the symbols defined by these macros into the output variable DEFS, which contains an option -Dsymbol=value for each symbol defined. Unlike in Autoconf version 1, there is no variable DEFS defined while configure is running. To check whether Autoconf macros have already defined a certain C preprocessor symbol, test the value of the appropriate cache variable, as in this example:

AC_CHECK_FUNC([vprintf], [AC_DEFINE([HAVE_VPRINTF], [1],
                          [Define if vprintf exists.])])
if test "x$ac_cv_func_vprintf" != xyes; then
  AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1],
                            [Define if _doprnt exists.])])
fi

If AC_CONFIG_HEADERS has been called, then instead of creating DEFS, AC_OUTPUT creates a header file by substituting the correct values into #define statements in a template file. See Configuration Header Files, for more information about this kind of output.

Macro: AC_DEFINE (variable, value, [description])
Macro: AC_DEFINE (variable)

Define variable to value (verbatim), by defining a C preprocessor macro for variable. variable should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis ‘...’ if C99-or-later syntax is employed. variable should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters.

value may contain backslash-escaped newlines, which will be preserved if you use AC_CONFIG_HEADERS but flattened if passed via @DEFS@ (with no effect on the compilation, since the preprocessor sees only one line in the first place). value should not contain raw newlines. If you are not using AC_CONFIG_HEADERS, value should not contain any ‘#’ characters, as make tends to eat them. To use a shell variable, use AC_DEFINE_UNQUOTED instead.

description is only useful if you are using AC_CONFIG_HEADERS. In this case, description is put into the generated config.h.in as the comment before the macro define. The following example defines the C preprocessor variable EQUATION to be the string constant ‘"$a > $b"’:

AC_DEFINE([EQUATION], ["$a > $b"],
  [Equation string.])

If neither value nor description are given, then value defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf.

If the variable is a literal string, it is passed to m4_pattern_allow (see Forbidden Patterns).

If multiple AC_DEFINE statements are executed for the same variable name (not counting any parenthesized argument list), the last one wins.

Macro: AC_DEFINE_UNQUOTED (variable, value, [description])
Macro: AC_DEFINE_UNQUOTED (variable)

Like AC_DEFINE, but three shell expansions are performed—once—on variable and value: variable expansion (‘$’), command substitution (‘`’), and backslash escaping (‘\’), as if in an unquoted here-document. Single and double quote characters in the value have no special meaning. Use this macro instead of AC_DEFINE when variable or value is a shell variable. Examples:

AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"],
  [Configuration machine file.])
AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups],
  [getgroups return type.])
AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1],
  [Translated header name.])

Due to a syntactical oddity of the Bourne shell, do not use semicolons to separate AC_DEFINE or AC_DEFINE_UNQUOTED calls from other macro calls or shell code; that can cause syntax errors in the resulting configure script. Use either blanks or newlines. That is, do this:

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"])

or this:

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4])
   LIBS="-lelf $LIBS"])

instead of this:

AC_CHECK_HEADER([elf.h],
  [AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"])

7.2 Setting Output Variables

Another way to record the results of tests is to set output variables, which are shell variables whose values are substituted into files that configure outputs. The two macros below create new output variables. See Preset Output Variables, for a list of output variables that are always available.

Macro: AC_SUBST (variable, [value])

Create an output variable from a shell variable. Make AC_OUTPUT substitute the variable variable into output files (typically one or more makefiles). This means that AC_OUTPUT replaces instances of ‘@variable@’ in input files with the value that the shell variable variable has when AC_OUTPUT is called. The value can contain any non-NUL character, including newline. If you are using Automake 1.11 or newer, for newlines in values you might want to consider using AM_SUBST_NOTMAKE to prevent automake from adding a line variable = @variable@ to the Makefile.in files (see Automake in Other things Automake recognizes).

Variable occurrences should not overlap: e.g., an input file should not contain ‘@var1@var2@’ if var1 and var2 are variable names. The substituted value is not rescanned for more output variables; occurrences of ‘@variable@’ in the value are inserted literally into the output file. (The algorithm uses the special marker |#_!!_#| internally, so neither the substituted value nor the output file may contain |#_!!_#|.)

If value is given, in addition assign it to variable.

The string variable is passed to m4_pattern_allow (see Forbidden Patterns). variable is not further expanded, even if there is another macro by the same name.

Macro: AC_SUBST_FILE (variable)

Another way to create an output variable from a shell variable. Make AC_OUTPUT insert (without substitutions) the contents of the file named by shell variable variable into output files. This means that AC_OUTPUT replaces instances of ‘@variable@’ in output files (such as Makefile.in) with the contents of the file that the shell variable variable names when AC_OUTPUT is called. Set the variable to /dev/null for cases that do not have a file to insert. This substitution occurs only when the ‘@variable@’ is on a line by itself, optionally surrounded by spaces and tabs. The substitution replaces the whole line, including the spaces, tabs, and the terminating newline.

This macro is useful for inserting makefile fragments containing special dependencies or other make directives for particular host or target types into makefiles. For example, configure.ac could contain:

AC_SUBST_FILE([host_frag])
host_frag=$srcdir/conf/sun4.mh

and then a Makefile.in could contain:

@host_frag@

The string variable is passed to m4_pattern_allow (see Forbidden Patterns).

Running configure in varying environments can be extremely dangerous. If for instance the user runs ‘CC=bizarre-cc ./configure’, then the cache, config.h, and many other output files depend upon bizarre-cc being the C compiler. If for some reason the user runs ./configure again, or if it is run via ‘./config.status --recheck’, (See Automatic Remaking, and see config.status Invocation), then the configuration can be inconsistent, composed of results depending upon two different compilers.

Environment variables that affect this situation, such as ‘CC’ above, are called precious variables, and can be declared as such by AC_ARG_VAR.

Macro: AC_ARG_VAR (variable, description)

Declare variable is a precious variable, and include its description in the variable section of ‘./configure --help’.

Being precious means that

  • - variable is substituted via AC_SUBST.
  • - The value of variable when configure was launched is saved in the cache, including if it was not specified on the command line but via the environment. Indeed, while configure can notice the definition of CC in ‘./configure CC=bizarre-cc’, it is impossible to notice it in ‘CC=bizarre-cc ./configure’, which, unfortunately, is what most users do.

    We emphasize that it is the initial value of variable which is saved, not that found during the execution of configure. Indeed, specifying ‘./configure FOO=foo’ and letting ‘./configure’ guess that FOO is foo can be two different things.

  • - variable is checked for consistency between two configure runs. For instance:
    $ ./configure --silent --config-cache
    $ CC=cc ./configure --silent --config-cache
    configure: error: 'CC' was not set in the previous run
    configure: error: changes in the environment can compromise \
    the build
    configure: error: run 'make distclean' and/or \
    'rm config.cache' and start over
    

    and similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused.

  • - variable is kept during automatic reconfiguration (see config.status Invocation) as if it had been passed as a command line argument, including when no cache is used:
    $ CC=/usr/bin/cc ./configure var=raboof --silent
    $ ./config.status --recheck
    running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \
      CC=/usr/bin/cc  --no-create --no-recursion
    

7.3 Special Characters in Output Variables

Many output variables are intended to be evaluated both by make and by the shell. Some characters are expanded differently in these two contexts, so to avoid confusion these variables’ values should not contain any of the following characters:

" # $ & ' ( ) * ; < > ? [ \ ^ ` |

Also, these variables’ values should neither contain newlines, nor start with ‘~’, nor contain white space or ‘:’ immediately followed by ‘~’. The values can contain nonempty sequences of white space characters like tabs and spaces, but each such sequence might arbitrarily be replaced by a single space during substitution.

These restrictions apply both to the values that configure computes, and to the values set directly by the user. For example, the following invocations of configure are problematic, since they attempt to use special characters within CPPFLAGS and white space within $(srcdir):

CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure'

'../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"'

7.4 Caching Results

To avoid checking for the same features repeatedly in various configure scripts (or in repeated runs of one script), configure can optionally save the results of many checks in a cache file (see Cache Files). If a configure script runs with caching enabled and finds a cache file, it reads the results of previous runs from the cache and avoids rerunning those checks. As a result, configure can then run much faster than if it had to perform all of the checks every time.

Macro: AC_CACHE_VAL (cache-id, commands-to-set-it)

Ensure that the results of the check identified by cache-id are available. If the results of the check were in the cache file that was read, and configure was not given the --quiet or --silent option, print a message saying that the result was cached; otherwise, run the shell commands commands-to-set-it. If the shell commands are run to determine the value, the value is saved in the cache file just before configure creates its output files. See Cache Variable Names, for how to choose the name of the cache-id variable.

The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

Macro: AC_CACHE_CHECK (message, cache-id, commands-to-set-it)

A wrapper for AC_CACHE_VAL that takes care of printing the messages. This macro provides a convenient shorthand for the most common way to use these macros. It calls AC_MSG_CHECKING for message, then AC_CACHE_VAL with the cache-id and commands arguments, and AC_MSG_RESULT with cache-id.

The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.

It is common to find buggy macros using AC_CACHE_VAL or AC_CACHE_CHECK, because people are tempted to call AC_DEFINE in the commands-to-set-it. Instead, the code that follows the call to AC_CACHE_VAL should call AC_DEFINE, by examining the value of the cache variable. For instance, the following macro is broken:

AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works],
                [my_cv_shell_true_works=no
                 (true) 2>/dev/null && my_cv_shell_true_works=yes
                 if test "x$my_cv_shell_true_works" = xyes; then
                   AC_DEFINE([TRUE_WORKS], [1],
                             [Define if 'true(1)' works properly.])
                 fi])
])

This fails if the cache is enabled: the second time this macro is run, TRUE_WORKS will not be defined. The proper implementation is:

AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works],
                [my_cv_shell_true_works=no
                 (true) 2>/dev/null && my_cv_shell_true_works=yes])
 if test "x$my_cv_shell_true_works" = xyes; then
   AC_DEFINE([TRUE_WORKS], [1],
             [Define if 'true(1)' works properly.])
 fi
])

Also, commands-to-set-it should not print any messages, for example with AC_MSG_CHECKING; do that before calling AC_CACHE_VAL, so the messages are printed regardless of whether the results of the check are retrieved from the cache or determined by running the shell commands.


7.4.1 Cache Variable Names

The names of cache variables should have the following format:

package-prefix_cv_value-type_specific-value_[additional-options]

for example, ‘ac_cv_header_stat_broken’ or ‘ac_cv_prog_gcc_traditional’. The parts of the variable name are:

package-prefix

An abbreviation for your package or organization; the same prefix you begin local Autoconf macros with, except lowercase by convention. For cache values used by the distributed Autoconf macros, this value is ‘ac’.

_cv_

Indicates that this shell variable is a cache value. This string must be present in the variable name, including the leading underscore.

value-type

A convention for classifying cache values, to produce a rational naming system. The values used in Autoconf are listed in Macro Names.

specific-value

Which member of the class of cache values this test applies to. For example, which function (‘alloca’), program (‘gcc’), or output variable (‘INSTALL’).

additional-options

Any particular behavior of the specific member that this test applies to. For example, ‘broken’ or ‘set’. This part of the name may be omitted if it does not apply.

The values assigned to cache variables may not contain newlines. Usually, their values are Boolean (‘yes’ or ‘no’) or the names of files or functions; so this is not an important restriction. Cache Variable Index for an index of cache variables with documented semantics.


7.4.2 Cache Files

A cache file is a shell script that caches the results of configure tests run on one system so they can be shared between configure scripts and configure runs. It is not useful on other systems. If its contents are invalid for some reason, the user may delete or edit it, or override documented cache variables on the configure command line.

By default, configure uses no cache file, to avoid problems caused by accidental use of stale cache files.

To enable caching, configure accepts --config-cache (or -C) to cache results in the file config.cache. Alternatively, --cache-file=file specifies that file be the cache file. The cache file is created if it does not exist already. When configure calls configure scripts in subdirectories, it uses the --cache-file argument so that they share the same cache. See Configuring Other Packages in Subdirectories, for information on configuring subdirectories with the AC_CONFIG_SUBDIRS macro.

config.status only pays attention to the cache file if it is given the --recheck option, which makes it rerun configure.

It is wrong to try to distribute cache files for particular system types. There is too much room for error in doing that, and too much administrative overhead in maintaining them. For any features that can’t be guessed automatically, use the standard method of the canonical system type and linking files (see Manual Configuration).

The site initialization script can specify a site-wide cache file to use, instead of the usual per-program cache. In this case, the cache file gradually accumulates information whenever someone runs a new configure script. (Running configure merges the new cache results with the existing cache file.) This may cause problems, however, if the system configuration (e.g., the installed libraries or compilers) changes and the stale cache file is not deleted.

If configure is interrupted at the right time when it updates a cache file outside of the build directory where the configure script is run, it may leave behind a temporary file named after the cache file with digits following it. You may safely delete such a file.


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7.4.3 Cache Checkpointing

If your configure script, or a macro called from configure.ac, happens to abort the configure process, it may be useful to checkpoint the cache a few times at key points using AC_CACHE_SAVE. Doing so reduces the amount of time it takes to rerun the configure script with (hopefully) the error that caused the previous abort corrected.

Macro: AC_CACHE_LOAD

Loads values from existing cache file, or creates a new cache file if a cache file is not found. Called automatically from AC_INIT.

Macro: AC_CACHE_SAVE

Flushes all cached values to the cache file. Called automatically from AC_OUTPUT, but it can be quite useful to call AC_CACHE_SAVE at key points in configure.ac.

For instance:

 … AC_INIT, etc. …
# Checks for programs.
AC_PROG_CC
AC_PROG_AWK
 … more program checks …
AC_CACHE_SAVE

# Checks for libraries.
AC_CHECK_LIB([nsl], [gethostbyname])
AC_CHECK_LIB([socket], [connect])
 … more lib checks …
AC_CACHE_SAVE

# Might abort…
AM_PATH_GTK([1.0.2], [], [AC_MSG_ERROR([GTK not in path])])
AM_PATH_GTKMM([0.9.5], [], [AC_MSG_ERROR([GTK not in path])])
 … AC_OUTPUT, etc. …

7.5 Printing Messages

configure scripts need to give users running them several kinds of information. The following macros print messages in ways appropriate for each kind. The arguments to all of them get enclosed in shell double quotes, so the shell performs variable and back-quote substitution on them.

These macros are all wrappers around the echo shell command. They direct output to the appropriate file descriptor (see File Descriptor Macros). configure scripts should rarely need to run echo directly to print messages for the user. Using these macros makes it easy to change how and when each kind of message is printed; such changes need only be made to the macro definitions and all the callers change automatically.

To diagnose static issues, i.e., when autoconf is run, see Diagnostic messages from M4sugar.

Macro: AC_MSG_CHECKING (feature-description)

Notify the user that configure is checking for a particular feature. This macro prints a message that starts with ‘checking ’ and ends with ‘...’ and no newline. It must be followed by a call to AC_MSG_RESULT to print the result of the check and the newline. The feature-description should be something like ‘whether the Fortran compiler accepts C++ comments’ or ‘for _Alignof’.

This macro prints nothing if configure is run with the --quiet or --silent option.

Macro: AC_MSG_RESULT (result-description)

Notify the user of the results of a check. result-description is almost always the value of the cache variable for the check, typically ‘yes’, ‘no’, or a file name. This macro should follow a call to AC_MSG_CHECKING, and the result-description should be the completion of the message printed by the call to AC_MSG_CHECKING.

This macro prints nothing if configure is run with the --quiet or --silent option.

Macro: AC_MSG_NOTICE (message)

Deliver the message to the user. It is useful mainly to print a general description of the overall purpose of a group of feature checks, e.g.,

AC_MSG_NOTICE([checking if stack overflow is detectable])

This macro prints nothing if configure is run with the --quiet or --silent option.

Macro: AC_MSG_ERROR (error-description, [exit-status = ‘$?/1])

Notify the user of an error that prevents configure from completing. This macro prints an error message to the standard error output and exits configure with exit-status (‘$?’ by default, except that ‘0’ is converted to ‘1’). error-description should be something like ‘invalid value $HOME for \$HOME’.

The error-description should start with a lower-case letter, and “cannot” is preferred to “can’t”.

Macro: AC_MSG_FAILURE (error-description, [exit-status])

This AC_MSG_ERROR wrapper notifies the user of an error that prevents configure from completing and that additional details are provided in config.log. This is typically used when abnormal results are found during a compilation.

Macro: AC_MSG_WARN (problem-description)

Notify the configure user of a possible problem. This macro prints the message to the standard error output; configure continues running afterward, so macros that call AC_MSG_WARN should provide a default (back-up) behavior for the situations they warn about. problem-description should be something like ‘ln -s seems to make hard links’.


8 Programming in M4

Autoconf is written on top of two layers: M4sugar, which provides convenient macros for pure M4 programming, and M4sh, which provides macros dedicated to shell script generation.

As of this version of Autoconf, these two layers still contain experimental macros, whose interface might change in the future. As a matter of fact, anything that is not documented must not be used.


8.1 M4 Quotation

The most common problem with existing macros is an improper quotation. This section, which users of Autoconf can skip, but which macro writers must read, first justifies the quotation scheme that was chosen for Autoconf and then ends with a rule of thumb. Understanding the former helps one to follow the latter.


8.1.1 Active Characters

To fully understand where proper quotation is important, you first need to know what the special characters are in Autoconf: ‘#’ introduces a comment inside which no macro expansion is performed, ‘,’ separates arguments, ‘[’ and ‘]’ are the quotes themselves3, ‘(’ and ‘)’ (which M4 tries to match by pairs), and finally ‘$’ inside a macro definition.

In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are “obvious-ified”, but when you find them in real life, they are usually in disguise.

Comments, introduced by a hash and running up to the newline, are opaque tokens to the top level: active characters are turned off, and there is no macro expansion:

# define([def], ine)
⇒# define([def], ine)

Each time there can be a macro expansion, there is a quotation expansion, i.e., one level of quotes is stripped:

int tab[10];
⇒int tab10;
[int tab[10];]
⇒int tab[10];

Without this in mind, the reader might try hopelessly to use her macro array:

define([array], [int tab[10];])
array
⇒int tab10;
[array]
⇒array

How can you correctly output the intended results4?


8.1.2 One Macro Call

Let’s proceed on the interaction between active characters and macros with this small macro, which just returns its first argument:

define([car], [$1])

The two pairs of quotes above are not part of the arguments of define; rather, they are understood by the top level when it tries to find the arguments of define. Therefore, assuming car is not already defined, it is equivalent to write:

define(car, $1)

But, while it is acceptable for a configure.ac to avoid unnecessary quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style.

At the top level, there are only two possibilities: either you quote or you don’t:

car(foo, bar, baz)
⇒foo
[car(foo, bar, baz)]
⇒car(foo, bar, baz)

Let’s pay attention to the special characters:

car(#)
error→EOF in argument list

The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way:

car([#)]

Proper quotation, of course, fixes the problem:

car([#])
⇒#

Here are more examples:

car(foo, bar)
⇒foo
car([foo, bar])
⇒foo, bar
car((foo, bar))
⇒(foo, bar)
car([(foo], [bar)])
⇒(foo
define([a], [b])
⇒
car(a)
⇒b
car([a])
⇒b
car([[a]])
⇒a
car([[[a]]])
⇒[a]

8.1.3 Quoting and Parameters

When M4 encounters ‘$’ within a macro definition, followed immediately by a character it recognizes (‘0’…‘9’, ‘#’, ‘@’, or ‘*’), it will perform M4 parameter expansion. This happens regardless of how many layers of quotes the parameter expansion is nested within, or even if it occurs in text that will be rescanned as a comment.

define([none], [$1])
⇒
define([one], [[$1]])
⇒
define([two], [[[$1]]])
⇒
define([comment], [# $1])
⇒
define([active], [ACTIVE])
⇒
none([active])
⇒ACTIVE
one([active])
⇒active
two([active])
⇒[active]
comment([active])
⇒# active

On the other hand, since autoconf generates shell code, you often want to output shell variable expansion, rather than performing M4 parameter expansion. To do this, you must use M4 quoting to separate the ‘$’ from the next character in the definition of your macro. If the macro definition occurs in single-quoted text, then insert another level of quoting; if the usage is already inside a double-quoted string, then split it into concatenated strings.

define([foo], [a single-quoted $[]1 definition])
⇒
define([bar], [[a double-quoted $][1 definition]])
⇒
foo
⇒a single-quoted $1 definition
bar
⇒a double-quoted $1 definition

Posix states that M4 implementations are free to provide implementation extensions when ‘${’ is encountered in a macro definition. Autoconf reserves the longer sequence ‘${{’ for use with planned extensions that will be available in the future GNU M4 2.0, but guarantees that all other instances of ‘${’ will be output literally. Therefore, this idiom can also be used to output shell code parameter references:

define([first], [${1}])first
⇒${1}

Posix also states that ‘$11’ should expand to the first parameter concatenated with a literal ‘1’, although some versions of GNU M4 expand the eleventh parameter instead. For portability, you should only use single-digit M4 parameter expansion.

With this in mind, we can explore the cases where macros invoke macros...


8.1.4 Quotation and Nested Macros

The examples below use the following macros:

define([car], [$1])
define([active], [ACT, IVE])
define([array], [int tab[10]])

Each additional embedded macro call introduces other possible interesting quotations:

car(active)
⇒ACT
car([active])
⇒ACT, IVE
car([[active]])
⇒active

In the first case, the top level looks for the arguments of car, and finds ‘active’. Because M4 evaluates its arguments before applying the macro, ‘active’ is expanded, which results in:

car(ACT, IVE)
⇒ACT

In the second case, the top level gives ‘active’ as first and only argument of car, which results in:

active
⇒ACT, IVE

i.e., the argument is evaluated after the macro that invokes it. In the third case, car receives ‘[active]’, which results in:

[active]
⇒active

exactly as we already saw above.

The example above, applied to a more realistic example, gives:

car(int tab[10];)
⇒int tab10;
car([int tab[10];])
⇒int tab10;
car([[int tab[10];]])
⇒int tab[10];

Huh? The first case is easily understood, but why is the second wrong, and the third right? To understand that, you must know that after M4 expands a macro, the resulting text is immediately subjected to macro expansion and quote removal. This means that the quote removal occurs twice—first before the argument is passed to the car macro, and second after the car macro expands to the first argument.

As the author of the Autoconf macro car, you then consider it to be incorrect that your users have to double-quote the arguments of car, so you “fix” your macro. Let’s call it qar for quoted car:

define([qar], [[$1]])

and check that qar is properly fixed:

qar([int tab[10];])
⇒int tab[10];

Ahhh! That’s much better.

But note what you’ve done: now that the result of qar is always a literal string, the only time a user can use nested macros is if she relies on an unquoted macro call:

qar(active)
⇒ACT
qar([active])
⇒active

leaving no way for her to reproduce what she used to do with car:

car([active])
⇒ACT, IVE

Worse yet: she wants to use a macro that produces a set of cpp macros:

define([my_includes], [#include <stdio.h>])
car([my_includes])
⇒#include <stdio.h>
qar(my_includes)
error→EOF in argument list

This macro, qar, because it double quotes its arguments, forces its users to leave their macro calls unquoted, which is dangerous. Commas and other active symbols are interpreted by M4 before they are given to the macro, often not in the way the users expect. Also, because qar behaves differently from the other macros, it’s an exception that should be avoided in Autoconf.


8.1.5 changequote is Evil

The temptation is often high to bypass proper quotation, in particular when it’s late at night. Then, many experienced Autoconf hackers finally surrender to the dark side of the force and use the ultimate weapon: changequote.

The M4 builtin changequote belongs to a set of primitives that allow one to adjust the syntax of the language to adjust it to one’s needs. For instance, by default M4 uses ‘`’ and ‘'’ as quotes, but in the context of shell programming (and actually of most programming languages), that’s about the worst choice one can make: because of strings and back-quoted expressions in shell code (such as ‘'this'’ and ‘`that`’), and because of literal characters in usual programming languages (as in ‘'0'’), there are many unbalanced ‘`’ and ‘'’. Proper M4 quotation then becomes a nightmare, if not impossible. In order to make M4 useful in such a context, its designers have equipped it with changequote, which makes it possible to choose another pair of quotes. M4sugar, M4sh, Autoconf, and Autotest all have chosen to use ‘[’ and ‘]’. Not especially because they are unlikely characters, but because they are characters unlikely to be unbalanced.

There are other magic primitives, such as changecom to specify what syntactic forms are comments (it is common to see ‘changecom(<!--, -->)’ when M4 is used to produce HTML pages), changeword and changesyntax to change other syntactic details (such as the character to denote the nth argument, ‘$’ by default, the parentheses around arguments, etc.).

These primitives are really meant to make M4 more useful for specific domains: they should be considered like command line options: --quotes, --comments, --words, and --syntax. Nevertheless, they are implemented as M4 builtins, as it makes M4 libraries self contained (no need for additional options).

There lies the problem...


The problem is that it is then tempting to use them in the middle of an M4 script, as opposed to its initialization. This, if not carefully thought out, can lead to disastrous effects: you are changing the language in the middle of the execution. Changing and restoring the syntax is often not enough: if you happened to invoke macros in between, these macros are lost, as the current syntax is probably not the one they were implemented with.


8.1.6 Quadrigraphs

When writing an Autoconf macro you may occasionally need to generate special characters that are difficult to express with the standard Autoconf quoting rules. For example, you may need to output the regular expression ‘[^[]’, which matches any character other than ‘[’. This expression contains unbalanced brackets so it cannot be put easily into an M4 macro.

Additionally, there are a few m4sugar macros (such as m4_split and m4_expand) which internally use special markers in addition to the regular quoting characters. If the arguments to these macros contain the literal strings ‘-=<{(’ or ‘)}>=-’, the macros might behave incorrectly.

You can work around these problems by using one of the following quadrigraphs:

@<:@

[

@:>@

]

@S|@

$

@%:@

#

@{:@

(

@:}@

)

@&t@

Expands to nothing.

Quadrigraphs are replaced at a late stage of the translation process, after m4 is run, so they do not get in the way of M4 quoting. For example, the string ‘^@<:@’, independently of its quotation, appears as ‘^[’ in the output.

The empty quadrigraph can be used:

  • - to mark trailing spaces explicitly

    Trailing spaces are smashed by autom4te. This is a feature.

  • - to produce quadrigraphs and other strings reserved by m4sugar

    For instance ‘@<@&t@:@’ produces ‘@<:@’. For a more contrived example:

    m4_define([a], [A])m4_define([b], [B])m4_define([c], [C])dnl
    m4_split([a )}>=- b -=<{( c])
    ⇒[a], [], [B], [], [c]
    m4_split([a )}@&t@>=- b -=<@&t@{( c])
    ⇒[a], [)}>=-], [b], [-=<{(], [c]
    
  • - to escape occurrences of forbidden patterns

    For instance you might want to mention AC_FOO in a comment, while still being sure that autom4te still catches unexpanded ‘AC_*’. Then write ‘AC@&t@_FOO’.

The name ‘@&t@’ was suggested by Paul Eggert:

I should give some credit to the ‘@&t@’ pun. The ‘&’ is my own invention, but the ‘t’ came from the source code of the ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), and which used ‘mt’ to denote the empty string. In C, it would have looked like something like:

char const mt[] = "";

but of course the source code was written in Algol 68.

I don’t know where he got ‘mt’ from: it could have been his own invention, and I suppose it could have been a common pun around the Cambridge University computer lab at the time.


8.1.7 Dealing with unbalanced parentheses

One of the pitfalls of portable shell programming is that if you intend your script to run with obsolescent shells, case statements require unbalanced parentheses. See Limitations of Shell Builtins. With syntax highlighting editors, the presence of unbalanced ‘)’ can interfere with editors that perform syntax highlighting of macro contents based on finding the matching ‘(’. Another concern is how much editing must be done when transferring code snippets between shell scripts and macro definitions. But most importantly, the presence of unbalanced parentheses can introduce expansion bugs.

For an example, here is an underquoted attempt to use the macro my_case, which happens to expand to a portable case statement:

AC_DEFUN([my_case],
[case $file_name in
  *.c) echo "C source code";;
esac])
AS_IF(:, my_case)

In the above example, the AS_IF call under-quotes its arguments. As a result, the unbalanced ‘)’ generated by the premature expansion of my_case results in expanding AS_IF with a truncated parameter, and the expansion is syntactically invalid:

if :; then
  case $file_name in
  *.c
fi echo "C source code";;
esac)

If nothing else, this should emphasize the importance of the quoting arguments to macro calls. On the other hand, there are several variations for defining my_case to be more robust, even when used without proper quoting, each with some benefits and some drawbacks.

  • Use left parenthesis before pattern
    AC_DEFUN([my_case],
    [case $file_name in
      (*.c) echo "C source code";;
    esac])
    

    This is simple and provides balanced parentheses. Although this is not portable to obsolescent shells (notably Solaris 10 /bin/sh), platforms with these shells invariably have a more-modern shell available somewhere so this approach typically suffices nowadays.

  • Creative literal shell comment
    AC_DEFUN([my_case],
    [case $file_name in #(
      *.c) echo "C source code";;
    esac])
    

    This version provides balanced parentheses to several editors, and can be copied and pasted into a terminal as is. Unfortunately, it is still unbalanced as an Autoconf argument, since ‘#(’ is an M4 comment that masks the normal properties of ‘(’.

  • Quadrigraph shell comment
    AC_DEFUN([my_case],
    [case $file_name in @%:@(
      *.c) echo "C source code";;
    esac])
    

    This version provides balanced parentheses to even more editors, and can be used as a balanced Autoconf argument. Unfortunately, it requires some editing before it can be copied and pasted into a terminal, and the use of the quadrigraph ‘@%:@’ for ‘#’ reduces readability.

  • Quoting just the parenthesis
    AC_DEFUN([my_case],
    [case $file_name in
      *.c[)] echo "C source code";;
    esac])
    

    This version quotes the ‘)’, so that it can be used as a balanced Autoconf argument. As written, this is not balanced to an editor, but it can be coupled with ‘[#(]’ to meet that need, too. However, it still requires some edits before it can be copied and pasted into a terminal.

  • Double-quoting the entire statement
    AC_DEFUN([my_case],
    [[case $file_name in #(
      *.c) echo "C source code";;
    esac]])
    

    Since the entire macro is double-quoted, there is no problem with using this as an Autoconf argument; and since the double-quoting is over the entire statement, this code can be easily copied and pasted into a terminal. However, the double quoting prevents the expansion of any macros inside the case statement, which may cause its own set of problems.

  • Using AS_CASE
    AC_DEFUN([my_case],
    [AS_CASE([$file_name],
      [*.c], [echo "C source code"])])
    

    This version avoids the balancing issue altogether, by relying on AS_CASE (see Common Shell Constructs); it also allows for the expansion of AC_REQUIRE to occur prior to the entire case statement, rather than within a branch of the case statement that might not be taken. However, the abstraction comes with a penalty that it is no longer a quick copy, paste, and edit to get back to shell code.


8.1.8 Quotation Rule Of Thumb

To conclude, the quotation rule of thumb is:

One pair of quotes per pair of parentheses.

Never over-quote, never under-quote, in particular in the definition of macros. In the few places where the macros need to use brackets (usually in C program text or regular expressions), properly quote the arguments!

It is common to read Autoconf programs with snippets like:

AC_TRY_LINK(
changequote(<<, >>)dnl
<<#include <time.h>
#ifndef tzname /* For SGI.  */
extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
#endif>>,
changequote([, ])dnl
[atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no)

which is incredibly useless since AC_TRY_LINK is already double quoting, so you just need:

AC_TRY_LINK(
[#include <time.h>
#ifndef tzname /* For SGI.  */
extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
#endif],
            [atoi (*tzname);],
            [ac_cv_var_tzname=yes],
            [ac_cv_var_tzname=no])

The M4-fluent reader might note that these two examples are rigorously equivalent, since M4 swallows both the ‘changequote(<<, >>)’ and ‘<<’ ‘>>’ when it collects the arguments: these quotes are not part of the arguments!

Simplified, the example above is just doing this:

changequote(<<, >>)dnl
<<[]>>
changequote([, ])dnl

instead of simply:

[[]]

With macros that do not double quote their arguments (which is the rule), double-quote the (risky) literals:

AC_LINK_IFELSE([AC_LANG_PROGRAM(
[[#include <time.h>
#ifndef tzname /* For SGI.  */
extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
#endif]],
                                [atoi (*tzname);])],
               [ac_cv_var_tzname=yes],
               [ac_cv_var_tzname=no])

Please note that the macro AC_TRY_LINK is obsolete, so you really should be using AC_LINK_IFELSE instead.

See Quadrigraphs, for what to do if you run into a hopeless case where quoting does not suffice.

When you create a configure script using newly written macros, examine it carefully to check whether you need to add more quotes in your macros. If one or more words have disappeared in the M4 output, you need more quotes. When in doubt, quote.

However, it’s also possible to put on too many layers of quotes. If this happens, the resulting configure script may contain unexpanded macros. The autoconf program checks for this problem by looking for the string ‘AC_’ in configure. However, this heuristic does not work in general: for example, it does not catch overquoting in AC_DEFINE descriptions.


8.2 Using autom4te

The Autoconf suite, including M4sugar, M4sh, and Autotest, in addition to Autoconf per se, heavily rely on M4. All these different uses revealed common needs factored into a layer over M4: autom4te5.

autom4te is a preprocessor that is like m4. It supports M4 extensions designed for use in tools like Autoconf.


8.2.1 Invoking autom4te

The command line arguments are modeled after M4’s:

autom4te options files

where the files are directly passed to m4. By default, GNU M4 is found during configuration, but the environment variable M4 can be set to tell autom4te where to look. In addition to the regular expansion, it handles the replacement of the quadrigraphs (see Quadrigraphs), and of ‘__oline__’, the current line in the output. It supports an extended syntax for the files:

file.m4f

This file is an M4 frozen file. Note that all the previous files are ignored. See the --melt option for the rationale.

file?

If found in the library path, the file is included for expansion, otherwise it is ignored instead of triggering a failure.


Of course, it supports the Autoconf common subset of options:

--help
-h

Print a summary of the command line options and exit.

--version
-V

Print the version number of Autoconf and exit.

--verbose
-v

Report processing steps.

--debug
-d

Don’t remove the temporary files and be even more verbose.

--include=dir
-I dir

Also look for input files in dir. Multiple invocations accumulate.

--output=file
-o file

Save output (script or trace) to file. The file - stands for the standard output.


As an extension of m4, it includes the following options:

--warnings=category[,category...]
-Wcategory[,category...]

Enable or disable warnings related to each category. See m4_warn, for a comprehensive list of categories. Special values include:

all

Enable all categories of warnings.

none

Disable all categories of warnings.

error

Treat all warnings as errors.

no-category

Disable warnings falling into category.

The enviroment variable WARNINGS may also be set to a comma-separated list of warning categories to enable or disable. It is interpreted exactly the same way as the argument of --warnings, but unknown categories are silently ignored. The command line takes precedence; for instance, if WARNINGS is set to obsolete, but -Wnone is given on the command line, no warnings will be issued.

Some categories of warnings are on by default. Again, for details see m4_warn.

--melt
-M

Do not use frozen files. Any argument file.m4f is replaced by file.m4. This helps tracing the macros which are executed only when the files are frozen, typically m4_define. For instance, running:

autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4

is roughly equivalent to running:

m4 1.m4 2.m4 3.m4 4.m4 input.m4

while

autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4

is equivalent to:

m4 --reload-state=4.m4f input.m4
--freeze
-F

Produce a frozen state file. autom4te freezing is stricter than M4’s: it must produce no warnings, and no output other than empty lines (a line with white space is not empty) and comments (starting with ‘#’). Unlike m4’s similarly-named option, this option takes no argument:

autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f

corresponds to

m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f
--mode=octal-mode
-m octal-mode

Set the mode of the non-traces output to octal-mode; by default ‘0666’.


As another additional feature over m4, autom4te caches its results. GNU M4 is able to produce a regular output and traces at the same time. Traces are heavily used in the GNU Build System: autoheader uses them to build config.h.in, autoreconf to determine what GNU Build System components are used, automake to “parse” configure.ac etc. To avoid recomputation, traces are cached while performing regular expansion, and conversely. This cache is (actually, the caches are) stored in the directory autom4te.cache. It can safely be removed at any moment (especially if for some reason autom4te considers it trashed).

--cache=directory
-C directory

Specify the name of the directory where the result should be cached. Passing an empty value disables caching. Be sure to pass a relative file name, as for the time being, global caches are not supported.

--no-cache

Don’t cache the results.

--force
-f

If a cache is used, consider it obsolete (but update it anyway).


Because traces are so important to the GNU Build System, autom4te provides high level tracing features as compared to M4, and helps exploiting the cache:

--trace=macro[:format]
-t macro[:format]

Trace the invocations of macro according to the format. Multiple --trace arguments can be used to list several macros. Multiple --trace arguments for a single macro are not cumulative; instead, you should just make format as long as needed.

The format is a regular string, with newlines if desired, and several special escape codes. It defaults to ‘$f:$l:$n:$%’. It can use the following special escapes:

$$

The character ‘$’.

$f

The file name from which macro is called.

$l

The line number from which macro is called.

$d

The depth of the macro call. This is an M4 technical detail that you probably don’t want to know about.

$n

The name of the macro.

$num

The numth argument of the call to macro.

$@
$sep@
${separator}@

All the arguments passed to macro, separated by the character sep or the string separator (‘,’ by default). Each argument is quoted, i.e., enclosed in a pair of square brackets.

$*
$sep*
${separator}*

As above, but the arguments are not quoted.

$%
$sep%
${separator}%

As above, but the arguments are not quoted, all new line characters in the arguments are smashed, and the default separator is ‘:’.

The escape ‘$%’ produces single-line trace outputs (unless you put newlines in the ‘separator’), while ‘$@’ and ‘$*’ do not.

See Using autoconf to Create configure, for examples of trace uses.

--preselect=macro
-p macro

Cache the traces of macro, but do not enable traces. This is especially important to save CPU cycles in the future. For instance, when invoked, autoconf pre-selects all the macros that autoheader, automake, autoreconf, etc., trace, so that running m4 is not needed to trace them: the cache suffices. This results in a huge speed-up.


Finally, autom4te introduces the concept of Autom4te libraries. They consists in a powerful yet extremely simple feature: sets of combined command line arguments:

--language=language
-l language

Use the language Autom4te library. Current languages include:

M4sugar

create M4sugar output.

M4sh

create M4sh executable shell scripts.

Autotest

create Autotest executable test suites.

Autoconf-without-aclocal-m4

create Autoconf executable configure scripts without reading aclocal.m4.

Autoconf

create Autoconf executable configure scripts. This language inherits all the characteristics of Autoconf-without-aclocal-m4 and additionally reads aclocal.m4.

--prepend-include=dir
-B dir

Prepend directory dir to the search path. This is used to include the language-specific files before any third-party macros.

As an example, if Autoconf is installed in its default location, /usr/local, the command ‘autom4te -l m4sugar foo.m4’ is strictly equivalent to the command:

autom4te --prepend-include /usr/local/share/autoconf \
  m4sugar/m4sugar.m4f foo.m4

Recursive expansion applies here: the command ‘autom4te -l m4sh foo.m4’ is the same as ‘autom4te --language M4sugar m4sugar/m4sh.m4f foo.m4’, i.e.:

autom4te --prepend-include /usr/local/share/autoconf \
  m4sugar/m4sugar.m4f m4sugar/m4sh.m4f --mode 777 foo.m4

The definition of the languages is stored in autom4te.cfg.


8.2.2 Customizing autom4te

One can customize autom4te via ~/.autom4te.cfg (i.e., as found in the user home directory), and ./.autom4te.cfg (i.e., as found in the directory from which autom4te is run). The order is first reading autom4te.cfg, then ~/.autom4te.cfg, then ./.autom4te.cfg, and finally the command line arguments.

In these text files, comments are introduced with #, and empty lines are ignored. Customization is performed on a per-language basis, wrapped in between a ‘begin-language: "language"’, ‘end-language: "language"’ pair.

Customizing a language stands for appending options (see Invoking autom4te) to the current definition of the language. Options, and more generally arguments, are introduced by ‘args: arguments’. You may use the traditional shell syntax to quote the arguments.

As an example, to disable Autoconf caches (autom4te.cache) globally, include the following lines in ~/.autom4te.cfg:

## ------------------ ##
## User Preferences.  ##
## ------------------ ##

begin-language: "Autoconf-without-aclocal-m4"
args: --no-cache
end-language: "Autoconf-without-aclocal-m4"

8.3 Programming in M4sugar

M4 by itself provides only a small, but sufficient, set of all-purpose macros. M4sugar introduces additional generic macros. Its name was coined by Lars J. Aas: “Readability And Greater Understanding Stands 4 M4sugar”.

M4sugar reserves the macro namespace ‘^_m4_’ for internal use, and the macro namespace ‘^m4_’ for M4sugar macros. You should not define your own macros into these namespaces.


8.3.1 Redefined M4 Macros

With a few exceptions, all the M4 native macros are moved in the ‘m4_’ pseudo-namespace, e.g., M4sugar renames define as m4_define etc.

The list of macros unchanged from M4, except for their name, is:

  • - m4_builtin
  • - m4_changecom
  • - m4_changequote
  • - m4_debugfile
  • - m4_debugmode
  • - m4_decr
  • - m4_define
  • - m4_divnum
  • - m4_errprint
  • - m4_esyscmd
  • - m4_eval
  • - m4_format
  • - m4_ifdef
  • - m4_incr
  • - m4_index
  • - m4_indir
  • - m4_len
  • - m4_pushdef
  • - m4_shift
  • - m4_substr
  • - m4_syscmd
  • - m4_sysval
  • - m4_traceoff
  • - m4_traceon
  • - m4_translit

Some M4 macros are redefined, and are slightly incompatible with their native equivalent.

Macro: __file__
Macro: __line__

All M4 macros starting with ‘__’ retain their original name: for example, no m4__file__ is defined.

Macro: __oline__

This is not technically a macro, but a feature of Autom4te. The sequence __oline__ can be used similarly to the other m4sugar location macros, but rather than expanding to the location of the input file, it is translated to the line number where it appears in the output file after all other M4 expansions.

Macro: dnl

This macro kept its original name: no m4_dnl is defined.

Macro: m4_bpatsubst (string, regexp, [replacement])

This macro corresponds to patsubst. The name m4_patsubst is kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax.

Macro: m4_bregexp (string, regexp, [replacement])

This macro corresponds to regexp. The name m4_regexp is kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax.

Macro: m4_copy (source, dest)
Macro: m4_copy_force (source, dest)
Macro: m4_rename (source, dest)
Macro: m4_rename_force (source, dest)

These macros aren’t directly builtins, but are closely related to m4_pushdef and m4_defn. m4_copy and m4_rename ensure that dest is undefined, while m4_copy_force and m4_rename_force overwrite any existing definition. All four macros then proceed to copy the entire pushdef stack of definitions of source over to dest. m4_copy and m4_copy_force preserve the source (including in the special case where source is undefined), while m4_rename and m4_rename_force undefine the original macro name (making it an error to rename an undefined source).

Note that attempting to invoke a renamed macro might not work, since the macro may have a dependence on helper macros accessed via composition of ‘$0’ but that were not also renamed; likewise, other macros may have a hard-coded dependence on source and could break if source has been deleted. On the other hand, it is always safe to rename a macro to temporarily move it out of the way, then rename it back later to restore original semantics.

Macro: m4_defn (macro…)

This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See m4_undefine. Unfortunately, in order to support these older versions of M4, there are some situations involving unbalanced quotes where concatenating multiple macros together will work in newer M4 but not in m4sugar; use quadrigraphs to work around this.

Macro: m4_divert (diversion)

M4sugar relies heavily on diversions, so rather than behaving as a primitive, m4_divert behaves like:

m4_divert_pop()m4_divert_push([diversion])

See Diversion support, for more details about the use of the diversion stack. In particular, this implies that diversion should be a named diversion rather than a raw number. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.

Macro: m4_dumpdef (name…)
Macro: m4_dumpdefs (name…)

m4_dumpdef is like the M4 builtin, except that this version requires at least one argument, output always goes to standard error rather than the current debug file, no sorting is done on multiple arguments, and an error is issued if any name is undefined. m4_dumpdefs is a convenience macro that calls m4_dumpdef for all of the m4_pushdef stack of definitions, starting with the current, and silently does nothing if name is undefined.

Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a builtin is output as the empty string. This behavior is rectified by using M4 1.6 or newer. However, this behavior difference means that m4_dumpdef should only be used while developing m4sugar macros, and never in the final published form of a macro.

Macro: m4_esyscmd_s (command)

Like m4_esyscmd, this macro expands to the result of running command in a shell. The difference is that any trailing newlines are removed, so that the output behaves more like shell command substitution.

Macro: m4_exit (exit-status)

This macro corresponds to m4exit.

Macro: m4_if (comment)
Macro: m4_if (string-1, string-2, equal, [not-equal])
Macro: m4_if (string-1, string-2, equal-1, string-3, string-4, equal-2, …, [not-equal])

This macro corresponds to ifelse. string-1 and string-2 are compared literally, so usually one of the two arguments is passed unquoted. See Conditional constructs, for more conditional idioms.

Macro: m4_include (file)
Macro: m4_sinclude (file)

Like the M4 builtins, but warn against multiple inclusions of file.

Macro: m4_mkstemp (template)
Macro: m4_maketemp (template)

Posix requires maketemp to replace the trailing ‘X’ characters in template with the process id, without regards to the existence of a file by that name, but this a security hole. When this was pointed out to the Posix folks, they agreed to invent a new macro mkstemp that always creates a uniquely named file, but not all versions of GNU M4 support the new macro. In M4sugar, m4_maketemp and m4_mkstemp are synonyms for each other, and both have the secure semantics regardless of which macro the underlying M4 provides.

Macro: m4_popdef (macro…)

This macro fails if macro is not defined, even when using older versions of M4 that did not warn. See m4_undefine.

Macro: m4_undefine (macro…)

This macro fails if macro is not defined, even when using older versions of M4 that did not warn. Use

m4_ifdef([macro], [m4_undefine([macro])])

if you are not sure whether macro is defined.

Macro: m4_undivert (diversion…)

Unlike the M4 builtin, at least one diversion must be specified. Also, since the M4sugar diversion stack prefers named diversions, the use of m4_undivert to include files is risky. See Diversion support, for more details about the use of the diversion stack. But be aware that it is seldom necessary to explicitly change the diversion stack, and that when done incorrectly, it can lead to syntactically invalid scripts.

Macro: m4_wrap (text)
Macro: m4_wrap_lifo (text)

These macros correspond to m4wrap. Posix requires arguments of multiple wrap calls to be reprocessed at EOF in the same order as the original calls (first-in, first-out). GNU M4 versions through 1.4.10, however, reprocess them in reverse order (last-in, first-out). Both orders are useful, therefore, you can rely on m4_wrap to provide FIFO semantics and m4_wrap_lifo for LIFO semantics, regardless of the underlying GNU M4 version.

Unlike the GNU M4 builtin, these macros only recognize one argument, and avoid token pasting between consecutive invocations. On the other hand, nested calls to m4_wrap from within wrapped text work just as in the builtin.


8.3.2 Diagnostic messages from M4sugar

When macros statically diagnose abnormal situations, benign or fatal, they should report them using these macros. For issuing dynamic issues, i.e., when configure is run, see Printing Messages.

Macro: m4_assert (expression, [exit-status = ‘1])

Assert that the arithmetic expression evaluates to non-zero. Otherwise, issue a fatal error, and exit autom4te with exit-status.

Macro: m4_errprintn (message)

Similar to the builtin m4_errprint, except that a newline is guaranteed after message.

Macro: m4_fatal (message)

Report a severe error message prefixed with the current location, and have autom4te die.

Macro: m4_location

Useful as a prefix in a message line. Short for:

__file__:__line__
Macro: m4_warn (category, message)

Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. If the message is emitted, it is prefixed with the current location, and followed by a call trace of all macros defined via AC_DEFUN used to get to the current expansion.

The category must be one of:

cross

Warnings about constructs that may interfere with cross-compilation, such as using AC_RUN_IFELSE without a default.

gnu

Warnings related to the GNU Coding Standards (see The GNU Coding Standards). On by default.

obsolete

Warnings about obsolete features. On by default.

override

Warnings about redefinitions of Autoconf internals.

portability

Warnings about non-portable constructs.

portability-recursive

Warnings about recursive Make variable expansions ($(foo$(x))).

extra-portability

Extra warnings about non-portable constructs, covering rarely-used tools.

syntax

Warnings about questionable syntactic constructs, incorrectly ordered macro calls, typos, etc. On by default.

unsupported

Warnings about unsupported features. On by default.

Hacking Note: The set of categories is defined by code in autom4te, not by M4sugar itself. Additions should be coordinated with Automake, so that both sets of tools accept the same options.


8.3.3 Diversion support

M4sugar makes heavy use of diversions under the hood, because it is often the case that text that must appear early in the output is not discovered until late in the input. Additionally, some of the topological sorting algorithms used in resolving macro dependencies use diversions. However, most macros should not need to change diversions directly, but rather rely on higher-level M4sugar macros to manage diversions transparently. If you change diversions improperly, you risk generating a syntactically invalid script, because an incorrect diversion will violate assumptions made by many macros about whether prerequisite text has been previously output. In short, if you manually change the diversion, you should not expect any macros provided by the Autoconf package to work until you have restored the diversion stack back to its original state.

In the rare case that it is necessary to write a macro that explicitly outputs text to a different diversion, it is important to be aware of an M4 limitation regarding diversions: text only goes to a diversion if it is not part of argument collection. Therefore, any macro that changes the current diversion cannot be used as an unquoted argument to another macro, but must be expanded at the top level. The macro m4_expand will diagnose any attempt to change diversions, since it is generally useful only as an argument to another macro. The following example shows what happens when diversion manipulation is attempted within macro arguments:

m4_do([normal text]
m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL])
[m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl
⇒normal text
⇒unwanted

Notice that the unquoted text unwanted is output, even though it was processed while the current diversion was KILL, because it was collected as part of the argument to m4_do. However, the text discarded disappeared as desired, because the diversion changes were single-quoted, and were not expanded until the top-level rescan of the output of m4_do.

To make diversion management easier, M4sugar uses the concept of named diversions. Rather than using diversion numbers directly, it is nicer to associate a name with each diversion. The diversion number associated with a particular diversion name is an implementation detail, and a syntax warning is issued if a diversion number is used instead of a name. In general, you should not output text to a named diversion until after calling the appropriate initialization routine for your language (m4_init, AS_INIT, AT_INIT, …), although there are some exceptions documented below.

M4sugar defines two named diversions.

KILL

Text written to this diversion is discarded. This is the default diversion once M4sugar is initialized.

GROW

This diversion is used behind the scenes by topological sorting macros, such as AC_REQUIRE.

M4sh adds several more named diversions.

BINSH

This diversion is reserved for the ‘#!’ interpreter line.

HEADER-REVISION

This diversion holds text from AC_REVISION.

HEADER-COMMENT

This diversion holds comments about the purpose of a file.

HEADER-COPYRIGHT

This diversion is managed by AC_COPYRIGHT.

M4SH-SANITIZE

This diversion contains M4sh sanitization code, used to ensure M4sh is executing in a reasonable shell environment.

M4SH-INIT

This diversion contains M4sh initialization code, initializing variables that are required by other M4sh macros.

BODY

This diversion contains the body of the shell code, and is the default diversion once M4sh is initialized.

Autotest inherits diversions from M4sh, and changes the default diversion from BODY back to KILL. It also adds several more named diversions, with the following subset designed for developer use.

PREPARE_TESTS

This diversion contains initialization sequences which are executed after atconfig and atlocal, and after all command line arguments have been parsed, but prior to running any tests. It can be used to set up state that is required across all tests. This diversion will work even before AT_INIT.

Autoconf inherits diversions from M4sh, and adds the following named diversions which developers can utilize.

DEFAULTS

This diversion contains shell variable assignments to set defaults that must be in place before arguments are parsed. This diversion is placed early enough in configure that it is unsafe to expand any autoconf macros into this diversion.

HELP_ENABLE

If AC_PRESERVE_HELP_ORDER was used, then text placed in this diversion will be included as part of a quoted here-doc providing all of the --help output of configure related to options created by AC_ARG_WITH and AC_ARG_ENABLE.

INIT_PREPARE

This diversion occurs after all command line options have been parsed, but prior to the main body of the configure script. This diversion is the last chance to insert shell code such as variable assignments or shell function declarations that will used by the expansion of other macros.

For now, the remaining named diversions of Autoconf, Autoheader, and Autotest are not documented. In other words, intentionally outputting text into an undocumented diversion is subject to breakage in a future release of Autoconf.

Macro: m4_cleardivert (diversion…)

Permanently discard any text that has been diverted into diversion.

Macro: m4_divert_once (diversion, [content])

Similar to m4_divert_text, except that content is only output to diversion if this is the first time that m4_divert_once has been called with its particular arguments.

Macro: m4_divert_pop ([diversion])

If provided, check that the current diversion is indeed diversion. Then change to the diversion located earlier on the stack, giving an error if an attempt is made to pop beyond the initial m4sugar diversion of KILL.

Macro: m4_divert_push (diversion)

Remember the former diversion on the diversion stack, and output subsequent text into diversion. M4sugar maintains a diversion stack, and issues an error if there is not a matching pop for every push.

Macro: m4_divert_text (diversion, [content])

Output content and a newline into diversion, without affecting the current diversion. Shorthand for:

m4_divert_push([diversion])content
m4_divert_pop([diversion])dnl

One use of m4_divert_text is to develop two related macros, where macro ‘MY_A’ does the work, but adjusts what work is performed based on whether the optional macro ‘MY_B’ has also been expanded. Of course, it is possible to use AC_BEFORE within MY_A to require that ‘MY_B’ occurs first, if it occurs at all. But this imposes an ordering restriction on the user; it would be nicer if macros ‘MY_A’ and ‘MY_B’ can be invoked in either order. The trick is to let ‘MY_B’ leave a breadcrumb in an early diversion, which ‘MY_A’ can then use to determine whether ‘MY_B’ has been expanded.

AC_DEFUN([MY_A],
[# various actions
if test -n "$b_was_used"; then
  # extra action
fi])
AC_DEFUN([MY_B],
[AC_REQUIRE([MY_A])dnl
m4_divert_text([INIT_PREPARE], [b_was_used=true])])
Macro: m4_init

Initialize the M4sugar environment, setting up the default named diversion to be KILL.


8.3.4 Conditional constructs

The following macros provide additional conditional constructs as convenience wrappers around m4_if.

Macro: m4_bmatch (string, regex-1, value-1, [regex-2], [value-2], …, [default])

The string string is repeatedly compared against a series of regex arguments; if a match is found, the expansion is the corresponding value, otherwise, the macro moves on to the next regex. If no regex match, then the result is the optional default, or nothing.

Macro: m4_bpatsubsts (string, regex-1, subst-1, [regex-2], [subst-2], …)

The string string is altered by regex-1 and subst-1, as if by:

m4_bpatsubst([[string]], [regex], [subst])

The result of the substitution is then passed through the next set of regex and subst, and so forth. An empty subst implies deletion of any matched portions in the current string. Note that this macro over-quotes string; this behavior is intentional, so that the result of each step of the recursion remains as a quoted string. However, it means that anchors (‘^’ and ‘$’ in the regex will line up with the extra quotations, and not the characters of the original string. The overquoting is removed after the final substitution.

Macro: m4_case (string, value-1, if-value-1, [value-2], [if-value-2], …, [default])

Test string against multiple value possibilities, resulting in the first if-value for a match, or in the optional default. This is shorthand for:

m4_if([string], [value-1], [if-value-1],
      [string], [value-2], [if-value-2], …,
      [default])
Macro: m4_cond (test-1, value-1, if-value-1, [test-2], [value-2], [if-value-2], …, [default])

This macro was introduced in Autoconf 2.62. Similar to m4_if, except that each test is expanded only when it is encountered. This is useful for short-circuiting expensive tests; while m4_if requires all its strings to be expanded up front before doing comparisons, m4_cond only expands a test when all earlier tests have failed.

For an example, these two sequences give the same result, but in the case where ‘$1’ does not contain a backslash, the m4_cond version only expands m4_index once, instead of five times, for faster computation if this is a common case for ‘$1’. Notice that every third argument is unquoted for m4_if, and quoted for m4_cond:

m4_if(m4_index([$1], [\]), [-1], [$2],
      m4_eval(m4_index([$1], [\\]) >= 0), [1], [$2],
      m4_eval(m4_index([$1], [\$]) >= 0), [1], [$2],
      m4_eval(m4_index([$1], [\`]) >= 0), [1], [$3],
      m4_eval(m4_index([$1], [\"]) >= 0), [1], [$3],
      [$2])
m4_cond([m4_index([$1], [\])], [-1], [$2],
        [m4_eval(m4_index([$1], [\\]) >= 0)], [1], [$2],
        [m4_eval(m4_index([$1], [\$]) >= 0)], [1], [$2],
        [m4_eval(m4_index([$1], [\`]) >= 0)], [1], [$3],
        [m4_eval(m4_index([$1], [\"]) >= 0)], [1], [$3],
        [$2])
Macro: m4_default (expr-1, expr-2)
Macro: m4_default_quoted (expr-1, expr-2)
Macro: m4_default_nblank (expr-1, [expr-2])
Macro: m4_default_nblank_quoted (expr-1, [expr-2])

If expr-1 contains text, use it. Otherwise, select expr-2. m4_default expands the result, while m4_default_quoted does not. Useful for providing a fixed default if the expression that results in expr-1 would otherwise be empty. The difference between m4_default and m4_default_nblank is whether an argument consisting of just blanks (space, tab, newline) is significant. When using the expanding versions, note that an argument may contain text but still expand to an empty string.

m4_define([active], [ACTIVE])dnl
m4_define([empty], [])dnl
m4_define([demo1], [m4_default([$1], [$2])])dnl
m4_define([demo2], [m4_default_quoted([$1], [$2])])dnl
m4_define([demo3], [m4_default_nblank([$1], [$2])])dnl
m4_define([demo4], [m4_default_nblank_quoted([$1], [$2])])dnl
demo1([active], [default])
⇒ACTIVE
demo1([], [active])
⇒ACTIVE
demo1([empty], [text])
⇒
-demo1([ ], [active])-
⇒- -
demo2([active], [default])
⇒active
demo2([], [active])
⇒active
demo2([empty], [text])
⇒empty
-demo2([ ], [active])-
⇒- -
demo3([active], [default])
⇒ACTIVE
demo3([], [active])
⇒ACTIVE
demo3([empty], [text])
⇒
-demo3([ ], [active])-
⇒-ACTIVE-
demo4([active], [default])
⇒active
demo4([], [active])
⇒active
demo4([empty], [text])
⇒empty
-demo4([ ], [active])-
⇒-active-
Macro: m4_define_default (macro, [default-definition])

If macro does not already have a definition, then define it to default-definition.

Macro: m4_ifblank (cond, [if-blank], [if-text])
Macro: m4_ifnblank (cond, [if-text], [if-blank])

If cond is empty or consists only of blanks (space, tab, newline), then expand if-blank; otherwise, expand if-text. Two variants exist, in order to make it easier to select the correct logical sense when using only two parameters. Note that this is more efficient than the equivalent behavior of:

m4_ifval(m4_normalize([cond]), if-text, if-blank)
Macro: m4_ifndef (macro, if-not-defined, [if-defined])

This is shorthand for:

m4_ifdef([macro], [if-defined], [if-not-defined])
Macro: m4_ifset (macro, [if-true], [if-false])

If macro is undefined, or is defined as the empty string, expand to if-false. Otherwise, expands to if-true. Similar to:

m4_ifval(m4_defn([macro]), [if-true], [if-false])

except that it is not an error if macro is undefined.

Macro: m4_ifval (cond, [if-true], [if-false])

Expands to if-true if cond is not empty, otherwise to if-false. This is shorthand for:

m4_if([cond], [], [if-false], [if-true])
Macro: m4_ifvaln (cond, [if-true], [if-false])

Similar to m4_ifval, except guarantee that a newline is present after any non-empty expansion. Often followed by dnl.

Macro: m4_n (text)

Expand to text, and add a newline if text is not empty. Often followed by dnl.


8.3.5 Looping constructs

The following macros are useful in implementing recursive algorithms in M4, including loop operations. An M4 list is formed by quoting a list of quoted elements; generally the lists are comma-separated, although m4_foreach_w is whitespace-separated. For example, the list ‘[[a], [b,c]]’ contains two elements: ‘[a]’ and ‘[b,c]’. It is common to see lists with unquoted elements when those elements are not likely to be macro names, as in ‘[fputc_unlocked, fgetc_unlocked]’.

Although not generally recommended, it is possible for quoted lists to have side effects; all side effects are expanded only once, and prior to visiting any list element. On the other hand, the fact that unquoted macros are expanded exactly once means that macros without side effects can be used to generate lists. For example,

m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i])
error→hi
⇒123
m4_define([list], [[1], [2], [3]])
⇒
m4_foreach([i], [list], [i])
⇒123
Macro: m4_argn (n, [arg]…)

Extracts argument n (larger than 0) from the remaining arguments. If there are too few arguments, the empty string is used. For any n besides 1, this is more efficient than the similar ‘m4_car(m4_shiftn([n], [], [arg…]))’.

Macro: m4_car (arg…)

Expands to the quoted first arg. Can be used with m4_cdr to recursively iterate through a list. Generally, when using quoted lists of quoted elements, m4_car should be called without any extra quotes.

Macro: m4_cdr (arg…)

Expands to a quoted list of all but the first arg, or the empty string if there was only one argument. Generally, when using quoted lists of quoted elements, m4_cdr should be called without any extra quotes.

For example, this is a simple implementation of m4_map; note how each iteration checks for the end of recursion, then merely applies the first argument to the first element of the list, then repeats with the rest of the list. (The actual implementation in M4sugar is a bit more involved, to gain some speed and share code with m4_map_sep, and also to avoid expanding side effects in ‘$2’ twice).

m4_define([m4_map], [m4_ifval([$2],
  [m4_apply([$1], m4_car($2))[]$0([$1], m4_cdr($2))])])dnl
m4_map([ m4_eval], [[[1]], [[1+1]], [[10],[16]]])
⇒ 1 2 a
Macro: m4_for (var, first, last, [step], expression)

Loop over the numeric values between first and last including bounds by increments of step. For each iteration, expand expression with the numeric value assigned to var. If step is omitted, it defaults to ‘1’ or ‘-1’ depending on the order of the limits. If given, step has to match this order. The number of iterations is determined independently from definition of var; iteration cannot be short-circuited or lengthened by modifying var from within expression.

Macro: m4_foreach (var, list, expression)

Loop over the comma-separated M4 list list, assigning each value to var, and expand expression. The following example outputs two lines:

m4_foreach([myvar], [[foo], [bar, baz]],
           [echo myvar
])dnl
⇒echo foo
⇒echo bar, baz

Note that for some forms of expression, it may be faster to use m4_map_args.

Macro: m4_foreach_w (var, list, expression)

Loop over the white-space-separated list list, assigning each value to var, and expand expression. If var is only referenced once in expression, it is more efficient to use m4_map_args_w.

The deprecated macro AC_FOREACH is an alias of m4_foreach_w.

Macro: m4_map (macro, list)
Macro: m4_mapall (macro, list)
Macro: m4_map_sep (macro, separator, list)
Macro: m4_mapall_sep (macro, separator, list)

Loop over the comma separated quoted list of argument descriptions in list, and invoke macro with the arguments. An argument description is in turn a comma-separated quoted list of quoted elements, suitable for m4_apply. The macros m4_map and m4_map_sep ignore empty argument descriptions, while m4_mapall and m4_mapall_sep invoke macro with no arguments. The macros m4_map_sep and m4_mapall_sep additionally expand separator between invocations of macro.

Note that separator is expanded, unlike in m4_join. When separating output with commas, this means that the map result can be used as a series of arguments, by using a single-quoted comma as separator, or as a single string, by using a double-quoted comma.

m4_map([m4_count], [])
⇒
m4_map([ m4_count], [[],
                     [[1]],
                     [[1], [2]]])
⇒ 1 2
m4_mapall([ m4_count], [[],
                        [[1]],
                        [[1], [2]]])
⇒ 0 1 2
m4_map_sep([m4_eval], [,], [[[1+2]],
                            [[10], [16]]])
⇒3,a
m4_map_sep([m4_echo], [,], [[[a]], [[b]]])
⇒a,b
m4_count(m4_map_sep([m4_echo], [,], [[[a]], [[b]]]))
⇒2
m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]])
⇒a,b
m4_count(m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]]))
⇒1
Macro: m4_map_args (macro, arg…)

Repeatedly invoke macro with each successive arg as its only argument. In the following example, three solutions are presented with the same expansion; the solution using m4_map_args is the most efficient.

m4_define([active], [ACTIVE])dnl
m4_foreach([var], [[plain], [active]], [ m4_echo(m4_defn([var]))])
⇒ plain active
m4_map([ m4_echo], [[[plain]], [[active]]])
⇒ plain active
m4_map_args([ m4_echo], [plain], [active])
⇒ plain active

In cases where it is useful to operate on additional parameters besides the list elements, the macro m4_curry can be used in macro to supply the argument currying necessary to generate the desired argument list. In the following example, list_add_n is more efficient than list_add_x. On the other hand, using m4_map_args_sep can be even more efficient.

m4_define([list], [[1], [2], [3]])dnl
m4_define([add], [m4_eval(([$1]) + ([$2]))])dnl
dnl list_add_n(N, ARG...)
dnl Output a list consisting of each ARG added to N
m4_define([list_add_n],
[m4_shift(m4_map_args([,m4_curry([add], [$1])], m4_shift($@)))])dnl
list_add_n([1], list)
⇒2,3,4
list_add_n([2], list)
⇒3,4,5
m4_define([list_add_x],
[m4_shift(m4_foreach([var], m4_dquote(m4_shift($@)),
  [,add([$1],m4_defn([var]))]))])dnl
list_add_x([1], list)
⇒2,3,4
Macro: m4_map_args_pair (macro, [macro-end = macro]

arg…) For every pair of arguments arg, invoke macro with two arguments. If there is an odd number of arguments, invoke macro-end, which defaults to macro, with the remaining argument.

m4_map_args_pair([, m4_reverse], [], [1], [2], [3])
⇒, 2, 1, 3
m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3])
⇒, 2, 1, [3]
m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3], [4])
⇒, 2, 1, 4, 3
Macro: m4_map_args_sep ([pre], [post], [sep], arg…)

Expand the sequence pre[arg]post for each argument, additionally expanding sep between arguments. One common use of this macro is constructing a macro call, where the opening and closing parentheses are split between pre and post; in particular, m4_map_args([macro], [arg]) is equivalent to m4_map_args_sep([macro(], [)], [], [arg]). This macro provides the most efficient means for iterating over an arbitrary list of arguments, particularly when repeatedly constructing a macro call with more arguments than arg.

Macro: m4_map_args_w (string, [pre], [post], [sep])

Expand the sequence pre[word]post for each word in the whitespace-separated string, additionally expanding sep between words. This macro provides the most efficient means for iterating over a whitespace-separated string. In particular, m4_map_args_w([string], [action(], [)]) is more efficient than m4_foreach_w([var], [string], [action(m4_defn([var]))]).

Macro: m4_shiftn (count, …)
Macro: m4_shift2 (…)
Macro: m4_shift3 (…)

m4_shiftn performs count iterations of m4_shift, along with validation that enough arguments were passed in to match the shift count, and that the count is positive. m4_shift2 and m4_shift3 are specializations of m4_shiftn, introduced in Autoconf 2.62, and are more efficient for two and three shifts, respectively.

Macro: m4_stack_foreach (macro, action)
Macro: m4_stack_foreach_lifo (macro, action)

For each of the m4_pushdef definitions of macro, expand action with the single argument of a definition of macro. m4_stack_foreach starts with the oldest definition, while m4_stack_foreach_lifo starts with the current definition. action should not push or pop definitions of macro, nor is there any guarantee that the current definition of macro matches the argument that was passed to action. The macro m4_curry can be used if action needs more than one argument, although in that case it is more efficient to use m4_stack_foreach_sep.

Due to technical limitations, there are a few low-level m4sugar functions, such as m4_pushdef, that cannot be used as the macro argument.

m4_pushdef([a], [1])m4_pushdef([a], [2])dnl
m4_stack_foreach([a], [ m4_incr])
⇒ 2 3
m4_stack_foreach_lifo([a], [ m4_curry([m4_substr], [abcd])])
⇒ cd bcd
Macro: m4_stack_foreach_sep (macro, [pre], [post], [sep])
Macro: m4_stack_foreach_sep_lifo (macro, [pre], [post], [sep])

Expand the sequence pre[definition]post for each m4_pushdef definition of macro, additionally expanding sep between definitions. m4_stack_foreach_sep visits the oldest definition first, while m4_stack_foreach_sep_lifo visits the current definition first. This macro provides the most efficient means for iterating over a pushdef stack. In particular, m4_stack_foreach([macro], [action]) is short for m4_stack_foreach_sep([macro], [action(], [)]).


8.3.6 Evaluation Macros

The following macros give some control over the order of the evaluation by adding or removing levels of quotes.

Macro: m4_apply (macro, list)

Apply the elements of the quoted, comma-separated list as the arguments to macro. If list is empty, invoke macro without arguments. Note the difference between m4_indir, which expects its first argument to be a macro name but can use names that are otherwise invalid, and m4_apply, where macro can contain other text, but must end in a valid macro name.

m4_apply([m4_count], [])
⇒0
m4_apply([m4_count], [[]])
⇒1
m4_apply([m4_count], [[1], [2]])
⇒2
m4_apply([m4_join], [[|], [1], [2]])
⇒1|2
Macro: m4_count (arg, …)

This macro returns the decimal count of the number of arguments it was passed.

Macro: m4_curry (macro, arg…)

This macro performs argument currying. The expansion of this macro is another macro name that expects exactly one argument; that argument is then appended to the arg list, and then macro is expanded with the resulting argument list.

m4_curry([m4_curry], [m4_reverse], [1])([2])([3])
⇒3, 2, 1

Unfortunately, due to a limitation in M4 1.4.x, it is not possible to pass the definition of a builtin macro as the argument to the output of m4_curry; the empty string is used instead of the builtin token. This behavior is rectified by using M4 1.6 or newer.

Macro: m4_do (arg, …)

This macro loops over its arguments and expands each arg in sequence. Its main use is for readability; it allows the use of indentation and fewer dnl to result in the same expansion. This macro guarantees that no expansion will be concatenated with subsequent text; to achieve full concatenation, use m4_unquote(m4_join([], arg…)).

m4_define([ab],[1])m4_define([bc],[2])m4_define([abc],[3])dnl
m4_do([a],[b])c
⇒abc
m4_unquote(m4_join([],[a],[b]))c
⇒3
m4_define([a],[A])m4_define([b],[B])m4_define([c],[C])dnl
m4_define([AB],[4])m4_define([BC],[5])m4_define([ABC],[6])dnl
m4_do([a],[b])c
⇒ABC
m4_unquote(m4_join([],[a],[b]))c
⇒3
Macro: m4_dquote (arg, …)

Return the arguments as a quoted list of quoted arguments. Conveniently, if there is just one arg, this effectively adds a level of quoting.

Macro: m4_dquote_elt (arg, …)

Return the arguments as a series of double-quoted arguments. Whereas m4_dquote returns a single argument, m4_dquote_elt returns as many arguments as it was passed.

Macro: m4_echo (arg, …)

Return the arguments, with the same level of quoting. Other than discarding whitespace after unquoted commas, this macro is a no-op.

Macro: m4_expand (arg)

Return the expansion of arg as a quoted string. Whereas m4_quote is designed to collect expanded text into a single argument, m4_expand is designed to perform one level of expansion on quoted text. One distinction is in the treatment of whitespace following a comma in the original arg. Any time multiple arguments are collected into one with m4_quote, the M4 argument collection rules discard the whitespace. However, with m4_expand, whitespace is preserved, even after the expansion of macros contained in arg. Additionally, m4_expand is able to expand text that would involve an unterminated comment, whereas expanding that same text as the argument to m4_quote runs into difficulty in finding the end of the argument. Since manipulating diversions during argument collection is inherently unsafe, m4_expand issues an error if arg attempts to change the current diversion (see Diversion support).

m4_define([active], [ACT, IVE])dnl
m4_define([active2], [[ACT, IVE]])dnl
m4_quote(active, active)
⇒ACT,IVE,ACT,IVE
m4_expand([active, active])
⇒ACT, IVE, ACT, IVE
m4_quote(active2, active2)
⇒ACT, IVE,ACT, IVE
m4_expand([active2, active2])
⇒ACT, IVE, ACT, IVE
m4_expand([# m4_echo])
⇒# m4_echo
m4_quote(# m4_echo)
)
⇒# m4_echo)
⇒

Note that m4_expand cannot handle an arg that expands to literal unbalanced quotes, but that quadrigraphs can be used when unbalanced output is necessary. Likewise, unbalanced parentheses should be supplied with double quoting or a quadrigraph.

m4_define([pattern], [[!@<:@]])dnl
m4_define([bar], [BAR])dnl
m4_expand([case $foo in
  m4_defn([pattern])@:}@ bar ;;
  *[)] blah ;;
esac])
⇒case $foo in
⇒  [![]) BAR ;;
⇒  *) blah ;;
⇒esac
Macro: m4_ignore (…)

This macro was introduced in Autoconf 2.62. Expands to nothing, ignoring all of its arguments. By itself, this isn’t very useful. However, it can be used to conditionally ignore an arbitrary number of arguments, by deciding which macro name to apply to a list of arguments.

dnl foo outputs a message only if [debug] is defined.
m4_define([foo],
[m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])])

Note that for earlier versions of Autoconf, the macro __gnu__ can serve the same purpose, although it is less readable.

Macro: m4_make_list (arg, …)

This macro exists to aid debugging of M4sugar algorithms. Its net effect is similar to m4_dquote—it produces a quoted list of quoted arguments, for each arg. The difference is that this version uses a comma-newline separator instead of just comma, to improve readability of the list; with the result that it is less efficient than m4_dquote.

m4_define([zero],[0])m4_define([one],[1])m4_define([two],[2])dnl
m4_dquote(zero, [one], [[two]])
⇒[0],[one],[[two]]
m4_make_list(zero, [one], [[two]])
⇒[0],
⇒[one],
⇒[[two]]
m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number])
⇒ 0 1 two
m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number])
⇒ 0 1 two
Macro: m4_quote (arg, …)

Return the arguments as a single entity, i.e., wrap them into a pair of quotes. This effectively collapses multiple arguments into one, although it loses whitespace after unquoted commas in the process.

Macro: m4_reverse (arg, …)

Outputs each argument with the same level of quoting, but in reverse order, and with space following each comma for readability.

m4_define([active], [ACT,IVE])
⇒
m4_reverse(active, [active])
⇒active, IVE, ACT
Macro: m4_unquote (arg, …)

This macro was introduced in Autoconf 2.62. Expand each argument, separated by commas. For a single arg, this effectively removes a layer of quoting, and m4_unquote([arg]) is more efficient than the equivalent m4_do([arg]). For multiple arguments, this results in an unquoted list of expansions. This is commonly used with m4_split, in order to convert a single quoted list into a series of quoted elements.

The following example aims at emphasizing the difference between several scenarios: not using these macros, using m4_defn, using m4_quote, using m4_dquote, and using m4_expand.

$ cat example.m4
dnl Overquote, so that quotes are visible.
m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
m4_define([a], [A])
m4_define([mkargs], [1, 2[,] 3])
m4_define([arg1], [[$1]])
m4_divert([0])dnl
show(a, b)
show([a, b])
show(m4_quote(a, b))
show(m4_dquote(a, b))
show(m4_expand([a, b]))

arg1(mkargs)
arg1([mkargs])
arg1(m4_defn([mkargs]))
arg1(m4_quote(mkargs))
arg1(m4_dquote(mkargs))
arg1(m4_expand([mkargs]))
$ autom4te -l m4sugar example.m4
$1 = A, $@ = [A],[b]
$1 = a, b, $@ = [a, b]
$1 = A,b, $@ = [A,b]
$1 = [A],[b], $@ = [[A],[b]]
$1 = A, b, $@ = [A, b]

1
mkargs
1, 2[,] 3
1,2, 3
[1],[2, 3]
1, 2, 3

8.3.7 String manipulation in M4

The following macros may be used to manipulate strings in M4. Many of the macros in this section intentionally result in quoted strings as output, rather than subjecting the arguments to further expansions. As a result, if you are manipulating text that contains active M4 characters, the arguments are passed with single quoting rather than double.

Macro: m4_append (macro-name, string, [separator])
Macro: m4_append_uniq (macro-name, string, [separator] [if-uniq], [if-duplicate])

Redefine macro-name to its former contents with separator and string added at the end. If macro-name was undefined before (but not if it was defined but empty), then no separator is added. As of Autoconf 2.62, neither string nor separator are expanded during this macro; instead, they are expanded when macro-name is invoked.

m4_append can be used to grow strings, and m4_append_uniq to grow strings without duplicating substrings. Additionally, m4_append_uniq takes two optional parameters as of Autoconf 2.62; if-uniq is expanded if string was appended, and if-duplicate is expanded if string was already present. Also, m4_append_uniq warns if separator is not empty, but occurs within string, since that can lead to duplicates.

Note that m4_append can scale linearly in the length of the final string, depending on the quality of the underlying M4 implementation, while m4_append_uniq has an inherent quadratic scaling factor. If an algorithm can tolerate duplicates in the final string, use the former for speed. If duplicates must be avoided, consider using m4_set_add instead (see Set manipulation in M4).

m4_define([active], [ACTIVE])dnl
m4_append([sentence], [This is an])dnl
m4_append([sentence], [ active ])dnl
m4_append([sentence], [symbol.])dnl
sentence
⇒This is an ACTIVE symbol.
m4_undefine([active])dnl
⇒This is an active symbol.
m4_append_uniq([list], [one], [, ], [new], [existing])
⇒new
m4_append_uniq([list], [one], [, ], [new], [existing])
⇒existing
m4_append_uniq([list], [two], [, ], [new], [existing])
⇒new
m4_append_uniq([list], [three], [, ], [new], [existing])
⇒new
m4_append_uniq([list], [two], [, ], [new], [existing])
⇒existing
list
⇒one, two, three
m4_dquote(list)
⇒[one],[two],[three]
m4_append([list2], [one], [[, ]])dnl
m4_append_uniq([list2], [two], [[, ]])dnl
m4_append([list2], [three], [[, ]])dnl
list2
⇒one, two, three
m4_dquote(list2)
⇒[one, two, three]
Macro: m4_append_uniq_w (macro-name, strings)

This macro was introduced in Autoconf 2.62. It is similar to m4_append_uniq, but treats strings as a whitespace separated list of words to append, and only appends unique words. macro-name is updated with a single space between new words.

m4_append_uniq_w([numbers], [1 1 2])dnl
m4_append_uniq_w([numbers], [ 2 3 ])dnl
numbers
⇒1 2 3
Macro: m4_chomp (string)
Macro: m4_chomp_all (string)

Output string in quotes, but without a trailing newline. The macro m4_chomp is slightly faster, and removes at most one newline; the macro m4_chomp_all removes all consecutive trailing newlines. Unlike m4_flatten, embedded newlines are left intact, and backslash does not influence the result.

Macro: m4_combine ([separator], prefix-list, [infix], suffix-1, [suffix-2], …)

This macro produces a quoted string containing the pairwise combination of every element of the quoted, comma-separated prefix-list, and every element from the suffix arguments. Each pairwise combination is joined with infix in the middle, and successive pairs are joined by separator. No expansion occurs on any of the arguments. No output occurs if either the prefix or suffix list is empty, but the lists can contain empty elements.

m4_define([a], [oops])dnl
m4_combine([, ], [[a], [b], [c]], [-], [1], [2], [3])
⇒a-1, a-2, a-3, b-1, b-2, b-3, c-1, c-2, c-3
m4_combine([, ], [[a], [b]], [-])
⇒
m4_combine([, ], [[a], [b]], [-], [])
⇒a-, b-
m4_combine([, ], [], [-], [1], [2])
⇒
m4_combine([, ], [[]], [-], [1], [2])
⇒-1, -2
Macro: m4_escape (string)

Convert all instances of ‘[’, ‘]’, ‘#’, and ‘$’ within string into their respective quadrigraphs. The result is still a quoted string.

Macro: m4_flatten (string)

Flatten string into a single line. Delete all backslash-newline pairs, and replace all remaining newlines with a space. The result is still a quoted string.

Macro: m4_join ([separator], args…)
Macro: m4_joinall ([separator], args…)

Concatenate each arg, separated by separator. joinall uses every argument, while join omits empty arguments so that there are no back-to-back separators in the output. The result is a quoted string.

m4_define([active], [ACTIVE])dnl
m4_join([|], [one], [], [active], [two])
⇒one|active|two
m4_joinall([|], [one], [], [active], [two])
⇒one||active|two

Note that if all you intend to do is join args with commas between them, to form a quoted list suitable for m4_foreach, it is more efficient to use m4_dquote.

Macro: m4_newline ([text])

This macro was introduced in Autoconf 2.62, and expands to a newline, followed by any text. It is primarily useful for maintaining macro formatting, and ensuring that M4 does not discard leading whitespace during argument collection.

Macro: m4_normalize (string)

Remove leading and trailing spaces and tabs, sequences of backslash-then-newline, and replace multiple spaces, tabs, and newlines with a single space. This is a combination of m4_flatten and m4_strip. To determine if string consists only of bytes that would be removed by m4_normalize, you can use m4_ifblank.

Macro: m4_re_escape (string)

Backslash-escape all characters in string that are active in regexps.

Macro: m4_split (string, [regexp = ‘[\t ]+])

Split string into an M4 list of elements quoted by ‘[’ and ‘]’, while keeping white space at the beginning and at the end. If regexp is given, use it instead of ‘[\t ]+’ for splitting. If string is empty, the result is an empty list.

Macro: m4_strip (string)

Strip whitespace from string. Sequences of spaces and tabs are reduced to a single space, then leading and trailing spaces are removed. The result is still a quoted string. Note that this does not interfere with newlines; if you want newlines stripped as well, consider m4_flatten, or do it all at once with m4_normalize. To quickly test if string has only whitespace, use m4_ifblank.

Macro: m4_text_box (message, [frame = ‘-])

Add a text box around message, using frame as the border character above and below the message. The frame argument must be a single byte, and does not support quadrigraphs. The frame correctly accounts for the subsequent expansion of message. For example:

m4_define([macro], [abc])dnl
m4_text_box([macro])
⇒## --- ##
⇒## abc ##
⇒## --- ##

The message must contain balanced quotes and parentheses, although quadrigraphs can be used to work around this.

Macro: m4_text_wrap (string, [prefix], [prefix1 = prefix]

Break string into a series of whitespace-separated words, then output those words separated by spaces, and wrapping lines any time the output would exceed width columns. If given, prefix1 begins the first line, and prefix begins all wrapped lines. If prefix1 is longer than prefix, then the first line consists of just prefix1. If prefix is longer than prefix1, padding is inserted so that the first word of string begins at the same indentation as all wrapped lines. Note that using literal tab characters in any of the arguments will interfere with the calculation of width. No expansions occur on prefix, prefix1, or the words of string, although quadrigraphs are recognized.

For some examples:

m4_text_wrap([Short string */], [   ], [/* ], [20])
⇒/* Short string */
m4_text_wrap([Much longer string */], [   ], [/* ], [20])
⇒/* Much longer
⇒   string */
m4_text_wrap([Short doc.], [          ], [  --short ], [30])
⇒  --short Short doc.
m4_text_wrap([Short doc.], [          ], [  --too-wide ], [30])
⇒  --too-wide
⇒          Short doc.
m4_text_wrap([Super long documentation.], [     ],
             [  --too-wide ], 30)
⇒  --too-wide
⇒     Super long
⇒     documentation.
Macro: m4_tolower (string)
Macro: m4_toupper (string)

Return string with letters converted to upper or lower case, respectively.


8.3.8 Arithmetic computation in M4

The following macros facilitate integer arithmetic operations. Where a parameter is documented as taking an arithmetic expression, you can use anything that can be parsed by m4_eval.

Macro: m4_cmp (expr-1, expr-2)

Compare the arithmetic expressions expr-1 and expr-2, and expand to ‘-1’ if expr-1 is smaller, ‘0’ if they are equal, and ‘1’ if expr-1 is larger.

Macro: m4_list_cmp (list-1, list-2)

Compare the two M4 lists consisting of comma-separated arithmetic expressions, left to right. Expand to ‘-1’ for the first element pairing where the value from list-1 is smaller, ‘1’ where the value from list-2 is smaller, or ‘0’ if both lists have the same values. If one list is shorter than the other, the remaining elements of the longer list are compared against zero.

m4_list_cmp([1, 0],       [1])
⇒0
m4_list_cmp([1, [1 * 0]], [1, 0])
⇒0
m4_list_cmp([1, 2],       [1, 0])
⇒1
m4_list_cmp([1, [1+1], 3],[1, 2])
⇒1
m4_list_cmp([1, 2, -3],   [1, 2])
⇒-1
m4_list_cmp([1, 0],       [1, 2])
⇒-1
m4_list_cmp([1],          [1, 2])
⇒-1
Macro: m4_max (arg, …)

This macro was introduced in Autoconf 2.62. Expand to the decimal value of the maximum arithmetic expression among all the arguments.

Macro: m4_min (arg, …)

This macro was introduced in Autoconf 2.62. Expand to the decimal value of the minimum arithmetic expression among all the arguments.

Macro: m4_sign (expr)

Expand to ‘-1’ if the arithmetic expression expr is negative, ‘1’ if it is positive, and ‘0’ if it is zero.

Macro: m4_version_compare (version-1, version-2)

This macro was introduced in Autoconf 2.53, but had a number of usability limitations that were not lifted until Autoconf 2.62. Compare the version strings version-1 and version-2, and expand to ‘-1’ if version-1 is smaller, ‘0’ if they are the same, or ‘1version-2 is smaller. Version strings must be a list of elements separated by ‘.’, ‘,’ or ‘-’, where each element is a number along with optional case-insensitive letters designating beta releases. The comparison stops at the leftmost element that contains a difference, although a 0 element compares equal to a missing element.

It is permissible to include commit identifiers in version, such as an abbreviated SHA1 of the commit, provided there is still a monotonically increasing prefix to allow for accurate version-based comparisons. For example, this paragraph was written when the development snapshot of autoconf claimed to be at version ‘2.61a-248-dc51’, or 248 commits after the 2.61a release, with an abbreviated commit identification of ‘dc51’.

m4_version_compare([1.1], [2.0])
⇒-1
m4_version_compare([2.0b], [2.0a])
⇒1
m4_version_compare([1.1.1], [1.1.1a])
⇒-1
m4_version_compare([1.2], [1.1.1a])
⇒1
m4_version_compare([1.0], [1])
⇒0
m4_version_compare([1.1pre], [1.1PRE])
⇒0
m4_version_compare([1.1a], [1,10])
⇒-1
m4_version_compare([2.61a], [2.61a-248-dc51])
⇒-1
m4_version_compare([2.61b], [2.61a-248-dc51])
⇒1
Macro: m4_version_prereq (version, [if-new-enough], [if-old = ‘m4_fatal])

Compares version against the version of Autoconf currently running. If the running version is at version or newer, expand if-new-enough, but if version is larger than the version currently executing, expand if-old, which defaults to printing an error message and exiting m4sugar with status 63. When given only one argument, this behaves like AC_PREREQ (see Dealing with Autoconf versions). Remember that the autoconf philosophy favors feature checks over version checks.


8.3.9 Set manipulation in M4

Sometimes, it is necessary to track a set of data, where the order does not matter and where there are no duplicates in the set. The following macros facilitate set manipulations. Each set is an opaque object, which can only be accessed via these basic operations. The underlying implementation guarantees linear scaling for set creation, which is more efficient than using the quadratic m4_append_uniq. Both set names and values can be arbitrary strings, except for unbalanced quotes. This implementation ties up memory for removed elements until the next operation that must traverse all the elements of a set; and although that may slow down some operations until the memory for removed elements is pruned, it still guarantees linear performance.

Macro: m4_set_add (set, value, [if-uniq], [if-dup])

Adds the string value as a member of set set. Expand if-uniq if the element was added, or if-dup if it was previously in the set. Operates in amortized constant time, so that set creation scales linearly.

Macro: m4_set_add_all (set, value…)

Adds each value to the set set. This is slightly more efficient than repeatedly invoking m4_set_add.

Macro: m4_set_contains (set, value, [if-present], [if-absent])

Expands if-present if the string value is a member of set, otherwise if-absent.

m4_set_contains([a], [1], [yes], [no])
⇒no
m4_set_add([a], [1], [added], [dup])
⇒added
m4_set_add([a], [1], [added], [dup])
⇒dup
m4_set_contains([a], [1], [yes], [no])
⇒yes
m4_set_remove([a], [1], [removed], [missing])
⇒removed
m4_set_contains([a], [1], [yes], [no])
⇒no
m4_set_remove([a], [1], [removed], [missing])
⇒missing
Macro: m4_set_contents (set, [sep])
Macro: m4_set_dump (set, [sep])

Expands to a single string consisting of all the members of the set set, each separated by sep, which is not expanded. m4_set_contents leaves the elements in set but reclaims any memory occupied by removed elements, while m4_set_dump is a faster one-shot action that also deletes the set. No provision is made for disambiguating members that contain a non-empty sep as a substring; use m4_set_empty to distinguish between an empty set and the set containing only the empty string. The order of the output is unspecified; in the current implementation, part of the speed of m4_set_dump results from using a different output order than m4_set_contents. These macros scale linearly in the size of the set before memory pruning, and m4_set_contents([set], [sep]) is faster than m4_joinall([sep]m4_set_listc([set])).

m4_set_add_all([a], [1], [2], [3])
⇒
m4_set_contents([a], [-])
⇒1-2-3
m4_joinall([-]m4_set_listc([a]))
⇒1-2-3
m4_set_dump([a], [-])
⇒3-2-1
m4_set_contents([a])
⇒
m4_set_add([a], [])
⇒
m4_set_contents([a], [-])
⇒
Macro: m4_set_delete (set)

Delete all elements and memory associated with set. This is linear in the set size, and faster than removing one element at a time.

Macro: m4_set_difference (seta, setb)
Macro: m4_set_intersection (seta, setb)
Macro: m4_set_union (seta, setb)

Compute the relation between seta and setb, and output the result as a list of quoted arguments without duplicates and with a leading comma. Set difference selects the elements in seta but not setb, intersection selects only elements in both sets, and union selects elements in either set. These actions are linear in the sum of the set sizes. The leading comma is necessary to distinguish between no elements and the empty string as the only element.

m4_set_add_all([a], [1], [2], [3])
⇒
m4_set_add_all([b], [3], [], [4])
⇒
m4_set_difference([a], [b])
⇒,1,2
m4_set_difference([b], [a])
⇒,,4
m4_set_intersection([a], [b])
⇒,3
m4_set_union([a], [b])
⇒,1,2,3,,4
Macro: m4_set_empty (set, [if-empty], [if-elements])

Expand if-empty if the set set has no elements, otherwise expand if-elements. This macro operates in constant time. Using this macro can help disambiguate output from m4_set_contents or m4_set_list.

Macro: m4_set_foreach (set, variable, action)

For each element in the set set, expand action with the macro variable defined as the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element currently contained in variable. This macro is faster than the corresponding m4_foreach([variable], m4_indir([m4_dquote]m4_set_listc([set])), [action]), although m4_set_map might be faster still.

m4_set_add_all([a]m4_for([i], [1], [5], [], [,i]))
⇒
m4_set_contents([a])
⇒12345
m4_set_foreach([a], [i],
  [m4_if(m4_eval(i&1), [0], [m4_set_remove([a], i, [i])])])
⇒24
m4_set_contents([a])
⇒135
Macro: m4_set_list (set)
Macro: m4_set_listc (set)

Produce a list of arguments, where each argument is a quoted element from the set set. The variant m4_set_listc is unambiguous, by adding a leading comma if there are any set elements, whereas the variant m4_set_list cannot distinguish between an empty set and a set containing only the empty string. These can be directly used in macros that take multiple arguments, such as m4_join or m4_set_add_all, or wrapped by m4_dquote for macros that take a quoted list, such as m4_map or m4_foreach. Any memory occupied by removed elements is reclaimed during these macros.

m4_set_add_all([a], [1], [2], [3])
⇒
m4_set_list([a])
⇒1,2,3
m4_set_list([b])
⇒
m4_set_listc([b])
⇒
m4_count(m4_set_list([b]))
⇒1
m4_set_empty([b], [0], [m4_count(m4_set_list([b]))])
⇒0
m4_set_add([b], [])
⇒
m4_set_list([b])
⇒
m4_set_listc([b])
⇒,
m4_count(m4_set_list([b]))
⇒1
m4_set_empty([b], [0], [m4_count(m4_set_list([b]))])
⇒1
Macro: m4_set_map (set, action)

For each element in the set set, expand action with a single argument of the set element. Behavior is unspecified if action recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element passed as an argument. This macro is faster than either corresponding counterpart of m4_map_args([action]m4_set_listc([set])) or m4_set_foreach([set], [var], [action(m4_defn([var]))]). It is possible to use m4_curry if more than one argument is needed for action, although it is more efficient to use m4_set_map_sep in that case.

Macro: m4_set_map_sep (set, [pre], [post], [sep])

For each element in the set set, expand pre[element]post, additionally expanding sep between elements. Behavior is unspecified if the expansion recursively lists the contents of set (although listing other sets is acceptable), or if it modifies the set in any way other than removing the element visited by the expansion. This macro provides the most efficient means for non-destructively visiting the elements of a set; in particular, m4_set_map([set], [action]) is equivalent to m4_set_map_sep([set], [action(], [)]).

Macro: m4_set_remove (set, value, [if-present], [if-absent])

If value is an element in the set set, then remove it and expand if-present. Otherwise expand if-absent. This macro operates in constant time so that multiple removals will scale linearly rather than quadratically; but when used outside of m4_set_foreach or m4_set_map, it leaves memory occupied until the set is later compacted by m4_set_contents or m4_set_list. Several other set operations are then less efficient between the time of element removal and subsequent memory compaction, but still maintain their guaranteed scaling performance.

Macro: m4_set_size (set)

Expand to the size of the set set. This implementation operates in constant time, and is thus more efficient than m4_eval(m4_count(m4_set_listc([set])) - 1).


8.3.10 Forbidden Patterns

M4sugar provides a means to define suspicious patterns, patterns describing tokens which should not be found in the output. For instance, if an Autoconf configure script includes tokens such as ‘AC_DEFINE’, or ‘dnl’, then most probably something went wrong (typically a macro was not evaluated because of overquotation).

M4sugar forbids all the tokens matching ‘^_?m4_’ and ‘^dnl$’. Additional layers, such as M4sh and Autoconf, add additional forbidden patterns to the list.

Macro: m4_pattern_forbid (pattern)

Declare that no token matching pattern must be found in the output. The output file is (temporarily) split into one word per line as part of the autom4te post-processing, with each line (and therefore word) then being checked against the Perl regular expression pattern. If the regular expression matches, and m4_pattern_allow does not also match, then an error is raised.

Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an ‘#include’. No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn’t make sense to the authors of this documentation: input, such as macros, should be documented by ‘dnl’ comments; reserving ‘#’-comments to document the output).

As an example, if you define your own macros that begin with ‘M_’ and are composed from capital letters and underscores, the specification of m4_pattern_forbid([^M_[A-Z_]+]) will ensure all your macros are expanded when not used in comments.

As an example of a common use of this macro, consider what happens in packages that want to use the pkg-config script via the third-party PKG_CHECK_MODULES macro. By default, if a developer checks out the development tree but has not yet installed the pkg-config macros locally, they can manage to successfully run autoconf on the package, but the resulting configure file will likely result in a confusing shell message about a syntax error on the line mentioning the unexpanded PKG_CHECK_MODULES macro. On the other hand, if configure.ac includes m4_pattern_forbid([^PKG_]), the missing pkg-config macros will be detected immediately without allowing autoconf to succeed.

Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to ‘$m4_flags’.

Macro: m4_pattern_allow (pattern)

Any token matching pattern is allowed, including if it matches an m4_pattern_forbid pattern.

For example, Gnulib uses m4_pattern_forbid([^gl_]) to reserve the gl_ namespace for itself, but also uses m4_pattern_allow([^gl_ES$]) to avoid a false negative on the valid locale name.


8.4 Debugging via autom4te

At times, it is desirable to see what was happening inside m4, to see why output was not matching expectations. However, post-processing done by autom4te means that directly using the m4 builtin m4_traceon is likely to interfere with operation. Also, frequent diversion changes and the concept of forbidden tokens make it difficult to use m4_defn to generate inline comments in the final output.

There are a couple of tools to help with this. One is the use of the --trace option provided by autom4te (as well as each of the programs that wrap autom4te, such as autoconf), in order to inspect when a macro is called and with which arguments. For example, when this paragraph was written, the autoconf version could be found by:

$ autoconf --trace=AC_INIT
configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@gnu.org
$ autoconf --trace='AC_INIT:version is $2'
version is 2.63b.95-3963

Another trick is to print out the expansion of various m4 expressions to standard error or to an independent file, with no further m4 expansion, and without interfering with diversion changes or the post-processing done to standard output. m4_errprintn shows a given expression on standard error. For example, if you want to see the expansion of an autoconf primitive or of one of your autoconf macros, you can do it like this:

$ cat <<\EOF > configure.ac
AC_INIT
m4_errprintn([The definition of AC_DEFINE_UNQUOTED:])
m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED]))
AC_OUTPUT
EOF
$ autoconf
error→The definition of AC_DEFINE_UNQUOTED:
error→_AC_DEFINE_Q([], $@)

9 Programming in M4sh

M4sh, pronounced “mash”, is aiming at producing portable Bourne shell scripts. This name was coined by Lars J. Aas, who notes that, according to the Webster’s Revised Unabridged Dictionary (1913):

Mash \Mash\, n. [Akin to G. meisch, maisch, meische, maische, mash, wash, and prob. to AS. miscian to mix. See “Mix”.]

  1. A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure...
  2. A mixture of meal or bran and water fed to animals.
  3. A mess; trouble. [Obs.] –Beau. & Fl.

M4sh reserves the M4 macro namespace ‘^_AS_’ for internal use, and the namespace ‘^AS_’ for M4sh macros. It also reserves the shell and environment variable namespace ‘^as_’, and the here-document delimiter namespace ‘^_AS[A-Z]’ in the output file. You should not define your own macros or output shell code that conflicts with these namespaces.


9.1 Common Shell Constructs

M4sh provides portable alternatives for some common shell constructs that unfortunately are not portable in practice.

Macro: AS_BOX (text, [char = ‘-])

Expand into shell code that will output text surrounded by a box with char in the top and bottom border. text should not contain a newline, but may contain shell expansions valid for unquoted here-documents. char defaults to ‘-’, but can be any character except ‘/’, ‘'’, ‘"’, ‘\’, ‘&’, or ‘`’. This is useful for outputting a comment box into log files to separate distinct phases of script operation.

Macro: AS_CASE (word, [pattern1], [if-matched1], …, [default])

Expand into a shell ‘case’ statement, where word is matched against one or more patterns. if-matched is run if the corresponding pattern matched word, else default is run. See Prerequisite Macros for why this macro should be used instead of plain ‘case’ in code outside of an AC_DEFUN macro, when the contents of the ‘case’ use AC_REQUIRE directly or indirectly. See Limitations of Shell Builtins, for how this macro avoids some portability issues. See Dealing with unbalanced parentheses for how this macro lets you write code with balanced parentheses even if your code must run on obsolescent shells.

Macro: AS_DIRNAME (file-name)

Output the directory portion of file-name. For example, if $file is ‘/one/two/three’, the command dir=`AS_DIRNAME(["$file"])` sets dir to ‘/one/two’.

This interface may be improved in the future to avoid forks and losing trailing newlines.

Macro: AS_ECHO (word)

Emits word to the standard output, followed by a newline. word must be a single shell word (typically a quoted string). The bytes of word are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation. This is much more portable than using echo (see Limitations of Shell Builtins).

Macro: AS_ECHO_N (word)

Emits word to the standard output, without a following newline. word must be a single shell word (typically a quoted string) and, for portability, should not include more than one newline. The bytes of word are output as-is, even if it starts with "-" or contains "\". Redirections can be placed outside the macro invocation.

Macro: AS_ESCAPE (string, [chars = ‘`\"$])

Expands to string, with any characters in chars escaped with a backslash (‘\’). chars should be at most four bytes long, and only contain characters from the set ‘`\"$’; however, characters may be safely listed more than once in chars for the sake of syntax highlighting editors. The current implementation expands string after adding escapes; if string contains macro calls that in turn expand to text needing shell quoting, you can use AS_ESCAPE(m4_dquote(m4_expand([string]))).

The default for chars (‘\"$`’) is the set of characters needing escapes when string will be used literally within double quotes. One common variant is the set of characters to protect when string will be used literally within back-ticks or an unquoted here-document (‘\$`’). Another common variant is ‘""’, which can be used to form a double-quoted string containing the same expansions that would have occurred if string were expanded in an unquoted here-document; however, when using this variant, care must be taken that string does not use double quotes within complex variable expansions (such as ‘${foo-`echo "hi"`}’) that would be broken with improper escapes.

This macro is often used with AS_ECHO. For an example, observe the output generated by the shell code generated from this snippet:

foo=bar
AS_ECHO(["AS_ESCAPE(["$foo" = ])AS_ESCAPE(["$foo"], [""])"])
⇒"$foo" = "bar"
m4_define([macro], [a, [\b]])
AS_ECHO(["AS_ESCAPE([[macro]])"])
⇒macro
AS_ECHO(["AS_ESCAPE([macro])"])
⇒a, b
AS_ECHO(["AS_ESCAPE(m4_dquote(m4_expand([macro])))"])
⇒a, \b

To escape a string that will be placed within single quotes, use:

m4_bpatsubst([[string]], ['], ['\\''])
Macro: AS_EXECUTABLE_P (file)

Emit code to probe whether file is a regular file with executable permissions (and not a directory with search permissions). The caller is responsible for quoting file.

Macro: AS_EXIT ([status = ‘$?])

Emit code to exit the shell with status, defaulting to ‘$?’. This macro works around shells that see the exit status of the command prior to exit inside a ‘trap 0’ handler (see Limitations of Shell Builtins).

Macro: AS_IF (test1, [run-if-true1], …, [run-if-false])

Run shell code test1. If test1 exits with a zero status then run shell code run-if-true1, else examine further tests. If no test exits with a zero status, run shell code run-if-false, with simplifications if either run-if-true1 or run-if-false is empty. For example,

AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])],
      [test "x$foo" != xno], [HANDLE_FOO([maybe])],
      [echo foo not specified])

ensures any required macros of HANDLE_FOO are expanded before the first test.

This macro should be used instead of plain ‘if’ in code outside of an AC_DEFUN macro, when the contents of the ‘if’ use AC_REQUIRE directly or indirectly (see Prerequisite Macros).

Macro: AS_MKDIR_P (file-name)

Make the directory file-name, including intervening directories as necessary. This is equivalent to ‘mkdir -p -- file-name’, except that it is portable to older versions of mkdir that lack support for the -p option or for the -- delimiter (see Limitations of Usual Tools). Also, AS_MKDIR_P succeeds if file-name is a symbolic link to an existing directory, even though Posix is unclear whether ‘mkdir -p’ should succeed in that case. If creation of file-name fails, exit the script.

Also see the AC_PROG_MKDIR_P macro (see Particular Program Checks).

Macro: AS_SET_STATUS (status)

Emit shell code to set the value of ‘$?’ to status, as efficiently as possible. However, this is not guaranteed to abort a shell running with set -e (see Limitations of Shell Builtins). This should also be used at the end of a complex shell function instead of ‘return’ (see Shell Functions) to avoid a DJGPP shell bug.

Macro: AS_TR_CPP (expression)

Transform expression into a valid right-hand side for a C #define. For example:

# This outputs "#define HAVE_CHAR_P 1".
# Notice the m4 quoting around #, to prevent an m4 comment
type="char *"
echo "[#]define AS_TR_CPP([HAVE_$type]) 1"
Macro: AS_TR_SH (expression)

Transform expression into shell code that generates a valid shell variable name. The result is literal when possible at m4 time, but must be used with eval if expression causes shell indirections. For example:

# This outputs "Have it!".
header="sys/some file.h"
eval AS_TR_SH([HAVE_$header])=yes
if test "x$HAVE_sys_some_file_h" = xyes; then echo "Have it!"; fi
Macro: AS_SET_CATFILE (var, dir, file)

Set the polymorphic shell variable var to dir/file, but optimizing the common cases (dir or file is ‘.’, file is absolute, etc.).

Macro: AS_UNSET (var)

Unsets the shell variable var, working around bugs in older shells (see Limitations of Shell Builtins). var can be a literal or indirect variable name.

Macro: AS_VERSION_COMPARE (version-1, version-2, [action-if-less], [action-if-equal], [action-if-greater])

Compare two strings version-1 and version-2, possibly containing shell variables, as version strings, and expand action-if-less, action-if-equal, or action-if-greater depending upon the result. The algorithm to compare is similar to the one used by strverscmp in glibc (see String/Array Comparison in The GNU C Library).


9.2 Support for indirect variable names

Often, it is convenient to write a macro that will emit shell code operating on a shell variable. The simplest case is when the variable name is known. But a more powerful idiom is writing shell code that can work through an indirection, where another variable or command substitution produces the name of the variable to actually manipulate. M4sh supports the notion of polymorphic shell variables, making it easy to write a macro that can deal with either literal or indirect variable names and output shell code appropriate for both use cases. Behavior is undefined if expansion of an indirect variable does not result in a literal variable name.

Macro: AS_LITERAL_IF (expression, [if-literal], [if-not], [if-simple-ref = if-not]
Macro: AS_LITERAL_WORD_IF (expression, [if-literal], [if-not], [if-simple-ref = if-not]

If the expansion of expression is definitely a shell literal, expand if-literal. If the expansion of expression looks like it might contain shell indirections (such as $var or `expr`), then if-not is expanded. Sometimes, it is possible to output optimized code if expression consists only of shell variable expansions (such as ${var}), in which case if-simple-ref can be provided; but defaulting to if-not should always be safe. AS_LITERAL_WORD_IF only expands if-literal if expression looks like a single shell word, containing no whitespace; while AS_LITERAL_IF allows whitespace in expression.

In order to reduce the time spent recognizing whether an expression qualifies as a literal or a simple indirection, the implementation is somewhat conservative: expression must be a single shell word (possibly after stripping whitespace), consisting only of bytes that would have the same meaning whether unquoted or enclosed in double quotes (for example, ‘a.b’ results in if-literal, even though it is not a valid shell variable name; while both ‘'a'’ and ‘[$]’ result in if-not, because they behave differently than ‘"'a'"’ and ‘"[$]"’). This macro can be used in contexts for recognizing portable file names (such as in the implementation of AC_LIBSOURCE), or coupled with some transliterations for forming valid variable names (such as in the implementation of AS_TR_SH, which uses an additional m4_translit to convert ‘.’ to ‘_’).

This example shows how to read the contents of the shell variable bar, exercising all three arguments to AS_LITERAL_IF. It results in a script that will output the line ‘hello’ three times.

AC_DEFUN([MY_ACTION],
[AS_LITERAL_IF([$1],
  [echo "$$1"],
  [AS_VAR_COPY([var], [$1])
   echo "$var"],
  [eval 'echo "$'"$1"\"])])
foo=bar bar=hello
MY_ACTION([bar])
MY_ACTION([`echo bar`])
MY_ACTION([$foo])
Macro: AS_VAR_APPEND (var, text)

Emit shell code to append the shell expansion of text to the end of the current contents of the polymorphic shell variable var, taking advantage of shells that provide the ‘+=’ extension for more efficient scaling.

For situations where the final contents of var are relatively short (less than 256 bytes), it is more efficient to use the simpler code sequence of var=${var}text (or its polymorphic equivalent of AS_VAR_COPY([t], [var]) and AS_VAR_SET([var], ["$t"text])). But in the case when the script will be repeatedly appending text into var, issues of scaling start to become apparent. A naive implementation requires execution time linear to the length of the current contents of var as well as the length of text for a single append, for an overall quadratic scaling with multiple appends. This macro takes advantage of shells which provide the extension var+=text, which can provide amortized constant time for a single append, for an overall linear scaling with multiple appends. Note that unlike AS_VAR_SET, this macro requires that text be quoted properly to avoid field splitting and file name expansion.

Macro: AS_VAR_ARITH (var, expression)

Emit shell code to compute the arithmetic expansion of expression, assigning the result as the contents of the polymorphic shell variable var. The code takes advantage of shells that provide ‘$(())’ for fewer forks, but uses expr as a fallback. Therefore, the syntax for a valid expression is rather limited: all operators must occur as separate shell arguments and with proper quoting, there is no portable equality operator, all variables containing numeric values must be expanded prior to the computation, all numeric values must be provided in decimal without leading zeroes, and the first shell argument should not be a negative number. In the following example, this snippet will print ‘(2+3)*4 == 20’.

bar=3
AS_VAR_ARITH([foo], [\( 2 + $bar \) \* 4])
echo "(2+$bar)*4 == $foo"
Macro: AS_VAR_COPY (dest, source)

Emit shell code to assign the contents of the polymorphic shell variable source to the polymorphic shell variable dest. For example, executing this M4sh snippet will output ‘bar hi’:

foo=bar bar=hi
AS_VAR_COPY([a], [foo])
AS_VAR_COPY([b], [$foo])
echo "$a $b"

When it is necessary to access the contents of an indirect variable inside a shell double-quoted context, the recommended idiom is to first copy the contents into a temporary literal shell variable.

for header in stdint_h inttypes_h ; do
  AS_VAR_COPY([var], [ac_cv_header_$header])
  echo "$header detected: $var"
done
Macro: AS_VAR_IF (var, [word], [if-equal], [if-not-equal])

Output a shell conditional statement. If the contents of the polymorphic shell variable var match the string word, execute if-equal; otherwise execute if-not-equal. word must be a single shell word (typically a quoted string). Avoids shell bugs if an interrupt signal arrives while a command substitution in var is being expanded.

Macro: AS_VAR_PUSHDEF (m4-name, value)
Macro: AS_VAR_POPDEF (m4-name)

A common M4sh idiom involves composing shell variable names from an m4 argument (for example, writing a macro that uses a cache variable). value can be an arbitrary string, which will be transliterated into a valid shell name by AS_TR_SH. In order to access the composed variable name based on value, it is easier to declare a temporary m4 macro m4-name with AS_VAR_PUSHDEF, then use that macro as the argument to subsequent AS_VAR macros as a polymorphic variable name, and finally free the temporary macro with AS_VAR_POPDEF. These macros are often followed with dnl, to avoid excess newlines in the output.

Here is an involved example, that shows the power of writing macros that can handle composed shell variable names:

m4_define([MY_CHECK_HEADER],
[AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl
AS_VAR_IF([my_Header], [yes], [echo "header $1 detected"])dnl
AS_VAR_POPDEF([my_Header])dnl
])
MY_CHECK_HEADER([stdint.h])
for header in inttypes.h stdlib.h ; do
  MY_CHECK_HEADER([$header])
done

In the above example, MY_CHECK_HEADER can operate on polymorphic variable names. In the first invocation, the m4 argument is stdint.h, which transliterates into a literal stdint_h. As a result, the temporary macro my_Header expands to the literal shell name ‘ac_cv_header_stdint_h’. In the second invocation, the m4 argument to MY_CHECK_HEADER is $header, and the temporary macro my_Header expands to the indirect shell name ‘$as_my_Header’. During the shell execution of the for loop, when ‘$header’ contains ‘inttypes.h’, then ‘$as_my_Header’ contains ‘ac_cv_header_inttypes_h’. If this script is then run on a platform where all three headers have been previously detected, the output of the script will include:

header stdint.h detected
header inttypes.h detected
header stdlib.h detected
Macro: AS_VAR_SET (var, [value])

Emit shell code to assign the contents of the polymorphic shell variable var to the shell expansion of value. value is not subject to field splitting or file name expansion, so if command substitution is used, it may be done with ‘`""`’ rather than using an intermediate variable (see Shell Substitutions). However, value does undergo rescanning for additional macro names; behavior is unspecified if late expansion results in any shell meta-characters.

Macro: AS_VAR_SET_IF (var, [if-set], [if-undef])

Emit a shell conditional statement, which executes if-set if the polymorphic shell variable var is set to any value, and if-undef otherwise.

Macro: AS_VAR_TEST_SET (var)

Emit a shell statement that results in a successful exit status only if the polymorphic shell variable var is set.


9.3 Initialization Macros

Macro: AS_BOURNE_COMPATIBLE

Set up the shell to be more compatible with the Bourne shell as standardized by Posix, if possible. This may involve setting environment variables, or setting options, or similar implementation-specific actions. This macro is deprecated, since AS_INIT already invokes it.

Macro: AS_INIT

Initialize the M4sh environment. This macro calls m4_init, then outputs the #! /bin/sh line, a notice about where the output was generated from, and code to sanitize the environment for the rest of the script. Among other initializations, this sets SHELL to the shell chosen to run the script (see CONFIG_SHELL), and LC_ALL to ensure the C locale. Finally, it changes the current diversion to BODY. AS_INIT is called automatically by AC_INIT and AT_INIT, so shell code in configure, config.status, and testsuite all benefit from a sanitized shell environment.

Macro: AS_INIT_GENERATED (file, [comment])

Emit shell code to start the creation of a subsidiary shell script in file, including changing file to be executable. This macro populates the child script with information learned from the parent (thus, the emitted code is equivalent in effect, but more efficient, than the code output by AS_INIT, AS_BOURNE_COMPATIBLE, and AS_SHELL_SANITIZE). If present, comment is output near the beginning of the child, prior to the shell initialization code, and is subject to parameter expansion, command substitution, and backslash quote removal. The parent script should check the exit status after this macro, in case file could not be properly created (for example, if the disk was full). If successfully created, the parent script can then proceed to append additional M4sh constructs into the child script.

Note that the child script starts life without a log file open, so if the parent script uses logging (see AS_MESSAGE_LOG_FD), you must temporarily disable any attempts to use the log file until after emitting code to open a log within the child. On the other hand, if the parent script has AS_MESSAGE_FD redirected somewhere besides ‘1’, then the child script already has code that copies stdout to that descriptor. Currently, the suggested idiom for writing a M4sh shell script from within another script is:

AS_INIT_GENERATED([file], [[# My child script.
]]) || { AS_ECHO(["Failed to create child script"]); AS_EXIT; }
m4_pushdef([AS_MESSAGE_LOG_FD])dnl
cat >> "file" <<\__EOF__
# Code to initialize AS_MESSAGE_LOG_FD
m4_popdef([AS_MESSAGE_LOG_FD])dnl
# Additional code
__EOF__

This, however, may change in the future as the M4sh interface is stabilized further.

Also, be aware that use of LINENO within the child script may report line numbers relative to their location in the parent script, even when using AS_LINENO_PREPARE, if the parent script was unable to locate a shell with working LINENO support.

Macro: AS_LINENO_PREPARE

Find a shell that supports the special variable LINENO, which contains the number of the currently executing line. This macro is automatically invoked by AC_INIT in configure scripts.

Macro: AS_ME_PREPARE

Set up variable as_me to be the basename of the currently executing script. This macro is automatically invoked by AC_INIT in configure scripts.

Macro: AS_TMPDIR (prefix, [dir = ‘${TMPDIR:=/tmp}])

Create, as safely as possible, a temporary sub-directory within dir with a name starting with prefix. prefix should be 2–4 characters, to make it slightly easier to identify the owner of the directory. If dir is omitted, then the value of TMPDIR will be used (defaulting to ‘/tmp’). On success, the name of the newly created directory is stored in the shell variable tmp. On error, the script is aborted.

Typically, this macro is coupled with some exit traps to delete the created directory and its contents on exit or interrupt. However, there is a slight window between when the directory is created and when the name is actually known to the shell, so an interrupt at the right moment might leave the temporary directory behind. Hence it is important to use a prefix that makes it easier to determine if a leftover temporary directory from an interrupted script is safe to delete.

The use of the output variable ‘$tmp’ rather than something in the ‘as_’ namespace is historical; it has the unfortunate consequence that reusing this otherwise common name for any other purpose inside your script has the potential to break any cleanup traps designed to remove the temporary directory.

Macro: AS_SHELL_SANITIZE

Initialize the shell suitably for configure scripts. This has the effect of AS_BOURNE_COMPATIBLE, and sets some other environment variables for predictable results from configuration tests. For example, it sets LC_ALL to change to the default C locale. See Special Shell Variables. This macro is deprecated, since AS_INIT already invokes it.


9.4 File Descriptor Macros

The following macros define file descriptors used to output messages (or input values) from configure scripts. For example:

echo "$wombats found" >&AS_MESSAGE_LOG_FD
echo 'Enter desired kangaroo count:' >&AS_MESSAGE_FD
read kangaroos <&AS_ORIGINAL_STDIN_FD`

However doing so is seldom needed, because Autoconf provides higher level macros as described below.

Macro: AS_MESSAGE_FD

The file descriptor for ‘checking for...’ messages and results. By default, AS_INIT sets this to ‘1’ for standalone M4sh clients. However, AC_INIT shuffles things around to another file descriptor, in order to allow the -q option of configure to choose whether messages should go to the script’s standard output or be discarded.

If you want to display some messages, consider using one of the printing macros (see Printing Messages) instead. Copies of messages output via these macros are also recorded in config.log.

Macro: AS_MESSAGE_LOG_FD

This must either be empty, or expand to a file descriptor for log messages. By default, AS_INIT sets this macro to the empty string for standalone M4sh clients, thus disabling logging. However, AC_INIT shuffles things around so that both configure and config.status use config.log for log messages. Macros that run tools, like AC_COMPILE_IFELSE (see Running the Compiler), redirect all output to this descriptor. You may want to do so if you develop such a low-level macro.

Macro: AS_ORIGINAL_STDIN_FD

This must expand to a file descriptor for the original standard input. By default, AS_INIT sets this macro to ‘0’ for standalone M4sh clients. However, AC_INIT shuffles things around for safety.

When configure runs, it may accidentally execute an interactive command that has the same name as the non-interactive meant to be used or checked. If the standard input was the terminal, such interactive programs would cause configure to stop, pending some user input. Therefore configure redirects its standard input from /dev/null during its initialization. This is not normally a problem, since configure normally does not need user input.

In the extreme case where your configure script really needs to obtain some values from the original standard input, you can read them explicitly from AS_ORIGINAL_STDIN_FD.


10 Writing Autoconf Macros

When you write a feature test that could be applicable to more than one software package, the best thing to do is encapsulate it in a new macro. Here are some instructions and guidelines for writing Autoconf macros. You should also familiarize yourself with M4sugar (see Programming in M4) and M4sh (see Programming in M4sh).


10.1 Macro Definitions

Macro: AC_DEFUN (name, [body])

Autoconf macros are defined using the AC_DEFUN macro, which is similar to the M4 builtin m4_define macro; this creates a macro named name and with body as its expansion. In addition to defining a macro, AC_DEFUN adds to it some code that is used to constrain the order in which macros are called, while avoiding redundant output (see Prerequisite Macros).

An Autoconf macro definition looks like this:

AC_DEFUN(macro-name, macro-body)

You can refer to any arguments passed to the macro as ‘$1’, ‘$2’, etc. See How to define new macros in GNU M4, for more complete information on writing M4 macros.

Most macros fall in one of two general categories. The first category includes macros which take arguments, in order to generate output parameterized by those arguments. Macros in this category are designed to be directly expanded, often multiple times, and should not be used as the argument to AC_REQUIRE. The other category includes macros which are shorthand for a fixed block of text, and therefore do not take arguments. For this category of macros, directly expanding the macro multiple times results in redundant output, so it is more common to use the macro as the argument to AC_REQUIRE, or to declare the macro with AC_DEFUN_ONCE (see One-Shot Macros).

Be sure to properly quote both the macro-body and the macro-name to avoid any problems if the macro happens to have been previously defined.

Each macro should have a header comment that gives its prototype, and a brief description. When arguments have default values, display them in the prototype. For example:

# AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1])
# --------------------------------------
m4_define([AC_MSG_ERROR],
  [{ AS_MESSAGE([error: $1], [2])
     exit m4_default([$2], [1]); }])

Comments about the macro should be left in the header comment. Most other comments make their way into configure, so just keep using ‘#’ to introduce comments.

If you have some special comments about pure M4 code, comments that make no sense in configure and in the header comment, then use the builtin dnl: it causes M4 to discard the text through the next newline.

Keep in mind that dnl is rarely needed to introduce comments; dnl is more useful to get rid of the newlines following macros that produce no output, such as AC_REQUIRE.

Public third-party macros need to use AC_DEFUN, and not m4_define, in order to be found by aclocal (see Extending aclocal in GNU Automake). Additionally, if it is ever determined that a macro should be made obsolete, it is easy to convert from AC_DEFUN to AU_DEFUN in order to have autoupdate assist the user in choosing a better alternative, but there is no corresponding way to make m4_define issue an upgrade notice (see AU_DEFUN).

There is another subtle, but important, difference between using m4_define and AC_DEFUN: only the former is unaffected by AC_REQUIRE. When writing a file, it is always safe to replace a block of text with a m4_define macro that will expand to the same text. But replacing a block of text with an AC_DEFUN macro with the same content does not necessarily give the same results, because it changes the location where any embedded but unsatisfied AC_REQUIRE invocations within the block will be expanded. For an example of this, see Expanded Before Required.


10.2 Macro Names

All of the public Autoconf macros have all-uppercase names in the namespace ‘^AC_’ to prevent them from accidentally conflicting with other text; Autoconf also reserves the namespace ‘^_AC_’ for internal macros. All shell variables that they use for internal purposes have mostly-lowercase names starting with ‘ac_’. Autoconf also uses here-document delimiters in the namespace ‘^_AC[A-Z]’. During configure, files produced by Autoconf make heavy use of the file system namespace ‘^conf’.

Since Autoconf is built on top of M4sugar (see Programming in M4sugar) and M4sh (see Programming in M4sh), you must also be aware of those namespaces (‘^_?\(m4\|AS\)_’). And since configure.ac is also designed to be scanned by Autoheader, Autoscan, Autoupdate, and Automake, you should be aware of the ‘^_?A[HNUM]_’ namespaces. In general, you should not use the namespace of a package that does not own the macro or shell code you are writing.

To ensure that your macros don’t conflict with present or future Autoconf macros, you should prefix your own macro names and any shell variables they use with some other sequence. Possibilities include your initials, or an abbreviation for the name of your organization or software package. Historically, people have not always followed the rule of using a namespace appropriate for their package, and this has made it difficult for determining the origin of a macro (and where to report bugs about that macro), as well as difficult for the true namespace owner to add new macros without interference from pre-existing uses of third-party macros. Perhaps the best example of this confusion is the AM_GNU_GETTEXT macro, which belongs, not to Automake, but to Gettext.

Most of the Autoconf macros’ names follow a structured naming convention that indicates the kind of feature check by the name. The macro names consist of several words, separated by underscores, going from most general to most specific. The names of their cache variables use the same convention (see Cache Variable Names, for more information on them).

The first word of the name after the namespace initials (such as ‘AC_’) usually tells the category of the feature being tested. Here are the categories used in Autoconf for specific test macros, the kind of macro that you are more likely to write. They are also used for cache variables, in all-lowercase. Use them where applicable; where they’re not, invent your own categories.

C

C language builtin features.

DECL

Declarations of C variables in header files.

FUNC

Functions in libraries.

GROUP

Posix group owners of files.

HEADER

Header files.

LIB

C libraries.

PROG

The base names of programs.

MEMBER

Members of aggregates.

SYS

Operating system features.

TYPE

C builtin or declared types.

VAR

C variables in libraries.

After the category comes the name of the particular feature being tested. Any further words in the macro name indicate particular aspects of the feature. For example, AC_PROG_MAKE_SET checks whether make sets a variable to its own name.

An internal macro should have a name that starts with an underscore; Autoconf internals should therefore start with ‘_AC_’. Additionally, a macro that is an internal subroutine of another macro should have a name that starts with an underscore and the name of that other macro, followed by one or more words saying what the internal macro does. For example, AC_PATH_X has internal macros _AC_PATH_X_XMKMF and _AC_PATH_X_DIRECT.


10.3 Dependencies Between Macros

Some Autoconf macros depend on other macros having been called first in order to work correctly. Autoconf provides a way to ensure that certain macros are called if needed and a way to warn the user if macros are called in an order that might cause incorrect operation.


10.3.1 Prerequisite Macros

A macro that you write might need to use values that have previously been computed by other macros. For example, AC_DECL_YYTEXT examines the output of flex or lex, so it depends on AC_PROG_LEX having been called first to set the shell variable LEX.

Rather than forcing the user of the macros to keep track of the dependencies between them, you can use the AC_REQUIRE macro to do it automatically. AC_REQUIRE can ensure that a macro is only called if it is needed, and only called once.

Macro: AC_REQUIRE (macro-name)

If the M4 macro macro-name has not already been called, call it (without any arguments). Make sure to quote macro-name with square brackets. macro-name must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

AC_REQUIRE must be used inside a macro defined by AC_DEFUN; it must not be called from the top level. Also, it does not make sense to require a macro that takes parameters.

AC_REQUIRE is often misunderstood. It really implements dependencies between macros in the sense that if one macro depends upon another, the latter is expanded before the body of the former. To be more precise, the required macro is expanded before the outermost defined macro in the current expansion stack. In particular, ‘AC_REQUIRE([FOO])’ is not replaced with the body of FOO. For instance, this definition of macros:

AC_DEFUN([TRAVOLTA],
[test "$body_temperature_in_celsius" -gt 38 &&
  dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "x$hair_style" = xcurly &&
  dance_floor=occupied])

AC_DEFUN([RESERVE_DANCE_FLOOR],
[if test "x`date +%A`" = xSaturday; then
  AC_REQUIRE([TRAVOLTA])
  AC_REQUIRE([NEWTON_JOHN])
fi])

with this configure.ac

AC_INIT([Dance Manager], [1.0], [bug-dance@example.org])
RESERVE_DANCE_FLOOR
if test "x$dance_floor" = xoccupied; then
  AC_MSG_ERROR([cannot pick up here, let's move])
fi

does not leave you with a better chance to meet a kindred soul on days other than Saturday, since the call to RESERVE_DANCE_FLOOR expands to:

test "$body_temperature_in_Celsius" -gt 38 &&
  dance_floor=occupied
test "x$hair_style" = xcurly &&
  dance_floor=occupied
fi
if test "x`date +%A`" = xSaturday; then


fi

This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:

if …; then
  AC_REQUIRE([SOME_CHECK])
fi
…
SOME_CHECK

However, this implementation can lead to another class of problems. Consider the case where an outer macro first expands, then indirectly requires, an inner macro:

AC_DEFUN([TESTA], [[echo in A
if test -n "$SEEN_A" ; then echo duplicate ; fi
SEEN_A=:]])
AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B
if test -z "$SEEN_A" ; then echo bug ; fi]])
AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
AC_DEFUN([OUTER], [[echo in OUTER]
TESTA
TESTC])
OUTER

Prior to Autoconf 2.64, the implementation of AC_REQUIRE recognized that TESTB needed to be hoisted prior to the expansion of OUTER, but because TESTA had already been directly expanded, it failed to hoist TESTA. Therefore, the expansion of TESTB occurs prior to its prerequisites, leading to the following output:

in B
bug
in OUTER
in A
in C

Newer Autoconf is smart enough to recognize this situation, and hoists TESTA even though it has already been expanded, but issues a syntax warning in the process. This is because the hoisted expansion of TESTA defeats the purpose of using AC_REQUIRE to avoid redundant code, and causes its own set of problems if the hoisted macro is not idempotent:

in A
in B
in OUTER
in A
duplicate
in C

The bug is not in Autoconf, but in the macro definitions. If you ever pass a particular macro name to AC_REQUIRE, then you are implying that the macro only needs to be expanded once. But to enforce this, either the macro must be declared with AC_DEFUN_ONCE (although this only helps in Autoconf 2.64 or newer), or all uses of that macro should be through AC_REQUIRE; directly expanding the macro defeats the point of using AC_REQUIRE to eliminate redundant expansion. In the example, this rule of thumb was violated because TESTB requires TESTA while OUTER directly expands it. One way of fixing the bug is to factor TESTA into two macros, the portion designed for direct and repeated use (here, named TESTA), and the portion designed for one-shot output and used only inside AC_REQUIRE (here, named TESTA_PREREQ). Then, by fixing all clients to use the correct calling convention according to their needs:

AC_DEFUN([TESTA], [AC_REQUIRE([TESTA_PREREQ])[echo in A]])
AC_DEFUN([TESTA_PREREQ], [[echo in A_PREREQ
if test -n "$SEEN_A" ; then echo duplicate ; fi
SEEN_A=:]])
AC_DEFUN([TESTB], [AC_REQUIRE([TESTA_PREREQ])[echo in B
if test -z "$SEEN_A" ; then echo bug ; fi]])
AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]])
AC_DEFUN([OUTER], [[echo in OUTER]
TESTA
TESTC])
OUTER

the resulting output will then obey all dependency rules and avoid any syntax warnings, whether the script is built with old or new Autoconf versions:

in A_PREREQ
in B
in OUTER
in A
in C

You can use the helper macros AS_IF and AS_CASE in top-level code to enforce expansion of required macros outside of shell conditional constructs; these helpers are not needed in the bodies of macros defined by AC_DEFUN. You are furthermore encouraged, although not required, to put all AC_REQUIRE calls at the beginning of a macro. You can use dnl to avoid the empty lines they leave.

Autoconf will normally warn if an AC_REQUIRE call refers to a macro that has not been defined. However, the aclocal tool relies on parsing an incomplete set of input files to trace which macros have been required, in order to then pull in additional files that provide those macros; for this particular use case, pre-defining the macro m4_require_silent_probe will avoid the warnings.


10.3.2 Suggested Ordering

Some macros should be run before another macro if both are called, but neither requires that the other be called. For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler. Many of these dependencies are noted in the documentation.

Autoconf provides the AC_BEFORE macro to warn users when macros with this kind of dependency appear out of order in a configure.ac file. The warning occurs when creating configure from configure.ac, not when running configure.

For example, AC_PROG_CPP checks whether the C compiler can run the C preprocessor when given the -E option. It should therefore be called after any macros that change which C compiler is being used, such as AC_PROG_CC. So AC_PROG_CC contains:

AC_BEFORE([$0], [AC_PROG_CPP])dnl

This warns the user if a call to AC_PROG_CPP has already occurred when AC_PROG_CC is called.

Macro: AC_BEFORE (this-macro-name, called-macro-name)

Make M4 print a warning message to the standard error output if called-macro-name has already been called. this-macro-name should be the name of the macro that is calling AC_BEFORE. The macro called-macro-name must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.


10.3.3 One-Shot Macros

Some macros should be called only once, either because calling them multiple time is unsafe, or because it is bad style. For instance Autoconf ensures that AC_CANONICAL_BUILD and cousins (see Getting the Canonical System Type) are evaluated only once, because it makes no sense to run these expensive checks more than once. Such one-shot macros can be defined using AC_DEFUN_ONCE.

Macro: AC_DEFUN_ONCE (macro-name, macro-body)

Declare macro macro-name like AC_DEFUN would (see Macro Definitions), but add additional logic that guarantees that only the first use of the macro (whether by direct expansion or AC_REQUIRE) causes an expansion of macro-body; the expansion will occur before the start of any enclosing macro defined by AC_DEFUN. Subsequent expansions are silently ignored. Generally, it does not make sense for macro-body to use parameters such as $1.

Prior to Autoconf 2.64, a macro defined by AC_DEFUN_ONCE would emit a warning if it was directly expanded a second time, so for portability, it is better to use AC_REQUIRE than direct invocation of macro-name inside a macro defined by AC_DEFUN (see Prerequisite Macros).


10.4 Obsoleting Macros

Configuration and portability technology has evolved over the years. Often better ways of solving a particular problem are developed, or ad-hoc approaches are systematized. This process has occurred in many parts of Autoconf. One result is that some of the macros are now considered obsolete; they still work, but are no longer considered the best thing to do, hence they should be replaced with more modern macros. Ideally, autoupdate should replace the old macro calls with their modern implementation.

Autoconf provides a simple means to obsolete a macro.

Macro: AU_DEFUN (old-macro, implementation, [message], [silent])

Define old-macro as implementation, just like AC_DEFUN, but also declare old-macro to be obsolete. When autoupdate is run, occurrences of old-macro will be replaced by the text of implementation in the updated configure.ac file.

If a simple textual replacement is not enough to finish the job of updating a configure.ac to modern style, provide instructions for whatever additional manual work is required as message. These instructions will be printed by autoupdate, and embedded in the updated configure.ac file, next to the text of implementation.

Normally, autoconf will also issue a warning (in the “obsolete” category) when it expands old-macro. This warning does not include message; it only advises the maintainer to run autoupdate. If it is inappropriate to issue this warning, set the silent argument to the word silent. One might want to use a silent AU_DEFUN when old-macro is used in a widely-distributed third-party macro. If that macro’s maintainers are aware of the need to update their code, it’s unnecessary to nag all of the transitive users of old-macro as well. This capability was added to AU_DEFUN in Autoconf 2.70; older versions of autoconf will ignore the silent argument and issue the warning anyway.

Caution: If implementation contains M4 or M4sugar macros, they will be evaluated when autoupdate is run, not emitted verbatim like the rest of implementation. This cannot be avoided with extra quotation, because then old-macro will not work when it is called normally. See the definition of AC_FOREACH in general.m4 for a workaround.

Macro: AU_ALIAS (old-name, new-name, [silent])

A shorthand version of AU_DEFUN, to be used when a macro has simply been renamed. autoupdate will replace calls to old-name with calls to new-name, keeping any arguments intact. No instructions for additional manual work will be printed.

The silent argument works the same as the silent argument to AU_DEFUN. It was added to AU_ALIAS in Autoconf 2.70.

Caution: AU_ALIAS cannot be used when new-name is an M4 or M4sugar macro. See above.


10.5 Coding Style

The Autoconf macros follow a strict coding style. You are encouraged to follow this style, especially if you intend to distribute your macro, either by contributing it to Autoconf itself or the Autoconf Macro Archive, or by other means.

The first requirement is to pay great attention to the quotation. For more details, see The Autoconf Language, and M4 Quotation.

Do not try to invent new interfaces. It is likely that there is a macro in Autoconf that resembles the macro you are defining: try to stick to this existing interface (order of arguments, default values, etc.). We are conscious that some of these interfaces are not perfect; nevertheless, when harmless, homogeneity should be preferred over creativity.

Be careful about clashes both between M4 symbols and between shell variables.

If you stick to the suggested M4 naming scheme (see Macro Names), you are unlikely to generate conflicts. Nevertheless, when you need to set a special value, avoid using a regular macro name; rather, use an “impossible” name. For instance, up to version 2.13, the macro AC_SUBST used to remember what symbol macros were already defined by setting AC_SUBST_symbol, which is a regular macro name. But since there is a macro named AC_SUBST_FILE, it was just impossible to ‘AC_SUBST(FILE)’! In this case, AC_SUBST(symbol) or _AC_SUBST(symbol) should have been used (yes, with the parentheses).

No Autoconf macro should ever enter the user-variable name space; i.e., except for the variables that are the actual result of running the macro, all shell variables should start with ac_. In addition, small macros or any macro that is likely to be embedded in other macros should be careful not to use obvious names.

Do not use dnl to introduce comments: most of the comments you are likely to write are either header comments which are not output anyway, or comments that should make their way into configure. There are exceptional cases where you do want to comment special M4 constructs, in which case dnl is right, but keep in mind that it is unlikely.

M4 ignores the leading blanks and newlines before each argument. Use this feature to indent in such a way that arguments are (more or less) aligned with the opening parenthesis of the macro being called. For instance, instead of

AC_CACHE_CHECK(for EMX OS/2 environment,
ac_cv_emxos2,
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])],
[ac_cv_emxos2=yes], [ac_cv_emxos2=no])])

write

AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                   [ac_cv_emxos2=yes],
                   [ac_cv_emxos2=no])])

or even

AC_CACHE_CHECK([for EMX OS/2 environment],
               [ac_cv_emxos2],
               [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
                                                   [return __EMX__;])],
                                  [ac_cv_emxos2=yes],
                                  [ac_cv_emxos2=no])])

When using AC_RUN_IFELSE or any macro that cannot work when cross-compiling, provide a pessimistic value (typically ‘no’).

Feel free to use various tricks to prevent auxiliary tools, such as syntax-highlighting editors, from behaving improperly. For instance, instead of:

m4_bpatsubst([$1], [$"])

use

m4_bpatsubst([$1], [$""])

so that Emacsen do not open an endless “string” at the first quote. For the same reasons, avoid:

test $[#] != 0

and use:

test $[@%:@] != 0

Otherwise, the closing bracket would be hidden inside a ‘#’-comment, breaking the bracket-matching highlighting from Emacsen. Note the preferred style to escape from M4: ‘$[1]’, ‘$[@]’, etc. Do not escape when it is unnecessary. Common examples of useless quotation are ‘[$]$1’ (write ‘$$1’), ‘[$]var’ (use ‘$var’), etc. If you add portability issues to the picture, you’ll prefer ‘${1+"$[@]"}’ to ‘"[$]@"’, and you’ll prefer do something better than hacking Autoconf :-).

When using sed, don’t use -e except for indenting purposes. With the s and y commands, the preferred separator is ‘/’ unless ‘/’ itself might appear in the pattern or replacement, in which case you should use ‘|’, or optionally ‘,’ if you know the pattern and replacement cannot contain a file name. If none of these characters will do, choose a printable character that cannot appear in the pattern or replacement. Characters from the set ‘"#$&'()*;<=>?`|~’ are good choices if the pattern or replacement might contain a file name, since they have special meaning to the shell and are less likely to occur in file names.

See Macro Definitions, for details on how to define a macro. If a macro doesn’t use AC_REQUIRE, is expected to never be the object of an AC_REQUIRE directive, and macros required by other macros inside arguments do not need to be expanded before this macro, then use m4_define. In case of doubt, use AC_DEFUN. Also take into account that public third-party macros need to use AC_DEFUN in order to be found by aclocal (see Extending aclocal in GNU Automake). All the AC_REQUIRE statements should be at the beginning of the macro, and each statement should be followed by dnl.

You should not rely on the number of arguments: instead of checking whether an argument is missing, test that it is not empty. It provides both a simpler and a more predictable interface to the user, and saves room for further arguments.

Unless the macro is short, try to leave the closing ‘])’ at the beginning of a line, followed by a comment that repeats the name of the macro being defined. This introduces an additional newline in configure; normally, that is not a problem, but if you want to remove it you can use ‘[]dnl’ on the last line. You can similarly use ‘[]dnl’ after a macro call to remove its newline. ‘[]dnl’ is recommended instead of ‘dnl’ to ensure that M4 does not interpret the ‘dnl’ as being attached to the preceding text or macro output. For example, instead of:

AC_DEFUN([AC_PATH_X],
[AC_MSG_CHECKING([for X])
AC_REQUIRE_CPP()
# …omitted…
  AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
fi])

you would write:

AC_DEFUN([AC_PATH_X],
[AC_REQUIRE_CPP()[]dnl
AC_MSG_CHECKING([for X])
# …omitted…
  AC_MSG_RESULT([libraries $x_libraries, headers $x_includes])
fi[]dnl
])# AC_PATH_X

If the macro is long, try to split it into logical chunks. Typically, macros that check for a bug in a function and prepare its AC_LIBOBJ replacement should have an auxiliary macro to perform this setup. Do not hesitate to introduce auxiliary macros to factor your code.

In order to highlight the recommended coding style, here is a macro written the old way:

dnl Check for EMX on OS/2.
dnl _AC_EMXOS2
AC_DEFUN(_AC_EMXOS2,
[AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2,
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)],
ac_cv_emxos2=yes, ac_cv_emxos2=no)])
test "x$ac_cv_emxos2" = xyes && EMXOS2=yes])

and the new way:

# _AC_EMXOS2
# ----------
# Check for EMX on OS/2.
m4_define([_AC_EMXOS2],
[AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                   [ac_cv_emxos2=yes],
                   [ac_cv_emxos2=no])])
test "x$ac_cv_emxos2" = xyes && EMXOS2=yes[]dnl
])# _AC_EMXOS2

11 Portable Shell Programming

When writing your own checks, there are some shell-script programming techniques you should avoid in order to make your code portable. The Bourne shell and upward-compatible shells like the Korn shell and Bash have evolved over the years, and many features added to the original System7 shell are now supported on all interesting porting targets. However, the following discussion between Russ Allbery and Robert Lipe is worth reading:

Russ Allbery:

The GNU assumption that /bin/sh is the one and only shell leads to a permanent deadlock. Vendors don’t want to break users’ existing shell scripts, and there are some corner cases in the Bourne shell that are not completely compatible with a Posix shell. Thus, vendors who have taken this route will never (OK…“never say never”) replace the Bourne shell (as /bin/sh) with a Posix shell.

Robert Lipe:

This is exactly the problem. While most (at least most System V’s) do have a Bourne shell that accepts shell functions most vendor /bin/sh programs are not the Posix shell.

So while most modern systems do have a shell somewhere that meets the Posix standard, the challenge is to find it.

For this reason, part of the job of M4sh (see Programming in M4sh) is to find such a shell. But to prevent trouble, if you’re not using M4sh you should not take advantage of features that were added after Unix version 7, circa 1977 (see Systemology); you should not use aliases, negated character classes, or even unset. # comments, while not in Unix version 7, were retrofitted in the original Bourne shell and can be assumed to be part of the least common denominator.

On the other hand, if you’re using M4sh you can assume that the shell has the features that were added in SVR2 (circa 1984), including shell functions, return, unset, and I/O redirection for builtins. For more information, refer to https://www.in-ulm.de/~mascheck/bourne/. However, some pitfalls have to be avoided for portable use of these constructs; these will be documented in the rest of this chapter. See in particular Shell Functions and Limitations of Shell Builtins.

Some ancient systems have quite small limits on the length of the ‘#!’ line; for instance, 32 bytes (not including the newline) on SunOS 4. However, these ancient systems are no longer of practical concern.

The set of external programs you should run in a configure script is fairly small. See Utilities in Makefiles in The GNU Coding Standards, for the list. This restriction allows users to start out with a fairly small set of programs and build the rest, avoiding too many interdependencies between packages.

Some of these external utilities have a portable subset of features; see Limitations of Usual Tools.

There are other sources of documentation about shells. The specification for the Posix Shell Command Language, though more generous than the restrictive shell subset described above, is fairly portable nowadays. Also please see the Shell FAQs.


11.1 Shellology

There are several families of shells, most prominently the Bourne family and the C shell family which are deeply incompatible. If you want to write portable shell scripts, avoid members of the C shell family. The the Shell difference FAQ includes a small history of Posix shells, and a comparison between several of them.

Below we describe some of the members of the Bourne shell family.

Ash

Ash is often used on GNU/Linux and BSD systems as a light-weight Bourne-compatible shell. Ash 0.2 has some bugs that are fixed in the 0.3.x series, but portable shell scripts should work around them, since version 0.2 is still shipped with many GNU/Linux distributions.

To be compatible with Ash 0.2:

  • - don’t use ‘$?’ after expanding empty or unset variables, or at the start of an eval:
    foo=
    false
    $foo
    echo "Do not use it: $?"
    false
    eval 'echo "Do not use it: $?"'
    
  • - don’t use command substitution within variable expansion:
    cat ${FOO=`bar`}
    
  • - beware that single builtin substitutions are not performed by a subshell, hence their effect applies to the current shell! See Shell Substitutions, item “Command Substitution”.
Bash

To detect whether you are running Bash, test whether BASH_VERSION is set. To require Posix compatibility, run ‘set -o posix’. See Bash Posix Mode in The GNU Bash Reference Manual, for details.

Bash 2.05 and later

Versions 2.05 and later of Bash use a different format for the output of the set builtin, designed to make evaluating its output easier. However, this output is not compatible with earlier versions of Bash (or with many other shells, probably). So if you use Bash 2.05 or higher to execute configure, you’ll need to use Bash 2.05 for all other build tasks as well.

Ksh

The Korn shell is compatible with the Bourne family and it mostly conforms to Posix. It has two major variants commonly called ‘ksh88’ and ‘ksh93’, named after the years of initial release. It is usually called ksh, but is called sh on some hosts if you set your path appropriately.

On Solaris 11, /bin/sh and /usr/bin/ksh are both ‘ksh93’. On Solaris 10 and earlier, /bin/sh is a pre-Posix Bourne shell and the Korn shell is found elsewhere: /usr/bin/ksh is ‘ksh88’ on Solaris 2.0 through 10, /usr/xpg4/bin/sh is a Posix-compliant variant of ‘ksh88’ on Solaris 9 and later, and /usr/dt/bin/dtksh is ‘ksh93’. Variants that are not standard may be parts of optional packages. There is no extra charge for these packages, but they are not part of a minimal OS install and therefore some installations may not have it.

Starting with Tru64 Version 4.0, the Korn shell /usr/bin/ksh is also available as /usr/bin/posix/sh. If the environment variable BIN_SH is set to xpg4, subsidiary invocations of the standard shell conform to Posix.

Pdksh

A public-domain clone of the Korn shell called pdksh is widely available: it has most of the ‘ksh88’ features along with a few of its own. It usually sets KSH_VERSION, except if invoked as /bin/sh on OpenBSD, and similarly to Bash you can require Posix compatibility by running ‘set -o posix’. Unfortunately, with pdksh 5.2.14 (the latest stable version as of January 2007) Posix mode is buggy and causes pdksh to depart from Posix in at least one respect, see Shell Substitutions.

Zsh

To detect whether you are running zsh, test whether ZSH_VERSION is set. By default zsh is not compatible with the Bourne shell: you must execute ‘emulate sh’, and for zsh versions before 3.1.6-dev-18 you must also set NULLCMD to ‘:’. See Compatibility in The Z Shell Manual, for details.

The default Mac OS X sh was originally Zsh; it was changed to Bash in Mac OS X 10.2.


11.2 Invoking the Shell

The Korn shell (up to at least version M-12/28/93d) has a bug when invoked on a file whose name does not contain a slash. It first searches for the file’s name in PATH, and if found it executes that rather than the original file. For example, assuming there is a binary executable /usr/bin/script in your PATH, the last command in the following example fails because the Korn shell finds /usr/bin/script and refuses to execute it as a shell script:

$ touch xxyzzyz script
$ ksh xxyzzyz
$ ksh ./script
$ ksh script
ksh: script: cannot execute

Bash 2.03 has a bug when invoked with the -c option: if the option-argument ends in backslash-newline, Bash incorrectly reports a syntax error. The problem does not occur if a character follows the backslash:

$ $ bash -c 'echo foo \
> '
bash: -c: line 2: syntax error: unexpected end of file
$ bash -c 'echo foo \
>  '
foo

See Backslash-Newline Before Empty Lines, for how this can cause problems in makefiles.


11.3 Here-Documents

Don’t rely on ‘\’ being preserved just because it has no special meaning together with the next symbol. In the native sh on OpenBSD 2.7 ‘\"’ expands to ‘"’ in here-documents with unquoted delimiter. As a general rule, if ‘\\’ expands to ‘\’ use ‘\\’ to get ‘\’.

With OpenBSD 2.7’s sh

$ cat <<EOF
> \" \\
> EOF
" \

and with Bash:

bash-2.04$ cat <<EOF
> \" \\
> EOF
\" \

Using command substitutions in a here-document that is fed to a shell function is not portable. For example, with Solaris 10 /bin/sh:

$ kitty () { cat; }
$ kitty <<EOF
> `echo ok`
> EOF
/tmp/sh199886: cannot open
$ echo $?
1

Some shells mishandle large here-documents: for example, Solaris 10 dtksh and the UnixWare 7.1.1 Posix shell, which are derived from Korn shell version M-12/28/93d, mishandle braced variable expansion that crosses a 1024- or 4096-byte buffer boundary within a here-document. Only the part of the variable name after the boundary is used. For example, ${variable} could be replaced by the expansion of ${ble}. If the end of the variable name is aligned with the block boundary, the shell reports an error, as if you used ${}. Instead of ${variable-default}, the shell may expand ${riable-default}, or even ${fault}. This bug can often be worked around by omitting the braces: $variable. The bug was fixed in ‘ksh93g’ (1998-04-30) but as of 2006 many operating systems were still shipping older versions with the bug.

Empty here-documents are not portable either; with the following code, zsh up to at least version 4.3.10 creates a file with a single newline, whereas other shells create an empty file:

cat >file <<EOF
EOF

Many shells (including the Bourne shell) implement here-documents inefficiently. In particular, some shells can be extremely inefficient when a single statement contains many here-documents. For instance if your configure.ac includes something like:

if <cross_compiling>; then
  assume this and that
else
  check this
  check that
  check something else
  …
  on and on forever
  …
fi

A shell parses the whole if/fi construct, creating temporary files for each here-document in it. Some shells create links for such here-documents on every fork, so that the clean-up code they had installed correctly removes them. It is creating the links that can take the shell forever.

Moving the tests out of the if/fi, or creating multiple if/fi constructs, would improve the performance significantly. Anyway, this kind of construct is not exactly the typical use of Autoconf. In fact, it’s even not recommended, because M4 macros can’t look into shell conditionals, so we may fail to expand a macro when it was expanded before in a conditional path, and the condition turned out to be false at runtime, and we end up not executing the macro at all.

Be careful with the use of ‘<<-’ to unindent here-documents. The behavior is only portable for stripping leading TABs, and things can silently break if an overzealous editor converts to using leading spaces (not all shells are nice enough to warn about unterminated here-documents).

$ printf 'cat <<-x\n\t1\n\t 2\n\tx\n' | bash && echo done
1
 2
done
$ printf 'cat <<-x\n 1\n  2\n x\n' | bash-3.2 && echo done
 1
  2
 x
done

11.4 File Descriptors

Most shells, if not all (including Bash, Zsh, Ash), output traces on stderr, even for subshells. This might result in undesirable content if you meant to capture the standard-error output of the inner command:

$ ash -x -c '(eval "echo foo >&2") 2>stderr'
$ cat stderr
+ eval echo foo >&2
+ echo foo
foo
$ bash -x -c '(eval "echo foo >&2") 2>stderr'
$ cat stderr
+ eval 'echo foo >&2'
++ echo foo
foo
$ zsh -x -c '(eval "echo foo >&2") 2>stderr'
# Traces on startup files deleted here.
$ cat stderr
+zsh:1> eval echo foo >&2
+zsh:1> echo foo
foo

One workaround is to grep out uninteresting lines, hoping not to remove good ones.

If you intend to redirect both standard error and standard output, redirect standard output first. This works better with HP-UX, since its shell mishandles tracing if standard error is redirected first:

$ sh -x -c ': 2>err >out'
+ :
+ 2> err $ cat err
1> out

Don’t try to redirect the standard error of a command substitution. It must be done inside the command substitution. When running ‘: `cd /zorglub` 2>/dev/null’ expect the error message to escape, while ‘: `cd /zorglub 2>/dev/null`’ works properly.

On the other hand, some shells, such as Solaris or FreeBSD /bin/sh, warn about missing programs before performing redirections. Therefore, to silently check whether a program exists, it is necessary to perform redirections on a subshell or brace group:

$ /bin/sh -c 'nosuch 2>/dev/null'
nosuch: not found
$ /bin/sh -c '(nosuch) 2>/dev/null'
$ /bin/sh -c '{ nosuch; } 2>/dev/null'
$ bash -c 'nosuch 2>/dev/null'

FreeBSD 6.2 sh may mix the trace output lines from the statements in a shell pipeline.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible in assignments though: ‘foo=`cd /zorglub` 2>/dev/null’.

Some shells, like ash, don’t recognize bi-directional redirection (‘<>’). And even on shells that recognize it, it is not portable to use on fifos: Posix does not require read-write support for named pipes, and Cygwin does not support it:

$ mkfifo fifo
$ exec 5<>fifo
$ echo hi >&5
bash: echo: write error: Communication error on send

Furthermore, versions of dash before 0.5.6 mistakenly truncate regular files when using ‘<>’:

$ echo a > file
$ bash -c ': 1<>file'; cat file
a
$ dash -c ': 1<>file'; cat file
$ rm a

Solaris 10 /bin/sh executes redirected compound commands in a subshell, while other shells don’t:

$ /bin/sh -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
0
$ ksh -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
1
$ bash -c 'foo=0; { foo=1; } 2>/dev/null; echo $foo'
1

When catering to old systems, don’t redirect the same file descriptor several times, as you are doomed to failure under Ultrix.

ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995
UWS V4.4 (Rev. 11)
$ eval 'echo matter >fullness' >void
illegal io
$ eval '(echo matter >fullness)' >void
illegal io
$ (eval '(echo matter >fullness)') >void
Ambiguous output redirect.

In each case the expected result is of course fullness containing ‘matter’ and void being empty. However, this bug is probably not of practical concern to modern platforms.

Solaris 10 sh will try to optimize away a : command (even if it is redirected) in a loop after the first iteration, or in a shell function after the first call:

$ for i in 1 2 3 ; do : >x$i; done
$ ls x*
x1
$ f () { : >$1; }; f y1; f y2; f y3;
$ ls y*
y1

As a workaround, echo or eval can be used.

Don’t rely on file descriptors 0, 1, and 2 remaining closed in a subsidiary program. If any of these descriptors is closed, the operating system may open an unspecified file for the descriptor in the new process image. Posix 2008 says this may be done only if the subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does it even for ordinary programs, and the next version of Posix will allow HP-UX behavior.

If you want a file descriptor above 2 to be inherited into a child process, then you must use redirections specific to that command or a containing subshell or command group, rather than relying on exec in the shell. In ksh as well as HP-UX sh, file descriptors above 2 which are opened using ‘exec n>file’ are closed by a subsequent ‘exec’ (such as that involved in the fork-and-exec which runs a program or script):

$ echo 'echo hello >&5' >k
$ /bin/sh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
hello
$ bash -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
hello
$ ksh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t
./k[1]: 5: cannot open [Bad file number]
$ ksh -c '(ksh ./k) 5>t; cat t'
hello
$ ksh -c '{ ksh ./k; } 5>t; cat t'
hello
$ ksh -c '5>t ksh ./k; cat t
hello

Don’t rely on duplicating a closed file descriptor to cause an error. With Solaris 10 /bin/sh, failed duplication is silently ignored, which can cause unintended leaks to the original file descriptor. In this example, observe the leak to standard output:

$ bash -c 'echo hi >&3' 3>&-; echo $?
bash: 3: Bad file descriptor
1
$ /bin/sh -c 'echo hi >&3' 3>&-; echo $?
hi
0

Fortunately, an attempt to close an already closed file descriptor will portably succeed. Likewise, it is safe to use either style of ‘n<&-’ or ‘n>&-’ for closing a file descriptor, even if it doesn’t match the read/write mode that the file descriptor was opened with.

DOS variants cannot rename or remove open files, such as in ‘mv foo bar >foo’ or ‘rm foo >foo’, even though this is perfectly portable among Posix hosts.

A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to /dev/tty when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don’t now remember what it was. Both these systems are obsolete, so it’s now safe to treat file descriptors 3 and 4 like any other file descriptors.

On the other hand, you can’t portably use multi-digit file descriptors. dash and Solaris ksh don’t understand any file descriptor larger than ‘9’:

$ bash -c 'exec 10>&-'; echo $?
0
$ ksh -c 'exec 9>&-'; echo $?
0
$ ksh -c 'exec 10>&-'; echo $?
ksh[1]: exec: 10: not found
127
$ dash -c 'exec 9>&-'; echo $?
0
$ dash -c 'exec 10>&-'; echo $?
exec: 1: 10: not found
2

11.5 Signal Handling

Portable handling of signals within the shell is another major source of headaches. This is worsened by the fact that various different, mutually incompatible approaches are possible in this area, each with its distinctive merits and demerits. A detailed description of these possible approaches, as well as of their pros and cons, can be found in this article.

Solaris 10 /bin/sh automatically traps most signals by default; the shell still exits with error upon termination by one of those signals, but in such a case the exit status might be somewhat unexpected (even if allowed by POSIX, strictly speaking):

$ bash -c 'kill -1 $$'; echo $? # Will exit 128 + (signal number).
Hangup
129
$ /bin/ksh -c 'kill -15 $$'; echo $? # Likewise.
Terminated
143
$ for sig in 1 2 3 15; do
>   echo $sig:
>   /bin/sh -c "kill -$s \$\$"; echo $?
> done
signal 1:
Hangup
129
signal 2:
208
signal 3:
208
signal 15:
208

This gets even worse if one is using the POSIX “wait” interface to get details about the shell process terminations: it will result in the shell having exited normally, rather than by receiving a signal.

$ cat > foo.c <<'END'
#include <stdio.h>    /* for printf */
#include <stdlib.h>   /* for system */
#include <sys/wait.h> /* for WIF* macros */
int main(void)
{
  int status = system ("kill -15 $$");
  printf ("Terminated by signal: %s\n",
          WIFSIGNALED (status) ? "yes" : "no");
  printf ("Exited normally: %s\n",
          WIFEXITED (status) ? "yes" : "no");
  return 0;
}
END
$ cc -o foo foo.c
$ ./a.out # On GNU/Linux
Terminated by signal: no
Exited normally: yes
$ ./a.out # On Solaris 10
Terminated by signal: yes
Exited normally: no

Various shells seem to handle SIGQUIT specially: they ignore it even if it is not blocked, and even if the shell is not running interactively (in fact, even if the shell has no attached tty); among these shells are at least Bash (from version 2 onward), Zsh 4.3.12, Solaris 10 /bin/ksh and /usr/xpg4/bin/sh, and AT&T ksh93 (2011). Still, SIGQUIT seems to be trappable quite portably within all these shells. OTOH, some other shells doesn’t special-case the handling of SIGQUIT; among these shells are at least pdksh 5.2.14, Solaris 10 and NetBSD 5.1 /bin/sh, and the Almquist Shell 0.5.5.1.

Some shells (especially Korn shells and derivatives) might try to propagate to themselves a signal that has killed a child process; this is not a bug, but a conscious design choice (although its overall value might be debatable). The exact details of how this is attained vary from shell to shell. For example, upon running perl -e 'kill 2, $$', after the perl process has been interrupted, AT&T ksh93 (2011) will proceed to send itself a SIGINT, while Solaris 10 /bin/ksh and /usr/xpg4/bin/sh will proceed to exit with status 130 (i.e., 128 + 2). In any case, if there is an active trap associated with SIGINT, those shells will correctly execute it.

Some Korn shells, when a child process die due receiving a signal with signal number n, can leave in ‘$?’ an exit status of 256+n instead of the more common 128+n. Observe the difference between AT&T ksh93 (2011) and bash 4.1.5 on Debian:

$ /bin/ksh -c 'sh -c "kill -1 \$\$"; echo $?'
/bin/ksh: line 1: 7837: Hangup
257
$ /bin/bash -c 'sh -c "kill -1 \$\$"; echo $?'
/bin/bash: line 1:  7861 Hangup        (sh -c "kill -1 \$\$")
129

This ksh behavior is allowed by POSIX, if implemented with due care; see this Austin Group discussion for more background. However, if it is not implemented with proper care, such a behavior might cause problems in some corner cases. To see why, assume we have a “wrapper” script like this:

#!/bin/sh
# Ignore some signals in the shell only, not in its child processes.
trap : 1 2 13 15
wrapped_command "$@"
ret=$?
other_command
exit $ret

If wrapped_command is interrupted by a SIGHUP (which has signal number 1), ret will be set to 257. Unless the exit shell builtin is smart enough to understand that such a value can only have originated from a signal, and adjust the final wait status of the shell appropriately, the value 257 will just get truncated to 1 by the closing exit call, so that a caller of the script will have no way to determine that termination by a signal was involved. Observe the different behavior of AT&T ksh93 (2011) and bash 4.1.5 on Debian:

$ cat foo.sh
#!/bin/sh
sh -c 'kill -1 $$'
ret=$?
echo $ret
exit $ret
$ /bin/ksh foo.sh; echo $?
foo.sh: line 2: 12479: Hangup
257
1
$ /bin/bash foo.sh; echo $?
foo.sh: line 2: 12487 Hangup        (sh -c 'kill -1 $$')
129
129

11.6 File System Conventions

Autoconf uses shell-script processing extensively, so the file names that it processes should not contain characters that are special to the shell. Special characters include space, tab, newline, NUL, and the following:

" # $ & ' ( ) * ; < = > ? [ \ ` |

Also, file names should not begin with ‘~’ or ‘-’, and should contain neither ‘-’ immediately after ‘/’ nor ‘~’ immediately after ‘:’. On Posix-like platforms, directory names should not contain ‘:’, as this runs afoul of ‘:’ used as the path separator.

These restrictions apply not only to the files that you distribute, but also to the absolute file names of your source, build, and destination directories.

On some Posix-like platforms, ‘!’ and ‘^’ are special too, so they should be avoided.

Posix lets implementations treat leading // specially, but requires leading /// and beyond to be equivalent to /. Most Unix variants treat // like /. However, some treat // as a “super-root” that can provide access to files that are not otherwise reachable from /. The super-root tradition began with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin has revived it.

While autoconf and friends are usually run on some Posix variety, they can be used on other systems, most notably DOS variants. This impacts several assumptions regarding file names.

For example, the following code:

case $foo_dir in
  /*) # Absolute
     ;;
  *)
     foo_dir=$dots$foo_dir ;;
esac

fails to properly detect absolute file names on those systems, because they can use a drivespec, and usually use a backslash as directory separator. If you want to be portable to DOS variants (at the price of rejecting valid but oddball Posix file names like a:\b), you can check for absolute file names like this:

case $foo_dir in
  [\\/]* | ?:[\\/]* ) # Absolute
     ;;
  *)
     foo_dir=$dots$foo_dir ;;
esac

Make sure you quote the brackets if appropriate and keep the backslash as first character. See Limitations of Shell Builtins.

Also, because the colon is used as part of a drivespec, these systems don’t use it as path separator. When creating or accessing paths, you can use the PATH_SEPARATOR output variable instead. configure sets this to the appropriate value for the build system (‘:’ or ‘;’) when it starts up.

File names need extra care as well. While DOS variants that are Posixy enough to run autoconf (such as DJGPP) are usually able to handle long file names properly, there are still limitations that can seriously break packages. Several of these issues can be easily detected by the doschk package.

A short overview follows; problems are marked with SFN/LFN to indicate where they apply: SFN means the issues are only relevant to plain DOS, not to DOS under Microsoft Windows variants, while LFN identifies problems that exist even under Microsoft Windows variants.

No multiple dots (SFN)

DOS cannot handle multiple dots in file names. This is an especially important thing to remember when building a portable configure script, as autoconf uses a .in suffix for template files.

This is perfectly OK on Posix variants:

AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([source.c foo.bar])
AC_OUTPUT

but it causes problems on DOS, as it requires ‘config.h.in’, ‘source.c.in’ and ‘foo.bar.in’. To make your package more portable to DOS-based environments, you should use this instead:

AC_CONFIG_HEADERS([config.h:config.hin])
AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
AC_OUTPUT
No leading dot (SFN)

DOS cannot handle file names that start with a dot. This is usually not important for autoconf.

Case insensitivity (LFN)

DOS is case insensitive, so you cannot, for example, have both a file called ‘INSTALL’ and a directory called ‘install’. This also affects make; if there’s a file called ‘INSTALL’ in the directory, ‘make install’ does nothing (unless the ‘install’ target is marked as PHONY).

The 8+3 limit (SFN)

Because the DOS file system only stores the first 8 characters of the file name and the first 3 of the extension, those must be unique. That means that foobar-part1.c, foobar-part2.c and foobar-prettybird.c all resolve to the same file name (FOOBAR-P.C). The same goes for foo.bar and foo.bartender.

The 8+3 limit is not usually a problem under Microsoft Windows, as it uses numeric tails in the short version of file names to make them unique. However, a registry setting can turn this behavior off. While this makes it possible to share file trees containing long file names between SFN and LFN environments, it also means the above problem applies there as well.

Invalid characters (LFN)

Some characters are invalid in DOS file names, and should therefore be avoided. In a LFN environment, these are ‘/’, ‘\’, ‘?’, ‘*’, ‘:’, ‘<’, ‘>’, ‘|’ and ‘"’. In a SFN environment, other characters are also invalid. These include ‘+’, ‘,’, ‘[’ and ‘]’.

Invalid names (LFN)

Some DOS file names are reserved, and cause problems if you try to use files with those names. These names include CON, AUX, COM1, COM2, COM3, COM4, LPT1, LPT2, LPT3, NUL, and PRN. File names are case insensitive, so even names like aux/config.guess are disallowed.


11.7 Shell Pattern Matching

Nowadays portable patterns can use negated character classes like ‘[!-aeiou]’. The older syntax ‘[^-aeiou]’ is supported by some shells but not others; hence portable scripts should never use ‘^’ as the first character of a bracket pattern.

Outside the C locale, patterns like ‘[a-z]’ are problematic since they may match characters that are not lower-case letters.


11.8 Shell Substitutions

Contrary to a persistent urban legend, the Bourne shell does not systematically split variables and back-quoted expressions, in particular on the right-hand side of assignments and in the argument of case. For instance, the following code:

case "$given_srcdir" in
.)  top_srcdir="`echo "$dots" | sed 's|/$||'`" ;;
*)  top_srcdir="$dots$given_srcdir" ;;
esac

is more readable when written as:

case $given_srcdir in
.)  top_srcdir=`echo "$dots" | sed 's|/$||'` ;;
*)  top_srcdir=$dots$given_srcdir ;;
esac

and in fact it is even more portable: in the first case of the first attempt, the computation of top_srcdir is not portable, since not all shells properly understand "`…"…"…`", for example Solaris 10 ksh:

$ foo="`echo " bar" | sed 's, ,,'`"
ksh: : cannot execute
ksh: bar | sed 's, ,,': cannot execute

Posix does not specify behavior for this sequence. On the other hand, behavior for "`…\"…\"…`" is specified by Posix, but in practice, not all shells understand it the same way: pdksh 5.2.14 prints spurious quotes when in Posix mode:

$ echo "`echo \"hello\"`"
hello
$ set -o posix
$ echo "`echo \"hello\"`"
"hello"

There is just no portable way to use double-quoted strings inside double-quoted back-quoted expressions (pfew!).

Bash 4.1 has a bug where quoted empty strings adjacent to unquoted parameter expansions are elided during word splitting. Meanwhile, zsh does not perform word splitting except when in Bourne compatibility mode. In the example below, the correct behavior is to have five arguments to the function, and exactly two spaces on either side of the middle ‘-’, since word splitting collapses multiple spaces in ‘$f’ but leaves empty arguments intact.

$ bash -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
3- - -
$ ksh -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
5-  -  -
$ zsh -c 'n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
3-   -   -
$ zsh -c 'emulate sh;
> n() { echo "$#$@"; }; f="  -  "; n - ""$f"" -'
5-  -  -

You can work around this by doing manual word splitting, such as using ‘"$str" $list’ rather than ‘"$str"$list’.

There are also portability pitfalls with particular expansions:

$@

One of the most famous shell-portability issues is related to ‘"$@"’. When there are no positional arguments, Posix says that ‘"$@"’ is supposed to be equivalent to nothing, but the original Unix version 7 Bourne shell treated it as equivalent to ‘""’ instead, and this behavior survives in later implementations like Digital Unix 5.0.

The traditional way to work around this portability problem is to use ‘${1+"$@"}’. Unfortunately this method does not work with Zsh (3.x and 4.x), which is used on Mac OS X. When emulating the Bourne shell, Zsh performs word splitting on ‘${1+"$@"}’:

zsh $ emulate sh
zsh $ for i in "$@"; do echo $i; done
Hello World
!
zsh $ for i in ${1+"$@"}; do echo $i; done
Hello
World
!

Zsh handles plain ‘"$@"’ properly, but we can’t use plain ‘"$@"’ because of the portability problems mentioned above. One workaround relies on Zsh’s “global aliases” to convert ‘${1+"$@"}’ into ‘"$@"’ by itself:

test ${ZSH_VERSION+y} && alias -g '${1+"$@"}'='"$@"'

Zsh only recognizes this alias when a shell word matches it exactly; ‘"foo"${1+"$@"}’ remains subject to word splitting. Since this case always yields at least one shell word, use plain ‘"$@"’.

A more conservative workaround is to avoid ‘"$@"’ if it is possible that there may be no positional arguments. For example, instead of:

cat conftest.c "$@"

you can use this instead:

case $# in
0) cat conftest.c;;
*) cat conftest.c "$@";;
esac

Autoconf macros often use the set command to update ‘$@’, so if you are writing shell code intended for configure you should not assume that the value of ‘$@’ persists for any length of time.

${10}

The 10th, 11th, … positional parameters can be accessed only after a shift. The 7th Edition shell reported an error if given ${10}, and Solaris 10 /bin/sh still acts that way:

$ set 1 2 3 4 5 6 7 8 9 10
$ echo ${10}
bad substitution

Conversely, not all shells obey the Posix rule that when braces are omitted, multiple digits beyond a ‘$’ imply the single-digit positional parameter expansion concatenated with the remaining literal digits. To work around the issue, you must use braces.

$ bash -c 'set a b c d e f g h i j; echo $10 ${1}0'
a0 a0
$ dash -c 'set a b c d e f g h i j; echo $10 ${1}0'
j a0
${var:-value}
${var:=value}
${var:?value}
${var:+value}

Old BSD shells, including the Ultrix sh, don’t accept the colon for any shell substitution, and complain and die. Similarly for ${var:=value}, ${var:?value}, etc. However, all shells that support functions allow the use of colon in shell substitution, and since m4sh requires functions, you can portably use null variable substitution patterns in configure scripts.

${var-value}
${var:-value}
${var=value}
${var:=value}
${var?value}
${var:?value}
${var+value}
${var:+value}

When using ‘${var-value}’ or similar notations that modify a parameter expansion, Posix requires that value must be a single shell word, which can contain quoted strings but cannot contain unquoted spaces. If this requirement is not met Solaris 10 /bin/sh sometimes complains, and anyway the behavior is not portable.

$ /bin/sh -c 'echo ${a-b c}'
/bin/sh: bad substitution
$ /bin/sh -c 'echo ${a-'\''b c'\''}'
b c
$ /bin/sh -c 'echo "${a-b c}"'
b c
$ /bin/sh -c 'cat <<EOF
${a-b c}
EOF
b c

Most shells treat the special parameters * and @ as being unset if there are no positional parameters. However, some shells treat them as being set to the empty string. Posix does not clearly specify either behavior.

$ bash -c 'echo "* is ${*-unset}."'
* is unset.
$ dash -c 'echo "* is ${*-unset}."'
* is .

According to Posix, if an expansion occurs inside double quotes, then the use of unquoted double quotes within value is unspecified, and any single quotes become literal characters; in that case, escaping must be done with backslash. Likewise, the use of unquoted here-documents is a case where double quotes have unspecified results:

$ /bin/sh -c 'echo "${a-"b  c"}"'
/bin/sh: bad substitution
$ ksh -c 'echo "${a-"b  c"}"'
b c
$ bash -c 'echo "${a-"b  c"}"'
b  c
$ /bin/sh -c 'a=; echo ${a+'\''b  c'\''}'
b  c
$ /bin/sh -c 'a=; echo "${a+'\''b  c'\''}"'
'b  c'
$ /bin/sh -c 'a=; echo "${a+\"b  c\"}"'
"b  c"
$ /bin/sh -c 'a=; echo "${a+b  c}"'
b  c
$ /bin/sh -c 'cat <<EOF
${a-"b  c"}
EOF'
"b  c"
$ /bin/sh -c 'cat <<EOF
${a-'b  c'}
EOF'
'b  c'
$ bash -c 'cat <<EOF
${a-"b  c"}
EOF'
b  c
$ bash -c 'cat <<EOF
${a-'b  c'}
EOF'
'b  c'

Perhaps the easiest way to work around quoting issues in a manner portable to all shells is to place the results in a temporary variable, then use ‘$t’ as the value, rather than trying to inline the expression needing quoting.

$ /bin/sh -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
b  c"'}\
$ ksh -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
b  c"'}\
$ bash -c 't="b  c\"'\''}\\"; echo "${a-$t}"'
b  c"'}\
${var=value}

When using ‘${var=value}’ to assign a default value to var, remember that even though the assignment to var does not undergo file name expansion, the result of the variable expansion does unless the expansion occurred within double quotes. In particular, when using : followed by unquoted variable expansion for the side effect of setting a default value, if the final value of ‘$var’ contains any globbing characters (either from value or from prior contents), the shell has to spend time performing file name expansion and field splitting even though those results will not be used. Therefore, it is a good idea to consider double quotes when performing default initialization; while remembering how this impacts any quoting characters appearing in value.

$ time bash -c ': "${a=/usr/bin/*}"; echo "$a"'
/usr/bin/*

real	0m0.005s
user	0m0.002s
sys	0m0.003s
$ time bash -c ': ${a=/usr/bin/*}; echo "$a"'
/usr/bin/*

real	0m0.039s
user	0m0.026s
sys	0m0.009s
$ time bash -c 'a=/usr/bin/*; : ${a=noglob}; echo "$a"'
/usr/bin/*

real	0m0.031s
user	0m0.020s
sys	0m0.010s

$ time bash -c 'a=/usr/bin/*; : "${a=noglob}"; echo "$a"'
/usr/bin/*

real	0m0.006s
user	0m0.002s
sys	0m0.003s

As with ‘+’ and ‘-’, value must be a single shell word, otherwise some shells, such as Solaris 10 /bin/sh or on Digital Unix V 5.0, die because of a “bad substitution”. Meanwhile, Posix requires that with ‘=’, quote removal happens prior to the assignment, and the expansion be the final contents of var without quoting (and thus subject to field splitting), in contrast to the behavior with ‘-’ passing the quoting through to the final expansion. However, bash 4.1 does not obey this rule.

$ ksh -c 'echo ${var-a\ \ b}'
a  b
$ ksh -c 'echo ${var=a\ \ b}'
a b
$ bash -c 'echo ${var=a\ \ b}'
a  b

Finally, Posix states that when mixing ‘${a=b}’ with regular commands, it is unspecified whether the assignments affect the parent shell environment. It is best to perform assignments independently from commands, to avoid the problems demonstrated in this example:

$ bash -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
+b+b+
-b-
$ /bin/sh -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
++b+
--
$ ksh -c 'x= y=${x:=b} sh -c "echo +\$x+\$y+";echo -$x-'
+b+b+
--
${var=value}

Solaris 10 /bin/sh has a frightening bug in its handling of literal assignments. Imagine you need set a variable to a string containing ‘}’. This ‘}’ character confuses Solaris 10 /bin/sh when the affected variable was already set. This bug can be exercised by running:

$ unset foo
$ foo=${foo='}'}
$ echo $foo
}
$ foo=${foo='}'   # no error; this hints to what the bug is
$ echo $foo
}
$ foo=${foo='}'}
$ echo $foo
}}
 ^ ugh!

It seems that ‘}’ is interpreted as matching ‘${’, even though it is enclosed in single quotes. The problem doesn’t happen using double quotes, or when using a temporary variable holding the problematic string.

${var=expanded-value}

On Ultrix, running

default="yu,yaa"
: ${var="$default"}

sets var to ‘M-yM-uM-,M-yM-aM-a’, i.e., the 8th bit of each char is set. You don’t observe the phenomenon using a simple ‘echo $var’ since apparently the shell resets the 8th bit when it expands $var. Here are two means to make this shell confess its sins:

$ cat -v <<EOF
$var
EOF

and

$ set | grep '^var=' | cat -v

One classic incarnation of this bug is:

default="a b c"
: ${list="$default"}
for c in $list; do
  echo $c
done

You’ll get ‘a b c’ on a single line. Why? Because there are no spaces in ‘$list’: there are ‘M- ’, i.e., spaces with the 8th bit set, hence no IFS splitting is performed!!!

One piece of good news is that Ultrix works fine with ‘: ${list=$default}’; i.e., if you don’t quote. The bad news is then that QNX 4.25 then sets list to the last item of default!

The portable way out consists in using a double assignment, to switch the 8th bit twice on Ultrix:

list=${list="$default"}

…but beware of the ‘}’ bug from Solaris 10 (see above). For safety, use:

test ${var+y} || var={value}
${#var}
${var%word}
${var%%word}
${var#word}
${var##word}

Posix requires support for these usages, but they do not work with many traditional shells, e.g., Solaris 10 /bin/sh.

Also, pdksh 5.2.14 mishandles some word forms. For example if ‘$1’ is ‘a/b’ and ‘$2’ is ‘a’, then ‘${1#$2}’ should yield ‘/b’, but with pdksh it yields the empty string.

`commands`

Posix requires shells to trim all trailing newlines from command output before substituting it, so assignments like ‘dir=`echo "$file" | tr a A`’ do not work as expected if ‘$file’ ends in a newline.

While in general it makes no sense, do not substitute a single builtin with side effects, because Ash 0.2, trying to optimize, does not fork a subshell to perform the command.

For instance, if you wanted to check that cd is silent, do not use ‘test -z "`cd /`"’ because the following can happen:

$ pwd
/tmp
$ test -z "`cd /`" && pwd
/

The result of ‘foo=`exit 1`’ is left as an exercise to the reader.

The MSYS shell leaves a stray byte in the expansion of a double-quoted command substitution of a native program, if the end of the substitution is not aligned with the end of the double quote. This may be worked around by inserting another pair of quotes:

$ echo "`printf 'foo\r\n'` bar" > broken
$ echo "`printf 'foo\r\n'`"" bar" | cmp - broken
- broken differ: char 4, line 1

Upon interrupt or SIGTERM, some shells may abort a command substitution, replace it with a null string, and wrongly evaluate the enclosing command before entering the trap or ending the script. This can lead to spurious errors:

$ sh -c 'if test `sleep 5; echo hi` = hi; then echo yes; fi'
$ ^C
sh: test: hi: unexpected operator/operand

You can avoid this by assigning the command substitution to a temporary variable:

$ sh -c 'res=`sleep 5; echo hi`
         if test "x$res" = xhi; then echo yes; fi'
$ ^C
$(commands)

This construct is meant to replace ‘`commands`’, and it has most of the problems listed under `commands`.

This construct can be nested while this is impossible to do portably with back quotes. Although it is almost universally supported, unfortunately Solaris 10 and earlier releases lack it:

$ showrev -c /bin/sh | grep version
Command version: SunOS 5.10 Generic 142251-02 Sep 2010
$ echo $(echo blah)
syntax error: `(' unexpected

nor does IRIX 6.5’s Bourne shell:

$ uname -a
IRIX firebird-image 6.5 07151432 IP22
$ echo $(echo blah)
$(echo blah)

If you do use ‘$(commands)’, make sure that the commands do not start with a parenthesis, as that would cause confusion with a different notation ‘$((expression))’ that in modern shells is an arithmetic expression not a command. To avoid the confusion, insert a space between the two opening parentheses.

Avoid commands that contain unbalanced parentheses in here-documents, comments, or case statement patterns, as many shells mishandle them. For example, Bash 3.1, ‘ksh88’, pdksh 5.2.14, and Zsh 4.2.6 all mishandle the following valid command:

echo $(case x in x) echo hello;; esac)
$((expression))

Arithmetic expansion is not portable as some shells (most notably Solaris 10 /bin/sh) don’t support it.

Among shells that do support ‘$(( ))’, not all of them obey the Posix rule that octal and hexadecimal constants must be recognized:

$ bash -c 'echo $(( 010 + 0x10 ))'
24
$ zsh -c 'echo $(( 010 + 0x10 ))'
26
$ zsh -c 'emulate sh; echo $(( 010 + 0x10 ))'
24
$ pdksh -c 'echo $(( 010 + 0x10 ))'
pdksh:  010 + 0x10 : bad number `0x10'
$ pdksh -c 'echo $(( 010 ))'
10

When it is available, using arithmetic expansion provides a noticeable speedup in script execution; but testing for support requires eval to avoid syntax errors. The following construct is used by AS_VAR_ARITH to provide arithmetic computation when all arguments are provided in decimal and without a leading zero, and all operators are properly quoted and appear as distinct arguments:

if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then
  eval 'func_arith ()
  {
    func_arith_result=$(( $* ))
  }'
else
  func_arith ()
  {
    func_arith_result=`expr "$@"`
  }
fi
func_arith 1 + 1
foo=$func_arith_result
^

Always quote ‘^’, otherwise traditional shells such as /bin/sh on Solaris 10 treat this like ‘|’.


11.9 Assignments

When setting several variables in a row, be aware that the order of the evaluation is undefined. For instance ‘foo=1 foo=2; echo $foo’ gives ‘1’ with Solaris 10 /bin/sh, but ‘2’ with Bash. You must use ‘;’ to enforce the order: ‘foo=1; foo=2; echo $foo’.

Don’t rely on the following to find subdir/program:

PATH=subdir$PATH_SEPARATOR$PATH program

as this does not work with Zsh 3.0.6. Use something like this instead:

(PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)

Don’t rely on the exit status of an assignment: Ash 0.2 does not change the status and propagates that of the last statement:

$ false || foo=bar; echo $?
1
$ false || foo=`:`; echo $?
0

and to make things even worse, QNX 4.25 just sets the exit status to 0 in any case:

$ foo=`exit 1`; echo $?
0

To assign default values, follow this algorithm:

  1. If the default value is a literal and does not contain any closing brace, use:
    : "${var='my literal'}"
    
  2. If the default value contains no closing brace, has to be expanded, and the variable being initialized is not intended to be IFS-split (i.e., it’s not a list), then use:
    : ${var="$default"}
    
  3. If the default value contains no closing brace, has to be expanded, and the variable being initialized is intended to be IFS-split (i.e., it’s a list), then use:
    var=${var="$default"}
    
  4. If the default value contains a closing brace, then use:
    test ${var+y} || var="has a '}'"
    

In most cases ‘var=${var="$default"}’ is fine, but in case of doubt, just use the last form. See Shell Substitutions, items ‘${var:-value}’ and ‘${var=value}’ for the rationale.


11.10 Parentheses in Shell Scripts

Beware of two opening parentheses in a row, as many shell implementations treat them specially, and Posix says that a portable script cannot use ‘((’ outside the ‘$((’ form used for shell arithmetic. In traditional shells, ‘((cat))’ behaves like ‘(cat)’; but many shells, including Bash and the Korn shell, treat ‘((cat))’ as an arithmetic expression equivalent to ‘let "cat"’, and may or may not report an error when they detect that ‘cat’ is not a number. As another example, ‘pdksh’ 5.2.14 does not treat the following code as a traditional shell would:

if ((true) || false); then
  echo ok
fi

To work around this problem, insert a space between the two opening parentheses. There is a similar problem and workaround with ‘$((’; see Shell Substitutions.


11.11 Slashes in Shell Scripts

Unpatched Tru64 5.1 sh omits the last slash of command-line arguments that contain two trailing slashes:

$ echo / // /// //// .// //.
/ / // /// ./ //.
$ x=//
$ eval "echo \$x"
/
$ set -x
$ echo abc | tr -t ab //
+ echo abc
+ tr -t ab /
/bc

Unpatched Tru64 4.0 sh adds a slash after ‘"$var"’ if the variable is empty and the second double-quote is followed by a word that begins and ends with slash:

$ sh -xc 'p=; echo "$p"/ouch/'
p=
+ echo //ouch/
//ouch/

However, our understanding is that patches are available, so perhaps it’s not worth worrying about working around these horrendous bugs.


11.12 Special Shell Variables

Some shell variables should not be used, since they can have a deep influence on the behavior of the shell. In order to recover a sane behavior from the shell, some variables should be unset; M4sh takes care of this and provides fallback values, whenever needed, to cater for a very old /bin/sh that does not support unset. (see Portable Shell Programming).

As a general rule, shell variable names containing a lower-case letter are safe; you can define and use these variables without worrying about their effect on the underlying system, and without worrying about whether the shell changes them unexpectedly. (The exception is the shell variable status, as described below.)

Here is a list of names that are known to cause trouble. This list is not exhaustive, but you should be safe if you avoid the name status and names containing only upper-case letters and underscores.

?

Not all shells correctly reset ‘$?’ after conditionals (see Limitations of Shell Builtins). Not all shells manage ‘$?’ correctly in shell functions (see Shell Functions) or in traps (see Limitations of Shell Builtins). Not all shells reset ‘$?’ to zero after an empty command.

$ bash -c 'false; $empty; echo $?'
0
$ zsh -c 'false; $empty; echo $?'
1
_

Many shells reserve ‘$_’ for various purposes, e.g., the name of the last command executed.

BIN_SH

In Tru64, if BIN_SH is set to xpg4, subsidiary invocations of the standard shell conform to Posix.

CDPATH

When this variable is set it specifies a list of directories to search when invoking cd with a relative file name that did not start with ‘./’ or ‘../’. Posix 1003.1-2001 says that if a nonempty directory name from CDPATH is used successfully, cd prints the resulting absolute file name. Unfortunately this output can break idioms like ‘abs=`cd src && pwd`’ because abs receives the name twice. Also, many shells do not conform to this part of Posix; for example, zsh prints the result only if a directory name other than . was chosen from CDPATH.

In practice the shells that have this problem also support unset, so you can work around the problem as follows:

(unset CDPATH) >/dev/null 2>&1 && unset CDPATH

You can also avoid output by ensuring that your directory name is absolute or anchored at ‘./’, as in ‘abs=`cd ./src && pwd`’.

Configure scripts use M4sh, which automatically unsets CDPATH if possible, so you need not worry about this problem in those scripts.

CLICOLOR_FORCE

When this variable is set, some implementations of tools like ls attempt to add color to their output via terminal escape sequences, even when the output is not directed to a terminal, and can thus cause spurious failures in scripts. Configure scripts use M4sh, which automatically unsets this variable.

DUALCASE

In the MKS shell, case statements and file name generation are case-insensitive unless DUALCASE is nonzero. Autoconf-generated scripts export this variable when they start up.

ENV
MAIL
MAILPATH
PS1
PS2
PS4

These variables should not matter for shell scripts, since they are supposed to affect only interactive shells. However, at least one shell (the pre-3.0 UWIN Korn shell) gets confused about whether it is interactive, which means that (for example) a PS1 with a side effect can unexpectedly modify ‘$?’. To work around this bug, M4sh scripts (including configure scripts) do something like this:

(unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH
PS1='$ '
PS2='> '
PS4='+ '

(actually, there is some complication due to bugs in unset; see Limitations of Shell Builtins).

FPATH

The Korn shell uses FPATH to find shell functions, so avoid FPATH in portable scripts. FPATH is consulted after PATH, but you still need to be wary of tests that use PATH to find whether a command exists, since they might report the wrong result if FPATH is also set.

GREP_OPTIONS

When this variable is set, some implementations of grep honor these options, even if the options include direction to enable colored output via terminal escape sequences, and the result can cause spurious failures when the output is not directed to a terminal. Configure scripts use M4sh, which automatically unsets this variable.

IFS

Long ago, shell scripts inherited IFS from the environment, but this caused many problems so modern shells ignore any environment settings for IFS.

Don’t set the first character of IFS to backslash. Indeed, Bourne shells use the first character (backslash) when joining the components in ‘"$@"’ and some shells then reinterpret (!) the backslash escapes, so you can end up with backspace and other strange characters.

The proper value for IFS (in regular code, not when performing splits) is ‘SPCTABRET’. The first character is especially important, as it is used to join the arguments in ‘$*’; however, note that traditional shells, but also bash-2.04, fail to adhere to this and join with a space anyway.

M4sh guarantees that IFS will have the default value at the beginning of a script, and many macros within autoconf rely on this setting. It is okay to use blocks of shell code that temporarily change the value of IFS in order to split on another character, but remember to restore it before expanding further macros.

Unsetting IFS instead of resetting it to the default sequence is not suggested, since code that tries to save and restore the variable’s value will incorrectly reset it to an empty value, thus disabling field splitting:

unset IFS
# default separators used for field splitting

save_IFS=$IFS
IFS=:
# ...
IFS=$save_IFS
# no field splitting performed
LANG
LC_ALL
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LC_MONETARY
LC_NUMERIC
LC_TIME

You should set all these variables to ‘C’ because so much configuration code assumes the C locale and Posix requires that locale environment variables be set to ‘C’ if the C locale is desired; configure scripts and M4sh do that for you. Export these variables after setting them.

LANGUAGE

LANGUAGE is not specified by Posix, but it is a GNU extension that overrides LC_ALL in some cases, so you (or M4sh) should set it too.

LC_ADDRESS
LC_IDENTIFICATION
LC_MEASUREMENT
LC_NAME
LC_PAPER
LC_TELEPHONE

These locale environment variables are GNU extensions. They are treated like their Posix brethren (LC_COLLATE, etc.) as described above.

LINENO

Most modern shells provide the current line number in LINENO. Its value is the line number of the beginning of the current command. M4sh, and hence Autoconf, attempts to execute configure with a shell that supports LINENO. If no such shell is available, it attempts to implement LINENO with a Sed prepass that replaces each instance of the string $LINENO (not followed by an alphanumeric character) with the line’s number. In M4sh scripts you should execute AS_LINENO_PREPARE so that these workarounds are included in your script; configure scripts do this automatically in AC_INIT.

You should not rely on LINENO within eval or shell functions, as the behavior differs in practice. The presence of a quoted newline within simple commands can alter which line number is used as the starting point for $LINENO substitutions within that command. Also, the possibility of the Sed prepass means that you should not rely on $LINENO when quoted, when in here-documents, or when line continuations are used. Subshells should be OK, though. In the following example, lines 1, 9, and 14 are portable, but the other instances of $LINENO do not have deterministic values:

$ cat lineno
echo 1. $LINENO
echo "2. $LINENO
3. $LINENO"
cat <<EOF
5. $LINENO
6. $LINENO
7. \$LINENO
EOF
( echo 9. $LINENO )
eval 'echo 10. $LINENO'
eval 'echo 11. $LINENO
echo 12. $LINENO'
echo 13. '$LINENO'
echo 14. $LINENO '
15.' $LINENO
f () { echo $1 $LINENO;
echo $1 $LINENO }
f 18.
echo 19. \
$LINENO
$ bash-3.2 ./lineno
1. 1
2. 3
3. 3
5. 4
6. 4
7. $LINENO
9. 9
10. 10
11. 12
12. 13
13. $LINENO
14. 14
15. 14
18. 16
18. 17
19. 19
$ zsh-4.3.4 ./lineno
1. 1
2. 2
3. 2
5. 4
6. 4
7. $LINENO
9. 9
10. 1
11. 1
12. 2
13. $LINENO
14. 14
15. 14
18. 0
18. 1
19. 19
$ pdksh-5.2.14 ./lineno
1. 1
2. 2
3. 2
5. 4
6. 4
7. $LINENO
9. 9
10. 0
11. 0
12. 0
13. $LINENO
14. 14
15. 14
18. 16
18. 17
19. 19
$ sed '=' <lineno |
>   sed '
>     N
>     s,$,-,
>     t loop
>     :loop
>     s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,
>     t loop
>     s,-$,,
>     s,^[0-9]*\n,,
>   ' |
>   sh
1. 1
2. 2
3. 3
5. 5
6. 6
7. \7
9. 9
10. 10
11. 11
12. 12
13. 13
14. 14
15. 15
18. 16
18. 17
19. 20

In particular, note that config.status (and any other subsidiary script created by AS_INIT_GENERATED) might report line numbers relative to the parent script as a result of the potential Sed pass.

NULLCMD

When executing the command ‘>foo’, zsh executes ‘$NULLCMD >foo’ unless it is operating in Bourne shell compatibility mode and the zsh version is newer than 3.1.6-dev-18. If you are using an older zsh and forget to set NULLCMD, your script might be suspended waiting for data on its standard input.

options

For zsh 4.3.10, options is treated as an associative array even after emulate sh, so it should not be used.

PATH_SEPARATOR

On DJGPP systems, the PATH_SEPARATOR environment variable can be set to either ‘:’ or ‘;’ to control the path separator Bash uses to set up certain environment variables (such as PATH). You can set this variable to ‘;’ if you want configure to use ‘;’ as a separator; this might be useful if you plan to use non-Posix shells to execute files. See File System Conventions, for more information about PATH_SEPARATOR.

POSIXLY_CORRECT

In the GNU environment, exporting POSIXLY_CORRECT with any value (even empty) causes programs to try harder to conform to Posix. Autoconf does not directly manipulate this variable, but bash ties the shell variable POSIXLY_CORRECT to whether the script is running in Posix mode. Therefore, take care when exporting or unsetting this variable, so as not to change whether bash is in Posix mode.

$ bash --posix -c 'set -o | grep posix
> unset POSIXLY_CORRECT
> set -o | grep posix'
posix           on
posix           off
PWD

Posix 1003.1-2001 requires that cd and pwd must update the PWD environment variable to point to the logical name of the current directory, but traditional shells do not support this. This can cause confusion if one shell instance maintains PWD but a subsidiary and different shell does not know about PWD and executes cd; in this case PWD points to the wrong directory. Use ‘`pwd`’ rather than ‘$PWD’.

RANDOM

Many shells provide RANDOM, a variable that returns a different integer each time it is used. Most of the time, its value does not change when it is not used, but on IRIX 6.5 the value changes all the time. This can be observed by using set. It is common practice to use $RANDOM as part of a file name, but code shouldn’t rely on $RANDOM expanding to a nonempty string.

status

This variable is an alias to ‘$?’ for zsh (at least 3.1.6), hence read-only. Do not use it.


11.13 Shell Functions

Nowadays, it is difficult to find a shell that does not support shell functions at all. However, some differences should be expected.

When declaring a shell function, you must include whitespace between the ‘)’ after the function name and the start of the compound expression, to avoid upsetting ksh. While it is possible to use any compound command, most scripts use ‘{…}’.

$ /bin/sh -c 'a(){ echo hi;}; a'
hi
$ ksh -c 'a(){ echo hi;}; a'
ksh: syntax error at line 1: `}' unexpected
$ ksh -c 'a() { echo hi;}; a'
hi

Inside a shell function, you should not rely on the error status of a subshell if the last command of that subshell was exit or trap, as this triggers bugs in zsh 4.x; while Autoconf tries to find a shell that does not exhibit the bug, zsh might be the only shell present on the user’s machine.

Likewise, the state of ‘$?’ is not reliable when entering a shell function. This has the effect that using a function as the first command in a trap handler can cause problems.

$ bash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
2
2
$ ash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
0
2

DJGPP bash 2.04 has a bug in that return from a shell function which also used a command substitution causes a segmentation fault. To work around the issue, you can use return from a subshell, or ‘AS_SET_STATUS’ as last command in the execution flow of the function (see Common Shell Constructs).

Not all shells treat shell functions as simple commands impacted by ‘set -e’, for example with Solaris 10 /bin/sh:

$ bash -c 'f() { return 1; }; set -e; f; echo oops'
$ /bin/sh -c 'f() { return 1; }; set -e; f; echo oops'
oops

Shell variables and functions may share the same namespace, for example with Solaris 10 /bin/sh:

$ f () { :; }; f=; f
f: not found

For this reason, Autoconf (actually M4sh, see Programming in M4sh) uses the prefix ‘as_fn_’ for its functions.

Handling of positional parameters and shell options varies among shells. For example, Korn shells reset and restore trace output (‘set -x’) and other options upon function entry and exit. Inside a function, IRIX sh sets ‘$0’ to the function name.

It is not portable to pass temporary environment variables to shell functions. Solaris 10 /bin/sh does not see the variable. Meanwhile, not all shells follow the Posix rule that the assignment must affect the current environment in the same manner as special built-ins.

$ /bin/sh -c 'func() { echo $a;}; a=1 func; echo $a'
⇒
⇒
$ ash -c 'func() { echo $a;}; a=1 func; echo $a'
⇒1
⇒
$ bash -c 'set -o posix; func() { echo $a;}; a=1 func; echo $a'
⇒1
⇒1

Some ancient Bourne shell variants with function support did not reset ‘$i, i >= 0’, upon function exit, so effectively the arguments of the script were lost after the first function invocation. It is probably not worth worrying about these shells any more.

With AIX sh, a trap on 0 installed in a shell function triggers at function exit rather than at script exit. See Limitations of Shell Builtins.


11.14 Limitations of Shell Builtins

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any builtin or command may support options, and therefore differ in behavior with arguments starting with a dash. For instance, even the innocent ‘echo "$word"’ can give unexpected results when word starts with a dash. It is often possible to avoid this problem using ‘echo "x$word"’, taking the ‘x’ into account later in the pipe. Many of these limitations can be worked around using M4sh (see Programming in M4sh).

.

Use . only with regular files (use ‘test -f’). Bash 2.03, for instance, chokes on ‘. /dev/null’. Remember that . uses PATH if its argument contains no slashes. Also, some shells, including bash 3.2, implicitly append the current directory to this PATH search, even though Posix forbids it. So if you want to use . on a file foo in the current directory, you must use ‘. ./foo’.

Not all shells gracefully handle syntax errors within a sourced file. On one extreme, some non-interactive shells abort the entire script. On the other, zsh 4.3.10 has a bug where it fails to react to the syntax error.

$ echo 'fi' > syntax
$ bash -c '. ./syntax; echo $?'
./syntax: line 1: syntax error near unexpected token `fi'
./syntax: line 1: `fi'
1
$ ash -c '. ./syntax; echo $?'
./syntax: 1: Syntax error: "fi" unexpected
$ zsh -c '. ./syntax; echo $?'
./syntax:1: parse error near `fi'
0
!

The Unix version 7 shell did not support negating the exit status of commands with !, and this feature is still absent from some shells (e.g., Solaris 10 /bin/sh). Other shells, such as FreeBSD /bin/sh or ash, have bugs when using !:

$ sh -c '! : | :'; echo $?
1
$ ash -c '! : | :'; echo $?
0
$ sh -c '! { :; }'; echo $?
1
$ ash -c '! { :; }'; echo $?
{: not found
Syntax error: "}" unexpected
2

Shell code like this:

if ! cmp file1 file2 >/dev/null 2>&1; then
  echo files differ or trouble
fi

is therefore not portable in practice. Typically it is easy to rewrite such code, e.g.:

cmp file1 file2 >/dev/null 2>&1 ||
  echo files differ or trouble

More generally, one can always rewrite ‘! command’ as:

if command; then (exit 1); else :; fi
{...}

Bash 3.2 (and earlier versions) sometimes does not properly set ‘$?’ when failing to write redirected output of a compound command. This problem is most commonly observed with ‘{…}’; it does not occur with ‘(…)’. For example:

$ bash -c '{ echo foo; } >/bad; echo $?'
bash: line 1: /bad: Permission denied
0
$ bash -c 'while :; do echo; done >/bad; echo $?'
bash: line 1: /bad: Permission denied
0

To work around the bug, prepend ‘:;’:

$ bash -c ':;{ echo foo; } >/bad; echo $?'
bash: line 1: /bad: Permission denied
1

Posix requires a syntax error if a brace list has no contents. However, not all shells obey this rule; and on shells where empty lists are permitted, the effect on ‘$?’ is inconsistent. To avoid problems, ensure that a brace list is never empty.

$ bash -c 'false; { }; echo $?' || echo $?
bash: line 1: syntax error near unexpected token `}'
bash: line 1: `false; { }; echo $?'
2
$ zsh -c 'false; { }; echo $?' || echo $?
1
$ pdksh -c 'false; { }; echo $?' || echo $?
0
break

The use of ‘break 2’ etc. is safe.

case

You don’t need to quote the argument; no splitting is performed.

You don’t need the final ‘;;’, but you should use it.

Posix requires support for case patterns with opening parentheses like this:

case $file_name in
  (*.c) echo "C source code";;
esac

but the ( in this example is not portable to a few obsolescent Bourne shell implementations, which is a pity for those of us using tools that rely on balanced parentheses. For instance, with Solaris 10 /bin/sh:

$ case foo in (foo) echo foo;; esac
error→syntax error: `(' unexpected

The leading ‘(’ can be omitted safely. Unfortunately, there are contexts where unbalanced parentheses cause other problems, such as when using a syntax-highlighting editor that searches for the balancing counterpart, or more importantly, when using a case statement as an underquoted argument to an Autoconf macro. See Dealing with unbalanced parentheses, for trade-offs involved in various styles of dealing with unbalanced ‘)’.

Zsh handles pattern fragments derived from parameter expansions or command substitutions as though quoted:

$ pat=\?; case aa in ?$pat) echo match;; esac
$ pat=\?; case a? in ?$pat) echo match;; esac
match

Because of a bug in its fnmatch, Bash fails to properly handle backslashes in character classes:

bash-2.02$ case /tmp in [/\\]*) echo OK;; esac
bash-2.02$

This is extremely unfortunate, since you are likely to use this code to handle Posix or MS-DOS absolute file names. To work around this bug, always put the backslash first:

bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac
OK
bash-2.02$ case /tmp in [\\/]*) echo OK;; esac
OK

Many Bourne shells cannot handle closing brackets in character classes correctly.

Some shells also have problems with backslash escaping in case you do not want to match the backslash: both a backslash and the escaped character match this pattern. To work around this, specify the character class in a variable, so that quote removal does not apply afterwards, and the special characters don’t have to be backslash-escaped:

$ case '\' in [\<]) echo OK;; esac
OK
$ scanset='[<]'; case '\' in $scanset) echo OK;; esac
$

Even with this, Solaris ksh matches a backslash if the set contains any of the characters ‘|’, ‘&’, ‘(’, or ‘)’.

Conversely, Tru64 ksh (circa 2003) erroneously always matches a closing parenthesis if not specified in a character class:

$ case foo in *\)*) echo fail ;; esac
fail
$ case foo in *')'*) echo fail ;; esac
fail

Some shells, such as Ash 0.3.8, are confused by an empty case/esac:

ash-0.3.8 $ case foo in esac;
error→Syntax error: ";" unexpected (expecting ")")

Posix requires case to give an exit status of 0 if no cases match. However, /bin/sh in Solaris 10 does not obey this rule. Meanwhile, it is unclear whether a case that matches, but contains no statements, must also change the exit status to 0. The M4sh macro AS_CASE works around these inconsistencies.

$ bash -c 'case `false` in ?) ;; esac; echo $?'
0
$ /bin/sh -c 'case `false` in ?) ;; esac; echo $?'
255
cd

Posix 1003.1-2001 requires that cd must support the -L (“logical”) and -P (“physical”) options, with -L being the default. However, traditional shells do not support these options, and their cd command has the -P behavior.

Portable scripts should assume neither option is supported, and should assume neither behavior is the default. This can be a bit tricky, since the Posix default behavior means that, for example, ‘ls ..’ and ‘cd ..’ may refer to different directories if the current logical directory is a symbolic link. It is safe to use cd dir if dir contains no .. components. Also, Autoconf-generated scripts check for this problem when computing variables like ac_top_srcdir (see Performing Configuration Actions), so it is safe to cd to these variables.

Posix states that behavior is undefined if cd is given an explicit empty argument. Some shells do nothing, some change to the first entry in CDPATH, some change to HOME, and some exit the shell rather than returning an error. Unfortunately, this means that if ‘$var’ is empty, then ‘cd "$var"’ is less predictable than ‘cd $var’ (at least the latter is well-behaved in all shells at changing to HOME, although this is probably not what you wanted in a script). You should check that a directory name was supplied before trying to change locations.

See Special Shell Variables, for portability problems involving cd and the CDPATH environment variable. Also please see the discussion of the pwd command.

echo

The simple echo is probably the most surprising source of portability troubles. It is not possible to use ‘echo’ portably unless both options and escape sequences are omitted. Don’t expect any option.

Do not use backslashes in the arguments, as there is no consensus on their handling. For ‘echo '\n' | wc -l’, the sh of Solaris 10 outputs 2, but Bash and Zsh (in sh emulation mode) output 1. The problem is truly echo: all the shells understand ‘'\n'’ as the string composed of a backslash and an ‘n’. Within a command substitution, ‘echo 'string\c'’ will mess up the internal state of ksh88 on AIX 6.1 so that it will print the first character ‘s’ only, followed by a newline, and then entirely drop the output of the next echo in a command substitution.

Because of these problems, do not pass a string containing arbitrary characters to echo. For example, ‘echo "$foo"’ is safe only if you know that foo’s value cannot contain backslashes and cannot start with ‘-’.

Normally, printf is safer and easier to use than echo and echo -n. Thus, you should use printf "%s\n" instead of echo, and similarly use printf %s instead of echo -n.

Older scripts, written before printf was portable, sometimes used a here-document as a safer alternative to echo, like this:

cat <<EOF
$foo
EOF
eval

The eval command is useful in limited circumstances, e.g., using commands like ‘eval table_$key=\$value’ and ‘eval value=table_$key’ to simulate a hash table when the key is known to be alphanumeric.

You should also be wary of common bugs in eval implementations. In some shell implementations (e.g., older ash, OpenBSD 3.8 sh, pdksh v5.2.14 99/07/13.2, and zsh 4.2.5), the arguments of ‘eval’ are evaluated in a context where ‘$?’ is 0, so they exhibit behavior like this:

$ false; eval 'echo $?'
0

The correct behavior here is to output a nonzero value, but portable scripts should not rely on this.

You should not rely on LINENO within eval. See Special Shell Variables.

Note that, even though these bugs are easily avoided, eval is tricky to use on arbitrary arguments. It is obviously unwise to use ‘eval $cmd’ if the string value of ‘cmd’ was derived from an untrustworthy source. But even if the string value is valid, ‘eval $cmd’ might not work as intended, since it causes field splitting and file name expansion to occur twice, once for the eval and once for the command itself. It is therefore safer to use ‘eval "$cmd"’. For example, if cmd has the value ‘cat test?.c’, ‘eval $cmd’ might expand to the equivalent of ‘cat test;.c’ if there happens to be a file named test;.c in the current directory; and this in turn mistakenly attempts to invoke cat on the file test and then execute the command .c. To avoid this problem, use ‘eval "$cmd"’ rather than ‘eval $cmd’.

However, suppose that you want to output the text of the evaluated command just before executing it. Assuming the previous example, ‘echo "Executing: $cmd"’ outputs ‘Executing: cat test?.c’, but this output doesn’t show the user that ‘test;.c’ is the actual name of the copied file. Conversely, ‘eval "echo Executing: $cmd"’ works on this example, but it fails with ‘cmd='cat foo >bar'’, since it mistakenly replaces the contents of bar by the string ‘cat foo’. No simple, general, and portable solution to this problem is known.

exec

Posix describes several categories of shell built-ins. Special built-ins (such as exit) must impact the environment of the current shell, and need not be available through exec. All other built-ins are regular, and must not propagate variable assignments to the environment of the current shell. However, the group of regular built-ins is further distinguished by commands that do not require a PATH search (such as cd), in contrast to built-ins that are offered as a more efficient version of something that must still be found in a PATH search (such as echo). Posix is not clear on whether exec must work with the list of 17 utilities that are invoked without a PATH search, and many platforms lack an executable for some of those built-ins:

$ sh -c 'exec cd /tmp'
sh: line 0: exec: cd: not found

All other built-ins that provide utilities specified by Posix must have a counterpart executable that exists on PATH, although Posix allows exec to use the built-in instead of the executable. For example, contrast bash 3.2 and pdksh 5.2.14:

$ bash -c 'pwd --version' | head -n1
bash: line 0: pwd: --: invalid option
pwd: usage: pwd [-LP]
$ bash -c 'exec pwd --version' | head -n1
pwd (GNU coreutils) 6.10
$ pdksh -c 'exec pwd --version' | head -n1
pdksh: pwd: --: unknown option

When it is desired to avoid a regular shell built-in, the workaround is to use some other forwarding command, such as env or nice, that will ensure a path search:

$ pdksh -c 'exec true --version' | head -n1
$ pdksh -c 'nice true --version' | head -n1
true (GNU coreutils) 6.10
$ pdksh -c 'env true --version' | head -n1
true (GNU coreutils) 6.10
exit

The default value of exit is supposed to be $?; unfortunately, some shells, such as the DJGPP port of Bash 2.04, just perform ‘exit 0’.

bash-2.04$ foo=`exit 1` || echo fail
fail
bash-2.04$ foo=`(exit 1)` || echo fail
fail
bash-2.04$ foo=`(exit 1); exit` || echo fail
bash-2.04$

Using ‘exit $?’ restores the expected behavior.

Some shell scripts, such as those generated by autoconf, use a trap to clean up before exiting. If the last shell command exited with nonzero status, the trap also exits with nonzero status so that the invoker can tell that an error occurred.

Unfortunately, in some shells, such as Solaris 10 /bin/sh, an exit trap ignores the exit command’s argument. In these shells, a trap cannot determine whether it was invoked by plain exit or by exit 1. Instead of calling exit directly, use the AC_MSG_ERROR macro that has a workaround for this problem.

export

The builtin export dubs a shell variable environment variable. Each update of exported variables corresponds to an update of the environment variables. Conversely, each environment variable received by the shell when it is launched should be imported as a shell variable marked as exported.

Alas, many shells, such as Solaris 10 /bin/sh, IRIX 6.3, IRIX 5.2, AIX 4.1.5, and Digital Unix 4.0, forget to export the environment variables they receive. As a result, two variables coexist: the environment variable and the shell variable. The following code demonstrates this failure:

#!/bin/sh
echo $FOO
FOO=bar
echo $FOO
exec /bin/sh $0

when run with ‘FOO=foo’ in the environment, these shells print alternately ‘foo’ and ‘bar’, although they should print only ‘foo’ and then a sequence of ‘bar’s.

Therefore you should export again each environment variable that you update; the export can occur before or after the assignment.

Posix is not clear on whether the export of an undefined variable causes the variable to be defined with the value of an empty string, or merely marks any future definition of a variable by that name for export. Various shells behave differently in this regard:

$ sh -c 'export foo; env | grep foo'
$ ash -c 'export foo; env | grep foo'
foo=

Posix requires export to honor assignments made as arguments, but older shells do not support this, including /bin/sh in Solaris 10. Portable scripts should separate assignments and exports into different statements.

$ bash -c 'export foo=bar; echo $foo'
bar
$ /bin/sh -c 'export foo=bar; echo $foo'
/bin/sh: foo=bar: is not an identifier
$ /bin/sh -c 'export foo; foo=bar; echo $foo'
bar

Posix requires export to work with any arbitrary value for the contents of the variable being exported, as long as the total size of the environment combined with arguments doesn’t exceed ARG_MAX when executing a child process. However, some shells have extensions that involve interpreting some environment values specially, regardless of the variable name. We currently know of one case: all versions of Bash released prior to 27 September 2014 interpret an environment variable with an initial content substring of () { as an exported function definition (this is the “Shellshock” remote execution bug, CVE-2014-6271 and friends, where it was possible to exploit the function parser to cause remote code execution on child bash startup; newer versions of Bash use special environment variable names instead of values to implement the same feature).

There may be entries inherited into the environment that are not valid as shell variable names; Posix states that processes should be tolerant of these names. Some shells such as dash do this by removing those names from the environment at startup, while others such as bash hide the entry from shell access but still pass it on to child processes. While you can set such names using env for a direct child process, you cannot rely on them being preserved through an intermediate pass through the shell.

false

Don’t expect false to exit with status 1: in native Solaris /bin/false exits with status 255.

for

To loop over positional arguments, use:

for arg
do
  echo "$arg"
done

You may not leave the do on the same line as for, since some shells improperly grok:

for arg; do
  echo "$arg"
done

If you want to explicitly refer to the positional arguments, given the ‘$@’ bug (see Shell Substitutions), use:

for arg in ${1+"$@"}; do
  echo "$arg"
done

But keep in mind that Zsh, even in Bourne shell emulation mode, performs word splitting on ‘${1+"$@"}’; see Shell Substitutions, item ‘$@’, for more.

Posix requires support for a for loop with no list after in. However, Solaris 10 /bin/sh treats that as a syntax error. It is possible to work around this by providing any shell word that expands to nothing, or by ignoring an obvious sentinel.

$ /bin/sh -c 'for a in $empty; do echo hi; done'
$ /bin/sh -c 'for a in ; do echo hi; done'
/bin/sh: syntax error at line 1: `;' unexpected

This syntax problem is most frequently encountered in code that goes through several layers of expansion, such as an m4 macro or makefile variable used as a list body, where the first layer of expansion (m4 or make) can end up expanding to nothing in the version handed to the shell. In the makefile context, one common workaround is to use a shell variable rather than a make variable as the source of the list.

$ cat Makefile
list =
bad:
	@for arg in $(list); do echo $$arg; done
good:
	@list='$(list)'; for arg in $$list; do echo $$arg; done
$ make bad 2&>1 | head -n1
sh: syntax error at line 1: `;' unexpected
$ make bad list='a b'
a
b
$ make good
$ make good list='a b'
a
b

In Solaris 10 /bin/sh, when the list of arguments of a for loop starts with unquoted tokens looking like variable assignments, the loop is not executed on those tokens:

$ /bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done'
x
e=f

Thankfully, quoting the assignment-like tokens, or starting the list with other tokens (including unquoted variable expansion that results in an assignment-like result), avoids the problem, so it is easy to work around:

$ /bin/sh -c 'for v in "a=b"; do echo $v; done'
a=b
$ /bin/sh -c 'x=a=b; for v in $x c=d; do echo $v; done'
a=b
c=d
if

Using ‘!’ is not portable. Instead of:

if ! cmp -s file file.new; then
  mv file.new file
fi

use:

if cmp -s file file.new; then :; else
  mv file.new file
fi

Or, especially if the else branch is short, you can use ||. In M4sh, the AS_IF macro provides an easy way to write these kinds of conditionals:

AS_IF([cmp -s file file.new], [], [mv file.new file])

This is especially useful in other M4 macros, where the then and else branches might be macro arguments.

Some very old shells did not reset the exit status from an if with no else:

$ if (exit 42); then true; fi; echo $?
42

whereas a proper shell should have printed ‘0’. But this is no longer a portability problem; any shell that supports functions gets it correct. However, it explains why some makefiles have lengthy constructs:

if test -f "$file"; then
  install "$file" "$dest"
else
  :
fi
printf

A format string starting with a ‘-’ can cause problems. Bash interprets it as an option and gives an error. And ‘--’ to mark the end of options is not good in the NetBSD Almquist shell (e.g., 0.4.6) which takes that literally as the format string. Putting the ‘-’ in a ‘%c’ or ‘%s’ is probably easiest:

printf %s -foo

AIX 7.2 sh mishandles octal escapes in multi-byte locales by treating them as characters instead of bytes. For example, in a locale using the UTF-8 encoding, ‘printf '\351'’ outputs the two bytes C3, A9 (the UTF-8 encoding for U+00E9) instead of the desired single byte E9. To work around the bug, use the C locale.

Bash 2.03 mishandles an escape sequence that happens to evaluate to ‘%’:

$ printf '\045'
bash: printf: `%': missing format character

Large outputs may cause trouble. On Solaris 2.5.1 through 10, for example, /usr/bin/printf is buggy, so when using /bin/sh the command ‘printf %010000x 123’ normally dumps core.

Since printf is not always a shell builtin, there is a potential speed penalty for using printf '%s\n' as a replacement for an echo that does not interpret ‘\’ or leading ‘-’. With Solaris ksh, it is possible to use print -r -- for this role instead.

See Limitations of Shell Builtins, for a discussion of portable alternatives to both printf and echo.

pwd

With modern shells, plain pwd outputs a “logical” directory name, some of whose components may be symbolic links. These directory names are in contrast to “physical” directory names, whose components are all directories.

Posix 1003.1-2001 requires that pwd must support the -L (“logical”) and -P (“physical”) options, with -L being the default. However, traditional shells do not support these options, and their pwd command has the -P behavior.

Portable scripts should assume neither option is supported, and should assume neither behavior is the default. Also, on many hosts ‘/bin/pwd’ is equivalent to ‘pwd -P’, but Posix does not require this behavior and portable scripts should not rely on it.

Typically it’s best to use plain pwd. On modern hosts this outputs logical directory names, which have the following advantages:

  • Logical names are what the user specified.
  • Physical names may not be portable from one installation host to another due to network file system gymnastics.
  • On modern hosts ‘pwd -P’ may fail due to lack of permissions to some parent directory, but plain pwd cannot fail for this reason.

Also please see the discussion of the cd command.

read

No options are portable, not even support -r (Solaris 10 /bin/sh for example). Tru64/OSF 5.1 sh treats read as a special built-in, so it may exit if input is redirected from a non-existent or unreadable file.

set

With the FreeBSD 6.0 shell, the set command (without any options) does not sort its output.

The set builtin faces the usual problem with arguments starting with a dash. Modern shells such as Bash or Zsh understand -- to specify the end of the options (any argument after -- is a parameter, even ‘-x’ for instance), but many traditional shells (e.g., Solaris 10 /bin/sh) simply stop option processing as soon as a non-option argument is found. Therefore, use ‘dummy’ or simply ‘x’ to end the option processing, and use shift to pop it out:

set x $my_list; shift

Avoid ‘set -’, e.g., ‘set - $my_list’. Posix no longer requires support for this command, and in traditional shells ‘set - $my_list’ resets the -v and -x options, which makes scripts harder to debug.

Some nonstandard shells do not recognize more than one option (e.g., ‘set -e -x’ assigns ‘-x’ to the command line). It is better to combine them:

set -ex

The -e option has historically been under-specified, with enough ambiguities to cause numerous differences across various shell implementations; see for example this overview, or this link, documenting a change to Posix 2008 to match ksh88 behavior. Note that mixing set -e and shell functions is asking for surprises:

set -e
doit()
{
  rm file
  echo one
}
doit || echo two

According to the recommendation, ‘one’ should always be output regardless of whether the rm failed, because it occurs within the body of the shell function ‘doit’ invoked on the left side of ‘||’, where the effects of ‘set -e’ are not enforced. Likewise, ‘two’ should never be printed, since the failure of rm does not abort the function, such that the status of ‘doit’ is 0.

The BSD shell has had several problems with the -e option. Older versions of the BSD shell (circa 1990) mishandled ‘&&’, ‘||’, ‘if’, and ‘case’ when -e was in effect, causing the shell to exit unexpectedly in some cases. This was particularly a problem with makefiles, and led to circumlocutions like ‘sh -c 'test -f file || touch file'’, where the seemingly-unnecessary ‘sh -c '…'’ wrapper works around the bug (see Failure in Make Rules).

Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4) wrongly exit with -e if the last command within a compound statement fails and is guarded by an ‘&&’ only. For example:

#! /bin/sh
set -e
foo=''
test -n "$foo" && exit 1
echo one
if :; then
  test -n "$foo" && exit 1
  echo two
  test -n "$foo" && exit 1
fi
echo three

does not print ‘three’. One workaround is to change the last instance of ‘test -n "$foo" && exit 1’ to be ‘if test -n "$foo"; then exit 1; fi’ instead. Another possibility is to warn BSD users not to use ‘sh -e’.

When ‘set -e’ is in effect, a failed command substitution in Solaris 10 /bin/sh cannot be ignored, even with ‘||’.

$ /bin/sh -c 'set -e; foo=`false` || echo foo; echo bar'
$ bash -c 'set -e; foo=`false` || echo foo; echo bar'
foo
bar

Moreover, a command substitution, successful or not, causes this shell to exit from a failing outer command even in presence of an ‘&&’ list:

$ bash -c 'set -e; false `true` && echo notreached; echo ok'
ok
$ sh -c 'set -e; false `true` && echo notreached; echo ok'
$

Portable scripts should not use ‘set -e’ if trap is used to install an exit handler. This is because Tru64/OSF 5.1 sh sometimes enters the trap handler with the exit status of the command prior to the one that triggered the errexit handler:

$ sh -ec 'trap '\''echo $?'\'' 0; false'
0
$ sh -c 'set -e; trap '\''echo $?'\'' 0; false'
1

Thus, when writing a script in M4sh, rather than trying to rely on ‘set -e’, it is better to append ‘|| AS_EXIT’ to any statement where it is desirable to abort on failure.

Job control is not provided by all shells, so the use of ‘set -m’ or ‘set -b’ must be done with care. When using zsh in native mode, asynchronous notification (‘set -b’) is enabled by default, and using ‘emulate sh’ to switch to Posix mode does not clear this setting (although asynchronous notification has no impact unless job monitoring is also enabled). Also, zsh 4.3.10 and earlier have a bug where job control can be manipulated in interactive shells, but not in subshells or scripts. Furthermore, some shells, like pdksh, fail to treat subshells as interactive, even though the parent shell was.

$ echo $ZSH_VERSION
4.3.10
$ set -m; echo $?
0
$ zsh -c 'set -m; echo $?'
set: can't change option: -m
$ (set -m); echo $?
set: can't change option: -m
1
$ pdksh -ci 'echo $-; (echo $-)'
cim
c

Use of set -n (typically via sh -n script) to validate a script is not foolproof. Modern ksh93 tries to be helpful by informing you about better syntax, but switching the script to use the suggested syntax in order to silence the warnings would render the script no longer portable to older shells:

$ ksh -nc '``'
ksh: warning: line 1: `...` obsolete, use $(...)
0

Furthermore, on ancient hosts, such as SunOS 4, sh -n could go into an infinite loop; even with that bug fixed, Solaris 8 /bin/sh takes extremely long to parse large scripts. Autoconf itself uses sh -n within its testsuite to check that correct scripts were generated, but only after first probing for other shell features (such as test ${BASH_VERSION+y}) that indicate a reasonably fast and working implementation.

shift

Not only is shifting a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it.

Don’t use ‘shift 2’ etc.; while it in the SVR1 shell (1983), it is also absent in many pre-Posix shells.

source

This command is not portable, as Posix does not require it; use . instead.

test

The test program is the way to perform many file and string tests. It is often invoked by the alternate name ‘[’, but using that name in Autoconf code is asking for trouble since it is an M4 quote character.

The -a, -o, ‘(’, and ‘)’ operands are not present in all implementations, and have been marked obsolete by Posix 2008. This is because there are inherent ambiguities in using them. For example, ‘test "$1" -a "$2"’ looks like a binary operator to check whether two strings are both non-empty, but if ‘$1’ is the literal ‘!’, then some implementations of test treat it as a negation of the unary operator -a.

Thus, portable uses of test should never have more than four arguments, and scripts should use shell constructs like ‘&&’ and ‘||’ instead. If you combine ‘&&’ and ‘||’ in the same statement, keep in mind that they have equal precedence, so it is often better to parenthesize even when this is redundant. For example:

# Not portable:
test "X$a" = "X$b" -a \
  '(' "X$c" != "X$d" -o "X$e" = "X$f" ')'

# Portable:
test "X$a" = "X$b" &&
  { test "X$c" != "X$d" || test "X$e" = "X$f"; }

test does not process options like most other commands do; for example, it does not recognize the -- argument as marking the end of options.

It is safe to use ‘!’ as a test operator. For example, ‘if test ! -d foo; …’ is portable even though ‘if ! test -d foo; …’ is not.

test (files)

To enable configure scripts to support cross-compilation, they shouldn’t do anything that tests features of the build system instead of the host system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use ‘test -f’, ‘test -r’, or ‘test -x’. Do not use ‘test -e’, because Solaris 10 /bin/sh lacks it. To test for symbolic links on systems that have them, use ‘test -h’ rather than ‘test -L’; either form conforms to Posix 1003.1-2001, but older shells like Solaris 8 /bin/sh support only -h.

For historical reasons, Posix reluctantly allows implementations of ‘test -x’ that will succeed for the root user, even if no execute permissions are present. Furthermore, shells do not all agree on whether Access Control Lists should affect ‘test -r’, ‘test -w’, and ‘test -x’; some shells base test results strictly on the current user id compared to file owner and mode, as if by stat(2); while other shells base test results on whether the current user has the given right, even if that right is only granted by an ACL, as if by faccessat(2). Furthermore, there is a classic time of check to time of use race between any use of test followed by operating on the just-checked file. Therefore, it is a good idea to write scripts that actually attempt an operation, and are prepared for the resulting failure if permission is denied, rather than trying to avoid an operation based solely on whether test guessed that it might not be permitted.

test (strings)

Posix says that ‘test "string"’ succeeds if string is not null, but this usage is not portable to traditional platforms like Solaris 10 /bin/sh, which mishandle strings like ‘!’ and ‘-n’. However, it is portable to test if a variable is set to a non-empty value, by using ‘test ${var+y}’, since all known implementations properly distinguish between no arguments and a known-safe string of ‘y’.

Posix also says that ‘test ! "string"’, ‘test -n "string"’ and ‘test -z "string"’ work with any string, but many shells (such as Solaris 10, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4, etc.) get confused if string looks like an operator:

$ test -n =
test: argument expected
$ test ! -n
test: argument expected
$ test -z ")"; echo $?
0

Similarly, Posix says that both ‘test "string1" = "string2"’ and ‘test "string1" != "string2"’ work for any pairs of strings, but in practice this is not true for troublesome strings that look like operators or parentheses, or that begin with ‘-’.

It is best to protect such strings with a leading ‘X’, e.g., ‘test "Xstring" != X’ rather than ‘test -n "string"’ or ‘test ! "string"’.

It is common to find variations of the following idiom:

test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&
  action

to take an action when a token matches a given pattern. Such constructs should be avoided by using:

case $ac_feature in
  *[!-a-zA-Z0-9_]*) action;;
esac

If the pattern is a complicated regular expression that cannot be expressed as a shell pattern, use something like this instead:

expr "X$ac_feature" : 'X.*[^-a-zA-Z0-9_]' >/dev/null &&
  action

expr "Xfoo" : "Xbar"’ is more robust than ‘echo "Xfoo" | grep "^Xbar"’, because it avoids problems when ‘foo’ contains backslashes.

trap

It is safe to trap at least the signals 1, 2, 13, and 15. You can also trap 0, i.e., have the trap run when the script ends (either via an explicit exit, or the end of the script). The trap for 0 should be installed outside of a shell function, or AIX 5.3 /bin/sh will invoke the trap at the end of this function.

Posix says that ‘trap - 1 2 13 15’ resets the traps for the specified signals to their default values, but many common shells (e.g., Solaris 10 /bin/sh) misinterpret this and attempt to execute a “command” named - when the specified conditions arise. Posix 2008 also added a requirement to support ‘trap 1 2 13 15’ to reset traps, as this is supported by a larger set of shells, but there are still shells like dash that mistakenly try to execute 1 instead of resetting the traps. Therefore, there is no portable workaround, except for ‘trap - 0’, for which ‘trap '' 0’ is a portable substitute.

Although Posix is not absolutely clear on this point, it is widely admitted that when entering the trap ‘$?’ should be set to the exit status of the last command run before the trap. The ambiguity can be summarized as: “when the trap is launched by an exit, what is the last command run: that before exit, or exit itself?”

Bash considers exit to be the last command, while Zsh and Solaris 10 /bin/sh consider that when the trap is run it is still in the exit, hence it is the previous exit status that the trap receives:

$ cat trap.sh
trap 'echo $?' 0
(exit 42); exit 0
$ zsh trap.sh
42
$ bash trap.sh
0

The portable solution is then simple: when you want to ‘exit 42’, run ‘(exit 42); exit 42’, the first exit being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash. In M4sh, this is covered by using AS_EXIT.

The shell in FreeBSD 4.0 has the following bug: ‘$?’ is reset to 0 by empty lines if the code is inside trap.

$ trap 'false

echo $?' 0
$ exit
0

Fortunately, this bug only affects trap.

Several shells fail to execute an exit trap that is defined inside a subshell, when the last command of that subshell is not a builtin. A workaround is to use ‘exit $?’ as the shell builtin.

$ bash -c '(trap "echo hi" 0; /bin/true)'
hi
$ /bin/sh -c '(trap "echo hi" 0; /bin/true)'
$ /bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)'
hi

Likewise, older implementations of bash failed to preserve ‘$?’ across an exit trap consisting of a single cleanup command.

$ bash -c 'trap "/bin/true" 0; exit 2'; echo $?
2
$ bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $?
0
$ bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $?
2

Be aware that a trap can be called from any number of places in your script, and therefore the trap handler should not make assumptions about shell state. For some examples, if your script temporarily modifies IFS, then the trap should include an initialization back to its typical value of space-tab-newline (autoconf does this for generated configure files). Likewise, if your script changes the current working directory at some point after the trap is installed, then your trap cannot assume which directory it is in, and should begin by changing directories to an absolute path if that is important to the cleanup efforts (autotest does this for generated testsuite files).

true

Don’t worry: as far as we know true is portable. Nevertheless, it’s not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using :. This has a funny side effect: when asked whether false is more portable than true Alexandre Oliva answered:

In a sense, yes, because if it doesn’t exist, the shell will produce an exit status of failure, which is correct for false, but not for true.

Remember that even though ‘:’ ignores its arguments, it still takes time to compute those arguments. It is a good idea to use double quotes around any arguments to ‘:’ to avoid time spent in field splitting and file name expansion.

unset

In some nonconforming shells (e.g., Solaris 10 /bin/ksh and /usr/xpg4/bin/sh, NetBSD 5.99.43 sh, or Bash 2.05a), unset FOO fails when FOO is not set. This can interfere with set -e operation. You can use

FOO=; unset FOO

if you are not sure that FOO is set.

A few ancient shells lack unset entirely. For some variables such as PS1, you can use a neutralizing value instead:

PS1='$ '

Usually, shells that do not support unset need less effort to make the environment sane, so for example is not a problem if you cannot unset CDPATH on those shells. However, Bash 2.01 mishandles unset MAIL and unset MAILPATH in some cases and dumps core. So, you should do something like

( (unset MAIL) || exit 1) >/dev/null 2>&1 && unset MAIL || :

See Special Shell Variables, for some neutralizing values. Also, see Limitations of Builtins, for the case of environment variables.

wait

The exit status of wait is not always reliable.


11.15 Limitations of Usual Tools

The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.

awk

Don’t leave white space before the opening parenthesis in a user function call. Posix does not allow this and GNU Awk rejects it:

$ gawk 'function die () { print "Aaaaarg!"  }
        BEGIN { die () }'
gawk: cmd. line:2:         BEGIN { die () }
gawk: cmd. line:2:                      ^ parse error
$ gawk 'function die () { print "Aaaaarg!"  }
        BEGIN { die() }'
Aaaaarg!

Posix says that if a program contains only ‘BEGIN’ actions, and contains no instances of getline, then the program merely executes the actions without reading input. However, traditional Awk implementations (such as Solaris 10 awk) read and discard input in this case. Portable scripts can redirect input from /dev/null to work around the problem. For example:

awk 'BEGIN {print "hello world"}' </dev/null

Posix says that in an ‘END’ action, ‘$NF’ (and presumably, ‘$1’) retain their value from the last record read, if no intervening ‘getline’ occurred. However, some implementations (such as Solaris 10 ‘/usr/bin/awk’, ‘nawk’, or Darwin ‘awk’) reset these variables. A workaround is to use an intermediate variable prior to the ‘END’ block. For example:

$ cat end.awk
{ tmp = $1 }
END { print "a", $1, $NF, "b", tmp }
$ echo 1 | awk -f end.awk
a   b 1
$ echo 1 | gawk -f end.awk
a 1 1 b 1

If you want your program to be deterministic, don’t depend on for on arrays:

$ cat for.awk
END {
  arr["foo"] = 1
  arr["bar"] = 1
  for (i in arr)
    print i
}
$ gawk -f for.awk </dev/null
foo
bar
$ nawk -f for.awk </dev/null
bar
foo

Some Awk implementations, such as HP-UX 11.0’s native one, mishandle anchors:

$ echo xfoo | $AWK '/foo|^bar/ { print }'
$ echo bar | $AWK '/foo|^bar/ { print }'
bar
$ echo xfoo | $AWK '/^bar|foo/ { print }'
xfoo
$ echo bar | $AWK '/^bar|foo/ { print }'
bar

Either do not depend on such patterns (i.e., use ‘/^(.*foo|bar)/’, or use a simple test to reject such implementations.

On ‘ia64-hp-hpux11.23’, Awk mishandles printf conversions after %u:

$ awk 'BEGIN { printf "%u %d\n", 0, -1 }'
0 0

AIX version 5.2 has an arbitrary limit of 399 on the length of regular expressions and literal strings in an Awk program.

Traditional Awk implementations derived from Unix version 7, such as Solaris /bin/awk, have many limitations and do not conform to Posix. Nowadays AC_PROG_AWK (see Particular Program Checks) finds you an Awk that doesn’t have these problems, but if for some reason you prefer not to use AC_PROG_AWK you may need to address them. For more detailed descriptions, see awk language history in GNU Awk User’s Guide.

Traditional Awk does not support multidimensional arrays or user-defined functions.

Traditional Awk does not support the -v option. You can use assignments after the program instead, e.g., $AWK '{print v $1}' v=x; however, don’t forget that such assignments are not evaluated until they are encountered (e.g., after any BEGIN action).

Traditional Awk does not support the keywords delete or do.

Traditional Awk does not support the expressions a?b:c, !a, a^b, or a^=b.

Traditional Awk does not support the predefined CONVFMT or ENVIRON variables.

Traditional Awk supports only the predefined functions exp, index, int, length, log, split, sprintf, sqrt, and substr.

Traditional Awk getline is not at all compatible with Posix; avoid it.

Traditional Awk has for (i in a) … but no other uses of the in keyword. For example, it lacks if (i in a) ….

In code portable to both traditional and modern Awk, FS must be a string containing just one ordinary character, and similarly for the field-separator argument to split.

Traditional Awk has a limit of 99 fields in a record. Since some Awk implementations, like Tru64’s, split the input even if you don’t refer to any field in the script, to circumvent this problem, set ‘FS’ to an unusual character and use split.

Traditional Awk has a limit of at most 99 bytes in a number formatted by OFMT; for example, OFMT="%.300e"; print 0.1; typically dumps core.

The original version of Awk had a limit of at most 99 bytes per split field, 99 bytes per substr substring, and 99 bytes per run of non-special characters in a printf format, but these bugs have been fixed on all practical hosts that we know of.

HP-UX 11.00 and IRIX 6.5 Awk require that input files have a line length of at most 3070 bytes.

basename

Not all hosts have a working basename. You can use expr instead.

cat

Don’t rely on any option.

cc

The command ‘cc -c foo.c’ traditionally produces an object file named foo.o. Most compilers allow -c to be combined with -o to specify a different object file name, but Posix does not require this combination and a few compilers lack support for it. See C Compiler Characteristics, for how GNU Make tests for this feature with AC_PROG_CC_C_O.

When a compilation such as ‘cc -o foo foo.c’ fails, some compilers (such as CDS on Reliant Unix) leave a foo.o.

HP-UX cc doesn’t accept .S files to preprocess and assemble. ‘cc -c foo.S’ appears to succeed, but in fact does nothing.

The default executable, produced by ‘cc foo.c’, can be

  • a.out – usual Posix convention.
  • b.out – i960 compilers (including gcc).
  • a.exe – DJGPP port of gcc.
  • a_out.exe – GNV cc wrapper for DEC C on OpenVMS.
  • foo.exe – various MS-DOS compilers.

The C compiler’s traditional name is cc, but other names like gcc are common. Posix 1003.1-2001 and 1003.1-2008 specify the name c99, but older Posix editions specified c89, future POSIX standards will likely specify c11, and anyway these standard names are rarely used in practice. Typically the C compiler is invoked from makefiles that use ‘$(CC)’, so the value of the ‘CC’ make variable selects the compiler name.

chgrp
chown

It is not portable to change a file’s group to a group that the owner does not belong to.

chmod

Avoid usages like ‘chmod -w file’; use ‘chmod a-w file’ instead, for two reasons. First, plain -w does not necessarily make the file unwritable, since it does not affect mode bits that correspond to bits in the file mode creation mask. Second, Posix says that the -w might be interpreted as an implementation-specific option, not as a mode; Posix suggests using ‘chmod -- -w file’ to avoid this confusion, but unfortunately ‘--’ does not work on some older hosts.

cmp

cmp performs a raw data comparison of two files, while diff compares two text files. Therefore, if you might compare DOS files, even if only checking whether two files are different, use diff to avoid spurious differences due to differences of newline encoding.

cp

Avoid the -r option, since Posix 1003.1-2004 marks it as obsolescent and its behavior on special files is implementation-defined. Use -R instead. On GNU hosts the two options are equivalent, but on Solaris hosts (for example) cp -r reads from pipes instead of replicating them. AIX 5.3 cp -R may corrupt its own memory with some directory hierarchies and error out or dump core:

mkdir -p 12345678/12345678/12345678/12345678
touch 12345678/12345678/x
cp -R 12345678 t
cp: 0653-440 12345678/12345678/: name too long.

Some cp implementations (e.g., BSD/OS 4.2) do not allow trailing slashes at the end of nonexistent destination directories. To avoid this problem, omit the trailing slashes. For example, use ‘cp -R source /tmp/newdir’ rather than ‘cp -R source /tmp/newdir/’ if /tmp/newdir does not exist.

The -f option is portable nowadays.

Traditionally, file timestamps had 1-second resolution, and ‘cp -p’ copied the timestamps exactly. However, many modern file systems have timestamps with 1-nanosecond resolution. Unfortunately, some older ‘cp -p’ implementations truncate timestamps when copying files, which can cause the destination file to appear to be older than the source. The exact amount of truncation depends on the resolution of the system calls that cp uses. Traditionally this was utime, which has 1-second resolution. Less-ancient cp implementations such as GNU Core Utilities 5.0.91 (2003) use utimes, which has 1-microsecond resolution. Modern implementations such as GNU Core Utilities 6.12 (2008) can set timestamps to the full nanosecond resolution, using the modern system calls futimens and utimensat when they are available. As of 2011, though, many platforms do not yet fully support these new system calls.

Bob Proulx notes that ‘cp -p’ always tries to copy ownerships. But whether it actually does copy ownerships or not is a system dependent policy decision implemented by the kernel. If the kernel allows it then it happens. If the kernel does not allow it then it does not happen. It is not something cp itself has control over.

In Unix System V any user can chown files to any other user, and System V also has a non-sticky /tmp. That probably derives from the heritage of System V in a business environment without hostile users. BSD changed this to be a more secure model where only root can chown files and a sticky /tmp is used. That undoubtedly derives from the heritage of BSD in a campus environment.

GNU/Linux and Solaris by default follow BSD, but can be configured to allow a System V style chown. On the other hand, HP-UX follows System V, but can be configured to use the modern security model and disallow chown. Since it is an administrator-configurable parameter you can’t use the name of the kernel as an indicator of the behavior.

date

Some versions of date do not recognize special ‘%’ directives, and unfortunately, instead of complaining, they just pass them through, and exit with success:

$ uname -a
OSF1 medusa.sis.pasteur.fr V5.1 732 alpha
$ date "+%s"
%s
diff

Option -u is nonportable.

Some implementations, such as Tru64’s, fail when comparing to /dev/null. Use an empty file instead.

dirname

Not all hosts have a working dirname, and you should instead use AS_DIRNAME (see Programming in M4sh). For example:

dir=`dirname "$file"`       # This is not portable.
dir=`AS_DIRNAME(["$file"])` # This is more portable.
egrep

Although Posix stopped requiring egrep in 2001, a few traditional hosts (notably Solaris) do not support the Posix replacement grep -E. Also, some traditional implementations do not work on long input lines. To work around these problems, invoke AC_PROG_EGREP and then use $EGREP.

Portable extended regular expressions should use ‘\’ only to escape characters in the string ‘$()*+.?[\^{|’. For example, ‘\}’ is not portable, even though it typically matches ‘}’.

The empty alternative is not portable. Use ‘?’ instead. For instance with Digital Unix v5.0:

> printf "foo\n|foo\n" | $EGREP '^(|foo|bar)$'
|foo
> printf "bar\nbar|\n" | $EGREP '^(foo|bar|)$'
bar|
> printf "foo\nfoo|\n|bar\nbar\n" | $EGREP '^(foo||bar)$'
foo
|bar

$EGREP also suffers the limitations of grep (see Limitations of Usual Tools).

expr

Not all implementations obey the Posix rule that ‘--’ separates options from arguments; likewise, not all implementations provide the extension to Posix that the first argument can be treated as part of a valid expression rather than an invalid option if it begins with ‘-’. When performing arithmetic, use ‘expr 0 + $var’ if ‘$var’ might be a negative number, to keep expr from interpreting it as an option.

No expr keyword starts with ‘X’, so use ‘expr X"word" : 'Xregex'’ to keep expr from misinterpreting word.

Don’t use length, substr, match and index.

expr (‘|’)

You can use ‘|’. Although Posix does require that ‘expr ''’ return the empty string, it does not specify the result when you ‘|’ together the empty string (or zero) with the empty string. For example:

expr '' \| ''

Posix 1003.2-1992 returns the empty string for this case, but traditional Unix returns ‘0’ (Solaris is one such example). In Posix 1003.1-2001, the specification was changed to match traditional Unix’s behavior (which is bizarre, but it’s too late to fix this). Please note that the same problem does arise when the empty string results from a computation, as in:

expr bar : foo \| foo : bar

Avoid this portability problem by avoiding the empty string.

expr (‘:’)

Portable expr regular expressions should use ‘\’ to escape only characters in the string ‘$()*.0123456789[\^n{}’. For example, alternation, ‘\|’, is common but Posix does not require its support, so it should be avoided in portable scripts. Similarly, ‘\+’ and ‘\?’ should be avoided.

Portable expr regular expressions should not begin with ‘^’. Patterns are automatically anchored so leading ‘^’ is not needed anyway.

On the other hand, the behavior of the ‘$’ anchor is not portable on multi-line strings. Posix is ambiguous whether the anchor applies to each line, as was done in older versions of the GNU Core Utilities, or whether it applies only to the end of the overall string, as in Coreutils 6.0 and most other implementations.

$ baz='foo
> bar'
$ expr "X$baz" : 'X\(foo\)$'

$ expr-5.97 "X$baz" : 'X\(foo\)$'
foo

The Posix standard is ambiguous as to whether ‘expr 'a' : '\(b\)'’ outputs ‘0’ or the empty string. In practice, it outputs the empty string on most platforms, but portable scripts should not assume this. For instance, the QNX 4.25 native expr returns ‘0’.

One might think that a way to get a uniform behavior would be to use the empty string as a default value:

expr a : '\(b\)' \| ''

Unfortunately this behaves exactly as the original expression; see the expr (‘|’) entry for more information.

Some ancient expr implementations (e.g., SunOS 4 expr and Solaris 8 /usr/ucb/expr) have a silly length limit that causes expr to fail if the matched substring is longer than 120 bytes. In this case, you might want to fall back on ‘echo|sed’ if expr fails. Nowadays this is of practical importance only for the rare installer who mistakenly puts /usr/ucb before /usr/bin in PATH.

On Mac OS X 10.4, expr mishandles the pattern ‘[^-]’ in some cases. For example, the command

expr Xpowerpc-apple-darwin8.1.0 : 'X[^-]*-[^-]*-\(.*\)'

outputs ‘apple-darwin8.1.0’ rather than the correct ‘darwin8.1.0’. This particular case can be worked around by substituting ‘[^--]’ for ‘[^-]’.

Don’t leave, there is some more!

The QNX 4.25 expr, in addition of preferring ‘0’ to the empty string, has a funny behavior in its exit status: it’s always 1 when parentheses are used!

$ val=`expr 'a' : 'a'`; echo "$?: $val"
0: 1
$ val=`expr 'a' : 'b'`; echo "$?: $val"
1: 0

$ val=`expr 'a' : '\(a\)'`; echo "?: $val"
1: a
$ val=`expr 'a' : '\(b\)'`; echo "?: $val"
1: 0

In practice this can be a big problem if you are ready to catch failures of expr programs with some other method (such as using sed), since you may get twice the result. For instance

$ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'

outputs ‘a’ on most hosts, but ‘aa’ on QNX 4.25. A simple workaround consists of testing expr and using a variable set to expr or to false according to the result.

Tru64 expr incorrectly treats the result as a number, if it can be interpreted that way:

$ expr 00001 : '.*\(...\)'
1

On HP-UX 11, expr only supports a single sub-expression.

$ expr 'Xfoo' : 'X\(f\(oo\)*\)$'
expr: More than one '\(' was used.
fgrep

Although Posix stopped requiring fgrep in 2001, a few traditional hosts (notably Solaris) do not support the Posix replacement grep -F. Also, some traditional implementations do not work on long input lines. To work around these problems, invoke AC_PROG_FGREP and then use $FGREP.

Tru64/OSF 5.1 fgrep does not match an empty pattern.

find

The -maxdepth option seems to be GNU specific. Tru64 v5.1, NetBSD 1.5 and Solaris find commands do not understand it.

The replacement of ‘{}’ is guaranteed only if the argument is exactly {}, not if it’s only a part of an argument. For instance on DU, and HP-UX 10.20 and HP-UX 11:

$ touch foo
$ find . -name foo -exec echo "{}-{}" \;
{}-{}

while GNU find reports ‘./foo-./foo’.

grep

Portable scripts can rely on the grep options -c, -l, -n, and -v, but should avoid other options. For example, don’t use -w, as Posix does not require it and Irix 6.5.16m’s grep does not support it. Also, portable scripts should not combine -c with -l, as Posix does not allow this.

Some of the options required by Posix are not portable in practice. Don’t use ‘grep -q’ to suppress output, because traditional grep implementations (e.g., Solaris) do not support -q. Don’t use ‘grep -s’ to suppress output either, because Posix says -s does not suppress output, only some error messages; also, the -s option of traditional grep behaved like -q does in most modern implementations. Instead, redirect the standard output and standard error (in case the file doesn’t exist) of grep to /dev/null. Check the exit status of grep to determine whether it found a match.

The QNX4 implementation fails to count lines with grep -c '$', but works with grep -c '^'. Other alternatives for counting lines are to use sed -n '$=' or wc -l.

Some traditional grep implementations do not work on long input lines. On AIX the default grep silently truncates long lines on the input before matching.

Also, traditional implementations do not support multiple regexps with -e: they either reject -e entirely (e.g., Solaris) or honor only the last pattern (e.g., IRIX 6.5 and NeXT). To work around these problems, invoke AC_PROG_GREP and then use $GREP.

Another possible workaround for the multiple -e problem is to separate the patterns by newlines, for example:

grep 'foo
bar' in.txt

except that this fails with traditional grep implementations and with OpenBSD 3.8 grep.

Traditional grep implementations (e.g., Solaris) do not support the -E or -F options. To work around these problems, invoke AC_PROG_EGREP and then use $EGREP, and similarly for AC_PROG_FGREP and $FGREP. Even if you are willing to require support for Posix grep, your script should not use both -E and -F, since Posix does not allow this combination.

Portable grep regular expressions should use ‘\’ only to escape characters in the string ‘$()*.0123456789[\^{}’. For example, alternation, ‘\|’, is common but Posix does not require its support in basic regular expressions, so it should be avoided in portable scripts. Solaris and HP-UX grep do not support it. Similarly, the following escape sequences should also be avoided: ‘\<’, ‘\>’, ‘\+’, ‘\?’, ‘\`’, ‘\'’, ‘\B’, ‘\b’, ‘\S’, ‘\s’, ‘\W’, and ‘\w’.

Posix does not specify the behavior of grep on binary files. An example where this matters is using BSD grep to search text that includes embedded ANSI escape sequences for colored output to terminals (‘\033[m’ is the sequence to restore normal output); the behavior depends on whether input is seekable:

$ printf 'esc\033[mape\n' > sample
$ grep . sample
Binary file sample matches
$ cat sample | grep .
escape
join

Solaris 8 join has bugs when the second operand is standard input, and when standard input is a pipe. For example, the following shell script causes Solaris 8 join to loop forever:

cat >file <<'EOF'
1 x
2 y
EOF
cat file | join file -

Use ‘join - file’ instead.

On NetBSD, join -a 1 file1 file2 mistakenly behaves like join -a 1 -a 2 1 file1 file2, resulting in a usage warning; the workaround is to use join -a1 file1 file2 instead.

ln

The -f option is portable nowadays.

Symbolic links are not available on some systems; use ‘$(LN_S)’ as a portable substitute.

For versions of the DJGPP before 2.04, ln emulates symbolic links to executables by generating a stub that in turn calls the real program. This feature also works with nonexistent files like in the Posix spec. So ‘ln -s file link’ generates link.exe, which attempts to call file.exe if run. But this feature only works for executables, so ‘cp -p’ is used instead for these systems. DJGPP versions 2.04 and later have full support for symbolic links.

ls

The portable options are -acdilrtu. Current practice is for -l to output both owner and group, even though ancient versions of ls omitted the group.

On ancient hosts, ‘ls foo’ sent the diagnostic ‘foo not found’ to standard output if foo did not exist. Hence a shell command like ‘sources=`ls *.c 2>/dev/null`’ did not always work, since it was equivalent to ‘sources='*.c not found'’ in the absence of ‘.c’ files. This is no longer a practical problem, since current ls implementations send diagnostics to standard error.

The behavior of ls