This manual documents the libraries used to compose and display MIME messages.
This manual is directed at users who want to modify the behavior of the MIME encoding/decoding process or want a more detailed picture of how the Emacs MIME library works, and people who want to write functions and commands that manipulate MIME elements.
MIME is short for Multipurpose Internet Mail Extensions. This standard is documented in a number of RFCs; mainly RFC2045 (Format of Internet Message Bodies), RFC2046 (Media Types), RFC2047 (Message Header Extensions for Non-ASCII Text), RFC2048 (Registration Procedures), RFC2049 (Conformance Criteria and Examples). It is highly recommended that anyone who intends writing MIME-compliant software read at least RFC2045 and RFC2047.
This file documents the Emacs MIME interface functionality.
Copyright © 1998–2024 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, with the Front-Cover Texts being “A GNU Manual”, and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”.
(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual.”
This chapter deals with decoding and viewing MIME messages on a higher level.
The main idea is to first analyze a MIME article, and then allow other programs to do things based on the list of handles that are returned as a result of this analysis.
The mm-dissect-buffer is the function responsible for dissecting
a MIME article. If given a multipart message, it will recursively
descend the message, following the structure, and return a tree of
MIME handles that describes the structure of the message.
Gnus also understands some non-MIME attachments, such as
postscript, uuencode, binhex, yenc, shar, forward, gnatsweb, pgp,
diff. Each of these features can be disabled by add an item into
mm-uu-configure-list. For example,
(require 'mm-uu) (add-to-list 'mm-uu-configure-list '(pgp-signed . disabled))
postscript ¶PostScript file.
uu ¶Uuencoded file.
binhex ¶Binhex encoded file.
yenc ¶Yenc encoded file.
shar ¶Shar archive file.
forward ¶Non-MIME forwarded message.
gnatsweb ¶Gnatsweb attachment.
pgp-signed ¶PGP signed clear text.
pgp-encrypted ¶PGP encrypted clear text.
pgp-key ¶PGP public keys.
emacs-sources ¶Emacs source code. This item works only in the groups matching
mm-uu-emacs-sources-regexp.
diff ¶Patches. This is intended for groups where diffs of committed files
are automatically sent to. It only works in groups matching
mm-uu-diff-groups-regexp.
verbatim-marks ¶Slrn-style verbatim marks.
LaTeX ¶LaTeX documents. It only works in groups matching
mm-uu-tex-groups-regexp.
Some inlined non-MIME attachments are displayed using the face
mm-uu-extract. By default, no MIME button for these
parts is displayed. You can force displaying a button using K b
(gnus-summary-display-buttonized) or add text/x-verbatim
to gnus-buttonized-mime-types, See MIME
Commands in Gnus Manual.
A MIME handle is a list that fully describes a MIME component.
The following macros can be used to access elements in a handle:
mm-handle-buffer ¶Return the buffer that holds the contents of the undecoded MIME part.
mm-handle-type ¶Return the parsed Content-Type of the part.
mm-handle-encoding ¶Return the Content-Transfer-Encoding of the part.
mm-handle-undisplayer ¶Return the object that can be used to remove the displayed part (if it has been displayed).
mm-handle-set-undisplayer ¶Set the undisplayer object.
mm-handle-disposition ¶Return the parsed Content-Disposition of the part.
mm-get-content-idReturns the handle(s) referred to by Content-ID.
Functions for displaying, removing and saving.
mm-display-part ¶Display the part.
mm-remove-part ¶Remove the part (if it has been displayed).
mm-inlinable-p ¶Say whether a MIME type can be displayed inline.
mm-automatic-display-p ¶Say whether a MIME type should be displayed automatically.
mm-destroy-part ¶Free all resources occupied by a part.
mm-save-part ¶Offer to save the part in a file.
mm-pipe-part ¶Offer to pipe the part to some process.
mm-interactively-view-part ¶Prompt for a mailcap method to use to view the part.
mm-inline-media-tests ¶This is an alist where the key is a MIME type, the second element
is a function to display the part inline (i.e., inside Emacs), and
the third element is a form to be evaled to say whether the part
can be displayed inline.
This variable specifies whether a part can be displayed inline, and, if so, how to do it. It does not say whether parts are actually displayed inline.
mm-inlined-types ¶This, on the other hand, says what types are to be displayed inline, if they satisfy the conditions set by the variable above. It’s a list of MIME media types.
mm-automatic-display ¶This is a list of types that are to be displayed “automatically”, but only if the above variable allows it. That is, only inlinable parts can be displayed automatically.
mm-automatic-external-display ¶This is a list of types that will be displayed automatically in an external viewer.
mm-keep-viewer-alive-types ¶This is a list of media types for which the external viewer will not be killed when selecting a different article.
mm-attachment-override-types ¶Some MIME agents create parts that have a content-disposition of ‘attachment’. This variable allows overriding that disposition and displaying the part inline. (Note that the disposition is only overridden if we are able to, and want to, display the part inline.)
mm-discouraged-alternatives ¶List of MIME types that are discouraged when viewing ‘multipart/alternative’. Viewing agents are supposed to view the last possible part of a message, as that is supposed to be the richest. However, users may prefer other types instead, and this list says what types are most unwanted. If, for instance, ‘text/html’ parts are very unwanted, and ‘text/richtext’ parts are somewhat unwanted, you could say something like:
(setq mm-discouraged-alternatives
'("text/html" "text/richtext")
mm-automatic-display
(remove "text/html" mm-automatic-display))
Adding "image/.*" might also be useful. Spammers use images as
the preferred part of ‘multipart/alternative’ messages, so you might
not notice there are other parts. See also
gnus-buttonized-mime-types, MIME Commands in Gnus Manual. After adding "multipart/alternative" to
gnus-buttonized-mime-types you can choose manually which
alternative you’d like to view. For example, you can set those
variables like:
(setq gnus-buttonized-mime-types
'("multipart/alternative" "multipart/signed")
mm-discouraged-alternatives
'("text/html" "image/.*"))
In this case, Gnus will display radio buttons for such a kind of spam message as follows:
1. (*) multipart/alternative ( ) image/gif 2. (*) text/plain ( ) text/html
mm-inline-large-images ¶This variable is resize by default, which means that images
that are bigger than the Emacs window are resized so that they fit.
If you set this to nil, large images are not displayed in
Emacs, but can instead be displayed externally (e.g., with
‘ImageMagick’ or ‘xv’). Setting this variable to t
disables this check and makes the library display all inline images as
inline, regardless of their size.
mm-inline-large-images-proportion ¶The proportion used when resizing large images.
mm-inline-override-types ¶mm-inlined-types may include regular expressions, for example to
specify that all ‘text/.*’ parts be displayed inline. If a user
prefers to have a type that matches such a regular expression be treated
as an attachment, that can be accomplished by setting this variable to a
list containing that type. For example assuming mm-inlined-types
includes ‘text/.*’, then including ‘text/html’ in this
variable will cause ‘text/html’ parts to be treated as attachments.
mm-text-html-renderer ¶This selects the function used to render HTML. The
predefined renderers are selected by the symbols shr,
gnus-w3m, w3m1, links, lynx, or w3m-standalone. You
can also specify a function, which will be called with a
MIME handle as the argument.
mm-html-inhibit-images ¶If this is non-nil, inhibit displaying of images inline in the
article body. It is effective to images in HTML articles
rendered when mm-text-html-renderer (see Display Customization) is shr or w3m. In Gnus, this is
overridden by the value of gnus-inhibit-images (see Misc Article in Gnus manual). The default is nil.
mm-html-blocked-images ¶External images that have URLs that match this regexp won’t be fetched and displayed. For instance, to block all URLs that have the string “ads” in them, do the following:
(setq mm-html-blocked-images "ads")
It is effective when mm-text-html-renderer (see Display Customization) is shr. In Gnus, this is overridden by the value
of gnus-blocked-images or the return value of the function that
gnus-blocked-images is set to (see HTML in Gnus
manual).
Some HTML mails might have the trick of spammers using
‘<img>’ tags. It is likely to be intended to verify whether you
have read the mail. You can prevent your personal information from
leaking by setting this option to "" (which is the default).
mm-w3m-safe-url-regexp ¶A regular expression that matches safe URL names, i.e., URLs that are
unlikely to leak personal information when rendering HTML
email (the default value is ‘\\`cid:’). If nil consider
all URLs safe. In Gnus, this will be overridden according to the value
of the variable gnus-safe-html-newsgroups, See Various Various in Gnus Manual.
mm-inline-text-html-with-w3m-keymap ¶You can use emacs-w3m command keys in the inlined text/html part by
setting this option to non-nil. The default value is t.
mm-external-terminal-program ¶This should be a list of strings; typically something like ‘("xterm" "-e")’ or ‘("gnome-terminal" "--")’.
mm-enable-external ¶Indicate whether external MIME handlers should be used.
If t, all defined external MIME handlers are used. If
nil, files are saved to disk (mailcap-save-binary-file).
If it is the symbol ask, you are prompted before the external
MIME handler is invoked.
When you launch an attachment through mailcap (see mailcap) an
attempt is made to use a safe viewer with the safest options—this isn’t
the case if you save it to disk and launch it in a different way
(command line or double-clicking). Anyhow, if you want to be sure not
to launch any external programs, set this variable to nil or
ask.
mm-inline-font-lock ¶If non-nil, inlined parts that support font locking (for
instance, patches or code snippets) will be font-locked. This may be
overridden by callers that have their own ways of enabling/inhibiting
font locking.
mm-default-directory ¶The default directory for saving attachments. If nil use
default-directory.
mm-tmp-directory ¶Directory for storing temporary files.
mm-file-name-rewrite-functions ¶A list of functions used for rewriting file names of MIME parts. Each function is applied successively to the file name. Ready-made functions include
mm-file-name-delete-control ¶Delete all control characters.
mm-file-name-delete-gotchas ¶Delete characters that could have unintended consequences when used with flawed shell scripts, i.e., ‘|’, ‘>’ and ‘<’; and ‘-’, ‘.’ as the first character.
mm-file-name-delete-whitespace ¶Remove all whitespace.
mm-file-name-trim-whitespace ¶Remove leading and trailing whitespace.
mm-file-name-collapse-whitespace ¶Collapse multiple whitespace characters.
mm-file-name-replace-whitespace ¶Replace whitespace with underscores. Set the variable
mm-file-name-replace-whitespace to any other string if you do
not like underscores.
The standard Emacs functions capitalize, downcase,
upcase and upcase-initials might also prove useful.
mm-path-name-rewrite-functions ¶List of functions used for rewriting the full file names of MIME parts. This is used when viewing parts externally, and is meant for transforming the absolute name so that non-compliant programs can find the file where it’s saved.
Here’s an example viewer for displaying text/enriched inline:
(defun mm-display-enriched-inline (handle)
(let (text)
(with-temp-buffer
(mm-insert-part handle)
(save-window-excursion
(enriched-decode (point-min) (point-max))
(setq text (buffer-string))))
(mm-insert-inline handle text)))
We see that the function takes a MIME handle as its parameter. It then goes to a temporary buffer, inserts the text of the part, does some work on the text, stores the result, goes back to the buffer it was called from and inserts the result.
The two important helper functions here are mm-insert-part and
mm-insert-inline. The first function inserts the text of the
handle in the current buffer. It handles charset and/or content
transfer decoding. The second function just inserts whatever text you
tell it to insert, but it also sets things up so that the text can be
“undisplayed” in a convenient manner.
Creating a MIME message is boring and non-trivial. Therefore,
a library called mml has been defined that parses a language
called MML (MIME Meta Language) and generates
MIME messages.
The main interface function is mml-generate-mime. It will
examine the contents of the current (narrowed-to) buffer and return a
string containing the MIME message.
Here’s a simple ‘multipart/alternative’:
<#multipart type=alternative> This is a plain text part. <#part type=text/enriched> <center>This is a centered enriched part</center> <#/multipart>
After running this through mml-generate-mime, we get this:
Content-Type: multipart/alternative; boundary="=-=-=" --=-=-= This is a plain text part. --=-=-= Content-Type: text/enriched <center>This is a centered enriched part</center> --=-=-=--
The MML language is very simple. It looks a bit like an SGML application, but it’s not.
The main concept of MML is the part. Each part can be of a different type or use a different charset. The way to delineate a part is with a ‘<#part ...>’ tag. Multipart parts can be introduced with the ‘<#multipart ...>’ tag. Parts are ended by the ‘<#/part>’ or ‘<#/multipart>’ tags. Parts started with the ‘<#part ...>’ tags are also closed by the next open tag.
There’s also the ‘<#external ...>’ tag. These introduce ‘external/message-body’ parts.
Each tag can contain zero or more parameters on the form ‘parameter=value’. The values may be enclosed in quotation marks, but that’s not necessary unless the value contains white space. So ‘filename=/home/user/#hello$^yes’ is perfectly valid.
If you want to talk about MML in a message, you need a way to “quote” these tags. The way to do that is to include an exclamation point after the opening two characters; i. e. ‘<#!part ...>’.
The following parameters have meaning in MML; parameters that have no meaning are ignored. The MML parameter names are the same as the MIME parameter names; the things in the parentheses say which header it will be used in.
The MIME type of the part (Content-Type).
Use the contents of the file in the body of the part
(Content-Disposition).
Use this as the file name in the generated MIME message for
the recipient. That is, even if the file is called foo.txt
locally, use this name instead in the Content-Disposition in
the sent message.
The contents of the body of the part are to be encoded in the character
set specified (Content-Type). See Charset Translation.
Might be used to suggest a file name if the part is to be saved
to a file (Content-Type).
Valid values are ‘inline’ and ‘attachment’
(Content-Disposition).
Valid values are ‘7bit’, ‘8bit’, ‘quoted-printable’ and
‘base64’. See Charset Translation. This parameter says what
Content-Transfer-Encoding to use when sending the part, and is
normally computed automatically.
This parameter says what encoding has been used on the data, and the data will be decoded before use. Valid values are ‘quoted-printable’ and ‘base64’. This is useful when you have a part with binary data (for instance an image) inserted directly into the Message buffer inside the ‘"<#part>...<#/part>"’ tags.
A description of the part (Content-Description).
Date when the part was created (Content-Disposition).
This uses the format of RFC 822 or its successors.
RFC 822 (or later) date when the part was modified
(Content-Disposition).
RFC 822 (or later) date when the part was read (Content-Disposition).
Who to encrypt/sign the part to. This field is used to override any auto-detection based on the To/Cc headers.
Identity used to sign the part. This field is used to override the default key used.
The size (in octets) of the part (Content-Disposition).
What technology to sign this MML part with (smime, pgp
or pgpmime)
What technology to encrypt this MML part with (smime,
pgp or pgpmime)
Parameters for ‘text/plain’:
Formatting parameter for the text, valid values include ‘fixed’ (the default) and ‘flowed’. Normally you do not specify this manually, since it requires the textual body to be formatted in a special way described in RFC 2646. See Flowed text.
Parameters for ‘application/octet-stream’:
Type of the part; informal—meant for human readers
(Content-Type).
Parameters for ‘message/external-body’:
A word indicating the supported access mechanism by which the file may
be obtained. Values include ‘ftp’, ‘anon-ftp’, ‘tftp’,
‘localfile’, and ‘mailserver’. (Content-Type.)
RFC 822 (or later) date after which the file may no longer be fetched.
(Content-Type.)
The size (in octets) of the file. (Content-Type.)
Valid values are ‘read’ and ‘read-write’
(Content-Type).
Parameters for ‘sign=smime’:
File containing key and certificate for signer.
Parameters for ‘encrypt=smime’:
File containing certificate for recipient.
Here’s a complex multipart message. It’s a ‘multipart/mixed’ that contains many parts, one of which is a ‘multipart/alternative’.
<#multipart type=mixed> <#part type=image/jpeg filename=~/rms.jpg disposition=inline> <#multipart type=alternative> This is a plain text part. <#part type=text/enriched name=enriched.txt> <center>This is a centered enriched part</center> <#/multipart> This is a new plain text part. <#part disposition=attachment> This plain text part is an attachment. <#/multipart>
And this is the resulting MIME message:
Content-Type: multipart/mixed; boundary="=-=-=" --=-=-= --=-=-= Content-Type: image/jpeg; filename="~/rms.jpg" Content-Disposition: inline; filename="~/rms.jpg" Content-Transfer-Encoding: base64 /9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/wAALCAAwADABAREA/8QAHwAA AQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQR BRIhMUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RF RkdISUpTVFVWV1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ip qrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uHi4+Tl5ufo6erx8vP09fb3+Pn6/9oACAEB AAA/AO/rifFHjldNuGsrDa0qcSSHkA+gHrXKw+LtWLrMb+RgTyhbr+HSug07xNqV9fQtZrNI AyiaE/NuBPOOOP0rvRNE880KOC8TbXXGCv1FPqjrF4LDR7u5L7SkTFT/ALWOP1xXgTuXfc7E sx6nua6rwp4IvvEM8chCxWxOdzn7wz6V9AaB4S07w9p5itow0rDLSY5Pt9K43xO66P4xs71m 2QXiGCbA4yOVJ9+1aYORkdK434lyNH4ahCnG66VT9Nj15JFbPdX0MS43M4VQf5/yr2vSpLnw 5ZW8dlCZ8KFXjOPX0/mK6rSPEGt3Angu44fNEReHYNvIH3TzXDeKNO8RX+kSX2ouZkicTIOc L+g7E810ulFjpVtv3bwgB3HJyK5L4quY/C9sVxk3ij/xx6850u7t1mtp/wDlpEw3An3Jr3Dw 34gsbWza4nBlhC5LDsaW6+IFgupQyCF3iHH7gA7c9R9ay7zx6t7aX9jHC4smhfBkGCvHGfrm tLQ7hbnRrV1GPkAP1x1/Hr+Ncr8Vzjwrbf8AX6v/AKA9eQRyYlQk8Yx9K6XTNbkgia2ciSIn 7p5Ga9Atte0LTLKO6it4i7dVRFJDcZ4PvXN+JvEMF9bILVGXJLSZ4zkjivRPDaeX4b08HOTC pOffmua+KkbS+GLVUGT9tT/0B68eeIpIFYjB70+OOVXyoOM9+M1eaWeCLzHPyHGO/NVWvJJm jQ8KGH1NfQWhXSXmh2c8eArRLwO3HSv/2Q== --=-=-= Content-Type: multipart/alternative; boundary="==-=-=" --==-=-= This is a plain text part. --==-=-= Content-Type: text/enriched; name="enriched.txt" <center>This is a centered enriched part</center> --==-=-=-- --=-=-= This is a new plain text part. --=-=-= Content-Disposition: attachment This plain text part is an attachment. --=-=-=--
mm-body-charset-encoding-alist ¶Mapping from MIME charset to encoding to use. This variable is usually used except, e.g., when other requirements force a specific encoding (digitally signed messages require 7bit encodings). The default is
((iso-2022-jp . 7bit) (iso-2022-jp-2 . 7bit) (utf-16 . base64) (utf-16be . base64) (utf-16le . base64))
As an example, if you do not want to have ISO-8859-1 characters
quoted-printable encoded, you may add (iso-8859-1 . 8bit) to
this variable. You can override this setting on a per-message basis
by using the encoding MML tag (see MML Definition).
mm-coding-system-priorities ¶Prioritize coding systems to use for outgoing messages. The default
is nil, which means to use the defaults in Emacs, but is
(iso-8859-1 iso-2022-jp utf-8) when running Emacs in the Japanese
language environment. It is a list of coding system symbols (aliases of
coding systems are also allowed, use M-x describe-coding-system to
make sure you are specifying correct coding system names). For example,
if you have configured Emacs to prefer UTF-8, but wish that outgoing
messages should be sent in ISO-8859-1 if possible, you can set this
variable to (iso-8859-1). You can override this setting on a
per-message basis by using the charset MML tag
(see MML Definition).
As different hierarchies prefer different charsets, you may want to set
mm-coding-system-priorities according to the hierarchy in Gnus.
Here’s an example:
(add-to-list 'gnus-newsgroup-variables 'mm-coding-system-priorities)
(setq gnus-parameters
(append
;; Some charsets are just examples!
'(("^cn\\." ;; Chinese
(mm-coding-system-priorities
'(iso-8859-1 cn-big5 chinese-iso-7bit utf-8)))
("^cz\\.\\|^pl\\." ;; Central and Eastern European
(mm-coding-system-priorities '(iso-8859-2 utf-8)))
("^de\\." ;; German language
(mm-coding-system-priorities '(iso-8859-1 iso-8859-15 utf-8)))
("^fr\\." ;; French
(mm-coding-system-priorities '(iso-8859-15 iso-8859-1 utf-8)))
("^fj\\." ;; Japanese
(mm-coding-system-priorities
'(iso-8859-1 iso-2022-jp utf-8)))
("^ru\\." ;; Cyrillic
(mm-coding-system-priorities
'(koi8-r iso-8859-5 iso-8859-1 utf-8))))
gnus-parameters))
mm-content-transfer-encoding-defaults ¶Mapping from MIME types to encoding to use. This variable is usually
used except, e.g., when other requirements force a safer encoding
(digitally signed messages require 7bit encoding). Besides the normal
MIME encodings, qp-or-base64 may be used to indicate that for
each case the most efficient of quoted-printable and base64 should be
used.
qp-or-base64 has another effect. It will fold long lines so that
MIME parts may not be broken by MTA. So do quoted-printable and
base64.
Note that it affects body encoding only when a part is a raw forwarded
message (which will be made by gnus-summary-mail-forward with the
arg 2 for example) or is neither the ‘text/*’ type nor the
‘message/*’ type. Even though in those cases, you can override
this setting on a per-message basis by using the encoding
MML tag (see MML Definition).
mm-use-ultra-safe-encoding ¶When this is non-nil, it means that textual parts are encoded as
quoted-printable if they contain lines longer than 76 characters or
starting with "From " in the body. Non-7bit encodings (8bit, binary)
are generally disallowed. This reduce the probability that a non-8bit
clean MTA or MDA changes the message. This should never be set
directly, but bound by other functions when necessary (e.g., when
encoding messages that are to be digitally signed).
During translation from MML to MIME, for each MIME part which has been composed inside Emacs, an appropriate charset has to be chosen.
If you are running a non-MULE Emacs, this process is simple: If the
part contains any non-ASCII (8-bit) characters, the MIME charset
given by mail-parse-charset (a symbol) is used. (Never set this
variable directly, though. If you want to change the default charset,
please consult the documentation of the package which you use to process
MIME messages.
See Various Message Variables in Message Manual, for example.)
If there are only ASCII characters, the MIME charset US-ASCII is
used, of course.
Things are slightly more complicated when running Emacs with MULE
support. In this case, a list of the MULE charsets used in the
part is obtained, and the MULE charsets are translated to
MIME charsets by consulting the table provided by Emacs itself.
If this results in a single MIME charset, this is used to encode
the part. But if the resulting list of MIME charsets contains more
than one element, two things can happen: If it is possible to encode the
part via UTF-8, this charset is used. (For this, Emacs must support
the utf-8 coding system, and the part must consist entirely of
characters which have Unicode counterparts.) If UTF-8 is not available
for some reason, the part is split into several ones, so that each one
can be encoded with a single MIME charset. The part can only be
split at line boundaries, though—if more than one MIME charset is
required to encode a single line, it is not possible to encode the part.
When running Emacs with MULE support, the preferences for which
coding system to use is inherited from Emacs itself. This means that
if Emacs is set up to prefer UTF-8, it will be used when encoding
messages. You can modify this by altering the
mm-coding-system-priorities variable though (see Encoding Customization).
The charset to be used can be overridden by setting the charset
MML tag (see MML Definition) when composing the message.
The encoding of characters (quoted-printable, 8bit, etc.) is orthogonal
to the discussion here, and is controlled by the variables
mm-body-charset-encoding-alist and
mm-content-transfer-encoding-defaults (see Encoding Customization).
A (multipart) MIME message can be converted to MML
with the mime-to-mml function. It works on the message in the
current buffer, and substitutes MML markup for MIME
boundaries. Non-textual parts do not have their contents in the buffer,
but instead have the contents in separate buffers that are referred to
from the MML tags.
An MML message can be converted back to MIME by the
mml-to-mime function.
These functions are in certain senses “lossy”—you will not get back
an identical message if you run mime-to-mml and then
mml-to-mime. Not only will trivial things like the order of the
headers differ, but the contents of the headers may also be different.
For instance, the original message may use base64 encoding on text,
while mml-to-mime may decide to use quoted-printable encoding, and
so on.
In essence, however, these two functions should be the inverse of each other. The resulting contents of the message should remain equivalent, if not identical.
The Emacs MIME library will respect the use-hard-newlines
variable (see Hard and Soft Newlines in Emacs Manual) when encoding a message, and the
“format=flowed” Content-Type parameter when decoding a message.
On encoding text, regardless of use-hard-newlines, lines
terminated by soft newline characters are filled together and wrapped
after the column decided by fill-flowed-encode-column.
Quotation marks (matching ‘^>* ?’) are respected. The variable
controls how the text will look in a client that does not support
flowed text, the default is to wrap after 66 characters. If hard
newline characters are not present in the buffer, no flow encoding
occurs.
You can customize the value of the mml-enable-flowed variable
to enable or disable the flowed encoding usage when newline
characters are present in the buffer.
On decoding flowed text, lines with soft newline characters are filled
together and wrapped after the column decided by
fill-flowed-display-column. The default is to wrap after
fill-column.
mm-fill-flowed ¶If non-nil a format=flowed article will be displayed flowed.
The mail-parse library is an abstraction over the actual
low-level libraries that are described in the next chapter.
Standards change, and so programs have to change to fit in the new
mold. For instance, RFC2045 describes a syntax for the
Content-Type header that only allows ASCII characters in the
parameter list. RFC2231 expands on RFC2045 syntax to provide a scheme
for continuation headers and non-ASCII characters.
The traditional way to deal with this is just to update the library functions to parse the new syntax. However, this is sometimes the wrong thing to do. In some instances it may be vital to be able to understand both the old syntax as well as the new syntax, and if there is only one library, one must choose between the old version of the library and the new version of the library.
The Emacs MIME library takes a different tack. It defines a
series of low-level libraries (rfc2047.el, rfc2231.el
and so on) that parses strictly according to the corresponding
standard. However, normal programs would not use the functions
provided by these libraries directly, but instead use the functions
provided by the mail-parse library. The functions in this
library are just aliases to the corresponding functions in the latest
low-level libraries. Using this scheme, programs get a consistent
interface they can use, and library developers are free to create
write code that handles new standards.
The following functions are defined by this library:
mail-header-parse-content-type ¶Parse a Content-Type header and return a list on the following
format:
("type/subtype"
(attribute1 . value1)
(attribute2 . value2)
...)
Here’s an example:
(mail-header-parse-content-type
"image/gif; name=\"b980912.gif\"")
⇒ ("image/gif" (name . "b980912.gif"))
mail-header-parse-content-disposition ¶Parse a Content-Disposition header and return a list on the same
format as the function above.
mail-content-type-get ¶Takes two parameters—a list on the format above, and an attribute. Returns the value of the attribute.
(mail-content-type-get
'("image/gif" (name . "b980912.gif")) 'name)
⇒ "b980912.gif"
mail-header-encode-parameter ¶Takes a parameter string and returns an encoded version of the string.
This is used for parameters in headers like Content-Type and
Content-Disposition.
mail-header-remove-comments ¶Return a comment-free version of a header.
(mail-header-remove-comments "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)") ⇒ "Gnus/5.070027 "
mail-header-remove-whitespace ¶Remove linear white space from a header. Space inside quoted strings and comments is preserved.
(mail-header-remove-whitespace "image/gif; name=\"Name with spaces\"") ⇒ "image/gif;name=\"Name with spaces\""
mail-header-get-comment ¶Return the last comment in a header.
(mail-header-get-comment "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)") ⇒ "Finnish Landrace"
mail-header-parse-address ¶Parse an address and return a list containing the mailbox and the plaintext name.
(mail-header-parse-address
"Hrvoje Nikšić <hniksic@srce.hr>")
⇒ ("hniksic@srce.hr" . "Hrvoje Nikšić")
mail-header-parse-addresses ¶Parse a string with list of addresses and return a list of elements like the one described above.
(mail-header-parse-addresses
"Hrvoje Nikšić <hniksic@srce.hr>, Steinar Bang <sb@metis.no>")
⇒ (("hniksic@srce.hr" . "Hrvoje Nikšić")
("sb@metis.no" . "Steinar Bang"))
mail-header-parse-date ¶Parse a date string and return an Emacs time structure.
mail-narrow-to-head ¶Narrow the buffer to the header section of the buffer. Point is placed at the beginning of the narrowed buffer.
mail-header-narrow-to-field ¶Narrow the buffer to the header under point. Understands continuation headers.
mail-header-fold-field ¶Fold the header under point.
mail-header-unfold-field ¶Unfold the header under point.
mail-header-field-value ¶Return the value of the field under point.
mail-encode-encoded-word-region ¶Encode the non-ASCII words in the region. For instance, ‘Naïve’ is encoded as ‘=?iso-8859-1?q?Na=EFve?=’.
mail-encode-encoded-word-buffer ¶Encode the non-ASCII words in the current buffer. This function is meant to be called narrowed to the headers of a message.
mail-encode-encoded-word-string ¶Encode the words that need encoding in a string, and return the result.
(mail-encode-encoded-word-string "This is naïve, baby") ⇒ "This is =?iso-8859-1?q?na=EFve,?= baby"
mail-decode-encoded-word-region ¶Decode the encoded words in the region.
mail-decode-encoded-word-string ¶Decode the encoded words in the string and return the result.
(mail-decode-encoded-word-string "This is =?iso-8859-1?q?na=EFve,?= baby") ⇒ "This is naïve, baby"
Currently, mail-parse is an abstraction over ietf-drums,
rfc2047, rfc2045 and rfc2231. These are documented
in the subsequent sections.
This chapter describes the basic, ground-level functions for parsing and
handling. Covered here is parsing From lines, removing comments
from header lines, decoding encoded words, parsing date headers and so
on. High-level functionality is dealt with in the first chapter
(see Decoding and Viewing).
RFC2045 is the “main” MIME document, and as such, one would imagine that there would be a lot to implement. But there isn’t, since most of the implementation details are delegated to the subsequent RFCs.
So rfc2045.el has only a single function:
rfc2045-encode-string ¶Takes a parameter and a value and returns a ‘PARAM=VALUE’ string. value will be quoted if there are non-safe characters in it.
RFC2231 defines a syntax for the Content-Type and
Content-Disposition headers. Its snappy name is MIME
Parameter Value and Encoded Word Extensions: Character Sets, Languages,
and Continuations.
In short, these headers look something like this:
Content-Type: application/x-stuff; title*0*=us-ascii'en'This%20is%20even%20more%20; title*1*=%2A%2A%2Afun%2A%2A%2A%20; title*2="isn't it!"
They usually aren’t this bad, though.
The following functions are defined by this library:
rfc2231-parse-string ¶Parse a Content-Type header and return a list describing its
elements.
(rfc2231-parse-string
"application/x-stuff;
title*0*=us-ascii'en'This%20is%20even%20more%20;
title*1*=%2A%2A%2Afun%2A%2A%2A%20;
title*2=\"isn't it!\"")
⇒ ("application/x-stuff"
(title . "This is even more ***fun*** isn't it!"))
rfc2231-get-value ¶Takes one of the lists on the format above and returns the value of the specified attribute.
rfc2231-encode-string ¶Encode a parameter in headers likes Content-Type and
Content-Disposition.
drums was an IETF working group that worked on Internet RFC 2822, the first successor to RFC 822 and a predecessor of the current email standard.
The functions provided by this library include:
ietf-drums-remove-comments ¶Remove the comments from the argument and return the results.
ietf-drums-remove-whitespace ¶Remove linear white space from the string and return the results. Spaces inside quoted strings and comments are left untouched.
ietf-drums-get-comment ¶Return the last most comment from the string.
ietf-drums-parse-address ¶Parse an address string and return a list that contains the mailbox and the plain text name.
ietf-drums-parse-addresses ¶Parse a string that contains any number of comma-separated addresses and return a list that contains mailbox/plain text pairs.
ietf-drums-parse-date ¶Parse a date string and return an Emacs time structure.
ietf-drums-narrow-to-header ¶Narrow the buffer to the header section of the current buffer.
RFC2047 (Message Header Extensions for Non-ASCII Text) specifies how non-ASCII text in headers are to be encoded. This is actually rather complicated, so a number of variables are necessary to tweak what this library does.
The following variables are tweakable:
rfc2047-header-encoding-alist ¶This is an alist of header / encoding-type pairs. Its main purpose is to prevent encoding of certain headers.
The keys can either be header regexps, or t.
The values can be nil, in which case the header(s) in question
won’t be encoded, mime, which means that they will be encoded, or
address-mime, which means the header(s) will be encoded carefully
assuming they contain addresses.
rfc2047-charset-encoding-alist ¶RFC2047 specifies two forms of encoding—Q (a
Quoted-Printable-like encoding) and B (base64). This alist
specifies which charset should use which encoding.
rfc2047-encode-function-alist ¶This is an alist of encoding / function pairs. The encodings are
Q, B and nil.
rfc2047-encoded-word-regexp ¶When decoding words, this library looks for matches to this regexp.
rfc2047-encoded-word-regexp-loose ¶This is a version from which the regexp for the Q encoding pattern of
rfc2047-encoded-word-regexp is made loose.
rfc2047-encode-encoded-words ¶The boolean variable specifies whether encoded words
(e.g., ‘=?us-ascii?q?hello?=’) should be encoded again.
rfc2047-encoded-word-regexp is used to look for such words.
rfc2047-allow-irregular-q-encoded-words ¶The boolean variable specifies whether irregular Q encoded words
(e.g., ‘=?us-ascii?q?hello??=’) should be decoded. If it is
non-nil, rfc2047-encoded-word-regexp-loose is used instead
of rfc2047-encoded-word-regexp to look for encoded words.
Those were the variables, and these are this functions:
rfc2047-narrow-to-field ¶Narrow the buffer to the header on the current line.
rfc2047-encode-message-header ¶Should be called narrowed to the header of a message. Encodes according
to rfc2047-header-encoding-alist.
rfc2047-encode-region ¶Encodes all encodable words in the region specified.
rfc2047-encode-string ¶Encode a string and return the results.
rfc2047-decode-region ¶Decode the encoded words in the region.
rfc2047-decode-string ¶Decode a string and return the results.
rfc2047-encode-parameter ¶Encode a parameter in the RFC2047-like style. This is a substitution
for the rfc2231-encode-string function, that is the standard but
many mailers don’t support it. See rfc2231.
While not really a part of the MIME library, it is convenient to
document time conversion functions often used when parsing Date headers
and manipulating time. (Not by using tesseracts, though, I’m sorry to
say.)
These functions convert between five formats: A date string, a Lisp timestamp, a decoded time list, a second number, and a day number.
Here’s a bunch of time/date/second/day examples:
(parse-time-string "Sat Sep 12 12:21:54 1998 +0200")
⇒ (54 21 12 12 9 1998 6 -1 7200)
(time-convert
(date-to-time "Sat Sep 12 12:21:54 1998 +0200")
'integer)
⇒ 905595714
(float-time '(905595714000000 . 1000000))
⇒ 905595714.0
(time-convert 905595714.0 1000000)
⇒ (905595714000000 . 1000000)
(time-to-days '(905595714000000 . 1000000))
⇒ 729644
(time-convert (days-to-time 729644) 'integer)
⇒ 63041241600
(time-convert (time-since '(905595714000000 . 1000000))
1000000)
⇒ (631963244775642171 . 1000000000)
(time-less-p '(905595714000000 . 1000000)
'(905595593000000000 . 1000000000))
⇒ nil
(time-equal-p '(905595593000000000 . 1000000000)
'(905595593000000 . 1000000 ))
⇒ t
(time-subtract '(905595714000000 . 1000000)
'(905595593000000000 . 1000000000))
⇒ (121000000 . 1000000)
(days-between "Sat Sep 12 12:21:54 1998 +0200"
"Sat Sep 07 12:21:54 1998 +0200")
⇒ 5
(date-leap-year-p 2000)
⇒ t
(time-to-day-in-year '(905595714000000 . 1000000))
⇒ 255
(time-to-number-of-days
(time-since
(date-to-time "Mon, 01 Jan 2001 02:22:26 GMT")))
⇒ 6472.722661506652
And finally, we have safe-date-to-time, which does the same as
date-to-time, but returns a zero time if the date is
syntactically malformed.
The five data representations used are the following:
An RFC 822 (or similar) date string. For instance: "Sat Sep 12
12:21:54 1998 +0200".
A Lisp timestamp.
For instance: (905595714000000 . 1000000).
An integer or floating point count of seconds. For instance:
905595714.0, 905595714.
An integer number representing the number of days since Sunday,
December 31, 1 BC (Gregorian). For instance: 729644.
A list of decoded time. For instance: (54 21 12 12 9 1998 6 nil
7200).
All the examples above represent the same moment, except that days represents the day containing the moment.
These are the functions available:
date-to-timeTake a date and return a time.
time-convertTake a time and return a timestamp in a specified form.
float-timeTake a time and return seconds.
encode-timeTake a decoded time and return a timestamp.
time-to-daysTake a time and return days.
days-to-timeTake days and return a time.
date-to-dayTake a date and return days.
time-to-number-of-daysTake a time and return the number of days that represents.
safe-date-to-timeTake a date and return a time. If the date is not syntactically valid, return a “zero” time.
time-less-pTake two times and say whether the first time is less (i.e., earlier) than the second time. (This is a built-in function.)
time-equal-pCheck whether two time values are equal. The time values need not be in the same format. (This is a built-in function.)
time-sinceTake a time and return a time saying how long it was since that time.
time-subtractTake two times and subtract the second from the first. I.e., return the time between the two times. (This is a built-in function.)
days-betweenTake two days and return the number of days between those two days.
date-leap-year-pTake a year number and say whether it’s a leap year.
time-to-day-in-yearTake a time and return the day number within the year that the time is in.
This library deals with decoding and encoding Quoted-Printable text.
Very briefly explained, qp encoding means translating all 8-bit characters (and lots of control characters) into things that look like ‘=EF’; that is, an equal sign followed by the byte encoded as a hex string.
The following functions are defined by the library:
quoted-printable-decode-region ¶QP-decode all the encoded text in the specified region.
quoted-printable-decode-string ¶Decode the QP-encoded text in a string and return the results.
quoted-printable-encode-region ¶QP-encode all the encodable characters in the specified region. The third optional parameter fold specifies whether to fold long lines. (Long here means 72.)
quoted-printable-encode-string ¶QP-encode all the encodable characters in a string and return the results.
Base64 is an encoding that encodes three bytes into four characters, thereby increasing the size by about 33%. The alphabet used for encoding is very resistant to mangling during transit.
The following functions are defined by this library:
base64-encode-region ¶base64 encode the selected region. Return the length of the encoded text. Optional third argument no-line-break means do not break long lines into shorter lines.
base64-encode-string ¶base64 encode a string and return the result.
base64-decode-region ¶base64 decode the selected region. Return the length of the decoded
text. If the region can’t be decoded, return nil and don’t
modify the buffer.
base64-decode-string ¶base64 decode a string and return the result. If the string can’t be
decoded, nil is returned.
binhex is an encoding that originated in Macintosh environments.
The following function is supplied to deal with these:
binhex-decode-region ¶Decode the encoded text in the region. If given a third parameter, only
decode the binhex header and return the filename.
uuencode is probably still the most popular encoding of binaries
used on Usenet, although base64 rules the mail world.
The following function is supplied by this package:
uudecode-decode-region ¶Decode the text in the region.
yenc is used for encoding binaries on Usenet. The following
function is supplied by this package:
yenc-decode-region ¶Decode the encoded text in the region.
RFC1843 deals with mixing Chinese and ASCII characters in messages. In essence, RFC1843 switches between ASCII and Chinese by doing this:
This sentence is in ASCII.
The next sentence is in GB.~{<:Ky2;S{#,NpJ)l6HK!#~}Bye.
Simple enough, and widely used in China.
The following functions are available to handle this encoding:
rfc1843-decode-regionDecode HZ-encoded text in the region.
rfc1843-decode-stringDecode a HZ-encoded string and return the result.
The ~/.mailcap file is parsed by most MIME-aware message handlers and describes how elements are supposed to be displayed. Here’s an example file:
image/*; gimp -8 %s audio/wav; wavplayer %s application/msword; catdoc %s ; copiousoutput ; nametemplate=%s.doc
This says that all image files should be displayed with gimp,
that WAVE audio files should be played by wavplayer, and that
MS-WORD files should be inlined by catdoc.
The mailcap library parses this file, and provides functions for
matching types.
mailcap-mime-data ¶This variable is an alist of alists containing backup viewing rules.
mailcap-user-mime-data ¶A customizable list of viewers that take preference over
mailcap-mime-data.
Interface functions:
mailcap-view-file ¶Prompt for a file name, and start a viewer applicable for the file type in question.
mailcap-parse-mailcaps ¶Parse the ~/.mailcap file.
mailcap-mime-infoTakes a MIME type as its argument and returns the matching viewer.
The mailcap-prefer-mailcap-viewers variable controls which
viewer is chosen. The default non-nil value means that
settings from ~/.mailcap is preferred over system-wide or
Emacs-provided viewer settings.
If nil, Emacs-provided viewer settings have precedence. Next,
the most specific viewer has precedence over less specific settings,
no matter if they’re system-provided or private, so ‘image/gif’
in /etc/mailcap will “win” over an ‘image/*’ setting in
~/.mailcap.
The Emacs MIME library implements handling of various elements according to a (somewhat) large number of RFCs, drafts and standards documents. This chapter lists the relevant ones. They can all be fetched from https://www.rfc-editor.org.
Internet Message Format
Netnews Article Format
Format of Internet Message Bodies
Media Types
Message Header Extensions for Non-ASCII Text
Media Type Specifications and Registration Procedures
Registration Procedures (obsoleting RFC 2048)
Conformance Criteria and Examples
MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
HZ—A Data Format for Exchanging Files of Arbitrarily Mixed Chinese and ASCII characters
The MIME Multipart/Related Content-type
The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages
Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field
The Text/Plain Format and DelSp Parameters
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If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
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