This manual documents an API for usage of D-Bus in Emacs. D-Bus is a message bus system, a simple way for applications to talk to one another. An overview of D-Bus can be found at https://dbus.freedesktop.org/.
Copyright © 2007–2024 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.”
D-Bus is an inter-process communication mechanism for applications residing on the same host. The communication is based on messages. Data in the messages is carried in a structured way, it is not just a byte stream.
The communication is connection oriented to two kinds of message buses: a so called system bus, and a session bus. On a given machine, there is always one single system bus for miscellaneous system-wide communication, like changing of hardware configuration. On the other hand, the session bus is always related to a single user’s session.
Every client application, which is connected to a bus, registers under a unique name at the bus. This name is used for identifying the client application. Such a unique name starts always with a colon, and looks like ‘:1.42’.
Additionally, a client application can register itself to a so called known name, which is a series of identifiers separated by dots, as in ‘org.gnu.Emacs’. If several applications register to the same known name, these registrations are queued, and only the first application which has registered for the known name is reachable via this name. If this application disconnects from the bus, the next queued unique name becomes the owner of this known name.
An application can install one or several objects under its name. Such objects are identified by an object path, which looks similar to paths in a filesystem. An example of such an object path could be ‘/org/gnu/Emacs/’.
Applications might send a request to an object, that means sending a message with some data as input parameters, and receiving a message from that object with the result of this message, the output parameters. Such a request is called method in D-Bus.
The other form of communication are signals. The underlying message is emitted from an object and will be received by all other applications which have registered for such a signal.
All methods and signals an object supports are called interface of the object. Interfaces are specified under a hierarchical name in D-Bus; an object can support several interfaces. Such an interface name could be ‘org.gnu.Emacs.TextEditor’ or ‘org.gnu.Emacs.FileManager’.
D-Bus has evolved over the years. New features have been added with new D-Bus versions. There are two variables, which allow the determination of the D-Bus version used.
This variable, a string, determines the version of D-Bus Emacs is
compiled against. If it cannot be determined the value is nil.
The other D-Bus version to be checked is the version of D-Bus Emacs
runs with. This string can be different from dbus-compiled-version.
It is also nil, if it cannot be determined at runtime.
There are several basic functions which inspect the buses for registered names. Internally they use the basic interface ‘org.freedesktop.DBus’, which is supported by all objects of a bus.
This function returns the D-Bus service names, which can be activated
for bus. It must be either the keyword :system (the
default) or the keyword :session. An activatable service is
described in a service registration file. Under GNU/Linux, such files
are located at /usr/share/dbus-1/system-services/ (for the
:system bus) or /usr/share/dbus-1/services/. An
activatable service is not necessarily registered at bus
already.
The result is a list of strings, which is nil when there are no
activatable service names at all. Example:
;; Check, whether the document viewer can be accessed via D-Bus.
(member "org.gnome.evince.Daemon"
(dbus-list-activatable-names :session))
This function returns all service names, which are registered at D-Bus
bus. The result is a list of strings, which is nil when
there are no registered service names at all. Well known names are
strings like ‘org.freedesktop.DBus’. Names starting with
‘:’ are unique names for services.
bus must be either the keyword :system or the keyword
:session.
This function retrieves all registered services which correspond to a
known name in bus. A service has a known name if it doesn’t
start with ‘:’. The result is a list of strings, which is
nil when there are no known names at all.
bus must be either the keyword :system or the keyword
:session.
For a given service, registered at D-Bus bus under the name
service, this function returns all queued unique names. The
result is a list of strings, or nil when there are no queued
names for service at all.
bus must be either the keyword :system or the keyword
:session. service must be a known service name as
string.
For a given service, registered at D-Bus bus under the name
service, this function returns the unique name of the name
owner. The result is a string, or nil when there is no name
owner of service.
bus must be either the keyword :system or the keyword
:session. service must be a known service name as
string.
This function checks whether the service name service is
registered at D-Bus bus. If service has not yet started,
it is autostarted if possible. The result is either t or
nil.
bus must be either the keyword :system or the keyword
:session. service must be a string. timeout, a
nonnegative integer, specifies the maximum number of milliseconds
before dbus-ping must return. The default value is 25,000.
Example:
(message "%s screensaver on board." (cond ((dbus-ping :session "org.gnome.ScreenSaver" 100) "Gnome") ((dbus-ping :session "org.freedesktop.ScreenSaver" 100) "KDE") (t "No")))
To check whether service is already running without autostarting it, you can instead write:
(member service (dbus-list-known-names bus))
This function returns the unique name, under which Emacs is registered at D-Bus bus, as a string.
bus must be either the keyword :system or the keyword
:session.
D-Bus services publish their interfaces. This can be retrieved and analyzed during runtime, in order to understand the used implementation.
The resulting introspection data are in XML format. The root
introspection element is always a node element. It might have
a name attribute, which denotes the (absolute) object path an
interface is introspected.
The root node element may have node and interface
children. A child node element must have a name
attribute, this case it is the relative object path to the root
node element.
An interface element has just one attribute, name, which
is the full name of that interface. The default interface
‘org.freedesktop.DBus.Introspectable’ is always present. Example:
<node name="/org/bluez">
<interface name="org.freedesktop.DBus.Introspectable">
...
</interface>
<interface name="org.bluez.Manager">
...
</interface>
<interface name="org.bluez.Database">
...
</interface>
<interface name="org.bluez.Security">
...
</interface>
<node name="service_audio"/>
<node name="service_input"/>
<node name="service_network"/>
<node name="service_serial"/>
</node>
Children of an interface element can be method,
signal and property elements. A method element
stands for a D-Bus method of the surrounding interface. The element
itself has a name attribute, showing the method name. Children
elements arg stand for the arguments of a method. Example:
<method name="ResolveHostName"> <arg name="interface" type="i" direction="in"/> <arg name="protocol" type="i" direction="in"/> <arg name="name" type="s" direction="in"/> <arg name="aprotocol" type="i" direction="in"/> <arg name="flags" type="u" direction="in"/> <arg name="interface" type="i" direction="out"/> <arg name="protocol" type="i" direction="out"/> <arg name="name" type="s" direction="out"/> <arg name="aprotocol" type="i" direction="out"/> <arg name="address" type="s" direction="out"/> <arg name="flags" type="u" direction="out"/> </method>
arg elements can have the attributes name, type
and direction. The name attribute is optional. The
type attribute stands for the signature of the argument
in D-Bus. For a discussion of D-Bus types and their Lisp
representation see Mapping Lisp types and D-Bus types..1
The direction attribute of an arg element can be only
‘in’ or ‘out’; in case it is omitted, it defaults to
‘in’.
A signal element of an interface has a similar
structure. The direction attribute of an arg child
element can be only ‘out’ here; which is also the default value.
Example:
<signal name="StateChanged"> <arg name="state" type="i"/> <arg name="error" type="s"/> </signal>
A property element has no arg child
element. It just has the attributes name, type and
access, which are all mandatory. The access attribute
allows the values ‘readwrite’, ‘read’, and ‘write’.
Example:
<property name="Status" type="u" direction="read"/>
annotation elements can be children of interface,
method, signal, and property elements. Unlike
properties, which can change their values during lifetime of a D-Bus
object, annotations are static. Often they are used for code
generators of D-Bus language bindings. Example:
<annotation name="de.berlios.Pinot.GetStatistics" value="pinotDBus"/>
Annotations have just name and value attributes, both
must be strings.
This function returns all interfaces and sub-nodes of service, registered at object path path at bus bus.
bus must be either the keyword :system or the keyword
:session. service must be a known service name, and
path must be a valid object path. The last two parameters are
strings. The result, the introspection data, is a string in XML
format. Example:
(dbus-introspect
:system "org.freedesktop.Hal"
"/org/freedesktop/Hal/devices/computer")
⇒ "<!DOCTYPE node PUBLIC
"-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<interface name="org.freedesktop.Hal.Device">
<method name="GetAllProperties">
<arg name="properties" direction="out" type="a{sv}"/>
</method>
...
<signal name="PropertyModified">
<arg name="num_updates" type="i"/>
<arg name="updates" type="a(sbb)"/>
</signal>
</interface>
...
</node>"
This example informs us, that the service ‘org.freedesktop.Hal’ at object path ‘/org/freedesktop/Hal/devices/computer’ offers the interface ‘org.freedesktop.Hal.Device’ (and 2 other interfaces not documented here). This interface contains the method ‘GetAllProperties’, which needs no input parameters, but returns as output parameter an array of dictionary entries (key-value pairs). Every dictionary entry has a string as key, and a variant as value.
The interface offers also a signal, which returns 2 parameters: an integer, and an array consisting of elements which are a struct of a string and 2 boolean values.2
This function serves a similar purpose to the function
dbus-introspect. The returned value is a parsed XML tree,
which can be used for further analysis. Example:
(dbus-introspect-xml
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main")
⇒ (node ((name . "/org/freedesktop/xesam/searcher/main"))
(interface ((name . "org.freedesktop.xesam.Search"))
(method ((name . "GetHitData"))
(arg ((name . "search")
(type . "s")
(direction . "in")))
(arg ((name . "hit_ids")
(type . "au")
(direction . "in")))
(arg ((name . "fields")
(type . "as")
(direction . "in")))
(arg ((name . "hit_data")
(type . "aav")
(direction . "out"))))
...
(signal ((name . "HitsAdded"))
(arg ((name . "search") (type . "s")))
(arg ((name . "count") (type . "u")))))
...)
This function returns the attribute value of a D-Bus
introspection object. The value of object can be any
subtree of a parsed XML tree as retrieved with
dbus-introspect-xml. attribute must be a string
according to the attribute names in the D-Bus specification. Example:
(dbus-introspect-get-attribute (dbus-introspect-xml :system "org.freedesktop.SystemToolsBackends" "/org/freedesktop/SystemToolsBackends/UsersConfig") "name") ⇒ "/org/freedesktop/SystemToolsBackends/UsersConfig"
If object has no attribute, the function returns
nil.
The first elements, to be introspected for a D-Bus object, are further object paths and interfaces.
This function returns all node names of service in D-Bus bus at object path path as a list of strings. Example:
(dbus-introspect-get-node-names
:session "org.gnome.seahorse" "/org/gnome/seahorse")
⇒ ("crypto" "keys")
The node names stand for further object paths of the D-Bus service, relative to path. In the example, ‘/org/gnome/seahorse/crypto’ and ‘/org/gnome/seahorse/keys’ are also object paths of the D-Bus service ‘org.gnome.seahorse’.
This function returns all node names of service in D-Bus bus at object path path. It returns a list of strings with all object paths of service, starting at path. Example:
(dbus-introspect-get-all-nodes :session "org.gnome.seahorse" "/")
⇒ ("/" "/org" "/org/gnome" "/org/gnome/seahorse"
"/org/gnome/seahorse/crypto"
"/org/gnome/seahorse/keys"
"/org/gnome/seahorse/keys/openpgp"
"/org/gnome/seahorse/keys/openpgp/local"
"/org/gnome/seahorse/keys/openssh"
"/org/gnome/seahorse/keys/openssh/local")
This function returns a list strings of all interface names of service in D-Bus bus at object path path. This list will contain the default interface ‘org.freedesktop.DBus.Introspectable’.
Another default interface is ‘org.freedesktop.DBus.Properties’.
If present, interface elements can also have property
children. Example:
(dbus-introspect-get-interface-names
:system "org.freedesktop.Hal"
"/org/freedesktop/Hal/devices/computer")
⇒ ("org.freedesktop.DBus.Introspectable"
"org.freedesktop.Hal.Device"
"org.freedesktop.Hal.Device.SystemPowerManagement"
"org.freedesktop.Hal.Device.CPUFreq")
This function returns interface of service in D-Bus
bus at object path path. The return value is an XML
element. interface must be a string and a member of the list
returned by dbus-introspect-get-interface-names. Example:
(dbus-introspect-get-interface
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search")
⇒ (interface ((name . "org.freedesktop.xesam.Search"))
(method ((name . "GetHitData"))
(arg ((name . "search") (type . "s") (direction . "in")))
(arg ((name . "hit_ids") (type . "au") (direction . "in")))
(arg ((name . "fields") (type . "as") (direction . "in")))
(arg ((name . "hit_data") (type . "aav") (direction . "out"))))
...
(signal ((name . "HitsAdded"))
(arg ((name . "search") (type . "s")))
(arg ((name . "count") (type . "u")))))
With these functions, it is possible to retrieve all introspection data from a running system:
(progn
(pop-to-buffer "*introspect*")
(erase-buffer)
(dolist (service (dbus-list-known-names :session))
(dolist (path (dbus-introspect-get-all-nodes :session service "/"))
;; We want to introspect only elements, which have more than
;; the default interface "org.freedesktop.DBus.Introspectable".
(when (delete
"org.freedesktop.DBus.Introspectable"
(dbus-introspect-get-interface-names :session service path))
(insert (format "\nservice: \"%s\" path: \"%s\"\n" service path)
(dbus-introspect :session service path))
(redisplay t)))))
Methods and signals are the communication means to D-Bus. The following functions return their specifications.
This function returns a list of strings of all method names of interface of service in D-Bus bus at object path path. Example:
(dbus-introspect-get-method-names
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search")
⇒ ("GetState" "StartSearch" "GetHitCount" "GetHits" "NewSession"
"CloseSession" "GetHitData" "SetProperty" "NewSearch"
"GetProperty" "CloseSearch")
This function returns method of interface as an XML
element. It must be located at service in D-Bus bus at
object path path. method must be a string and a member of
the list returned by dbus-introspect-get-method-names.
Example:
(dbus-introspect-get-method
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search" "GetHitData")
⇒ (method ((name . "GetHitData"))
(arg ((name . "search") (type . "s") (direction . "in")))
(arg ((name . "hit_ids") (type . "au") (direction . "in")))
(arg ((name . "fields") (type . "as") (direction . "in")))
(arg ((name . "hit_data") (type . "aav") (direction . "out"))))
This function returns a list of strings of all signal names of interface of service in D-Bus bus at object path path. Example:
(dbus-introspect-get-signal-names
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search")
⇒ ("StateChanged" "SearchDone" "HitsModified"
"HitsRemoved" "HitsAdded")
This function returns signal of interface as an XML
element. It must be located at service in D-Bus bus at
object path path. signal must be a string and a member of
the list returned by dbus-introspect-get-signal-names.
Example:
(dbus-introspect-get-signal
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search" "HitsAdded")
⇒ (signal ((name . "HitsAdded"))
(arg ((name . "search") (type . "s")))
(arg ((name . "count") (type . "u"))))
Interfaces can have properties. These can be exposed via the ‘org.freedesktop.DBus.Properties’ interface3. That is, properties can be retrieved and changed during the lifetime of an element.
A generalized interface is ‘org.freedesktop.DBus.Objectmanager’4, which returns objects, their interfaces and properties for a given service in just one call.
Annotations, on the other hand, are static values for an element. Often, they are used to instruct generators, how to generate code from the interface for a given language binding.
This function returns a list of strings with all property names of interface of service in D-Bus bus at object path path. Example:
(dbus-introspect-get-property-names
:session "org.kde.kded" "/modules/networkstatus"
"org.kde.Solid.Networking.Client")
⇒ ("Status")
If an interface declares properties, the corresponding element supports also the ‘org.freedesktop.DBus.Properties’ interface.
This function returns property of interface as an XML
element. It must be located at service in D-Bus bus at
object path path. property must be a string and a member
of the list returned by dbus-introspect-get-property-names.
A property value can be retrieved by the function
dbus-introspect-get-attribute. Example:
(dbus-introspect-get-property :session "org.kde.kded" "/modules/networkstatus" "org.kde.Solid.Networking.Client" "Status") ⇒ (property ((access . "read") (type . "u") (name . "Status"))) (dbus-introspect-get-attribute (dbus-introspect-get-property :session "org.kde.kded" "/modules/networkstatus" "org.kde.Solid.Networking.Client" "Status") "access") ⇒ "read"
This function returns the value of property of interface. It will be checked at bus, service, path. The result can be any valid D-Bus value. If there is no property, or property cannot be read, an error is raised. Example:
(dbus-get-property :session "org.kde.kded" "/modules/networkstatus" "org.kde.Solid.Networking.Client" "Status") ⇒ 4
This function sets the value of property of interface to value. It will be checked at bus, service, path. value can be preceded by a type keyword. When the value is successfully set, this function returns value. Example:
(dbus-set-property :session "org.kde.kaccess" "/MainApplication" "com.trolltech.Qt.QApplication" "doubleClickInterval" :uint16 500) ⇒ 500
This function returns all readable properties of interface. It
will be checked at bus, service, path. The result
is a list of cons cells. Every cons cell contains the name of the
property, and its value. If there are no properties, nil is
returned. Example:
(dbus-get-all-properties
:session "org.kde.kaccess" "/MainApplication"
"com.trolltech.Qt.QApplication")
⇒ (("cursorFlashTime" . 1000) ("doubleClickInterval" . 500)
("keyboardInputInterval" . 400) ("wheelScrollLines" . 3)
("globalStrut" 0 0) ("startDragTime" . 500)
("startDragDistance" . 4) ("quitOnLastWindowClosed" . t)
("styleSheet" . ""))
This function returns all objects at bus, service,
path, and the children of path. The result is a list of
objects. Every object is a cons cell of an existing path name, and
the list of available interface objects. An interface object is
another cons, whose car is the interface name and cdr is the list of
properties as returned by dbus-get-all-properties for that path
and interface. Example:
(dbus-get-all-managed-objects
:session "org.gnome.SettingsDaemon" "/")
⇒ (("/org/gnome/SettingsDaemon/Power"
("org.gnome.SettingsDaemon.Power.Keyboard")
("org.gnome.SettingsDaemon.Power.Screen")
("org.gnome.SettingsDaemon.Power"
("Icon" . ". GThemedIcon battery-full-charged-symbolic ")
("Tooltip" . "Laptop battery is charged"))
("org.freedesktop.DBus.Peer")
("org.freedesktop.DBus.Introspectable")
("org.freedesktop.DBus.Properties")
("org.freedesktop.DBus.ObjectManager"))
...)
If possible, ‘org.freedesktop.DBus.ObjectManager.GetManagedObjects’ is used for retrieving the information. Otherwise, the information is collected via ‘org.freedesktop.DBus.Introspectable.Introspect’ and ‘org.freedesktop.DBus.Properties.GetAll’, which is slow.
An overview of all existing object paths, their interfaces and properties could be retrieved by the following code:
(let ((result (mapcar (lambda (service)
(cons service
(dbus-get-all-managed-objects
:session service "/")))
(dbus-list-known-names :session))))
(pop-to-buffer "*objectmanager*")
(erase-buffer)
(pp result (current-buffer)))
This function returns a list of all annotation names as list of
strings. If name is nil, the annotations are children of
interface, otherwise name must be a method,
signal, or property XML element, where the annotations
belong to. Example:
(dbus-introspect-get-annotation-names
:session "de.berlios.Pinot" "/de/berlios/Pinot"
"de.berlios.Pinot" "GetStatistics")
⇒ ("de.berlios.Pinot.GetStatistics")
Default annotation names5 are
Whether or not the entity is deprecated; defaults to nil
The C symbol; may be used for methods and interfaces
If set, don’t expect a reply to the method call; defaults to nil
This function returns annotation as an XML object. If
name is nil, annotation is a child of
interface, otherwise name must be the name of a
method, signal, or property XML element, where
the annotation belongs to.
An attribute value can be retrieved by
dbus-introspect-get-attribute. Example:
(dbus-introspect-get-annotation
:session "de.berlios.Pinot" "/de/berlios/Pinot"
"de.berlios.Pinot" "GetStatistics"
"de.berlios.Pinot.GetStatistics")
⇒ (annotation ((name . "de.berlios.Pinot.GetStatistics")
(value . "pinotDBus")))
(dbus-introspect-get-attribute
(dbus-introspect-get-annotation
:session "de.berlios.Pinot" "/de/berlios/Pinot"
"de.berlios.Pinot" "GetStatistics"
"de.berlios.Pinot.GetStatistics")
"value")
⇒ "pinotDBus"
Methods and signals have arguments. They are described in the
arg XML elements.
This function returns a list of all argument names as strings.
name must be a method or signal XML element.
Example:
(dbus-introspect-get-argument-names
:session "org.freedesktop.xesam.searcher"
"/org/freedesktop/xesam/searcher/main"
"org.freedesktop.xesam.Search" "GetHitData")
⇒ ("search" "hit_ids" "fields" "hit_data")
Argument names are optional; the function can therefore return
nil, even if the method or signal has arguments.
This function returns the argument arg as an XML object.
name must be a method or signal XML element.
Example:
(dbus-introspect-get-argument :session "org.freedesktop.xesam.searcher" "/org/freedesktop/xesam/searcher/main" "org.freedesktop.xesam.Search" "GetHitData" "search") ⇒ (arg ((name . "search") (type . "s") (direction . "in")))
This function returns the signature of a method or
signal, represented by name, as a string.
If name is a method, direction can be either
‘in’ or ‘out’. If direction is nil, ‘in’
is assumed.
If name is a signal, and direction is
non-nil, direction must be ‘out’. Example:
(dbus-introspect-get-signature :session "org.freedesktop.xesam.searcher" "/org/freedesktop/xesam/searcher/main" "org.freedesktop.xesam.Search" "GetHitData" "in") ⇒ "sauas" (dbus-introspect-get-signature :session "org.freedesktop.xesam.searcher" "/org/freedesktop/xesam/searcher/main" "org.freedesktop.xesam.Search" "HitsAdded") ⇒ "su"
D-Bus method calls and signals accept usually several arguments as parameters, either as input parameter, or as output parameter. Every argument belongs to a D-Bus type.
Such arguments must be mapped between the value encoded as a D-Bus type, and the corresponding type of Lisp objects. The mapping is applied Lisp object → D-Bus type for input parameters, and D-Bus type → Lisp object for output parameters.
Input parameters for D-Bus methods and signals occur as arguments of a Lisp function call. The following mapping to D-Bus types is applied, when the corresponding D-Bus message is created:
Lisp type | D-Bus type | |
| → | DBUS_TYPE_BOOLEAN |
natural number | → | DBUS_TYPE_UINT32 |
negative integer | → | DBUS_TYPE_INT32 |
float | → | DBUS_TYPE_DOUBLE |
string | → | DBUS_TYPE_STRING |
list | → | DBUS_TYPE_ARRAY |
Other Lisp objects, like symbols or hash tables, are not accepted as input parameters.
If it is necessary to use another D-Bus type, a corresponding type
keyword can be prepended to the corresponding Lisp object. Basic
D-Bus types are represented by the type keywords :byte,
:boolean, :int16, :uint16, :int32,
:uint32, :int64, :uint64, :double,
:string, :object-path, :signature and
:unix-fd.
Example:
(dbus-call-method ... nat-number string)
is equivalent to
(dbus-call-method ... :uint32 nat-number :string string)
but different to
(dbus-call-method ... :int32 nat-number :signature string)
The value for a D-Bus byte type can be any natural number. If the
number is larger than 255, it is truncated to the least significant
byte. For example, :byte 1025 is equal to :byte 1. If
a character is used as argument, modifiers represented outside this
range are stripped off. For example, :byte ?x is equal to
:byte ?\M-x, but it is not equal to :byte ?\C-x or
:byte ?\M-\C-x.
Signed and unsigned D-Bus integer types expect a corresponding integer value. A unix file descriptor is restricted to the values 0…9.
If typed explicitly, a non-nil boolean value like
:boolean 'symbol is handled like t or :boolean t.
A D-Bus compound type is always represented as a list. The CAR
of this list can be the type keyword :array, :variant,
:struct or :dict-entry, which would result in a
corresponding D-Bus container. :array is optional, because
this is the default compound D-Bus type for a list.
The objects being elements of the list are checked according to the D-Bus compound type rules.
If an empty array needs an element D-Bus type other than string, it
can contain exactly one element of D-Bus type :signature. The
value of this element (a string) is used as the signature of the
elements of this array. Example:
(dbus-call-method
:session "org.freedesktop.Notifications"
"/org/freedesktop/Notifications"
"org.freedesktop.Notifications" "Notify"
"GNU Emacs" ; Application name.
0 ; No replacement of other notifications.
"" ; No icon.
"Notification summary" ; Summary.
(format ; Body.
"This is a test notification, raised from\n%S" (emacs-version))
'(:array) ; No actions (empty array of strings).
'(:array :signature "{sv}") ; No hints
; (empty array of dictionary entries).
:int32 -1) ; Default timeout.
⇒ 3
Sometimes, D-Bus methods require as input parameter an array of bytes, instead of a string. If it is guaranteed, that string is a UTF-8 string, this function performs the conversion. Example:
(dbus-string-to-byte-array "/etc/hosts")
⇒ (:array :byte 47 :byte 101 :byte 116 :byte 99 :byte 47
:byte 104 :byte 111 :byte 115 :byte 116 :byte 115)
This function escapes an arbitrary string so it follows the rules for a C identifier. The escaped string can be used as object path component, interface element component, bus name component or member name in D-Bus.
The escaping consists of replacing all non-alphanumerics, and the first character if it’s a digit, with an underscore and two lower-case hex digits. As a special case, "" is escaped to "_". Example:
(dbus-escape-as-identifier "0123abc_xyz\x01\xff") ⇒ "_30123abc_5fxyz_01_ff"
Output parameters of D-Bus methods and signals are mapped to Lisp objects.
D-Bus type | Lisp type | |
DBUS_TYPE_BOOLEAN | → |
|
DBUS_TYPE_BYTE | → | natural number |
DBUS_TYPE_UINT16 | → | natural number |
DBUS_TYPE_INT16 | → | integer |
DBUS_TYPE_UINT32 | → | natural number |
DBUS_TYPE_UNIX_FD | → | natural number |
DBUS_TYPE_INT32 | → | integer |
DBUS_TYPE_UINT64 | → | natural number |
DBUS_TYPE_INT64 | → | integer |
DBUS_TYPE_DOUBLE | → | float |
DBUS_TYPE_STRING | → | string |
DBUS_TYPE_OBJECT_PATH | → | string |
DBUS_TYPE_SIGNATURE | → | string |
DBUS_TYPE_ARRAY | → | list |
DBUS_TYPE_VARIANT | → | list |
DBUS_TYPE_STRUCT | → | list |
DBUS_TYPE_DICT_ENTRY | → | list |
The resulting list of the last 4 D-Bus compound types contains as elements the elements of the D-Bus container, mapped according to the same rules.
The signal PropertyModified, discussed as an example in
Inspection of D-Bus services., would offer as Lisp data the following object
(bool stands here for either nil or t):
(integer ((string bool bool) (string bool bool) ...))
If a D-Bus method or signal returns an array of bytes, which are known
to represent a UTF-8 string, this function converts byte-array
to the corresponding string. The string is unibyte encoded, unless
multibyte is non-nil. Example:
(dbus-byte-array-to-string '(47 101 116 99 47 104 111 115 116 115)) ⇒ "/etc/hosts"
This function retrieves the original string from the encoded
string as a unibyte string. The value of string must have
been encoded with dbus-escape-as-identifier. Example:
(dbus-unescape-from-identifier "_30123abc_5fxyz_01_ff") ⇒ "0123abc_xyz\x01\xff"
If the original string used in dbus-escape-as-identifier is a
multibyte string, it cannot be expected that this function returns
that string:
(string-equal (dbus-unescape-from-identifier (dbus-escape-as-identifier "Grüß Göttin")) "Grüß Göttin") ⇒ nil
Methods can be called synchronously (blocking) or asynchronously (non-blocking).
At the D-Bus level, a method call consist of two messages: one message which carries the input parameters to the object owning the method to be called, and a reply message returning the resulting output parameters from the object.
This function calls method on the D-Bus bus. bus is
either the keyword :system or the keyword :session.
service is the D-Bus service name to be used. path is the D-Bus object path, service is registered at. interface is an interface offered by service. It must provide method.
If the parameter :timeout is given, the following integer
timeout specifies the maximum number of milliseconds before the
method call must return. The default value is 25,000. If the method
call doesn’t return in time, a D-Bus error is raised (see Errors and events.).
The remaining arguments args are passed to method as arguments. They are converted into D-Bus types as described in Mapping Lisp types and D-Bus types..
The function returns the resulting values of method as a list of Lisp objects, according to the type conversion rules described in Mapping Lisp types and D-Bus types.. Example:
(dbus-call-method
:session "org.gnome.seahorse" "/org/gnome/seahorse/keys/openpgp"
"org.gnome.seahorse.Keys" "GetKeyField"
"openpgp:657984B8C7A966DD" "simple-name")
⇒ (t ("Philip R. Zimmermann"))
If the result of the method call is just one value, the converted Lisp object is returned instead of a list containing this single Lisp object. Example:
(dbus-call-method :system "org.freedesktop.Hal" "/org/freedesktop/Hal/devices/computer" "org.freedesktop.Hal.Device" "GetPropertyString" "system.kernel.machine") ⇒ "i686"
With the dbus-introspect function it is possible to explore the
interfaces of ‘org.freedesktop.Hal’ service. It offers the
interfaces ‘org.freedesktop.Hal.Manager’ for the object at the
path ‘/org/freedesktop/Hal/Manager’ as well as the interface
‘org.freedesktop.Hal.Device’ for all objects prefixed with the
path ‘/org/freedesktop/Hal/devices’. With the methods
‘GetAllDevices’ and ‘GetAllProperties’, it is simple to
emulate the lshal command on GNU/Linux systems:
(dolist (device
(dbus-call-method
:system "org.freedesktop.Hal"
"/org/freedesktop/Hal/Manager"
"org.freedesktop.Hal.Manager" "GetAllDevices"))
(message "\nudi = %s" device)
(dolist (properties
(dbus-call-method
:system "org.freedesktop.Hal" device
"org.freedesktop.Hal.Device" "GetAllProperties"))
(message " %s = %S"
(car properties) (or (caadr properties) ""))))
-| "udi = /org/freedesktop/Hal/devices/computer
info.addons = (\"hald-addon-acpi\")
info.bus = \"unknown\"
info.product = \"Computer\"
info.subsystem = \"unknown\"
info.udi = \"/org/freedesktop/Hal/devices/computer\"
linux.sysfs_path_device = \"(none)\"
power_management.acpi.linux.version = \"20051216\"
power_management.can_suspend_to_disk = t
power_management.can_suspend_to_ram = \"\"
power_management.type = \"acpi\"
smbios.bios.release_date = \"11/07/2001\"
system.chassis.manufacturer = \"COMPAL\"
system.chassis.type = \"Notebook\"
system.firmware.release_date = \"03/19/2005\"
..."
This function calls method on the D-Bus bus
asynchronously. bus is either the keyword :system or the
keyword :session.
service is the D-Bus service name to be used. path is the D-Bus object path, service is registered at. interface is an interface offered by service. It must provide method.
handler is a Lisp function, which is called when the
corresponding return message arrives. If handler is nil,
no return message will be expected.
If the parameter :timeout is given, the following integer
timeout specifies the maximum number of milliseconds before a
reply message must arrive. The default value is 25,000. If there is
no reply message in time, a D-Bus error is raised (see Errors and events.).
The remaining arguments args are passed to method as arguments. They are converted into D-Bus types as described in Mapping Lisp types and D-Bus types..
If handler is a Lisp function, the function returns a key into
the hash table dbus-registered-objects-table. The
corresponding entry in the hash table is removed, when the return
message arrives, and handler is called. Example:
(dbus-call-method-asynchronously :system "org.freedesktop.Hal" "/org/freedesktop/Hal/devices/computer" "org.freedesktop.Hal.Device" "GetPropertyString" (lambda (msg) (message "%s" msg)) "system.kernel.machine") -| i686 ⇒ (:serial :system 2)
You can offer an own service in D-Bus, which will be visible by other D-Bus clients. See https://dbus.freedesktop.org/doc/dbus-api-design.html for a discussion of the design.
In order to register methods on the D-Bus, Emacs has to request a well known name on the D-Bus under which it will be available for other clients. Names on the D-Bus can be registered and unregistered using the following functions:
This function registers the known name service on D-Bus bus.
bus is either the keyword :system or the keyword
:session.
service is the service name to be registered on the D-Bus. It must be a known name.
flags is a subset of the following keywords:
:allow-replacementAllow another service to become the primary owner if requested.
:replace-existingRequest to replace the current primary owner.
:do-not-queueIf we can not become the primary owner do not place us in the queue.
One of the following keywords is returned:
:primary-ownerWe have become the primary owner of the name service.
:in-queueWe could not become the primary owner and have been placed in the queue.
:existsWe already are in the queue.
:already-ownerWe already are the primary owner.
This function unregisters all objects from D-Bus bus, that were registered by Emacs for service.
bus is either the keyword :system or the keyword
:session.
service is the D-Bus service name of the D-Bus. It must be a known name. Emacs releases its association to service from D-Bus.
One of the following keywords is returned:
:releasedWe successfully released the name service.
:non-existentThe name service does not exist on the bus.
:not-ownerWe are not an owner of the name service.
When service is not a known name but a unique name, the function returns nil.
When a name has been chosen, Emacs can offer its own methods, which can be called by other applications. These methods could be an implementation of an interface of a well known service, like ‘org.freedesktop.TextEditor’.
They could also be an implementation of its own interface. In this
case, the service name must be ‘org.gnu.Emacs’. The object path
shall begin with ‘/org/gnu/Emacs/application’, and the
interface name shall be org.gnu.Emacs.application, where
application is the name of the application which provides the
interface.
The well known service name ‘org.gnu.Emacs’ of Emacs.
The object path namespace ‘/org/gnu/Emacs’ used by Emacs.
The interface namespace org.gnu.Emacs used by Emacs.
With this function, an application registers method on the D-Bus bus.
bus is either the keyword :system or the keyword
:session.
service is the D-Bus service name of the D-Bus object method is registered for. It must be a known name (see discussion of dont-register-service below).
path is the D-Bus object path service is registered (see discussion of dont-register-service below).
interface is the interface offered by service. It must provide method.
handler is a Lisp function to be called when a method call is received. It must accept as arguments the input arguments of method. handler should return a list, whose elements are to be used as arguments for the reply message of method. This list can be composed like the input parameters in Mapping Lisp types and D-Bus types..
If handler wants to return just one Lisp object and it is not a cons cell, handler can return this object directly, instead of returning a list containing the object.
If handler returns a reply message with an empty argument list,
handler must return the keyword :ignore in order to
distinguish it from nil (the boolean false).
If handler detects an error, it shall return the list
(:error error-name error-message).
error-name is a namespaced string which characterizes the error
type, and error-message is a free text string. Alternatively,
any Emacs signal dbus-error in handler raises a D-Bus
error message with the error name ‘org.freedesktop.DBus.Error.Failed’.
When dont-register-service is non-nil, the known name
service is not registered. This means that other D-Bus clients
have no way of noticing the newly registered method. When interfaces
are constructed incrementally by adding single methods or properties
at a time, dont-register-service can be used to prevent other
clients from discovering the still incomplete interface.
The default D-Bus timeout when waiting for a message reply is 25 seconds. This value could be even smaller, depending on the calling client. Therefore, handler should not last longer than absolutely necessary.
dbus-register-method returns a Lisp object, which can be used
as argument in dbus-unregister-object for removing the
registration for method. Example:
(defun my-dbus-method-handler (filename)
(if (find-file filename)
'(:boolean t)
'(:boolean nil)))
(dbus-register-method
:session "org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"
"org.freedesktop.TextEditor" "OpenFile"
#'my-dbus-method-handler)
⇒ ((:method :session "org.freedesktop.TextEditor" "OpenFile")
("org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"
my-dbus-method-handler))
If you invoke the method ‘org.freedesktop.TextEditor.OpenFile’
from another D-Bus application with a file name as parameter, the file
is opened in Emacs, and the method returns either true or
false, indicating the success of the method. As a test tool one
could use the command line tool dbus-send in a shell:
# dbus-send --session --print-reply \
--dest="org.freedesktop.TextEditor" \
"/org/freedesktop/TextEditor" \
"org.freedesktop.TextEditor.OpenFile" string:"/etc/hosts"
-| method return sender=:1.22 -> dest=:1.23 reply_serial=2
boolean true
You can indicate an error by returning an :error list reply, or
by raising the Emacs signal dbus-error. The handler above
could be changed like this:
(defun my-dbus-method-handler (&rest args)
(if (not (and (= (length args) 1) (stringp (car args))))
(list :error
"org.freedesktop.TextEditor.Error.InvalidArgs"
(format "Wrong argument list: %S" args))
(condition-case err
(find-file (car args))
(error (signal 'dbus-error (cdr err))))
t))
The test then runs
# dbus-send --session --print-reply \
--dest="org.freedesktop.TextEditor" \
"/org/freedesktop/TextEditor" \
"org.freedesktop.TextEditor.OpenFile" \
string:"/etc/hosts" string:"/etc/passwd"
-| Error org.freedesktop.TextEditor.Error.InvalidArgs:
Wrong argument list: ("/etc/hosts" "/etc/passwd")
# dbus-send --session --print-reply \
--dest="org.freedesktop.TextEditor" \
"/org/freedesktop/TextEditor" \
"org.freedesktop.TextEditor.OpenFile" \
string:"/etc/crypttab"
-| Error org.freedesktop.DBus.Error.Failed:
D-Bus error: "File is not readable", "/etc/crypttab"
With this function, an application declares a property on the D-Bus bus.
bus is either the keyword :system or the keyword
:session.
service is the D-Bus service name of the D-Bus. It must be a known name.
path is the D-Bus object path service is registered (see discussion of dont-register-service below).
interface is the name of the interface used at path, property is the name of the property of interface.
access indicates, whether the property can be changed by other
services via D-Bus. It must be either the keyword :read,
:write or :readwrite.
value is the initial value of the property, it can be of any valid type (See dbus-call-method, for details). value can be preceded by a type keyword.
If property already exists on path, it will be
overwritten. For properties with access type :read this is the
only way to change their values. Properties with access type
:write or :readwrite can be changed by
dbus-set-property.
The interface ‘org.freedesktop.DBus.Properties’ is added to
path, including a default handler for the ‘Get’,
‘GetAll’ and ‘Set’ methods of this interface. When
emits-signal is non-nil, the signal
‘PropertiesChanged’ is sent when the property is changed by
dbus-set-property.
When dont-register-service is non-nil, the known name
service is not registered. This means that other D-Bus clients
have no way of noticing the newly registered method. When interfaces
are constructed incrementally by adding single methods or properties
at a time, dont-register-service can be used to prevent other
clients from discovering the still incomplete interface.
dbus-register-property returns a Lisp object, which can be used
as argument in dbus-unregister-object for removing the
registration for property. Example:
(dbus-register-property
:session "org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"
"org.freedesktop.TextEditor" "name" :read "GNU Emacs")
⇒ ((:property :session "org.freedesktop.TextEditor" "name")
("org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"))
(dbus-register-property
:session "org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"
"org.freedesktop.TextEditor" "version" :readwrite emacs-version t)
⇒ ((:property :session "org.freedesktop.TextEditor" "version")
("org.freedesktop.TextEditor" "/org/freedesktop/TextEditor"))
Other D-Bus applications can read the property via the default methods
‘org.freedesktop.DBus.Properties.Get’ and
‘org.freedesktop.DBus.Properties.GetAll’. Testing is also
possible via the command line tool dbus-send in a shell:
# dbus-send --session --print-reply \
--dest="org.freedesktop.TextEditor" \
"/org/freedesktop/TextEditor" \
"org.freedesktop.DBus.Properties.GetAll" \
string:"org.freedesktop.TextEditor"
-| method return sender=:1.22 -> dest=:1.23 reply_serial=3
array [
dict entry(
string "name"
variant string "GNU Emacs"
)
dict entry(
string "version"
variant string "23.1.50.5"
)
]
It is also possible to apply the dbus-get-property,
dbus-get-all-properties and dbus-set-property functions
(see What else to know about interfaces.).
(dbus-set-property :session "org.freedesktop.TextEditor" "/org/freedesktop/TextEditor" "org.freedesktop.TextEditor" "version" "23.1.50") ⇒ "23.1.50" (dbus-get-property :session "org.freedesktop.TextEditor" "/org/freedesktop/TextEditor" "org.freedesktop.TextEditor" "version") ⇒ "23.1.50"
This function unregisters object from the D-Bus. object
must be the result of a preceding dbus-register-method,
dbus-register-property, dbus-register-signal
(see Sending and receiving signals.) or dbus-register-monitor call. It returns
t if object has been unregistered, nil otherwise.
When object identifies the last method or property, which is registered for the respective service, Emacs releases its association to the service from D-Bus.
Signals are one way messages. They carry input parameters, which are received by all objects which have registered for such a signal.
This function is similar to dbus-call-method. The difference
is, that there are no returning output parameters.
The function emits signal on the D-Bus bus. bus is
either the keyword :system or the keyword :session. It
doesn’t matter whether another object has registered for signal.
Signals can be unicast or broadcast messages. For broadcast messages,
service must be nil. Otherwise, service is the
D-Bus service name the signal is sent to as a unicast
message.6 path is the D-Bus object path
signal is sent from. interface is an interface available
at path. It must provide signal.
The remaining arguments args are passed to signal as arguments. They are converted into D-Bus types as described in Mapping Lisp types and D-Bus types.. Example:
(dbus-send-signal :session nil dbus-path-emacs (concat dbus-interface-emacs ".FileManager") "FileModified" "/home/albinus/.emacs")
With this function, an application registers for a signal on the D-Bus bus.
bus is either the keyword :system or the keyword
:session.
service is the D-Bus service name used by the sending D-Bus object. It can be either a known name or the unique name of the D-Bus object sending the signal. A known name will be mapped onto the unique name of the object, owning service at registration time. When the corresponding D-Bus object disappears, signals will no longer be received.
path is the corresponding D-Bus object path that service is registered at. interface is an interface offered by service. It must provide signal.
service, path, interface and signal can be
nil. This is interpreted as a wildcard for the respective
argument.
handler is a Lisp function to be called when the signal is received. It must accept as arguments the output parameters signal is sending.
The remaining arguments args can be keywords or keyword string pairs.7 Their meaning is as follows:
:argn string:pathn stringThis stands for the nth argument of the signal.
:pathn arguments can be used for object path wildcard
matches as specified by D-Bus, while an :argN argument requires
an exact match.
:arg-namespace stringRegister for those signals, whose first argument names a service or interface within the namespace string.
:path-namespace stringRegister for the object path namespace string. All signals sent
from an object path, which has string as the preceding string,
are matched. This requires path to be nil.
:eavesdropRegister for unicast signals which are not directed to the D-Bus object Emacs is registered at D-Bus BUS, if the security policy of BUS allows this. Otherwise, this argument is ignored.
dbus-register-signal returns a Lisp object, which can be used
as argument in dbus-unregister-object for removing the
registration for signal. Example:
(defun my-dbus-signal-handler (device)
(message "Device %s added" device))
(dbus-register-signal
:system "org.freedesktop.Hal" "/org/freedesktop/Hal/Manager"
"org.freedesktop.Hal.Manager" "DeviceAdded"
#'my-dbus-signal-handler)
⇒ ((:signal :system "org.freedesktop.Hal.Manager" "DeviceAdded")
("org.freedesktop.Hal" "/org/freedesktop/Hal/Manager"
my-signal-handler))
As we know from the introspection data of interface
‘org.freedesktop.Hal.Manager’, the signal ‘DeviceAdded’
provides one single parameter, which is mapped into a Lisp string.
The callback function my-dbus-signal-handler must therefore
define a single string argument. Plugging a USB device into your
machine, when registered for signal ‘DeviceAdded’, will show you
which objects the GNU/Linux hal daemon adds.
Some of the match rules have been added to a later version of D-Bus. In order to test the availability of such features, you could register for a dummy signal, and check the result:
(dbus-ignore-errors (dbus-register-signal :system nil nil nil nil #'ignore :path-namespace "/invalid/path")) ⇒ nil
Until now, we have spoken about the system and the session buses,
which are the default buses to be connected to. However, it is
possible to connect to any bus with a known address. This is a UNIX
domain or TCP/IP socket. Everywhere, where a bus is mentioned
as argument of a function (the keyword :system or the keyword
:session), this address can be used instead. The connection to
this bus must be initialized first.
This function establishes the connection to D-Bus bus.
bus can be either the keyword :system or the keyword
:session, or it can be a string denoting the address of the
corresponding bus. For the system and session buses, this function is
called when loading dbus.el, there is no need to call it again.
If Emacs was invoked when there was no D-Bus session bus available
yet, you can set the environment variable
DBUS_SESSION_BUS_ADDRESS once the session bus daemon is running
and offering the address. Calling dbus-init-bus initializes
the connection to the session bus.
(setenv "DBUS_SESSION_BUS_ADDRESS" "unix:path=/run/user/1000/bus") ⇒ "unix:path=/run/user/1000/bus" (dbus-init-bus :session) ⇒ 2
dbus-init-bus returns the number of connections this Emacs
session has established to the bus under the same unique name
(see dbus-get-unique-name). It depends on the libraries Emacs is
linked with, and on the environment Emacs is running. For example, if
Emacs is linked with the GTK+ toolkit, and it runs in a GTK+-aware
environment like GNOME, another connection might already be
established.
When private is non-nil, a new connection is established
instead of reusing an existing one. It results in a new unique name
at the bus. This can be used, if it is necessary to distinguish
from another connection used in the same Emacs process, like the one
established by GTK+. If bus is the keyword :system or
the keyword :session, the new private connection is identified
by the keywords :system-private or :session-private,
respectively.
Example: You initialize a connection to the AT-SPI bus on your host:
(setq my-bus
(dbus-call-method
:session "org.a11y.Bus" "/org/a11y/bus"
"org.a11y.Bus" "GetAddress"))
⇒ "unix:abstract=/tmp/dbus-2yzWHOCdSD,guid=a490dd26625870ca1298b6e10000fd7f"
;; If Emacs is built with GTK+ support, and you run in a GTK+-enabled
;; environment (like a GNOME session), the initialization reuses the
;; connection established by GTK+'s atk bindings.
(dbus-init-bus my-bus)
⇒ 2
(dbus-get-unique-name my-bus)
⇒ ":1.19"
;; Open a new connection to the same bus. This supersedes the
;; previous one.
(dbus-init-bus my-bus 'private)
⇒ 1
(dbus-get-unique-name my-bus)
⇒ ":1.20"
D-Bus addresses can specify a different transport. A possible address could be based on TCP/IP sockets, see next example. Which transport is supported depends on the bus daemon configuration, however.
This function sets the value of the bus environment variable to value.
bus is either a Lisp keyword, :system or :session,
or a string denoting the bus address. Both variable and
value should be strings.
Normally, services inherit the environment of the bus daemon. This function adds to or modifies that environment when activating services.
Some bus instances, such as :system, may disable setting the
environment. In such cases, or if this feature is not available in
older D-Bus versions, this function signals a dbus-error.
As an example, it might be desirable to start X11 enabled services on a remote host’s bus on the same X11 server the local Emacs is running. This could be achieved by
(setq my-bus "unix:host=example.gnu.org,port=4711") ⇒ "unix:host=example.gnu.org,port=4711" (dbus-init-bus my-bus) ⇒ 1 (dbus-setenv my-bus "DISPLAY" (getenv "DISPLAY")) ⇒ nil
The internal actions can be traced by running in a debug mode.
If this variable is non-nil, D-Bus specific debug messages are raised.
Input parameters of dbus-call-method,
dbus-call-method-asynchronously, dbus-send-signal,
dbus-register-method, dbus-register-property and
dbus-register-signal are checked for correct D-Bus types. If
there is a type mismatch, the Lisp error wrong-type-argument
D-Bus arg is raised.
All errors raised by D-Bus are signaled with the error symbol
dbus-error. If possible, error messages from D-Bus are
appended to the dbus-error.
This executes forms exactly like a progn, except that
dbus-error errors are ignored during the forms (the macro
returns nil then). These errors can be made visible when
dbus-debug is set to non-nil.
Incoming D-Bus messages are handled as Emacs events, see (elisp)Misc Events. They are retrieved only, when Emacs runs in interactive mode. The generated event has this form:
(dbus-event bus type serial service destination path interface member
handler &rest args)
bus identifies the D-Bus the message is coming from. It is
either a Lisp keyword, :system, :session,
:system-private or :session-private, or a string
denoting the bus address.
type is the D-Bus message type which has caused the event. It
can be dbus-message-type-invalid,
dbus-message-type-method-call,
dbus-message-type-method-return,
dbus-message-type-error, or dbus-message-type-signal.
serial is the serial number of the received D-Bus message,
unless type is equal dbus-message-type-error.
service and path are the unique name and the object path
of the D-Bus object emitting the message. destination is the
D-Bus name the message is dedicated to, or nil in case the
message is a broadcast signal.
interface and member denote the message which has been
sent. When type is dbus-message-type-error, member
is the error name.
handler is the callback function which has been registered for
this message (see Sending and receiving signals.). args are the typed arguments as
returned from the message. They are passed to handler without
type information, when it is called during event handling in
dbus-handle-event.
In order to inspect the dbus-event data, you could extend the
definition of the callback function in Sending and receiving signals.:
(defun my-dbus-signal-handler (&rest args) (message "my-dbus-signal-handler: %S" last-input-event))
There exist convenience functions which could be called inside a callback function in order to retrieve the information from the event.
This function returns the bus name event is coming from. The
result is either the keyword :system or the keyword
:session.
This function returns the message type of the corresponding D-Bus message. The result is a natural number.
This function returns the serial number of the corresponding D-Bus message. The result is a natural number.
This function returns the unique name of the D-Bus object event is coming from.
This function returns the unique name of the D-Bus object event is dedicated to.
This function returns the object path of the D-Bus object event is coming from.
This function returns the interface name of the D-Bus object event is coming from.
This function returns the member name of the D-Bus object event is coming from. It is either a signal name or a method name.
This function returns the handler the D-Bus object event is applied with.
This function returns the arguments the D-Bus object event is carrying on.
D-Bus errors are not propagated during event handling, because it is
usually not desired. D-Bus errors in events can be made visible by
setting the variable dbus-debug to non-nil. They can
also be handled by a hook function.
This hook variable keeps a list of functions, which are called when a
D-Bus error happens in the event handler. Every function must accept
two arguments, the event and the error variable caught in
condition-case by dbus-error.
Such functions can be used to adapt the error signal to be raised. Example:
(defun my-dbus-event-error-handler (event error)
(when (string-equal (concat dbus-interface-emacs ".FileManager")
(dbus-event-interface-name event))
(message "my-dbus-event-error-handler: %S %S" event error)
(signal 'file-error (cdr error))))
(add-hook 'dbus-event-error-functions #'my-dbus-event-error-handler)
Hook functions should take into account that there might be other D-Bus applications running. They should therefore check carefully, whether a given D-Bus error is related to them.
This function registers handler for monitoring messages on the D-Bus bus.
bus is either a Lisp keyword, :system or :session,
or a string denoting the bus address.
handler is the function to be called when a D-Bus event to be
monitored arrives. It is called with the args slot of the D-Bus
event (see Errors and events.), which are stripped off the type
keywords. If handler is nil, the default handler
dbus-monitor-handler is applied. This default handler behaves
similar to the dbus-monitor program.
The other arguments are keyword-value pairs. :type type
defines the message type to be monitored. If given, it must be equal
one of the strings ‘method_call’, ‘method_return’,
‘error’ or ‘signal’.
:sender sender and :destination destination
are D-Bus names. They can be unique names, or well-known service
names.
:path path is the D-Bus object to be monitored.
:interface interface is the name of an interface, and
:member member is either a method name, a signal name, or
an error name.
dbus-register-monitor returns a Lisp object, which can be used
as argument in dbus-unregister-object for removing the monitor.
The following form shows all D-Bus events on the session bus in buffer ‘*D-Bus Monitor*’:
(dbus-register-monitor :session)
And this form restricts the monitoring on D-Bus errors:
(dbus-register-monitor :session nil :type "error")
This command invokes dbus-register-monitor interactively, and
switches to the monitor buffer.
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D-Bus signatures are explained in the D-Bus specification https://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-signatures.
The interfaces of the service ‘org.freedesktop.Hal’ are described in the HAL specification.
See https://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-properties
See https://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager
See https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format
For backward compatibility, a broadcast message is also emitted if service is the known or unique name Emacs is registered at D-Bus bus.
For backward compatibility, the arguments args
can also be just strings. They stand for the respective arguments of
signal in their order, and are used for filtering as well. A
nil argument might be used to preserve the order.