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8 Alternative buses and environments.

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.

Function: dbus-init-bus bus &optional private ¶

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.

Function: dbus-setenv bus variable value ¶

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