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, from which the address is known. This is a UNIX domain or TCP/IP socket. Everywhere, where a bus is mentioned as argument of a function (the symbol :system or the symbol :session), this address can be used instead. The connection to this bus must be initialized first.

Function: dbus-init-bus bus &optional private

Establish the connection to D-Bus bus.

bus can be either the symbol :system or the symbol :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.

The function returns a number, which counts the 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+. It should be used with care for at least the :system and :session buses, because other Emacs Lisp packages might already use this connection to those buses.

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 obsoletes the
;; previous one.
(dbus-init-bus my-bus 'private)

⇒ 1

(dbus-get-unique-name my-bus)

⇒ ":1.20"

D-Bus addresses can specify different transport. A possible address could be based on TCP/IP sockets, see next example. However, it depends on the bus daemon configuration, which transport is supported.

Function: dbus-setenv bus variable value

Set the value of the bus environment variable variable to value.

bus is either a Lisp symbol, :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, a dbus-error error is raised.

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