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1667 lines
63 KiB
XML
1667 lines
63 KiB
XML
<?xml version="1.0" standalone="no"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
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[
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]>
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<article id="index">
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<articleinfo>
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<title>D-Bus Tutorial</title>
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<releaseinfo>Version 0.5.0</releaseinfo>
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<date>20 August 2006</date>
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<authorgroup>
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<author>
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<firstname>Havoc</firstname>
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<surname>Pennington</surname>
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<affiliation>
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<orgname>Red Hat, Inc.</orgname>
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<address><email>hp@pobox.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>David</firstname>
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<surname>Wheeler</surname>
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</author>
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<author>
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<firstname>John</firstname>
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<surname>Palmieri</surname>
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<affiliation>
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<orgname>Red Hat, Inc.</orgname>
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<address><email>johnp@redhat.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>Colin</firstname>
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<surname>Walters</surname>
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<affiliation>
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<orgname>Red Hat, Inc.</orgname>
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<address><email>walters@redhat.com</email></address>
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</affiliation>
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</author>
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</authorgroup>
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</articleinfo>
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<sect1 id="meta">
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<title>Tutorial Work In Progress</title>
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<para>
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This tutorial is not complete; it probably contains some useful information, but
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also has plenty of gaps. Right now, you'll also need to refer to the D-Bus specification,
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Doxygen reference documentation, and look at some examples of how other apps use D-Bus.
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</para>
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<para>
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Enhancing the tutorial is definitely encouraged - send your patches or suggestions to the
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mailing list. If you create a D-Bus binding, please add a section to the tutorial for your
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binding, if only a short section with a couple of examples.
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</para>
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</sect1>
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<sect1 id="whatis">
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<title>What is D-Bus?</title>
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<para>
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D-Bus is a system for <firstterm>interprocess communication</firstterm>
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(IPC). Architecturally, it has several layers:
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<itemizedlist>
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<listitem>
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<para>
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A library, <firstterm>libdbus</firstterm>, that allows two
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applications to connect to each other and exchange messages.
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</para>
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</listitem>
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<listitem>
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<para>
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A <firstterm>message bus daemon</firstterm> executable, built on
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libdbus, that multiple applications can connect to. The daemon can
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route messages from one application to zero or more other
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applications.
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</para>
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</listitem>
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<listitem>
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<para>
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<firstterm>Wrapper libraries</firstterm> or <firstterm>bindings</firstterm>
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based on particular application frameworks. For example, libdbus-glib and
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libdbus-qt. There are also bindings to languages such as
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Python. These wrapper libraries are the API most people should use,
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as they simplify the details of D-Bus programming. libdbus is
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intended to be a low-level backend for the higher level bindings.
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Much of the libdbus API is only useful for binding implementation.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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libdbus only supports one-to-one connections, just like a raw network
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socket. However, rather than sending byte streams over the connection, you
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send <firstterm>messages</firstterm>. Messages have a header identifying
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the kind of message, and a body containing a data payload. libdbus also
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abstracts the exact transport used (sockets vs. whatever else), and
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handles details such as authentication.
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</para>
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<para>
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The message bus daemon forms the hub of a wheel. Each spoke of the wheel
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is a one-to-one connection to an application using libdbus. An
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application sends a message to the bus daemon over its spoke, and the bus
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daemon forwards the message to other connected applications as
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appropriate. Think of the daemon as a router.
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</para>
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<para>
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The bus daemon has multiple instances on a typical computer. The
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first instance is a machine-global singleton, that is, a system daemon
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similar to sendmail or Apache. This instance has heavy security
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restrictions on what messages it will accept, and is used for systemwide
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communication. The other instances are created one per user login session.
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These instances allow applications in the user's session to communicate
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with one another.
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</para>
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<para>
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The systemwide and per-user daemons are separate. Normal within-session
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IPC does not involve the systemwide message bus process and vice versa.
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</para>
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<sect2 id="uses">
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<title>D-Bus applications</title>
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<para>
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There are many, many technologies in the world that have "Inter-process
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communication" or "networking" in their stated purpose: <ulink
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url="http://www.omg.org">CORBA</ulink>, <ulink
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url="http://www.opengroup.org/dce/">DCE</ulink>, <ulink
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url="http://www.microsoft.com/com/">DCOM</ulink>, <ulink
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url="http://developer.kde.org/documentation/library/kdeqt/dcop.html">DCOP</ulink>, <ulink
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url="http://www.xmlrpc.com">XML-RPC</ulink>, <ulink
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url="http://www.w3.org/TR/SOAP/">SOAP</ulink>, <ulink
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url="http://www.mbus.org/">MBUS</ulink>, <ulink
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url="http://www.zeroc.com/ice.html">Internet Communications Engine (ICE)</ulink>,
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and probably hundreds more.
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Each of these is tailored for particular kinds of application.
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D-Bus is designed for two specific cases:
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<itemizedlist>
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<listitem>
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<para>
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Communication between desktop applications in the same desktop
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session; to allow integration of the desktop session as a whole,
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and address issues of process lifecycle (when do desktop components
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start and stop running).
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</para>
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</listitem>
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<listitem>
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<para>
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Communication between the desktop session and the operating system,
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where the operating system would typically include the kernel
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and any system daemons or processes.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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For the within-desktop-session use case, the GNOME and KDE desktops
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have significant previous experience with different IPC solutions
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such as CORBA and DCOP. D-Bus is built on that experience and
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carefully tailored to meet the needs of these desktop projects
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in particular. D-Bus may or may not be appropriate for other
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applications; the FAQ has some comparisons to other IPC systems.
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</para>
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<para>
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The problem solved by the systemwide or communication-with-the-OS case
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is explained well by the following text from the Linux Hotplug project:
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<blockquote>
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<para>
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A gap in current Linux support is that policies with any sort of
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dynamic "interact with user" component aren't currently
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supported. For example, that's often needed the first time a network
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adapter or printer is connected, and to determine appropriate places
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to mount disk drives. It would seem that such actions could be
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supported for any case where a responsible human can be identified:
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single user workstations, or any system which is remotely
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administered.
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</para>
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<para>
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This is a classic "remote sysadmin" problem, where in this case
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hotplugging needs to deliver an event from one security domain
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(operating system kernel, in this case) to another (desktop for
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logged-in user, or remote sysadmin). Any effective response must go
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the other way: the remote domain taking some action that lets the
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kernel expose the desired device capabilities. (The action can often
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be taken asynchronously, for example letting new hardware be idle
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until a meeting finishes.) At this writing, Linux doesn't have
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widely adopted solutions to such problems. However, the new D-Bus
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work may begin to solve that problem.
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</para>
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</blockquote>
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</para>
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<para>
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D-Bus may happen to be useful for purposes other than the one it was
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designed for. Its general properties that distinguish it from
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other forms of IPC are:
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<itemizedlist>
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<listitem>
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<para>
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Binary protocol designed to be used asynchronously
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(similar in spirit to the X Window System protocol).
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</para>
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</listitem>
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<listitem>
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<para>
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Stateful, reliable connections held open over time.
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</para>
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</listitem>
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<listitem>
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<para>
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The message bus is a daemon, not a "swarm" or
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distributed architecture.
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</para>
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</listitem>
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<listitem>
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<para>
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Many implementation and deployment issues are specified rather
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than left ambiguous/configurable/pluggable.
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</para>
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</listitem>
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<listitem>
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<para>
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Semantics are similar to the existing DCOP system, allowing
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KDE to adopt it more easily.
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</para>
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</listitem>
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<listitem>
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<para>
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Security features to support the systemwide mode of the
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message bus.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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</sect1>
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<sect1 id="concepts">
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<title>Concepts</title>
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<para>
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Some basic concepts apply no matter what application framework you're
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using to write a D-Bus application. The exact code you write will be
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different for GLib vs. Qt vs. Python applications, however.
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</para>
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<para>
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Here is a diagram (<ulink url="diagram.png">png</ulink> <ulink
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url="diagram.svg">svg</ulink>) that may help you visualize the concepts
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that follow.
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</para>
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<sect2 id="objects">
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<title>Native Objects and Object Paths</title>
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<para>
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Your programming framework probably defines what an "object" is like;
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usually with a base class. For example: java.lang.Object, GObject, QObject,
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python's base Object, or whatever. Let's call this a <firstterm>native object</firstterm>.
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</para>
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<para>
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The low-level D-Bus protocol, and corresponding libdbus API, does not care about native objects.
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However, it provides a concept called an
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<firstterm>object path</firstterm>. The idea of an object path is that
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higher-level bindings can name native object instances, and allow remote applications
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to refer to them.
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</para>
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<para>
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The object path
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looks like a filesystem path, for example an object could be
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named <literal>/org/kde/kspread/sheets/3/cells/4/5</literal>.
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Human-readable paths are nice, but you are free to create an
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object named <literal>/com/mycompany/c5yo817y0c1y1c5b</literal>
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if it makes sense for your application.
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</para>
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<para>
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Namespacing object paths is smart, by starting them with the components
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of a domain name you own (e.g. <literal>/org/kde</literal>). This
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keeps different code modules in the same process from stepping
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on one another's toes.
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</para>
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</sect2>
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<sect2 id="members">
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<title>Methods and Signals</title>
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<para>
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Each object has <firstterm>members</firstterm>; the two kinds of member
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are <firstterm>methods</firstterm> and
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<firstterm>signals</firstterm>. Methods are operations that can be
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invoked on an object, with optional input (aka arguments or "in
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parameters") and output (aka return values or "out parameters").
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Signals are broadcasts from the object to any interested observers
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of the object; signals may contain a data payload.
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</para>
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<para>
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Both methods and signals are referred to by name, such as
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"Frobate" or "OnClicked".
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</para>
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</sect2>
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<sect2 id="interfaces">
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<title>Interfaces</title>
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<para>
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Each object supports one or more <firstterm>interfaces</firstterm>.
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Think of an interface as a named group of methods and signals,
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just as it is in GLib or Qt or Java. Interfaces define the
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<emphasis>type</emphasis> of an object instance.
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</para>
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<para>
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DBus identifies interfaces with a simple namespaced string,
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something like <literal>org.freedesktop.Introspectable</literal>.
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Most bindings will map these interface names directly to
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the appropriate programming language construct, for example
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to Java interfaces or C++ pure virtual classes.
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</para>
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</sect2>
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<sect2 id="proxies">
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<title>Proxies</title>
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<para>
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A <firstterm>proxy object</firstterm> is a convenient native object created to
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represent a remote object in another process. The low-level DBus API involves manually creating
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a method call message, sending it, then manually receiving and processing
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the method reply message. Higher-level bindings provide proxies as an alternative.
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Proxies look like a normal native object; but when you invoke a method on the proxy
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object, the binding converts it into a DBus method call message, waits for the reply
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message, unpacks the return value, and returns it from the native method..
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</para>
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<para>
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In pseudocode, programming without proxies might look like this:
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<programlisting>
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Message message = new Message("/remote/object/path", "MethodName", arg1, arg2);
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Connection connection = getBusConnection();
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connection.send(message);
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Message reply = connection.waitForReply(message);
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if (reply.isError()) {
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} else {
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Object returnValue = reply.getReturnValue();
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}
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</programlisting>
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</para>
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<para>
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Programming with proxies might look like this:
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<programlisting>
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Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path");
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Object returnValue = proxy.MethodName(arg1, arg2);
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</programlisting>
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</para>
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</sect2>
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<sect2 id="bus-names">
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<title>Bus Names</title>
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<para>
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When each application connects to the bus daemon, the daemon immediately
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assigns it a name, called the <firstterm>unique connection name</firstterm>.
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A unique name begins with a ':' (colon) character. These names are never
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reused during the lifetime of the bus daemon - that is, you know
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a given name will always refer to the same application.
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An example of a unique name might be
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<literal>:34-907</literal>. The numbers after the colon have
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no meaning other than their uniqueness.
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</para>
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<para>
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When a name is mapped
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to a particular application's connection, that application is said to
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<firstterm>own</firstterm> that name.
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</para>
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<para>
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Applications may ask to own additional <firstterm>well-known
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names</firstterm>. For example, you could write a specification to
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define a name called <literal>com.mycompany.TextEditor</literal>.
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Your definition could specify that to own this name, an application
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should have an object at the path
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<literal>/com/mycompany/TextFileManager</literal> supporting the
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interface <literal>org.freedesktop.FileHandler</literal>.
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</para>
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<para>
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Applications could then send messages to this bus name,
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object, and interface to execute method calls.
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</para>
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<para>
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You could think of the unique names as IP addresses, and the
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well-known names as domain names. So
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<literal>com.mycompany.TextEditor</literal> might map to something like
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<literal>:34-907</literal> just as <literal>mycompany.com</literal> maps
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to something like <literal>192.168.0.5</literal>.
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</para>
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<para>
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Names have a second important use, other than routing messages. They
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are used to track lifecycle. When an application exits (or crashes), its
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connection to the message bus will be closed by the operating system
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kernel. The message bus then sends out notification messages telling
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remaining applications that the application's names have lost their
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owner. By tracking these notifications, your application can reliably
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monitor the lifetime of other applications.
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</para>
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<para>
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Bus names can also be used to coordinate single-instance applications.
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If you want to be sure only one
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<literal>com.mycompany.TextEditor</literal> application is running for
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example, have the text editor application exit if the bus name already
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has an owner.
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</para>
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</sect2>
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<sect2 id="addresses">
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<title>Addresses</title>
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<para>
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Applications using D-Bus are either servers or clients. A server
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listens for incoming connections; a client connects to a server. Once
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the connection is established, it is a symmetric flow of messages; the
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client-server distinction only matters when setting up the
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connection.
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</para>
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<para>
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If you're using the bus daemon, as you probably are, your application
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will be a client of the bus daemon. That is, the bus daemon listens
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for connections and your application initiates a connection to the bus
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daemon.
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</para>
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<para>
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A D-Bus <firstterm>address</firstterm> specifies where a server will
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listen, and where a client will connect. For example, the address
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<literal>unix:path=/tmp/abcdef</literal> specifies that the server will
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listen on a UNIX domain socket at the path
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<literal>/tmp/abcdef</literal> and the client will connect to that
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socket. An address can also specify TCP/IP sockets, or any other
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transport defined in future iterations of the D-Bus specification.
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</para>
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<para>
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When using D-Bus with a message bus daemon,
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libdbus automatically discovers the address of the per-session bus
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daemon by reading an environment variable. It discovers the
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systemwide bus daemon by checking a well-known UNIX domain socket path
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(though you can override this address with an environment variable).
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|
</para>
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|
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<para>
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|
If you're using D-Bus without a bus daemon, it's up to you to
|
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define which application will be the server and which will be
|
|
the client, and specify a mechanism for them to agree on
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the server's address. This is an unusual case.
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|
</para>
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|
|
</sect2>
|
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|
|
<sect2 id="bigpicture">
|
|
<title>Big Conceptual Picture</title>
|
|
|
|
<para>
|
|
Pulling all these concepts together, to specify a particular
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|
method call on a particular object instance, a number of
|
|
nested components have to be named:
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|
<programlisting>
|
|
Address -> [Bus Name] -> Path -> Interface -> Method
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|
</programlisting>
|
|
The bus name is in brackets to indicate that it's optional -- you only
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|
provide a name to route the method call to the right application
|
|
when using the bus daemon. If you have a direct connection to another
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|
application, bus names aren't used; there's no bus daemon.
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|
</para>
|
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|
|
<para>
|
|
The interface is also optional, primarily for historical
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reasons; DCOP does not require specifying the interface,
|
|
instead simply forbidding duplicate method names
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|
on the same object instance. D-Bus will thus let you
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|
omit the interface, but if your method name is ambiguous
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|
it is undefined which method will be invoked.
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</para>
|
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|
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</sect2>
|
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|
|
<sect2 id="messages">
|
|
<title>Messages - Behind the Scenes</title>
|
|
<para>
|
|
D-Bus works by sending messages between processes. If you're using
|
|
a sufficiently high-level binding, you may never work with messages directly.
|
|
</para>
|
|
<para>
|
|
There are 4 message types:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Method call messages ask to invoke a method
|
|
on an object.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Method return messages return the results
|
|
of invoking a method.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Error messages return an exception caused by
|
|
invoking a method.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Signal messages are notifications that a given signal
|
|
has been emitted (that an event has occurred).
|
|
You could also think of these as "event" messages.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
A method call maps very simply to messages: you send a method call
|
|
message, and receive either a method return message or an error message
|
|
in reply.
|
|
</para>
|
|
<para>
|
|
Each message has a <firstterm>header</firstterm>, including <firstterm>fields</firstterm>,
|
|
and a <firstterm>body</firstterm>, including <firstterm>arguments</firstterm>. You can think
|
|
of the header as the routing information for the message, and the body as the payload.
|
|
Header fields might include the sender bus name, destination bus name, method or signal name,
|
|
and so forth. One of the header fields is a <firstterm>type signature</firstterm> describing the
|
|
values found in the body. For example, the letter "i" means "32-bit integer" so the signature
|
|
"ii" means the payload has two 32-bit integers.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="callprocedure">
|
|
<title>Calling a Method - Behind the Scenes</title>
|
|
|
|
<para>
|
|
A method call in DBus consists of two messages; a method call message sent from process A to process B,
|
|
and a matching method reply message sent from process B to process A. Both the call and the reply messages
|
|
are routed through the bus daemon. The caller includes a different serial number in each call message, and the
|
|
reply message includes this number to allow the caller to match replies to calls.
|
|
</para>
|
|
|
|
<para>
|
|
The call message will contain any arguments to the method.
|
|
The reply message may indicate an error, or may contain data returned by the method.
|
|
</para>
|
|
|
|
<para>
|
|
A method invocation in DBus happens as follows:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
The language binding may provide a proxy, such that invoking a method on
|
|
an in-process object invokes a method on a remote object in another process. If so, the
|
|
application calls a method on the proxy, and the proxy
|
|
constructs a method call message to send to the remote process.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
For more low-level APIs, the application may construct a method call message itself, without
|
|
using a proxy.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
In either case, the method call message contains: a bus name belonging to the remote process; the name of the method;
|
|
the arguments to the method; an object path inside the remote process; and optionally the name of the
|
|
interface that specifies the method.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The method call message is sent to the bus daemon.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The bus daemon looks at the destination bus name. If a process owns that name,
|
|
the bus daemon forwards the method call to that process. Otherwise, the bus daemon
|
|
creates an error message and sends it back as the reply to the method call message.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The receiving process unpacks the method call message. In a simple low-level API situation, it
|
|
may immediately run the method and send a method reply message to the bus daemon.
|
|
When using a high-level binding API, the binding might examine the object path, interface,
|
|
and method name, and convert the method call message into an invocation of a method on
|
|
a native object (GObject, java.lang.Object, QObject, etc.), then convert the return
|
|
value from the native method into a method reply message.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The bus daemon receives the method reply message and sends it to the process that
|
|
made the method call.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The process that made the method call looks at the method reply and makes use of any
|
|
return values included in the reply. The reply may also indicate that an error occurred.
|
|
When using a binding, the method reply message may be converted into the return value of
|
|
of a proxy method, or into an exception.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
The bus daemon never reorders messages. That is, if you send two method call messages to the same recipient,
|
|
they will be received in the order they were sent. The recipient is not required to reply to the calls
|
|
in order, however; for example, it may process each method call in a separate thread, and return reply messages
|
|
in an undefined order depending on when the threads complete. Method calls have a unique serial
|
|
number used by the method caller to match reply messages to call messages.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="signalprocedure">
|
|
<title>Emitting a Signal - Behind the Scenes</title>
|
|
|
|
<para>
|
|
A signal in DBus consists of a single message, sent by one process to any number of other processes.
|
|
That is, a signal is a unidirectional broadcast. The signal may contain arguments (a data payload), but
|
|
because it is a broadcast, it never has a "return value." Contrast this with a method call
|
|
(see <xref linkend="callprocedure"/>) where the method call message has a matching method reply message.
|
|
</para>
|
|
|
|
<para>
|
|
The emitter (aka sender) of a signal has no knowledge of the signal recipients. Recipients register
|
|
with the bus daemon to receive signals based on "match rules" - these rules would typically include the sender and
|
|
the signal name. The bus daemon sends each signal only to recipients who have expressed interest in that
|
|
signal.
|
|
</para>
|
|
|
|
<para>
|
|
A signal in DBus happens as follows:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A signal message is created and sent to the bus daemon. When using the low-level API this may be
|
|
done manually, with certain bindings it may be done for you by the binding when a native object
|
|
emits a native signal or event.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The signal message contains the name of the interface that specifies the signal;
|
|
the name of the signal; the bus name of the process sending the signal; and
|
|
any arguments
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Any process on the message bus can register "match rules" indicating which signals it
|
|
is interested in. The bus has a list of registered match rules.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The bus daemon examines the signal and determines which processes are interested in it.
|
|
It sends the signal message to these processes.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Each process receiving the signal decides what to do with it; if using a binding,
|
|
the binding may choose to emit a native signal on a proxy object. If using the
|
|
low-level API, the process may just look at the signal sender and name and decide
|
|
what to do based on that.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="introspection">
|
|
<title>Introspection</title>
|
|
|
|
<para>
|
|
D-Bus objects may support the interface <literal>org.freedesktop.DBus.Introspectable</literal>.
|
|
This interface has one method <literal>Introspect</literal> which takes no arguments and returns
|
|
an XML string. The XML string describes the interfaces, methods, and signals of the object.
|
|
See the D-Bus specification for more details on this introspection format.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="glib-client">
|
|
<title>GLib API: Using Remote Objects</title>
|
|
|
|
<para>
|
|
The GLib binding is defined in the header file
|
|
<literal><dbus/dbus-glib.h></literal>.
|
|
</para>
|
|
|
|
<sect2 id="glib-typemappings">
|
|
<title>D-Bus - GLib type mappings</title>
|
|
<para>
|
|
The heart of the GLib bindings for D-Bus is the mapping it
|
|
provides between D-Bus "type signatures" and GLib types
|
|
(<literal>GType</literal>). The D-Bus type system is composed of
|
|
a number of "basic" types, along with several "container" types.
|
|
</para>
|
|
<sect3 id="glib-basic-typemappings">
|
|
<title>Basic type mappings</title>
|
|
<para>
|
|
Below is a list of the basic types, along with their associated
|
|
mapping to a <literal>GType</literal>.
|
|
<informaltable>
|
|
<tgroup cols="4">
|
|
<thead>
|
|
<row>
|
|
<entry>D-Bus basic type</entry>
|
|
<entry>GType</entry>
|
|
<entry>Free function</entry>
|
|
<entry>Notes</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><literal>BYTE</literal></entry>
|
|
<entry><literal>G_TYPE_UCHAR</literal></entry>
|
|
<entry></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>BOOLEAN</literal></entry>
|
|
<entry><literal>G_TYPE_BOOLEAN</literal></entry>
|
|
<entry></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>INT16</literal></entry>
|
|
<entry><literal>G_TYPE_INT</literal></entry>
|
|
<entry></entry>
|
|
<entry>Will be changed to a <literal>G_TYPE_INT16</literal> once GLib has it</entry>
|
|
</row><row>
|
|
<entry><literal>UINT16</literal></entry>
|
|
<entry><literal>G_TYPE_UINT</literal></entry>
|
|
<entry></entry>
|
|
<entry>Will be changed to a <literal>G_TYPE_UINT16</literal> once GLib has it</entry>
|
|
</row><row>
|
|
<entry><literal>INT32</literal></entry>
|
|
<entry><literal>G_TYPE_INT</literal></entry>
|
|
<entry></entry>
|
|
<entry>Will be changed to a <literal>G_TYPE_INT32</literal> once GLib has it</entry>
|
|
</row><row>
|
|
<entry><literal>UINT32</literal></entry>
|
|
<entry><literal>G_TYPE_UINT</literal></entry>
|
|
<entry></entry>
|
|
<entry>Will be changed to a <literal>G_TYPE_UINT32</literal> once GLib has it</entry>
|
|
</row><row>
|
|
<entry><literal>INT64</literal></entry>
|
|
<entry><literal>G_TYPE_GINT64</literal></entry>
|
|
<entry></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>UINT64</literal></entry>
|
|
<entry><literal>G_TYPE_GUINT64</literal></entry>
|
|
<entry></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>DOUBLE</literal></entry>
|
|
<entry><literal>G_TYPE_DOUBLE</literal></entry>
|
|
<entry></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>STRING</literal></entry>
|
|
<entry><literal>G_TYPE_STRING</literal></entry>
|
|
<entry><literal>g_free</literal></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>OBJECT_PATH</literal></entry>
|
|
<entry><literal>DBUS_TYPE_G_PROXY</literal></entry>
|
|
<entry><literal>g_object_unref</literal></entry>
|
|
<entry>The returned proxy does not have an interface set; use <literal>dbus_g_proxy_set_interface</literal> to invoke methods</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
As you can see, the basic mapping is fairly straightforward.
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-container-typemappings">
|
|
<title>Container type mappings</title>
|
|
<para>
|
|
The D-Bus type system also has a number of "container"
|
|
types, such as <literal>DBUS_TYPE_ARRAY</literal> and
|
|
<literal>DBUS_TYPE_STRUCT</literal>. The D-Bus type system
|
|
is fully recursive, so one can for example have an array of
|
|
array of strings (i.e. type signature
|
|
<literal>aas</literal>).
|
|
</para>
|
|
<para>
|
|
However, not all of these types are in common use; for
|
|
example, at the time of this writing the author knows of no
|
|
one using <literal>DBUS_TYPE_STRUCT</literal>, or a
|
|
<literal>DBUS_TYPE_ARRAY</literal> containing any non-basic
|
|
type. The approach the GLib bindings take is pragmatic; try
|
|
to map the most common types in the most obvious way, and
|
|
let using less common and more complex types be less
|
|
"natural".
|
|
</para>
|
|
<para>
|
|
First, D-Bus type signatures which have an "obvious"
|
|
corresponding built-in GLib type are mapped using that type:
|
|
<informaltable>
|
|
<tgroup cols="6">
|
|
<thead>
|
|
<row>
|
|
<entry>D-Bus type signature</entry>
|
|
<entry>Description</entry>
|
|
<entry>GType</entry>
|
|
<entry>C typedef</entry>
|
|
<entry>Free function</entry>
|
|
<entry>Notes</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><literal>as</literal></entry>
|
|
<entry>Array of strings</entry>
|
|
<entry><literal>G_TYPE_STRV</literal></entry>
|
|
<entry><literal>char **</literal></entry>
|
|
<entry><literal>g_strfreev</literal></entry>
|
|
<entry></entry>
|
|
</row><row>
|
|
<entry><literal>v</literal></entry>
|
|
<entry>Generic value container</entry>
|
|
<entry><literal>G_TYPE_VALUE</literal></entry>
|
|
<entry><literal>GValue *</literal></entry>
|
|
<entry><literal>g_value_unset</literal></entry>
|
|
<entry>The calling conventions for values expect that method callers have allocated return values; see below.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
</para>
|
|
<para>
|
|
The next most common recursive type signatures are arrays of
|
|
basic values. The most obvious mapping for arrays of basic
|
|
types is a <literal>GArray</literal>. Now, GLib does not
|
|
provide a builtin <literal>GType</literal> for
|
|
<literal>GArray</literal>. However, we actually need more than
|
|
that - we need a "parameterized" type which includes the
|
|
contained type. Why we need this we will see below.
|
|
</para>
|
|
<para>
|
|
The approach taken is to create these types in the D-Bus GLib
|
|
bindings; however, there is nothing D-Bus specific about them.
|
|
In the future, we hope to include such "fundamental" types in GLib
|
|
itself.
|
|
<informaltable>
|
|
<tgroup cols="6">
|
|
<thead>
|
|
<row>
|
|
<entry>D-Bus type signature</entry>
|
|
<entry>Description</entry>
|
|
<entry>GType</entry>
|
|
<entry>C typedef</entry>
|
|
<entry>Free function</entry>
|
|
<entry>Notes</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><literal>ay</literal></entry>
|
|
<entry>Array of bytes</entry>
|
|
<entry><literal>DBUS_TYPE_G_BYTE_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>au</literal></entry>
|
|
<entry>Array of uint</entry>
|
|
<entry><literal>DBUS_TYPE_G_UINT_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>ai</literal></entry>
|
|
<entry>Array of int</entry>
|
|
<entry><literal>DBUS_TYPE_G_INT_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>ax</literal></entry>
|
|
<entry>Array of int64</entry>
|
|
<entry><literal>DBUS_TYPE_G_INT64_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>at</literal></entry>
|
|
<entry>Array of uint64</entry>
|
|
<entry><literal>DBUS_TYPE_G_UINT64_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>ad</literal></entry>
|
|
<entry>Array of double</entry>
|
|
<entry><literal>DBUS_TYPE_G_DOUBLE_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
<row>
|
|
<entry><literal>ab</literal></entry>
|
|
<entry>Array of boolean</entry>
|
|
<entry><literal>DBUS_TYPE_G_BOOLEAN_ARRAY</literal></entry>
|
|
<entry><literal>GArray *</literal></entry>
|
|
<entry>g_array_free</entry>
|
|
<entry></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
</para>
|
|
<para>
|
|
D-Bus also includes a special type DBUS_TYPE_DICT_ENTRY which
|
|
is only valid in arrays. It's intended to be mapped to a "dictionary"
|
|
type by bindings. The obvious GLib mapping here is GHashTable. Again,
|
|
however, there is no builtin <literal>GType</literal> for a GHashTable.
|
|
Moreover, just like for arrays, we need a parameterized type so that
|
|
the bindings can communiate which types are contained in the hash table.
|
|
</para>
|
|
<para>
|
|
At present, only strings are supported. Work is in progress to
|
|
include more types.
|
|
<informaltable>
|
|
<tgroup cols="6">
|
|
<thead>
|
|
<row>
|
|
<entry>D-Bus type signature</entry>
|
|
<entry>Description</entry>
|
|
<entry>GType</entry>
|
|
<entry>C typedef</entry>
|
|
<entry>Free function</entry>
|
|
<entry>Notes</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><literal>a{ss}</literal></entry>
|
|
<entry>Dictionary mapping strings to strings</entry>
|
|
<entry><literal>DBUS_TYPE_G_STRING_STRING_HASHTABLE</literal></entry>
|
|
<entry><literal>GHashTable *</literal></entry>
|
|
<entry>g_hash_table_destroy</entry>
|
|
<entry></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-generic-typemappings">
|
|
<title>Arbitrarily recursive type mappings</title>
|
|
<para>
|
|
Finally, it is possible users will want to write or invoke D-Bus
|
|
methods which have arbitrarily complex type signatures not
|
|
directly supported by these bindings. For this case, we have a
|
|
<literal>DBusGValue</literal> which acts as a kind of special
|
|
variant value which may be iterated over manually. The
|
|
<literal>GType</literal> associated is
|
|
<literal>DBUS_TYPE_G_VALUE</literal>.
|
|
</para>
|
|
<para>
|
|
TODO insert usage of <literal>DBUS_TYPE_G_VALUE</literal> here.
|
|
</para>
|
|
</sect3>
|
|
</sect2>
|
|
<sect2 id="sample-program-1">
|
|
<title>A sample program</title>
|
|
<para>Here is a D-Bus program using the GLib bindings.
|
|
<programlisting>
|
|
int
|
|
main (int argc, char **argv)
|
|
{
|
|
DBusGConnection *connection;
|
|
GError *error;
|
|
DBusGProxy *proxy;
|
|
char **name_list;
|
|
char **name_list_ptr;
|
|
|
|
g_type_init ();
|
|
|
|
error = NULL;
|
|
connection = dbus_g_bus_get (DBUS_BUS_SESSION,
|
|
&error);
|
|
if (connection == NULL)
|
|
{
|
|
g_printerr ("Failed to open connection to bus: %s\n",
|
|
error->message);
|
|
g_error_free (error);
|
|
exit (1);
|
|
}
|
|
|
|
/* Create a proxy object for the "bus driver" (name "org.freedesktop.DBus") */
|
|
|
|
proxy = dbus_g_proxy_new_for_name (connection,
|
|
DBUS_SERVICE_DBUS,
|
|
DBUS_PATH_DBUS,
|
|
DBUS_INTERFACE_DBUS);
|
|
|
|
/* Call ListNames method, wait for reply */
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "ListNames", &error, G_TYPE_INVALID,
|
|
G_TYPE_STRV, &name_list, G_TYPE_INVALID))
|
|
{
|
|
/* Just do demonstrate remote exceptions versus regular GError */
|
|
if (error->domain == DBUS_GERROR && error->code == DBUS_GERROR_REMOTE_EXCEPTION)
|
|
g_printerr ("Caught remote method exception %s: %s",
|
|
dbus_g_error_get_name (error),
|
|
error->message);
|
|
else
|
|
g_printerr ("Error: %s\n", error->message);
|
|
g_error_free (error);
|
|
exit (1);
|
|
}
|
|
|
|
/* Print the results */
|
|
|
|
g_print ("Names on the message bus:\n");
|
|
|
|
for (name_list_ptr = name_list; *name_list_ptr; name_list_ptr++)
|
|
{
|
|
g_print (" %s\n", *name_list_ptr);
|
|
}
|
|
g_strfreev (name_list);
|
|
|
|
g_object_unref (proxy);
|
|
|
|
return 0;
|
|
}
|
|
</programlisting>
|
|
</para>
|
|
</sect2>
|
|
<sect2 id="glib-program-setup">
|
|
<title>Program initalization</title>
|
|
<para>
|
|
A connection to the bus is acquired using
|
|
<literal>dbus_g_bus_get</literal>. Next, a proxy
|
|
is created for the object "/org/freedesktop/DBus" with
|
|
interface <literal>org.freedesktop.DBus</literal>
|
|
on the service <literal>org.freedesktop.DBus</literal>.
|
|
This is a proxy for the message bus itself.
|
|
</para>
|
|
</sect2>
|
|
<sect2 id="glib-method-invocation">
|
|
<title>Understanding method invocation</title>
|
|
<para>
|
|
You have a number of choices for method invocation. First, as
|
|
used above, <literal>dbus_g_proxy_call</literal> sends a
|
|
method call to the remote object, and blocks until a reply is
|
|
recieved. The outgoing arguments are specified in the varargs
|
|
array, terminated with <literal>G_TYPE_INVALID</literal>.
|
|
Next, pointers to return values are specified, followed again
|
|
by <literal>G_TYPE_INVALID</literal>.
|
|
</para>
|
|
<para>
|
|
To invoke a method asynchronously, use
|
|
<literal>dbus_g_proxy_begin_call</literal>. This returns a
|
|
<literal>DBusGPendingCall</literal> object; you may then set a
|
|
notification function using
|
|
<literal>dbus_g_pending_call_set_notify</literal>.
|
|
</para>
|
|
</sect2>
|
|
<sect2 id="glib-signal-connection">
|
|
<title>Connecting to object signals</title>
|
|
<para>
|
|
You may connect to signals using
|
|
<literal>dbus_g_proxy_add_signal</literal> and
|
|
<literal>dbus_g_proxy_connect_signal</literal>. You must
|
|
invoke <literal>dbus_g_proxy_add_signal</literal> to specify
|
|
the signature of your signal handlers; you may then invoke
|
|
<literal>dbus_g_proxy_connect_signal</literal> multiple times.
|
|
</para>
|
|
<para>
|
|
Note that it will often be the case that there is no builtin
|
|
marshaller for the type signature of a remote signal. In that
|
|
case, you must generate a marshaller yourself by using
|
|
<application>glib-genmarshal</application>, and then register
|
|
it using <literal>dbus_g_object_register_marshaller</literal>.
|
|
</para>
|
|
</sect2>
|
|
<sect2 id="glib-error-handling">
|
|
<title>Error handling and remote exceptions</title>
|
|
<para>
|
|
All of the GLib binding methods such as
|
|
<literal>dbus_g_proxy_end_call</literal> return a
|
|
<literal>GError</literal>. This <literal>GError</literal> can
|
|
represent two different things:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
An internal D-Bus error, such as an out-of-memory
|
|
condition, an I/O error, or a network timeout. Errors
|
|
generated by the D-Bus library itself have the domain
|
|
<literal>DBUS_GERROR</literal>, and a corresponding code
|
|
such as <literal>DBUS_GERROR_NO_MEMORY</literal>. It will
|
|
not be typical for applications to handle these errors
|
|
specifically.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
A remote D-Bus exception, thrown by the peer, bus, or
|
|
service. D-Bus remote exceptions have both a textual
|
|
"name" and a "message". The GLib bindings store this
|
|
information in the <literal>GError</literal>, but some
|
|
special rules apply.
|
|
</para>
|
|
<para>
|
|
The set error will have the domain
|
|
<literal>DBUS_GERROR</literal> as above, and will also
|
|
have the code
|
|
<literal>DBUS_GERROR_REMOTE_EXCEPTION</literal>. In order
|
|
to access the remote exception name, you must use a
|
|
special accessor, such as
|
|
<literal>dbus_g_error_has_name</literal> or
|
|
<literal>dbus_g_error_get_name</literal>. The remote
|
|
exception detailed message is accessible via the regular
|
|
GError <literal>message</literal> member.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</sect2>
|
|
<sect2 id="glib-more-examples">
|
|
<title>More examples of method invocation</title>
|
|
<sect3 id="glib-sending-stuff">
|
|
<title>Sending an integer and string, receiving an array of bytes</title>
|
|
<para>
|
|
<programlisting>
|
|
GArray *arr;
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "Foobar", &error,
|
|
G_TYPE_INT, 42, G_TYPE_STRING, "hello",
|
|
G_TYPE_INVALID,
|
|
DBUS_TYPE_G_UCHAR_ARRAY, &arr, G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
g_assert (arr != NULL);
|
|
printf ("got back %u values", arr->len);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-sending-hash">
|
|
<title>Sending a GHashTable</title>
|
|
<para>
|
|
<programlisting>
|
|
GHashTable *hash = g_hash_table_new (g_str_hash, g_str_equal);
|
|
guint32 ret;
|
|
|
|
g_hash_table_insert (hash, "foo", "bar");
|
|
g_hash_table_insert (hash, "baz", "whee");
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "HashSize", &error,
|
|
DBUS_TYPE_G_STRING_STRING_HASH, hash, G_TYPE_INVALID,
|
|
G_TYPE_UINT, &ret, G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
g_assert (ret == 2);
|
|
g_hash_table_destroy (hash);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-receiving-bool-int">
|
|
<title>Receiving a boolean and a string</title>
|
|
<para>
|
|
<programlisting>
|
|
gboolean boolret;
|
|
char *strret;
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "GetStuff", &error,
|
|
G_TYPE_INVALID,
|
|
G_TYPE_BOOLEAN, &boolret,
|
|
G_TYPE_STRING, &strret,
|
|
G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
printf ("%s %s", boolret ? "TRUE" : "FALSE", strret);
|
|
g_free (strret);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-sending-str-arrays">
|
|
<title>Sending two arrays of strings</title>
|
|
<para>
|
|
<programlisting>
|
|
/* NULL terminate */
|
|
char *strs_static[] = {"foo", "bar", "baz", NULL};
|
|
/* Take pointer to array; cannot pass array directly */
|
|
char **strs_static_p = strs_static;
|
|
char **strs_dynamic;
|
|
|
|
strs_dynamic = g_new (char *, 4);
|
|
strs_dynamic[0] = g_strdup ("hello");
|
|
strs_dynamic[1] = g_strdup ("world");
|
|
strs_dynamic[2] = g_strdup ("!");
|
|
/* NULL terminate */
|
|
strs_dynamic[3] = NULL;
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "TwoStrArrays", &error,
|
|
G_TYPE_STRV, strs_static_p,
|
|
G_TYPE_STRV, strs_dynamic,
|
|
G_TYPE_INVALID,
|
|
G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
g_strfreev (strs_dynamic);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-getting-str-array">
|
|
<title>Sending a boolean, receiving an array of strings</title>
|
|
<para>
|
|
<programlisting>
|
|
char **strs;
|
|
char **strs_p;
|
|
gboolean blah;
|
|
|
|
error = NULL;
|
|
blah = TRUE;
|
|
if (!dbus_g_proxy_call (proxy, "GetStrs", &error,
|
|
G_TYPE_BOOLEAN, blah,
|
|
G_TYPE_INVALID,
|
|
G_TYPE_STRV, &strs,
|
|
G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
for (strs_p = strs; *strs_p; strs_p++)
|
|
printf ("got string: \"%s\"", *strs_p);
|
|
g_strfreev (strs);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-sending-variant">
|
|
<title>Sending a variant</title>
|
|
<para>
|
|
<programlisting>
|
|
GValue val = {0, };
|
|
|
|
g_value_init (&val, G_TYPE_STRING);
|
|
g_value_set_string (&val, "hello world");
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "SendVariant", &error,
|
|
G_TYPE_VALUE, &val, G_TYPE_INVALID,
|
|
G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
g_assert (ret == 2);
|
|
g_value_unset (&val);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="glib-receiving-variant">
|
|
<title>Receiving a variant</title>
|
|
<para>
|
|
<programlisting>
|
|
GValue val = {0, };
|
|
|
|
error = NULL;
|
|
if (!dbus_g_proxy_call (proxy, "GetVariant", &error, G_TYPE_INVALID,
|
|
G_TYPE_VALUE, &val, G_TYPE_INVALID))
|
|
{
|
|
/* Handle error */
|
|
}
|
|
if (G_VALUE_TYPE (&val) == G_TYPE_STRING)
|
|
printf ("%s\n", g_value_get_string (&val));
|
|
else if (G_VALUE_TYPE (&val) == G_TYPE_INT)
|
|
printf ("%d\n", g_value_get_int (&val));
|
|
else
|
|
...
|
|
g_value_unset (&val);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="glib-generated-bindings">
|
|
<title>Generated Bindings</title>
|
|
<para>
|
|
By using the Introspection XML files, convenient client-side bindings
|
|
can be automatically created to ease the use of a remote DBus object.
|
|
</para>
|
|
<para>
|
|
Here is a sample XML file which describes an object that exposes
|
|
one method, named <literal>ManyArgs</literal>.
|
|
<programlisting>
|
|
<?xml version="1.0" encoding="UTF-8" ?>
|
|
<node name="/com/example/MyObject">
|
|
<interface name="com.example.MyObject">
|
|
<method name="ManyArgs">
|
|
<arg type="u" name="x" direction="in" />
|
|
<arg type="s" name="str" direction="in" />
|
|
<arg type="d" name="trouble" direction="in" />
|
|
<arg type="d" name="d_ret" direction="out" />
|
|
<arg type="s" name="str_ret" direction="out" />
|
|
</method>
|
|
</interface>
|
|
</node>
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
Run <literal>dbus-binding-tool --mode=glib-client
|
|
<replaceable>FILENAME</replaceable> >
|
|
<replaceable>HEADER_NAME</replaceable></literal> to generate the header
|
|
file. For example: <command>dbus-binding-tool --mode=glib-client
|
|
my-object.xml > my-object-bindings.h</command>. This will generate
|
|
inline functions with the following prototypes:
|
|
<programlisting>
|
|
/* This is a blocking call */
|
|
gboolean
|
|
com_example_MyObject_many_args (DBusGProxy *proxy, const guint IN_x,
|
|
const char * IN_str, const gdouble IN_trouble,
|
|
gdouble* OUT_d_ret, char ** OUT_str_ret,
|
|
GError **error);
|
|
|
|
/* This is a non-blocking call */
|
|
DBusGProxyCall*
|
|
com_example_MyObject_many_args_async (DBusGProxy *proxy, const guint IN_x,
|
|
const char * IN_str, const gdouble IN_trouble,
|
|
com_example_MyObject_many_args_reply callback,
|
|
gpointer userdata);
|
|
|
|
/* This is the typedef for the non-blocking callback */
|
|
typedef void
|
|
(*com_example_MyObject_many_args_reply)
|
|
(DBusGProxy *proxy, gdouble OUT_d_ret, char * OUT_str_ret,
|
|
GError *error, gpointer userdata);
|
|
</programlisting>
|
|
The first argument in all functions is a <literal>DBusGProxy
|
|
*</literal>, which you should create with the usual
|
|
<literal>dbus_g_proxy_new_*</literal> functions. Following that are the
|
|
"in" arguments, and then either the "out" arguments and a
|
|
<literal>GError *</literal> for the synchronous (blocking) function, or
|
|
callback and user data arguments for the asynchronous (non-blocking)
|
|
function. The callback in the asynchronous function passes the
|
|
<literal>DBusGProxy *</literal>, the returned "out" arguments, an
|
|
<literal>GError *</literal> which is set if there was an error otherwise
|
|
<literal>NULL</literal>, and the user data.
|
|
</para>
|
|
<para>
|
|
As with the server-side bindings support (see <xref
|
|
linkend="glib-server"/>), the exact behaviour of the client-side
|
|
bindings can be manipulated using "annotations". Currently the only
|
|
annotation used by the client bindings is
|
|
<literal>org.freedesktop.DBus.GLib.NoReply</literal>, which sets the
|
|
flag indicating that the client isn't expecting a reply to the method
|
|
call, so a reply shouldn't be sent. This is often used to speed up
|
|
rapid method calls where there are no "out" arguments, and not knowing
|
|
if the method succeeded is an acceptable compromise to half the traffic
|
|
on the bus.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="glib-server">
|
|
<title>GLib API: Implementing Objects</title>
|
|
<para>
|
|
At the moment, to expose a GObject via D-Bus, you must
|
|
write XML by hand which describes the methods exported
|
|
by the object. In the future, this manual step will
|
|
be obviated by the upcoming GLib introspection support.
|
|
</para>
|
|
<para>
|
|
Here is a sample XML file which describes an object that exposes
|
|
one method, named <literal>ManyArgs</literal>.
|
|
<programlisting>
|
|
<?xml version="1.0" encoding="UTF-8" ?>
|
|
|
|
<node name="/com/example/MyObject">
|
|
|
|
<interface name="com.example.MyObject">
|
|
<annotation name="org.freedesktop.DBus.GLib.CSymbol" value="my_object"/>
|
|
<method name="ManyArgs">
|
|
<!-- This is optional, and in this case is redunundant -->
|
|
<annotation name="org.freedesktop.DBus.GLib.CSymbol" value="my_object_many_args"/>
|
|
<arg type="u" name="x" direction="in" />
|
|
<arg type="s" name="str" direction="in" />
|
|
<arg type="d" name="trouble" direction="in" />
|
|
<arg type="d" name="d_ret" direction="out" />
|
|
<arg type="s" name="str_ret" direction="out" />
|
|
</method>
|
|
</interface>
|
|
</node>
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
This XML is in the same format as the D-Bus introspection XML
|
|
format. Except we must include an "annotation" which give the C
|
|
symbols corresponding to the object implementation prefix
|
|
(<literal>my_object</literal>). In addition, if particular
|
|
methods symbol names deviate from C convention
|
|
(i.e. <literal>ManyArgs</literal> ->
|
|
<literal>many_args</literal>), you may specify an annotation
|
|
giving the C symbol.
|
|
</para>
|
|
<para>
|
|
Once you have written this XML, run <literal>dbus-binding-tool --mode=glib-server <replaceable>FILENAME</replaceable> > <replaceable>HEADER_NAME</replaceable>.</literal> to
|
|
generate a header file. For example: <command>dbus-binding-tool --mode=glib-server my-object.xml > my-object-glue.h</command>.
|
|
</para>
|
|
<para>
|
|
Next, include the generated header in your program, and invoke
|
|
<literal>dbus_g_object_class_install_info</literal> in the class
|
|
initializer, passing the object class and "object info" included in the
|
|
header. For example:
|
|
<programlisting>
|
|
dbus_g_object_type_install_info (COM_FOO_TYPE_MY_OBJECT, &com_foo_my_object_info);
|
|
</programlisting>
|
|
This should be done exactly once per object class.
|
|
</para>
|
|
<para>
|
|
To actually implement the method, just define a C function named e.g.
|
|
<literal>my_object_many_args</literal> in the same file as the info
|
|
header is included. At the moment, it is required that this function
|
|
conform to the following rules:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
The function must return a value of type <literal>gboolean</literal>;
|
|
<literal>TRUE</literal> on success, and <literal>FALSE</literal>
|
|
otherwise.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The first parameter is a pointer to an instance of the object.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Following the object instance pointer are the method
|
|
input values.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Following the input values are pointers to return values.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The final parameter must be a <literal>GError **</literal>.
|
|
If the function returns <literal>FALSE</literal> for an
|
|
error, the error parameter must be initalized with
|
|
<literal>g_set_error</literal>.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
Finally, you can export an object using <literal>dbus_g_connection_register_g_object</literal>. For example:
|
|
<programlisting>
|
|
dbus_g_connection_register_g_object (connection,
|
|
"/com/foo/MyObject",
|
|
obj);
|
|
</programlisting>
|
|
</para>
|
|
|
|
<sect2 id="glib-annotations">
|
|
<title>Server-side Annotations</title>
|
|
<para>
|
|
There are several annotations that are used when generating the
|
|
server-side bindings. The most common annotation is
|
|
<literal>org.freedesktop.DBus.GLib.CSymbol</literal> but there are other
|
|
annotations which are often useful.
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><literal>org.freedesktop.DBus.GLib.CSymbol</literal></term>
|
|
<listitem>
|
|
<para>
|
|
This annotation is used to specify the C symbol names for
|
|
the various types (interface, method, etc), if it differs from the
|
|
name DBus generates.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><literal>org.freedesktop.DBus.GLib.Async</literal></term>
|
|
<listitem>
|
|
<para>
|
|
This annotation marks the method implementation as an
|
|
asynchronous function, which doesn't return a response straight
|
|
away but will send the response at some later point to complete
|
|
the call. This is used to implement non-blocking services where
|
|
method calls can take time.
|
|
</para>
|
|
<para>
|
|
When a method is asynchronous, the function prototype is
|
|
different. It is required that the function conform to the
|
|
following rules:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
The function must return a value of type <literal>gboolean</literal>;
|
|
<literal>TRUE</literal> on success, and <literal>FALSE</literal>
|
|
otherwise. TODO: the return value is currently ignored.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The first parameter is a pointer to an instance of the object.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Following the object instance pointer are the method
|
|
input values.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The final parameter must be a
|
|
<literal>DBusGMethodInvocation *</literal>. This is used
|
|
when sending the response message back to the client, by
|
|
calling <literal>dbus_g_method_return</literal> or
|
|
<literal>dbus_g_method_return_error</literal>.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><literal>org.freedesktop.DBus.GLib.Const</literal></term>
|
|
<listitem>
|
|
<para>This attribute can only be applied to "out"
|
|
<literal><arg></literal> nodes, and specifies that the
|
|
parameter isn't being copied when returned. For example, this
|
|
turns a 's' argument from a <literal>char **</literal> to a
|
|
<literal>const char **</literal>, and results in the argument not
|
|
being freed by DBus after the message is sent.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><literal>org.freedesktop.DBus.GLib.ReturnVal</literal></term>
|
|
<listitem>
|
|
<para>
|
|
This attribute can only be applied to "out"
|
|
<literal><arg></literal> nodes, and alters the expected
|
|
function signature. It currently can be set to two values:
|
|
<literal>""</literal> or <literal>"error"</literal>. The
|
|
argument marked with this attribute is not returned via a
|
|
pointer argument, but by the function's return value. If the
|
|
attribute's value is the empty string, the <literal>GError
|
|
*</literal> argument is also omitted so there is no standard way
|
|
to return an error value. This is very useful for interfacing
|
|
with existing code, as it is possible to match existing APIs.
|
|
If the attribute's value is <literal>"error"</literal>, then the
|
|
final argument is a <literal>GError *</literal> as usual.
|
|
</para>
|
|
<para>
|
|
Some examples to demonstrate the usage. This introspection XML:
|
|
<programlisting>
|
|
<method name="Increment">
|
|
<arg type="u" name="x" />
|
|
<arg type="u" direction="out" />
|
|
</method>
|
|
</programlisting>
|
|
Expects the following function declaration:
|
|
<programlisting>
|
|
gboolean
|
|
my_object_increment (MyObject *obj, gint32 x, gint32 *ret, GError **error);
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
This introspection XML:
|
|
<programlisting>
|
|
<method name="IncrementRetval">
|
|
<arg type="u" name="x" />
|
|
<arg type="u" direction="out" >
|
|
<annotation name="org.freedesktop.DBus.GLib.ReturnVal" value=""/>
|
|
</arg>
|
|
</method>
|
|
</programlisting>
|
|
Expects the following function declaration:
|
|
<programlisting>
|
|
gint32
|
|
my_object_increment_retval (MyObject *obj, gint32 x)
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
This introspection XML:
|
|
<programlisting>
|
|
<method name="IncrementRetvalError">
|
|
<arg type="u" name="x" />
|
|
<arg type="u" direction="out" >
|
|
<annotation name="org.freedesktop.DBus.GLib.ReturnVal" value="error"/>
|
|
</arg>
|
|
</method>
|
|
</programlisting>
|
|
Expects the following function declaration:
|
|
<programlisting>
|
|
gint32
|
|
my_object_increment_retval_error (MyObject *obj, gint32 x, GError **error)
|
|
</programlisting>
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="python-client">
|
|
<title>Python API</title>
|
|
<para>
|
|
The Python API, dbus-python, is now documented separately in
|
|
<ulink url="http://dbus.freedesktop.org/doc/dbus-python/doc/tutorial.html">the dbus-python tutorial</ulink> (also available in doc/tutorial.txt,
|
|
and doc/tutorial.html if built with python-docutils, in the dbus-python
|
|
source distribution).
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="qt-client">
|
|
<title>Qt API: Using Remote Objects</title>
|
|
<para>
|
|
|
|
The Qt bindings are not yet documented.
|
|
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="qt-server">
|
|
<title>Qt API: Implementing Objects</title>
|
|
<para>
|
|
The Qt bindings are not yet documented.
|
|
</para>
|
|
</sect1>
|
|
</article>
|