<article id="index">
<articleinfo>
- <title>D-BUS Tutorial</title>
- <releaseinfo>Version 0.1</releaseinfo>
- <date>29 September 2003</date>
+ <title>D-Bus Tutorial</title>
+ <releaseinfo>Version 0.5.0</releaseinfo>
+ <date>20 August 2006</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
<surname>Pennington</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
- <address>
- <email>hp@pobox.com</email>
- </address>
+ <address><email>hp@pobox.com</email></address>
+ </affiliation>
+ </author>
+ <author>
+ <firstname>David</firstname>
+ <surname>Wheeler</surname>
+ </author>
+ <author>
+ <firstname>John</firstname>
+ <surname>Palmieri</surname>
+ <affiliation>
+ <orgname>Red Hat, Inc.</orgname>
+ <address><email>johnp@redhat.com</email></address>
+ </affiliation>
+ </author>
+ <author>
+ <firstname>Colin</firstname>
+ <surname>Walters</surname>
+ <affiliation>
+ <orgname>Red Hat, Inc.</orgname>
+ <address><email>walters@redhat.com</email></address>
</affiliation>
</author>
</authorgroup>
</articleinfo>
- <sect1 id="introduction">
- <title>Introduction</title>
+ <sect1 id="meta">
+ <title>Tutorial Work In Progress</title>
+
+ <para>
+ This tutorial is not complete; it probably contains some useful information, but
+ also has plenty of gaps. Right now, you'll also need to refer to the D-Bus specification,
+ Doxygen reference documentation, and look at some examples of how other apps use D-Bus.
+ </para>
+
+ <para>
+ Enhancing the tutorial is definitely encouraged - send your patches or suggestions to the
+ mailing list. If you create a D-Bus binding, please add a section to the tutorial for your
+ binding, if only a short section with a couple of examples.
+ </para>
+
+ </sect1>
+
+ <sect1 id="whatis">
+ <title>What is D-Bus?</title>
<para>
- D-BUS blah blah blah
+ D-Bus is a system for <firstterm>interprocess communication</firstterm>
+ (IPC). Architecturally, it has several layers:
+
<itemizedlist>
<listitem>
<para>
- foo
+ A library, <firstterm>libdbus</firstterm>, that allows two
+ applications to connect to each other and exchange messages.
</para>
</listitem>
<listitem>
<para>
- bar
+ A <firstterm>message bus daemon</firstterm> executable, built on
+ libdbus, that multiple applications can connect to. The daemon can
+ route messages from one application to zero or more other
+ applications.
</para>
</listitem>
+ <listitem>
+ <para>
+ <firstterm>Wrapper libraries</firstterm> or <firstterm>bindings</firstterm>
+ based on particular application frameworks. For example, libdbus-glib and
+ libdbus-qt. There are also bindings to languages such as
+ Python. These wrapper libraries are the API most people should use,
+ as they simplify the details of D-Bus programming. libdbus is
+ intended to be a low-level backend for the higher level bindings.
+ Much of the libdbus API is only useful for binding implementation.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ libdbus only supports one-to-one connections, just like a raw network
+ socket. However, rather than sending byte streams over the connection, you
+ send <firstterm>messages</firstterm>. Messages have a header identifying
+ the kind of message, and a body containing a data payload. libdbus also
+ abstracts the exact transport used (sockets vs. whatever else), and
+ handles details such as authentication.
+ </para>
+
+ <para>
+ The message bus daemon forms the hub of a wheel. Each spoke of the wheel
+ is a one-to-one connection to an application using libdbus. An
+ application sends a message to the bus daemon over its spoke, and the bus
+ daemon forwards the message to other connected applications as
+ appropriate. Think of the daemon as a router.
+ </para>
+
+ <para>
+ The bus daemon has multiple instances on a typical computer. The
+ first instance is a machine-global singleton, that is, a system daemon
+ similar to sendmail or Apache. This instance has heavy security
+ restrictions on what messages it will accept, and is used for systemwide
+ communication. The other instances are created one per user login session.
+ These instances allow applications in the user's session to communicate
+ with one another.
+ </para>
+
+ <para>
+ The systemwide and per-user daemons are separate. Normal within-session
+ IPC does not involve the systemwide message bus process and vice versa.
+ </para>
+
+ <sect2 id="uses">
+ <title>D-Bus applications</title>
+ <para>
+ There are many, many technologies in the world that have "Inter-process
+ communication" or "networking" in their stated purpose: <ulink
+ url="http://www.omg.org">CORBA</ulink>, <ulink
+ url="http://www.opengroup.org/dce/">DCE</ulink>, <ulink
+ url="http://www.microsoft.com/com/">DCOM</ulink>, <ulink
+ url="http://developer.kde.org/documentation/library/kdeqt/dcop.html">DCOP</ulink>, <ulink
+ url="http://www.xmlrpc.com">XML-RPC</ulink>, <ulink
+ url="http://www.w3.org/TR/SOAP/">SOAP</ulink>, <ulink
+ url="http://www.mbus.org/">MBUS</ulink>, <ulink
+ url="http://www.zeroc.com/ice.html">Internet Communications Engine (ICE)</ulink>,
+ and probably hundreds more.
+ Each of these is tailored for particular kinds of application.
+ D-Bus is designed for two specific cases:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Communication between desktop applications in the same desktop
+ session; to allow integration of the desktop session as a whole,
+ and address issues of process lifecycle (when do desktop components
+ start and stop running).
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Communication between the desktop session and the operating system,
+ where the operating system would typically include the kernel
+ and any system daemons or processes.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ <para>
+ For the within-desktop-session use case, the GNOME and KDE desktops
+ have significant previous experience with different IPC solutions
+ such as CORBA and DCOP. D-Bus is built on that experience and
+ carefully tailored to meet the needs of these desktop projects
+ in particular. D-Bus may or may not be appropriate for other
+ applications; the FAQ has some comparisons to other IPC systems.
+ </para>
+ <para>
+ The problem solved by the systemwide or communication-with-the-OS case
+ is explained well by the following text from the Linux Hotplug project:
+ <blockquote>
+ <para>
+ A gap in current Linux support is that policies with any sort of
+ dynamic "interact with user" component aren't currently
+ supported. For example, that's often needed the first time a network
+ adapter or printer is connected, and to determine appropriate places
+ to mount disk drives. It would seem that such actions could be
+ supported for any case where a responsible human can be identified:
+ single user workstations, or any system which is remotely
+ administered.
+ </para>
+
+ <para>
+ This is a classic "remote sysadmin" problem, where in this case
+ hotplugging needs to deliver an event from one security domain
+ (operating system kernel, in this case) to another (desktop for
+ logged-in user, or remote sysadmin). Any effective response must go
+ the other way: the remote domain taking some action that lets the
+ kernel expose the desired device capabilities. (The action can often
+ be taken asynchronously, for example letting new hardware be idle
+ until a meeting finishes.) At this writing, Linux doesn't have
+ widely adopted solutions to such problems. However, the new D-Bus
+ work may begin to solve that problem.
+ </para>
+ </blockquote>
+ </para>
+ <para>
+ D-Bus may happen to be useful for purposes other than the one it was
+ designed for. Its general properties that distinguish it from
+ other forms of IPC are:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Binary protocol designed to be used asynchronously
+ (similar in spirit to the X Window System protocol).
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Stateful, reliable connections held open over time.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ The message bus is a daemon, not a "swarm" or
+ distributed architecture.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Many implementation and deployment issues are specified rather
+ than left ambiguous/configurable/pluggable.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Semantics are similar to the existing DCOP system, allowing
+ KDE to adopt it more easily.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Security features to support the systemwide mode of the
+ message bus.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect2>
+ </sect1>
+ <sect1 id="concepts">
+ <title>Concepts</title>
+ <para>
+ Some basic concepts apply no matter what application framework you're
+ using to write a D-Bus application. The exact code you write will be
+ different for GLib vs. Qt vs. Python applications, however.
+ </para>
+
+ <para>
+ Here is a diagram (<ulink url="diagram.png">png</ulink> <ulink
+ url="diagram.svg">svg</ulink>) that may help you visualize the concepts
+ that follow.
+ </para>
+
+ <sect2 id="objects">
+ <title>Native Objects and Object Paths</title>
+ <para>
+ Your programming framework probably defines what an "object" is like;
+ usually with a base class. For example: java.lang.Object, GObject, QObject,
+ python's base Object, or whatever. Let's call this a <firstterm>native object</firstterm>.
+ </para>
+ <para>
+ The low-level D-Bus protocol, and corresponding libdbus API, does not care about native objects.
+ However, it provides a concept called an
+ <firstterm>object path</firstterm>. The idea of an object path is that
+ higher-level bindings can name native object instances, and allow remote applications
+ to refer to them.
+ </para>
+ <para>
+ The object path
+ looks like a filesystem path, for example an object could be
+ named <literal>/org/kde/kspread/sheets/3/cells/4/5</literal>.
+ Human-readable paths are nice, but you are free to create an
+ object named <literal>/com/mycompany/c5yo817y0c1y1c5b</literal>
+ if it makes sense for your application.
+ </para>
+ <para>
+ Namespacing object paths is smart, by starting them with the components
+ of a domain name you own (e.g. <literal>/org/kde</literal>). This
+ keeps different code modules in the same process from stepping
+ on one another's toes.
+ </para>
+ </sect2>
+
+ <sect2 id="members">
+ <title>Methods and Signals</title>
+
+ <para>
+ Each object has <firstterm>members</firstterm>; the two kinds of member
+ are <firstterm>methods</firstterm> and
+ <firstterm>signals</firstterm>. Methods are operations that can be
+ invoked on an object, with optional input (aka arguments or "in
+ parameters") and output (aka return values or "out parameters").
+ Signals are broadcasts from the object to any interested observers
+ of the object; signals may contain a data payload.
+ </para>
+
+ <para>
+ Both methods and signals are referred to by name, such as
+ "Frobate" or "OnClicked".
+ </para>
+
+ </sect2>
+
+ <sect2 id="interfaces">
+ <title>Interfaces</title>
+ <para>
+ Each object supports one or more <firstterm>interfaces</firstterm>.
+ Think of an interface as a named group of methods and signals,
+ just as it is in GLib or Qt or Java. Interfaces define the
+ <emphasis>type</emphasis> of an object instance.
+ </para>
+ <para>
+ DBus identifies interfaces with a simple namespaced string,
+ something like <literal>org.freedesktop.Introspectable</literal>.
+ Most bindings will map these interface names directly to
+ the appropriate programming language construct, for example
+ to Java interfaces or C++ pure virtual classes.
+ </para>
+ </sect2>
+
+ <sect2 id="proxies">
+ <title>Proxies</title>
+ <para>
+ A <firstterm>proxy object</firstterm> is a convenient native object created to
+ represent a remote object in another process. The low-level DBus API involves manually creating
+ a method call message, sending it, then manually receiving and processing
+ the method reply message. Higher-level bindings provide proxies as an alternative.
+ Proxies look like a normal native object; but when you invoke a method on the proxy
+ object, the binding converts it into a DBus method call message, waits for the reply
+ message, unpacks the return value, and returns it from the native method..
+ </para>
+ <para>
+ In pseudocode, programming without proxies might look like this:
+ <programlisting>
+ Message message = new Message("/remote/object/path", "MethodName", arg1, arg2);
+ Connection connection = getBusConnection();
+ connection.send(message);
+ Message reply = connection.waitForReply(message);
+ if (reply.isError()) {
+
+ } else {
+ Object returnValue = reply.getReturnValue();
+ }
+ </programlisting>
+ </para>
+ <para>
+ Programming with proxies might look like this:
+ <programlisting>
+ Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path");
+ Object returnValue = proxy.MethodName(arg1, arg2);
+ </programlisting>
+ </para>
+ </sect2>
+
+ <sect2 id="bus-names">
+ <title>Bus Names</title>
+
+ <para>
+ When each application connects to the bus daemon, the daemon immediately
+ assigns it a name, called the <firstterm>unique connection name</firstterm>.
+ A unique name begins with a ':' (colon) character. These names are never
+ reused during the lifetime of the bus daemon - that is, you know
+ a given name will always refer to the same application.
+ An example of a unique name might be
+ <literal>:34-907</literal>. The numbers after the colon have
+ no meaning other than their uniqueness.
+ </para>
+
+ <para>
+ When a name is mapped
+ to a particular application's connection, that application is said to
+ <firstterm>own</firstterm> that name.
+ </para>
+
+ <para>
+ Applications may ask to own additional <firstterm>well-known
+ names</firstterm>. For example, you could write a specification to
+ define a name called <literal>com.mycompany.TextEditor</literal>.
+ Your definition could specify that to own this name, an application
+ should have an object at the path
+ <literal>/com/mycompany/TextFileManager</literal> supporting the
+ interface <literal>org.freedesktop.FileHandler</literal>.
+ </para>
+
+ <para>
+ Applications could then send messages to this bus name,
+ object, and interface to execute method calls.
+ </para>
+
+ <para>
+ You could think of the unique names as IP addresses, and the
+ well-known names as domain names. So
+ <literal>com.mycompany.TextEditor</literal> might map to something like
+ <literal>:34-907</literal> just as <literal>mycompany.com</literal> maps
+ to something like <literal>192.168.0.5</literal>.
+ </para>
+
+ <para>
+ Names have a second important use, other than routing messages. They
+ are used to track lifecycle. When an application exits (or crashes), its
+ connection to the message bus will be closed by the operating system
+ kernel. The message bus then sends out notification messages telling
+ remaining applications that the application's names have lost their
+ owner. By tracking these notifications, your application can reliably
+ monitor the lifetime of other applications.
+ </para>
+
+ <para>
+ Bus names can also be used to coordinate single-instance applications.
+ If you want to be sure only one
+ <literal>com.mycompany.TextEditor</literal> application is running for
+ example, have the text editor application exit if the bus name already
+ has an owner.
+ </para>
+
+ </sect2>
+
+ <sect2 id="addresses">
+ <title>Addresses</title>
+
+ <para>
+ Applications using D-Bus are either servers or clients. A server
+ listens for incoming connections; a client connects to a server. Once
+ the connection is established, it is a symmetric flow of messages; the
+ client-server distinction only matters when setting up the
+ connection.
+ </para>
+
+ <para>
+ If you're using the bus daemon, as you probably are, your application
+ will be a client of the bus daemon. That is, the bus daemon listens
+ for connections and your application initiates a connection to the bus
+ daemon.
+ </para>
+
+ <para>
+ A D-Bus <firstterm>address</firstterm> specifies where a server will
+ listen, and where a client will connect. For example, the address
+ <literal>unix:path=/tmp/abcdef</literal> specifies that the server will
+ listen on a UNIX domain socket at the path
+ <literal>/tmp/abcdef</literal> and the client will connect to that
+ socket. An address can also specify TCP/IP sockets, or any other
+ transport defined in future iterations of the D-Bus specification.
+ </para>
+
+ <para>
+ When using D-Bus with a message bus daemon,
+ libdbus automatically discovers the address of the per-session bus
+ daemon by reading an environment variable. It discovers the
+ systemwide bus daemon by checking a well-known UNIX domain socket path
+ (though you can override this address with an environment variable).
+ </para>
+
+ <para>
+ If you're using D-Bus without a bus daemon, it's up to you to
+ define which application will be the server and which will be
+ the client, and specify a mechanism for them to agree on
+ the server's address. This is an unusual case.
+ </para>
+
+ </sect2>
+
+ <sect2 id="bigpicture">
+ <title>Big Conceptual Picture</title>
+
+ <para>
+ Pulling all these concepts together, to specify a particular
+ method call on a particular object instance, a number of
+ nested components have to be named:
+ <programlisting>
+ Address -> [Bus Name] -> Path -> Interface -> Method
+ </programlisting>
+ The bus name is in brackets to indicate that it's optional -- you only
+ 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
+ application, bus names aren't used; there's no bus daemon.
+ </para>
+
+ <para>
+ The interface is also optional, primarily for historical
+ reasons; DCOP does not require specifying the interface,
+ instead simply forbidding duplicate method names
+ on the same object instance. D-Bus will thus let you
+ omit the interface, but if your method name is ambiguous
+ it is undefined which method will be invoked.
+ </para>
+
+ </sect2>
+
+ <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>
- blah blah blah
+ 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>
- blah blah blah
+ 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>
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