<article id="index">
<articleinfo>
- <title>D-BUS Tutorial</title>
- <releaseinfo>Version 0.2</releaseinfo>
- <date>10 August 2004</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="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>
+ <title>What is D-Bus?</title>
<para>
- D-BUS is a system for <firstterm>interprocess communication</firstterm>
+ D-Bus is a system for <firstterm>interprocess communication</firstterm>
(IPC). Architecturally, it has several layers:
<itemizedlist>
</listitem>
<listitem>
<para>
- <firstterm>Wrapper libraries</firstterm> based on particular
- application frameworks. For example, libdbus-glib and
+ <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
+ 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>
</para>
<para>
- If you just want to use D-BUS and don't care how it works, jump directly
- to <xref linkend="concepts"/>.
- Otherwise, read on.
- </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
</para>
<sect2 id="uses">
- <title>D-BUS applications</title>
+ <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.mbus.org/">MBUS</ulink>, <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>, and probably hundreds
- more. Each of these is tailored for particular kinds of application.
- D-BUS is designed for two specific cases:
+ 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>
<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
+ 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.
+ 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
</blockquote>
</para>
<para>
- D-BUS may happen to be useful for purposes other than the one it was
+ 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>
Many implementation and deployment issues are specified rather
- than left ambiguous.
+ than left ambiguous/configurable/pluggable.
</para>
</listitem>
<listitem>
<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
+ 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>Objects and Object Paths</title>
+ <title>Native Objects and Object Paths</title>
<para>
- Each application using D-BUS contains <firstterm>objects</firstterm>,
- which generally map to GObject, QObject, C++ objects, or Python objects
- (but need not). An object is an <emphasis>instance</emphasis> rather
- than a type. When messages are received over a D-BUS connection, they
- are sent to a specific object, not to the application as a whole.
+ 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>
- To allow messages to specify their destination object, there has to be a
- way to refer to an object. In your favorite programming language, this
- is normally called a <firstterm>pointer</firstterm> or
- <firstterm>reference</firstterm>. However, these references are
- implemented as memory addresses relative to the address space of your
- application, and thus can't be passed from one application to another.
+ 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>
- To solve this, D-BUS introduces a name for each object. The name
+ 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
</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>
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="messages">
- <title>Message Types</title>
+
+ <sect2 id="proxies">
+ <title>Proxies</title>
<para>
- Messages are not all the same; in particular, D-BUS has
- 4 built-in 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>
+ 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>
- A method call maps very simply to messages, then: you send a method call
- message, and receive either a method return message or an error message
- in reply.
+ 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>
- </sect2>
-
- <sect2 id="services">
- <title>Services</title>
-
<para>
- Object paths, interfaces, and messages exist on the level of
- libdbus and the D-BUS protocol; they are used even in the
- 1-to-1 case with no message bus involved.
+ 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>
- Services, on the other hand, are a property of the message bus daemon.
- A <firstterm>service</firstterm> is simply a name mapped to
- some application connected to the message bus daemon.
- These names are used to specify the origin and destination
- of messages passing through the message bus. When a name is mapped
- to a particular application, the application is said to
- <firstterm>own</firstterm> that service.
+ 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>
- On connecting to the bus daemon, each application immediately owns a
- special name called the <firstterm>base service</firstterm>. A base
- service begins with a ':' (colon) character; no other services are
- allowed to begin with that character. Base services are special because
- each one is unique. They are created dynamically, and are never re-used
- during the lifetime of the same bus daemon. You know that a given
- base service name will have the same owner at all times.
- An example of a base service name might be <literal>:34-907</literal>.
+ 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
- services</firstterm>. For example, you could write a specification to
- define a service called <literal>com.mycompany.TextEditor</literal>.
- Your definition could specify that to own this service, an application
+ 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>.
+ interface <literal>org.freedesktop.FileHandler</literal>.
</para>
<para>
- Applications could then send messages to this service,
+ Applications could then send messages to this bus name,
object, and interface to execute method calls.
</para>
<para>
- You could think of the base service names as IP addresses, and the
- well-known services as domain names. So
+ 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>
- Services have a second important use, other than routing messages. They
+ 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 services have lost their
+ 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
+ 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
</para>
<para>
- A D-BUS <firstterm>address</firstterm> specifies where a server will
+ 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.
+ transport defined in future iterations of the D-Bus specification.
</para>
<para>
- When using D-BUS with a message bus, the bus daemon is a server
- and all other applications are clients of the bus daemon.
+ 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
</para>
<para>
- If you're using D-BUS without a bus daemon, it's up to you to
+ 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.
+ the server's address. This is an unusual case.
</para>
</sect2>
method call on a particular object instance, a number of
nested components have to be named:
<programlisting>
- Address -> [Service] -> Path -> Interface -> Method
+ Address -> [Bus Name] -> Path -> Interface -> Method
</programlisting>
- The service is in brackets to indicate that it's optional -- you only
- provide a service name to route the method call to the right application
+ 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, services aren't used; there's no bus daemon.
+ 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
+ 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>
<title>GLib API: Using Remote Objects</title>
<para>
-
The GLib binding is defined in the header file
- <dbus/dbus-glib.h>. The API is very small, in sharp contrast to the
- low-level <dbus/dbus.h>.
-
+ <literal><dbus/dbus-glib.h></literal>.
</para>
- <para>
- The GLib bindings are incomplete, see the TODO file and comments in the
- source code.
- </para>
-
- <para>
-Here is a D-BUS program using the GLib bindings.
+ <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;
- DBusGPendingCall *call;
- char **service_list;
- int service_list_len;
- int i;
+ char **name_list;
+ char **name_list_ptr;
g_type_init ();
if (connection == NULL)
{
g_printerr ("Failed to open connection to bus: %s\n",
- error->message);
+ error->message);
g_error_free (error);
exit (1);
}
- /* Create a proxy object for the "bus driver" (service org.freedesktop.DBus) */
-
- proxy = dbus_g_proxy_new_for_service (connection,
- DBUS_SERVICE_ORG_FREEDESKTOP_DBUS,
- DBUS_PATH_ORG_FREEDESKTOP_DBUS,
- DBUS_INTERFACE_ORG_FREEDESKTOP_DBUS);
-
- /* Call ListServices method */
+ /* Create a proxy object for the "bus driver" (name "org.freedesktop.DBus") */
- call = dbus_g_proxy_begin_call (proxy, "ListServices", DBUS_TYPE_INVALID);
+ 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_end_call (proxy, call, &error,
- DBUS_TYPE_ARRAY, DBUS_TYPE_STRING,
- &service_list, &service_list_len,
- DBUS_TYPE_INVALID))
+ if (!dbus_g_proxy_call (proxy, "ListNames", &error, G_TYPE_INVALID,
+ G_TYPE_STRV, &name_list, G_TYPE_INVALID))
{
- g_printerr ("Failed to complete ListServices call: %s\n",
- error->message);
+ /* 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 ("Services on the message bus:\n");
- i = 0;
- while (i < service_list_len)
+ g_print ("Names on the message bus:\n");
+
+ for (name_list_ptr = name_list; *name_list_ptr; name_list_ptr++)
{
- g_assert (service_list[i] != NULL);
- g_print (" %s\n", service_list[i]);
- ++i;
+ g_print (" %s\n", *name_list_ptr);
}
- g_assert (service_list[i] == NULL);
+ g_strfreev (name_list);
- g_strfreev (service_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");
- <para>
+ 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;
- DBusGProxy represents a remote object. dbus_g_proxy_begin_call() sends
- a method call to the remote object, and dbus_g_proxy_end_call() retrieves
- any return values or exceptions resulting from the method call.
- There are also DBusGProxy functions to connect and disconnect signals,
- not shown in the code example.
+ 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;
- </para>
+ 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, };
- <para>
-
- dbus_g_bus_get() assumes that the application will use GMainLoop. The
- created connection will be associated with the main loop such that
- messages will be sent and received when the main loop runs. However, in
- the above code example the main loop never runs; D-BUS will not run the
- loop implicitly. Instead, dbus_g_proxy_end_call() will block until the
- method call has been sent and the reply received. A more complex GUI
- application might run the main loop while waiting for the method call
- reply. (DBusGPendingCall is currently missing the "notify me when the
- call is complete" functionality found in DBusPendingCall, but it should be
- added.)
+ 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, };
- </para>
+ 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>
- <para>
-
- Future plans (see doc/TODO) are to use G_TYPE_STRING in place of
- DBUS_TYPE_STRING and so forth. In fact the above code is slightly
- incorrect at the moment, since it uses g_strfreev() to free a string array
- that was not allocated with g_malloc(). dbus_free_string_array() should
- really be used. However, once the GLib bindings are complete the returned
- data from dbus_g_proxy_end_call() will be allocated with g_malloc().
+ <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);
- </para>
+/* 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>
-
- The GLib binding is defined in the header file
- <dbus/dbus-glib.h>. To implement an object, it's also necessary
- to use the dbus-glib-tool command line tool.
+ 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 GLib bindings are incomplete. Implementing an object is not yet
- possible, see the TODO file and comments in the source code for details
- on what work needs doing.
+ 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">
The Qt bindings are not yet documented.
</para>
</sect1>
-
-
- <sect1 id="python-client">
- <title>Python API: Using Remote Objects</title>
- <para>
- The Python bindings are not yet documented, but the
- bindings themselves are in good shape.
- </para>
- </sect1>
-
- <sect1 id="python-server">
- <title>Python API: Implementing Objects</title>
- <para>
- The Python bindings are not yet documented, but the
- bindings themselves are in good shape.
- </para>
- </sect1>
-
</article>
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