<?xml version="1.0" standalone="no"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
[
]>
<article id="index">
<articleinfo>
<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>
</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>
<para>
D-Bus is a system for <firstterm>interprocess communication</firstterm>
(IPC). Architecturally, it has several layers:
<itemizedlist>
<listitem>
<para>
A library, <firstterm>libdbus</firstterm>, that allows two
applications to connect to each other and exchange messages.
</para>
</listitem>
<listitem>
<para>
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>
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: Using Remote Objects</title>
<para>
The Python bindings provide a simple to use interface for talking over D-Bus.
Where possible much of the inner-workings of D-Bus are hidden behind what looks
like normal Python objects.
</para>
<sect2 id="python-typemappings">
<title>D-Bus - Python type mappings</title>
<para>
While python itself is a largely untyped language D-Bus provides a simple type system
for talking with other languages which may be strongly typed. Python for the most part
tries automatically map python objects to types on the bus. It is none the less good to
know what the type mappings are so one can better utilize services over the bus.
</para>
<sect3 id="python-basic-typemappings">
<title>Basic type mappings</title>
<para>
Below is a list of the basic types, along with their associated
mapping to a Python object.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>D-Bus basic type</entry>
<entry>Python wrapper</entry>
<entry>Notes</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>BYTE</literal></entry>
<entry><literal>dbus.Byte</literal></entry>
<entry></entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry><literal>dbus.Boolean</literal></entry>
<entry>Any variable assigned a True or False boolean value will automatically be converted into a BOOLEAN over the bus</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry><literal>dbus.Int16</literal></entry>
<entry></entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry><literal>dbus.UInt16</literal></entry>
<entry></entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry><literal>dbus.Int32</literal></entry>
<entry>This is the default mapping for Python integers</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry><literal>dbus.UInt32</literal></entry>
<entry></entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry><literal>dbus.Int64</literal></entry>
<entry></entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry><literal>dbus.UInt64</literal></entry>
<entry></entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry><literal>dbus.Double</literal></entry>
<entry>Any variable assigned a floating point number will automatically be converted into a DOUBLE over the bus</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry><literal>dbus.String</literal></entry>
<entry>Any variable assigned a quoted string will automatically be converted into a STRING over the bus</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry><literal>dbus.ObjectPath</literal></entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="python-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>
D-Bus container types have native corresponding built-in Python types
so it is easy to use them.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>D-Bus type</entry>
<entry>Python type</entry>
<entry>Python wrapper</entry>
<entry>Notes</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>ARRAY</literal></entry>
<entry><literal>Python lists</literal></entry>
<entry><literal>dbus.Array</literal></entry>
<entry>Python lists, denoted by square brackets [], are converted into arrays and visa versa.
The one restriction is that when sending a Python list each element of the list must be of the same
type. This is because D-Bus arrays can contain only one element type. Use Python tuples for mixed types.
When using the wrapper you may also specify a type or signature of the elements contained in the Array.
This is manditory when passing an empty Array to a method on the bus because Python can not guess at the
contents of an empty array. For example if a method is expecting an Array of int32's and you need to pass
it an empty Array you would do it as such:
<programlisting>emptyint32array = dbus.Array([], type=dbus.Int32)</programlisting>
or
<programlisting>emptyint32array = dbus.Array([], signature="i")</programlisting>
Note that dbus.Array derives from list so it acts just like a python list.
</entry>
</row>
<row>
<entry><literal>STRUCT</literal></entry>
<entry><literal>Python tuple</literal></entry>
<entry><literal>dbus.Struct</literal></entry>
<entry>Python tuples, denoted by parentheses (,), are converted into structs and visa versa.
Tuples can have mixed types.</entry>
</row>
<row>
<entry><literal>DICTIONARY</literal></entry>
<entry><literal>Python dictionary</literal></entry>
<entry><literal>dbus.Dictionary</literal></entry>
<entry>D-Bus doesn't have an explicit dictionary type. Instead it uses LISTS of DICT_ENTRIES to
represent a dictionary. A DICT_ENTRY is simply a two element struct containing a key/value pair.
Python dictionaries are automatically converted to a LIST of DICT_ENTRIES and visa versa.
Since dictonaries are described as lists of dict_entries we also need the signature in order
to pass empty dictionaries. The wrapper provides a way of specifying this through the key_type/value_type
type parameters or the signature parameters. To send an empty Dictionary where the key is a string
and the value is a string you would do it as such:
<programlisting>emptystringstringdict = dbus.Dictionary({}, key_type=dbus.String, value_type=dbus.Value)</programlisting>
or
<programlisting>emptystringstringdict = dbus.Dictionary({}, signature="ss")</programlisting>
Note that dbus.Dictionary derives from dict so it acts just like a python dictionary.
</entry>
</row>
<row>
<entry><literal>VARIANT</literal></entry>
<entry><literal>any type</literal></entry>
<entry><literal>dbus.Variant</literal></entry>
<entry>A variant is a container for any type. Python exports its methods to accept only variants
since we are an untyped language and can demarshal into any Python type.
To send a variant you must first wrap it in a<literal>dbus.Variant</literal>. If no type or signiture is
given to the variant the marshaler will get the type from the contents.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="python-invoking-methods">
<title>Invoking Methods</title>
<para>Here is a D-Bus program using the Python bindings to get a listing of all names on the session bus.
<programlisting>
import dbus
bus = dbus.SessionBus()
proxy_obj = bus.bus.get_object('org.freedesktop.DBus', '/org/freedesktop/DBus')
dbus_iface = dbus.Interface(proxy_obj, 'org.freedesktop.DBus')
print dbus_iface.ListNames()
</programlisting>
</para>
<para>
Notice I get an interface on the proxy object and use that to make the call. While the specifications
state that you do not need to specify an interface if the call is unambiguous (i.e. only one method implements
that name) due to a bug on the bus that drops messages which don't have an interface field you need to specify
interfaces at this time. In any event it is always good practice to specify the interface of the method you
wish to call to avoid any side effects should a method of the same name be implemented on another interface.
</para>
<para>
You can specify the interface for a single call using the dbus_interface keyword.
<programlisting>
proxy_obj.ListNames(dbus_interface = 'org.freedesktop.DBus')
</programlisting>
</para>
<para>
This is all fine and good if all you want to do is call methods on the bus and then exit. In order to
do more complex things such as use a GUI or make asynchronous calls you will need a mainloop. You would use
asynchronous calls because in GUI applications it is very bad to block for any long period of time. This cause
the GUI to seem to freeze. Since replies to D-Bus messages can take an indeterminate amount of time using async
calls allows you to return control to the GUI while you wait for the reply. This is exceedingly easy to do in
Python. Here is an example using the GLib/GTK+ mainloop.
<programlisting>
import gobject
import dbus
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
import dbus.glib
def print_list_names_reply(list):
print str(list)
def print_error(e):
print str(e)
bus = dbus.SessionBus()
proxy_obj = bus.bus.get_object('org.freedesktop.DBus', '/org/freedesktop/DBus')
dbus_iface = dbus.Interface(proxy_obj, 'org.freedesktop.DBus')
dbus_iface.ListNames(reply_handler=print_list_names_reply, error_handler=print_error)
mainloop = gobject.MainLoop()
mainloop.run()
</programlisting>
</para>
<para>
In the above listing you will notice the reply_handler and error_handler keywords. These tell the method that
it should be called async and to call print_list_names_reply or print_error depending if you get a reply or an error.
The signature for replys depends on the number of arguments being sent back. Error handlers always take one parameter
which is the error object returned.
</para>
<para>
You will also notice that I check the version of the dbus bindings before importing dbus.glib. In older versions
glib was the only available mainloop. As of version 0.41.0 we split out the glib dependency to allow for other mainloops
to be implemented. Notice also the python binding version does not match up with the D-Bus version. Once we reach 1.0
this should change with Python changes simply tracking the D-Bus changes.
While the glib mainloop is the only mainloop currently implemented, integrating other mainloops should
be very easy to do. There are plans for creating a a generic mainloop to be the default for non gui programs.
</para>
</sect2>
<sect2 id="python-listening-for-signals">
<title>Listening for Signals</title>
<para>
Signals are emitted by objects on the bus to notify listening programs that an event has occurred. There are a couple of ways
to register a signal handler on the bus. One way is to attach to an already created proxy using the connect_to_signal method
which takes a signal name and handler as arguments. Let us look at an example of connecting to the HAL service to receive
signals when devices are added and removed and when devices register a capability. This example assumes you have HAL already running.
<programlisting>
import gobject
import dbus
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
import dbus.glib
def device_added_callback(udi):
print 'Device with udi %s was added' % (udi)
def device_removed_callback(udi):
print 'Device with udi %s was added' % (udi)
def device_capability_callback(udi, capability):
print 'Device with udi %s added capability %s' % (udi, capability)
bus = dbus.SystemBus()
hal_manager_obj = bus.get_object('org.freedesktop.Hal',
'/org/freedesktop/Hal/Manager')
hal_manager = dbus.Interface(hal_manager_obj,
'org.freedesktop.Hal.Manager')
hal_manager.connect_to_signal('DeviceAdded', device_added_callback)
hal_manager.connect_to_signal('DeviceRemoved', device_removed_callback)
hal_manager.connect_to_signal('NewCapability', device_capability_callback)
mainloop = gobject.MainLoop()
mainloop.run()
</programlisting>
</para>
<para>
The drawback of using this method is that the service that you are connecting to has to be around when you register
your signal handler. While HAL is guaranteed to be around on systems that use it this is not always the case for every
service on the bus. Say our program started up before HAL, we could connect to the signal by adding a signal receiver
directly to the bus.
<programlisting>
bus.add_signal_receiver(device_added_callback,
'DeviceAdded',
'org.freedesktop.Hal.Manager',
'org.freedesktop.Hal',
'/org/freedesktop/Hal/Manager')
bus.add_signal_receiver(device_removed_callback,
'DeviceRemoved',
'org.freedesktop.Hal.Manager',
'org.freedesktop.Hal',
'/org/freedesktop/Hal/Manager')
bus.add_signal_receiver(device_capability_callback,
'DeviceAdded',
'org.freedesktop.Hal.Manager',
'org.freedesktop.Hal',
'/org/freedesktop/Hal/Manager')
</programlisting>
</para>
<para>
All this can be done without creating the proxy object if one wanted to but in most cases you would want to have
a reference to the object so once a signal was received operations could be executed on the object.
</para>
<sidebar>
<title>Signal matching on arguments</title>
<para>
Starting with D-Bus 0.36 and the (0, 43, 0) version of the python
bindings you can now add a match on arguments being sent in a signal.
This is useful for instance for only getting NameOwnerChanged
signals for your service. Lets say we create a name on the bus called
'org.foo.MyName' we could also add a match to just get
NameOwnerChanges for that name as such:
<programlisting>
bus.add_signal_receiver(myname_changed,
'NameOwnerChanged',
'org.freedesktop.DBus',
'org.freedesktop.DBus',
'/org/freedesktop/DBus',
arg0='org.foo.MyName')
</programlisting>
It is as simple as that. To match the second arg you would use arg1=,
the third arg2=, etc.
</para>
</sidebar>
<sidebar>
<title>Cost of Creating a Proxy Object</title>
<para>
Note that creating proxy objects can have an associated processing cost. When introspection is implemented
a proxy may wait for introspection data before processing any requests. It is generally good practice to
create proxies once and reuse the proxy when calling into the object. Constantly creating the same proxy
over and over again can become a bottleneck for your program.
</para>
</sidebar>
<para>
TODO: example of getting information about devices from HAL
</para>
</sect2>
</sect1>
<sect1 id="python-server">
<title>Python API: Implementing Objects</title>
<para>
Implementing object on the bus is just as easy as invoking methods or listening for signals on the bus.
</para>
<sidebar>
<title>Version Alert</title>
<para>
The Python D-Bus bindings require version 2.4 or greater of Python when creating D-Bus objects.
</para>
</sidebar>
<sect2 id="python-inheriting-from-dbus-object">
<title>Inheriting From dbus.service.Object</title>
<para>
In order to export a Python object over the bus one must first get a bus name and then create
a Python object that inherits from dbus.service.Object. The following is the start of an example
HelloWorld object that we want to export over the session bus.
<programlisting>
import gobject
import dbus
import dbus.service
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
import dbus.glib
class HelloWorldObject(dbus.service.Object):
def __init__(self, bus_name, object_path='/org/freedesktop/HelloWorldObject'):
dbus.service.Object.__init__(self, bus_name, object_path)
session_bus = dbus.SessionBus()
bus_name = dbus.service.BusName('org.freedesktop.HelloWorld', bus=session_bus)
object = HelloWorldObject(bus_name)
mainloop = gobject.MainLoop()
mainloop.run()
</programlisting>
</para>
<para>
Here we got the session bus, then created a BusName object which requests a name on the bus.
We pass that bus name to the HelloWorldObject object which inherits from dbus.service.Object.
We now have an object on the bus but it is pretty useless.
</para>
</sect2>
<sect2 id="python-exporting-methods">
<title>Exporting Methods Over The Bus</title>
<para>
Let's make this object do something and export a method over the bus.
<programlisting>
import gobject
import dbus
import dbus.service
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
import dbus.glib
class HelloWorldObject(dbus.service.Object):
def __init__(self, bus_name, object_path='/org/freedesktop/HelloWorldObject'):
dbus.service.Object.__init__(self, bus_name, object_path)
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def hello(self):
return 'Hello from the HelloWorldObject'
session_bus = dbus.SessionBus()
bus_name = dbus.service.BusName('org.freedesktop.HelloWorld', bus=session_bus)
object = HelloWorldObject(bus_name)
mainloop = gobject.MainLoop()
mainloop.run()
</programlisting>
</para>
<sidebar>
<title>Python Decorators</title>
<para>
Notice the @ symbol on the line before the hello method. This is a new directive introduced in
Python 2.4. It is called a decorator and it "decorates" methods. All you have to know is that
it provides metadata that can then be used to alter the behavior of the method being decorated.
In this case we are telling the bindings that the hello method should be exported as a D-Bus method
over the bus.
</para>
</sidebar>
<para>
As you can see we exported the hello method as part of the org.freedesktop.HelloWorldIFace interface.
It takes no arguments and returns a string to the calling program. Let's create a proxy and invoke this
method.
<programlisting>
import dbus
bus = dbus.SessionBus()
proxy_obj = bus.bus.get_object('org.freedesktop.HelloWorld', '/org/freedesktop/HelloWorldObject')
iface = dbus.Interface(proxy_obj, 'org.freedesktop.HelloWorldIFace')
print iface.hello()
</programlisting>
</para>
<para>
When invoking methods exported over the bus the bindings automatically know how many parameters
the method exports. You can even make a method that exports an arbitrary number of parameters.
Also, whatever you return will automatically be transfered as a reply over the bus. Some examples.
<programlisting>
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def one_arg(self, first_arg):
return 'I got arg %s' % first_arg
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def two_args(self, first_arg, second_arg):
return ('I got 2 args', first_arg, second_arg)
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def return_list(self):
return [1, 2, 3, 4, 5, 6]
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def return_dict(self):
return {one: '1ne', two: '2wo', three: '3ree'}
</programlisting>
</para>
</sect2>
<sect2 id="python-emitting-signals">
<title>Emitting Signals</title>
<para>
Setting up signals to emit is just as easy as exporting methods. It uses the same syntax as methods.
<programlisting>
import gobject
import dbus
import dbus.service
if getattr(dbus, 'version', (0,0,0)) >= (0,41,0):
import dbus.glib
class HelloWorldObject(dbus.service.Object):
def __init__(self, bus_name, object_path='/org/freedesktop/HelloWorldObject'):
dbus.service.Object.__init__(self, bus_name, object_path)
@dbus.service.method('org.freedesktop.HelloWorldIFace')
def hello(self):
return 'Hello from the HelloWorldObject'
@dbus.service.signal('org.freedesktop.HelloWorldIFace')
def hello_signal(self, message):
pass
session_bus = dbus.SessionBus()
bus_name = dbus.service.BusName('org.freedesktop.HelloWorld', bus=session_bus)
object = HelloWorldObject(bus_name)
object.hello_signal('I sent a hello signal')
mainloop = gobject.MainLoop()
mainloop.run()
</programlisting>
</para>
<para>
Adding a @dbus.service.signal decorator to a method turns it into a signal emitter. You can put code
in this method to do things like keep track of how many times you call the emitter or to print out debug
messages but for the most part a pass noop will do. Whenever you call the emitter a signal will be emitted
with the parameters you passed in as arguments. In the above example we send the message 'I sent a hello signal'
with the signal.
</para>
</sect2>
<sect2 id="python-inheriting-and-overriding">
<title>Inheriting from HelloWorldObject</title>
<para>
One of the cool things you can do in Python is inherit from another D-Bus object. We use this trick in
the bindings to provide a default implementation for the org.freedesktop.DBus.Introspectable interface.
Let's inherit from the HelloWorldObject example above and overide the hello method to say goodbye.
<programlisting>
class HelloWorldGoodbyeObject(HelloWorldObject):
def __init__(self, bus_name, object_path='/org/freedesktop/HelloWorldGoodbyeObject'):
HelloWorldObject.__init__(self, bus_name, object_path)
@dbus.service.method('org.freedesktop.HelloWorldGoodbyeIFace')
def hello(self):
return 'Goodbye'
goodbye_object = HelloWorldGoodbyeObject(bus_name)
</programlisting>
</para>
<para>
Let's now call both methods with a little help from interfaces.
<programlisting>
import dbus
bus = dbus.SessionBus()
proxy_obj = bus.bus.get_object('org.freedesktop.HelloWorld', '/org/freedesktop/HelloWorldGoodbyeObject')
print proxy_obj.hello(dbus_interface='org.freedesktop.HelloWorldIFace')
print proxy_obj.hello(dbus_interface='org.freedesktop.HelloWorldGoodbyeIFace')
</programlisting>
</para>
<para>
This should print out 'Hello from the HelloWorldObject' followed by a 'Goodbye'.
</para>
</sect2>
<sect2 id="python-conclusion">
<title>Conclusion</title>
<para>
As you can see, using D-Bus from Python is an extremely easy proposition. Hopefully
the tutorial has been helpful in getting you started. If you need anymore help please
feel free to post on the <ulink url="http://lists.freedesktop.org/mailman/listinfo/dbus/">mailing list</ulink>.
The Python bindings are still in a state of flux and there may be API changes in the future.
This tutorial will be updated if such changes occur.
</para>
</sect2>
</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>