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diff --git a/doc/dbus-tutorial.xml b/doc/dbus-tutorial.xml new file mode 100644 index 00000000..5c385f0e --- /dev/null +++ b/doc/dbus-tutorial.xml @@ -0,0 +1,1667 @@ +<?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</title> + <para> + The Python API, dbus-python, is now documented separately in + <ulink url="http://dbus.freedesktop.org/doc/dbus-python/doc/tutorial.html">the dbus-python tutorial</ulink> (also available in doc/tutorial.txt, + and doc/tutorial.html if built with python-docutils, in the dbus-python + source distribution). + </para> + </sect1> + + <sect1 id="qt-client"> + <title>Qt API: Using Remote Objects</title> + <para> + + The Qt bindings are not yet documented. + + </para> + </sect1> + + <sect1 id="qt-server"> + <title>Qt API: Implementing Objects</title> + <para> + The Qt bindings are not yet documented. + </para> + </sect1> +</article> |