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diff --git a/doc/dbus-tutorial.html b/doc/dbus-tutorial.html new file mode 100644 index 00000000..957afaf1 --- /dev/null +++ b/doc/dbus-tutorial.html @@ -0,0 +1,991 @@ +<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>D-Bus Tutorial</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article" title="D-Bus Tutorial"><div class="titlepage"><div><div><h2 class="title"><a name="index"></a>D-Bus Tutorial</h2></div><div><div class="authorgroup"><div class="author"><h3 class="author"><span class="firstname">Havoc</span> <span class="surname">Pennington</span></h3><div class="affiliation"><span class="orgname">Red Hat, Inc.<br></span><div class="address"><p><code class="email"><<a class="email" href="mailto:hp@pobox.com">hp@pobox.com</a>></code></p></div></div></div><div class="author"><h3 class="author"><span class="firstname">David</span> <span class="surname">Wheeler</span></h3></div><div class="author"><h3 class="author"><span class="firstname">John</span> <span class="surname">Palmieri</span></h3><div class="affiliation"><span class="orgname">Red Hat, Inc.<br></span><div class="address"><p><code class="email"><<a class="email" href="mailto:johnp@redhat.com">johnp@redhat.com</a>></code></p></div></div></div><div class="author"><h3 class="author"><span class="firstname">Colin</span> <span class="surname">Walters</span></h3><div class="affiliation"><span class="orgname">Red Hat, Inc.<br></span><div class="address"><p><code class="email"><<a class="email" href="mailto:walters@redhat.com">walters@redhat.com</a>></code></p></div></div></div></div></div><div><p class="releaseinfo">Version 0.5.0</p></div></div><hr></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="#meta">Tutorial Work In Progress</a></span></dt><dt><span class="sect1"><a href="#whatis">What is D-Bus?</a></span></dt><dd><dl><dt><span class="sect2"><a href="#uses">D-Bus applications</a></span></dt></dl></dd><dt><span class="sect1"><a href="#concepts">Concepts</a></span></dt><dd><dl><dt><span class="sect2"><a href="#objects">Native Objects and Object Paths</a></span></dt><dt><span class="sect2"><a href="#members">Methods and Signals</a></span></dt><dt><span class="sect2"><a href="#interfaces">Interfaces</a></span></dt><dt><span class="sect2"><a href="#proxies">Proxies</a></span></dt><dt><span class="sect2"><a href="#bus-names">Bus Names</a></span></dt><dt><span class="sect2"><a href="#addresses">Addresses</a></span></dt><dt><span class="sect2"><a href="#bigpicture">Big Conceptual Picture</a></span></dt><dt><span class="sect2"><a href="#messages">Messages - Behind the Scenes</a></span></dt><dt><span class="sect2"><a href="#callprocedure">Calling a Method - Behind the Scenes</a></span></dt><dt><span class="sect2"><a href="#signalprocedure">Emitting a Signal - Behind the Scenes</a></span></dt><dt><span class="sect2"><a href="#introspection">Introspection</a></span></dt></dl></dd><dt><span class="sect1"><a href="#glib-client">GLib API: Using Remote Objects</a></span></dt><dd><dl><dt><span class="sect2"><a href="#glib-typemappings">D-Bus - GLib type mappings</a></span></dt><dt><span class="sect2"><a href="#sample-program-1">A sample program</a></span></dt><dt><span class="sect2"><a href="#glib-program-setup">Program initalization</a></span></dt><dt><span class="sect2"><a href="#glib-method-invocation">Understanding method invocation</a></span></dt><dt><span class="sect2"><a href="#glib-signal-connection">Connecting to object signals</a></span></dt><dt><span class="sect2"><a href="#glib-error-handling">Error handling and remote exceptions</a></span></dt><dt><span class="sect2"><a href="#glib-more-examples">More examples of method invocation</a></span></dt><dt><span class="sect2"><a href="#glib-generated-bindings">Generated Bindings</a></span></dt></dl></dd><dt><span class="sect1"><a href="#glib-server">GLib API: Implementing Objects</a></span></dt><dd><dl><dt><span class="sect2"><a href="#glib-annotations">Server-side Annotations</a></span></dt></dl></dd><dt><span class="sect1"><a href="#python-client">Python API</a></span></dt><dt><span class="sect1"><a href="#qt-client">Qt API: Using Remote Objects</a></span></dt><dt><span class="sect1"><a href="#qt-server">Qt API: Implementing Objects</a></span></dt></dl></div><div class="sect1" title="Tutorial Work In Progress"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="meta"></a>Tutorial Work In Progress</h2></div></div></div><p> + 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. + </p><p> + 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. + </p></div><div class="sect1" title="What is D-Bus?"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="whatis"></a>What is D-Bus?</h2></div></div></div><p> + D-Bus is a system for <em class="firstterm">interprocess communication</em> + (IPC). Architecturally, it has several layers: + + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + A library, <em class="firstterm">libdbus</em>, that allows two + applications to connect to each other and exchange messages. + </p></li><li class="listitem"><p> + A <em class="firstterm">message bus daemon</em> executable, built on + libdbus, that multiple applications can connect to. The daemon can + route messages from one application to zero or more other + applications. + </p></li><li class="listitem"><p> + <em class="firstterm">Wrapper libraries</em> or <em class="firstterm">bindings</em> + 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. + </p></li></ul></div><p> + </p><p> + libdbus only supports one-to-one connections, just like a raw network + socket. However, rather than sending byte streams over the connection, you + send <em class="firstterm">messages</em>. 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. + </p><p> + 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. + </p><p> + 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. + </p><p> + The systemwide and per-user daemons are separate. Normal within-session + IPC does not involve the systemwide message bus process and vice versa. + </p><div class="sect2" title="D-Bus applications"><div class="titlepage"><div><div><h3 class="title"><a name="uses"></a>D-Bus applications</h3></div></div></div><p> + There are many, many technologies in the world that have "Inter-process + communication" or "networking" in their stated purpose: <a class="ulink" href="http://www.omg.org" target="_top">CORBA</a>, <a class="ulink" href="http://www.opengroup.org/dce/" target="_top">DCE</a>, <a class="ulink" href="http://www.microsoft.com/com/" target="_top">DCOM</a>, <a class="ulink" href="http://developer.kde.org/documentation/library/kdeqt/dcop.html" target="_top">DCOP</a>, <a class="ulink" href="http://www.xmlrpc.com" target="_top">XML-RPC</a>, <a class="ulink" href="http://www.w3.org/TR/SOAP/" target="_top">SOAP</a>, <a class="ulink" href="http://www.mbus.org/" target="_top">MBUS</a>, <a class="ulink" href="http://www.zeroc.com/ice.html" target="_top">Internet Communications Engine (ICE)</a>, + and probably hundreds more. + Each of these is tailored for particular kinds of application. + D-Bus is designed for two specific cases: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + 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). + </p></li><li class="listitem"><p> + Communication between the desktop session and the operating system, + where the operating system would typically include the kernel + and any system daemons or processes. + </p></li></ul></div><p> + </p><p> + 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. + </p><p> + The problem solved by the systemwide or communication-with-the-OS case + is explained well by the following text from the Linux Hotplug project: + </p><div class="blockquote"><blockquote class="blockquote"><p> + 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. + </p><p> + 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. + </p></blockquote></div><p> + </p><p> + 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: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + Binary protocol designed to be used asynchronously + (similar in spirit to the X Window System protocol). + </p></li><li class="listitem"><p> + Stateful, reliable connections held open over time. + </p></li><li class="listitem"><p> + The message bus is a daemon, not a "swarm" or + distributed architecture. + </p></li><li class="listitem"><p> + Many implementation and deployment issues are specified rather + than left ambiguous/configurable/pluggable. + </p></li><li class="listitem"><p> + Semantics are similar to the existing DCOP system, allowing + KDE to adopt it more easily. + </p></li><li class="listitem"><p> + Security features to support the systemwide mode of the + message bus. + </p></li></ul></div><p> + </p></div></div><div class="sect1" title="Concepts"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="concepts"></a>Concepts</h2></div></div></div><p> + 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. + </p><p> + Here is a diagram (<a class="ulink" href="diagram.png" target="_top">png</a> <a class="ulink" href="diagram.svg" target="_top">svg</a>) that may help you visualize the concepts + that follow. + </p><div class="sect2" title="Native Objects and Object Paths"><div class="titlepage"><div><div><h3 class="title"><a name="objects"></a>Native Objects and Object Paths</h3></div></div></div><p> + 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 <em class="firstterm">native object</em>. + </p><p> + The low-level D-Bus protocol, and corresponding libdbus API, does not care about native objects. + However, it provides a concept called an + <em class="firstterm">object path</em>. The idea of an object path is that + higher-level bindings can name native object instances, and allow remote applications + to refer to them. + </p><p> + The object path + looks like a filesystem path, for example an object could be + named <code class="literal">/org/kde/kspread/sheets/3/cells/4/5</code>. + Human-readable paths are nice, but you are free to create an + object named <code class="literal">/com/mycompany/c5yo817y0c1y1c5b</code> + if it makes sense for your application. + </p><p> + Namespacing object paths is smart, by starting them with the components + of a domain name you own (e.g. <code class="literal">/org/kde</code>). This + keeps different code modules in the same process from stepping + on one another's toes. + </p></div><div class="sect2" title="Methods and Signals"><div class="titlepage"><div><div><h3 class="title"><a name="members"></a>Methods and Signals</h3></div></div></div><p> + Each object has <em class="firstterm">members</em>; the two kinds of member + are <em class="firstterm">methods</em> and + <em class="firstterm">signals</em>. 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. + </p><p> + Both methods and signals are referred to by name, such as + "Frobate" or "OnClicked". + </p></div><div class="sect2" title="Interfaces"><div class="titlepage"><div><div><h3 class="title"><a name="interfaces"></a>Interfaces</h3></div></div></div><p> + Each object supports one or more <em class="firstterm">interfaces</em>. + 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 + <span class="emphasis"><em>type</em></span> of an object instance. + </p><p> + DBus identifies interfaces with a simple namespaced string, + something like <code class="literal">org.freedesktop.Introspectable</code>. + Most bindings will map these interface names directly to + the appropriate programming language construct, for example + to Java interfaces or C++ pure virtual classes. + </p></div><div class="sect2" title="Proxies"><div class="titlepage"><div><div><h3 class="title"><a name="proxies"></a>Proxies</h3></div></div></div><p> + A <em class="firstterm">proxy object</em> 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.. + </p><p> + In pseudocode, programming without proxies might look like this: + </p><pre class="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(); + } + </pre><p> + </p><p> + Programming with proxies might look like this: + </p><pre class="programlisting"> + Proxy proxy = new Proxy(getBusConnection(), "/remote/object/path"); + Object returnValue = proxy.MethodName(arg1, arg2); + </pre><p> + </p></div><div class="sect2" title="Bus Names"><div class="titlepage"><div><div><h3 class="title"><a name="bus-names"></a>Bus Names</h3></div></div></div><p> + When each application connects to the bus daemon, the daemon immediately + assigns it a name, called the <em class="firstterm">unique connection name</em>. + 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 + <code class="literal">:34-907</code>. The numbers after the colon have + no meaning other than their uniqueness. + </p><p> + When a name is mapped + to a particular application's connection, that application is said to + <em class="firstterm">own</em> that name. + </p><p> + Applications may ask to own additional <em class="firstterm">well-known + names</em>. For example, you could write a specification to + define a name called <code class="literal">com.mycompany.TextEditor</code>. + Your definition could specify that to own this name, an application + should have an object at the path + <code class="literal">/com/mycompany/TextFileManager</code> supporting the + interface <code class="literal">org.freedesktop.FileHandler</code>. + </p><p> + Applications could then send messages to this bus name, + object, and interface to execute method calls. + </p><p> + You could think of the unique names as IP addresses, and the + well-known names as domain names. So + <code class="literal">com.mycompany.TextEditor</code> might map to something like + <code class="literal">:34-907</code> just as <code class="literal">mycompany.com</code> maps + to something like <code class="literal">192.168.0.5</code>. + </p><p> + 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. + </p><p> + Bus names can also be used to coordinate single-instance applications. + If you want to be sure only one + <code class="literal">com.mycompany.TextEditor</code> application is running for + example, have the text editor application exit if the bus name already + has an owner. + </p></div><div class="sect2" title="Addresses"><div class="titlepage"><div><div><h3 class="title"><a name="addresses"></a>Addresses</h3></div></div></div><p> + 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. + </p><p> + 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. + </p><p> + A D-Bus <em class="firstterm">address</em> specifies where a server will + listen, and where a client will connect. For example, the address + <code class="literal">unix:path=/tmp/abcdef</code> specifies that the server will + listen on a UNIX domain socket at the path + <code class="literal">/tmp/abcdef</code> 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. + </p><p> + 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). + </p><p> + 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. + </p></div><div class="sect2" title="Big Conceptual Picture"><div class="titlepage"><div><div><h3 class="title"><a name="bigpicture"></a>Big Conceptual Picture</h3></div></div></div><p> + 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: + </p><pre class="programlisting"> + Address -> [Bus Name] -> Path -> Interface -> Method + </pre><p> + 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. + </p><p> + 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. + </p></div><div class="sect2" title="Messages - Behind the Scenes"><div class="titlepage"><div><div><h3 class="title"><a name="messages"></a>Messages - Behind the Scenes</h3></div></div></div><p> + D-Bus works by sending messages between processes. If you're using + a sufficiently high-level binding, you may never work with messages directly. + </p><p> + There are 4 message types: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + Method call messages ask to invoke a method + on an object. + </p></li><li class="listitem"><p> + Method return messages return the results + of invoking a method. + </p></li><li class="listitem"><p> + Error messages return an exception caused by + invoking a method. + </p></li><li class="listitem"><p> + 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. + </p></li></ul></div><p> + </p><p> + 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. + </p><p> + Each message has a <em class="firstterm">header</em>, including <em class="firstterm">fields</em>, + and a <em class="firstterm">body</em>, including <em class="firstterm">arguments</em>. 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 <em class="firstterm">type signature</em> 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. + </p></div><div class="sect2" title="Calling a Method - Behind the Scenes"><div class="titlepage"><div><div><h3 class="title"><a name="callprocedure"></a>Calling a Method - Behind the Scenes</h3></div></div></div><p> + 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. + </p><p> + 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. + </p><p> + A method invocation in DBus happens as follows: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + For more low-level APIs, the application may construct a method call message itself, without + using a proxy. + </p></li><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + The method call message is sent to the bus daemon. + </p></li><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + The bus daemon receives the method reply message and sends it to the process that + made the method call. + </p></li><li class="listitem"><p> + 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. + </p></li></ul></div><p> + </p><p> + 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. + </p></div><div class="sect2" title="Emitting a Signal - Behind the Scenes"><div class="titlepage"><div><div><h3 class="title"><a name="signalprocedure"></a>Emitting a Signal - Behind the Scenes</h3></div></div></div><p> + 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 <a class="xref" href="#callprocedure" title="Calling a Method - Behind the Scenes">the section called “Calling a Method - Behind the Scenes”</a>) where the method call message has a matching method reply message. + </p><p> + 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. + </p><p> + A signal in DBus happens as follows: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + 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 + </p></li><li class="listitem"><p> + 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. + </p></li><li class="listitem"><p> + The bus daemon examines the signal and determines which processes are interested in it. + It sends the signal message to these processes. + </p></li><li class="listitem"><p> + 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. + </p></li></ul></div><p> + </p></div><div class="sect2" title="Introspection"><div class="titlepage"><div><div><h3 class="title"><a name="introspection"></a>Introspection</h3></div></div></div><p> + D-Bus objects may support the interface <code class="literal">org.freedesktop.DBus.Introspectable</code>. + This interface has one method <code class="literal">Introspect</code> 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. + </p></div></div><div class="sect1" title="GLib API: Using Remote Objects"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="glib-client"></a>GLib API: Using Remote Objects</h2></div></div></div><p> + The GLib binding is defined in the header file + <code class="literal"><dbus/dbus-glib.h></code>. + </p><div class="sect2" title="D-Bus - GLib type mappings"><div class="titlepage"><div><div><h3 class="title"><a name="glib-typemappings"></a>D-Bus - GLib type mappings</h3></div></div></div><p> + The heart of the GLib bindings for D-Bus is the mapping it + provides between D-Bus "type signatures" and GLib types + (<code class="literal">GType</code>). The D-Bus type system is composed of + a number of "basic" types, along with several "container" types. + </p><div class="sect3" title="Basic type mappings"><div class="titlepage"><div><div><h4 class="title"><a name="glib-basic-typemappings"></a>Basic type mappings</h4></div></div></div><p> + Below is a list of the basic types, along with their associated + mapping to a <code class="literal">GType</code>. + </p><div class="informaltable"><table border="1"><colgroup><col><col><col><col></colgroup><thead><tr><th>D-Bus basic type</th><th>GType</th><th>Free function</th><th>Notes</th></tr></thead><tbody><tr><td><code class="literal">BYTE</code></td><td><code class="literal">G_TYPE_UCHAR</code></td><td> </td><td> </td></tr><tr><td><code class="literal">BOOLEAN</code></td><td><code class="literal">G_TYPE_BOOLEAN</code></td><td> </td><td> </td></tr><tr><td><code class="literal">INT16</code></td><td><code class="literal">G_TYPE_INT</code></td><td> </td><td>Will be changed to a <code class="literal">G_TYPE_INT16</code> once GLib has it</td></tr><tr><td><code class="literal">UINT16</code></td><td><code class="literal">G_TYPE_UINT</code></td><td> </td><td>Will be changed to a <code class="literal">G_TYPE_UINT16</code> once GLib has it</td></tr><tr><td><code class="literal">INT32</code></td><td><code class="literal">G_TYPE_INT</code></td><td> </td><td>Will be changed to a <code class="literal">G_TYPE_INT32</code> once GLib has it</td></tr><tr><td><code class="literal">UINT32</code></td><td><code class="literal">G_TYPE_UINT</code></td><td> </td><td>Will be changed to a <code class="literal">G_TYPE_UINT32</code> once GLib has it</td></tr><tr><td><code class="literal">INT64</code></td><td><code class="literal">G_TYPE_GINT64</code></td><td> </td><td> </td></tr><tr><td><code class="literal">UINT64</code></td><td><code class="literal">G_TYPE_GUINT64</code></td><td> </td><td> </td></tr><tr><td><code class="literal">DOUBLE</code></td><td><code class="literal">G_TYPE_DOUBLE</code></td><td> </td><td> </td></tr><tr><td><code class="literal">STRING</code></td><td><code class="literal">G_TYPE_STRING</code></td><td><code class="literal">g_free</code></td><td> </td></tr><tr><td><code class="literal">OBJECT_PATH</code></td><td><code class="literal">DBUS_TYPE_G_PROXY</code></td><td><code class="literal">g_object_unref</code></td><td>The returned proxy does not have an interface set; use <code class="literal">dbus_g_proxy_set_interface</code> to invoke methods</td></tr></tbody></table></div><p> + As you can see, the basic mapping is fairly straightforward. + </p></div><div class="sect3" title="Container type mappings"><div class="titlepage"><div><div><h4 class="title"><a name="glib-container-typemappings"></a>Container type mappings</h4></div></div></div><p> + The D-Bus type system also has a number of "container" + types, such as <code class="literal">DBUS_TYPE_ARRAY</code> and + <code class="literal">DBUS_TYPE_STRUCT</code>. The D-Bus type system + is fully recursive, so one can for example have an array of + array of strings (i.e. type signature + <code class="literal">aas</code>). + </p><p> + 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 <code class="literal">DBUS_TYPE_STRUCT</code>, or a + <code class="literal">DBUS_TYPE_ARRAY</code> 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". + </p><p> + First, D-Bus type signatures which have an "obvious" + corresponding built-in GLib type are mapped using that type: + </p><div class="informaltable"><table border="1"><colgroup><col><col><col><col><col><col></colgroup><thead><tr><th>D-Bus type signature</th><th>Description</th><th>GType</th><th>C typedef</th><th>Free function</th><th>Notes</th></tr></thead><tbody><tr><td><code class="literal">as</code></td><td>Array of strings</td><td><code class="literal">G_TYPE_STRV</code></td><td><code class="literal">char **</code></td><td><code class="literal">g_strfreev</code></td><td> </td></tr><tr><td><code class="literal">v</code></td><td>Generic value container</td><td><code class="literal">G_TYPE_VALUE</code></td><td><code class="literal">GValue *</code></td><td><code class="literal">g_value_unset</code></td><td>The calling conventions for values expect that method callers have allocated return values; see below.</td></tr></tbody></table></div><p> + </p><p> + The next most common recursive type signatures are arrays of + basic values. The most obvious mapping for arrays of basic + types is a <code class="literal">GArray</code>. Now, GLib does not + provide a builtin <code class="literal">GType</code> for + <code class="literal">GArray</code>. 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. + </p><p> + 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. + </p><div class="informaltable"><table border="1"><colgroup><col><col><col><col><col><col></colgroup><thead><tr><th>D-Bus type signature</th><th>Description</th><th>GType</th><th>C typedef</th><th>Free function</th><th>Notes</th></tr></thead><tbody><tr><td><code class="literal">ay</code></td><td>Array of bytes</td><td><code class="literal">DBUS_TYPE_G_BYTE_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">au</code></td><td>Array of uint</td><td><code class="literal">DBUS_TYPE_G_UINT_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">ai</code></td><td>Array of int</td><td><code class="literal">DBUS_TYPE_G_INT_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">ax</code></td><td>Array of int64</td><td><code class="literal">DBUS_TYPE_G_INT64_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">at</code></td><td>Array of uint64</td><td><code class="literal">DBUS_TYPE_G_UINT64_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">ad</code></td><td>Array of double</td><td><code class="literal">DBUS_TYPE_G_DOUBLE_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr><tr><td><code class="literal">ab</code></td><td>Array of boolean</td><td><code class="literal">DBUS_TYPE_G_BOOLEAN_ARRAY</code></td><td><code class="literal">GArray *</code></td><td>g_array_free</td><td> </td></tr></tbody></table></div><p> + </p><p> + 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 <code class="literal">GType</code> 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. + </p><p> + At present, only strings are supported. Work is in progress to + include more types. + </p><div class="informaltable"><table border="1"><colgroup><col><col><col><col><col><col></colgroup><thead><tr><th>D-Bus type signature</th><th>Description</th><th>GType</th><th>C typedef</th><th>Free function</th><th>Notes</th></tr></thead><tbody><tr><td><code class="literal">a{ss}</code></td><td>Dictionary mapping strings to strings</td><td><code class="literal">DBUS_TYPE_G_STRING_STRING_HASHTABLE</code></td><td><code class="literal">GHashTable *</code></td><td>g_hash_table_destroy</td><td> </td></tr></tbody></table></div><p> + </p></div><div class="sect3" title="Arbitrarily recursive type mappings"><div class="titlepage"><div><div><h4 class="title"><a name="glib-generic-typemappings"></a>Arbitrarily recursive type mappings</h4></div></div></div><p> + 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 + <code class="literal">DBusGValue</code> which acts as a kind of special + variant value which may be iterated over manually. The + <code class="literal">GType</code> associated is + <code class="literal">DBUS_TYPE_G_VALUE</code>. + </p><p> + TODO insert usage of <code class="literal">DBUS_TYPE_G_VALUE</code> here. + </p></div></div><div class="sect2" title="A sample program"><div class="titlepage"><div><div><h3 class="title"><a name="sample-program-1"></a>A sample program</h3></div></div></div><p>Here is a D-Bus program using the GLib bindings. +</p><pre class="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; +} +</pre><p> + </p></div><div class="sect2" title="Program initalization"><div class="titlepage"><div><div><h3 class="title"><a name="glib-program-setup"></a>Program initalization</h3></div></div></div><p> + A connection to the bus is acquired using + <code class="literal">dbus_g_bus_get</code>. Next, a proxy + is created for the object "/org/freedesktop/DBus" with + interface <code class="literal">org.freedesktop.DBus</code> + on the service <code class="literal">org.freedesktop.DBus</code>. + This is a proxy for the message bus itself. + </p></div><div class="sect2" title="Understanding method invocation"><div class="titlepage"><div><div><h3 class="title"><a name="glib-method-invocation"></a>Understanding method invocation</h3></div></div></div><p> + You have a number of choices for method invocation. First, as + used above, <code class="literal">dbus_g_proxy_call</code> 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 <code class="literal">G_TYPE_INVALID</code>. + Next, pointers to return values are specified, followed again + by <code class="literal">G_TYPE_INVALID</code>. + </p><p> + To invoke a method asynchronously, use + <code class="literal">dbus_g_proxy_begin_call</code>. This returns a + <code class="literal">DBusGPendingCall</code> object; you may then set a + notification function using + <code class="literal">dbus_g_pending_call_set_notify</code>. + </p></div><div class="sect2" title="Connecting to object signals"><div class="titlepage"><div><div><h3 class="title"><a name="glib-signal-connection"></a>Connecting to object signals</h3></div></div></div><p> + You may connect to signals using + <code class="literal">dbus_g_proxy_add_signal</code> and + <code class="literal">dbus_g_proxy_connect_signal</code>. You must + invoke <code class="literal">dbus_g_proxy_add_signal</code> to specify + the signature of your signal handlers; you may then invoke + <code class="literal">dbus_g_proxy_connect_signal</code> multiple times. + </p><p> + 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 + <span class="application">glib-genmarshal</span>, and then register + it using <code class="literal">dbus_g_object_register_marshaller</code>. + </p></div><div class="sect2" title="Error handling and remote exceptions"><div class="titlepage"><div><div><h3 class="title"><a name="glib-error-handling"></a>Error handling and remote exceptions</h3></div></div></div><p> + All of the GLib binding methods such as + <code class="literal">dbus_g_proxy_end_call</code> return a + <code class="literal">GError</code>. This <code class="literal">GError</code> can + represent two different things: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + 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 + <code class="literal">DBUS_GERROR</code>, and a corresponding code + such as <code class="literal">DBUS_GERROR_NO_MEMORY</code>. It will + not be typical for applications to handle these errors + specifically. + </p></li><li class="listitem"><p> + 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 <code class="literal">GError</code>, but some + special rules apply. + </p><p> + The set error will have the domain + <code class="literal">DBUS_GERROR</code> as above, and will also + have the code + <code class="literal">DBUS_GERROR_REMOTE_EXCEPTION</code>. In order + to access the remote exception name, you must use a + special accessor, such as + <code class="literal">dbus_g_error_has_name</code> or + <code class="literal">dbus_g_error_get_name</code>. The remote + exception detailed message is accessible via the regular + GError <code class="literal">message</code> member. + </p></li></ul></div><p> + </p></div><div class="sect2" title="More examples of method invocation"><div class="titlepage"><div><div><h3 class="title"><a name="glib-more-examples"></a>More examples of method invocation</h3></div></div></div><div class="sect3" title="Sending an integer and string, receiving an array of bytes"><div class="titlepage"><div><div><h4 class="title"><a name="glib-sending-stuff"></a>Sending an integer and string, receiving an array of bytes</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Sending a GHashTable"><div class="titlepage"><div><div><h4 class="title"><a name="glib-sending-hash"></a>Sending a GHashTable</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Receiving a boolean and a string"><div class="titlepage"><div><div><h4 class="title"><a name="glib-receiving-bool-int"></a>Receiving a boolean and a string</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Sending two arrays of strings"><div class="titlepage"><div><div><h4 class="title"><a name="glib-sending-str-arrays"></a>Sending two arrays of strings</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Sending a boolean, receiving an array of strings"><div class="titlepage"><div><div><h4 class="title"><a name="glib-getting-str-array"></a>Sending a boolean, receiving an array of strings</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Sending a variant"><div class="titlepage"><div><div><h4 class="title"><a name="glib-sending-variant"></a>Sending a variant</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div><div class="sect3" title="Receiving a variant"><div class="titlepage"><div><div><h4 class="title"><a name="glib-receiving-variant"></a>Receiving a variant</h4></div></div></div><p> +</p><pre class="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); +</pre><p> + </p></div></div><div class="sect2" title="Generated Bindings"><div class="titlepage"><div><div><h3 class="title"><a name="glib-generated-bindings"></a>Generated Bindings</h3></div></div></div><p> + By using the Introspection XML files, convenient client-side bindings + can be automatically created to ease the use of a remote DBus object. + </p><p> + Here is a sample XML file which describes an object that exposes + one method, named <code class="literal">ManyArgs</code>. + </p><pre class="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> +</pre><p> + </p><p> + Run <code class="literal">dbus-binding-tool --mode=glib-client + <em class="replaceable"><code>FILENAME</code></em> > + <em class="replaceable"><code>HEADER_NAME</code></em></code> to generate the header + file. For example: <span class="command"><strong>dbus-binding-tool --mode=glib-client + my-object.xml > my-object-bindings.h</strong></span>. This will generate + inline functions with the following prototypes: + </p><pre class="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); +</pre><p> + The first argument in all functions is a <code class="literal">DBusGProxy + *</code>, which you should create with the usual + <code class="literal">dbus_g_proxy_new_*</code> functions. Following that are the + "in" arguments, and then either the "out" arguments and a + <code class="literal">GError *</code> 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 + <code class="literal">DBusGProxy *</code>, the returned "out" arguments, an + <code class="literal">GError *</code> which is set if there was an error otherwise + <code class="literal">NULL</code>, and the user data. + </p><p> + As with the server-side bindings support (see <a class="xref" href="#glib-server" title="GLib API: Implementing Objects">the section called “GLib API: Implementing Objects”</a>), the exact behaviour of the client-side + bindings can be manipulated using "annotations". Currently the only + annotation used by the client bindings is + <code class="literal">org.freedesktop.DBus.GLib.NoReply</code>, 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. + </p></div></div><div class="sect1" title="GLib API: Implementing Objects"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="glib-server"></a>GLib API: Implementing Objects</h2></div></div></div><p> + 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. + </p><p> + Here is a sample XML file which describes an object that exposes + one method, named <code class="literal">ManyArgs</code>. +</p><pre class="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> +</pre><p> + </p><p> + 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 + (<code class="literal">my_object</code>). In addition, if particular + methods symbol names deviate from C convention + (i.e. <code class="literal">ManyArgs</code> -> + <code class="literal">many_args</code>), you may specify an annotation + giving the C symbol. + </p><p> + Once you have written this XML, run <code class="literal">dbus-binding-tool --mode=glib-server <em class="replaceable"><code>FILENAME</code></em> > <em class="replaceable"><code>HEADER_NAME</code></em>.</code> to + generate a header file. For example: <span class="command"><strong>dbus-binding-tool --mode=glib-server my-object.xml > my-object-glue.h</strong></span>. + </p><p> + Next, include the generated header in your program, and invoke + <code class="literal">dbus_g_object_class_install_info</code> in the class + initializer, passing the object class and "object info" included in the + header. For example: + </p><pre class="programlisting"> + dbus_g_object_type_install_info (COM_FOO_TYPE_MY_OBJECT, &com_foo_my_object_info); + </pre><p> + This should be done exactly once per object class. + </p><p> + To actually implement the method, just define a C function named e.g. + <code class="literal">my_object_many_args</code> in the same file as the info + header is included. At the moment, it is required that this function + conform to the following rules: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + The function must return a value of type <code class="literal">gboolean</code>; + <code class="literal">TRUE</code> on success, and <code class="literal">FALSE</code> + otherwise. + </p></li><li class="listitem"><p> + The first parameter is a pointer to an instance of the object. + </p></li><li class="listitem"><p> + Following the object instance pointer are the method + input values. + </p></li><li class="listitem"><p> + Following the input values are pointers to return values. + </p></li><li class="listitem"><p> + The final parameter must be a <code class="literal">GError **</code>. + If the function returns <code class="literal">FALSE</code> for an + error, the error parameter must be initalized with + <code class="literal">g_set_error</code>. + </p></li></ul></div><p> + </p><p> + Finally, you can export an object using <code class="literal">dbus_g_connection_register_g_object</code>. For example: + </p><pre class="programlisting"> + dbus_g_connection_register_g_object (connection, + "/com/foo/MyObject", + obj); + </pre><p> + </p><div class="sect2" title="Server-side Annotations"><div class="titlepage"><div><div><h3 class="title"><a name="glib-annotations"></a>Server-side Annotations</h3></div></div></div><p> + There are several annotations that are used when generating the + server-side bindings. The most common annotation is + <code class="literal">org.freedesktop.DBus.GLib.CSymbol</code> but there are other + annotations which are often useful. + </p><div class="variablelist"><dl><dt><span class="term"><code class="literal">org.freedesktop.DBus.GLib.CSymbol</code></span></dt><dd><p> + 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. + </p></dd><dt><span class="term"><code class="literal">org.freedesktop.DBus.GLib.Async</code></span></dt><dd><p> + 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. + </p><p> + When a method is asynchronous, the function prototype is + different. It is required that the function conform to the + following rules: + </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p> + The function must return a value of type <code class="literal">gboolean</code>; + <code class="literal">TRUE</code> on success, and <code class="literal">FALSE</code> + otherwise. TODO: the return value is currently ignored. + </p></li><li class="listitem"><p> + The first parameter is a pointer to an instance of the object. + </p></li><li class="listitem"><p> + Following the object instance pointer are the method + input values. + </p></li><li class="listitem"><p> + The final parameter must be a + <code class="literal">DBusGMethodInvocation *</code>. This is used + when sending the response message back to the client, by + calling <code class="literal">dbus_g_method_return</code> or + <code class="literal">dbus_g_method_return_error</code>. + </p></li></ul></div><p> + </p></dd><dt><span class="term"><code class="literal">org.freedesktop.DBus.GLib.Const</code></span></dt><dd><p>This attribute can only be applied to "out" + <code class="literal"><arg></code> nodes, and specifies that the + parameter isn't being copied when returned. For example, this + turns a 's' argument from a <code class="literal">char **</code> to a + <code class="literal">const char **</code>, and results in the argument not + being freed by DBus after the message is sent. + </p></dd><dt><span class="term"><code class="literal">org.freedesktop.DBus.GLib.ReturnVal</code></span></dt><dd><p> + This attribute can only be applied to "out" + <code class="literal"><arg></code> nodes, and alters the expected + function signature. It currently can be set to two values: + <code class="literal">""</code> or <code class="literal">"error"</code>. 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 <code class="literal">GError + *</code> 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 <code class="literal">"error"</code>, then the + final argument is a <code class="literal">GError *</code> as usual. + </p><p> + Some examples to demonstrate the usage. This introspection XML: + </p><pre class="programlisting"> +<method name="Increment"> + <arg type="u" name="x" /> + <arg type="u" direction="out" /> +</method> + </pre><p> + Expects the following function declaration: + </p><pre class="programlisting"> +gboolean +my_object_increment (MyObject *obj, gint32 x, gint32 *ret, GError **error); + </pre><p> + </p><p> + This introspection XML: + </p><pre class="programlisting"> +<method name="IncrementRetval"> + <arg type="u" name="x" /> + <arg type="u" direction="out" > + <annotation name="org.freedesktop.DBus.GLib.ReturnVal" value=""/> + </arg> +</method> + </pre><p> + Expects the following function declaration: + </p><pre class="programlisting"> +gint32 +my_object_increment_retval (MyObject *obj, gint32 x) + </pre><p> + </p><p> + This introspection XML: + </p><pre class="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> + </pre><p> + Expects the following function declaration: + </p><pre class="programlisting"> +gint32 +my_object_increment_retval_error (MyObject *obj, gint32 x, GError **error) + </pre><p> + </p></dd></dl></div><p> + </p></div></div><div class="sect1" title="Python API"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="python-client"></a>Python API</h2></div></div></div><p> + The Python API, dbus-python, is now documented separately in + <a class="ulink" href="http://dbus.freedesktop.org/doc/dbus-python/doc/tutorial.html" target="_top">the dbus-python tutorial</a> (also available in doc/tutorial.txt, + and doc/tutorial.html if built with python-docutils, in the dbus-python + source distribution). + </p></div><div class="sect1" title="Qt API: Using Remote Objects"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="qt-client"></a>Qt API: Using Remote Objects</h2></div></div></div><p> + + The Qt bindings are not yet documented. + + </p></div><div class="sect1" title="Qt API: Implementing Objects"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="qt-server"></a>Qt API: Implementing Objects</h2></div></div></div><p> + The Qt bindings are not yet documented. + </p></div></div></body></html> |