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| <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> |