summaryrefslogtreecommitdiffstats
path: root/doc/dbus-tutorial.xml
blob: 44f2f7cadb70d2c3768f90ccf5512981bdabd1c0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
<?xml version="1.0" standalone="no"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
[
]>

<article id="index">
  <articleinfo>
    <title>D-BUS Tutorial</title>
    <releaseinfo>Version 0.3</releaseinfo>
    <date>18 January 2005</date>
    <authorgroup>
      <author>
	<firstname>Havoc</firstname>
	<surname>Pennington</surname>
	<affiliation>
	  <orgname>Red Hat, Inc.</orgname>
	  <address>
	    <email>hp@pobox.com</email>
	  </address>
	</affiliation>
      </author>
    </authorgroup>
  </articleinfo>

  <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> 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>
      If you just want to use D-BUS and don't care how it works, jump directly
      to <xref linkend="concepts"/>.
      Otherwise, read on.
    </para>

    <para>
      libdbus only supports one-to-one connections, just like a raw network
      socket. However, rather than sending byte streams over the connection, you
      send <firstterm>messages</firstterm>. Messages have a header identifying
      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.mbus.org/">MBUS</ulink>, <ulink
        url="http://www.omg.org">CORBA</ulink>, <ulink
        url="http://www.xmlrpc.com">XML-RPC</ulink>, <ulink
        url="http://www.w3.org/TR/SOAP/">SOAP</ulink>, and probably hundreds
        more.  Each of these is tailored for particular kinds of application.
        D-BUS is designed for two specific cases:
        <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.
      </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.
            </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>

    <sect2 id="objects">
      <title>Objects and Object Paths</title>
      <para>
        Each application using D-BUS contains <firstterm>objects</firstterm>,
        which generally map to GObject, QObject, C++ objects, or Python objects
        (but need not).  An object is an <emphasis>instance</emphasis> rather
        than a type.  When messages are received over a D-BUS connection, they
        are sent to a specific object, not to the application as a whole.
      </para>
      <para>
        To allow messages to specify their destination object, there has to be a
        way to refer to an object. In your favorite programming language, this
        is normally called a <firstterm>pointer</firstterm> or
        <firstterm>reference</firstterm>. However, these references are
        implemented as memory addresses relative to the address space of your
        application, and thus can't be passed from one application to another.
      </para>
      <para>
        To solve this, D-BUS introduces a name for each object. The name 
        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="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>
    </sect2>
      
    <sect2 id="messages">
      <title>Message Types</title>
      <para>
        Messages are not all the same; in particular, D-BUS has 
        4 built-in message types:
        <itemizedlist>
          <listitem>
            <para>
              Method call messages ask to invoke a method 
              on an object.
            </para>
          </listitem>
          <listitem>
            <para>
              Method return messages return the results 
              of invoking a method.
            </para>
          </listitem>
          <listitem>
            <para>
              Error messages return an exception caused by 
              invoking a method.
            </para>
          </listitem>
          <listitem>
            <para>
              Signal messages are notifications that a given signal 
              has been emitted (that an event has occurred). 
              You could also think of these as "event" messages.
            </para>
          </listitem>
        </itemizedlist>
      </para>
      <para>
        A method call maps very simply to messages, then: you send a method call
        message, and receive either a method return message or an error message
        in reply.
      </para>
    </sect2>

    <sect2 id="bus-names">
      <title>Bus Names</title>
      
      <para>
        Object paths, interfaces, and messages exist on the level of 
        libdbus and the D-BUS protocol; they are used even in the 
        1-to-1 case with no message bus involved.
      </para>

      <para>
        Bus names, on the other hand, are a property of the message bus daemon.
        The bus maintains a mapping from names to message bus connections.
        These names are used to specify the origin and destination
        of messages passing through the message bus. When a name is mapped 
        to a particular application's connection, that application is said to 
        <firstterm>own</firstterm> that name.
      </para>

      <para>
        On connecting to the bus daemon, each application immediately owns a
        special name called the <firstterm>unique connection name</firstterm>.
        A unique name begins with a ':' (colon) character; no other names are
        allowed to begin with that character. Unique names are special because
        they are created dynamically, and are never re-used during the lifetime
        of the same bus daemon. You know that a given unique name will have the
        same owner at all times.  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>
        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>

    </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>
        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, the bus daemon is a server 
        and all other applications are clients of the 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.
      </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>

  </sect1>

  <sect1 id="glib-client">
    <title>GLib API: Using Remote Objects</title>

    <para>
      
      The GLib binding is defined in the header file
      &lt;dbus/dbus-glib.h&gt;. The API is very small, in sharp contrast to the
      low-level &lt;dbus/dbus.h&gt;.

    </para>

    <para>
      The GLib bindings are incomplete, see the TODO file and comments in the
      source code.
    </para>

    <para>
Here is a D-BUS program using the GLib bindings.
<programlisting>      
int
main (int argc, char **argv)
{
  DBusGConnection *connection;
  GError *error;
  DBusGProxy *proxy;
  DBusGPendingCall *call;
  char **name_list;
  int name_list_len;
  int i;
  
  g_type_init ();

  error = NULL;
  connection = dbus_g_bus_get (DBUS_BUS_SESSION,
                               &amp;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_ORG_FREEDESKTOP_DBUS,
                                     DBUS_PATH_ORG_FREEDESKTOP_DBUS,
                                     DBUS_INTERFACE_ORG_FREEDESKTOP_DBUS);

  /* Call ListNames method */
  
  call = dbus_g_proxy_begin_call (proxy, "ListNames", DBUS_TYPE_INVALID);

  error = NULL;
  if (!dbus_g_proxy_end_call (proxy, call, &amp;error,
                              DBUS_TYPE_ARRAY, DBUS_TYPE_STRING,
                              &amp;name_list, &amp;name_list_len,
                              DBUS_TYPE_INVALID))
    {
      g_printerr ("Failed to complete ListNames call: %s\n",
                  error->message);
      g_error_free (error);
      exit (1);
    }

  /* Print the results */
 
  g_print ("Names on the message bus:\n");
  i = 0;
  while (i &lt; name_list_len)
    {
      g_assert (name_list[i] != NULL);
      g_print ("  %s\n", name_list[i]);
      ++i;
    }
  g_assert (name_list[i] == NULL);

  g_strfreev (name_list);

  return 0;
}
</programlisting>
    </para>

    <para>

      DBusGProxy represents a remote object. dbus_g_proxy_begin_call() sends 
      a method call to the remote object, and dbus_g_proxy_end_call() retrieves 
      any return values or exceptions resulting from the method call. 
      There are also DBusGProxy functions to connect and disconnect signals, 
      not shown in the code example.

    </para>

    <para>
      
      dbus_g_bus_get() assumes that the application will use GMainLoop. The
      created connection will be associated with the main loop such that
      messages will be sent and received when the main loop runs.  However, in
      the above code example the main loop never runs; D-BUS will not run the
      loop implicitly. Instead, dbus_g_proxy_end_call() will block until the
      method call has been sent and the reply received. A more complex GUI
      application might run the main loop while waiting for the method call
      reply.  (DBusGPendingCall is currently missing the "notify me when the
      call is complete" functionality found in DBusPendingCall, but it should be
      added.)

    </para>

    <para>
      
      Future plans (see doc/TODO) are to use G_TYPE_STRING in place of
      DBUS_TYPE_STRING and so forth. In fact the above code is slightly
      incorrect at the moment, since it uses g_strfreev() to free a string array
      that was not allocated with g_malloc(). dbus_free_string_array() should
      really be used. However, once the GLib bindings are complete the returned
      data from dbus_g_proxy_end_call() will be allocated with g_malloc().

    </para>

  </sect1>

  <sect1 id="glib-server">
    <title>GLib API: Implementing Objects</title>

    <para>
      
      The GLib binding is defined in the header file
      &lt;dbus/dbus-glib.h&gt;. To implement an object, it's also necessary
      to use the dbus-glib-tool command line tool.

    </para>

    <para>
      The GLib bindings are incomplete.  Implementing an object is not yet
      possible, see the TODO file and comments in the source code for details
      on what work needs doing.
    </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>


  <sect1 id="python-client">
    <title>Python API: Using Remote Objects</title>
    <para>
      The Python bindings are not yet documented, but the 
      bindings themselves are in good shape.
    </para>
  </sect1>

  <sect1 id="python-server">
    <title>Python API: Implementing Objects</title>
    <para>
      The Python bindings are not yet documented, but the 
      bindings themselves are in good shape.
    </para>
  </sect1>

</article>