gstreamer/docs/manual/basics-bus.xml

<chapter id="chapter-bus">
  <title>Bus</title>
  <para>
    A bus is a simple system that takes care of forwarding messages from
    the pipeline threads to an application in its own thread context. The
    advantage of a bus is that an application does not need to be
    thread-aware in order to use &GStreamer;, even though &GStreamer;
    itself is heavily threaded.
  </para>
  <para>
    Every pipeline contains a bus by default, so applications do not need
    to create a bus or anything. The only thing applications should do is
    set a message handler on a bus, which is similar to a signal handler
    to an object. When the mainloop is running, the bus will periodically
    be checked for new messages, and the callback will be called when any
    message is available.
  </para>

  <sect1 id="section-bus-howto">
    <title>How to use a bus</title>
    <para>
      To use a bus, attach a message handler to the default bus of a pipeline
      using <function>gst_bus_add_watch ()</function>. This handler will be
      called whenever the pipeline emits a message to the bus. In this
      handler, check the signal type (see next section) and do something
      accordingly. The return value of the handler should be TRUE to remove
      the message from the bus.
    </para>
    <programlisting><!-- example-begin bus.c a -->
#include &lt;gst/gst.h&gt;

static GMainLoop *loop;

static gboolean
my_bus_callback (GstBus     *bus,
		 GstMessage *message,
		 gpointer    data)
{
  switch (GST_MESSAGE_TYPE (message)) {
    case GST_MESSAGE_ERROR: {
      GError *err;
      gchar *debug;

      gst_message_parse_error (message, &amp;err, &amp;debug);
      g_print ("Error: %s\n", err-&gt;message);
      g_error_free (err);
      g_free (debug);

      g_main_loop_quit (loop);
      break;
    }
    case GST_MESSAGE_EOS:
      /* end-of-stream */
      g_main_loop_quit (loop);
      break;
    default:
      /* unhandled message */
      break;
  }

  /* remove message from the queue */
  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline;

  /* init */
  gst_init (&amp;argc, &amp;argv);

  /* create pipeline, add handler */
  pipeline = gst_pipeline_new ("my_pipeline");
  gst_bus_add_watch (gst_pipeline_get_bus (GST_PIPELINE (pipeline)),
		     my_bus_callback, NULL);
<!-- example-end bus.c a -->
[..]<!-- example-begin bus.c b -->
<!-- example-begin bus.c c -->
  /* in the mainloop, all messages posted to the bus by the pipeline
   * will automatically be sent to our callback. */
  loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (loop);

  return 0;
}
    <!-- example-end bus.c c -->
    </programlisting>
    <para>
      It is important to know that the handler will be called in the thread
      context of the mainloop. This means that the interaction between the
      pipeline and application over the bus is
      <emphasis>asynchronous</emphasis>, and thus not suited for some
      real-time purposes, such as cross-fading between audio tracks, doing
      (theoretically) gapless playback or video effects. All such things
      should be done in the pipeline context, which is easiest by writing
      a &GStreamer; plug-in. It is very useful for its primary purpose,
      though: passing messages from pipeline to application.
    </para>
  </sect1>

  <sect1 id="section-bus-message-types">
    <title>Message types</title>
    <para>
      &GStreamer; has a few pre-defined message types that can be passed
      over the bus. The messages are extendible, however. Plug-ins can
      define additional messages, and applications can decide to either
      have specific code for those or ignore them. All applications are
      strongly recommended to at least handle error messages by providing
      visual feedback to the user.
    </para>
    <para>
      All messages have a message source, type and timestamp. The message
      source can be used to see which element emitted the message. For some
      messages, for example, only the ones emitted by the top-level pipeline
      will be interesting to most applications (e.g. for state-change
      notifications). Below is a list of all messages and a short explanation
      of what they do and how to parse message-specific content.
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Error, warning and information notifications: those are used
          by elements if a message should be shown to the user about the
          state of the pipeline. Error messages are fatal and terminate
          the data-passing. The error should be repaired to resume pipeline
          acvitity. Warnings are not fatal, but imply a problem nevertheless.
          Information messages are for non-problem notifications. All those
          messages contain a <classname>GError</classname> with the main
          error type and message, and optionally a debug string. Both
          can be extracted using <function>gst_message_parse_error
          ()</function>, <function>_parse_warning ()</function> and
          <function>_parse_info ()</function>. Both error and debug string
          should be free'ed after use.
        </para>
      </listitem>
      <listitem>
        <para>
          End-of-stream notification: this is emitted when the stream has
          ended. The state of the pipeline will not change, but further
          media handling will stall. Applications can use this to skip to
          the next song in their playlist. After end-of-stream, it is also
          possible to seek back in the stream. Playback will then continue
          automatically. This message has no specific arguments.
        </para>
      </listitem>
      <listitem>
        <para>
          Tags: emitted when metadata was found in the stream. This can be
          emitted multiple times for a pipeline (e.g. once for descriptive
          metadata such as artist name or song title, and another one for
          stream-information, such as samplerate and bitrate). Applications
          should cache metadata internally. <function>gst_message_parse_tag
          ()</function> should be used to parse the taglist, which should
          be dereferenced after use.
        </para>
      </listitem>
      <listitem>
        <para>
          State-changes: emitted after a successful state change.
          <function>gst_message_parse_state_changed ()</function> can be
          used to parse the old and new state of this transition.
        </para>
      </listitem>
      <listitem>
        <para>
          Buffering: emitted during caching of network-streams. One can
          manually extract the progress (in percent) from the message by
          extracting the <quote>buffer-percent</quote> property from the
          structure returned by <function>gst_message_parse_structure
          ()</function>.
        </para>
      </listitem>
      <listitem>
        <para>
          Other application-specific messages: any information on those can
          be extracted by getting a structure (see above) and reading
          properties. In most cases, such messages can conveniently be
          ignored.
        </para>
      </listitem>
    </itemizedlist>
  </sect1>
</chapter>