Bus

A bus is a simple system that takes care of forwarding messages from the streaming 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.

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.

How to use a bus

There are two different ways to use a bus:

  • Run a GLib/Gtk+ main loop (or iterate the default GLib main context yourself regularly) and attach some kind of watch to the bus. This way the GLib main loop will check the bus for new messages and notify you whenever there are messages.

    Typically you would use gst_bus_add_watch () or gst_bus_add_signal_watch () in this case.

    To use a bus, attach a message handler to the bus of a pipeline using gst_bus_add_watch (). 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 keep the handler attached to the bus, return FALSE to remove it.

  • Check for messages on the bus yourself. This can be done using gst_bus_peek () and/or gst_bus_poll ().

#include <gst/gst.h>

static GMainLoop *loop;

static gboolean
my_bus_callback (GstBus     *bus,
         GstMessage *message,
         gpointer    data)
{
  g_print ("Got %s message\n", GST_MESSAGE_TYPE_NAME (message));

  switch (GST_MESSAGE_TYPE (message)) {
    case GST_MESSAGE_ERROR: {
      GError *err;
      gchar *debug;

      gst_message_parse_error (message, &err, &debug);
      g_print ("Error: %s\n", err->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;
  }

  /* we want to be notified again the next time there is a message
   * on the bus, so returning TRUE (FALSE means we want to stop watching
   * for messages on the bus and our callback should not be called again)
   */
  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GstElement *pipeline;
  GstBus *bus;
  guint bus_watch_id;

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

  /* create pipeline, add handler */
  pipeline = gst_pipeline_new ("my_pipeline");

  /* adds a watch for new message on our pipeline's message bus to
   * the default GLib main context, which is the main context that our
   * GLib main loop is attached to below
   */
  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  bus_watch_id = gst_bus_add_watch (bus, my_bus_callback, NULL);
  gst_object_unref (bus);

  /* [...] */

  /* create a mainloop that runs/iterates the default GLib main context
   * (context NULL), in other words: makes the context check if anything
   * it watches for has happened. When a message has been posted on the
   * bus, the default main context will automatically call our
   * my_bus_callback() function to notify us of that message.
   * The main loop will be run until someone calls g_main_loop_quit()
   */
  loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (loop);

  /* clean up */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  g_source_remove (bus_watch_id);
  g_main_loop_unref (loop);

  return 0;
}

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 asynchronous, 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. The advantage of this approach is that all the threading that GStreamer does internally is hidden from the application and the application developer does not have to worry about thread issues at all.

Note that if you're using the default GLib mainloop integration, you can, instead of attaching a watch, connect to the “message” signal on the bus. This way you don't have to switch() on all possible message types; just connect to the interesting signals in form of “message::<type>”, where <type> is a specific message type (see the next section for an explanation of message types).

The above snippet could then also be written as:

GstBus *bus;

[..]

bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline);
gst_bus_add_signal_watch (bus);
g_signal_connect (bus, "message::error", G_CALLBACK (cb_message_error), NULL);
g_signal_connect (bus, "message::eos", G_CALLBACK (cb_message_eos), NULL);

[..]

If you aren't using GLib mainloop, the asynchronous message signals won't be available by default. You can however install a custom sync handler that wakes up the custom mainloop and that uses gst_bus_async_signal_func () to emit the signals. (see also documentation for details)

Message types

GStreamer has a few pre-defined message types that can be passed over the bus. The messages are extensible, 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.

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.

  • 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 activity. Warnings are not fatal, but imply a problem nevertheless. Information messages are for non-problem notifications. All those messages contain a GError with the main error type and message, and optionally a debug string. Both can be extracted using gst_message_parse_error(), _parse_warning () and _parse_info (). Both error and debug strings should be freed after use.

  • 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.

  • 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. gst_message_parse_tag() should be used to parse the taglist, which should be gst_tag_list_unref ()'ed when no longer needed.

  • State-changes: emitted after a successful state change. gst_message_parse_state_changed () can be used to parse the old and new state of this transition.

  • Buffering: emitted during caching of network-streams. One can manually extract the progress (in percent) from the message by extracting the “buffer-percent” property from the structure returned by gst_message_get_structure(). See also Buffering

  • Element messages: these are special messages that are unique to certain elements and usually represent additional features. The element's documentation should mention in detail which element messages a particular element may send. As an example, the 'qtdemux' QuickTime demuxer element may send a 'redirect' element message on certain occasions if the stream contains a redirect instruction.

  • Application-specific messages: any information on those can be extracted by getting the message structure (see above) and reading its fields. Usually these messages can safely be ignored.

    Application messages are primarily meant for internal use in applications in case the application needs to marshal information from some thread into the main thread. This is particularly useful when the application is making use of element signals (as those signals will be emitted in the context of the streaming thread).

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