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<chapter id="chapter-queryevents">
<title>Position tracking and seeking</title>
So far, we've looked at how to create a pipeline to do media processing
and how to make it run. Most application developers will be interested
in providing feedback to the user on media progress. Media players, for
example, will want to show a slider showing the progress in the song,
and usually also a label indicating stream length. Transcoding
applications will want to show a progress bar on how much percent of
the task is done. &GStreamer; has built-in support for doing all this
using a concept known as <emphasis>querying</emphasis>. Since seeking
is very similar, it will be discussed here as well. Seeking is done
using the concept of <emphasis>events</emphasis>.
<sect1 id="section-querying">
<title>Querying: getting the position or length of a stream</title>
Querying is defined as requesting a specific stream property related
to progress tracking. This includes getting the length of a stream (if
available) or getting the current position. Those stream properties
can be retrieved in various formats such as time, audio samples, video
frames or bytes. The function most commonly used for this is
<function>gst_element_query ()</function>, although some convenience
wrappers are provided as well (such as
<function>gst_element_query_position ()</function> and
<function>gst_element_query_duration ()</function>). You can generally
query the pipeline directly, and it'll figure out the internal details
for you, like which element to query.
Internally, queries will be sent to the sinks, and
<quote>dispatched</quote> backwards until one element can handle it;
that result will be sent back to the function caller. Usually, that
is the demuxer, although with live sources (from a webcam), it is the
source itself.
<!-- example-begin query.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end query.c a -->
<!-- example-begin query.c b --><!--
static void
my_bus_message_cb (GstBus *bus,
GstMessage *message,
gpointer data)
GMainLoop *loop = (GMainLoop *) data;
switch (GST_MESSAGE_TYPE (message)) {
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);
/* end-of-stream */
g_main_loop_quit (loop);
<!-- example-end query.c b -->
<!-- example-begin query.c c -->
static gboolean
cb_print_position (GstElement *pipeline)
gint64 pos, len;
if (gst_element_query_position (pipeline, GST_FORMAT_TIME, &amp;pos)
&amp;&amp; gst_element_query_duration (pipeline, GST_FORMAT_TIME, &amp;len)) {
g_print ("Time: %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
/* call me again */
return TRUE;
main (gint argc,
gchar *argv[])
GstElement *pipeline;
<!-- example-end query.c c -->
[..]<!-- example-begin query.c d --><!--
GstStateChangeReturn ret;
GMainLoop *loop;
GError *err = NULL;
GstBus *bus;
gchar *l;
/* init */
gst_init (&amp;argc, &amp;argv);
/* args */
if (argc != 2) {
g_print ("Usage: %s &lt;filename&gt;\n", argv[0]);
return -1;
loop = g_main_loop_new (NULL, FALSE);
/* build pipeline, the easy way */
l = g_strdup_printf ("filesrc location=\"%s\" ! oggdemux ! vorbisdec ! "
"audioconvert ! audioresample ! alsasink",
pipeline = gst_parse_launch (l, &amp;err);
if (pipeline == NULL || err != NULL) {
g_printerr ("Cannot build pipeline: %s\n", err->message);
g_error_free (err);
g_free (l);
if (pipeline)
gst_object_unref (pipeline);
return -1;
g_free (l);
bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
gst_bus_add_signal_watch (bus);
g_signal_connect (bus, "message", G_CALLBACK (my_bus_message_cb), loop);
gst_object_unref (bus);
/* play */
ret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
g_error ("Failed to set pipeline to PLAYING.\n");
--><!-- example-end query.c d -->
<!-- example-begin query.c e -->
/* run pipeline */
g_timeout_add (200, (GSourceFunc) cb_print_position, pipeline);
g_main_loop_run (loop);
<!-- example-end query.c e -->
[..]<!-- example-begin query.c f --><!--
/* clean up */
gst_element_set_state (pipeline, GST_STATE_NULL);
gst_object_unref (GST_OBJECT (pipeline));
return 0;
--><!-- example-end query.c f -->
<!-- example-begin query.c g -->
<!-- example-end query.c g --></programlisting>
<sect1 id="section-eventsseek">
<title>Events: seeking (and more)</title>
Events work in a very similar way as queries. Dispatching, for
example, works exactly the same for events (and also has the same
limitations), and they can similarly be sent to the toplevel pipeline
and it will figure out everything for you. Although there are more
ways in which applications and elements can interact using events,
we will only focus on seeking here. This is done using the seek-event.
A seek-event contains a playback rate, a seek offset format (which is
the unit of the offsets to follow, e.g. time, audio samples, video
frames or bytes), optionally a set of seeking-related flags (e.g.
whether internal buffers should be flushed), a seek method (which
indicates relative to what the offset was given), and seek offsets.
The first offset (cur) is the new position to seek to, while
the second offset (stop) is optional and specifies a position where
streaming is supposed to stop. Usually it is fine to just specify
GST_SEEK_TYPE_NONE and -1 as end_method and end offset. The behaviour
of a seek is also wrapped in the <function>gst_element_seek ()</function>.
static void
seek_to_time (GstElement *pipeline,
gint64 time_nanoseconds)
if (!gst_element_seek (pipeline, 1.0, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH,
GST_SEEK_TYPE_SET, time_nanoseconds,
g_print ("Seek failed!\n");
Seeks with the GST_SEEK_FLAG_FLUSH should be done when the pipeline is
in PAUSED or PLAYING state. The pipeline will automatically go to preroll
state until the new data after the seek will cause the pipeline to preroll
again. After the pipeline is prerolled, it will go back to the state
(PAUSED or PLAYING) it was in when the seek was executed. You can wait
(blocking) for the seek to complete with
<function>gst_element_get_state()</function> or by waiting for the
ASYNC_DONE message to appear on the bus.
Seeks without the GST_SEEK_FLAG_FLUSH should only be done when the
pipeline is in the PLAYING state. Executing a non-flushing seek in the
PAUSED state might deadlock because the pipeline streaming threads might
be blocked in the sinks.
It is important to realise that seeks will not happen instantly in the
sense that they are finished when the function
<function>gst_element_seek ()</function> returns. Depending on the
specific elements involved, the actual seeking might be done later in
another thread (the streaming thread), and it might take a short time
until buffers from the new seek position will reach downstream elements
such as sinks (if the seek was non-flushing then it might take a bit
It is possible to do multiple seeks in short time-intervals, such as
a direct response to slider movement. After a seek, internally, the
pipeline will be paused (if it was playing), the position will be
re-set internally, the demuxers and decoders will decode from the new
position onwards and this will continue until all sinks have data
again. If it was playing originally, it will be set to playing again,
too. Since the new position is immediately available in a video output,
you will see the new frame, even if your pipeline is not in the playing