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<chapter id="cha-queues">
<title>Queues</title>
<para>
A <classname>GstQueue</classname> is a filter element.
Queues can be used to connect two elements in such way that the data can
be buffered.
</para>
<para>
A buffer that is sinked to a Queue will not automatically be pushed to the
next connected element but will be buffered. It will be pushed to the next
element as soon as a gst_pad_pull () is called on the queues srcpad.
</para>
<para>
Queues are mostly used in conjunction with a <classname>GstThread</classname> to
provide an external connection for the thread elements. You could have one
thread feeding buffers into a <classname>GstQueue</classname> and another
thread repeadedly calling gst_pad_pull () on the queue to feed its
internal elements.
</para>
<para>
Below is a figure of a two-threaded decoder. We have one thread (the main execution
thread) reading the data from a file, and another thread decoding the data.
</para>
<figure float="1" id="sec-queues-img">
<title>a two-threaded decoder with a queue</title>
<graphic fileref="images/queue" format="png"></graphic>
</figure>
<para>
The standard <application>GStreamer</application> queue implementation has some
properties that can be changed using the gtk_objet_set () method. To set the
maximum number of buffers that can be queued to 30, do:
</para>
<programlisting>
gtk_object_set (GTK_OBJECT (queue), "max_level", 30, NULL);
</programlisting>
<para>
The following mp3 player shows you how to create the above pipeline using a
thread and a queue.
</para>
<programlisting>
#include <stdlib.h>
#include <gst/gst.h>
gboolean playing;
/* eos will be called when the src element has an end of stream */
void
eos (GstElement *element, gpointer data)
{
g_print ("have eos, quitting\n");
playing = FALSE;
}
int
main (int argc, char *argv[])
{
GstElement *disksrc, *audiosink, *queue, *parse, *decode;
GstElement *bin;
GstElement *thread;
gst_init (&argc,&argv);
if (argc != 2) {
g_print ("usage: %s <filename>\n", argv[0]);
exit (-1);
}
/* create a new thread to hold the elements */
thread = gst_thread_new ("thread");
g_assert (thread != NULL);
/* create a new bin to hold the elements */
bin = gst_bin_new ("bin");
g_assert (bin != NULL);
/* create a disk reader */
disksrc = gst_elementfactory_make ("disksrc", "disk_source");
g_assert (disksrc != NULL);
gtk_object_set (GTK_OBJECT (disksrc), "location", argv[1], NULL);
gtk_signal_connect (GTK_OBJECT (disksrc), "eos",
GTK_SIGNAL_FUNC (eos), thread);
queue = gst_elementfactory_make ("queue", "queue");
/* and an audio sink */
audiosink = gst_elementfactory_make ("audiosink", "play_audio");
g_assert (audiosink != NULL);
parse = gst_elementfactory_make ("mp3parse", "parse");
decode = gst_elementfactory_make ("mpg123", "decode");
/* add objects to the main bin */
gst_bin_add (GST_BIN (bin), disksrc);
gst_bin_add (GST_BIN (bin), queue);
gst_bin_add (GST_BIN (thread), parse);
gst_bin_add (GST_BIN (thread), decode);
gst_bin_add (GST_BIN (thread), audiosink);
gst_pad_connect (gst_element_get_pad (disksrc,"src"),
gst_element_get_pad (queue,"sink"));
gst_pad_connect (gst_element_get_pad (queue, "src"),
gst_element_get_pad (parse, "sink"));
gst_pad_connect (gst_element_get_pad (parse, "src"),
gst_element_get_pad (decode, "sink"));
gst_pad_connect (gst_element_get_pad (decode, "src"),
gst_element_get_pad (audiosink, "sink"));
gst_bin_add (GST_BIN (bin), thread);
/* make it ready */
gst_element_set_state (GST_ELEMENT (bin), GST_STATE_READY);
/* start playing */
gst_element_set_state (GST_ELEMENT (bin), GST_STATE_PLAYING);
playing = TRUE;
while (playing) {
gst_bin_iterate (GST_BIN (bin));
}
gst_element_set_state (GST_ELEMENT (bin), GST_STATE_NULL);
exit (0);
}
</programlisting>
</chapter>
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