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>Chapter 22. Queues</H1
><P
>
A queue is a filter element.
Queues can be used to link two elements in such way that the data can
be buffered.
</P
><P
>
A buffer that is sinked to a Queue will not automatically be pushed to the
next linked element but will be buffered. It will be pushed to the next
element as soon as a gst_pad_pull () is called on the queue's source pad.
</P
><P
>
Queues are mostly used in conjunction with a thread bin to
provide an external link for the thread's elements. You could have one
thread feeding buffers into a queue and another
thread repeatedly pulling on the queue to feed its
internal elements.
</P
><P
>
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.
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>Figure 22-1. a two-threaded decoder with a queue</B
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The standard <SPAN
CLASS="application"
>GStreamer</SPAN
> queue implementation has some
properties that can be changed using the g_objet_set () method. To set the
maximum number of buffers that can be queued to 30, do:
</P
><PRE
CLASS="programlisting"
> g_object_set (G_OBJECT (queue), "max_level", 30, NULL);
</PRE
><P
>
The following MP3 player shows you how to create the above pipeline
using a thread and a queue.
</P
><PRE
CLASS="programlisting"
> /* example-begin queue.c */
#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 *filesrc, *audiosink, *queue, *decode;
GstElement *bin;
GstElement *thread;
gst_init (&argc,&argv);
if (argc != 2) {
g_print ("usage: %s <mp3 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 */
filesrc = gst_element_factory_make ("filesrc", "disk_source");
g_assert (filesrc != NULL);
g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
g_signal_connect (G_OBJECT (filesrc), "eos",
G_CALLBACK (eos), thread);
queue = gst_element_factory_make ("queue", "queue");
g_assert (queue != NULL);
/* and an audio sink */
audiosink = gst_element_factory_make ("osssink", "play_audio");
g_assert (audiosink != NULL);
decode = gst_element_factory_make ("mad", "decode");
/* add objects to the main bin */
gst_bin_add_many (GST_BIN (thread), decode, audiosink, NULL);
gst_bin_add_many (GST_BIN (bin), filesrc, queue, thread, NULL);
gst_element_link (filesrc, queue);
gst_element_link_many (queue, decode, audiosink, NULL);
/* 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);
return 0;
}
/* example-end queue.c */
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