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129 lines
3.5 KiB
XML
129 lines
3.5 KiB
XML
<chapter id="chapter-queues">
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<title>Queues</title>
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<para>
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A queue is a filter element.
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Queues can be used to link two elements in such way that the data can
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be buffered.
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</para>
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<para>
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A buffer that is sinked to a Queue will not automatically be pushed to the
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next linked element but will be buffered. It will be pushed to the next
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element as soon as a gst_pad_pull () is called on the queue's source pad.
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</para>
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<para>
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Queues are mostly used in conjunction with a thread bin to
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provide an external link for the thread's elements. You could have one
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thread feeding buffers into a queue and another
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thread repeatedly pulling on the queue to feed its
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internal elements.
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</para>
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<para>
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Below is a figure of a two-threaded decoder. We have one thread (the main execution
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thread) reading the data from a file, and another thread decoding the data.
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</para>
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<figure float="1" id="section-queues-img">
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<title>a two-threaded decoder with a queue</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/queue.ℑ" format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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The standard <application>GStreamer</application> queue implementation has some
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properties that can be changed using the g_objet_set () method. To set the
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maximum number of buffers that can be queued to 30, do:
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</para>
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<programlisting>
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g_object_set (G_OBJECT (queue), "max_level", 30, NULL);
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</programlisting>
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<para>
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The following MP3 player shows you how to create the above pipeline
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using a thread and a queue.
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</para>
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<programlisting>
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/* example-begin queue.c */
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#include <stdlib.h>
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#include <gst/gst.h>
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gboolean playing;
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/* eos will be called when the src element has an end of stream */
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void
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eos (GstElement *element, gpointer data)
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{
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g_print ("have eos, quitting\n");
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playing = FALSE;
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}
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int
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main (int argc, char *argv[])
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{
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GstElement *filesrc, *audiosink, *queue, *decode;
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GstElement *bin;
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GstElement *thread;
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gst_init (&argc,&argv);
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if (argc != 2) {
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g_print ("usage: %s <mp3 filename>\n", argv[0]);
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exit (-1);
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}
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/* create a new thread to hold the elements */
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thread = gst_thread_new ("thread");
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g_assert (thread != NULL);
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/* create a new bin to hold the elements */
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bin = gst_bin_new ("bin");
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g_assert (bin != NULL);
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/* create a disk reader */
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filesrc = gst_element_factory_make ("filesrc", "disk_source");
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g_assert (filesrc != NULL);
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g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
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g_signal_connect (G_OBJECT (filesrc), "eos",
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G_CALLBACK (eos), thread);
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queue = gst_element_factory_make ("queue", "queue");
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g_assert (queue != NULL);
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/* and an audio sink */
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audiosink = gst_element_factory_make ("osssink", "play_audio");
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g_assert (audiosink != NULL);
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decode = gst_element_factory_make ("mad", "decode");
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/* add objects to the main bin */
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gst_bin_add_many (GST_BIN (thread), decode, audiosink, NULL);
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gst_bin_add_many (GST_BIN (bin), filesrc, queue, thread, NULL);
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gst_element_link (filesrc, queue);
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gst_element_link_many (queue, decode, audiosink, NULL);
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/* start playing */
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gst_element_set_state (GST_ELEMENT (bin), GST_STATE_PLAYING);
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playing = TRUE;
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while (playing) {
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gst_bin_iterate (GST_BIN (bin));
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}
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gst_element_set_state (GST_ELEMENT (bin), GST_STATE_NULL);
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return 0;
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}
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/* example-end queue.c */
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</programlisting>
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</chapter>
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