<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML ><HEAD ><TITLE >Queues</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="GStreamer Application Development Manual" HREF="index.html"><LINK REL="UP" TITLE="Advanced GStreamer concepts" HREF="part-advanced.html"><LINK REL="PREVIOUS" TITLE="When would you want to use a thread?" HREF="section-threads-when.html"><LINK REL="NEXT" TITLE="Cothreads" HREF="chapter-cothreads.html"></HEAD ><BODY CLASS="chapter" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" ><SPAN CLASS="application" >GStreamer</SPAN > Application Development Manual</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="section-threads-when.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="chapter-cothreads.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="chapter" ><H1 ><A NAME="chapter-queues" ></A >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. </P ><DIV CLASS="figure" ><A NAME="section-queues-img" ></A ><P ><B >Figure 22-1. a two-threaded decoder with a queue</B ></P ><DIV CLASS="mediaobject" ><P ><IMG SRC="images/queue.png"></P ></DIV ></DIV ><P > 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 */ </PRE ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="section-threads-when.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="chapter-cothreads.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >When would you want to use a thread?</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="part-advanced.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Cothreads</TD ></TR ></TABLE ></DIV ></BODY ></HTML >