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>Chapter 31. XML in <SPAN
CLASS="application"
>GStreamer</SPAN
></H1
><P
>
<SPAN
CLASS="application"
>GStreamer</SPAN
> uses XML to store and load
its pipeline definitions. XML is also used internally to manage the
plugin registry. The plugin registry is a file that contains the definition
of all the plugins <SPAN
CLASS="application"
>GStreamer</SPAN
> knows about to have
quick access to the specifics of the plugins.
</P
><P
> We will show you how you can save a pipeline to XML and how you can reload that
XML file again for later use.
</P
><DIV
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><H1
CLASS="sect1"
><A
NAME="section-xml-write"
>31.1. Turning GstElements into XML</A
></H1
><P
> We create a simple pipeline and write it to stdout with
gst_xml_write_file (). The following code constructs an MP3 player
pipeline with two threads and then writes out the XML both to stdout
and to a file. Use this program with one argument: the MP3 file on disk.
</P
><PRE
CLASS="programlisting"
> /* example-begin xml-mp3.c */
#include <stdlib.h>
#include <gst/gst.h>
gboolean playing;
int
main (int argc, char *argv[])
{
GstElement *filesrc, *osssink, *queue, *queue2, *decode;
GstElement *bin;
GstElement *thread, *thread2;
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_element_factory_make ("thread", "thread");
g_assert (thread != NULL);
thread2 = gst_element_factory_make ("thread", "thread2");
g_assert (thread2 != 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);
queue = gst_element_factory_make ("queue", "queue");
queue2 = gst_element_factory_make ("queue", "queue2");
/* and an audio sink */
osssink = gst_element_factory_make ("osssink", "play_audio");
g_assert (osssink != NULL);
decode = gst_element_factory_make ("mad", "decode");
g_assert (decode != NULL);
/* add objects to the main bin */
gst_bin_add_many (GST_BIN (bin), filesrc, queue, NULL);
gst_bin_add_many (GST_BIN (thread), decode, queue2, NULL);
gst_bin_add (GST_BIN (thread2), osssink);
gst_element_link_many (filesrc, queue, decode, queue2, osssink, NULL);
gst_bin_add_many (GST_BIN (bin), thread, thread2, NULL);
/* write the bin to stdout */
gst_xml_write_file (GST_ELEMENT (bin), stdout);
/* write the bin to a file */
gst_xml_write_file (GST_ELEMENT (bin), fopen ("xmlTest.gst", "w"));
exit (0);
}
/* example-end xml-mp3.c */
</PRE
><P
> The most important line is:
</P
><PRE
CLASS="programlisting"
> gst_xml_write_file (GST_ELEMENT (bin), stdout);
</PRE
><P
> gst_xml_write_file () will turn the given element into an xmlDocPtr that
is then formatted and saved to a file. To save to disk, pass the result
of a fopen(2) as the second argument.
</P
><P
> The complete element hierarchy will be saved along with the inter element
pad links and the element parameters. Future <SPAN
CLASS="application"
>GStreamer</SPAN
>
versions will also allow you to store the signals in the XML file.
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