# XML in GStreamer

GStreamer 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 GStreamer knows about to have quick access to the specifics of the plugins.

We will show you how you can save a pipeline to XML and how you can reload that XML file again for later use.

## Turning GstElements into XML

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.

    /* 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 (GST_BIN (bin), filesrc);
      gst_bin_add (GST_BIN (bin), queue);

      gst_bin_add (GST_BIN (thread), decode);
      gst_bin_add (GST_BIN (thread), queue2);

      gst_bin_add (GST_BIN (thread2), osssink);

      gst_pad_link (gst_element_get_pad (filesrc,"src"),
                       gst_element_get_pad (queue,"sink"));

      gst_pad_link (gst_element_get_pad (queue,"src"),
                       gst_element_get_pad (decode,"sink"));
      gst_pad_link (gst_element_get_pad (decode,"src"),
                       gst_element_get_pad (queue2,"sink"));

      gst_pad_link (gst_element_get_pad (queue2,"src"),
                       gst_element_get_pad (osssink,"sink"));

      gst_bin_add (GST_BIN (bin), thread);
      gst_bin_add (GST_BIN (bin), thread2);

      /* 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 */

The most important line is:

      gst_xml_write_file (GST_ELEMENT (bin), stdout);

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.

The complete element hierarchy will be saved along with the inter element pad links and the element parameters. Future GStreamer versions will also allow you to store the signals in the XML file.

## Loading a GstElement from an XML file

Before an XML file can be loaded, you must create a GstXML object. A saved XML file can then be loaded with the gst_xml_parse_file (xml, filename, rootelement) method. The root element can optionally left NULL. The following code example loads the previously created XML file and runs it.

    #include <stdlib.h>
    #include <gst/gst.h>

    int
    main(int argc, char *argv[])
    {
      GstXML *xml;
      GstElement *bin;
      gboolean ret;

      gst_init (&argc, &argv);

      xml = gst_xml_new ();

      ret = gst_xml_parse_file(xml, "xmlTest.gst", NULL);
      g_assert (ret == TRUE);

      bin = gst_xml_get_element (xml, "bin");
      g_assert (bin != NULL);

      gst_element_set_state (bin, GST_STATE_PLAYING);

      while (gst_bin_iterate(GST_BIN(bin)));

      gst_element_set_state (bin, GST_STATE_NULL);

      exit (0);
    }

gst_xml_get_element (xml, "name") can be used to get a specific element from the XML file.

gst_xml_get_topelements (xml) can be used to get a list of all toplevel elements in the XML file.

In addition to loading a file, you can also load a from a xmlDocPtr and an in memory buffer using gst_xml_parse_doc and gst_xml_parse_memory respectively. Both of these methods return a gboolean indicating success or failure of the requested action.

## Adding custom XML tags into the core XML data

It is possible to add custom XML tags to the core XML created with gst_xml_write. This feature can be used by an application to add more information to the save plugins. The editor will for example insert the position of the elements on the screen using the custom XML tags.

It is strongly suggested to save and load the custom XML tags using a namespace. This will solve the problem of having your XML tags interfere with the core XML tags.

To insert a hook into the element saving procedure you can link a signal to the GstElement using the following piece of code:

    xmlNsPtr ns;

      ...
      ns = xmlNewNs (NULL, "http://gstreamer.net/gst-test/1.0/", "test");
        ...
      thread = gst_element_factory_make ("thread", "thread");
      g_signal_connect (G_OBJECT (thread), "object_saved",
      		     G_CALLBACK (object_saved), g_strdup ("decoder thread"));
        ...

When the thread is saved, the object_save method will be caled. Our example will insert a comment tag:

    static void
    object_saved (GstObject *object, xmlNodePtr parent, gpointer data)
    {
      xmlNodePtr child;

      child = xmlNewChild (parent, ns, "comment", NULL);
      xmlNewChild (child, ns, "text", (gchar *)data);
    }

Adding the custom tag code to the above example you will get an XML file with the custom tags in it. Here's an excerpt:

              ...
            <gst:element>
              <gst:name>thread</gst:name>
              <gst:type>thread</gst:type>
              <gst:version>0.1.0</gst:version>
    	  ...
            </gst:children>
            <test:comment>
              <test:text>decoder thread</test:text>
            </test:comment>
          </gst:element>
              ...

To retrieve the custom XML again, you need to attach a signal to the GstXML object used to load the XML data. You can then parse your custom XML from the XML tree whenever an object is loaded.

We can extend our previous example with the following piece of code.

      xml = gst_xml_new ();

      g_signal_connect (G_OBJECT (xml), "object_loaded",
      		     G_CALLBACK (xml_loaded), xml);

      ret = gst_xml_parse_file (xml, "xmlTest.gst", NULL);
      g_assert (ret == TRUE);

Whenever a new object has been loaded, the xml_loaded function will be called. This function looks like:

    static void
    xml_loaded (GstXML *xml, GstObject *object, xmlNodePtr self, gpointer data)
    {
      xmlNodePtr children = self->xmlChildrenNode;

      while (children) {
        if (!strcmp (children->name, "comment")) {
          xmlNodePtr nodes = children->xmlChildrenNode;

          while (nodes) {
            if (!strcmp (nodes->name, "text")) {
              gchar *name = g_strdup (xmlNodeGetContent (nodes));
              g_print ("object %s loaded with comment '%s'\n",
                       gst_object_get_name (object), name);
            }
            nodes = nodes->next;
          }
        }
        children = children->next;
      }
    }

As you can see, you'll get a handle to the GstXML object, the newly loaded GstObject and the xmlNodePtr that was used to create this object. In the above example we look for our special tag inside the XML tree that was used to load the object and we print our comment to the console.