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>Chapter 27. Type Detection</H1
><P
>
Sometimes the capabilities of a pad are not specificied. The filesrc
element, for example, does not know what type of file it is reading. Before
you can attach an element to the pad of the filesrc, you need to determine
the media type in order to be able to choose a compatible element.
</P
><P
>
To solve this problem, a plugin can provide the <SPAN
CLASS="application"
>GStreamer</SPAN
>
core library with a type definition. The type definition
will contain the following information:
<P
></P
><UL
><LI
><P
> The MIME type we are going to define.
</P
></LI
><LI
><P
> An optional string with a list of possible file extensions this
type usually is associated with. the list entries are separated with
a space. eg, ".mp3 .mpa .mpg".
</P
></LI
><LI
><P
> An optional typefind function.
</P
></LI
></UL
>
</P
><P
>
The typefind functions give a meaning to the MIME types that are used
in GStreamer. The typefind function is a function with the following definition:
</P
><PRE
CLASS="programlisting"
> typedef GstCaps *(*GstTypeFindFunc) (GstBuffer *buf, gpointer priv);
</PRE
><P
>
This typefind function will inspect a GstBuffer with data and will output
a GstCaps structure describing the type. If the typefind function does not
understand the buffer contents, it will return NULL.
</P
><P
> <SPAN
CLASS="application"
>GStreamer</SPAN
> has a typefind element in the set
of core elements
that can be used to determine the type of a given pad.
</P
><P
> The next example will show how a typefind element can be inserted into a pipeline
to detect the media type of a file. It will output the capabilities of the pad into
an XML representation.
</P
><PRE
CLASS="programlisting"
> #include <gst/gst.h>
void type_found (GstElement *typefind, GstCaps* caps);
int
main(int argc, char *argv[])
{
GstElement *bin, *filesrc, *typefind;
gst_init (&argc, &argv);
if (argc != 2) {
g_print ("usage: %s <filename>\n", argv[0]);
exit (-1);
}
/* 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);
/* create the typefind element */
typefind = gst_element_factory_make ("typefind", "typefind");
g_assert (typefind != NULL);
/* add objects to the main pipeline */
gst_bin_add_many (GST_BIN (bin), filesrc, typefind, NULL);
g_signal_connect (G_OBJECT (typefind), "have_type",
G_CALLBACK (type_found), NULL);
gst_element_link (filesrc, typefind);
/* start playing */
gst_element_set_state (GST_ELEMENT (bin), GST_STATE_PLAYING);
gst_bin_iterate (GST_BIN (bin));
gst_element_set_state (GST_ELEMENT (bin), GST_STATE_NULL);
exit (0);
}
</PRE
><P
> We create a very simple pipeline with only a filesrc and the typefind
element in it. The sinkpad of the typefind element has been linked
to the source pad of the filesrc.
</P
><P
> We attached a signal 'have_type' to the typefind element which will be called
when the type of the media stream as been detected.
</P
><P
> The typefind function will loop over all the registered types and will
execute each of the typefind functions. As soon as a function returns
a GstCaps pointer, the type_found function will be called:
</P
><PRE
CLASS="programlisting"
> void
type_found (GstElement *typefind, GstCaps* caps)
{
xmlDocPtr doc;
xmlNodePtr parent;
doc = xmlNewDoc ("1.0");
doc->root = xmlNewDocNode (doc, NULL, "Capabilities", NULL);
parent = xmlNewChild (doc->root, NULL, "Caps1", NULL);
gst_caps_save_thyself (caps, parent);
xmlDocDump (stdout, doc);
}
</PRE
><P
> In the type_found function we can print or inspect the type that has been
detected using the GstCaps APIs. In this example, we just print out the
XML representation of the caps structure to stdout.
</P
><P
> A more useful option would be to use the registry to look up an element
that can handle this particular caps structure, or we can also use the
autoplugger to link this caps structure to, for example, a videosink.
</P
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