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<!-- ############ chapter ############# -->
<chapter id="chapter-building-pads">
<title>Specifying the pads</title>
As explained before, pads are the port through which data goes in and out
of your element, and that makes them a very important item in the process
of element creation. In the boilerplate code, we have seen how static pad
templates take care of registering pad templates with the element class.
Here, we will see how to create actual elements, use an <function>_event
()</function>-function to configure for a particular format and how to
register functions to let data flow through the element.
In the element <function>_init ()</function> function, you create the pad
from the pad template that has been registered with the element class in
the <function>_class_init ()</function> function. After creating the pad,
you have to set a <function>_chain ()</function> function pointer that will
receive and process the input data on the sinkpad.
You can optionally also set an <function>_event ()</function> function
pointer and a <function>_query ()</function> function pointer.
Alternatively, pads can also operate in looping mode, which means that they
can pull data themselves. More on this topic later. After that, you have
to register the pad with the element. This happens like this:
<programlisting><!-- example-begin init.func a --><!--
#include "filter.h"
#include &lt;string.h&gt;
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition);
G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
static void
gst_my_filter_class_init (gpointer klass)
GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
static GstStaticPadTemplate sink_template =
static GstStaticPadTemplate src_template =
gst_element_class_set_static_metadata (element_class,
"An example plugin",
"Shows the basic structure of a plugin",
"your name <>");
gst_element_class_add_pad_template (element_class,
gst_static_pad_template_get (&src_template));
gst_element_class_add_pad_template (element_class,
gst_static_pad_template_get (&sink_template));
static void
gst_my_filter_class_init (GstMyFilterClass * klass)
GST_ELEMENT_CLASS (klass)->change_state = gst_my_filter_change_state;
--><!-- example-end init.func a -->
<!-- example-begin init.func c --><!--
static GstFlowReturn gst_my_filter_chain (GstPad *pad,
GstObject *parent,
GstBuffer *buf);
static gboolean gst_my_filter_sink_event (GstPad *pad,
GstObject *parent,
GstEvent *event);
static gboolean gst_my_filter_src_query (GstPad *pad,
GstObject *parent,
GstQuery *query);
static gboolean gst_my_filter_sink_query (GstPad *pad,
GstObject *parent,
GstQuery *query);
--><!-- example-end init.func c -->
<!-- example-begin init.func d -->
static void
gst_my_filter_init (GstMyFilter *filter)
/* pad through which data comes in to the element */
filter-&gt;sinkpad = gst_pad_new_from_static_template (
&amp;sink_template, "sink");
/* pads are configured here with gst_pad_set_*_function () */
<!-- example-end init.func d -->
<!-- example-begin init.func e --><!--
gst_pad_set_chain_function (filter-&gt;sinkpad, gst_my_filter_chain);
gst_pad_set_event_function (filter-&gt;sinkpad, gst_my_filter_sink_event);
gst_pad_set_query_function (filter-&gt;sinkpad, gst_my_filter_sink_query);
--><!-- example-end init.func e -->
<!-- example-begin init.func f -->
gst_element_add_pad (GST_ELEMENT (filter), filter-&gt;sinkpad);
/* pad through which data goes out of the element */
filter-&gt;srcpad = gst_pad_new_from_static_template (
&amp;src_template, "src");
/* pads are configured here with gst_pad_set_*_function () */
<!-- example-end init.func f -->
<!-- example-begin init.func g --><!--
gst_pad_set_query_function (filter-&gt;srcpad, gst_my_filter_src_query);
--><!-- example-end init.func g -->
<!-- example-begin init.func h -->
gst_element_add_pad (GST_ELEMENT (filter), filter-&gt;srcpad);
/* properties initial value */
filter->silent = FALSE;
<!-- example-end init.func h --></programlisting>
<!-- example-begin pads.c --><!--
#include "init.func"
static gboolean
gst_my_filter_event (GstPad * pad, GstObject * parent, GstEvent * event)
return gst_pad_event_default (pad, parent, event);
static GstFlowReturn
gst_my_filter_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
return gst_pad_push (GST_MY_FILTER (parent)->srcpad, buf);
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition)
change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
#include "register.func"
--><!-- example-end pads.c -->