docs/pwg/building-chainfn.xml

<!-- ############ chapter ############# -->

<chapter id="chapter-building-chainfn">
  <title>The chain function</title>
  <para>
    The chain function is the function in which all data processing takes
    place. In the case of a simple filter, <function>_chain ()</function>
    functions are mostly linear functions - so for each incoming buffer,
    one buffer will go out, too. Below is a very simple implementation of
    a chain function:
  </para>
  <programlisting><!-- example-begin chain.c a --><!--
#include "init.func"
#include "caps.func"
static gboolean
gst_my_filter_event (GstPad * pad, GstEvent * event)
{
  return gst_pad_event_default (pad, event);
}
--><!-- example-end chain.c a -->
<!-- example-begin chain.c b -->
static GstFlowReturn
gst_my_filter_chain (GstPad    *pad,
		     GstBuffer *buf)
{
  GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));

  if (!filter->silent)
    g_print ("Have data of size %u bytes!\n", GST_BUFFER_SIZE (buf));

  return gst_pad_push (filter->srcpad, buf);
}
<!-- example-end chain.c b -->
<!-- example-begin chain.c c --><!--
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition)
{
  return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
      change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
}
#include "register.func"
  --><!-- example-end chain.c c --></programlisting>
  <para>
    Obviously, the above doesn't do much useful. Instead of printing that the
    data is in, you would normally process the data there. Remember, however,
    that buffers are not always writeable. In more advanced elements (the ones
    that do event processing), you may want to additionally specify an event
    handling function, which will be called when stream-events are sent (such
    as end-of-stream, newsegment, tags, etc.).
  </para>
  <programlisting>
static void
gst_my_filter_init (GstMyFilter * filter)
{
[..]
  gst_pad_set_event_function (filter-&gt;sinkpad,
      gst_my_filter_event);
[..]
}
<!-- example-begin chain2.c a --><!--
#include "init.func"
#include "caps.func"
#include "chain.func"
--><!-- example-end chain2.c a -->
<!-- example-begin chain.func a --><!--
static void
gst_my_filter_stop_processing (GstMyFilter * filter)
{
}

static GstBuffer *
gst_my_filter_process_data (GstMyFilter * filter, const GstBuffer * buf)
{
  return NULL;
}
--><!-- example-end chain.func a -->
<!-- example-begin chain.func b -->
static gboolean
gst_my_filter_event (GstPad   *pad,
		     GstEvent *event)
{
  GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));

  switch (GST_EVENT_TYPE (event)) {
    case GST_EVENT_EOS:
      /* end-of-stream, we should close down all stream leftovers here */
      gst_my_filter_stop_processing (filter);
      break;
    default:
      break;
  }

  return gst_pad_event_default (pad, event);
}

static GstFlowReturn
gst_my_filter_chain (GstPad    *pad,
		     GstBuffer *buf)
{
  GstMyFilter *filter = GST_MY_FILTER (gst_pad_get_parent (pad));
  GstBuffer *outbuf;

  outbuf = gst_my_filter_process_data (filter, buf);
  gst_buffer_unref (buf);
  if (!outbuf) {
    /* something went wrong - signal an error */
    GST_ELEMENT_ERROR (GST_ELEMENT (filter), STREAM, FAILED, (NULL), (NULL));
    return GST_FLOW_ERROR;
  }

  return gst_pad_push (filter->srcpad, outbuf);
}
<!-- example-end chain.func b -->
<!-- example-begin chain2.c b --><!--
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition)
{
  return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
      change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
}
#include "register.func"
  --><!-- example-end chain2.c b --></programlisting>
  <para>
    In some cases, it might be useful for an element to have control over the
    input data rate, too. In that case, you probably want to write a so-called
    <emphasis>loop-based</emphasis> element. Source elements (with only source
    pads) can also be <emphasis>get-based</emphasis> elements. These concepts
    will be explained in the advanced section of this guide, and in the section
    that specifically discusses source pads.
  </para>
</chapter>