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>Chapter 5. The chain function</H1
><P
> The chain function is the function in which all data processing takes
place. In the case of a simple filter, <CODE
CLASS="function"
>_chain ()</CODE
>
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:
</P
><PRE
CLASS="programlisting"
>
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);
}
</PRE
><P
> 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.).
</P
><PRE
CLASS="programlisting"
> static void
gst_my_filter_init (GstMyFilter * filter)
{
[..]
gst_pad_set_event_function (filter->sinkpad,
gst_my_filter_event);
[..]
}
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);
}
</PRE
><P
> 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
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>loop-based</I
></SPAN
> element. Source elements (with only source
pads) can also be <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>get-based</I
></SPAN
> elements. These concepts
will be explained in the advanced section of this guide, and in the section
that specifically discusses source pads.
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