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>23.2. Loop-based elements</A
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><P
> As opposed to chain-based elements, loop-based elements enter an
infinite loop that looks like this:
<PRE
CLASS="programlisting"
> GstBuffer *buffer, *outbuffer;
while (1) {
buffer = gst_pad_pull (sinkpad);
...
// process buffer, create outbuffer
while (!done) {
....
// optionally request another buffer
buffer = gst_pad_pull (sinkpad);
....
}
...
gst_pad_push (srcpad, outbuffer);
}
</PRE
>
The loop-based elements request a buffer whenever they need one.
</P
><P
> When the request for a buffer cannot be immediately satisfied, the control
will be given to the source element of the loop-based element until it
performs a push on its source pad. At that time the control is handed
back to the loop-based element, etc... The execution trace can get
fairly complex using cothreads when there are multiple input/output
pads for the loop-based element. Cothread switches are performed within
the call to gst_pad_pull and gst_pad_push; from the perspective of
the loop-based element, it just "appears" that gst_pad_push (or _pull)
might take a long time to return.
</P
><P
> Loop based elements are mainly used for the more complex elements
that need a specific amount of data before they can start to produce
output. An example of such an element is the MPEG video decoder. The
element will pull a buffer, perform some decoding on it and optionally
request more buffers to decode, and when a complete video frame has
been decoded, a buffer is sent out. For example, any plugin using the
bytestream library will need to be loop-based.
</P
><P
> There is no problem in putting cothreaded elements into a <CODE
CLASS="classname"
>GstThread</CODE
> to
create even more complex pipelines with both user and kernel space threads.
</P
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