This function will be called on source pads when a peer element request a buffer at the specified offset and length. If this function returns GST_FLOW_OK, the result buffer will be stored in buffer. The contents of buffer is invalid for any other return value.
offset and length are always given in byte units. offset must normally be a value between 0 and the length in bytes of the data available on pad. The length (duration in bytes) can be retrieved with a GST_QUERY_DURATION or with a GST_QUERY_SEEKING
Any offset larger or equal than the length will make the function return GST_FLOW_EOS, which corresponds to EOS. In this case buffer does not contain a valid buffer.
The buffer size of buffer will only be smaller than length when offset is near the end of the stream. In all other cases, the size of buffer must be exactly the requested size.
It is allowed to call this function with a 0 length and valid offset, in which case buffer will contain a 0-sized buffer and the function returns GST_FLOW_OK
When this function is called with a -1 offset, the sequentially next buffer of length length in the stream is returned.
When this function is called with a -1 length, a buffer with a default optimal length is returned in buffer. The length might depend on the value of offset.