Chapter 23. Cothreads

Cothreads are user-space threads that greatly reduce context switching overhead introduced by regular kernel threads. Cothreads are also used to handle the more complex elements. They differ from other user-space threading libraries in that they are scheduled explictly by GStreamer.

A cothread is created by a GstBin whenever an element is found inside the bin that has one or more of the following properties:

The GstBin will create a cothread context for all the elements in the bin so that the elements will interact in cooperative multithreading.

Before proceding to the concept of loop-based elements we will first explain the chain-based elements.

23.1. Chain-based elements

Chain based elements receive a buffer of data and are supposed to handle the data and perform a gst_pad_push.

The basic main function of a chain-based element is like:


static void 
chain_function (GstPad *pad, GstBuffer *buffer)
{
  GstBuffer *outbuffer;

  ....
  // process the buffer, create a new outbuffer
  ...

  gst_pad_push (srcpad, outbuffer);
}
    

Chain based function are mainly used for elements that have a one to one relation between their input and output behaviour. An example of such an element can be a simple video blur filter. The filter takes a buffer in, performs the blur operation on it and sends out the resulting buffer.

Another element, for example, is a volume filter. The filter takes audio samples as input, performs the volume effect and sends out the resulting buffer.