gstreamer/docs/random/wtay/plugin_guidelines

This short document gives some guidelines to plugin writers.


chain-based plugins
===================

 - the plugin has a chain function on each sink pad (can be the same function)

! 
! chain (GstPad *pad, GstBuffer *buffer) 
! {
!   MyElement *elem = MY_ELEMENT (gst_pad_get_parent (pad));
!   GstBuffer *new_buffer;
! 
!   ...
!   (process the buffer)
!   ...
! 
!   /* you can push as many buffers (0 or more) as you want */
!   while (!done) {
!     ...
!     (process some more
!     ...
!     gst_pad_push (elem->some_sink_pad, new_buffer);
!     ...
!     /* if you're like going to send a large amount of buffers
!      * it's a good idea to call _yield from time to time after
!      * the buffer has been pushed */
!     (optional gst_element_yield (GST_ELEMENT (elem)); )
!   }
! }
! 

 - chain based functions are usually easy and recommended.
 - use gst_element_yield if you are going to push a lot of buffers.



loop-based plugins
==================

 - one loop function for the element.
 - required for bytestream based plugins.

simple case
-----------
  
 - one pull at the start

 - if you do something like this, you really should consider using a 
   chain function as it can be significantly optimised by the scheduler.

!
! loop (GstElement *element) 
! {
!   MyElement *elem = MY_ELEMENT (element);
!   GstBuffer *buffer;
!   GstBuffer *new_buffer;
! 
!   buffer = gst_pad_pull (elem->sinkpad);
!  
!   ...
!   (do something)
!   ...
!
!   /* you can push as many buffers (0 or more) as you want */
!   while (!done) {
!     ...
!     (process some more
!     ...
!     gst_pad_push (elem->some_sink_pad, new_buffer);
!     ...
!     /* if you're like going to send a large amount of buffers
!      * it's a good idea to call _yield from time to time after
!      * the buffer has been pushed */
!     (optional gst_element_yield (GST_ELEMENT (elem)); )
!   }
! }
!

DONT!!

- infinite loop

!
! loop (element)
! {
!   while (TRUE) {
!     ...
!     _pull ()
!     ...
!
!     ...
!     _push ()
!     ...
!   }
! }
!

* you can fix this by either:

 - setting the GST_ELEMENT_INFINITE_LOOP flag on the element. this is
   not recommended, if all plugins in the pipeline (or depending on the
   pipeline, some plugins) have this flag, the pipeline will not run.
 - calling break; from time to time to get out of the loop. (duh, then
   it's not an infinite loop anymore). beware that the next time the loop
   function is called, it will be started from the top.
 - calling gst_element_yield() from time to time (see NOTES). 
 - this is fine (albeit rather useless, use a chain function):
  !
  ! loop (element)
  ! {
  !   ...
  !   _pull ()
  !   ...
  !
  !   ...
  !   _push ()
  !   ...
  ! }
  !
 - so is this (albeit rather useless, consider removing the while and the _yield):
  !
  ! loop (element)
  ! {
  !   while (TRUE) {
  !     ...
  !     _pull ()
  !     ...
  !
  !     ...
  !     _push ()
  !     ...
  !     gst_element_yield (element);
  !   }
  ! }
  !


DONT!!

- allocate data, loop, free data

!
! loop (element)
! {
!   
!   (my funky malloc)
!
!   while (TRUE) {
!
!     ...
!     _pull ()
!     ...
!
!     ...
!     _push ()
!     ...
!
!     _yield ()
!   }
!
!   (my funky free)
!
! }
!

 - the free will NEVER happen!.

* You can fix this by:

 - allocating/freeing data in the state change function
 - you could think the following code would work too:
  !
  ! (*WRONG* example follows)
  !
  ! loop (element)
  ! {
  !   
  !   (my funky malloc)
  !
  !   while (TRUE) {
  !
  !     ...
  !     _pull ()
  !     if (EOS)
  !       break;
  !
  !     ...
  !     _push ()
  !     ...
  !
  !     _yield ()
  !   }
  !
  !   (my funky free)
  !
  ! }
  !
   but it'll only free the data if the pipeline was shut down with
   and EOS so don't try it. Besides, on EOS, a state change will happen
   anyway so free the data there.


bytestream/multiple pull case
-----------------------------

 - same as the simple case, but you can't use a chain based function unless
   you want to make things a little harder then they should be.


complicated case
----------------

 - push and pull are completely mixed.
 - the flow is usually something like this:

!
! loop (element)
! {
!   
!   while (TRUE) {
!
!     while (something) {
!       ...
!       do some _pull ()
!       ...
!       do some _push ()
!       ...
!       while (something_else) {
!         ...
!         if (testing)
!           do some _pull ()
!         ...
!         do some _push ()
!       }
!     }
!
!     while (something_useful) {
!       ...
!       _push ()
!       ...
!     }
!   }
! }
!
(example made complicated on purpose, but vorbisdec comes close)

 - you cannot call break to avoid infinite loops and there are loops that 
   take a significant amount of time to execute, possibly pushing/pulling
   a lot of buffers.

* You can fix this by:

 - inserting gst_element_yield () in sane places, don't exagerate because
   every yield can potentially never return so you need to keep track of
   allocations (see the NOTES below).
 


NOTES:
======

 - a call to _yield() can never return. if you have data allocated on the
   stack before the yield, keep a pointer to it in the element struct 
   and free it in the state change function.


IMPLEMENATION DETAILS
=====================

The infinite loops are only problematic if the scheduler chooses to select
the plugin as an entry point in the chain. _yield() will be a nop if this is
not the case. The scheduler will not select plugins with the INFINITE_LOOP
flag set as entries in a chain.

A _yield in an entry will hand over control to the main thread context, allowing
state changes and other actions to be performed. It will basically exit the 
_iterate() function. spending a long time in a loop will degrade app responsiveness
because _iterate will take a long time.

Calling yield, pulling, pushing can potentially never return because a state change
might have happened, killing off execution of the plugin. pulling/pushing buffers
will cause no leaks in this case because the core will free pending buffers in a
state change to READY. The plugin must free allocated data/buffers itself in the state
change function if the yield didn't retrun.