[gst-devel] Sources

Erik Walthinsen omega at cse.ogi.edu
Sat Nov 25 10:59:12 CET 2000

There are a number of different ways of coding a GstSrc.  I'll try to
outline them and how the function here: 

1a) Simple push-function based with single output

This is the style that all the existing sources use.  There is a single
output pad, and a _push function that's global to the whole element.  The
_push function simple constructs buffer and pushes them out the pad. 


Typically this will be the/an entry into the Bin.  The Bin's iterate
function simply calls the Src's _push function.  When the _push function
pushes a buffer out it's pad, the chain function of the peer pad is
called, presumably causing a push out the other side of that element, and
eventually data gets to the other end.  The stack unrolls, and the
iteration ends for that Src. 


Again, the source would generally be an entry into the Bin.  A loopfunc
will be constructed around it, which will simply loop calling the Src's
_push function as in the non-cothreaded case.  When the _push function
pushes a buffer, it finds a pushfunc attached to the pad, drops the buffer
in the pen, and calls the pushfunc provided by the Bin.  This causes a
switch to the next element, then the next, to the end, at which point a
buffer pull will travel back down the chain.  The _push function gets
context and finishes, at which point the loopfunc wrapper simply calls it
again in the next iteration. 

1b) Simple push-function based with multiple output


Similar to the single output variant, except several chains are spawned
off, one per push, hanging off whichever pad the buffer is pushed off of. 
The stack will grow and unwind as many times as buffers are pushed out. 


Also similar to the single output variant.  When the pull winds its way
back from the first push, execution returns to the Src's _push function,
which simply goes off and pushes out another buffer, causing another
series of context switches.  Eventually the loopfunc wrapper starts over,
round and round we go. 

2) Pull-function based with single output

Similar to a regular filter with a chain function associated with each
pad, this kind of source doesn't provide a src-wide push function, but
does provide pullfuncs for its pad.  A pullfunc puts a buffer in the pen
and exits. 


As usual, is likely to be an entry into a Bin.  The Bin iterate code must
explicitely pull a buffer and pass it on to the peer. 


---- ok, I'll finish this tomorrow when my brain's working again.... ----

         Erik Walthinsen <omega at cse.ogi.edu> - Staff Programmer @ OGI
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