[osg-users] Information on ffmpeg thread safety

[Garrett Potts]

Hello All:

Just recently I have been testing ffmpeg library (Not OSG's ffmpeg
plugin) use within some tools we are doing. Over the past couple of
days we have been hammering video in a multi threaded environment and
it has come to our attention that ffmpeg is not thread safe. I have
not locked down where in ffmpeg it uses global structures besides the
registerall calls. In our code we had to do a shared mutex lock on
all calls to the ffmpeg library. So for instance a call to
avcodec_open could fail if you are doing a code decode at the same time.

Example:

When we were grabbing a frame from one file and opening another file
the open could fail which gave strong indication of shared global data
within ffmpeg. After lock down with a comon critical section mutex we
were able to use the ffmpeg in a multi threaded environment.

I wanted to pass this along because I have seen the ffmpeg plugin
added to OSG.

If anyone knows the exact API calls or found docs specifying the exact
calls in ffmpeg library that is not thread safe please pass it along.
For now, I have just wrapped all calls as critical and everything
works robustly in a multi threaded environment.

Take care

Garrett

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[Robert Osfield]

Hi Garret,

Thanks for reporting this issue, I'm afraid I can't add anything to
the mix as I'm not too familiar with ffmpeg. However, it does sounds
like we'll also need to protect some or all of ffmpeg calls. I'll
have a look through the ffmpeg and have a think about how to tackle
this is a manageable way.

Cheers,
Robert.

[Garrett Potts]

Hello Robert:

After researching and talking with some others there is a way to make
ffmpeg thread safe but you need to add a call back via the lock
register api found in libavcodec and compile ffmpeg with thread
support enabled. Internally they use the lock callback to allow for
global synchronization access to the libavcodec library. I am going
to try and test this out this week.

Take care

Garrett

[Robert Osfield]

Hi Garret,

On Sat, Jan 9, 2010 at 3:27 PM, Garrett Potts <po...@cfl.rr.com> wrote:
> After researching and talking with some others there is a way to make ffmpeg
> thread safe but you need to add a call back via the lock register api found
> in libavcodec and compile ffmpeg with thread support enabled.  Internally
> they use the lock callback to allow for global synchronization access to the
> libavcodec library.  I am going to try and test this out this week.

Thanks for the updated. Could you tell us how you get on, and what
you found worked/didn't work so when we tackle the ffmpeg plugin we
can take the short cut straight to the solution.

Thanks,
Robert.