[Libav-user] Is libav thread-safe?

[Bruce Wheaton]

On Oct 8, 2013, at 12:05 PM, James Board <jpboard2 at yahoo.com> wrote:

> I want to create several pthreads in a C++ program (assuming you can call
> libav routines from a C++ program) that each call libav subroutines.  

Well, yes, you can call C from a C++ app. You need to mark the headers as C when you include them, but that's about it.

> Are the libav subroutines  thread-safe?  Specifically,
> can several pthreads open the same AVI file and decode different video
> frames concurrently?

No, not really. At least, you'd have to lock the access entirely, which would make it sequential. You can use each context in one thread, and there are a few overall functions that need locking, such as choosing the right codecs for a file.

> 
> I'm doing this so I can write a real-time video that
> can play the video forward and backwards, and the multiple threads will
> help decode and process each frame quickly so there will be no delay when
> I step through the video.

Most codecs are multi-threaded by now - so there already are threads running to decode frames. You can set the number of threads you want a codec to use when you open it.

There's a number of other things you could do... multiple codecs open etc. But with codecs that need B and P frames, you pretty much have to decode in order anyway, and file access is always going to be one at a time.

It sounds like you're trying to solve a problem that you don't have yet. Start with the basics - just getting going from a cold start in ffmpeg is enough trouble for most people, without trying to get clever.

Bruce




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