[Libav-user] Using custom asynchronous I/O with avio_alloc_context

Nicolas efyxps at gmail.com
Mon Mar 11 14:59:32 CET 2013


Thanks for these useful informations. It works great, I used libpcl.


On Sun, Mar 10, 2013 at 7:44 PM, Nicolas George <
nicolas.george at normalesup.org> wrote:

> Le nonidi 19 ventôse, an CCXXI, Nicolas a écrit :
> > I am using libav with custom I/O (with avio_alloc_context()) for reading
> > various stream.
> >
> > In my project, all my I/O are asynchronous but ffmpeg callbacks work in a
> > synchronous way. So right now, what I am doing is to use an internal
> buffer
> > 4 times bigger than my avio buffer and each time my read callback is
> called
> > I copy a part of my buffer and get new data asynchronously, this works
> well
> > for reading, but when I seek my media I don't have (yet) data in my
> > internal buffer. In such case what should I return in my read callback?
> >
> > Also, there is a flag AVIO_FLAG_NONBLOCKING that I could pass to
> > avio_alloc_context but i couldn't figure how to use it (what should I
> > return in my read callback?)
> >
> > is there any way to use custom I/O in an asynchronous fashion?
> Unfortunately, no, there is currently no way of working with asynchronous
> and non-blocking I/O. Some of the network protocols can be set to work
> non-blocking, but not all, and the demuxers themselves can not work in
> non-blocking mode.
> The usual solution for that is to use threads. You could, for example, run
> the demuxer in a separate thread. When the demuxer calls the read function
> in your custom I/O context, the thread becomes blocked waiting for data
> from
> the main thread, and when it has demuxed a packet, it can wake the main
> thread using some kind of signalfd/self-pipe. That way, the demuxing thread
> do not force you to abandon your async model (using a few threads has a
> tendency to force you to use threads everywhere).
> Also note that since you are using the thread not for concurrency but only
> to keep the local context of the demuxer, you do not have to use "real"
> threads, you can consider using, for examples, GNU Pth, or libpcl.
> Regards,
> --
>   Nicolas George
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