[FFmpeg-devel] [PATCH] lavf: AVFMT_HAS_NONBLOCK flag for (de)muxers

Ivo ivop
Thu Nov 26 21:56:08 CET 2009

On Thursday 26 November 2009, 21:10:35, Luca Abeni wrote:
> On Thu, 2009-11-26 at 18:08 +0100, Ivo wrote:
> > On Thursday 26 November 2009, 18:06:33, Ivo wrote:
> > > This patch...
> [...]
> >      while((res = snd_pcm_writei(s->h, buf, size / s->frame_size)) < 0)
> > { -        if (res == -EAGAIN) {
> > +        if (res == -EAGAIN && s1->flags & AVFMT_FLAG_NONBLOCK) {
> >
> >              return AVERROR(EAGAIN);
> >          }
> Are you sure about this hunk? I would not expect alsa to return -EAGAIN
> if the non-blocking flag has not been set previously...

I checked the Alsa API docs and -EGAIN isn't a valid return value in any 

[snd_pcm_writei()] Returns: a positive number of frames actually written 
otherwise a negative error code 

[negative] Return values: -EBADFD, -EPIPE, -ESTRPIPE

So, basically, the code is incorrect. Hopefully, alsa supports a way to 
check whether writing x frames will block, so we can return EAGAIN in that 
case, otherwise I think a non-blocking alsa muxer is not possible. I'll 
look into it.

> Also, I think you said that the OSS driver is not honouring the NONBLOCK
> flag... (or am I misremembering?). If this is the case, I suspect that
> simply adding AVFMT_HAS_NONBLOCK to the oss_demuxer flags is not enough.
> Or am I missing something?

I have checked the code and the OSS demuxer returns an empty packet 
(pkt->size=0; pkt->pts=av_gettime();) if it's in non-blocking mode. In 
blocking mode, it actually does the same if the select() times out.

The OSS muxer always blocks, hence I did not add the flag there.


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