[Libav-user] interrupt_callback issue

Selmeci Tamás tselmeci at open-st.eu
Fri Nov 22 17:03:34 CET 2013

Hello all!

I'm using ffmpeg libraries to demultiplex MPEG2/H264 streams in an IPTV 
player application. The demultiplexed frames are decoded by a special 
target HW on Freescale iMX535. Input is from a network stream.

My problem in general is that if I call av_read_frame(...) and currently 
there's no data available through TCP, it blocks and waits for new data. 
This can take up to 100-150 ms, and my program must not be blocked for 
such a long period. (I know I could do it on a separate thread, but for 
certain reasons I want to avoid it now)

The idea is to use the interrupt callback functionality and interrupt 
av_read_frame if it would last too long.

My program is something like this:

struct program_context prog_context;

ctx = avformat_alloc_context();
ctx->flags |= AVFMT_FLAG_NONBLOCK;
ctx->interrupt_callback.callback = interrupt_cb;
ctx->interrupt_callback.opaque   = &prog_context;
avformat_open_input(url, ...);
if (need_to_get_stream_info)

for (;;) {

int interrupt_cb(char *arg)
	struct program_context *c = (struct program_context *)arg;

	if (c->need_to_interrupt)
		return 1;
	return 0;

void read_packet(...)
	AVPacket p;

	av_read_frame(..., &p);

If c->need_to_interrupt is non-zero in interrupt_cb(...), the read 
operation is interrupted as expected. However, the next read operation 
will not call interrupt_cb(...) any more and returns with error. And all 
consecutive reads do fail. I can't interrupt the read operation.

I believe this is a bug, since in my home project I do use a 
significantly older ffmpeg version, which has a perfect and working 
interrupt mechanism; unfortunately it's not an option to switch the IPTV 
project to that ancient version.

I thought of filing a bug request on this issue, but it looked like to 
me Trac contains the command-line ffmpeg issues only.

Sorry if I sent this mail to a wrong place.

Could you please help me? I have some experience in ffmpeg patching, but 
I wouldn't begin wasting time on it if somebody had a quick solution to 
this problem.

Selmeci Tamás

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