Question

I am writing some software that uses ffmpeg extensively and it is multi threaded, with multiple class instances.

If the network connection drops out ffmpeg hangs on reading. I found a method to assign a callback that ffmpeg fires periodically to check if it should abort or not:

static int interrupt_cb(void *ctx) 
{ 

// do something 
    return 0;
} 

static const libffmpeg::AVIOInterruptCB int_cb = { interrupt_cb, NULL }; 

…

AVFormatContext* formatContext = libffmpeg::avformat_alloc_context( );
formatContext->interrupt_callback = int_cb; 
if ( libffmpeg::avformat_open_input( &formatContext, fileName, NULL, NULL ) !=0 ) {...}

This is all fine but nowhere on the web can i find what *ctx contains and how to determine whether the callback should return 1 or 0. I can’t assign a static “abort” flag as the class has many instances. I also can’t debug the code as for some reason visual studio refuses to set a breakpoint on the return 0; line, claiming no executable code is associated with the location. Any ideas?

Answer 1

Found in the ffmpeg documentation:

During blocking operations, callback is called with opaque as
parameter. If the callback returns 1, the blocking operation will
be aborted.

Here is declaration int_cb variable of type AVIOInterruptCB struct from your code:

static const libffmpeg::AVIOInterruptCB int_cb = { interrupt_cb, NULL };

You declared opaque parameter as NULL.

I’d recommend to rewrite initialization code like this:

AVFormatContext* formatContext = libffmpeg::avformat_alloc_context( );
formatContext->interrupt_callback.callback = interrupt_cb;
formatContext->interrupt_callback.opaque = formatContext;

you will be able to access formatContext instance inside interrupt_cb:

static int interrupt_cb(void *ctx) 
{ 
    AVFormatContext* formatContext = reinterpret_cast<AVFormatContext*>(ctx);
// do something 
    return 0;
}