[Libav-user] g++ 4.7.2 fails to compile av_err2str

Nicolas George nicolas.george at normalesup.org
Mon Jan 14 17:49:13 CET 2013

Le quintidi 25 nivôse, an CCXXI, Alex Cohn a écrit :
> Yes, alloca() should be OK. Now that I think about it, that's right:
> it is compatible with C, too.

I am far from sure that alloca() exists in all supported C compilers.

>				But Microsoft compiler only recognizes
> _alloca().

There you are.

There is another problem with alloca(): the scope of the allocation is
per function, while the scope of a compound literal, as used by the macro
currently, is per block.

That means that the following function:

void encode(void)
    while (packet = read_packet()) {
	if ((ret = avcodec_decode_video2(avc, frame, &out, packet)) < 0) {
	    fprintf(stderr, "Error decoding: %s\n", av_err2str(ret));
	if (out)

with the current macro, works as expected, whereas if the macro were based
on alloca(), the error strings would stay allocated until the loop is
completed. That is a memory leak. Probably not terrible, but still a leak.


  Nicolas George
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