[FFmpeg-devel] Can av_write_frame() modify pkt.data?

Michael Niedermayer michaelni
Tue Jun 17 15:05:07 CEST 2008

On Tue, Jun 17, 2008 at 02:23:00PM +0200, Luca Abeni wrote:
> Hi all,
> I just discovered that the mov muxer can modify the
> "data" field of the AVPacket structure that it receives
> as parameter (in case of H.264 video, mov_write_packet()
> calls ff_avc_parse_nal_units(pkt->data, &pkt->data, &pkt->size);
> which frees pkt->data and allocates a new buffer).
> Is this intended? This behaviour is not a problem for
> ffmpeg.c, because it dynamically allocates and deallocates
> pkt->data for every output packet, but is a problem for
> output_example.c (if you modify it for writing H.264 video
> in mov).
> So, should output_example.c be fixed, or should movenc.c
> be fixed? (or is everything ok, since output_example does
> not write H.264 in mov unless you modify it?)

movenc.c looks buggy, and a quick grep hints towards that its not
the only one.
pkt.data does not need to be a malloced() buffer. Thus a blind av_free()
is always wrong. The correct way to kill a packet is with pkt->destruct()
though that is of course not the muxers job to do.

Besides a packet could be passed to more than 1 muxer and thus its content
must not be changed by any muxer.

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The educated differ from the uneducated as much as the living from the
dead. -- Aristotle 
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