[libav-api] must av_init_packet(&dummy_packet) when draining final frames from video using avcodec_decode_video2()

Luca Barbato lu_zero at gentoo.org
Thu May 14 19:20:07 CEST 2015

On 13/05/15 20:49, Bernd Pfrommer wrote:
> I hit an issue today with draining the last few frames of a video
> stream, using libav v12_dev0:
> The documentation
> https://libav.org/doxygen/master/group__lavc__decoding.html says:
> "Some decoders (those marked with CODEC_CAP_DELAY) have a delay between
> input and output. This means that for some packets they will not
> immediately produce decoded output and need to be flushed at the end of
> decoding to get all the decoded data. Flushing is done by calling this
> function with packets with avpkt->data set to NULL and avpkt->size set
> to 0 until it stops returning samples."

> But that is not sufficient.
> I got a core dump with the following stack trace:
> #0  0x00007fffeaf9f6bd in av_packet_get_side_data (pkt=0x7fffffffc2d0,
> type=AV_PKT_DATA_PARAM_CHANGE, size=0x7fffffffc1fc) at
> libavcodec/avpacket.c:293
> #1  0x00007fffeb2c59e0 in apply_param_change (avpkt=0x7fffffffc2d0,
> avctx=0x1ddaca0) at libavcodec/utils.c:1549
> #2  avcodec_decode_video2 (avctx=0x1ddaca0, picture=0x9f0000,
> got_picture_ptr=0x7fffffffc260, avpkt=0x7fffffffc2d0) at
> libavcodec/utils.c:1657
> and in another run, I got a core dump where it tried to access the
> packets "buf" field. Setting side_data, side_data_elems, and buf to zero
> eventually worked. This can be accomplished by using av_init_packet():
>      AVPacket emptyPacket;
>      av_init_packet(&emptyPacket);  // <---------- must initialize side
> data and buf!!!!
>      emptyPacket.data = NULL;
>      emptyPacket.size = 0;
>      int frameFinished;
>      AVFrame *tmpFrame = av_frame_alloc();
>      int len = avcodec_decode_video2(context, tmpFrame, &frameFinished,
> &emptyPacket);

> I suggest updating the documents to say that av_init_packet() is
> required. This is particulary important because there is example code
> that does not call av_init_packet():

Absolutely! Thanks for noticing! I'll send a patch about it now.

Thanks a lot!


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