[Libav-user] How To Duplicate AVPacket

[Xiemin Chen]

Dear nfxjfg:
    I just do av_malloc(sizeof(AVPacket)) instead of define local variable
AVPacket packet before av_read_frame().
    And nothing else except for putting into queue, then seems that it does
solve my problem. Thanks for your professional answer.
    Also thanks for indicating my potential memory-leaking mistake.
    Thanks very much.

Sincerely



2014-06-26 8:33 GMT-04:00 wm4 <nfxjfg at googlemail.com>:

> On Wed, 25 Jun 2014 20:46:59 -0400
> Xiemin Chen <chenxiemin at gmail.com> wrote:
>
> > Dear all:
> >     I want to split av_read_frame() and avcodec_decode_video2() in to
> > different thread so I need to put a queue behind them. av_read_frame()
> read
> > a packet then put the packet into queue, avcodec_decode_video2() read a
> > packet from the queue then decode.
> >     But sometimes the decoded frames may contain rubbish, it's random.
> So I
> > think that AVPacket shares its data memory and overwrite by somebody
> before
> > I decode it. The following code like this:
>
> Why do you think that? It could be something else. Did you test with
> valgrind?
>
> >          1. av_read_frame(&packet);
> >          2.     av_dup_packet(&packet);
> >          3.     AVPacket *newPakcet = av_malloc(sizeof(AVPacket));
> >          4.     *newPacket = packet;
>
> It depends what you do with the original packet after this. If you
> unref it, the packet might be free'd. If you malloc the AVPacket before
> the av_read_frame call, and then put that into the queue, you should be
> fine.
>
> >          5.  put_queue(packet);
> >          6.  get_queue(&packet); (Another thread)
> >          7.  avcodec_decode_video2(packet, &frame);
> >          8.  show frame;
> >     Is there any mistake I took on these codes? Thanks very much.
> >
> > Sincerely
>
> AVPackets have other data than the packet buffer - at least side data.
> av_dup_packet() is an old API that AFAIK does nothing on packets
> returned by libavformat. (Yes, that is very misleading - and I'm not
> even sure if it _really_ does nothing.) Now you should use
> av_packet_ref(), which is a relatively recently added function. But
> it looks like you don't need it in your specific use-case.
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