[Libav-user] How to keep AVFrame data after codec close

[Paul B Mahol]

On 12/5/13, Eric Beuque <eric.beuque at gmail.com> wrote:
> Hi, i'm using libavcodec from FFMpeg 1.1.2.
>
> I'm decoding MJPEG stream. My problem is that i need to keep a reference to
> the decoded frame, after the codec close. But when using avcodec_close, it
> freed the AVFrame data.
>
> Here a sample program showing the function i use:
>
> int main(int argc, char **argv)
> {
>     int bGotPicture;
>
>     avcodec_register_all();
>
>     AVCodec* pCodec = avcodec_find_decoder (CODEC_ID_MJPEG);
>     AVCodecContext* pAVContext = avcodec_alloc_context3 (pCodec);
>
>     avcodec_open2(pAVContext, pCodec, NULL);
>
>     AVPacket packet;
>     av_init_packet (&packet);
>
>     size_t iSize;
>     std::string content = get_file_contents("../img1.jpg", iSize);
>     packet.data = (unsigned char*)content.c_str();
>     packet.size = iSize;
>
>     AVFrame* pFrame = avcodec_alloc_frame();
>     avcodec_decode_video2 (pAVContext, pFrame, &bGotPicture, &packet);
>
>     // Here pFrame->data is valid
>
>     av_free_packet(&packet);
>     avcodec_close(pAVContext);
>     av_free(pAVContext);
>
>     // Here pFrame->data has been freed, access may crash
>
>     av_free(pFrame);
>     pFrame = NULL;
> }
>
> How i can tell libavcodec that i want to become the owner of the data and
> be able to free it, to without codec context instance? I was thinking about
> using get/release_buffer functions, but i don't know how to compute size of
> the data.
>
> Is data memcpy the only way to do this?
>
> Note, that i also can't upgrade to new version of FFMPEG.
>
> Thanks for your help.
>

Use reference counting system.
They you need to manually free frame once you will not need it
(otherwise you leak memory), so you do not need to do copy.