Re: [Libav-user] OpenCV VideoCapture FFmpeg Module Bug on Linux using Video without a Codec

Johannes Rehm Thu, 05 Mar 2015 05:51:48 -0800

So this is the function where the error happens (look for av_free_packet (&packet);):

bool CvCapture_FFMPEG::grabFrame()
{
bool valid = false;
int got_picture;

int count_errs = 0;
const int max_number_of_attempts = 1 << 9;

if( !ic || !video_st ) return false;

    if( ic->streams[video_stream]->nb_frames > 0 &&
        frame_number > ic->streams[video_stream]->nb_frames )
        return false;

av_free_packet (&packet);

picture_pts = AV_NOPTS_VALUE_;

    // get the next frame
    while (!valid)
    {
        int ret = av_read_frame(ic, &packet);
        if (ret == AVERROR(EAGAIN)) continue;

/* else if (ret < 0) break; */

        if( packet.stream_index != video_stream )
        {
            av_free_packet (&packet);
            count_errs++;
            if (count_errs > max_number_of_attempts)
                break;
            continue;
        }

        // Decode video frame
        #if LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(53, 2, 0)
            avcodec_decode_video2(video_st->codec, picture, &got_picture, &packet);
        #elif LIBAVFORMAT_BUILD > 4628
                avcodec_decode_video(video_st->codec,
                                     picture, &got_picture,
                                     packet.data, packet.size);
        #else
                avcodec_decode_video(&video_st->codec,
                                     picture, &got_picture,
                                     packet.data, packet.size);
        #endif

        // Did we get a video frame?
        if(got_picture)
        {
            //picture_pts = picture->best_effort_timestamp;
            if( picture_pts == AV_NOPTS_VALUE_ )
                picture_pts = packet.pts != AV_NOPTS_VALUE_ && packet.pts != 0 ? packet.pts : packet.dts;
            frame_number++;
            valid = true;
        }
        else
        {
            count_errs++;
            if (count_errs > max_number_of_attempts)
                break;
        }

//I need to comment out the following line to prevent an segmentation fault when processing a raw video file without a codec:

av_free_packet (&packet);
}

if( valid && first_frame_number < 0 )
first_frame_number = dts_to_frame_number(picture_pts);

// return if we have a new picture or not
return valid;
}

I am not even sure if there will be a memory leak if I delete av_free_packet (&packet);

But they might put this command for a reason and I do not want to create a new bug.

Sorry for asking such a basic question: do you know how I can verify that there will be no memory leak (with or without adding av_init_packet before)?

Gesendet: Donnerstag, 05. März 2015 um 14:22 Uhr
Von: "Rafael Lúcio" <[email protected]>
An: "This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter." <[email protected]>
Betreff: Re: [Libav-user] OpenCV VideoCapture FFmpeg Module Bug on Linux using Video without a Codec

I'm not an expert but maybe I can give u a simpler solution.... just ignore if I'm talking nonsense stuffs..

maybe if u just av_init_packet before av_free_packet your problem is solved and you don't have memory leak.

I haven't tested this theory, its just an idea.

2015-03-05 8:30 GMT-03:00 Johannes Rehm <[email protected]>:

Hi All,

there is a known bug in the VideoCapture ffmpeg module of OpenCV. It occurs while loading videos without a codec:

http://code.opencv.org/issues/1915

I had some time now to debug this bug and found the issue in:

CvCapture_FFMPEG::grabFrame()

In the function

avcodec_decode_video2(video_st->codec, picture, &got_picture, &packet);

the AVFrame picture get set. But after

av_free_packet (&packet);

I get: "error: Cannot access memory at address..." for picture.data[0] which leads to a segmentation fault later in the function sws_scale().

I can encode this video with a video codec using ffmpeg or whatever and I get no segmentation fault (memory in picture.data[0] can still be accessed).

Of course there is an easy fix for this bug: Simply comment out / delete

av_free_packet (&packet);

This is working fine for me. Now I can process videos with and without codecs. But I am not sure if this will lead to memory issues if av_free_packet gets not called. Is there anybody who is familiar with the FFmpeg interface of OpenCV and can help me here?

Thanks a lot!

Best regards,

Johannes Rehm

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Re: [Libav-user] OpenCV VideoCapture FFmpeg Module Bug on Linux using Video without a Codec

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