[Libav-user] Encountering exception while reading all frames of an MKV video file

[Vaughn Treude]

From: Libav-user <libav-user-bounces at ffmpeg.org> On Behalf Of Vaughn Treude
Sent: Wednesday, September 27, 2023 3:56 PM
To: libav-user at ffmpeg.org
Subject: [Libav-user] Encountering exception while reading all frames of an MKV video file

Hello all,
I'm using the FFMPEG libraries within an FFmpeg-Autogen C# wrapper in Windows Framework 4.8 to decode a video from an MKV (Matroska) container.
I open the file and initialize the stream using the following sequence:
            ffmpeg.avformat_open_input(&pFormatContext, url, null, null).ThrowExceptionIfError();
            ffmpeg.avformat_find_stream_info(_pFormatContext, null).ThrowExceptionIfError();
            AVCodec* codec = null;
            _streamIndex = ffmpeg
                .av_find_best_stream(_pFormatContext, AVMediaType.AVMEDIA_TYPE_VIDEO, -1, -1, &codec, 0)
                .ThrowExceptionIfError();
            _pCodecContext = ffmpeg.avcodec_alloc_context3(codec);
            ffmpeg.avcodec_parameters_to_context(_pCodecContext, _pFormatContext->streams[_streamIndex]->codecpar)
                .ThrowExceptionIfError();
            ffmpeg.avcodec_open2(_pCodecContext, codec, null).ThrowExceptionIfError();

After initializing, I can read each frame of the video stream using av_read_frame. I was assuming that at the end, av_read_frame would return an EOF error code. But it doesn't return any kind of error, so I continue with the process, calling avcodec_send_packet with the packet of frame data I've supposedly just read, which gives me an illegal memory access exception.
My normal approach would be to get the length of the video in frames and not read beyond this number. Unfortunately, I've been unable to find a way to determine the length of an MKV video in frames.
The nb_frames member of the stream object is never set to anything but zero. Alternately, I could multiply the duration in seconds by the frame rate, but the duration member is always set to a large-valued negative number.
How can I read all the frames in the file and exit gracefully at the end of the stream?

Thank you kindly for any hints you can give me,
Vaughn T.


An update:
I believe the crash was due to my attempt to access the Ffmpeg-Autogen wrapper with multiple threads. That thing's static! I put a lock on the decoder routine to prevent simultaneous access and the crash went away.
That said, it would still be helpful to have a better way to determine the number of frames in the video without needing to read it until I encounter an error code.
If anyone knows of such an approach, please let me know.
Thanks again,
Vaughn T.


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