H264 extract SEI using FFMPEG

gistfile1.c

// It's actually quite simple to extract unregistered sei data using ffmpeg, but I couln't find any documentation about how it's done 

        AVFrameSideData *sd;
        sd = av_frame_get_side_data(frame, AV_FRAME_DATA_SEI_UNREGISTERED);
        if (sd) {
            // First 15 bytes is the uuid, unregistered payload start at index 16 and the
            // total size is given by sd->size
            printf("side-data size %ld first-byte %u\n", sd->size, sd->data[16]);
            

That was very helpful. It took me a fair bit of time to get it up and running though and I realised that a frame could hold multiple AV_FRAME_DATA_SEI_UNREGISTERED items. This required additional care and looping over the frame->side_data rather than calling av_frame_get_side_data. The result of my exploration incorporating this in two ffmpeg examples is here: https://gist.github.com/tvercaut/94c68e46d0d0321a2c83b88024d9cd69