if (pkt->data) {
			pkt->size -= len;
			pkt->data += len;
		}

#include <stdlib.h>
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>
#ifdef __cplusplus
}
#endif

#define NEEDED_FORMAT AV_PIX_FMT_GRAY8

int main(int argc, char** argv)
{
    int rv = 0, vstream, width, height, outSize;
    unsigned int i;
    AVFrame *frame = NULL;
    uint8_t * out[4] = { 0 };
    int outLinesize[4] = { 0 };
    AVPacket pkt;
    AVFormatContext * fmt = NULL;
    AVCodecContext * ctx = NULL;
    AVCodec * decoder = NULL;
    AVCodecParameters * codecpar = NULL;
    struct SwsContext * sws = NULL;
    const char * filename;
    char err[512];

    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s <file>\n", argv[0]);
        return -1;
    }
    filename = argv[1];

    // init FFmpeg
    av_register_all();
    avcodec_register_all();
    avformat_network_init();
    av_log_set_level(AV_LOG_DEBUG);

    // init packet with default values
    av_init_packet(&pkt);

    // alloc temporary frame
    frame = av_frame_alloc();
    if (!frame) { rv = AVERROR(ENOMEM); goto exit; }

    // open input file/stream
    rv = avformat_open_input(&fmt, filename, NULL, NULL);
    if (rv < 0) goto exit;

    // find streams
    rv = avformat_find_stream_info(fmt, NULL);
    if (rv < 0) goto exit;

    // log input streams
    av_dump_format(fmt, 0, filename, 0);

    // find video stream
    for (i = 0; i<fmt->nb_streams; ++i)
    {
        if (fmt->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&
            !codecpar)
        {
            vstream = i;
            codecpar = fmt->streams[i]->codecpar;
            width = codecpar->width;
            height = codecpar->height;
            break;
        }
    }
    if (!codecpar) { rv = AVERROR_STREAM_NOT_FOUND; goto exit; }

    // allocate sws context
    sws = sws_getContext(width, height, codecpar->format, width, height, NEEDED_FORMAT, SWS_POINT, NULL, NULL, NULL);
    if (!sws) { rv = AVERROR(ENOMEM); goto exit; }

    // find decoder description
    decoder = avcodec_find_decoder(codecpar->codec_id);
    if (!decoder) { rv = AVERROR_DECODER_NOT_FOUND; goto exit; }

    // alloc decoder context
    ctx = avcodec_alloc_context3(decoder);
    if (!ctx) { rv = AVERROR(ENOMEM); goto exit; }

    // copy codec parameters to context
    rv = avcodec_parameters_to_context(ctx, codecpar);
    if (rv < 0) goto exit; 

    // open decoder context
    rv = avcodec_open2(ctx, decoder, NULL);
    if (rv < 0) goto exit;

    // alloc data for output frame
    rv = av_image_get_buffer_size(NEEDED_FORMAT, width, height, 1);
    if (rv < 0) goto exit;
    outSize = rv;
    out[0] = (uint8_t*)av_malloc((size_t)outSize);
    if(!out[0]) { rv = AVERROR(ENOMEM); goto exit; }
    rv = av_image_fill_arrays(out, outLinesize, out[0], NEEDED_FORMAT, width, height, 1);
    if (rv < 0) goto exit;
    
    // frame read cycle
    while ((rv = av_read_frame(fmt, &pkt)) >= 0) // read packet from input
    {
        // ignore audio
        if (pkt.stream_index == vstream)
        {
            // send packet to decoder
            rv = avcodec_send_packet(ctx, &pkt);
            if (rv >= 0)
            {
                // receive frame from decoder
                while ((rv = avcodec_receive_frame(ctx, frame)) >= 0)
                {
                    // convert colorspace
                    rv = sws_scale(sws, frame->data, frame->linesize, 0, height, out, outLinesize);
                    if (rv >= 0)
                    {
                        // Now 'out[0]' contains first plane data
                    }
                }
            }
        }
        av_packet_unref(&pkt);
    }

    rv = 0;
exit:
    if (rv < 0)
    {
        av_strerror(rv, err, sizeof(err));;
        fprintf(stderr, "Error: %s\n", err);
    }
    if (out[0]) { av_freep(&out[0]); };
    if (frame) { av_frame_free(&frame); }
    if (ctx) { avcodec_free_context(&ctx); }
    if (fmt) { avformat_close_input(&fmt); }
    if (sws) { sws_freeContext(sws); }
    return rv < 0 ? rv : 0;
}

/tmp/main.c:27: error: unknown type name ‘AVCodecParameters’
  AVCodecParameters * codecpar = NULL;
  ^