libav open/read

    if (format != NULL){
        fmt = av_find_input_format(format);
        res = avformat_open_input(&oc, url, fmt , NULL);
    } else {
        res = avformat_open_input(&oc, url, NULL , NULL);
    }


    if (res < 0) {
        fprintf(stderr,"[LIBAV] Unable to open input file/url: %s\n", url);
        return res;
    }

    res = avformat_find_stream_info(oc, NULL);

    if (res < 0) {
        fprintf(stderr,"[LIBAV] Unable to read stream info for file/url: %s\n", url);
        libavClose();
        return res;
    }


    res = av_find_best_stream(oc, AVMEDIA_TYPE_VIDEO, -1, -1, &video_decoder, 0);

    if (res < 0){
        fprintf(stderr,"[LIBAV] No video stream found\n");
        libavClose();
        return -1;
    }


    video_st_index = res;
    video_st = oc->streams[video_st_index];
    video_codec = video_st->codec;

    if((res = avcodec_open2(video_codec, video_decoder, NULL))<0) {
        char error[1024];
        av_strerror(res, error, sizeof(error));

        fprintf(stderr,"[LIBAV] Could not open video decoder: %s\n", error);
        libavClose();
        return -1;
    }

    // Fetch Audio stream and Decoder
    res = av_find_best_stream(oc, AVMEDIA_TYPE_AUDIO, -1, -1, &audio_decoder, 0);

    if (res < 0){
        fprintf(stderr,"[LIBAV] No audio stream found\n");
        libavClose();
        return -1;
    }

    audio_st_index = res;
    audio_st = oc->streams[audio_st_index];
    audio_codec = audio_st->codec;


    if((res = avcodec_open2(audio_codec, audio_decoder, NULL))<0) {
        char error[1024];
        av_strerror(res, error, sizeof(error));

        fprintf(stderr,"[LIBAV] Could not open audio decoder: %s\n", error);
        libavClose();
        return -1;
    }

    // Dump source information
    av_dump_format(oc,0,url,0);


    // Setup audio/video
    video_buf = avcodec_alloc_frame();
    audio_buf = (u_int8_t *) av_malloc(AVCODEC_MAX_AUDIO_FRAME_SIZE);

    while ((res = av_read_frame(oc, &packet))>=0 ) {
        orig = packet;
        if (packet.stream_index == audio_st_index) {

            // Keep reading audio samples from the packet (may contain multiple)
            while (packet.size > 0) {
                int chunk = AVCODEC_MAX_AUDIO_FRAME_SIZE;
                int in_used = avcodec_decode_audio3(audio_codec, (int16_t *) ((u_int8_t *) audio_buf + (*audio_samples) * av_get_bytes_per_sample( audio_st->codec->sample_fmt) * audio_st->codec->channels ) , &chunk, &packet);

                if (in_used < 0) {
                    //printf("Invalid Audio Frame\n");
                    // Decoding error -- skip this frame.
                    break;
                }

                audio_samples += (chunk / av_get_bytes_per_sample( audio_st->codec->sample_fmt)) /  audio_st->codec->channels;

                packet.data += in_used;
                packet.size -= in_used;

            }

            // If audio_samples were written, then update pointer
            if (audio_samples > 0) {
                // audio_buf stores audio_samples number of samples
                sampleCount += audio_samples;
            }

        } else if (packet.stream_index == video_st_index) {


            if (packet.size > 0) {

                frameFinished = 0;

                int in_used = avcodec_decode_video2(video_codec,video_buf, &frameFinished, &packet);

                if (in_used < 0) {
                    //printf("Invalid Video Frame\n");
                    // Decoding error -- skip this frame.

                } else {

                    if(frameFinished) {

                        frameCount++;

                        // video_frame stores the current frame

                    }
                    packet.data += in_used;
                    packet.size -= in_used;
                }

            }

        }

        av_free_packet(&orig); // free the packet
    }