ffmpeg and the libopus encoder

gistfile1.txt

av_register_all(); // Initialize libavformat and register all the muxers, demuxers and protocols (but not the codecs)
avcodec_register_all(); // Register the codec and initialize libavcodec

SwrContext *swr;
AVCodec *codec;
AVCodecContext *context;
// get the libopus codec encoder
codec = avcodec_find_encoder_by_name("libopus");
if (codec) {
    swr = swr_alloc_set_opts(NULL,   // we're allocating a new context
                AV_CH_LAYOUT_STEREO, // out_ch_layout
                AV_SAMPLE_FMT_FLTP,  // out_sample_fmt
                48000,               // out_sample_rate
                AV_CH_LAYOUT_MONO,   // in_ch_layout
                AV_SAMPLE_FMT_S16,   // in_sample_fmt
                48000,               // in_sample_rate
                0,                   // log_offset
                NULL);               // log_ctx
    int results = swr_init(swr);
    AVDictionary *opts = NULL;
    av_dict_set(&opts, "strict", "experimental", 0);
    // Use variable bitrate encoding (default)
    //av_dict_set(&opts, "vbr", NULL, 0);
    // Target bitrate in kbit/sec (6-256/channel)
    //av_dict_set(&opts, "bitrate", (br / 1000), 0);
    // Encoding complexity (0-10, default: 10 (slowest))
    av_dict_set(&opts, "comp", "10", 0);
    // Maximum frame size in milliseconds (2.5, 5, 10, 20, 40, 60, default: 20)
    av_dict_set(&opts, "framesize", "20", 0);
    // Percentage packet loss to expect (default: 0)
    av_dict_set(&opts, "expect-loss", "0", 0);
    // Downmix to stereo (if >2 channels)
    //av_dict_set(&opts, "downmix-stereo", NULL, 0);
    // Maximum container delay in milliseconds (0-1000, default: 1000)
    av_dict_set(&opts, "max-delay", "1000", 0);
    context = avcodec_alloc_context3(codec);
    if (context) {
        //memset(context, 0, sizeof(context));
        context->sample_rate = 48000;
        context->time_base.den = 48000;
        context->time_base.num = 1;
        context->channels = 2;
        context->channel_layout = AV_CH_LAYOUT_STEREO;
        context->sample_fmt = AV_SAMPLE_FMT_FLTP;
        context->bit_rate = 32000;
    }
    // this fails just a quickly as without opts
    //results = avcodec_open2(context, codec, &opts);
    results = avcodec_open2(context, codec, 0);
    if (results < 0) {
        printf("codec not opened: %d\n", results);
    } else {
        if (context->codec != 0)  {
            printf("codec opened\n");
        } else {
            printf("setting codec\n");
            context->codec=codec;
        }
    }
} else {
    fprintf(stderr, "Could not find an Opus encoder\n");
}