c - How can I dynamically update metadata in audio output with libav so that updates appear in MPV? - Stack Overflow

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My ultimate goal is to proxy an internet radio station and programmatically add metadata to it during the stream that can be displayed and updated in MPV, the media player playing the audio.

The metadata I would like to add is primarily the song title, but ideally additional information including artist, composer, and album.

I envision running the proxy program like this:

$ curl .mp3 | ./proxy_add_metadata | mpv -

or maybe this:

$ ./proxy_add_metadata &
#=> <output>
$ mpv <output>

How could I make song metadata updates dynamically over time using libav?

I’m using FFmpeg version 7.1.

I first tried changing the metadata dictionary shortly before writing a frame with a few different container formats:

av_dict_set(&output_ctx->metadata, "title", "Title 1", 0);
/* [...] */
av_interleaved_write_frame(output_ctx, packet);

Setting metadata with av_dict_set(&output_ctx->metadata, "title", "Title 1", 0); only appears to work when done before writing the output header.

My next idea was to try setting metadata in AVPacket side data, but I’m unclear which container formats support this for the kind of metadata I’m working with.

I’m open to any output media container format or FFmpeg-originated network stream.

It’s unclear to me whether the metadata should be written to a separate stream within the media container or whether it should be written as side data in the output packets.

If what I’m trying to do is impossible, please explain why.

What I have so far reads audio from standard input and writes it to standard output. The input audio can be assumed to be in MP3 format. Non-working sections for metadata updates are commented out.

/* proxy_add_metadata.c */
#include <stdio.h>

#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>

int main() {
    int _err;

    /* MP3 input */
    AVFormatContext *input_ctx = avformat_alloc_context();
    _err = avformat_open_input(&input_ctx, "pipe:0", NULL, NULL);
    _err = avformat_find_stream_info(input_ctx, NULL);

    AVFormatContext *output_ctx;
    _err = avformat_alloc_output_context2(&output_ctx, NULL, "matroska", "pipe:1");

    AVStream *input_stream = input_ctx->streams[0];
    AVStream *output_stream = avformat_new_stream(output_ctx, NULL);

    _err = avcodec_parameters_copy(output_stream->codecpar, input_stream->codecpar);
    _err = avio_open(&output_ctx->pb, "pipe:1", AVIO_FLAG_WRITE);

    _err = avformat_write_header(output_ctx, NULL);

    AVPacket *packet = av_packet_alloc();

    /* Set up packet side data. */
    /*
    AVDictionary *packet_side_data_dict;
    av_dict_set(&packet_side_data_dict, "title", "Title 1", 0);

    size_t packet_side_data_size = 0;
    uint8_t *packet_side_data = av_packet_pack_dictionary(
        packet_side_data_dict,
        &packet_side_data_size
    );
    av_dict_free(&packet_side_data_dict);
    */

    while (1) {
        _err = av_read_frame(input_ctx, packet);
        if (_err < 0) {
            break;
        }

        /* Can metadata updates be made here? */

        /* Option 1: Attempt to write metadata to the container. */
        /*
        _err = av_dict_set(&output_ctx->metadata, "title", "Title 1", 0);
        if (_err < 0) {
            fprintf(stderr, "error: can't set metadata title in stream: %s\n", av_err2str(_err));
            break;
        }
        */

        /* Option 2: Attempt to write metadata to packet side data. */
        /*
        _err = av_packet_add_side_data(
            packet,
            AV_PKT_DATA_METADATA_UPDATE,
            packet_side_data,
            packet_side_data_size
        );
        if (_err < 0) {
            fprintf(stderr, "error: can't add side data to packet: %s\n", av_err2str(_err));
            break;
        }
        */

        AVStream *input_stream = input_ctx->streams[packet->stream_index];
        AVStream *output_stream = output_ctx->streams[packet->stream_index];

        av_packet_rescale_ts(packet, input_stream->time_base, output_stream->time_base);
        packet->pos = -1;

        _err = av_interleaved_write_frame(output_ctx, packet);
        if (_err < 0) {
            fprintf(stderr, "error: packet write: %s\n", av_err2str(_err));
            break;
        }
    }

    av_write_trailer(output_ctx);

    av_packet_free_side_data(packet);
    av_packet_free(&packet);

    avio_closep(&output_ctx->pb);
    avformat_free_context(output_ctx);

    avformat_close_input(&input_ctx);

    return 0;
}

cc \
    -Wall \
    -g \
    -I/.../ffmpeg7/include \
    -o proxy_add_metadata \
    proxy_add_metadata.c \
    -L/.../ffmpeg7/lib -lavformat -lavcodec

$ < sample.mp3 ./proxy_add_metadata | mpv -

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