Using libavformat to create MKV Container and having VLC Playback Issues

When running the remuxing code against an MP4 as input and MKV as output, the media plays in VLC media player but the progress bar does not update. If I run...

ffmpeg -i input.mp4 -map 0 -c copy output.mkv

The file works fine in VLC (the progress bar and duration info is updating correctly). I've been going nuts trying to figure out what's missing to get VLC to play the output media correctly. Any pointers? What am I missing here?

// remux.c
#include <stdlib.h>
#include <stdio.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

int main(int argc, char **argv)
{
  AVFormatContext *input_format_context = NULL, *output_format_context = NULL;
  AVDictionary *opts = NULL;
  AVPacket packet;
  const char *in_filename, *out_filename;
  int ret, i;

  if (argc < 3)
  {
    fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
    return 1;
  }

  in_filename = argv[1];
  out_filename = argv[2];

  if ((ret = avformat_open_input(&input_format_context, in_filename, NULL, NULL)) < 0)
  {
    fprintf(stderr, "Could not open input file '%s'", in_filename);
    goto end;
  }
  if ((ret = avformat_find_stream_info(input_format_context, NULL)) < 0)
  {
    fprintf(stderr, "Failed to retrieve input stream information");
    goto end;
  }

  avformat_alloc_output_context2(&output_format_context, NULL, NULL, out_filename);
  if (!output_format_context)
  {
    fprintf(stderr, "Could not create output context\n");
    ret = AVERROR_UNKNOWN;
    goto end;
  }

  for (i = 0; i < input_format_context->nb_streams; i++)
  {
    AVStream *out_stream;
    AVStream *in_stream = input_format_context->streams[i];
    AVCodecParameters *in_codecpar = in_stream->codecpar;

    out_stream = avformat_new_stream(output_format_context, NULL);
    if (!out_stream)
    {
      fprintf(stderr, "Failed allocating output stream\n");
      ret = AVERROR_UNKNOWN;
      goto end;
    }

    ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
    if (ret < 0)
    {
      fprintf(stderr, "Failed to copy codec parameters\n");
      goto end;
    }
  }
  // https://ffmpeg.org/doxygen/trunk/group__lavf__misc.html#gae2645941f2dc779c307eb6314fd39f10
  av_dump_format(output_format_context, 0, out_filename, 1);

  ret = avio_open(&output_format_context->pb, out_filename, AVIO_FLAG_WRITE);
  if (ret < 0)
  {
    fprintf(stderr, "Could not open output file '%s'", out_filename);
    goto end;
  }

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga18b7b10bb5b94c4842de18166bc677cb
  ret = avformat_write_header(output_format_context, &opts);
  if (ret < 0)
  {
    fprintf(stderr, "Error occurred when opening output file\n");
    goto end;
  }

  while (1)
  {
    AVStream *in_stream, *out_stream;
    ret = av_read_frame(input_format_context, &packet);
    if (ret < 0)
      break;
    in_stream = input_format_context->streams[packet.stream_index];
    out_stream = output_format_context->streams[packet.stream_index];

    // Adjust PTS/DTS
    packet.pts = av_rescale_q_rnd(packet.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);
    packet.dts = av_rescale_q_rnd(packet.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);
    packet.duration = av_rescale_q(packet.duration, in_stream->time_base, out_stream->time_base);
    // https://ffmpeg.org/doxygen/trunk/structAVPacket.html#ab5793d8195cf4789dfb3913b7a693903
    packet.pos = -1;

    // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga37352ed2c63493c38219d935e71db6c1
    ret = av_interleaved_write_frame(output_format_context, &packet);
    if (ret < 0)
    {
      fprintf(stderr, "Error muxing packet\n");
      break;
    }
    av_packet_unref(&packet);
  }

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga7f14007e7dc8f481f054b21614dfec13
  av_write_trailer(output_format_context);

end:
  avformat_close_input(&input_format_context);
  /* close output */
  if (output_format_context && !(output_format_context->oformat->flags & AVFMT_NOFILE))
    avio_closep(&output_format_context->pb);
  avformat_free_context(output_format_context);
  if (ret < 0 && ret != AVERROR_EOF)
  {
    fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
    return 1;
  }
  return 0;
}

I've tried various renditions of code based on snippets I have found around the net, but nothing seems to work. It seems like there's a flag or metadata tag missing that VLC requires to playback the video correctly. I did test the output using MKPlayer and that worked - so this is definitely an issue with the output media and VLC Player.

I finally figured out the issue. I set the packet flags with AV_PKT_FLAG_KEY and now the progress bar is advancing.

while (1)
{
  AVStream *in_stream, *out_stream;
  ret = av_read_frame(input_format_context, &packet);
  if (ret < 0)
    break;
  in_stream = input_format_context->streams[packet.stream_index];
  out_stream = (*output_format_context)->streams[packet.stream_index];

  // Rescale PTS/DTS
  av_packet_rescale_ts(&packet, in_stream->time_base, out_stream->time_base);
  packet.flags |= AV_PKT_FLAG_KEY;
  packet.pos = -1;

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga37352ed2c63493c38219d935e71db6c1
  ret = av_interleaved_write_frame(*output_format_context, &packet);
  if (ret < 0)
  {
    fprintf(stderr, "Error muxing packet\n");
    break;
  }
  av_packet_unref(&packet);
}