How to use ffmpeg faststart flag programmatically? - c++

Question

I try to convert videos for playback on Android using H264, AAC codecs and mp4 container. Video plays normaly with non-system players. But system player shows error "Can't play this video".
I found out that the problem is in moov atom, which is writed in the end of the file.
When I use "-movflags +faststart" ffmeg flag to convert video, it plays normal, but when I try to do that programmatically, it gives no result. I use following code:
av_dict_set( &dict, "movflags", "faststart", 0 );
ret = avformat_write_header( ofmt_ctx, &dict );

Answer 1

This code works fine:
av_dict_set( &dict, "movflags", "faststart", 0 );
ret = avformat_write_header( ofmt_ctx, &dict );
But problem is not solved. I still can't play converted videos on Android devices.

Answer 2

I assume that this answer is very late, but still, for anyone who might still be facing the same issue: this might be caused by the AV_CODEC_FLAG_GLOBAL_HEADER not being set in the audio/video AVCodecContext. A lot of guides show that it needs to be set in the AVFormatContext, but it needs to be set in the AVCodecContext before opening it using avcodec_open2.
if (format_context->oformat->flags & AVFMT_GLOBALHEADER) {
video_codec_context->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}
avcodec_open2(video_codec_context, video_codec, nullptr);

Answer 3

Maybe the video is not compatible with your android phone? Try to convert with h264 baseline profile.

Answer 4

TL;DR
Set url field of AVFormatContext before avformat_write_header.
Why
I hit same issue today, and I found there is a log when calling av_write_trailer:
Unable to re-open output file for the second pass (faststart)
In the movenc.c implementation, we can see it needs s->url to re-open the file:
avio_flush(s->pb);
ret = s->io_open(s, &read_pb, s->url, AVIO_FLAG_READ, NULL);
if (ret < 0) {
av_log(s, AV_LOG_ERROR, "Unable to re-open %s output file for "
"the second pass (faststart)\n", s->url);
goto end;
}