ffmpeg写mp4信息

如上图，ffmpeg转码完成后，会写文件信息。

    while (!received_sigterm) {

    。。。

     ret = transcode_step();

    for (i = 0; i < nb_output_files; i++) {
        os = output_files[i]->ctx;
        if ((ret = av_write_trailer(os)) < 0) {
            av_log(NULL, AV_LOG_ERROR, "Error writing trailer of %s: %s", os->filename, av_err2str(ret));
            if (exit_on_error)
                exit_program(1);
        }

    }

。。。

}

具体写时长是在movenc.c里。

static int mov_write_mvhd_tag(AVIOContext *pb, MOVMuxContext *mov)
{
    int max_track_id = 1, i;
    int64_t max_track_len = 0;
    int version;

    //获取max_track_len。

    for (i = 0; i < mov->nb_streams; i++) {
        if (mov->tracks[i].entry > 0 && mov->tracks[i].timescale) {
            int64_t max_track_len_temp = av_rescale_rnd(mov->tracks[i].track_duration,
                                                MOV_TIMESCALE,
                                                mov->tracks[i].timescale,
                                                AV_ROUND_UP);
            if (max_track_len < max_track_len_temp)
                max_track_len = max_track_len_temp;
            if (max_track_id < mov->tracks[i].track_id)
                max_track_id = mov->tracks[i].track_id;
        }
    }
    /* If using delay_moov, make sure the output is the same as if no
     * samples had been written yet. */
    if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV) {
        max_track_len = 0;
        max_track_id  = 1;
    }
    version = max_track_len < UINT32_MAX ? 0 : 1;
    avio_wb32(pb, version == 1 ? 120 : 108); /* size */
    ffio_wfourcc(pb, "mvhd");
    avio_w8(pb, version);
    avio_wb24(pb, 0); /* flags */
    if (version == 1) {
        avio_wb64(pb, mov->time);
        avio_wb64(pb, mov->time);
    } else {
        avio_wb32(pb, mov->time); /* creation time */
        avio_wb32(pb, mov->time); /* modification time */
    }
    avio_wb32(pb, MOV_TIMESCALE);
    (version == 1) ? avio_wb64(pb, max_track_len) : avio_wb32(pb, max_track_len); /* duration of longest track */

    avio_wb32(pb, 0x00010000); /* reserved (preferred rate) 1.0 = normal */
    avio_wb16(pb, 0x0100); /* reserved (preferred volume) 1.0 = normal */
    avio_wb16(pb, 0); /* reserved */
    avio_wb32(pb, 0); /* reserved */
    avio_wb32(pb, 0); /* reserved */

    /* Matrix structure */
    write_matrix(pb, 1, 0, 0, 1, 0, 0);

    avio_wb32(pb, 0); /* reserved (preview time) */
    avio_wb32(pb, 0); /* reserved (preview duration) */
    avio_wb32(pb, 0); /* reserved (poster time) */
    avio_wb32(pb, 0); /* reserved (selection time) */
    avio_wb32(pb, 0); /* reserved (selection duration) */
    avio_wb32(pb, 0); /* reserved (current time) */
    avio_wb32(pb, max_track_id + 1); /* Next track id */
    return 0x6c;
}