ffmpeg在mp4中是如何写入aac extradata的？

之前已知在mp4格式中，aac的extradata是以AudioSpecificConfig的形式存在的。搜索到的相关资料
https://stackoverflow.com/questions/3987850/mp4-atom-how-to-discriminate-the-audio-codec-is-it-aac-or-mp3

说是 esds atom 中的ASC就是AudioSpecificConfig。对应ffmpeg的代码，看ffmpeg是如何写入esds的
/libavformat/movenc.c

static int mov_write_esds_tag(AVIOContext *pb, MOVTrack *track) // Basic
{
    struct mpeg4_bit_rate_values bit_rates = calculate_mpeg4_bit_rates(track);
    int64_t pos = avio_tell(pb);
    int decoder_specific_info_len = track->vos_len ? 5 + track->vos_len : 0;

    avio_wb32(pb, 0); // size
    ffio_wfourcc(pb, "esds");
    avio_wb32(pb, 0); // Version

    // ES descriptor
    put_descr(pb, 0x03, 3 + 5+13 + decoder_specific_info_len + 5+1);
    avio_wb16(pb, track->track_id);
    avio_w8(pb, 0x00); // flags (= no flags)

    // DecoderConfig descriptor
    put_descr(pb, 0x04, 13 + decoder_specific_info_len);

    // Object type indication
    if ((track->par->codec_id == AV_CODEC_ID_MP2 ||
         track->par->codec_id == AV_CODEC_ID_MP3) &&
        track->par->sample_rate > 24000)
        avio_w8(pb, 0x6B); // 11172-3
    else
        avio_w8(pb, ff_codec_get_tag(ff_mp4_obj_type, track->par->codec_id));

    // the following fields is made of 6 bits to identify the streamtype (4 for video, 5 for audio)
    // plus 1 bit to indicate upstream and 1 bit set to 1 (reserved)
    if (track->par->codec_id == AV_CODEC_ID_DVD_SUBTITLE)
        avio_w8(pb, (0x38 << 2) | 1); // flags (= NeroSubpicStream)
    else if (track->par->codec_type == AVMEDIA_TYPE_AUDIO)
        avio_w8(pb, 0x15); // flags (= Audiostream)
    else
        avio_w8(pb, 0x11); // flags (= Visualstream)

    avio_wb24(pb, bit_rates.buffer_size); // Buffersize DB
    avio_wb32(pb, bit_rates.max_bit_rate); // maxbitrate
    avio_wb32(pb, bit_rates.avg_bit_rate);

    if (track->vos_len) {
        // DecoderSpecific info descriptor
        put_descr(pb, 0x05, track->vos_len);
        avio_write(pb, track->vos_data, track->vos_len);
    }

    // SL descriptor
    put_descr(pb, 0x06, 1);
    avio_w8(pb, 0x02);
    return update_size(pb, pos);
}

从代码可以大概看出 track->vos_data 是 AudioSpecificConfig。 mov_write_esds_tag 的调用链条为
mov_write_trailer -->
mov_write_moov_tag -->
mov_write_trak_tag -->
mov_write_mdia_tag -->
mov_write_minf_tag -->
mov_write_stbl_tag -->
mov_write_stsd_tag -->
mov_write_audio_tag -->
mov_write_wave_tag -->
mov_write_esds_tag

不得不吐槽一下，mp4格式的套娃现象也非常严重。继续看track->vos_data是怎么来的

static int mov_write_header(AVFormatContext *s)
{
    ...
    for (i = 0; i < s->nb_streams; i++) {
        int j;
        AVStream *st= s->streams[i];
        MOVTrack *track= &mov->tracks[i];

        /* copy extradata if it exists */
        if (st->codecpar->extradata_size) {
            if (st->codecpar->codec_id == AV_CODEC_ID_DVD_SUBTITLE)
                mov_create_dvd_sub_decoder_specific_info(track, st);
            else if (!TAG_IS_AVCI(track->tag) && st->codecpar->codec_id != AV_CODEC_ID_DNXHD) {
                track->vos_len  = st->codecpar->extradata_size;
                track->vos_data = av_malloc(track->vos_len + AV_INPUT_BUFFER_PADDING_SIZE);
                if (!track->vos_data) {
                    return AVERROR(ENOMEM);
                }
                memcpy(track->vos_data, st->codecpar->extradata, track->vos_len);
                memset(track->vos_data + track->vos_len, 0, AV_INPUT_BUFFER_PADDING_SIZE);
            }
        }

可见vos_data就是codec的extradata。顺便看一下mov解复用的时候是怎么处理esds tag的
libavformat/mov_esds.c

int ff_mov_read_esds(AVFormatContext *fc, AVIOContext *pb)
{
    AVStream *st;
    int tag, ret = 0;

    if (fc->nb_streams < 1)
        return 0;
    st = fc->streams[fc->nb_streams-1];

    avio_rb32(pb); /* version + flags */
    ff_mp4_read_descr(fc, pb, &tag);
    if (tag == MP4ESDescrTag) {
        ff_mp4_parse_es_descr(pb, NULL);
    } else
        avio_rb16(pb); /* ID */

    ff_mp4_read_descr(fc, pb, &tag);
    if (tag == MP4DecConfigDescrTag)
        ret = ff_mp4_read_dec_config_descr(fc, st, pb);

    return ret;
}

libavformat/isom.c

int ff_mp4_read_dec_config_descr(AVFormatContext *fc, AVStream *st, AVIOContext *pb)
{
    enum AVCodecID codec_id;
    int len, tag;
    int ret;
    int object_type_id = avio_r8(pb);
    avio_r8(pb); /* stream type */
    avio_rb24(pb); /* buffer size db */
    avio_rb32(pb); /* rc_max_rate */

    st->codecpar->bit_rate = avio_rb32(pb); /* avg bitrate */

    codec_id= ff_codec_get_id(ff_mp4_obj_type, object_type_id);
    if (codec_id)
        st->codecpar->codec_id = codec_id;
    av_log(fc, AV_LOG_TRACE, "esds object type id 0x%02x\n", object_type_id);
    len = ff_mp4_read_descr(fc, pb, &tag);
    if (tag == MP4DecSpecificDescrTag) {
        av_log(fc, AV_LOG_TRACE, "Specific MPEG-4 header len=%d\n", len);
        /* As per 14496-3:2009 9.D.2.2, No decSpecificInfo is defined
           for MPEG-1 Audio or MPEG-2 Audio; MPEG-2 AAC excluded. */
        if (object_type_id == 0x69 || object_type_id == 0x6b)
            return 0;
        if (!len || (uint64_t)len > (1<<30))
            return AVERROR_INVALIDDATA;
        if ((ret = ff_get_extradata(fc, st->codecpar, pb, len)) < 0)
            return ret;
        if (st->codecpar->codec_id == AV_CODEC_ID_AAC) {
            MPEG4AudioConfig cfg = {0};
            ret = avpriv_mpeg4audio_get_config2(&cfg, st->codecpar->extradata,
                                                st->codecpar->extradata_size, 1, fc);
            if (ret < 0)
                return ret;
            st->codecpar->ch_layout.order = AV_CHANNEL_ORDER_UNSPEC;
            st->codecpar->ch_layout.nb_channels = cfg.channels;
            if (cfg.object_type == 29 && cfg.sampling_index < 3) // old mp3on4
                st->codecpar->sample_rate = ff_mpa_freq_tab[cfg.sampling_index];
            else if (cfg.ext_sample_rate)
                st->codecpar->sample_rate = cfg.ext_sample_rate;
            else
                st->codecpar->sample_rate = cfg.sample_rate;
            av_log(fc, AV_LOG_TRACE, "mp4a config channels %d obj %d ext obj %d "
                    "sample rate %d ext sample rate %d\n", cfg.channels,
                    cfg.object_type, cfg.ext_object_type,
                    cfg.sample_rate, cfg.ext_sample_rate);
            if (!(st->codecpar->codec_id = ff_codec_get_id(mp4_audio_types,
                                                        cfg.object_type)))
                st->codecpar->codec_id = AV_CODEC_ID_AAC;
        }
    }
    return 0;
}

通过 ff_get_extradata，已经读到 st->codecpar 的extradata了。