AVInputFormat ff_flv_demuxer = {
    .name           = "flv",
    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
    .priv_data_size = sizeof(FLVContext),
    .read_probe     = flv_probe,
    .read_header    = flv_read_header,
    .read_packet    = flv_read_packet,
    .read_seek      = flv_read_seek,
    .read_close     = flv_read_close,
    .extensions     = "flv",
    .priv_class     = &flv_class,
};

static int flv_read_header(AVFormatContext *s)
{
    int offset, flags;

    avio_skip(s->pb, 4); //将flv的头去掉
    flags = avio_r8(s->pb);//读取flv的video和audio flag信息

    s->ctx_flags |= AVFMTCTX_NOHEADER;

    if (flags & FLV_HEADER_FLAG_HASVIDEO) //如果flv中包含video, 创建视频流
        if (!create_stream(s, AVMEDIA_TYPE_VIDEO))
            return AVERROR(ENOMEM);
    if (flags & FLV_HEADER_FLAG_HASAUDIO) //如果flv中包含audio,创建音频流
        if (!create_stream(s, AVMEDIA_TYPE_AUDIO))
            return AVERROR(ENOMEM);

    offset = avio_rb32(s->pb);//获取文件头长度,通常为9
    avio_seek(s->pb, offset, SEEK_SET);//跳过DataOffset字段
    avio_skip(s->pb, 4);//跳过Body中的PreviousTagSize0字段，大小即为4bytes

    s->start_time = 0;

    return 0;
}

static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    FLVContext *flv = s->priv_data;
    int ret, i, size, flags;
    enum FlvTagType type;
    int stream_type=-1;
    int64_t next, pos, meta_pos;
    int64_t dts, pts = AV_NOPTS_VALUE;
    int av_uninit(channels);
    int av_uninit(sample_rate);
    AVStream *st    = NULL;

    /* pkt size is repeated at end. skip it */
    for (;; avio_skip(s->pb, 4)) {
        pos  = avio_tell(s->pb);
        type = (avio_r8(s->pb) & 0x1F);//获取tag的类型，分为三种:0x08:AUDIO,0x09:VIDOE0x12:META
        size = avio_rb24(s->pb);//tag的长度
        dts  = avio_rb24(s->pb);//获取时间戳
        dts |= avio_r8(s->pb) << 24;//获取时间戳扩展
        av_log(s, AV_LOG_TRACE, "type:%d, size:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, dts, avio_tell(s->pb));
        if (avio_feof(s->pb))
            return AVERROR_EOF;
        avio_skip(s->pb, 3); /* stream id, always 0 */
        flags = 0;

        ...

        if (size == 0)
            continue;

        next = size + avio_tell(s->pb);

        if (type == FLV_TAG_TYPE_AUDIO) {
            stream_type = FLV_STREAM_TYPE_AUDIO;
            flags    = avio_r8(s->pb);
            size--;
        } else if (type == FLV_TAG_TYPE_VIDEO) {
            stream_type = FLV_STREAM_TYPE_VIDEO;
            flags    = avio_r8(s->pb);
            size--;
            if ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_VIDEO_INFO_CMD)
                goto skip;
        } else if (type == FLV_TAG_TYPE_META) {
            stream_type=FLV_STREAM_TYPE_DATA;
            if (size > 13 + 1 + 4) { // Header-type metadata stuff
                int type;
                meta_pos = avio_tell(s->pb);
                type = flv_read_metabody(s, next);
                if (type == 0 && dts == 0 || type < 0 || type == TYPE_UNKNOWN) {
                    goto skip;
                } else if (type == TYPE_ONTEXTDATA) {
                    avpriv_request_sample(s, "OnTextData packet");
                    return flv_data_packet(s, pkt, dts, next);
                } else if (type == TYPE_ONCAPTION) {
                    return flv_data_packet(s, pkt, dts, next);
                }
                avio_seek(s->pb, meta_pos, SEEK_SET);
            }
        } else {
            av_log(s, AV_LOG_DEBUG,
                   "Skipping flv packet: type %d, size %d, flags %d.\n",
                   type, size, flags);