FFmpeg源代码简单分析：av_write_trailer()

标签：ffmpeg   avpacket   flv   输出   源代码   

/**
 * Write the stream trailer to an output media file and free the
 * file private data.
 *
 * May only be called after a successful call to avformat_write_header.
 *
 * @param s media file handle
 * @return 0 if OK, AVERROR_xxx on error
 */
int av_write_trailer(AVFormatContext *s);

这2个函数最典型的例子可以参考：

最简单的基于FFMPEG的视频编码器（YUV编码为H.264）

函数调用关系图

av_write_trailer()的调用关系如下图所示。

av_write_trailer()

int av_write_trailer(AVFormatContext *s)
{
    int ret, i;

    for (;; ) {
        AVPacket pkt;
        ret = interleave_packet(s, &pkt, NULL, 1);
        if (ret < 0)
            goto fail;
        if (!ret)
            break;
        //写入AVPacket
        ret = write_packet(s, &pkt);
        if (ret >= 0)
            s->streams[pkt.stream_index]->nb_frames++;

        av_free_packet(&pkt);

        if (ret < 0)
            goto fail;
        if(s->pb && s->pb->error)
            goto fail;
    }

fail:
	//写文件尾
    if (s->oformat->write_trailer)
        if (ret >= 0) {
        ret = s->oformat->write_trailer(s);
        } else {
            s->oformat->write_trailer(s);
        }

    if (s->pb)
       avio_flush(s->pb);
    if (ret == 0)
       ret = s->pb ? s->pb->error : 0;
    for (i = 0; i < s->nb_streams; i++) {
        av_freep(&s->streams[i]->priv_data);
        av_freep(&s->streams[i]->index_entries);
    }
    if (s->oformat->priv_class)
        av_opt_free(s->priv_data);
    av_freep(&s->priv_data);
    return ret;
}

从源代码可以看出av_write_trailer()主要完成了以下两步工作：

（1）循环调用interleave_packet()以及write_packet()，将还未输出的AVPacket输出出来。

（2）调用AVOutputFormat的write_trailer()，输出文件尾。

其中第一步和av_write_frame()中的步骤大致是一样的（interleave_packet()这一部分在并不包含在av_write_frame()中，而是包含在av_interleaved_write_frame()中，这一部分源代码还没有分析），可以参考文章《FFmpeg源代码简单分析：av_write_frame()》。下面分析一下第二步。

AVOutputFormat->write_trailer()

AVOutputFormat ff_flv_muxer = {
    .name           = "flv",
    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
    .mime_type      = "video/x-flv",
    .extensions     = "flv",
    .priv_data_size = sizeof(FLVContext),
    .audio_codec    = CONFIG_LIBMP3LAME ? AV_CODEC_ID_MP3 : AV_CODEC_ID_ADPCM_SWF,
    .video_codec    = AV_CODEC_ID_FLV1,
    .write_header   = flv_write_header,
    .write_packet   = flv_write_packet,
    .write_trailer  = flv_write_trailer,
    .codec_tag      = (const AVCodecTag* const []) {
                          flv_video_codec_ids, flv_audio_codec_ids, 0
                      },
    .flags          = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
                      AVFMT_TS_NONSTRICT,
};

从FLV对应的AVOutputFormat结构体的定义我们可以看出，write_trailer()指向了flv_write_trailer()函数。

flv_write_trailer()

flv_write_trailer()函数的定义位于libavformat\flvenc.c，如下所示。

static int flv_write_trailer(AVFormatContext *s)
{
    int64_t file_size;

    AVIOContext *pb = s->pb;
    FLVContext *flv = s->priv_data;
    int i;

    /* Add EOS tag */
    for (i = 0; i < s->nb_streams; i++) {
        AVCodecContext *enc = s->streams[i]->codec;
        FLVStreamContext *sc = s->streams[i]->priv_data;
        if (enc->codec_type == AVMEDIA_TYPE_VIDEO &&
                (enc->codec_id == AV_CODEC_ID_H264 || enc->codec_id == AV_CODEC_ID_MPEG4))
            put_avc_eos_tag(pb, sc->last_ts);
    }

    file_size = avio_tell(pb);

    /* update information */
    if (avio_seek(pb, flv->duration_offset, SEEK_SET) < 0)
        av_log(s, AV_LOG_WARNING, "Failed to update header with correct duration.\n");
    else
        put_amf_double(pb, flv->duration / (double)1000);
    if (avio_seek(pb, flv->filesize_offset, SEEK_SET) < 0)
        av_log(s, AV_LOG_WARNING, "Failed to update header with correct filesize.\n");
    else
        put_amf_double(pb, file_size);

    avio_seek(pb, file_size, SEEK_SET);
    return 0;
}

static void put_avc_eos_tag(AVIOContext *pb, unsigned ts)
{
    avio_w8(pb, FLV_TAG_TYPE_VIDEO);
    avio_wb24(pb, 5);               /* Tag Data Size */
    avio_wb24(pb, ts);              /* lower 24 bits of timestamp in ms */
    avio_w8(pb, (ts >> 24) & 0x7F); /* MSB of ts in ms */
    avio_wb24(pb, 0);               /* StreamId = 0 */
    avio_w8(pb, 23);                /* ub[4] FrameType = 1, ub[4] CodecId = 7 */
    avio_w8(pb, 2);                 /* AVC end of sequence */
    avio_wb24(pb, 0);               /* Always 0 for AVC EOS. */
    avio_wb32(pb, 16);              /* Size of FLV tag */
}

 

可以看出包含EOS NALU的AVCVIDEOPACKET的AVCPacketType为2。在这种情况下，AVCVIDEOPACKET的CompositionTime字段取0，并且无需包含Data字段。

雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020

FFmpeg源代码简单分析：av_write_trailer()

标签：ffmpeg   avpacket   flv   输出   源代码   

原文地址：http://blog.csdn.net/leixiaohua1020/article/details/44201645