ffmpeg源码分析：av_write_trailer()

av_write_trailer()用于输出文件尾，它的声明位于libavformat\avformat.h，如下所示。

/**
 * 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);

av_write_trailer()的定义位于libavformat\mux.c，如下所示。

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;

        ret = write_packet(s, &pkt);
        if (ret >= 0)
            s->streams[pkt.stream_index]->nb_frames++;

        av_packet_unref(&pkt);

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

fail:
    if (s->oformat->write_trailer) {
        if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
            avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_TRAILER);
        if (ret >= 0) {
        ret = s->oformat->write_trailer(s);
        } else {
            s->oformat->write_trailer(s);
        }
    }

    if (s->oformat->deinit)
        s->oformat->deinit(s);

    s->internal->initialized =
    s->internal->streams_initialized = 0;

    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的write_trailer()是一个函数指针，指向特定的AVOutputFormat中的实现函数。我们以FLV对应的AVOutputFormat为例，看一下它的定义，如下所示。

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,
    .init           = flv_init,
    .write_header   = flv_write_header,
    .write_packet   = flv_write_packet,
    .write_trailer  = flv_write_trailer,
    .check_bitstream= flv_check_bitstream,
    .codec_tag      = (const AVCodecTag* const []) {
                          flv_video_codec_ids, flv_audio_codec_ids, 0
                      },
    .flags          = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
                      AVFMT_TS_NONSTRICT,
    .priv_class     = &flv_muxer_class,
};

      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 build_keyframes_idx = flv->flags & FLV_ADD_KEYFRAME_INDEX;
    int i, res;
    int64_t cur_pos = avio_tell(s->pb);

    if (build_keyframes_idx) {
        FLVFileposition *newflv_posinfo, *p;

        avio_seek(pb, flv->videosize_offset, SEEK_SET);
        put_amf_double(pb, flv->videosize);

        avio_seek(pb, flv->audiosize_offset, SEEK_SET);
        put_amf_double(pb, flv->audiosize);

        avio_seek(pb, flv->lasttimestamp_offset, SEEK_SET);
        put_amf_double(pb, flv->lasttimestamp);

        avio_seek(pb, flv->lastkeyframetimestamp_offset, SEEK_SET);
        put_amf_double(pb, flv->lastkeyframetimestamp);

        avio_seek(pb, flv->lastkeyframelocation_offset, SEEK_SET);
        put_amf_double(pb, flv->lastkeyframelocation + flv->keyframe_index_size);
        avio_seek(pb, cur_pos, SEEK_SET);

        res = shift_data(s);
        if (res < 0) {
             goto end;
        }
        avio_seek(pb, flv->keyframes_info_offset, SEEK_SET);
        put_amf_string(pb, "filepositions");
        put_amf_dword_array(pb, flv->filepositions_count);
        for (newflv_posinfo = flv->head_filepositions; newflv_posinfo; newflv_posinfo = newflv_posinfo->next) {
            put_amf_double(pb, newflv_posinfo->keyframe_position + flv->keyframe_index_size);
        }

        put_amf_string(pb, "times");
        put_amf_dword_array(pb, flv->filepositions_count);
        for (newflv_posinfo = flv->head_filepositions; newflv_posinfo; newflv_posinfo = newflv_posinfo->next) {
            put_amf_double(pb, newflv_posinfo->keyframe_timestamp);
        }

        newflv_posinfo = flv->head_filepositions;
        while (newflv_posinfo) {
            p = newflv_posinfo->next;
            if (p) {
                newflv_posinfo->next = p->next;
                av_free(p);
                p = NULL;
            } else {
                av_free(newflv_posinfo);
                newflv_posinfo = NULL;
            }
        }

        put_amf_string(pb, "");
        avio_w8(pb, AMF_END_OF_OBJECT);

        avio_seek(pb, cur_pos + flv->keyframe_index_size, SEEK_SET);
    }

end:
    if (flv->flags & FLV_NO_SEQUENCE_END) {
        av_log(s, AV_LOG_DEBUG, "FLV no sequence end mode open\n");
    } else {
        /* Add EOS tag */
        for (i = 0; i < s->nb_streams; i++) {
            AVCodecParameters *par = s->streams[i]->codecpar;
            FLVStreamContext *sc = s->streams[i]->priv_data;
            if (par->codec_type == AVMEDIA_TYPE_VIDEO &&
                    (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4))
                put_avc_eos_tag(pb, sc->last_ts);
        }
    }

    file_size = avio_tell(pb);

    if (build_keyframes_idx) {
        flv->datasize = file_size - flv->datastart_offset;
        avio_seek(pb, flv->datasize_offset, SEEK_SET);
        put_amf_double(pb, flv->datasize);
    }
    if (!(flv->flags & FLV_NO_METADATA)) {
        if (!(flv->flags & FLV_NO_DURATION_FILESIZE)) {
            /* 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);
            }
        }
    }

    return 0;
}

从flv_write_trailer()的源代码可以看出该函数做了以下两步工作：
    （1）如果视频流是H.264，则添加包含EOS（End Of Stream） NALU的Tag。
    （2）更新FLV的时长信息，以及文件大小信息。
其中，put_avc_eos_tag()函数用于添加包含EOS NALU的Tag（包含结尾的一个PreviousTagSize），如下所示。

static void put_avc_eos_tag(AVIOContext *pb, unsigned ts)
{
    avio_w8(pb, FLV_TAG_TYPE_VIDEO);
    avio_wb24(pb, 5);               /* Tag Data Size */
    put_timestamp(pb, ts);
    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 */
}