参考链接：

FFmpeg源代码简单分析：av_write_trailer()_雷霄骅的博客-CSDN博客_av_malloc

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);

它只需要指定一个参数，即用于输出的AVFormatContext。
函数正常执行后返回值等于0。
av_write_trailer()的定义位于libavformat\mux.c，如下所示。

int av_write_trailer(AVFormatContext *s)
{FFFormatContext *const si = ffformatcontext(s);AVPacket *const pkt = si->parse_pkt;int ret1, ret = 0;for (unsigned i = 0; i < s->nb_streams; i++) {AVStream *const st  = s->streams[i];FFStream *const sti = ffstream(st);if (sti->bsfc) {ret1 = write_packets_from_bsfs(s, st, pkt, 1/*interleaved*/);if (ret1 < 0)av_packet_unref(pkt);if (ret >= 0)ret = ret1;}}ret1 = interleaved_write_packet(s, pkt, 1, 0);if (ret >= 0)ret = ret1;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);}}deinit_muxer(s);if (s->pb)avio_flush(s->pb);if (ret == 0)ret = s->pb ? s->pb->error : 0;for (unsigned i = 0; i < s->nb_streams; i++) {av_freep(&s->streams[i]->priv_data);av_freep(&ffstream(s->streams[i])->index_entries);}if (s->oformat->priv_class)av_opt_free(s->priv_data);av_freep(&s->priv_data);av_packet_unref(si->pkt);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）循环调用输出将还未输出的AVPacket。
- （2）调用AVOutputFormat的write_trailer()，输出文件尾。

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。

const 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：位于end：之后的两段代码
- （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 */
}

可以参考FLV封装格式理解上述函数。AVCVIDEOPACKET的格式，如下所示

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。

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FFmpeg源代码简单分析-编码-av_write_trailer()

参考链接：

av_write_trailer()

AVOutputFormat->write_trailer()

flv_write_trailer()

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