参考链接

• FFmpeg源代码简单分析：av_write_frame()_雷霄骅的博客-CSDN博客_av_write_frame

av_write_frame()

• av_write_frame()用于输出一帧视音频数据，它的声明位于libavformat\avformat.h，如下所示。

/*** Write a packet to an output media file.** This function passes the packet directly to the muxer, without any buffering* or reordering. The caller is responsible for correctly interleaving the* packets if the format requires it. Callers that want libavformat to handle* the interleaving should call av_interleaved_write_frame() instead of this* function.** @param s media file handle* @param pkt The packet containing the data to be written. Note that unlike*            av_interleaved_write_frame(), this function does not take*            ownership of the packet passed to it (though some muxers may make*            an internal reference to the input packet).*            <br>*            This parameter can be NULL (at any time, not just at the end), in*            order to immediately flush data buffered within the muxer, for*            muxers that buffer up data internally before writing it to the*            output.*            <br>*            Packet's @ref AVPacket.stream_index "stream_index" field must be*            set to the index of the corresponding stream in @ref*            AVFormatContext.streams "s->streams".*            <br>*            The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")*            must be set to correct values in the stream's timebase (unless the*            output format is flagged with the AVFMT_NOTIMESTAMPS flag, then*            they can be set to AV_NOPTS_VALUE).*            The dts for subsequent packets passed to this function must be strictly*            increasing when compared in their respective timebases (unless the*            output format is flagged with the AVFMT_TS_NONSTRICT, then they*            merely have to be nondecreasing).  @ref AVPacket.duration*            "duration") should also be set if known.* @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush** @see av_interleaved_write_frame()*/
int av_write_frame(AVFormatContext *s, AVPacket *pkt);

• 简单解释一下它的参数的含义：
  • s：用于输出的AVFormatContext。
  • pkt：等待输出的AVPacket。
• 函数正常执行后返回值等于0。 
• av_write_frame()的定义位于libavformat\mux.c，如下所示。

int av_write_frame(AVFormatContext *s, AVPacket *in)
{FFFormatContext *const si = ffformatcontext(s);AVPacket *pkt = si->parse_pkt;int ret;if (!in) {if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {ret = s->oformat->write_packet(s, NULL);flush_if_needed(s);if (ret >= 0 && s->pb && s->pb->error < 0)ret = s->pb->error;return ret;}return 1;}if (in->flags & AV_PKT_FLAG_UNCODED_FRAME) {pkt = in;} else {/* We don't own in, so we have to make sure not to modify it.* (ff_write_chained() relies on this fact.)* The following avoids copying in's data unnecessarily.* Copying side data is unavoidable as a bitstream filter* may change it, e.g. free it on errors. */pkt->data = in->data;pkt->size = in->size;ret = av_packet_copy_props(pkt, in);if (ret < 0)return ret;if (in->buf) {pkt->buf = av_buffer_ref(in->buf);if (!pkt->buf) {ret = AVERROR(ENOMEM);goto fail;}}}ret = write_packets_common(s, pkt, 0/*non-interleaved*/);fail:// Uncoded frames using the noninterleaved codepath are also freed hereav_packet_unref(pkt);return ret;
}

• 从源代码可以看出，av_write_frame()主要完成了以下几步工作：版本差异
• （1）调用check_packet()做一些简单的检测，新版本目前将check_packet放进write_packets_common函数内
• （2）调用compute_pkt_fields2()设置AVPacket的一些属性值   compute_pkt_fields2函数感觉已被弃用，其功能合并进其余函数
• （3）调用write_packets_common()写入数据

• 如果AVPacket中的flag标记中包含AV_PKT_FLAG_UNCODED_FRAME，就会调用AVOutputFormat的write_uncoded_frame()函数（对应上文代码，pkt=in）；如果不包含那个标记，就会调用write_packet()函数（执行else里面的内容）。 

 av_write_frame

static int write_packets_common(AVFormatContext *s, AVPacket *pkt, int interleaved)
{AVStream *st;FFStream *sti;int ret = check_packet(s, pkt);if (ret < 0)return ret;st = s->streams[pkt->stream_index];sti = ffstream(st);ret = prepare_input_packet(s, st, pkt);if (ret < 0)return ret;ret = check_bitstream(s, sti, pkt);if (ret < 0)return ret;if (sti->bsfc) {return write_packets_from_bsfs(s, st, pkt, interleaved);} else {return write_packet_common(s, st, pkt, interleaved);}
}

check_packet()

• check_packet()定义位于libavformat\mux.c，如下所示。
• 从代码中可以看出，check_packet()的功能比较简单：
• 然后检查一下AVPacket的stream_index（标记了该AVPacket所属的AVStream）设置是否正常，如果为负数或者大于AVStream的个数，则返回错误信息；
• 最后检查AVPacket所属的AVStream是否属于attachment stream，这个地方没见过，目前还没有研究。

static int check_packet(AVFormatContext *s, AVPacket *pkt)
{if (pkt->stream_index < 0 || pkt->stream_index >= s->nb_streams) {av_log(s, AV_LOG_ERROR, "Invalid packet stream index: %d\n",pkt->stream_index);return AVERROR(EINVAL);}if (s->streams[pkt->stream_index]->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT) {av_log(s, AV_LOG_ERROR, "Received a packet for an attachment stream.\n");return AVERROR(EINVAL);}return 0;
}

AVOutputFormat->write_packet()

• write_packet()函数的定义位于libavformat\mux.c，如下所示。

    /*** Write a packet. If AVFMT_ALLOW_FLUSH is set in flags,* pkt can be NULL in order to flush data buffered in the muxer.* When flushing, return 0 if there still is more data to flush,* or 1 if everything was flushed and there is no more buffered* data.*/int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);

static int write_packet(AVFormatContext *s1, AVPacket *pkt)
{const V4L2Context *s = s1->priv_data;if (write(s->fd, pkt->data, pkt->size) == -1)return AVERROR(errno);return 0;
}

• write_packet()实际上是一个函数指针，指向特定的AVOutputFormat中的实现函数。
• 例如，我们看一下FLV对应的AVOutputFormat，位于libavformat\flvenc.c，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ff_flv_muxer的定义可以看出，write_packet()指向的是flv_write_packet()函数。
• 在看flv_write_packet()函数的定义之前，我们先回顾一下FLV封装格式的结构。

FLV封装格式

• FLV封装格式如下图所示。
• PS：原图是网上找的，感觉画的很清晰，比官方的Video File Format Specification更加通俗易懂。但是图中有一个错误，就是TagHeader中的StreamID字段的长度写错了（查看了一下官方标准，应该是3字节，现在已经改过来了）。

• 从FLV的封装格式结构可以看出，它的文件数据是一个一个的Tag连接起来的，中间间隔包含着Previous Tag Size。
• 因此，flv_write_packet()函数的任务就是写入一个Tag和Previous Tag Size。
• 下面简单记录一下Tag Data的格式。
• Tag Data根据Tag的Type不同而不同：可以分为音频Tag Data，视频Tag Data以及Script Tag Data。
• 下面简述一下音频Tag Data和视频Tag Data。

Audio Tag Data

• Audio Tag在官方标准中定义如下

• Audio Tag开始的第1个字节包含了音频数据的参数信息，从第2个字节开始为音频流数据。
• 第1个字节的前4位的数值表示了音频数据格式：
  • 0 = Linear PCM, platform endian
  • 1 = ADPCM 2 = MP3
  • 3 = Linear PCM, little endian
  • 4 = Nellymoser 16-kHz mono
  • 5 = Nellymoser 8-kHz mono
  • 6 = Nellymoser
  • 7 = G.711 A-law logarithmic PCM
  • 8 = G.711 mu-law logarithmic PCM
  • 9 = reserved
  • 10 = AAC
  • 14 = MP3 8-Khz
  • 15 = Device-specific sound
• 第1个字节的第5-6位的数值表示采样率：0 = 5.5kHz，1 = 11KHz，2 = 22 kHz，3 = 44 kHz。
• 第1个字节的第7位表示采样精度：0 = 8bits，1 = 16bits。
• 第1个字节的第8位表示音频类型：0 = sndMono，1 = sndStereo。
• 其中，当音频编码为AAC的时候，第一个字节后面存储的是AACAUDIODATA，格式如下所示。 

 Video Tag Data

• Video Tag在官方标准中的定义如下

•  Video Tag也用开始的第1个字节包含视频数据的参数信息，从第2个字节为视频流数据。
• 第1个字节的前4位的数值表示帧类型（FrameType）：
  • 1: keyframe (for AVC, a seekableframe)（关键帧）
  • 2: inter frame (for AVC, a nonseekableframe)
  • 3: disposable inter frame (H.263only)
  • 4: generated keyframe (reservedfor server use only)
  • 5: video info/command frame
• 第1个字节的后4位的数值表示视频编码ID（CodecID）：
  • 1: JPEG (currently unused)
  • 2: Sorenson H.263
  • 3: Screen video
  • 4: On2 VP6
  • 5: On2 VP6 with alpha channel
  • 6: Screen video version 2
  • 7: AVC
• 其中，当音频编码为AVC（H.264）的时候，第一个字节后面存储的是AVCVIDEOPACKET，格式如下所示

 flv_write_packet()

• 下面我们看一下FLV格式中write_packet()对应的实现函数flv_write_packet()的定义，位于libavformat\flvenc.c，如下所示。

static int flv_write_packet(AVFormatContext *s, AVPacket *pkt)
{AVIOContext *pb      = s->pb;AVCodecParameters *par = s->streams[pkt->stream_index]->codecpar;FLVContext *flv      = s->priv_data;FLVStreamContext *sc = s->streams[pkt->stream_index]->priv_data;unsigned ts;int size = pkt->size;uint8_t *data = NULL;int flags = -1, flags_size, ret = 0;int64_t cur_offset = avio_tell(pb);if (par->codec_type == AVMEDIA_TYPE_AUDIO && !pkt->size) {av_log(s, AV_LOG_WARNING, "Empty audio Packet\n");return AVERROR(EINVAL);}if (par->codec_id == AV_CODEC_ID_VP6F || par->codec_id == AV_CODEC_ID_VP6A ||par->codec_id == AV_CODEC_ID_VP6  || par->codec_id == AV_CODEC_ID_AAC)flags_size = 2;else if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4)flags_size = 5;elseflags_size = 1;if (par->codec_id == AV_CODEC_ID_AAC || par->codec_id == AV_CODEC_ID_H264|| par->codec_id == AV_CODEC_ID_MPEG4) {size_t side_size;uint8_t *side = av_packet_get_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);if (side && side_size > 0 && (side_size != par->extradata_size || memcmp(side, par->extradata, side_size))) {ret = ff_alloc_extradata(par, side_size);if (ret < 0)return ret;memcpy(par->extradata, side, side_size);flv_write_codec_header(s, par, pkt->dts);}}if (flv->delay == AV_NOPTS_VALUE)flv->delay = -pkt->dts;if (pkt->dts < -flv->delay) {av_log(s, AV_LOG_WARNING,"Packets are not in the proper order with respect to DTS\n");return AVERROR(EINVAL);}if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4) {if (pkt->pts == AV_NOPTS_VALUE) {av_log(s, AV_LOG_ERROR, "Packet is missing PTS\n");return AVERROR(EINVAL);}}ts = pkt->dts;if (s->event_flags & AVSTREAM_EVENT_FLAG_METADATA_UPDATED) {write_metadata(s, ts);s->event_flags &= ~AVSTREAM_EVENT_FLAG_METADATA_UPDATED;}avio_write_marker(pb, av_rescale(ts, AV_TIME_BASE, 1000),pkt->flags & AV_PKT_FLAG_KEY && (flv->video_par ? par->codec_type == AVMEDIA_TYPE_VIDEO : 1) ? AVIO_DATA_MARKER_SYNC_POINT : AVIO_DATA_MARKER_BOUNDARY_POINT);switch (par->codec_type) {case AVMEDIA_TYPE_VIDEO:avio_w8(pb, FLV_TAG_TYPE_VIDEO);flags = ff_codec_get_tag(flv_video_codec_ids, par->codec_id);flags |= pkt->flags & AV_PKT_FLAG_KEY ? FLV_FRAME_KEY : FLV_FRAME_INTER;break;case AVMEDIA_TYPE_AUDIO:flags = get_audio_flags(s, par);av_assert0(size);avio_w8(pb, FLV_TAG_TYPE_AUDIO);break;case AVMEDIA_TYPE_SUBTITLE:case AVMEDIA_TYPE_DATA:avio_w8(pb, FLV_TAG_TYPE_META);break;default:return AVERROR(EINVAL);}if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4) {/* check if extradata looks like mp4 formatted */if (par->extradata_size > 0 && *(uint8_t*)par->extradata != 1)if ((ret = ff_avc_parse_nal_units_buf(pkt->data, &data, &size)) < 0)return ret;} else if (par->codec_id == AV_CODEC_ID_AAC && pkt->size > 2 &&(AV_RB16(pkt->data) & 0xfff0) == 0xfff0) {if (!s->streams[pkt->stream_index]->nb_frames) {av_log(s, AV_LOG_ERROR, "Malformed AAC bitstream detected: ""use the audio bitstream filter 'aac_adtstoasc' to fix it ""('-bsf:a aac_adtstoasc' option with ffmpeg)\n");return AVERROR_INVALIDDATA;}av_log(s, AV_LOG_WARNING, "aac bitstream error\n");}/* check Speex packet duration */if (par->codec_id == AV_CODEC_ID_SPEEX && ts - sc->last_ts > 160)av_log(s, AV_LOG_WARNING, "Warning: Speex stream has more than ""8 frames per packet. Adobe Flash ""Player cannot handle this!\n");if (sc->last_ts < ts)sc->last_ts = ts;if (size + flags_size >= 1<<24) {av_log(s, AV_LOG_ERROR, "Too large packet with size %u >= %u\n",size + flags_size, 1<<24);ret = AVERROR(EINVAL);goto fail;}avio_wb24(pb, size + flags_size);put_timestamp(pb, ts);avio_wb24(pb, flv->reserved);if (par->codec_type == AVMEDIA_TYPE_DATA ||par->codec_type == AVMEDIA_TYPE_SUBTITLE ) {int data_size;int64_t metadata_size_pos = avio_tell(pb);if (par->codec_id == AV_CODEC_ID_TEXT) {// legacy FFmpeg magic?avio_w8(pb, AMF_DATA_TYPE_STRING);put_amf_string(pb, "onTextData");avio_w8(pb, AMF_DATA_TYPE_MIXEDARRAY);avio_wb32(pb, 2);put_amf_string(pb, "type");avio_w8(pb, AMF_DATA_TYPE_STRING);put_amf_string(pb, "Text");put_amf_string(pb, "text");avio_w8(pb, AMF_DATA_TYPE_STRING);put_amf_string(pb, pkt->data);put_amf_string(pb, "");avio_w8(pb, AMF_END_OF_OBJECT);} else {// just pass the metadata throughavio_write(pb, data ? data : pkt->data, size);}/* write total size of tag */data_size = avio_tell(pb) - metadata_size_pos;avio_seek(pb, metadata_size_pos - 10, SEEK_SET);avio_wb24(pb, data_size);avio_seek(pb, data_size + 10 - 3, SEEK_CUR);avio_wb32(pb, data_size + 11);} else {av_assert1(flags>=0);avio_w8(pb,flags);if (par->codec_id == AV_CODEC_ID_VP6)avio_w8(pb,0);if (par->codec_id == AV_CODEC_ID_VP6F || par->codec_id == AV_CODEC_ID_VP6A) {if (par->extradata_size)avio_w8(pb, par->extradata[0]);elseavio_w8(pb, ((FFALIGN(par->width,  16) - par->width) << 4) |(FFALIGN(par->height, 16) - par->height));} else if (par->codec_id == AV_CODEC_ID_AAC)avio_w8(pb, 1); // AAC rawelse if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4) {avio_w8(pb, 1); // AVC NALUavio_wb24(pb, pkt->pts - pkt->dts);}avio_write(pb, data ? data : pkt->data, size);avio_wb32(pb, size + flags_size + 11); // previous tag sizeflv->duration = FFMAX(flv->duration,pkt->pts + flv->delay + pkt->duration);}if (flv->flags & FLV_ADD_KEYFRAME_INDEX) {switch (par->codec_type) {case AVMEDIA_TYPE_VIDEO:flv->videosize += (avio_tell(pb) - cur_offset);flv->lasttimestamp = flv->acurframeindex / flv->framerate;flv->acurframeindex++;if (pkt->flags & AV_PKT_FLAG_KEY) {double ts = flv->lasttimestamp;int64_t pos = cur_offset;flv->lastkeyframetimestamp = ts;flv->lastkeyframelocation = pos;ret = flv_append_keyframe_info(s, flv, ts, pos);if (ret < 0)goto fail;}break;case AVMEDIA_TYPE_AUDIO:flv->audiosize += (avio_tell(pb) - cur_offset);break;default:av_log(s, AV_LOG_WARNING, "par->codec_type is type = [%d]\n", par->codec_type);break;}}
fail:av_free(data);return ret;
}

• 我们通过源代码简单梳理一下flv_write_packet()在写入H.264/AAC时候的流程：
• （1）写入Tag Header的Type，如果是视频，代码如下：avio_w8(pb, FLV_TAG_TYPE_VIDEO);   如果是音频，代码如下： avio_w8(pb, FLV_TAG_TYPE_AUDIO);
• （2）写入Tag Header的Datasize，Timestamp和StreamID（至此完成Tag Header）：

    //Tag Header - Datasizeavio_wb24(pb, size + flags_size);//Tag Header - Timestamp   无定义avio_wb24(pb, ts & 0xFFFFFF);avio_w8(pb, (ts >> 24) & 0x7F); // timestamps are 32 bits _signed_//StreamIDavio_wb24(pb, flv->reserved);

• （3）写入Tag Data的第一字节（其中flag已经在前面的代码中设置完毕）：
• //First Byte of Tag Data
• avio_w8(pb,flags);
• （4）如果编码格式VP6作相应的处理（不研究）；编码格式为AAC，写入AACAUDIODATA；编码格式为H.264，写入AVCVIDEOPACKET：

        if (enc->codec_id == AV_CODEC_ID_VP6F || enc->codec_id == AV_CODEC_ID_VP6A) {if (enc->extradata_size)avio_w8(pb, enc->extradata[0]);elseavio_w8(pb, ((FFALIGN(enc->width,  16) - enc->width) << 4) |(FFALIGN(enc->height, 16) - enc->height));} else if (enc->codec_id == AV_CODEC_ID_AAC)avio_w8(pb, 1); // AAC rawelse if (enc->codec_id == AV_CODEC_ID_H264 || enc->codec_id == AV_CODEC_ID_MPEG4) {//AVCVIDEOPACKET-AVCPacketTypeavio_w8(pb, 1); // AVC NALU//AVCVIDEOPACKET-CompositionTimeavio_wb24(pb, pkt->pts - pkt->dts);}

• （5）写入数据：avio_write(pb, data ? data : pkt->data, size);  //Data
• （6）  写入previous tag size：avio_wb32(pb, size + flags_size + 11); // previous tag size
• 至此，flv_write_packet()就完成了一个Tag的写入。