FFmpeg源码分析：写音视频帧av_write_frame()

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FFmpeg在libavformat模块提供音视频的muxer封装与demuxer解封装。其中muxer封装文件包括avformat_write_header()、av_write_frame()和av_write_trailer()。本文主要探讨av_write_frame函数如何写入音视频帧数据，包括音视频交错与音视频非交错两种情况。

av_write_frame函数位于libavformat/mux.c，写音视频帧的流程如下：

目录

1、av_write_frame

2、write_packets_common

3、write_packet_common

4、write_packet与interleaved_write_packet

5、s->oformat->write_packet

1、av_write_frame

av_write_frame的声明位于libavformat/avformat.h，具体如下：

/*** 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).* @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);

 大致翻译为：写一个数据包到输出媒体文件。这个函数直接传递数据包到封装器，没有任何缓存或重排序。如果格式需要，调用者要负责把数据包正确排列。如果调用者期望处理交错的数据包，应该调用av_interleaved_write_frame()函数而不是当前这个函数。

下面来看看无交错写音视频av_write_frame()函数的实现，位于libavformat/mux.c：

int av_write_frame(AVFormatContext *s, AVPacket *in)
{AVPacket *pkt = s->internal->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.* The following avoids copying in's data unnecessarily. */av_packet_unref(pkt);pkt->buf  = NULL;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:av_packet_unref(pkt);return ret;
}

再看看有交错写音视频帧av_interleaved_write_frame()函数的实现：

int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt)
{int ret;if (pkt) {ret = write_packets_common(s, pkt, 1/*interleaved*/);if (ret < 0)av_packet_unref(pkt);return ret;} else {av_log(s, AV_LOG_TRACE, "av_interleaved_write_frame FLUSH\n");return interleaved_write_packet(s, NULL, 1/*flush*/);}
}

经过对比，内部都是调用write_packets_common()函数，区别在于av_write_frame()传参是无交错，而av_interleaved_write_frame()传参是有交错。

2、write_packets_common

接下来看看write_packets_common()函数的实现：

static int write_packets_common(AVFormatContext *s, AVPacket *pkt, int interleaved)
{AVStream *st;// 检查pkt的stream_index和codec_typeint ret = check_packet(s, pkt);if (ret < 0)return ret;st = s->streams[pkt->stream_index];// 检查pkt的pts和dtsret = prepare_input_packet(s, st, pkt);if (ret < 0)return ret;// 调用s->oformat->check_bitstream()检查码流ret = check_bitstream(s, st, pkt);if (ret < 0)return ret;if (st->internal->bsfc) {// 经过bitstream filter处理再写数据包return write_packets_from_bsfs(s, st, pkt, interleaved);} else {// 通用方式写数据包return write_packet_common(s, st, pkt, interleaved);}
}

由源码可知，主要是分4个步骤处理：

1. 检查pkt的stream_index和codec_type；
2. 检查pkt的pts和dts；
3. 调用s->oformat->check_bitstream()检查码流；
4. 调用write_packets_from_bsfs或write_packet_common写数据包；

3、write_packet_common

write_packets_from_bsfs()函数主要是经过bitstream filter处理，比如h264要处理startcode起始码。我们主要探讨write_packet_common()函数的实现：

static int write_packet_common(AVFormatContext *s, AVStream *st, AVPacket *pkt, int interleaved)
{int ret;// 猜测pkt数据包的时长guess_pkt_duration(s, st, pkt);#if FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTXif ((ret = compute_muxer_pkt_fields(s, st, pkt)) < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))return ret;
#endifif (interleaved) {// 有交错写数据包if (pkt->dts == AV_NOPTS_VALUE && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))return AVERROR(EINVAL);return interleaved_write_packet(s, pkt, 0);} else {// 无交错写数据包return write_packet(s, pkt);}
}

由此可见，主要根据interleaved标志位判断，如果为有交错就调用interleaved_write_packet()函数写数据包，如果为无交错就调用write_packet()函数写数据包。

4、write_packet与interleaved_write_packet

先看看interleaved_write_packet()函数的实现：

static int interleaved_write_packet(AVFormatContext *s, AVPacket *pkt, int flush)
{for (;; ) {AVPacket opkt;int ret = interleave_packet(s, &opkt, pkt, flush);if (ret <= 0)return ret;pkt = NULL;ret = write_packet(s, &opkt);av_packet_unref(&opkt);if (ret < 0)return ret;}
}

由此可见，有交错写数据包先调用interleave_packet()函数进行交错排序，最终调用write_packet()函数写入数据包。接下来分析write_packet()函数的实现：

static int write_packet(AVFormatContext *s, AVPacket *pkt)
{int ret;// 根据output_ts_offset来校正dts和pts时间戳if (s->output_ts_offset) {AVStream *st = s->streams[pkt->stream_index];int64_t offset = av_rescale_q(s->output_ts_offset, AV_TIME_BASE_Q, st->time_base);if (pkt->dts != AV_NOPTS_VALUE)pkt->dts += offset;if (pkt->pts != AV_NOPTS_VALUE)pkt->pts += offset;}// 校正负数的时间戳if (s->avoid_negative_ts > 0) {AVStream *st = s->streams[pkt->stream_index];int64_t offset = st->internal->mux_ts_offset;int64_t ts = s->internal->avoid_negative_ts_use_pts ? pkt->pts : pkt->dts;if (s->internal->offset == AV_NOPTS_VALUE && ts != AV_NOPTS_VALUE &&(ts < 0 || s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_MAKE_ZERO)) {s->internal->offset = -ts;s->internal->offset_timebase = st->time_base;}if (s->internal->offset != AV_NOPTS_VALUE && !offset) {offset = st->internal->mux_ts_offset =av_rescale_q_rnd(s->internal->offset,s->internal->offset_timebase,st->time_base,AV_ROUND_UP);}if (pkt->dts != AV_NOPTS_VALUE)pkt->dts += offset;if (pkt->pts != AV_NOPTS_VALUE)pkt->pts += offset;}if ((pkt->flags & AV_PKT_FLAG_UNCODED_FRAME)) {AVFrame **frame = (AVFrame **)pkt->data;av_assert0(pkt->size == sizeof(*frame));// 写入未压缩编码的数据包ret = s->oformat->write_uncoded_frame(s, pkt->stream_index, frame, 0);} else {// 写入编码的数据包ret = s->oformat->write_packet(s, pkt);}if (s->pb && ret >= 0) {flush_if_needed(s);if (s->pb->error < 0)ret = s->pb->error;}if (ret >= 0)s->streams[pkt->stream_index]->nb_frames++;return ret;
}

 由源码可知，分3个步骤处理：

1. 根据output_ts_offset来校正dts和pts时间戳；
2. 校正负数的时间戳；
3. 写入未压缩编码的数据包或写入编码的数据包；

5、s->oformat->write_packet

 以mp4封装格式为例，位于libavformat/movenc.c，来看看mp4的AVOutputFormat：

AVOutputFormat ff_mp4_muxer = {.name              = "mp4",.long_name         = NULL_IF_CONFIG_SMALL("MP4 (MPEG-4 Part 14)"),.mime_type         = "video/mp4",.extensions        = "mp4",.priv_data_size    = sizeof(MOVMuxContext),.audio_codec       = AV_CODEC_ID_AAC,.video_codec       = CONFIG_LIBX264_ENCODER ?AV_CODEC_ID_H264 : AV_CODEC_ID_MPEG4,.init              = mov_init,.write_header      = mov_write_header,.write_packet      = mov_write_packet,.write_trailer     = mov_write_trailer,.deinit            = mov_free,.flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,.codec_tag         = mp4_codec_tags_list,.check_bitstream   = mov_check_bitstream,.priv_class        = &mp4_muxer_class,
};

此时write_packet函数指针指向mov_write_packet()，我们来探讨下mov_write_packet()函数实现：

static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
{MOVMuxContext *mov = s->priv_data;MOVTrack *trk;// 判断pkt是否为空if (!pkt) {mov_flush_fragment(s, 1);return 1;}trk = &mov->tracks[pkt->stream_index];if (is_cover_image(trk->st)) {int ret;if (trk->st->nb_frames >= 1) {return 0;}if ((ret = av_packet_ref(trk->cover_image, pkt)) < 0)return ret;return 0;} else {int i;if (!pkt->size)return mov_write_single_packet(s, pkt); /* Passthrough. *//** Subtitles require special handling.** 1) For full complaince, every track must have a sample at* dts == 0, which is rarely true for subtitles. So, as soon* as we see any packet with dts > 0, write an empty subtitle* at dts == 0 for any subtitle track with no samples in it.** 2) For each subtitle track, check if the current packet's* dts is past the duration of the last subtitle sample. If* so, we now need to write an end sample for that subtitle.** 3) See mov_write_trailer for how the final end sample is* handled.*/for (i = 0; i < mov->nb_streams; i++) {MOVTrack *trk = &mov->tracks[i];int ret;if (trk->par->codec_id == AV_CODEC_ID_MOV_TEXT &&trk->track_duration < pkt->dts &&(trk->entry == 0 || !trk->last_sample_is_subtitle_end)) {// 写入字幕流结束的数据包ret = mov_write_subtitle_end_packet(s, i, trk->track_duration);if (ret < 0) return ret;trk->last_sample_is_subtitle_end = 1;}}......// 写入单个数据包return mov_write_single_packet(s, pkt);}
}

由此可见，先判断pkt数据包是否为空，最终调用mov_write_single_packet()函数写单个数据包，函数实现如下：

static int mov_write_single_packet(AVFormatContext *s, AVPacket *pkt)
{MOVMuxContext *mov = s->priv_data;MOVTrack *trk = &mov->tracks[pkt->stream_index];AVCodecParameters *par = trk->par;int64_t frag_duration = 0;int size = pkt->size;// 判断pkt是否为空int ret = check_pkt(s, pkt);if (ret < 0)return ret;......return ff_mov_write_packet(s, pkt);
}

同样地，该函数也是先判断pk数据包是否为空，中间做一些逻辑处理，最终调用ff_mov_write_packet()函数来真正写数据包：

int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
{......// 如果存在extradata则进行拷贝if (trk->vos_len == 0 && par->extradata_size > 0 &&!TAG_IS_AVCI(trk->tag) &&(par->codec_id != AV_CODEC_ID_DNXHD)) {trk->vos_len  = par->extradata_size;trk->vos_data = av_malloc(trk->vos_len + AV_INPUT_BUFFER_PADDING_SIZE);if (!trk->vos_data) {ret = AVERROR(ENOMEM);goto err;}memcpy(trk->vos_data, par->extradata, trk->vos_len);memset(trk->vos_data + trk->vos_len, 0, AV_INPUT_BUFFER_PADDING_SIZE);}if ((par->codec_id == AV_CODEC_ID_DNXHD ||par->codec_id == AV_CODEC_ID_H264 ||par->codec_id == AV_CODEC_ID_HEVC ||par->codec_id == AV_CODEC_ID_TRUEHD ||par->codec_id == AV_CODEC_ID_AC3) && !trk->vos_len &&!TAG_IS_AVCI(trk->tag)) {/* copy frame to create needed atoms */trk->vos_len  = size;trk->vos_data = av_malloc(size + AV_INPUT_BUFFER_PADDING_SIZE);if (!trk->vos_data) {ret = AVERROR(ENOMEM);goto err;}memcpy(trk->vos_data, pkt->data, size);memset(trk->vos_data + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);}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) {return -1;}av_log(s, AV_LOG_WARNING, "aac bitstream error\n");}if (par->codec_id == AV_CODEC_ID_H264 && trk->vos_len > 0 && *(uint8_t *)trk->vos_data != 1 && !TAG_IS_AVCI(trk->tag)) {/* from x264 or from bytestream H.264 *//* NAL reformatting needed */if (trk->hint_track >= 0 && trk->hint_track < mov->nb_streams) {ret = ff_avc_parse_nal_units_buf(pkt->data, &reformatted_data,&size);if (ret < 0)return ret;avio_write(pb, reformatted_data, size);} else {if (trk->cenc.aes_ctr) {size = ff_mov_cenc_avc_parse_nal_units(&trk->cenc, pb, pkt->data, size);if (size < 0) {ret = size;goto err;}} else {size = ff_avc_parse_nal_units(pb, pkt->data, pkt->size);}}} else if (par->codec_id == AV_CODEC_ID_HEVC && trk->vos_len > 6 &&(AV_RB24(trk->vos_data) == 1 || AV_RB32(trk->vos_data) == 1)) {/* extradata is Annex B, assume the bitstream is too and convert it */if (trk->hint_track >= 0 && trk->hint_track < mov->nb_streams) {ret = ff_hevc_annexb2mp4_buf(pkt->data, &reformatted_data,&size, 0, NULL);if (ret < 0)return ret;avio_write(pb, reformatted_data, size);} else {size = ff_hevc_annexb2mp4(pb, pkt->data, pkt->size, 0, NULL);}} else if (par->codec_id == AV_CODEC_ID_AV1) {if (trk->hint_track >= 0 && trk->hint_track < mov->nb_streams) {ret = ff_av1_filter_obus_buf(pkt->data, &reformatted_data,&size, &offset);if (ret < 0)return ret;avio_write(pb, reformatted_data, size);} else {size = ff_av1_filter_obus(pb, pkt->data, pkt->size);}
#if CONFIG_AC3_PARSER} else if (par->codec_id == AV_CODEC_ID_EAC3) {size = handle_eac3(mov, pkt, trk);if (size < 0)return size;else if (!size)goto end;avio_write(pb, pkt->data, size);
#endif} else if (par->codec_id == AV_CODEC_ID_EIA_608) {size = 8;for (int i = 0; i < pkt->size; i += 3) {if (pkt->data[i] == 0xFC) {size += 2;}}avio_wb32(pb, size);ffio_wfourcc(pb, "cdat");for (int i = 0; i < pkt->size; i += 3) {if (pkt->data[i] == 0xFC) {avio_w8(pb, pkt->data[i + 1]);avio_w8(pb, pkt->data[i + 2]);}}} else {if (trk->cenc.aes_ctr) {if (par->codec_id == AV_CODEC_ID_H264 && par->extradata_size > 4) {int nal_size_length = (par->extradata[4] & 0x3) + 1;ret = ff_mov_cenc_avc_write_nal_units(s, &trk->cenc, nal_size_length, pb, pkt->data, size);} else {ret = ff_mov_cenc_write_packet(&trk->cenc, pb, pkt->data, size);}if (ret) {goto err;}} else {avio_write(pb, pkt->data, size);}}if (trk->entry >= trk->cluster_capacity) {unsigned new_capacity = trk->entry + MOV_INDEX_CLUSTER_SIZE;void *cluster = av_realloc_array(trk->cluster, new_capacity, sizeof(*trk->cluster));if (!cluster) {ret = AVERROR(ENOMEM);goto err;}trk->cluster          = cluster;trk->cluster_capacity = new_capacity;}trk->cluster[trk->entry].pos              = avio_tell(pb) - size;trk->cluster[trk->entry].samples_in_chunk = samples_in_chunk;trk->cluster[trk->entry].chunkNum         = 0;trk->cluster[trk->entry].size             = size;trk->cluster[trk->entry].entries          = samples_in_chunk;trk->cluster[trk->entry].dts              = pkt->dts;trk->cluster[trk->entry].pts              = pkt->pts;if (!trk->entry && trk->start_dts != AV_NOPTS_VALUE) {if (!trk->frag_discont) {trk->cluster[trk->entry].dts = trk->start_dts + trk->track_duration;if ((mov->flags & FF_MOV_FLAG_DASH &&!(mov->flags & (FF_MOV_FLAG_GLOBAL_SIDX | FF_MOV_FLAG_SKIP_SIDX))) ||mov->mode == MODE_ISM)pkt->pts = pkt->dts + trk->end_pts - trk->cluster[trk->entry].dts;} else {trk->frag_start = pkt->dts - trk->start_dts;trk->end_pts = AV_NOPTS_VALUE;trk->frag_discont = 0;}}if (!trk->entry && trk->start_dts == AV_NOPTS_VALUE && !mov->use_editlist &&s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_MAKE_ZERO) {trk->cluster[trk->entry].dts = trk->start_dts = 0;}if (trk->start_dts == AV_NOPTS_VALUE) {trk->start_dts = pkt->dts;if (trk->frag_discont) {if (mov->use_editlist) {trk->frag_start = pkt->pts;trk->start_dts  = pkt->dts - pkt->pts;} else {trk->frag_start = pkt->dts;trk->start_dts  = 0;}trk->frag_discont = 0;} else if (pkt->dts && mov->moov_written)av_log(s, AV_LOG_WARNING,"Track %d starts with a nonzero dts %"PRId64", while the moov ""already has been written.\n", pkt->stream_index, pkt->dts);}trk->track_duration = pkt->dts - trk->start_dts + pkt->duration;trk->last_sample_is_subtitle_end = 0;if (pkt->pts == AV_NOPTS_VALUE) {pkt->pts = pkt->dts;}if (pkt->dts != pkt->pts)trk->flags |= MOV_TRACK_CTTS;trk->cluster[trk->entry].cts   = pkt->pts - pkt->dts;trk->cluster[trk->entry].flags = 0;if (trk->start_cts == AV_NOPTS_VALUE)trk->start_cts = pkt->pts - pkt->dts;if (trk->end_pts == AV_NOPTS_VALUE)trk->end_pts = trk->cluster[trk->entry].dts +trk->cluster[trk->entry].cts + pkt->duration;elsetrk->end_pts = FFMAX(trk->end_pts, trk->cluster[trk->entry].dts +trk->cluster[trk->entry].cts +pkt->duration);if (par->codec_id == AV_CODEC_ID_VC1) {mov_parse_vc1_frame(pkt, trk);} else if (par->codec_id == AV_CODEC_ID_TRUEHD) {mov_parse_truehd_frame(pkt, trk);} else if (pkt->flags & AV_PKT_FLAG_KEY) {if (mov->mode == MODE_MOV && par->codec_id == AV_CODEC_ID_MPEG2VIDEO &&trk->entry > 0) { // force sync sample for the first key framemov_parse_mpeg2_frame(pkt, &trk->cluster[trk->entry].flags);if (trk->cluster[trk->entry].flags & MOV_PARTIAL_SYNC_SAMPLE)trk->flags |= MOV_TRACK_STPS;} else {trk->cluster[trk->entry].flags = MOV_SYNC_SAMPLE;}if (trk->cluster[trk->entry].flags & MOV_SYNC_SAMPLE)trk->has_keyframes++;}......end:
err:if (pkt->data != reformatted_data)av_free(reformatted_data);return ret;
}

至此，av_write_frame()写音视频函数过程分析完毕。下一篇文章将与大家探讨av_write_trailer()函数的实现。

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