【FFmpeg】av_read_frame函数

参考：
ffmpeg 源代码简单分析 ： av_read_frame()

FFmpeg相关记录：

示例工程：
【FFmpeg】调用ffmpeg库实现264软编
【FFmpeg】调用ffmpeg库实现264软解
【FFmpeg】调用ffmpeg库进行RTMP推流和拉流
【FFmpeg】调用ffmpeg库进行SDL2解码后渲染

流程分析：
【FFmpeg】编码链路上主要函数的简单分析
【FFmpeg】解码链路上主要函数的简单分析

结构体分析：
【FFmpeg】AVCodec结构体
【FFmpeg】AVCodecContext结构体
【FFmpeg】AVStream结构体
【FFmpeg】AVFormatContext结构体
【FFmpeg】AVIOContext结构体
【FFmpeg】AVPacket结构体

函数分析：
【FFmpeg】avformat_open_input函数
【FFmpeg】avformat_find_stream_info函数
【FFmpeg】avformat_alloc_output_context2函数
【FFmpeg】avio_open2函数
【FFmpeg】avformat_write_header函数

av_read_frame函数内调用关系如下

1.av_read_frame

av_read_frame用于读取输入数据流当中的下一个帧，具体实现方式如下

/*** Return the next frame of a stream.* This function returns what is stored in the file, and does not validate* that what is there are valid frames for the decoder. It will split what is* stored in the file into frames and return one for each call. It will not* omit invalid data between valid frames so as to give the decoder the maximum* information possible for decoding.** On success, the returned packet is reference-counted (pkt->buf is set) and* valid indefinitely. The packet must be freed with av_packet_unref() when* it is no longer needed. For video, the packet contains exactly one frame.* For audio, it contains an integer number of frames if each frame has* a known fixed size (e.g. PCM or ADPCM data). If the audio frames have* a variable size (e.g. MPEG audio), then it contains one frame.** pkt->pts, pkt->dts and pkt->duration are always set to correct* values in AVStream.time_base units (and guessed if the format cannot* provide them). pkt->pts can be AV_NOPTS_VALUE if the video format* has B-frames, so it is better to rely on pkt->dts if you do not* decompress the payload.** @return 0 if OK, < 0 on error or end of file. On error, pkt will be blank*         (as if it came from av_packet_alloc()).** @note pkt will be initialized, so it may be uninitialized, but it must not*       contain data that needs to be freed.*/
// 返回流的下一帧
// 1.此函数返回存储在文件中的内容，而不验证是否存在用于解码器的有效帧。它将把存储在文件中的内容分割成帧，
//		并为每次调用返回一个帧。它不会省略有效帧之间的无效数据，以便为解码器提供可能用于解码的最大信息
// 2.如果成功，返回的数据包将被引用计数(pkt->但已设置)并无限期有效。当不再需要该数据包时，必须使用
//		av_packet_unref()释放该数据包。对于视频，数据包只包含一帧。对于音频，如果每帧有一个已知的固定大小
//		(例如PCM或ADPCM数据)，它包含一个整数帧数。如果音频帧具有可变大小(例如MPEG音频)，则它包含一个帧
// 3.pkt->pts, pkt->dts和pkt->duration在AVStream中总是设置为正确的值。Time_base单位
//		(并猜测格式是否不能提供它们)。如果视频格式有b帧，pkt->pts可以是AV_NOPTS_VALUE，
//		所以如果不解压缩有效载荷，最好依赖pkt->dts
// @return: 返回0则成功，如果return小于0则出错或者到达文件末尾，如果出错pkt为空
// @note: PKT将被初始化，因此它可能是未初始化的，但它必须不包含需要释放的数据
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);// AVFMT_FLAG_GENPTS: 生成丢失的pts，即使它需要解析未来的帧const int genpts = s->flags & AVFMT_FLAG_GENPTS;int eof = 0;int ret;AVStream *st;// 1.数据包已被缓冲但未解码// 只有当数据包已经被缓冲但还没有解码时才需要这个缓冲区，例如在MPEG流中获取编解码器参数// genpts表示是否需要生成丢失的pts，一般情况下是不需要的，默认会进入下述分支if (!genpts) {ret = si->packet_buffer.head? avpriv_packet_list_get(&si->packet_buffer, pkt) // 从packet buffer中获取1帧: read_frame_internal(s, pkt); // 从流中读取一帧到pktif (ret < 0)return ret;goto return_packet;}// 需要生成丢失的ptsfor (;;) {PacketListEntry *pktl = si->packet_buffer.head;if (pktl) {AVPacket *next_pkt = &pktl->pkt;if (next_pkt->dts != AV_NOPTS_VALUE) {// pts_wrap_bits用于处理时间戳的溢出问题，它允许时间戳在溢出之前使用更多的位// 例如设置pts_wrap_bits为24，时间戳将在达到2^24时溢出int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;// last dts seen for this stream. if any of packets following// current one had no dts, we will set this to AV_NOPTS_VALUE.// 这条流的最后一次DTS。如果当前数据包之后的任何数据包没有dts，将其设置为AV_NOPTS_VALUEint64_t last_dts = next_pkt->dts;av_assert2(wrap_bits <= 64);while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {// 如果当前pkt和下一个pkt同属一个stream// av_compare_mod用于比较两个整数a和b相对于模数mod的大小关系// 如果返回值小于0，表示整数a在模数mod下的值小于整数bif (pktl->pkt.stream_index == next_pkt->stream_index &&av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2ULL << (wrap_bits - 1)) < 0) { if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) {// not B-frame// 修正ptsnext_pkt->pts = pktl->pkt.dts;}if (last_dts != AV_NOPTS_VALUE) {// Once last dts was set to AV_NOPTS_VALUE, we don't change it.last_dts = pktl->pkt.dts;}}pktl = pktl->next;}if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {// Fixing the last reference frame had none pts issue (For MXF etc).// We only do this when// 1. eof.// 2. we are not able to resolve a pts value for current packet.// 3. the packets for this stream at the end of the files had valid dts.// 修复最后一个参考帧的pts问题// 1.到达文件末尾// 2.无法解析当前数据包的PTS值// 3.该流在文件末尾的数据包具有有效的DTSnext_pkt->pts = last_dts + next_pkt->duration;}pktl = si->packet_buffer.head;}/* read packet from packet buffer, if there is data */// 如果有数据，则从packet buffer中取出数据st = s->streams[next_pkt->stream_index];if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL &&next_pkt->dts != AV_NOPTS_VALUE && !eof)) {ret = avpriv_packet_list_get(&si->packet_buffer, pkt);goto return_packet;}}// 读取帧ret = read_frame_internal(s, pkt);if (ret < 0) {if (pktl && ret != AVERROR(EAGAIN)) {eof = 1;continue;} elsereturn ret;}// 将pkt放入packet buffer当中ret = avpriv_packet_list_put(&si->packet_buffer,pkt, NULL, 0);if (ret < 0) {av_packet_unref(pkt);return ret;}}return_packet:st = s->streams[pkt->stream_index];if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) {ff_reduce_index(s, st->index);av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);}if (is_relative(pkt->dts))pkt->dts -= RELATIVE_TS_BASE;if (is_relative(pkt->pts))pkt->pts -= RELATIVE_TS_BASE;return ret;
}

1.2 从pkt buffer中读取帧（avpriv_packet_list_get）

函数会从pkt的buffer当中读取一帧，位于libavcodec\avpkt.c中

/*** Remove the oldest AVPacket in the list and return it.** @note The pkt will be overwritten completely on success. The caller*       owns the packet and must unref it by itself.** @param head A pointer to a PacketList struct* @param pkt  Pointer to an AVPacket struct* @return 0 on success, and a packet is returned. AVERROR(EAGAIN) if*         the list was empty.*/
// 删除列表中最早的AVPacket并返回它
// @note: 一旦成功，pkt将被完全覆盖。调用者拥有packet，必须由调用者释放
int avpriv_packet_list_get(PacketList *pkt_buffer,AVPacket      *pkt)
{PacketListEntry *pktl = pkt_buffer->head;if (!pktl)return AVERROR(EAGAIN);*pkt        = pktl->pkt;pkt_buffer->head = pktl->next;if (!pkt_buffer->head)pkt_buffer->tail = NULL;av_freep(&pktl);return 0;
}

1.3 从流当中读取帧（read_frame_internal）

函数的定义位于libavformat\demux.c中，其主要的目的是从流当中读取一帧，函数大体可以分为几个部分：
（1）读取帧并且解析
（2）如果有必要则更新context
（3）时间戳等信息的检查
（4）检查discard和side data

在这些流程之中，最核心的内容是读取帧（ff_read_packet）并且解析（parse_packet），其中解析packet是只在有必要时进行，这需要提前检查

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int ret, got_packet = 0;AVDictionary *metadata = NULL;// 没有获取packet则持续读取while (!got_packet && !si->parse_queue.head) {AVStream *st;FFStream *sti;// 1.读取帧并且考虑解析/* read next packet */// 读取packetret = ff_read_packet(s, pkt);if (ret < 0) {if (ret == AVERROR(EAGAIN))return ret;/* flush the parsers */for (unsigned i = 0; i < s->nb_streams; i++) {AVStream *const st  = s->streams[i];FFStream *const sti = ffstream(st);if (sti->parser && sti->need_parsing)// 解析packetparse_packet(s, pkt, st->index, 1);}/* all remaining packets are now in parse_queue =>* really terminate parsing */break;}ret = 0;st  = s->streams[pkt->stream_index];sti = ffstream(st);st->event_flags |= AVSTREAM_EVENT_FLAG_NEW_PACKETS;/* update context if required */// 2.如果有必要则更新contextif (sti->need_context_update) {if (avcodec_is_open(sti->avctx)) {av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n");ret = codec_close(sti);sti->info->found_decoder = 0;if (ret < 0)return ret;}/* close parser, because it depends on the codec */if (sti->parser && sti->avctx->codec_id != st->codecpar->codec_id) {av_parser_close(sti->parser);sti->parser = NULL;}ret = avcodec_parameters_to_context(sti->avctx, st->codecpar);if (ret < 0) {av_packet_unref(pkt);return ret;}sti->codec_desc = avcodec_descriptor_get(sti->avctx->codec_id);sti->need_context_update = 0;}// 3.时间戳等信息的检查if (pkt->pts != AV_NOPTS_VALUE &&pkt->dts != AV_NOPTS_VALUE &&pkt->pts < pkt->dts) {av_log(s, AV_LOG_WARNING,"Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",pkt->stream_index,av_ts2str(pkt->pts),av_ts2str(pkt->dts),pkt->size);}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_DEBUG,"ff_read_packet stream=%d, pts=%s, dts=%s, size=%d, duration=%"PRId64", flags=%d\n",pkt->stream_index,av_ts2str(pkt->pts),av_ts2str(pkt->dts),pkt->size, pkt->duration, pkt->flags);if (sti->need_parsing && !sti->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {sti->parser = av_parser_init(st->codecpar->codec_id);if (!sti->parser) {av_log(s, AV_LOG_VERBOSE, "parser not found for codec ""%s, packets or times may be invalid.\n",avcodec_get_name(st->codecpar->codec_id));/* no parser available: just output the raw packets */sti->need_parsing = AVSTREAM_PARSE_NONE;} else if (sti->need_parsing == AVSTREAM_PARSE_HEADERS)sti->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;else if (sti->need_parsing == AVSTREAM_PARSE_FULL_ONCE)sti->parser->flags |= PARSER_FLAG_ONCE;else if (sti->need_parsing == AVSTREAM_PARSE_FULL_RAW)sti->parser->flags |= PARSER_FLAG_USE_CODEC_TS;}if (!sti->need_parsing || !sti->parser) {/* no parsing needed: we just output the packet as is */compute_pkt_fields(s, st, NULL, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);if ((s->iformat->flags & AVFMT_GENERIC_INDEX) &&(pkt->flags & AV_PKT_FLAG_KEY) && pkt->dts != AV_NOPTS_VALUE) {ff_reduce_index(s, st->index);av_add_index_entry(st, pkt->pos, pkt->dts,0, 0, AVINDEX_KEYFRAME);}got_packet = 1;} else if (st->discard < AVDISCARD_ALL) {if ((ret = parse_packet(s, pkt, pkt->stream_index, 0)) < 0)return ret;st->codecpar->sample_rate = sti->avctx->sample_rate;st->codecpar->bit_rate = sti->avctx->bit_rate;ret = av_channel_layout_copy(&st->codecpar->ch_layout, &sti->avctx->ch_layout);if (ret < 0)return ret;st->codecpar->codec_id = sti->avctx->codec_id;} else {/* free packet */av_packet_unref(pkt);}if (pkt->flags & AV_PKT_FLAG_KEY)sti->skip_to_keyframe = 0;if (sti->skip_to_keyframe) {av_packet_unref(pkt);got_packet = 0;}}if (!got_packet && si->parse_queue.head)ret = avpriv_packet_list_get(&si->parse_queue, pkt);// 4.检查discard和side dataif (ret >= 0) {AVStream *const st  = s->streams[pkt->stream_index];FFStream *const sti = ffstream(st);int discard_padding = 0;if (sti->first_discard_sample && pkt->pts != AV_NOPTS_VALUE) {int64_t pts = pkt->pts - (is_relative(pkt->pts) ? RELATIVE_TS_BASE : 0);int64_t sample = ts_to_samples(st, pts);int64_t duration = ts_to_samples(st, pkt->duration);int64_t end_sample = sample + duration;if (duration > 0 && end_sample >= sti->first_discard_sample &&sample < sti->last_discard_sample)discard_padding = FFMIN(end_sample - sti->first_discard_sample, duration);}if (sti->start_skip_samples && (pkt->pts == 0 || pkt->pts == RELATIVE_TS_BASE))sti->skip_samples = sti->start_skip_samples;sti->skip_samples = FFMAX(0, sti->skip_samples);if (sti->skip_samples || discard_padding) {uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);if (p) {AV_WL32(p, sti->skip_samples);AV_WL32(p + 4, discard_padding);av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %u / discard %u\n",(unsigned)sti->skip_samples, (unsigned)discard_padding);}sti->skip_samples = 0;}#if FF_API_AVSTREAM_SIDE_DATAif (sti->inject_global_side_data) {for (int i = 0; i < st->codecpar->nb_coded_side_data; i++) {const AVPacketSideData *const src_sd = &st->codecpar->coded_side_data[i];uint8_t *dst_data;if (av_packet_get_side_data(pkt, src_sd->type, NULL))continue;dst_data = av_packet_new_side_data(pkt, src_sd->type, src_sd->size);if (!dst_data) {av_log(s, AV_LOG_WARNING, "Could not inject global side data\n");continue;}memcpy(dst_data, src_sd->data, src_sd->size);}sti->inject_global_side_data = 0;}
#endif}if (!si->metafree) {int metaret = av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);if (metadata) {s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;av_dict_copy(&s->metadata, metadata, 0);av_dict_free(&metadata);av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);}si->metafree = metaret == AVERROR_OPTION_NOT_FOUND;}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_DEBUG,"read_frame_internal stream=%d, pts=%s, dts=%s, ""size=%d, duration=%"PRId64", flags=%d\n",pkt->stream_index,av_ts2str(pkt->pts),av_ts2str(pkt->dts),pkt->size, pkt->duration, pkt->flags);/* A demuxer might have returned EOF because of an IO error, let's* propagate this back to the user. */if (ret == AVERROR_EOF && s->pb && s->pb->error < 0 && s->pb->error != AVERROR(EAGAIN))ret = s->pb->error;return ret;
}

1.3.1 读取帧（ff_read_packet）

ff_read_packet用于从媒体文件中读取数据包，该函数会根据具体输入的媒体格式，调用具体的read_packet函数进行，核心函数为.read_packet

/*** Read a transport packet from a media file.** @param s media file handle* @param pkt is filled* @return 0 if OK, AVERROR_xxx on error*/
// 从媒体文件中读取传输数据包
int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int err;#if FF_API_INIT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS // 禁用FFmpeg中的弃用警告pkt->data = NULL;pkt->size = 0;av_init_packet(pkt);
FF_ENABLE_DEPRECATION_WARNINGS // 启用FFmpeg中的弃用警告
#elseav_packet_unref(pkt);
#endiffor (;;) {PacketListEntry *pktl = si->raw_packet_buffer.head;if (pktl) {AVStream *const st = s->streams[pktl->pkt.stream_index];if (si->raw_packet_buffer_size >= s->probesize)if ((err = probe_codec(s, st, NULL)) < 0)return err;if (ffstream(st)->request_probe <= 0) {avpriv_packet_list_get(&si->raw_packet_buffer, pkt);si->raw_packet_buffer_size -= pkt->size;return 0;}}// 具体读取packet的函数err = ffifmt(s->iformat)->read_packet(s, pkt);if (err < 0) {av_packet_unref(pkt);/* Some demuxers return FFERROR_REDO when they consumedata and discard it (ignored streams, junk, extradata).We must re-call the demuxer to get the real packet. */if (err == FFERROR_REDO)continue;if (!pktl || err == AVERROR(EAGAIN))return err;for (unsigned i = 0; i < s->nb_streams; i++) {AVStream *const st  = s->streams[i];FFStream *const sti = ffstream(st);if (sti->probe_packets || sti->request_probe > 0)if ((err = probe_codec(s, st, NULL)) < 0)return err;av_assert0(sti->request_probe <= 0);}continue;}err = av_packet_make_refcounted(pkt);if (err < 0) {av_packet_unref(pkt);return err;}err = handle_new_packet(s, pkt, 1);if (err <= 0) /* Error or passthrough */return err;}
}

在上面的函数当中，最核心的地方是read_packet，会根据不同的FFInputFormat来调用不同的方式，例如FLV格式，会调用flv_read_packet，定义如下

const FFInputFormat ff_flv_demuxer = {.p.name         = "flv",.p.long_name    = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),.p.extensions   = "flv",.p.priv_class   = &flv_kux_class,.priv_data_size = sizeof(FLVContext),.read_probe     = flv_probe,.read_header    = flv_read_header,.read_packet    = flv_read_packet,.read_seek      = flv_read_seek,.read_close     = flv_read_close,
};

其中，flv_read_packet的定义如下。主要的工作是根据FLV格式，进行逐层的Tag解析以及TagData，获取Tag和TagData，流程如下
（1）解析tag header
（2）解析tag data
（3）其他信息配置

static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
{FLVContext *flv = s->priv_data;int ret, i, size, flags;enum FlvTagType type;int stream_type=-1;int64_t next, pos, meta_pos;int64_t dts, pts = AV_NOPTS_VALUE;int av_uninit(channels);int av_uninit(sample_rate);AVStream *st    = NULL;int last = -1;int orig_size;int enhanced_flv = 0;uint32_t video_codec_id = 0;retry:/* pkt size is repeated at end. skip it */// 1.解析tag headerpos  = avio_tell(s->pb);// tag的类型type = (avio_r8(s->pb) & 0x1F);// datasize的大小orig_size =size = avio_rb24(s->pb);flv->sum_flv_tag_size += size + 11LL;// 时间戳dts  = avio_rb24(s->pb);dts |= (unsigned)avio_r8(s->pb) << 24;av_log(s, AV_LOG_TRACE, "type:%d, size:%d, last:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, last, dts, avio_tell(s->pb));if (avio_feof(s->pb))return AVERROR_EOF;// 流idavio_skip(s->pb, 3); /* stream id, always 0 */// 解析tag header结束flags = 0;if (flv->validate_next < flv->validate_count) {int64_t validate_pos = flv->validate_index[flv->validate_next].pos;if (pos == validate_pos) {if (FFABS(dts - flv->validate_index[flv->validate_next].dts) <=VALIDATE_INDEX_TS_THRESH) {flv->validate_next++;} else {clear_index_entries(s, validate_pos);flv->validate_count = 0;}} else if (pos > validate_pos) {clear_index_entries(s, validate_pos);flv->validate_count = 0;}}if (size == 0) {ret = FFERROR_REDO;goto leave;}next = size + avio_tell(s->pb);if (type == FLV_TAG_TYPE_AUDIO) { // 音频stream_type = FLV_STREAM_TYPE_AUDIO;// tag data的第一个字节flags    = avio_r8(s->pb);size--;} else if (type == FLV_TAG_TYPE_VIDEO) { // 视频stream_type = FLV_STREAM_TYPE_VIDEO;// tag data的第一个字节flags    = avio_r8(s->pb);video_codec_id = flags & FLV_VIDEO_CODECID_MASK;/** Reference Enhancing FLV 2023-03-v1.0.0-B.8* https://github.com/veovera/enhanced-rtmp/blob/main/enhanced-rtmp-v1.pdf* */enhanced_flv = (flags >> 7) & 1;size--;if (enhanced_flv) {video_codec_id = avio_rb32(s->pb);size -= 4;}// 如果是增强的flv格式if (enhanced_flv && stream_type == FLV_STREAM_TYPE_VIDEO && (flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_VIDEO_INFO_CMD) 				{int pkt_type = flags & 0x0F;if (pkt_type == PacketTypeMetadata) {int ret = flv_parse_video_color_info(s, st, next);av_log(s, AV_LOG_DEBUG, "enhanced flv parse metadata ret %d and skip\n", ret);}goto skip;} else if ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_VIDEO_INFO_CMD) {goto skip;}} else if (type == FLV_TAG_TYPE_META) { // 如果是元数据stream_type=FLV_STREAM_TYPE_SUBTITLE; // 字幕if (size > 13 + 1 + 4) { // Header-type metadata stuffint type;meta_pos = avio_tell(s->pb);type = flv_read_metabody(s, next);if (type == 0 && dts == 0 || type < 0) {if (type < 0 && flv->validate_count &&flv->validate_index[0].pos     > next &&flv->validate_index[0].pos - 4 < next) {av_log(s, AV_LOG_WARNING, "Adjusting next position due to index mismatch\n");next = flv->validate_index[0].pos - 4;}goto skip;} else if (type == TYPE_ONTEXTDATA) {avpriv_request_sample(s, "OnTextData packet");return flv_data_packet(s, pkt, dts, next);} else if (type == TYPE_ONCAPTION) {return flv_data_packet(s, pkt, dts, next);} else if (type == TYPE_UNKNOWN) {stream_type = FLV_STREAM_TYPE_DATA;}avio_seek(s->pb, meta_pos, SEEK_SET);}} else { // 跳过av_log(s, AV_LOG_DEBUG,"Skipping flv packet: type %d, size %d, flags %d.\n",type, size, flags);
skip:if (avio_seek(s->pb, next, SEEK_SET) != next) {// This can happen if flv_read_metabody above read past// next, on a non-seekable input, and the preceding data has// been flushed out from the IO buffer.av_log(s, AV_LOG_ERROR, "Unable to seek to the next packet\n");return AVERROR_INVALIDDATA;}ret = FFERROR_REDO;goto leave;}/* skip empty data packets */if (!size) {ret = FFERROR_REDO;goto leave;}/* now find stream */// 寻找流for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];if (stream_type == FLV_STREAM_TYPE_AUDIO) {// flv_same_audio_codec用于确保在转换为FLV格式时音频编码格式不变if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&(s->audio_codec_id || flv_same_audio_codec(st->codecpar, flags)))break;} else if (stream_type == FLV_STREAM_TYPE_VIDEO) {// flv_same_viedo_codec用于确保在转换为FLV格式时保持视频编码格式不变if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&(s->video_codec_id || flv_same_video_codec(st->codecpar, video_codec_id)))break;} else if (stream_type == FLV_STREAM_TYPE_SUBTITLE) {if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE)break;} else if (stream_type == FLV_STREAM_TYPE_DATA) {if (st->codecpar->codec_type == AVMEDIA_TYPE_DATA)break;}}// 2.获取TagData// 根据读取的信息，创建一条流，将信息填充到流当中if (i == s->nb_streams) {static const enum AVMediaType stream_types[] = {AVMEDIA_TYPE_VIDEO, AVMEDIA_TYPE_AUDIO, AVMEDIA_TYPE_SUBTITLE, AVMEDIA_TYPE_DATA};st = create_stream(s, stream_types[stream_type]);if (!st)return AVERROR(ENOMEM);}av_log(s, AV_LOG_TRACE, "%d %X %d \n", stream_type, flags, st->discard);if (flv->time_pos <= pos) {dts += flv->time_offset;}if ((s->pb->seekable & AVIO_SEEKABLE_NORMAL) &&((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY ||stream_type == FLV_STREAM_TYPE_AUDIO))av_add_index_entry(st, pos, dts, size, 0, AVINDEX_KEYFRAME);if ((st->discard >= AVDISCARD_NONKEY && !((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY || stream_type == FLV_STREAM_TYPE_AUDIO)) ||(st->discard >= AVDISCARD_BIDIR && ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_DISP_INTER && stream_type == FLV_STREAM_TYPE_VIDEO)) ||st->discard >= AVDISCARD_ALL) {avio_seek(s->pb, next, SEEK_SET);ret = FFERROR_REDO;goto leave;}// if not streamed and no duration from metadata then seek to end to find// the duration from the timestamps// 如果没有流化并且没有从元数据中获取持续时间，则寻求结束以从时间戳中查找持续时间if ((s->pb->seekable & AVIO_SEEKABLE_NORMAL) &&(!s->duration || s->duration == AV_NOPTS_VALUE) &&!flv->searched_for_end) {int size;const int64_t pos   = avio_tell(s->pb);// Read the last 4 bytes of the file, this should be the size of the// previous FLV tag. Use the timestamp of its payload as duration.// 读取文件的最后4个字节，这应该是前一个FLV标记的大小。使用其有效负载的时间戳作为持续时间int64_t fsize       = avio_size(s->pb);
retry_duration:avio_seek(s->pb, fsize - 4, SEEK_SET);size = avio_rb32(s->pb);if (size > 0 && size < fsize) {// Seek to the start of the last FLV tag at position (fsize - 4 - size)// but skip the byte indicating the type.avio_seek(s->pb, fsize - 3 - size, SEEK_SET);if (size == avio_rb24(s->pb) + 11) {uint32_t ts = avio_rb24(s->pb);ts         |= (unsigned)avio_r8(s->pb) << 24;if (ts)s->duration = ts * (int64_t)AV_TIME_BASE / 1000;else if (fsize >= 8 && fsize - 8 >= size) {fsize -= size+4;goto retry_duration;}}}avio_seek(s->pb, pos, SEEK_SET);flv->searched_for_end = 1;}// 3.其他信息配置//	如果流类型是音频，进行一些检查和配置if (stream_type == FLV_STREAM_TYPE_AUDIO) {int bits_per_coded_sample;channels = (flags & FLV_AUDIO_CHANNEL_MASK) == FLV_STEREO ? 2 : 1;sample_rate = 44100 << ((flags & FLV_AUDIO_SAMPLERATE_MASK) >>FLV_AUDIO_SAMPLERATE_OFFSET) >> 3;bits_per_coded_sample = (flags & FLV_AUDIO_SAMPLESIZE_MASK) ? 16 : 8;if (!av_channel_layout_check(&st->codecpar->ch_layout) ||!st->codecpar->sample_rate ||!st->codecpar->bits_per_coded_sample) {av_channel_layout_default(&st->codecpar->ch_layout, channels);st->codecpar->sample_rate           = sample_rate;st->codecpar->bits_per_coded_sample = bits_per_coded_sample;}if (!st->codecpar->codec_id) {flv_set_audio_codec(s, st, st->codecpar,flags & FLV_AUDIO_CODECID_MASK);flv->last_sample_rate =sample_rate           = st->codecpar->sample_rate;flv->last_channels    =channels              = st->codecpar->ch_layout.nb_channels;} else {AVCodecParameters *par = avcodec_parameters_alloc();if (!par) {ret = AVERROR(ENOMEM);goto leave;}par->sample_rate = sample_rate;par->bits_per_coded_sample = bits_per_coded_sample;flv_set_audio_codec(s, st, par, flags & FLV_AUDIO_CODECID_MASK);sample_rate = par->sample_rate;avcodec_parameters_free(&par);}} else if (stream_type == FLV_STREAM_TYPE_VIDEO) { // 如果是视频，则进行video coedc的配置int ret = flv_set_video_codec(s, st, video_codec_id, 1);if (ret < 0)return ret;size -= ret;} else if (stream_type == FLV_STREAM_TYPE_SUBTITLE) {st->codecpar->codec_id = AV_CODEC_ID_TEXT;} else if (stream_type == FLV_STREAM_TYPE_DATA) {st->codecpar->codec_id = AV_CODEC_ID_NONE; // Opaque AMF data}// 相对比于雷博记录的版本，这里新增了HEVC、AV1和VP9if (st->codecpar->codec_id == AV_CODEC_ID_AAC ||st->codecpar->codec_id == AV_CODEC_ID_H264 ||st->codecpar->codec_id == AV_CODEC_ID_MPEG4 ||st->codecpar->codec_id == AV_CODEC_ID_HEVC ||st->codecpar->codec_id == AV_CODEC_ID_AV1 ||st->codecpar->codec_id == AV_CODEC_ID_VP9) {int type = 0;if (enhanced_flv && stream_type == FLV_STREAM_TYPE_VIDEO) {type = flags & 0x0F;} else {type = avio_r8(s->pb);size--;}if (size < 0) {ret = AVERROR_INVALIDDATA;goto leave;}if (enhanced_flv && stream_type == FLV_STREAM_TYPE_VIDEO && flv->meta_color_info_flag) {// 更新packet的side dataflv_update_video_color_info(s, st); // update av packet side dataflv->meta_color_info_flag = 0;}// H264 或 H265if (st->codecpar->codec_id == AV_CODEC_ID_H264 || st->codecpar->codec_id == AV_CODEC_ID_MPEG4 ||(st->codecpar->codec_id == AV_CODEC_ID_HEVC && type == PacketTypeCodedFrames)) {// sign extension// 对应的composition timeint32_t cts = (avio_rb24(s->pb) + 0xff800000) ^ 0xff800000;pts = av_sat_add64(dts, cts);if (cts < 0) { // dts might be wrongif (!flv->wrong_dts)av_log(s, AV_LOG_WARNING,"Negative cts, previous timestamps might be wrong.\n");flv->wrong_dts = 1;} else if (FFABS(dts - pts) > 1000*60*15) {av_log(s, AV_LOG_WARNING,"invalid timestamps %"PRId64" %"PRId64"\n", dts, pts);dts = pts = AV_NOPTS_VALUE;}size -= 3;}if (type == 0 && (!st->codecpar->extradata || st->codecpar->codec_id == AV_CODEC_ID_AAC ||st->codecpar->codec_id == AV_CODEC_ID_H264 || st->codecpar->codec_id == AV_CODEC_ID_HEVC ||st->codecpar->codec_id == AV_CODEC_ID_AV1 || st->codecpar->codec_id == AV_CODEC_ID_VP9)) {AVDictionaryEntry *t;if (st->codecpar->extradata) {if ((ret = flv_queue_extradata(flv, s->pb, stream_type, size)) < 0)return ret;ret = FFERROR_REDO;goto leave;}if ((ret = flv_get_extradata(s, st, size)) < 0)return ret;/* Workaround for buggy Omnia A/XE encoder */t = av_dict_get(s->metadata, "Encoder", NULL, 0);if (st->codecpar->codec_id == AV_CODEC_ID_AAC && t && !strcmp(t->value, "Omnia A/XE"))st->codecpar->extradata_size = 2;ret = FFERROR_REDO;goto leave;}}/* skip empty data packets */if (!size) {ret = FFERROR_REDO;goto leave;}// 获取pktret = av_get_packet(s->pb, pkt, size);if (ret < 0)return ret;// 配置dts和pts等信息pkt->dts          = dts;pkt->pts          = pts == AV_NOPTS_VALUE ? dts : pts;pkt->stream_index = st->index;pkt->pos          = pos;if (flv->new_extradata[stream_type]) {int ret = av_packet_add_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA,flv->new_extradata[stream_type],flv->new_extradata_size[stream_type]);if (ret >= 0) {flv->new_extradata[stream_type]      = NULL;flv->new_extradata_size[stream_type] = 0;}}if (stream_type == FLV_STREAM_TYPE_AUDIO &&(sample_rate != flv->last_sample_rate ||channels    != flv->last_channels)) {flv->last_sample_rate = sample_rate;flv->last_channels    = channels;ff_add_param_change(pkt, channels, 0, sample_rate, 0, 0);}if (stream_type == FLV_STREAM_TYPE_AUDIO ||(flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY ||stream_type == FLV_STREAM_TYPE_SUBTITLE ||stream_type == FLV_STREAM_TYPE_DATA)pkt->flags |= AV_PKT_FLAG_KEY; // 关键帧的配置leave:last = avio_rb32(s->pb);if (!flv->trust_datasize) {if (last != orig_size + 11 && last != orig_size + 10 &&!avio_feof(s->pb) &&(last != orig_size || !last) && last != flv->sum_flv_tag_size &&!flv->broken_sizes) {av_log(s, AV_LOG_ERROR, "Packet mismatch %d %d %"PRId64"\n", last, orig_size + 11, flv->sum_flv_tag_size);avio_seek(s->pb, pos + 1, SEEK_SET);ret = resync(s);av_packet_unref(pkt);if (ret >= 0) {goto retry;}}}if (ret >= 0)flv->last_ts = pkt->dts;return ret;
}

参考雷博的文章知道FLV格式如下

从上图可以看出，FLV格式的媒体文件包含几个部分：FLV Header和FLV Body，Header中包括了Signature，Version，Flags和Header Size，Body包括Previous Tag、Tag Header和Tag Data，Tag Header中又包括了Type、Datasize、Timestamp、Tiemstamp_ex和StreamID。其中，Video Tag Data的定义如下

第一个字节记录了FrameType和CodecID，从第二个字节开始记录VideoData。其中，第一个字节的前面4位表示的是帧类型：

	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

第一个字节的后面4位表示的是CodecID：

	1: JPEG (currently unused)2: Sorenson H.2633: Screen video4: On2 VP65: On2 VP6 with alpha channel6: Screen video version 27: AVC

1.3.2 解析packet（parse_packet）

函数的定义位于libavformat\demux.c中，用于解析packet，并且将解析出来的信息添加到parse_queue之中。函数最核心的内容是调用了av_parser_parse2进行pkt的解析，随后根据解析的信息对一些变量进行配置

/*** Parse a packet, add all split parts to parse_queue.** @param pkt   Packet to parse; must not be NULL.* @param flush Indicates whether to flush. If set, pkt must be blank.*/
static int parse_packet(AVFormatContext *s, AVPacket *pkt,int stream_index, int flush)
{FFFormatContext *const si = ffformatcontext(s);AVPacket *out_pkt = si->parse_pkt;AVStream *st = s->streams[stream_index];FFStream *const sti = ffstream(st);const uint8_t *data = pkt->data;int size = pkt->size;int ret = 0, got_output = flush;if (!size && !flush && sti->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {// preserve 0-size sync packets// 计算和设置AVPakcet中的属性值compute_pkt_fields(s, st, sti->parser, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);}while (size > 0 || (flush && got_output)) {int64_t next_pts = pkt->pts;int64_t next_dts = pkt->dts;int len;// 解析packetlen = av_parser_parse2(sti->parser, sti->avctx,&out_pkt->data, &out_pkt->size, data, size,pkt->pts, pkt->dts, pkt->pos);pkt->pts = pkt->dts = AV_NOPTS_VALUE;pkt->pos = -1;/* increment read pointer */av_assert1(data || !len);data  = len ? data + len : data;size -= len;got_output = !!out_pkt->size;if (!out_pkt->size)continue;if (pkt->buf && out_pkt->data == pkt->data) {/* reference pkt->buf only when out_pkt->data is guaranteed to point* to data in it and not in the parser's internal buffer. *//* XXX: Ensure this is the case with all parsers when sti->parser->flags* is PARSER_FLAG_COMPLETE_FRAMES and check for that instead? */out_pkt->buf = av_buffer_ref(pkt->buf);if (!out_pkt->buf) {ret = AVERROR(ENOMEM);goto fail;}} else {ret = av_packet_make_refcounted(out_pkt);if (ret < 0)goto fail;}if (pkt->side_data) {out_pkt->side_data       = pkt->side_data;out_pkt->side_data_elems = pkt->side_data_elems;pkt->side_data          = NULL;pkt->side_data_elems    = 0;}/* set the duration */// 设置durationout_pkt->duration = (sti->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->duration : 0;if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {if (sti->avctx->sample_rate > 0) {out_pkt->duration =av_rescale_q_rnd(sti->parser->duration,(AVRational) { 1, sti->avctx->sample_rate },st->time_base,AV_ROUND_DOWN);}} else if (st->codecpar->codec_id == AV_CODEC_ID_GIF) {if (st->time_base.num > 0 && st->time_base.den > 0 &&sti->parser->duration) {out_pkt->duration = sti->parser->duration;}}// 设置pkt的一些属性值out_pkt->stream_index = st->index;out_pkt->pts          = sti->parser->pts;out_pkt->dts          = sti->parser->dts;out_pkt->pos          = sti->parser->pos;out_pkt->flags       |= pkt->flags & (AV_PKT_FLAG_DISCARD | AV_PKT_FLAG_CORRUPT);if (sti->need_parsing == AVSTREAM_PARSE_FULL_RAW)out_pkt->pos = sti->parser->frame_offset;if (sti->parser->key_frame == 1 ||(sti->parser->key_frame == -1 &&sti->parser->pict_type == AV_PICTURE_TYPE_I))out_pkt->flags |= AV_PKT_FLAG_KEY;if (sti->parser->key_frame == -1 && sti->parser->pict_type ==AV_PICTURE_TYPE_NONE && (pkt->flags&AV_PKT_FLAG_KEY))out_pkt->flags |= AV_PKT_FLAG_KEY;compute_pkt_fields(s, st, sti->parser, out_pkt, next_dts, next_pts);// 将packet添加到parse_queue之中ret = avpriv_packet_list_put(&si->parse_queue,out_pkt, NULL, 0);if (ret < 0)goto fail;}/* end of the stream => close and free the parser */if (flush) {av_parser_close(sti->parser);sti->parser = NULL;}fail:if (ret < 0)av_packet_unref(out_pkt);av_packet_unref(pkt);return ret;
}

上面代码的核心函数是av_parser_parse2，下面进行记录

1.3.2.1 解析（av_parser_parse2）

av_parser_parse2的定义位于libavcodec\parser.c中，定义如下

// 解析数据获得一个Packet, 从输入的数据流中分离出一帧一帧的压缩编码数据
// poutbuf: 解析后输出的压缩编码数据帧
// buf: 解析前的压缩编码数据帧
// 如果函数执行完后输出数据为空（poutbuf_size为0），则代表解析还没有完成，还需要再次调用av_parser_parse2()解析一部分数据才可以得到解析后的数据帧。
// 当函数执行完后输出数据不为空的时候，代表解析完成，可以将poutbuf中的这帧数据取出来做后续处理
int av_parser_parse2(AVCodecParserContext *s, AVCodecContext *avctx,uint8_t **poutbuf, int *poutbuf_size,const uint8_t *buf, int buf_size,int64_t pts, int64_t dts, int64_t pos)
{int index, i;uint8_t dummy_buf[AV_INPUT_BUFFER_PADDING_SIZE];// 检查当前的codec id是否不为空av_assert1(avctx->codec_id != AV_CODEC_ID_NONE);/* Parsers only work for the specified codec ids. */// 检查当前的parser的codec类型av_assert1(avctx->codec_id == s->parser->codec_ids[0] ||avctx->codec_id == s->parser->codec_ids[1] ||avctx->codec_id == s->parser->codec_ids[2] ||avctx->codec_id == s->parser->codec_ids[3] ||avctx->codec_id == s->parser->codec_ids[4] ||avctx->codec_id == s->parser->codec_ids[5] ||avctx->codec_id == s->parser->codec_ids[6]);/* 第一次进入时，flags为0，会进入if将offset设置成当前pkt的pos */if (!(s->flags & PARSER_FLAG_FETCHED_OFFSET)) {s->next_frame_offset =s->cur_offset        = pos;s->flags            |= PARSER_FLAG_FETCHED_OFFSET;}if (buf_size == 0) {/* padding is always necessary even if EOF, so we add it here */memset(dummy_buf, 0, sizeof(dummy_buf));buf = dummy_buf;} else if (s->cur_offset + buf_size != s->cur_frame_end[s->cur_frame_start_index]) { /* skip remainder packets *//* add a new packet descriptor */// 保留一下cur_offset, frame_end 信息, 有4个槽位供使用, 方便知道数据偏移量i = (s->cur_frame_start_index + 1) & (AV_PARSER_PTS_NB - 1);s->cur_frame_start_index = i;s->cur_frame_offset[i]   = s->cur_offset;s->cur_frame_end[i]      = s->cur_offset + buf_size;s->cur_frame_pts[i]      = pts;s->cur_frame_dts[i]      = dts;s->cur_frame_pos[i]      = pos;}if (s->fetch_timestamp) {s->fetch_timestamp = 0;s->last_pts        = s->pts;s->last_dts        = s->dts;s->last_pos        = s->pos;ff_fetch_timestamp(s, 0, 0, 0);}/* WARNING: the returned index can be negative */index = s->parser->parser_parse(s, avctx, (const uint8_t **) poutbuf,poutbuf_size, buf, buf_size);av_assert0(index > -0x20000000); // The API does not allow returning AVERROR codes
#define FILL(name) if(s->name > 0 && avctx->name <= 0) avctx->name = s->nameif (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {FILL(field_order);FILL(coded_width);FILL(coded_height);FILL(width);FILL(height);}/* update the file pointer */if (*poutbuf_size) {/* fill the data for the current frame */s->frame_offset = s->next_frame_offset;/* offset of the next frame */s->next_frame_offset = s->cur_offset + index;s->fetch_timestamp   = 1;} else {/* Don't return a pointer to dummy_buf. */*poutbuf = NULL;}if (index < 0)index = 0;s->cur_offset += index;return index;
}

函数的核心是parser_parse，假设初始化解析器时使用的是264格式，则会调用h264_parse，264格式的解析器定义如下

const AVCodecParser ff_h264_parser = {.codec_ids      = { AV_CODEC_ID_H264 },.priv_data_size = sizeof(H264ParseContext),.parser_init    = init,.parser_parse   = h264_parse,.parser_close   = h264_close,
};

调用的h264_parse定义位于libavcodec\h264_parser.c中，如下所示

static int h264_parse(AVCodecParserContext *s,AVCodecContext *avctx,const uint8_t **poutbuf, int *poutbuf_size,const uint8_t *buf, int buf_size)
{H264ParseContext *p = s->priv_data;ParseContext *pc = &p->pc;AVRational time_base = { 0, 1 };int next;if (!p->got_first) {p->got_first = 1;if (avctx->extradata_size) {// h264解码额外数据// 用于解析AVCodecContext的extradata（里面实际上存储了H.264的SPS、PPS）ff_h264_decode_extradata(avctx->extradata, avctx->extradata_size,&p->ps, &p->is_avc, &p->nal_length_size,avctx->err_recognition, avctx);}}if (s->flags & PARSER_FLAG_COMPLETE_FRAMES) { // 解析成功一帧next = buf_size;} else {next = h264_find_frame_end(p, buf, buf_size, avctx); // 找到一帧的结尾if (ff_combine_frame(pc, next, &buf, &buf_size) < 0) {*poutbuf      = NULL;*poutbuf_size = 0;return buf_size;}if (next < 0 && next != END_NOT_FOUND) {av_assert1(pc->last_index + next >= 0);h264_find_frame_end(p, &pc->buffer[pc->last_index + next], -next, avctx); // update state}}// 解析nal数据parse_nal_units(s, avctx, buf, buf_size);if (avctx->framerate.num)time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){2, 1}));if (p->sei.picture_timing.cpb_removal_delay >= 0) {s->dts_sync_point    = p->sei.buffering_period.present;s->dts_ref_dts_delta = p->sei.picture_timing.cpb_removal_delay;s->pts_dts_delta     = p->sei.picture_timing.dpb_output_delay;} else {s->dts_sync_point    = INT_MIN;s->dts_ref_dts_delta = INT_MIN;s->pts_dts_delta     = INT_MIN;}if (s->flags & PARSER_FLAG_ONCE) {s->flags &= PARSER_FLAG_COMPLETE_FRAMES;}if (s->dts_sync_point >= 0) {int64_t den = time_base.den * (int64_t)avctx->pkt_timebase.num;if (den > 0) {int64_t num = time_base.num * (int64_t)avctx->pkt_timebase.den;if (s->dts != AV_NOPTS_VALUE) {// got DTS from the stream, update reference timestampp->reference_dts = av_sat_sub64(s->dts, av_rescale(s->dts_ref_dts_delta, num, den));} else if (p->reference_dts != AV_NOPTS_VALUE) {// compute DTS based on reference timestamps->dts = av_sat_add64(p->reference_dts, av_rescale(s->dts_ref_dts_delta, num, den));}if (p->reference_dts != AV_NOPTS_VALUE && s->pts == AV_NOPTS_VALUE)s->pts = s->dts + av_rescale(s->pts_dts_delta, num, den);if (s->dts_sync_point > 0)p->reference_dts = s->dts; // new reference}}*poutbuf      = buf;*poutbuf_size = buf_size;return next;
}

1.4 向pkt buffer中填充帧（avpriv_packet_list_put）

函数的主要作用是向pkt buffer中填充帧，位于libavcodec\avpkt.c中

int avpriv_packet_list_put(PacketList *packet_buffer,AVPacket      *pkt,int (*copy)(AVPacket *dst, const AVPacket *src),int flags)
{PacketListEntry *pktl = av_malloc(sizeof(*pktl));int ret;if (!pktl)return AVERROR(ENOMEM);if (copy) {get_packet_defaults(&pktl->pkt);ret = copy(&pktl->pkt, pkt);if (ret < 0) {av_free(pktl);return ret;}} else {ret = av_packet_make_refcounted(pkt);if (ret < 0) {av_free(pktl);return ret;}av_packet_move_ref(&pktl->pkt, pkt);}pktl->next = NULL;if (packet_buffer->head)packet_buffer->tail->next = pktl;elsepacket_buffer->head = pktl;/* Add the packet in the buffered packet list. */packet_buffer->tail = pktl;return 0;
}

CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen