ffmpeg 源代码简单分析： av_read

=====================================================

FFmpeg的库函数源代码分析文章列表：

【架构图】

FFmpeg源代码结构图 - 解码

FFmpeg源代码结构图 - 编码

【通用】

FFmpeg 源代码简单分析：av_register_all()

FFmpeg 源代码简单分析：avcodec_register_all()

FFmpeg 源代码简单分析：内存的分配和释放（av_malloc()、av_free()等）

FFmpeg 源代码简单分析：常见结构体的初始化和销毁（AVFormatContext，AVFrame等）

FFmpeg 源代码简单分析：avio_open2()

FFmpeg 源代码简单分析：av_find_decoder()和av_find_encoder()

FFmpeg 源代码简单分析：avcodec_open2()

FFmpeg 源代码简单分析：avcodec_close()

【解码】

图解FFMPEG打开媒体的函数avformat_open_input

FFmpeg 源代码简单分析：avformat_open_input()

FFmpeg 源代码简单分析：avformat_find_stream_info()

FFmpeg 源代码简单分析：av_read_frame()

FFmpeg 源代码简单分析：avcodec_decode_video2()

FFmpeg 源代码简单分析：avformat_close_input()

【编码】

FFmpeg 源代码简单分析：avformat_alloc_output_context2()

FFmpeg 源代码简单分析：avformat_write_header()

FFmpeg 源代码简单分析：avcodec_encode_video()

FFmpeg 源代码简单分析：av_write_frame()

FFmpeg 源代码简单分析：av_write_trailer()

【其它】

FFmpeg源代码简单分析：日志输出系统（av_log()等）

FFmpeg源代码简单分析：结构体成员管理系统-AVClass

FFmpeg源代码简单分析：结构体成员管理系统-AVOption

FFmpeg源代码简单分析：libswscale的sws_getContext()

FFmpeg源代码简单分析：libswscale的sws_scale()

FFmpeg源代码简单分析：libavdevice的avdevice_register_all()

FFmpeg源代码简单分析：libavdevice的gdigrab

【脚本】

FFmpeg源代码简单分析：makefile

FFmpeg源代码简单分析：configure

【H.264】

FFmpeg的H.264解码器源代码简单分析：概述

=====================================================

ffmpeg中的av_read_frame()的作用是读取码流中的音频若干帧或者视频一帧。例如，解码视频的时候，每解码一个视频帧，需要先调用 av_read_frame()获得一帧视频的压缩数据，然后才能对该数据进行解码（例如H.264中一帧压缩数据通常对应一个NAL）。

对该函数源代码的分析是很久之前做的了，现在翻出来，用博客记录一下。

上代码之前，先参考了其他人对av_read_frame()的解释，在此做一个参考：

通过av_read_packet(***)，读取一个包，需要说明的是此函数必须是包含整数帧的，不存在半帧的情况，以ts流为例，是读取一个完整的PES包（一个完整pes包包含若干视频或音频es包），读取完毕后，通过av_parser_parse2(***)分析出视频一帧（或音频若干帧），返回，下次进入循环的时候，如果上次的数据没有完全取完，则st = s->cur_st;不会是NULL，即再此进入av_parser_parse2(***)流程，而不是下面的av_read_packet（**）流程，这样就保证了，如果读取一次包含了N帧视频数据（以视频为例），则调用av_read_frame（***）N次都不会去读数据，而是返回第一次读取的数据，直到全部解析完毕。

av_read_frame()的声明位于libavformat\avformat.h，如下所示。

/*** 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.** If pkt->buf is NULL, then the packet is valid until the next* av_read_frame() or until avformat_close_input(). Otherwise the packet* is valid indefinitely. In both cases the packet must be freed with* av_free_packet 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*/
int av_read_frame(AVFormatContext *s, AVPacket *pkt);

av_read_frame()使用方法在注释中写得很详细，用中文简单描述一下它的两个参数：

s：输入的AVFormatContext

pkt：输出的AVPacket

如果返回0则说明读取正常。

函数调用结构图

函数调用结构图如下所示。

av_read_frame()

av_read_frame()的定义位于libavformat\utils.c，如下所示：

//获取一个AVPacket
/** av_read_frame - 新版本的ffmpeg用的是av_read_frame，而老版本的是av_read_packet* 。区别是av_read_packet读出的是包，它可能是半帧或多帧，不保证帧的完整性。av_read_frame对* av_read_packet进行了封装，使读出的数据总是完整的帧*/
int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{const int genpts = s->flags & AVFMT_FLAG_GENPTS;int          eof = 0;if (!genpts)/*** This buffer is only needed when packets were already buffered but* not decoded, for example to get the codec parameters in MPEG* streams.* 一般情况下会调用read_frame_internal(s, pkt)* 直接返回*/return s->packet_buffer ? read_from_packet_buffer(s, pkt) :read_frame_internal(s, pkt);for (;;) {int ret;AVPacketList *pktl = s->packet_buffer;if (pktl) {AVPacket *next_pkt = &pktl->pkt;if (next_pkt->dts != AV_NOPTS_VALUE) {int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {if (pktl->pkt.stream_index == next_pkt->stream_index &&(av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2LL << (wrap_bits - 1)) < 0) &&av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (wrap_bits - 1))) { //not b framenext_pkt->pts = pktl->pkt.dts;}pktl = pktl->next;}pktl = s->packet_buffer;}/* read packet from packet buffer, if there is data */if (!(next_pkt->pts == AV_NOPTS_VALUE &&next_pkt->dts != AV_NOPTS_VALUE && !eof))return read_from_packet_buffer(s, pkt);}ret = read_frame_internal(s, pkt);if (ret < 0) {if (pktl && ret != AVERROR(EAGAIN)) {eof = 1;continue;} elsereturn ret;}if (av_dup_packet(add_to_pktbuf(&s->packet_buffer, pkt,&s->packet_buffer_end)) < 0)return AVERROR(ENOMEM);}
}

可以从源代码中看出，av_read_frame()调用了read_frame_internal()。

read_frame_internal()

read_frame_internal()代码如下所示：

//av_read_frame对他进行了封装
static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{int ret = 0, i, got_packet = 0;AVDictionary *metadata = NULL;//初始化av_init_packet(pkt);while (!got_packet && !s->parse_queue) {AVStream *st;AVPacket cur_pkt;/* read next packet */ret = ff_read_packet(s, &cur_pkt);if (ret < 0) {if (ret == AVERROR(EAGAIN))return ret;/* flush the parsers */for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];//需要解析if (st->parser && st->need_parsing)parse_packet(s, NULL, st->index);}/* all remaining packets are now in parse_queue =>* really terminate parsing */break;}ret = 0;st  = s->streams[cur_pkt.stream_index];if (cur_pkt.pts != AV_NOPTS_VALUE &&cur_pkt.dts != AV_NOPTS_VALUE &&cur_pkt.pts < cur_pkt.dts) {av_log(s, AV_LOG_WARNING,"Invalid timestamps stream=%d, pts=%s, dts=%s, size=%d\n",cur_pkt.stream_index,av_ts2str(cur_pkt.pts),av_ts2str(cur_pkt.dts),cur_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=%d, flags=%d\n",cur_pkt.stream_index,av_ts2str(cur_pkt.pts),av_ts2str(cur_pkt.dts),cur_pkt.size, cur_pkt.duration, cur_pkt.flags);if (st->need_parsing && !st->parser && !(s->flags & AVFMT_FLAG_NOPARSE)) {st->parser = av_parser_init(st->codec->codec_id);if (!st->parser) {av_log(s, AV_LOG_VERBOSE, "parser not found for codec ""%s, packets or times may be invalid.\n",avcodec_get_name(st->codec->codec_id));/* no parser available: just output the raw packets */st->need_parsing = AVSTREAM_PARSE_NONE;} else if (st->need_parsing == AVSTREAM_PARSE_HEADERS)st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;else if (st->need_parsing == AVSTREAM_PARSE_FULL_ONCE)st->parser->flags |= PARSER_FLAG_ONCE;else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;}if (!st->need_parsing || !st->parser) {/* no parsing needed: we just output the packet as is */*pkt = cur_pkt;compute_pkt_fields(s, st, NULL, pkt);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, &cur_pkt, cur_pkt.stream_index)) < 0)return ret;} else {/* free packet */av_free_packet(&cur_pkt);}if (pkt->flags & AV_PKT_FLAG_KEY)st->skip_to_keyframe = 0;if (st->skip_to_keyframe) {av_free_packet(&cur_pkt);if (got_packet) {*pkt = cur_pkt;}got_packet = 0;}}if (!got_packet && s->parse_queue)ret = read_from_packet_buffer(&s->parse_queue, &s->parse_queue_end, pkt);if (ret >= 0) {AVStream *st = s->streams[pkt->stream_index];int discard_padding = 0;if (st->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);int duration = ts_to_samples(st, pkt->duration);int64_t end_sample = sample + duration;if (duration > 0 && end_sample >= st->first_discard_sample &&sample < st->last_discard_sample)discard_padding = FFMIN(end_sample - st->first_discard_sample, duration);}if (st->skip_samples || discard_padding) {uint8_t *p = av_packet_new_side_data(pkt, AV_PKT_DATA_SKIP_SAMPLES, 10);if (p) {AV_WL32(p, st->skip_samples);AV_WL32(p + 4, discard_padding);av_log(s, AV_LOG_DEBUG, "demuxer injecting skip %d\n", st->skip_samples);}st->skip_samples = 0;}if (st->inject_global_side_data) {for (i = 0; i < st->nb_side_data; i++) {AVPacketSideData *src_sd = &st->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);}st->inject_global_side_data = 0;}if (!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))av_packet_merge_side_data(pkt);}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);}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_DEBUG,"read_frame_internal stream=%d, pts=%s, dts=%s, ""size=%d, duration=%d, flags=%d\n",pkt->stream_index,av_ts2str(pkt->pts),av_ts2str(pkt->dts),pkt->size, pkt->duration, pkt->flags);return ret;
}

read_frame_internal()代码比较长，这里只简单看一下它前面的部分。它前面部分有2步是十分关键的：

（1）调用了ff_read_packet()从相应的AVInputFormat读取数据。

（2）如果媒体频流需要使用AVCodecParser，则调用parse_packet()解析相应的AVPacket。

下面我们分成分别看一下ff_read_packet()和parse_packet()的源代码。

ff_read_packet()

ff_read_packet()的代码比较长，如下所示。

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{int ret, i, err;AVStream *st;for (;;) {AVPacketList *pktl = s->raw_packet_buffer;if (pktl) {*pkt = pktl->pkt;st   = s->streams[pkt->stream_index];if (s->raw_packet_buffer_remaining_size <= 0)if ((err = probe_codec(s, st, NULL)) < 0)return err;if (st->request_probe <= 0) {s->raw_packet_buffer                 = pktl->next;s->raw_packet_buffer_remaining_size += pkt->size;av_free(pktl);return 0;}}pkt->data = NULL;pkt->size = 0;av_init_packet(pkt);//关键：读取Packetret = s->iformat->read_packet(s, pkt);if (ret < 0) {if (!pktl || ret == AVERROR(EAGAIN))return ret;for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];if (st->probe_packets)if ((err = probe_codec(s, st, NULL)) < 0)return err;av_assert0(st->request_probe <= 0);}continue;}if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&(pkt->flags & AV_PKT_FLAG_CORRUPT)) {av_log(s, AV_LOG_WARNING,"Dropped corrupted packet (stream = %d)\n",pkt->stream_index);av_free_packet(pkt);continue;}if (pkt->stream_index >= (unsigned)s->nb_streams) {av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);continue;}st = s->streams[pkt->stream_index];if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {// correct first time stamps to negative valuesif (!is_relative(st->first_dts))st->first_dts = wrap_timestamp(st, st->first_dts);if (!is_relative(st->start_time))st->start_time = wrap_timestamp(st, st->start_time);if (!is_relative(st->cur_dts))st->cur_dts = wrap_timestamp(st, st->cur_dts);}pkt->dts = wrap_timestamp(st, pkt->dts);pkt->pts = wrap_timestamp(st, pkt->pts);force_codec_ids(s, st);/* TODO: audio: time filter; video: frame reordering (pts != dts) */if (s->use_wallclock_as_timestamps)pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);if (!pktl && st->request_probe <= 0)return ret;add_to_pktbuf(&s->raw_packet_buffer, pkt, &s->raw_packet_buffer_end);s->raw_packet_buffer_remaining_size -= pkt->size;if ((err = probe_codec(s, st, pkt)) < 0)return err;}
}

ff_read_packet()中最关键的地方就是调用了AVInputFormat的read_packet()方法。AVInputFormat的read_packet()是一个函数指针，指向当前的AVInputFormat的读取数据的函数。在这里我们以FLV封装格式对应的AVInputFormat为例，看看read_packet()的实现函数是什么样子的。

FLV封装格式对应的AVInputFormat的定义位于libavformat\flvdec.c，如下所示。

AVInputFormat ff_flv_demuxer = {.name           = "flv",.long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),.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,.extensions     = "flv",.priv_class     = &flv_class,
};

从ff_flv_demuxer的定义可以看出，read_packet()对应的是flv_read_packet()函数。在看flv_read_packet()函数之前，我们先回顾一下FLV封装格式的结构，如下图所示。

从图中可以看出，FLV文件体部分是由一个一个的Tag连接起来的（中间间隔着Previous Tag Size）。每个Tag包含了Tag Header和Tag Data两个部分。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。

了解了FLV的基本格式之后，就可以看一下FLV解析Tag的函数flv_read_packet()了。

flv_read_packet()

flv_read_packet()的定义位于libavformat\flvdec.c，如下所示。

static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
{FLVContext *flv = s->priv_data;int ret, i, type, size, flags;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;/* pkt size is repeated at end. skip it */for (;; avio_skip(s->pb, 4)) {pos  = avio_tell(s->pb);//解析Tag Header==========//Tag类型type = (avio_r8(s->pb) & 0x1F);//Datasize数据大小size = avio_rb24(s->pb);//Timstamp时间戳dts  = avio_rb24(s->pb);dts |= avio_r8(s->pb) << 24;av_dlog(s, "type:%d, size:%d, dts:%"PRId64" pos:%"PRId64"\n", type, size, dts, avio_tell(s->pb));if (avio_feof(s->pb))return AVERROR_EOF;//StreamIDavio_skip(s->pb, 3); /* stream id, always 0 */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)continue;next = size + avio_tell(s->pb);if (type == FLV_TAG_TYPE_AUDIO) {//Type是音频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) {//Type是音频stream_type = FLV_STREAM_TYPE_VIDEO;//Tag Data的第一个字节flags    = avio_r8(s->pb);size--;if ((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_VIDEO_INFO_CMD)goto skip;} else if (type == FLV_TAG_TYPE_META) {stream_type=FLV_STREAM_TYPE_DATA;if (size > 13 + 1 + 4 && dts == 0) { // Header-type metadata stuffmeta_pos = avio_tell(s->pb);if (flv_read_metabody(s, next) <= 0) {goto skip;}avio_seek(s->pb, meta_pos, SEEK_SET);}} else {av_log(s, AV_LOG_DEBUG,"Skipping flv packet: type %d, size %d, flags %d.\n",type, size, flags);
skip:avio_seek(s->pb, next, SEEK_SET);continue;}/* skip empty data packets */if (!size)continue;/* now find stream */for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];if (stream_type == FLV_STREAM_TYPE_AUDIO) {if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO &&(s->audio_codec_id || flv_same_audio_codec(st->codec, flags)))break;} else if (stream_type == FLV_STREAM_TYPE_VIDEO) {if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO &&(s->video_codec_id || flv_same_video_codec(st->codec, flags)))break;} else if (stream_type == FLV_STREAM_TYPE_DATA) {if (st->codec->codec_type == AVMEDIA_TYPE_DATA)break;}}if (i == s->nb_streams) {static const enum AVMediaType stream_types[] = {AVMEDIA_TYPE_VIDEO, AVMEDIA_TYPE_AUDIO, AVMEDIA_TYPE_DATA};av_log(s, AV_LOG_WARNING, "Stream discovered after head already parsed\n");st = create_stream(s, stream_types[stream_type]);if (!st)return AVERROR(ENOMEM);}av_dlog(s, "%d %X %d \n", stream_type, flags, st->discard);if ((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);continue;}break;}// if not streamed and no duration from metadata then seek to end to find// the duration from the timestampsif (s->pb->seekable && (!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.int64_t fsize       = avio_size(s->pb);
retry_duration:avio_seek(s->pb, fsize - 4, SEEK_SET);size = avio_rb32(s->pb);// 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         |= 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;}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 (!st->codec->channels || !st->codec->sample_rate ||!st->codec->bits_per_coded_sample) {st->codec->channels              = channels;st->codec->channel_layout        = channels == 1? AV_CH_LAYOUT_MONO: AV_CH_LAYOUT_STEREO;st->codec->sample_rate           = sample_rate;st->codec->bits_per_coded_sample = bits_per_coded_sample;}if (!st->codec->codec_id) {flv_set_audio_codec(s, st, st->codec,flags & FLV_AUDIO_CODECID_MASK);flv->last_sample_rate =sample_rate           = st->codec->sample_rate;flv->last_channels    =channels              = st->codec->channels;} else {AVCodecContext ctx = {0};ctx.sample_rate = sample_rate;ctx.bits_per_coded_sample = bits_per_coded_sample;flv_set_audio_codec(s, st, &ctx, flags & FLV_AUDIO_CODECID_MASK);sample_rate = ctx.sample_rate;}} else if (stream_type == FLV_STREAM_TYPE_VIDEO) {size -= flv_set_video_codec(s, st, flags & FLV_VIDEO_CODECID_MASK, 1);}//几种特殊的格式if (st->codec->codec_id == AV_CODEC_ID_AAC ||st->codec->codec_id == AV_CODEC_ID_H264 ||st->codec->codec_id == AV_CODEC_ID_MPEG4) {//对应AACPacketType或者AVCPacketTypeint type = avio_r8(s->pb);size--;//H.264if (st->codec->codec_id == AV_CODEC_ID_H264 || st->codec->codec_id == AV_CODEC_ID_MPEG4) {// sign extension//对应CompositionTimeint32_t cts = (avio_rb24(s->pb) + 0xff800000) ^ 0xff800000;//计算PTSpts = 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;}}//如果编码器是AAC或者H.264if (type == 0 && (!st->codec->extradata || st->codec->codec_id == AV_CODEC_ID_AAC ||st->codec->codec_id == AV_CODEC_ID_H264)) {AVDictionaryEntry *t;if (st->codec->extradata) {if ((ret = flv_queue_extradata(flv, s->pb, stream_type, size)) < 0)return ret;ret = AVERROR(EAGAIN);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->codec->codec_id == AV_CODEC_ID_AAC && t && !strcmp(t->value, "Omnia A/XE"))st->codec->extradata_size = 2;//AACif (st->codec->codec_id == AV_CODEC_ID_AAC && 0) {MPEG4AudioConfig cfg;if (avpriv_mpeg4audio_get_config(&cfg, st->codec->extradata,st->codec->extradata_size * 8, 1) >= 0) {st->codec->channels       = cfg.channels;st->codec->channel_layout = 0;if (cfg.ext_sample_rate)st->codec->sample_rate = cfg.ext_sample_rate;elsest->codec->sample_rate = cfg.sample_rate;av_dlog(s, "mp4a config channels %d sample rate %d\n",st->codec->channels, st->codec->sample_rate);}}ret = AVERROR(EAGAIN);goto leave;}}/* skip empty data packets */if (!size) {ret = AVERROR(EAGAIN);goto leave;}ret = av_get_packet(s->pb, pkt, size);if (ret < 0)return ret;//设置PTS、DTS等等pkt->dts          = dts;pkt->pts          = pts == AV_NOPTS_VALUE ? dts : pts;pkt->stream_index = st->index;if (flv->new_extradata[stream_type]) {uint8_t *side = av_packet_new_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA,flv->new_extradata_size[stream_type]);if (side) {memcpy(side, flv->new_extradata[stream_type],flv->new_extradata_size[stream_type]);av_freep(&flv->new_extradata[stream_type]);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);}//标记上Keyframeif (    stream_type == FLV_STREAM_TYPE_AUDIO ||((flags & FLV_VIDEO_FRAMETYPE_MASK) == FLV_FRAME_KEY) ||stream_type == FLV_STREAM_TYPE_DATA)pkt->flags |= AV_PKT_FLAG_KEY;leave:avio_skip(s->pb, 4);return ret;
}

flv_read_packet()的代码比较长，但是逻辑比较简单。它的主要功能就是根据FLV文件格式的规范，逐层解析Tag以及TagData，获取Tag以及TagData中的信息。比较关键的地方已经写上了注释，不再详细叙述。

parse_packet()

parse_packet()给需要AVCodecParser的媒体流提供解析AVPacket的功能。它的代码如下所示：

/*** Parse a packet, add all split parts to parse_queue.** @param pkt Packet to parse, NULL when flushing the parser at end of stream.*/
static int parse_packet(AVFormatContext *s, AVPacket *pkt, int stream_index)
{AVPacket out_pkt = { 0 }, flush_pkt = { 0 };AVStream *st = s->streams[stream_index];uint8_t *data = pkt ? pkt->data : NULL;int size      = pkt ? pkt->size : 0;int ret = 0, got_output = 0;if (!pkt) {av_init_packet(&flush_pkt);pkt        = &flush_pkt;got_output = 1;} else if (!size && st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {// preserve 0-size sync packetscompute_pkt_fields(s, st, st->parser, pkt);}while (size > 0 || (pkt == &flush_pkt && got_output)) {int len;av_init_packet(&out_pkt);//解析len = av_parser_parse2(st->parser, st->codec,&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 */data += len;size -= len;got_output = !!out_pkt.size;//继续if (!out_pkt.size)continue;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 */out_pkt.duration = 0;if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {if (st->codec->sample_rate > 0) {out_pkt.duration =av_rescale_q_rnd(st->parser->duration,(AVRational) { 1, st->codec->sample_rate },st->time_base,AV_ROUND_DOWN);}}//设置属性值out_pkt.stream_index = st->index;out_pkt.pts          = st->parser->pts;out_pkt.dts          = st->parser->dts;out_pkt.pos          = st->parser->pos;if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW)out_pkt.pos = st->parser->frame_offset;if (st->parser->key_frame == 1 ||(st->parser->key_frame == -1 &&st->parser->pict_type == AV_PICTURE_TYPE_I))out_pkt.flags |= AV_PKT_FLAG_KEY;if (st->parser->key_frame == -1 && st->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, st->parser, &out_pkt);if (out_pkt.data == pkt->data && out_pkt.size == pkt->size) {out_pkt.buf = pkt->buf;pkt->buf    = NULL;
#if FF_API_DESTRUCT_PACKET
FF_DISABLE_DEPRECATION_WARNINGSout_pkt.destruct = pkt->destruct;pkt->destruct = NULL;
FF_ENABLE_DEPRECATION_WARNINGS
#endif}if ((ret = av_dup_packet(&out_pkt)) < 0)goto fail;if (!add_to_pktbuf(&s->parse_queue, &out_pkt, &s->parse_queue_end)) {av_free_packet(&out_pkt);ret = AVERROR(ENOMEM);goto fail;}}/* end of the stream => close and free the parser */if (pkt == &flush_pkt) {av_parser_close(st->parser);st->parser = NULL;}fail:av_free_packet(pkt);return ret;
}

从代码中可以看出，最终调用了相应AVCodecParser的av_parser_parse2()函数，解析出来AVPacket。此后根据解析的信息还进行了一系列的赋值工作，不再详细叙述。