参考链接

ffmpeg 源代码简单分析： av_read_frame()_雷霄骅的博客-CSDN博客_ffmpeg frame=

av_read_frame()

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）
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.** 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.*/
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

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

int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);const int genpts = s->flags & AVFMT_FLAG_GENPTS;int eof = 0;int ret;AVStream *st;if (!genpts) {ret = si->packet_buffer.head? avpriv_packet_list_get(&si->packet_buffer, pkt): read_frame_internal(s, pkt);if (ret < 0)return ret;goto return_packet;}for (;;) {PacketListEntry *pktl = si->packet_buffer.head;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;// last dts seen for this stream. if any of packets following// current one had no dts, we will set this to AV_NOPTS_VALUE.int64_t last_dts = next_pkt->dts;av_assert2(wrap_bits <= 64);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, 2ULL << (wrap_bits - 1)) < 0) {if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) {// not B-framenext_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.next_pkt->pts = last_dts + next_pkt->duration;}pktl = si->packet_buffer.head;}/* read packet from packet buffer, if there is data */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;}}ret = read_frame_internal(s, pkt);if (ret < 0) {if (pktl && ret != AVERROR(EAGAIN)) {eof = 1;continue;} elsereturn ret;}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;
}

read_frame_internal

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。

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int ret, got_packet = 0;AVDictionary *metadata = NULL;while (!got_packet && !si->parse_queue.head) {AVStream *st;FFStream *sti;/* read next packet */ret = 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)parse_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 */if (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");avcodec_close(sti->avctx);sti->info->found_decoder = 0;}/* 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->need_context_update = 0;}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;
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGSst->codecpar->channels = sti->avctx->ch_layout.nb_channels;st->codecpar->channel_layout = sti->avctx->ch_layout.order == AV_CHANNEL_ORDER_NATIVE ?sti->avctx->ch_layout.u.mask : 0;
FF_ENABLE_DEPRECATION_WARNINGS
#endifret = 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);if (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 (sti->inject_global_side_data) {for (int i = 0; i < st->nb_side_data; i++) {const AVPacketSideData *const 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);}sti->inject_global_side_data = 0;}}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;
}

ff_read_packet()

ff_read_packet()的代码比较长，如下所示。
ff_read_packet()中最关键的地方就是调用了AVInputFormat的read_packet()方法。

int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{FFFormatContext *const si = ffformatcontext(s);int err;#if FF_API_INIT_PACKET
FF_DISABLE_DEPRECATION_WARNINGSpkt->data = NULL;pkt->size = 0;av_init_packet(pkt);
FF_ENABLE_DEPRECATION_WARNINGS
#elseav_packet_unref(pkt);
#endiffor (;;) {PacketListEntry *pktl = si->raw_packet_buffer.head;AVStream *st;FFStream *sti;const AVPacket *pkt1;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;}}err = 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;}if (pkt->flags & AV_PKT_FLAG_CORRUPT) {av_log(s, AV_LOG_WARNING,"Packet corrupt (stream = %d, dts = %s)",pkt->stream_index, av_ts2str(pkt->dts));if (s->flags & AVFMT_FLAG_DISCARD_CORRUPT) {av_log(s, AV_LOG_WARNING, ", dropping it.\n");av_packet_unref(pkt);continue;}av_log(s, AV_LOG_WARNING, ".\n");}av_assert0(pkt->stream_index < (unsigned)s->nb_streams &&"Invalid stream index.\n");st  = s->streams[pkt->stream_index];sti = ffstream(st);if (update_wrap_reference(s, st, pkt->stream_index, pkt) && sti->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {// correct first time stamps to negative valuesif (!is_relative(sti->first_dts))sti->first_dts = wrap_timestamp(st, sti->first_dts);if (!is_relative(st->start_time))st->start_time = wrap_timestamp(st, st->start_time);if (!is_relative(sti->cur_dts))sti->cur_dts = wrap_timestamp(st, sti->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 && sti->request_probe <= 0)return 0;err = avpriv_packet_list_put(&si->raw_packet_buffer,pkt, NULL, 0);if (err < 0) {av_packet_unref(pkt);return err;}pkt1 = &si->raw_packet_buffer.tail->pkt;si->raw_packet_buffer_size += pkt1->size;if ((err = probe_codec(s, st, pkt1)) < 0)return err;}
}

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。

const 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_kux_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。

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, 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;retry:/* pkt size is repeated at end. skip it */pos  = avio_tell(s->pb);type = (avio_r8(s->pb) & 0x1F);orig_size =size = avio_rb24(s->pb);flv->sum_flv_tag_size += size + 11;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;avio_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) {ret = FFERROR_REDO;goto leave;}next = size + avio_tell(s->pb);if (type == FLV_TAG_TYPE_AUDIO) {stream_type = FLV_STREAM_TYPE_AUDIO;flags    = avio_r8(s->pb);size--;} else if (type == FLV_TAG_TYPE_VIDEO) {stream_type = FLV_STREAM_TYPE_VIDEO;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_SUBTITLE;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 {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 */for (i = 0; i < s->nb_streams; i++) {st = s->streams[i];if (stream_type == FLV_STREAM_TYPE_AUDIO) {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) {if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO &&(s->video_codec_id || flv_same_video_codec(st->codecpar, flags)))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;}}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 timestampsif ((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.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;}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) {int ret = flv_set_video_codec(s, st, flags & FLV_VIDEO_CODECID_MASK, 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}if (st->codecpar->codec_id == AV_CODEC_ID_AAC ||st->codecpar->codec_id == AV_CODEC_ID_H264 ||st->codecpar->codec_id == AV_CODEC_ID_MPEG4) {int type = avio_r8(s->pb);size--;if (size < 0) {ret = AVERROR_INVALIDDATA;goto leave;}if (st->codecpar->codec_id == AV_CODEC_ID_H264 || st->codecpar->codec_id == AV_CODEC_ID_MPEG4) {// sign extensionint32_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;}}if (type == 0 && (!st->codecpar->extradata || st->codecpar->codec_id == AV_CODEC_ID_AAC ||st->codecpar->codec_id == AV_CODEC_ID_H264)) {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;}ret = av_get_packet(s->pb, pkt, size);if (ret < 0)return ret;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;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 %d\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;
}

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：
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。

/*** 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 packetscompute_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;len = 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 */out_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);}}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);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;
}

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