FFmpeg源代码简单分析-解码-av_read_frame()

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-frame
                            next_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;
            } else
                return 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_WARNINGS
            st->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
#endif
            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);

    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_WARNINGS
    pkt->data = NULL;
    pkt->size = 0;
    av_init_packet(pkt);
FF_ENABLE_DEPRECATION_WARNINGS
#else
    av_packet_unref(pkt);
#endif

    for (;;) {
        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 consume
               data 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 values
            if (!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 stuff
            int 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 timestamps
    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.
        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 extension
            int32_t cts = (avio_rb24(s->pb) + 0xff800000) ^ 0xff800000;
            pts = av_sat_add64(dts, cts);
            if (cts < 0) { // dts might be wrong
                if (!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 packets
        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;

        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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