ijkplayer框架简析 -- av_read_frame

av_read_frame() 的作用是读取码流中的音频若干帧或者视频一帧

av_read_frame

int av_read_frame(AVFormatContext *s, AVPacket *pkt){

    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;

    if (!genpts) {
        ret = s->internal->packet_buffer
              ? read_from_packet_buffer(&s->internal->packet_buffer, // 从缓存中读取
                                        &s->internal->packet_buffer_end, pkt)
              : read_frame_internal(s, pkt); // 从文件，或探测缓存中读取
        if (ret < 0)
            return ret;
        goto return_packet;
    }

    for (;;) {
        AVPacketList *pktl = s->internal->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;
                // 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;
                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)) {
                        if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2LL << (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 = s->internal->packet_buffer;
            }

            /* 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 = read_from_packet_buffer(&s->internal->packet_buffer,
                                               &s->internal->packet_buffer_end, 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 = add_to_pktbuf(&s->internal->packet_buffer, pkt,
                            &s->internal->packet_buffer_end, 1);
        av_packet_unref(pkt);
        if (ret < 0)
            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;
}

av_read_frame() 会先判断未解码缓存中是否又数据，如果有的话则调用 read_from_packet_buffer() ，否则就调用 read_frame_internal()

read_frame_internal

static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
    int ret = 0, i, got_packet = 0;
    AVDictionary *metadata = NULL;

    av_init_packet(pkt);

    while (!got_packet && !s->internal->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];
                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];

        /* update context if required */
        if (st->internal->need_context_update) {
            if (avcodec_is_open(st->internal->avctx)) {
                av_log(s, AV_LOG_DEBUG, "Demuxer context update while decoder is open, closing and trying to re-open\n");
                avcodec_close(st->internal->avctx);
                st->info->found_decoder = 0;
            }

            /* close parser, because it depends on the codec */
            if (st->parser && st->internal->avctx->codec_id != st->codecpar->codec_id) {
                av_parser_close(st->parser);
                st->parser = NULL;
            }

            ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);
            if (ret < 0)
                return ret;

#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
            /* update deprecated public codec context */
            ret = avcodec_parameters_to_context(st->codec, st->codecpar);
            if (ret < 0)
                return ret;
FF_ENABLE_DEPRECATION_WARNINGS
#endif

            st->internal->need_context_update = 0;
        }

        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=%"PRId64", 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->codecpar->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->codecpar->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, 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, &cur_pkt, cur_pkt.stream_index)) < 0)
                return ret;
            st->codecpar->sample_rate = st->internal->avctx->sample_rate;
            st->codecpar->bit_rate = st->internal->avctx->bit_rate;
            st->codecpar->channels = st->internal->avctx->channels;
            st->codecpar->channel_layout = st->internal->avctx->channel_layout;
            st->codecpar->codec_id = st->internal->avctx->codec_id;
        } else {
            /* free packet */
            av_packet_unref(&cur_pkt);
        }
        if (pkt->flags & AV_PKT_FLAG_KEY)
            st->skip_to_keyframe = 0;
        if (st->skip_to_keyframe) {
            av_packet_unref(&cur_pkt);
            if (got_packet) {
                *pkt = cur_pkt;
            }
            got_packet = 0;
        }
    }

    if (!got_packet && s->internal->parse_queue)
        ret = read_from_packet_buffer(&s->internal->parse_queue, &s->internal->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->start_skip_samples && (pkt->pts == 0 || pkt->pts == RELATIVE_TS_BASE))
            st->skip_samples = st->start_skip_samples;
        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 / discard %d\n", st->skip_samples, discard_padding);
            }
            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 FF_API_LAVF_MERGE_SD
FF_DISABLE_DEPRECATION_WARNINGS
        if (!(s->flags & AVFMT_FLAG_KEEP_SIDE_DATA))
            av_packet_merge_side_data(pkt);
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    }

    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 FF_API_LAVF_AVCTX
    update_stream_avctx(s);
#endif

    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);

    return ret;
}

• 调用了 ff_read_packet() 从相应的 AVInputFormat 读取数据
• 如果媒体频流需要使用 AVCodecParser，则调用 parse_packet() 解析相应的 AVPacket

ff_read_packet

int ff_read_packet(AVFormatContext *s, AVPacket *pkt){

    int ret, i, err;
    AVStream *st;

    for (;;) {
        AVPacketList *pktl = s->internal->raw_packet_buffer;

        // 如果缓存中有数据，则到缓存中取
        if (pktl) {
            *pkt = pktl->pkt;
            st   = s->streams[pkt->stream_index];
            // raw_packet_buffer 剩下的有效buf大小小于0
            if (s->internal->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            // 如果 探测结束，则更新信息，并返回
            if (st->request_probe <= 0) {
                s->internal->raw_packet_buffer                 = pktl->next;
                s->internal->raw_packet_buffer_remaining_size += pkt->size;
                av_free(pktl);
                return 0;
            }
        }

        pkt->data = NULL;
        pkt->size = 0;
        // 用 AVInputFormat 的 read_packet 函数指针从文件读取数据
        av_init_packet(pkt);
        ret = s->iformat->read_packet(s, pkt);
        if (ret < 0) {
            /* 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. */
               data and discard it (ignored streams, junk, extradata).
               We must re-call the demuxer to get the real packet. */
            if (ret == FFERROR_REDO)
                continue;
            if (!pktl || ret == AVERROR(EAGAIN))
                return ret;
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
                if (st->probe_packets || st->request_probe > 0)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(st->request_probe <= 0);
            }
            continue;
        }

        if (!pkt->buf) {
            AVPacket tmp = { 0 };
            ret = av_packet_ref(&tmp, pkt);
            if (ret < 0)
                return ret;
            *pkt = tmp;
        }

        // 如果 AVFormatContext 的 flags 设置了丢弃损坏包，并且此包是损坏的，则丢弃
        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_packet_unref(pkt);
            continue;
        }

        // 读取到的 pkt 的 stream_index 不合法，抛弃
        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 values
            if (!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);

        // 如果不需要探测，则返回，如果需要探测，且 raw_packet_buffer 为空，则把刚读到的包加进去
        if (!pktl && st->request_probe <= 0)
            return ret;

        // 把刚读的包，加入raw_packet_buffe
        err = add_to_pktbuf(&s->internal->raw_packet_buffer, pkt,
                            &s->internal->raw_packet_buffer_end, 0);
        if (err)
            return err;
        s->internal->raw_packet_buffer_remaining_size -= pkt->size;

        if ((err = probe_codec(s, st, pkt)) < 0)
            return err;
    }
}

在 for 循环中调用了 iformat->read_packet(s, pkt) ，是 AVInputFormat 的指针函数，以 mp4 为例对应的是

AVInputFormat ff_mov_demuxer = {
    .name           = "mov,mp4,m4a,3gp,3g2,mj2",
    .long_name      = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
    .priv_class     = &mov_class,
    .priv_data_size = sizeof(MOVContext),
    .extensions     = "mov,mp4,m4a,3gp,3g2,mj2",
    .read_probe     = mov_probe,
    .read_header    = mov_read_header,
    .read_packet    = mov_read_packet,
    .read_close     = mov_read_close,
    .read_seek      = mov_read_seek,
    .flags          = AVFMT_NO_BYTE_SEEK,
};


static int mov_read_packet(AVFormatContext *s, AVPacket *pkt){

    MOVContext *mov = s->priv_data;
    MOVStreamContext *sc;
    AVIndexEntry *sample;
    AVStream *st = NULL;
    int64_t current_index;
    int ret;
    mov->fc = s;
 retry:
    // 查找下一个sample
    sample = mov_find_next_sample(s, &st);
    if (!sample || (mov->next_root_atom && sample->pos > mov->next_root_atom)) {
        if (!mov->next_root_atom)
            return AVERROR_EOF;
        if ((ret = mov_switch_root(s, mov->next_root_atom)) < 0)
            return ret;
        goto retry;
    }
    sc = st->priv_data;
    /* must be done just before reading, to avoid infinite loop on sample */
    current_index = sc->current_index;
    mov_current_sample_inc(sc);

    if (mov->next_root_atom) {
        sample->pos = FFMIN(sample->pos, mov->next_root_atom);
        sample->size = FFMIN(sample->size, (mov->next_root_atom - sample->pos));
    }

    if (st->discard != AVDISCARD_ALL) {
         // 通过 sample 中的 pos 找到媒体文件中的 sample 起始点与大小，然后根据这个去读取媒体文件的内容
        int64_t ret64 = avio_seek(sc->pb, sample->pos, SEEK_SET);
        if (ret64 != sample->pos) {
            av_log(mov->fc, AV_LOG_ERROR, "stream %d, offset 0x%"PRIx64": partial file\n",
                   sc->ffindex, sample->pos);
            if (should_retry(sc->pb, ret64)) {
                mov_current_sample_dec(sc);
            }
            return AVERROR_INVALIDDATA;
        }

        if( st->discard == AVDISCARD_NONKEY && 0==(sample->flags & AVINDEX_KEYFRAME) ) {
            av_log(mov->fc, AV_LOG_DEBUG, "Nonkey frame from stream %d discarded due to AVDISCARD_NONKEY\n", sc->ffindex);
            goto retry;
        }

        // 读取媒体文件的内容，大小是 sample->size
        ret = av_get_packet(sc->pb, pkt, sample->size);
        if (ret < 0) {
            if (should_retry(sc->pb, ret)) {
                mov_current_sample_dec(sc);
            }
            return ret;
        }
        if (sc->has_palette) {
            uint8_t *pal;

            pal = av_packet_new_side_data(pkt, AV_PKT_DATA_PALETTE, AVPALETTE_SIZE);
            if (!pal) {
                av_log(mov->fc, AV_LOG_ERROR, "Cannot append palette to packet\n");
            } else {
                memcpy(pal, sc->palette, AVPALETTE_SIZE);
                sc->has_palette = 0;
            }
        }
#if CONFIG_DV_DEMUXER
        if (mov->dv_demux && sc->dv_audio_container) {
            avpriv_dv_produce_packet(mov->dv_demux, pkt, pkt->data, pkt->size, pkt->pos);
            av_freep(&pkt->data);
            pkt->size = 0;
            ret = avpriv_dv_get_packet(mov->dv_demux, pkt);
            if (ret < 0)
                return ret;
        }
#endif
        if (st->codecpar->codec_id == AV_CODEC_ID_MP3 && !st->need_parsing && pkt->size > 4) {
            if (ff_mpa_check_header(AV_RB32(pkt->data)) < 0)
                st->need_parsing = AVSTREAM_PARSE_FULL;
        }
    }

    pkt->stream_index = sc->ffindex;
    // dts 是计算出来的 start_time + sampleduration
    pkt->dts = sample->timestamp;
    if (sample->flags & AVINDEX_DISCARD_FRAME) {
        pkt->flags |= AV_PKT_FLAG_DISCARD;
    }
    if (sc->ctts_data && sc->ctts_index < sc->ctts_count) {
        pkt->pts = pkt->dts + sc->dts_shift + sc->ctts_data[sc->ctts_index].duration;
        /* update ctts context */
        sc->ctts_sample++;
        if (sc->ctts_index < sc->ctts_count &&
            sc->ctts_data[sc->ctts_index].count == sc->ctts_sample) {
            sc->ctts_index++;
            sc->ctts_sample = 0;
        }
    } else {
        int64_t next_dts = (sc->current_sample < st->nb_index_entries) ?
            st->index_entries[sc->current_sample].timestamp : st->duration;
        pkt->duration = next_dts - pkt->dts;
        pkt->pts = pkt->dts;
    }
    if (st->discard == AVDISCARD_ALL)
        goto retry;
    pkt->flags |= sample->flags & AVINDEX_KEYFRAME ? AV_PKT_FLAG_KEY : 0;
    pkt->pos = sample->pos;

    /* Multiple stsd handling. */
    if (sc->stsc_data) {
        /* Keep track of the stsc index for the given sample, then check        * if the stsd index is different from the last used one. */
        * if the stsd index is different from the last used one. */
        sc->stsc_sample++;
        if (mov_stsc_index_valid(sc->stsc_index, sc->stsc_count) &&
            mov_get_stsc_samples(sc, sc->stsc_index) == sc->stsc_sample) {
            sc->stsc_index++;
            sc->stsc_sample = 0;
        /* Do not check indexes after a switch. */
        } else if (sc->stsc_data[sc->stsc_index].id > 0 &&
                   sc->stsc_data[sc->stsc_index].id - 1 < sc->stsd_count &&
                   sc->stsc_data[sc->stsc_index].id - 1 != sc->last_stsd_index) {
            ret = mov_change_extradata(sc, pkt);
            if (ret < 0)
                return ret;
        }
    }

    if (mov->aax_mode)
        aax_filter(pkt->data, pkt->size, mov);

    if (sc->cenc.aes_ctr) {
        ret = cenc_filter(mov, sc, current_index, pkt->data, pkt->size);
        if (ret) {
            return ret;
        }
    }

    return 0;
}

• mov_find_next_sample() 找到要读取的文件的偏移 + size
• avio_seek(sc->pb, sample->pos, SEEK_SET) 将文件的指针定位到文件的偏移处
• av_get_packet(sc->pb, pkt, sample->size) 从文件的偏移读取 size 的数据

parse_packet

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 packets
        compute_pkt_fields(s, st, st->parser, pkt, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
    }

    while (size > 0 || (pkt == &flush_pkt && got_output)) {
        int len;
        int64_t next_pts = pkt->pts;
        int64_t next_dts = pkt->dts;

        av_init_packet(&out_pkt);
        len = av_parser_parse2(st->parser, st->internal->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 */
        data += len;
        size -= len;

        got_output = !!out_pkt.size;

        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 = (st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->duration : 0;
        if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (st->internal->avctx->sample_rate > 0) {
                out_pkt.duration =
                    av_rescale_q_rnd(st->parser->duration,
                                     (AVRational) { 1, st->internal->avctx->sample_rate },
                                     st->time_base,
                                     AV_ROUND_DOWN);
            }
        }

        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, next_dts, next_pts);

        ret = add_to_pktbuf(&s->internal->parse_queue, &out_pkt,
                            &s->internal->parse_queue_end, 1);
        av_packet_unref(&out_pkt);
        if (ret < 0)
            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_packet_unref(pkt);
    return ret;
}

parse_packet() 的作用是给需要 AVCodecParser 的媒体流提供解析 AVPacket 的功能，主要是通过 av_parser_parse2() 来解析 AVPacket

调用链

参考

• 雷霄骅 - FFMPEG