ffmpeg中av_read_frame 超时设置

https://trac.ffmpeg.org/
ffmpeg wiki

ffmpeg中avformat_open_input超时设置
这里有日志可以参考:日志
y也可以参考:
https://blog.csdn.net/qpx0033/article/details/51290535

接着上篇文章,我们来看看av_read_frame,这个雷神有一篇文章:
https://blog.csdn.net/leixiaohua1020/article/details/12678577
https://www.freesion.com/article/6303637278/
这个比较有价值
https://blog.csdn.net/finewind/article/details/39433055

ffmpeg官网
https://www.ffmpeg.org/doxygen/trunk/avio_8h.html#aea3131c92c70177e463538e9a6b4308e

av_read_frame()-->read_frame_internal(s, pkt)-->





int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
//正常情况下这个数字都为0
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;

    if (!genpts) {
    //如果packet_buffer中还有数据,就直接进入其中去取数据,这个
    //函数单纯的取数据
        ret = s->internal->packet_buffer
              ? ff_packet_list_get(&s->internal->packet_buffer,
                                        &s->internal->packet_buffer_end, pkt)
              : read_frame_internal(s, pkt);//一般都是这里,然后goto
        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;
                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 = 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 = ff_packet_list_get(&s->internal->packet_buffer,
                                               &s->internal->packet_buffer_end, pkt);
                goto return_packet;
            }
        }
//在这里执行下一步
        ret = read_frame_internal(s, pkt);
        if (ret < 0) {
            if (pktl && ret != AVERROR(EAGAIN)) {
                eof = 1;
                continue;
            } else
                return ret;
        }

        ret = ff_packet_list_put(&s->internal->packet_buffer,
                                 &s->internal->packet_buffer_end,
                                 pkt, 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;
}


static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, got_packet = 0;
    AVDictionary *metadata = NULL;
//while循环,好多次才能从中获取到一个完整的帧,所以当你设置了超时比如200ms,但是实际超过200就是这个原因。
//一般两次或者三次就能读完整的一帧
    while (!got_packet && !s->internal->parse_queue) {
        AVStream *st;

        /* read next packet */
        //在这里执行读包
        ret = ff_read_packet(s, 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)
                //如果出错,将所有包全部发出去
                    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];

        /* 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) {
                av_packet_unref(pkt);
                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) {
                av_packet_unref(pkt);
                return ret;
            }
FF_ENABLE_DEPRECATION_WARNINGS
#endif

            st->internal->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 (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 */
            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 = 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(pkt);
        }
        if (pkt->flags & AV_PKT_FLAG_KEY)
            st->skip_to_keyframe = 0;
        if (st->skip_to_keyframe) {
            av_packet_unref(pkt);
            got_packet = 0;
        }
    }

    if (!got_packet && s->internal->parse_queue)
        ret = ff_packet_list_get(&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;
        }
    }

    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);

    /* 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;
}



int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, err;
    AVStream *st;

    pkt->data = NULL;
    pkt->size = 0;
    av_init_packet(pkt);

    for (;;) {
        AVPacketList *pktl = s->internal->raw_packet_buffer;
        const AVPacket *pkt1;

        if (pktl) {
            st = s->streams[pktl->pkt.stream_index];
            if (s->internal->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            if (st->request_probe <= 0) {
                ff_packet_list_get(&s->internal->raw_packet_buffer,
                                   &s->internal->raw_packet_buffer_end, pkt);
                s->internal->raw_packet_buffer_remaining_size += pkt->size;
                return 0;
            }
        }
        //在这里执行rtsp_demux的read_packet,这个函数是个static函数
        ret = s->iformat->read_packet(s, pkt);
        if (ret < 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 (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);
            }
            //注意这里的continue
            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];

        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);

        if (!pktl && st->request_probe <= 0)
            return ret;

        err = ff_packet_list_put(&s->internal->raw_packet_buffer,
                                 &s->internal->raw_packet_buffer_end,
                                 pkt, 0);
        if (err < 0) {
            av_packet_unref(pkt);
            return err;
        }
        pkt1 = &s->internal->raw_packet_buffer_end->pkt;
        s->internal->raw_packet_buffer_remaining_size -= pkt1->size;

        if ((err = probe_codec(s, st, pkt1)) < 0)
            return err;
    }
}



AVInputFormat ff_rtsp_demuxer = {
    .name           = "rtsp",
    .long_name      = NULL_IF_CONFIG_SMALL("RTSP input"),
    .priv_data_size = sizeof(RTSPState),
    .read_probe     = rtsp_probe,
    .read_header    = rtsp_read_header,
    .read_packet    = rtsp_read_packet,
    .read_close     = rtsp_read_close,
    .read_seek      = rtsp_read_seek,
    .flags          = AVFMT_NOFILE,
    .read_play      = rtsp_read_play,
    .read_pause     = rtsp_read_pause,
    .priv_class     = &rtsp_demuxer_class,
};



static int rtsp_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    RTSPState *rt = s->priv_data;
    int ret;
    RTSPMessageHeader reply1, *reply = &reply1;
    char cmd[1024];

retry:
    if (rt->server_type == RTSP_SERVER_REAL) {
        int i;

        for (i = 0; i < s->nb_streams; i++)
            rt->real_setup[i] = s->streams[i]->discard;

        if (!rt->need_subscription) {
            if (memcmp (rt->real_setup, rt->real_setup_cache,
                        sizeof(enum AVDiscard) * s->nb_streams)) {
                snprintf(cmd, sizeof(cmd),
                         "Unsubscribe: %s\r\n",
                         rt->last_subscription);
                ff_rtsp_send_cmd(s, "SET_PARAMETER", rt->control_uri,
                                 cmd, reply, NULL);
                if (reply->status_code != RTSP_STATUS_OK)
                    return ff_rtsp_averror(reply->status_code, AVERROR_INVALIDDATA);
                rt->need_subscription = 1;
            }
        }

        if (rt->need_subscription) {
            int r, rule_nr, first = 1;

            memcpy(rt->real_setup_cache, rt->real_setup,
                   sizeof(enum AVDiscard) * s->nb_streams);
            rt->last_subscription[0] = 0;

            snprintf(cmd, sizeof(cmd),
                     "Subscribe: ");
            for (i = 0; i < rt->nb_rtsp_streams; i++) {
                rule_nr = 0;
                for (r = 0; r < s->nb_streams; r++) {
                    if (s->streams[r]->id == i) {
                        if (s->streams[r]->discard != AVDISCARD_ALL) {
                            if (!first)
                                av_strlcat(rt->last_subscription, ",",
                                           sizeof(rt->last_subscription));
                            ff_rdt_subscribe_rule(
                                rt->last_subscription,
                                sizeof(rt->last_subscription), i, rule_nr);
                            first = 0;
                        }
                        rule_nr++;
                    }
                }
            }
            av_strlcatf(cmd, sizeof(cmd), "%s\r\n", rt->last_subscription);
            ff_rtsp_send_cmd(s, "SET_PARAMETER", rt->control_uri,
                             cmd, reply, NULL);
            if (reply->status_code != RTSP_STATUS_OK)
                return ff_rtsp_averror(reply->status_code, AVERROR_INVALIDDATA);
            rt->need_subscription = 0;

            if (rt->state == RTSP_STATE_STREAMING)
                rtsp_read_play (s);
        }
    }
//在这里获取新的packet
    ret = ff_rtsp_fetch_packet(s, pkt);
    if (ret < 0) {
        if (ret == AVERROR(ETIMEDOUT) && !rt->packets) {
            if (rt->lower_transport == RTSP_LOWER_TRANSPORT_UDP &&
                rt->lower_transport_mask & (1 << RTSP_LOWER_TRANSPORT_TCP)) {
                RTSPMessageHeader reply1, *reply = &reply1;
                av_log(s, AV_LOG_WARNING, "UDP timeout, retrying with TCP\n");
                if (rtsp_read_pause(s) != 0)
                    return -1;
                // TEARDOWN is required on Real-RTSP, but might make
                // other servers close the connection.
                if (rt->server_type == RTSP_SERVER_REAL)
                    ff_rtsp_send_cmd(s, "TEARDOWN", rt->control_uri, NULL,
                                     reply, NULL);
                rt->session_id[0] = '\0';
                if (resetup_tcp(s) == 0) {
                    rt->state = RTSP_STATE_IDLE;
                    rt->need_subscription = 1;
                    if (rtsp_read_play(s) != 0)
                        return -1;
                    goto retry;
                }
            }
        }
        return ret;
    }
    rt->packets++;

    if (!(rt->rtsp_flags & RTSP_FLAG_LISTEN)) {
        /* send dummy request to keep TCP connection alive */
        if ((av_gettime_relative() - rt->last_cmd_time) / 1000000 >= rt->timeout / 2 ||
            rt->auth_state.stale) {
            if (rt->server_type == RTSP_SERVER_WMS ||
                (rt->server_type != RTSP_SERVER_REAL &&
                 rt->get_parameter_supported)) {
                ff_rtsp_send_cmd_async(s, "GET_PARAMETER", rt->control_uri, NULL);
            } else {
                ff_rtsp_send_cmd_async(s, "OPTIONS", rt->control_uri, NULL);
            }
            /* The stale flag should be reset when creating the auth response in
             * ff_rtsp_send_cmd_async, but reset it here just in case we never
             * called the auth code (if we didn't have any credentials set). */
            rt->auth_state.stale = 0;
        }
    }

    return 0;
}


int ff_rtsp_fetch_packet(AVFormatContext *s, AVPacket *pkt)
{
//我们在av_open_input的时候不是说s->priv_data是
    RTSPState *rt = s->priv_data;
    int ret, len;
    RTSPStream *rtsp_st, *first_queue_st = NULL;
    int64_t wait_end = 0;

    if (rt->nb_byes == rt->nb_rtsp_streams)
        return AVERROR_EOF;

    /* get next frames from the same RTP packet */
    if (rt->cur_transport_priv) {
        if (rt->transport == RTSP_TRANSPORT_RDT) {
            ret = ff_rdt_parse_packet(rt->cur_transport_priv, pkt, NULL, 0);
        } else if (rt->transport == RTSP_TRANSPORT_RTP) {
            ret = ff_rtp_parse_packet(rt->cur_transport_priv, pkt, NULL, 0);
        } else if (CONFIG_RTPDEC && rt->ts) {
            ret = avpriv_mpegts_parse_packet(rt->ts, pkt, rt->recvbuf + rt->recvbuf_pos, rt->recvbuf_len - rt->recvbuf_pos);
            if (ret >= 0) {
                rt->recvbuf_pos += ret;
                ret = rt->recvbuf_pos < rt->recvbuf_len;
            }
        } else
            ret = -1;
        if (ret == 0) {
            rt->cur_transport_priv = NULL;
            return 0;
        } else if (ret == 1) {
            return 0;
        } else
            rt->cur_transport_priv = NULL;
    }

redo:
    if (rt->transport == RTSP_TRANSPORT_RTP) {
        int i;
        int64_t first_queue_time = 0;
        for (i = 0; i < rt->nb_rtsp_streams; i++) {
            RTPDemuxContext *rtpctx = rt->rtsp_streams[i]->transport_priv;
            int64_t queue_time;
            if (!rtpctx)
                continue;
            queue_time = ff_rtp_queued_packet_time(rtpctx);
            if (queue_time && (queue_time - first_queue_time < 0 ||
                               !first_queue_time)) {
                first_queue_time = queue_time;
                first_queue_st   = rt->rtsp_streams[i];
            }
        }
        if (first_queue_time) {
            wait_end = first_queue_time + s->max_delay;
        } else {
            wait_end = 0;
            first_queue_st = NULL;
        }
    }

    /* read next RTP packet */
    if (!rt->recvbuf) {
        rt->recvbuf = av_malloc(RECVBUF_SIZE);
        if (!rt->recvbuf)
            return AVERROR(ENOMEM);
    }
//在这里度packet
    len = read_packet(s, &rtsp_st, first_queue_st, wait_end);
    if (len == AVERROR(EAGAIN) && first_queue_st &&
        rt->transport == RTSP_TRANSPORT_RTP) {
        av_log(s, AV_LOG_WARNING,
                "max delay reached. need to consume packet\n");
        rtsp_st = first_queue_st;
        ret = ff_rtp_parse_packet(rtsp_st->transport_priv, pkt, NULL, 0);
        goto end;
    }
    if (len < 0)
        return len;

    if (rt->transport == RTSP_TRANSPORT_RDT) {
        ret = ff_rdt_parse_packet(rtsp_st->transport_priv, pkt, &rt->recvbuf, len);
    } else if (rt->transport == RTSP_TRANSPORT_RTP) {
        ret = ff_rtp_parse_packet(rtsp_st->transport_priv, pkt, &rt->recvbuf, len);
        if (rtsp_st->feedback) {
            AVIOContext *pb = NULL;
            if (rt->lower_transport == RTSP_LOWER_TRANSPORT_CUSTOM)
                pb = s->pb;
            ff_rtp_send_rtcp_feedback(rtsp_st->transport_priv, rtsp_st->rtp_handle, pb);
        }
        if (ret < 0) {
            /* Either bad packet, or a RTCP packet. Check if the
             * first_rtcp_ntp_time field was initialized. */
            RTPDemuxContext *rtpctx = rtsp_st->transport_priv;
            if (rtpctx->first_rtcp_ntp_time != AV_NOPTS_VALUE) {
                /* first_rtcp_ntp_time has been initialized for this stream,
                 * copy the same value to all other uninitialized streams,
                 * in order to map their timestamp origin to the same ntp time
                 * as this one. */
                int i;
                AVStream *st = NULL;
                if (rtsp_st->stream_index >= 0)
                    st = s->streams[rtsp_st->stream_index];
                for (i = 0; i < rt->nb_rtsp_streams; i++) {
                    RTPDemuxContext *rtpctx2 = rt->rtsp_streams[i]->transport_priv;
                    AVStream *st2 = NULL;
                    if (rt->rtsp_streams[i]->stream_index >= 0)
                        st2 = s->streams[rt->rtsp_streams[i]->stream_index];
                    if (rtpctx2 && st && st2 &&
                        rtpctx2->first_rtcp_ntp_time == AV_NOPTS_VALUE) {
                        rtpctx2->first_rtcp_ntp_time = rtpctx->first_rtcp_ntp_time;
                        rtpctx2->rtcp_ts_offset = av_rescale_q(
                            rtpctx->rtcp_ts_offset, st->time_base,
                            st2->time_base);
                    }
                }
                // Make real NTP start time available in AVFormatContext
                if (s->start_time_realtime == AV_NOPTS_VALUE) {
                    s->start_time_realtime = av_rescale (rtpctx->first_rtcp_ntp_time - (NTP_OFFSET << 32), 1000000, 1LL << 32);
                    if (rtpctx->st) {
                        s->start_time_realtime -=
                            av_rescale (rtpctx->rtcp_ts_offset,
                                        (uint64_t) rtpctx->st->time_base.num * 1000000,
                                                   rtpctx->st->time_base.den);
                    }
                }
            }
            if (ret == -RTCP_BYE) {
                rt->nb_byes++;

                av_log(s, AV_LOG_DEBUG, "Received BYE for stream %d (%d/%d)\n",
                       rtsp_st->stream_index, rt->nb_byes, rt->nb_rtsp_streams);

                if (rt->nb_byes == rt->nb_rtsp_streams)
                    return AVERROR_EOF;
            }
        }
    } else if (CONFIG_RTPDEC && rt->ts) {
        ret = avpriv_mpegts_parse_packet(rt->ts, pkt, rt->recvbuf, len);
        if (ret >= 0) {
            if (ret < len) {
                rt->recvbuf_len = len;
                rt->recvbuf_pos = ret;
                rt->cur_transport_priv = rt->ts;
                return 1;
            } else {
                ret = 0;
            }
        }
    } else {
        return AVERROR_INVALIDDATA;
    }
end:
    if (ret < 0)
        goto redo;
    if (ret == 1)
        /* more packets may follow, so we save the RTP context */
        rt->cur_transport_priv = rtsp_st->transport_priv;

    return ret;
}
#endif /* CONFIG_RTPDEC */



static int read_packet(AVFormatContext *s,
                       RTSPStream **rtsp_st, RTSPStream *first_queue_st,
                       int64_t wait_end)
{
    RTSPState *rt = s->priv_data;
    int len;

    switch(rt->lower_transport) {
    default:
#if CONFIG_RTSP_DEMUXER
    case RTSP_LOWER_TRANSPORT_TCP:
    //如果设置为tcp的话,进入这个函数
        len = ff_rtsp_tcp_read_packet(s, rtsp_st, rt->recvbuf, RECVBUF_SIZE);
        break;
#endif
    case RTSP_LOWER_TRANSPORT_UDP:
    case RTSP_LOWER_TRANSPORT_UDP_MULTICAST:
        len = udp_read_packet(s, rtsp_st, rt->recvbuf, RECVBUF_SIZE, wait_end);
        if (len > 0 && (*rtsp_st)->transport_priv && rt->transport == RTSP_TRANSPORT_RTP)
            ff_rtp_check_and_send_back_rr((*rtsp_st)->transport_priv, (*rtsp_st)->rtp_handle, NULL, len);
        break;
    case RTSP_LOWER_TRANSPORT_CUSTOM:
        if (first_queue_st && rt->transport == RTSP_TRANSPORT_RTP &&
            wait_end && wait_end < av_gettime_relative())
            len = AVERROR(EAGAIN);
        else
            len = avio_read_partial(s->pb, rt->recvbuf, RECVBUF_SIZE);
        len = pick_stream(s, rtsp_st, rt->recvbuf, len);
        if (len > 0 && (*rtsp_st)->transport_priv && rt->transport == RTSP_TRANSPORT_RTP)
            ff_rtp_check_and_send_back_rr((*rtsp_st)->transport_priv, NULL, s->pb, len);
        break;
    }

    if (len == 0)
        return AVERROR_EOF;

    return len;
}

int ff_rtsp_tcp_read_packet(AVFormatContext *s, RTSPStream **prtsp_st,
                            uint8_t *buf, int buf_size)
{
    RTSPState *rt = s->priv_data;
    int id, len, i, ret;
    RTSPStream *rtsp_st;

    av_log(s, AV_LOG_TRACE, "tcp_read_packet:\n");
redo:
//这里是为了寻找rtp包头的位置,一直循环去找
    for (;;) {
        RTSPMessageHeader reply;
//https://blog.csdn.net/weixin_35804181/article/details/87383444
//RTP(tcp)  interleaverframe
//|- Byte[0]:'$'-|- Byte[1]:channel-|-Byte[2:3]:package len-|-RTP common head-|
//第二个字节指示通道号,0表示RTP数据,1表示RTCP数据;后面两个字节用来指示插入的有效数据长度
        ret = ff_rtsp_read_reply(s, &reply, NULL, 1, NULL);
        if (ret < 0)
            return ret;
        if (ret == 1) /* received '$' */
            break;
        /* XXX: parse message */
        if (rt->state != RTSP_STATE_STREAMING)
            return 0;
    }
    //在这里获取数据长度,前三个字节是长度
    ret = ffurl_read_complete(rt->rtsp_hd, buf, 3);
    if (ret != 3)
        return -1;
    id  = buf[0];
    len = AV_RB16(buf + 1);
    av_log(s, AV_LOG_TRACE, "id=%d len=%d\n", id, len);
    if (len > buf_size || len < 8)
        goto redo;
    /* get the data */
    //在这里真正读取数据,传入的数据就是RTSPState中的URLContext
    ret = ffurl_read_complete(rt->rtsp_hd, buf, len);
    if (ret != len)
        return -1;
    if (rt->transport == RTSP_TRANSPORT_RDT &&
        ff_rdt_parse_header(buf, len, &id, NULL, NULL, NULL, NULL) < 0)
        return -1;

    /* find the matching stream */
    for (i = 0; i < rt->nb_rtsp_streams; i++) {
        rtsp_st = rt->rtsp_streams[i];
        if (id >= rtsp_st->interleaved_min &&
            id <= rtsp_st->interleaved_max)
            goto found;
    }
    goto redo;
found:
    *prtsp_st = rtsp_st;
    return len;
}


int ff_rtsp_read_reply(AVFormatContext *s, RTSPMessageHeader *reply,
                       unsigned char **content_ptr,
                       int return_on_interleaved_data, const char *method)
{
    RTSPState *rt = s->priv_data;
    char buf[4096], buf1[1024], *q;
    unsigned char ch;
    const char *p;
    int ret, content_length, line_count = 0, request = 0;
    unsigned char *content = NULL;

start:
    line_count = 0;
    request = 0;
    content = NULL;
    memset(reply, 0, sizeof(*reply));

    /* parse reply (XXX: use buffers) */
    rt->last_reply[0] = '\0';
    for (;;) {
        q = buf;
        for (;;) {
        rtp基于tcp的包头比基于udp的包头多了4个字节: 
* magic固定为0x24* channel用来区分音视频等多路流媒体的通道,其中偶数通道为流媒体内容,奇数通道为RTCP 
* len表示数据包的长度减去开始的4个字节,即len字段之后的数据长度
* https://blog.csdn.net/qq_23552895/article/details/88226339
            ret = ffurl_read_complete(rt->rtsp_hd, &ch, 1);
            av_log(s, AV_LOG_TRACE, "ret=%d c=%02x [%c]\n", ret, ch, ch);
            //如果没有读到直接返回错误-50532
            if (ret != 1)
                return AVERROR_EOF;
                //什么时候返回\n 
                //rtsp 501 NoITimplementedrtsp 停止发送
            if (ch == '\n')
                break;
            if (ch == '$' && q == buf) {
                if (return_on_interleaved_data) {
                    return 1;
                } else
                    ff_rtsp_skip_packet(s);
            } else if (ch != '\r') {
                if ((q - buf) < sizeof(buf) - 1)
                    *q++ = ch;
            }
        }
        *q = '\0';

        av_log(s, AV_LOG_TRACE, "line='%s'\n", buf);

        /* test if last line */
        if (buf[0] == '\0')
            break;
        p = buf;
        if (line_count == 0) {
            /* get reply code */
            get_word(buf1, sizeof(buf1), &p);
            if (!strncmp(buf1, "RTSP/", 5)) {
                get_word(buf1, sizeof(buf1), &p);
                reply->status_code = atoi(buf1);
                av_strlcpy(reply->reason, p, sizeof(reply->reason));
            } else {
                av_strlcpy(reply->reason, buf1, sizeof(reply->reason)); // method
                get_word(buf1, sizeof(buf1), &p); // object
                request = 1;
            }
        } else {
            ff_rtsp_parse_line(s, reply, p, rt, method);
            av_strlcat(rt->last_reply, p,    sizeof(rt->last_reply));
            av_strlcat(rt->last_reply, "\n", sizeof(rt->last_reply));
        }
        line_count++;
    }

    if (rt->session_id[0] == '\0' && reply->session_id[0] != '\0' && !request)
        av_strlcpy(rt->session_id, reply->session_id, sizeof(rt->session_id));

    content_length = reply->content_length;
    if (content_length > 0) {
        /* leave some room for a trailing '\0' (useful for simple parsing) */
        content = av_malloc(content_length + 1);
        if (!content)
            return AVERROR(ENOMEM);
        ffurl_read_complete(rt->rtsp_hd, content, content_length);
        content[content_length] = '\0';
    }
    if (content_ptr)
        *content_ptr = content;
    else
        av_freep(&content);

    if (request) {
        char buf[1024];
        char base64buf[AV_BASE64_SIZE(sizeof(buf))];
        const char* ptr = buf;

        if (!strcmp(reply->reason, "OPTIONS")) {
            snprintf(buf, sizeof(buf), "RTSP/1.0 200 OK\r\n");
            if (reply->seq)
                av_strlcatf(buf, sizeof(buf), "CSeq: %d\r\n", reply->seq);
            if (reply->session_id[0])
                av_strlcatf(buf, sizeof(buf), "Session: %s\r\n",
                                              reply->session_id);
        } else {
            snprintf(buf, sizeof(buf), "RTSP/1.0 501 Not Implemented\r\n");
        }
        av_strlcat(buf, "\r\n", sizeof(buf));

        if (rt->control_transport == RTSP_MODE_TUNNEL) {
            av_base64_encode(base64buf, sizeof(base64buf), buf, strlen(buf));
            ptr = base64buf;
        }
        ffurl_write(rt->rtsp_hd_out, ptr, strlen(ptr));

        rt->last_cmd_time = av_gettime_relative();
        /* Even if the request from the server had data, it is not the data
         * that the caller wants or expects. The memory could also be leaked
         * if the actual following reply has content data. */
        if (content_ptr)
            av_freep(content_ptr);
        /* If method is set, this is called from ff_rtsp_send_cmd,
         * where a reply to exactly this request is awaited. For
         * callers from within packet receiving, we just want to
         * return to the caller and go back to receiving packets. */
        if (method)
            goto start;
        return 0;
    }

    if (rt->seq != reply->seq) {
        av_log(s, AV_LOG_WARNING, "CSeq %d expected, %d received.\n",
            rt->seq, reply->seq);
    }

    /* EOS */
    if (reply->notice == 2101 /* End-of-Stream Reached */      ||
        reply->notice == 2104 /* Start-of-Stream Reached */    ||
        reply->notice == 2306 /* Continuous Feed Terminated */) {
        rt->state = RTSP_STATE_IDLE;
    } else if (reply->notice >= 4400 && reply->notice < 5500) {
        return AVERROR(EIO); /* data or server error */
    } else if (reply->notice == 2401 /* Ticket Expired */ ||
             (reply->notice >= 5500 && reply->notice < 5600) /* end of term */ )
        return AVERROR(EPERM);

    return 0;
}


int ffurl_read_complete(URLContext *h, unsigned char *buf, int size)
{
    if (!(h->flags & AVIO_FLAG_READ))
        return AVERROR(EIO);
        //注意这里的 h->prot->url_read是tcp read
    return retry_transfer_wrapper(h, buf, size, size, h->prot->url_read);
}












static inline int retry_transfer_wrapper(URLContext *h, uint8_t *buf,
                                         int size, int size_min,
                                         int (*transfer_func)(URLContext *h,
                                                              uint8_t *buf,
                                                              int size))
{
    int ret, len;
    int fast_retries = 5;
    int64_t wait_since = 0;

    len = 0;
    while (len < size_min) {
    //在这里执行打断回调函数
        if (ff_check_interrupt(&h->interrupt_callback))
            return AVERROR_EXIT;
   //tcp read 回调
        ret = transfer_func(h, buf + len, size - len);
        if (ret == AVERROR(EINTR))
            continue;
            //如果是非阻塞字节返回
        if (h->flags & AVIO_FLAG_NONBLOCK)
            return ret;
        if (ret == AVERROR(EAGAIN)) {
            ret = 0;
            //循环5次
            if (fast_retries) {
                fast_retries--;
            } else {
                if (h->rw_timeout) {
                    if (!wait_since)
                        wait_since = av_gettime_relative();
                    else if (av_gettime_relative() > wait_since + h->rw_timeout)
                        return AVERROR(EIO);
                }
                av_usleep(1000);
            }
        } else if (ret == AVERROR_EOF)
            return (len > 0) ? len : AVERROR_EOF;
        else if (ret < 0)
            return ret;
        if (ret) {
            fast_retries = FFMAX(fast_retries, 2);
            wait_since = 0;
        }
        len += ret;
    }
    return len;
}

static int tcp_read(URLContext *h, uint8_t *buf, int size)
{
    TCPContext *s = h->priv_data;
    int ret;
//这里设置非阻塞后就不会执行了,直接去recv
/**
 * Use non-blocking mode.
 * If this flag is set, operations on the context will return
 * AVERROR(EAGAIN) if they can not be performed immediately.
 * If this flag is not set, operations on the context will never return
 * AVERROR(EAGAIN).
 * Note that this flag does not affect the opening/connecting of the
 * context. Connecting a protocol will always block if necessary (e.g. on
 * network protocols) but never hang (e.g. on busy devices).
 * Warning: non-blocking protocols is work-in-progress; this flag may be
 * silently ignored.
 */
//#define AVIO_FLAG_NONBLOCK 8
    if (!(h->flags & AVIO_FLAG_NONBLOCK)) {
        ret = ff_network_wait_fd_timeout(s->fd, 0, h->rw_timeout, &h->interrupt_callback);
        if (ret)
            return ret;
    }
    成功执行时,返回接收到的字节数。

另一端已关闭则返回0。

失败返回-1,

errno被设为以下的某个值

EAGAIN:套接字已标记为非阻塞,而接收操作被阻塞或者接收超时

EBADF:sock不是有效的描述词

ECONNREFUSE:远程主机阻绝网络连接

EFAULT:内存空间访问出错

EINTR:操作被信号中断

EINVAL:参数无效

ENOMEM:内存不足

ENOTCONN:与面向连接关联的套接字尚未被连接上

ENOTSOCK:sock索引的不是套接字 当返回值是0时,为正常关闭连接
    ret = recv(s->fd, buf, size, 0);
    if (ret == 0)
        return AVERROR_EOF;
    return ret < 0 ? ff_neterrno() : ret;
}

int ff_network_wait_fd_timeout(int fd, int write, int64_t timeout, AVIOInterruptCB *int_cb)
{
    int ret;
    int64_t wait_start = 0;

    while (1) {
        if (ff_check_interrupt(int_cb))
            return AVERROR_EXIT;//负数
        ret = ff_network_wait_fd(fd, write);
        if (ret != AVERROR(EAGAIN))
            return ret;
           //相当于执行若干次循环,timeout就是我们传入的时间stimeout
        if (timeout > 0) {
            if (!wait_start)
                wait_start = av_gettime_relative();
            else if (av_gettime_relative() - wait_start > timeout)
                return AVERROR(ETIMEDOUT);
        }
    }
}

int ff_network_wait_fd(int fd, int write)
{
    int ev = write ? POLLOUT : POLLIN;
    struct pollfd p = { .fd = fd, .events = ev, .revents = 0 };
    int ret;
    ret = poll(&p, 1, POLLING_TIME);
    return ret < 0 ? ff_neterrno() : p.revents & (ev | POLLERR | POLLHUP) ? 0 : AVERROR(EAGAIN);
}

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