ZLMedia中RTSP协议的处理简要分析（2）--SDP解析函数

通过void SdpParser::load(const string &sdp)解析。

void SdpParser::load(const string &sdp) {
    {
        _track_vec.clear();
        SdpTrack::Ptr track = std::make_shared<SdpTrack>();
        track->_type = TrackTitle;
        _track_vec.emplace_back(track);

        auto lines = split(sdp, "\n");
        for (auto &line : lines) {
            trim(line);
            if (line.size() < 2 || line[1] != '=') {
                continue;
            }
            char opt = line[0];
            string opt_val = line.substr(2);
            switch (opt) {
                case 't':
                    track->_t = opt_val;
                    break;
                case 'b':
                    track->_b = opt_val;
                    break;
                case 'm': {
                    track = std::make_shared<SdpTrack>();
                    int pt, port;
                    char rtp[16] = {0}, type[16];
                    if (4 == sscanf(opt_val.data(), " %15[^ ] %d %15[^ ] %d", type, &port, rtp, &pt)) {
                        track->_pt = pt;
                        track->_samplerate = RtpPayload::getClockRate(pt);
                        track->_channel = RtpPayload::getAudioChannel(pt);
                        track->_type = toTrackType(type);
                        track->_port = port;
                        _track_vec.emplace_back(track);
                    }
                    break;
                }
                case 'a': {
                    string attr = FindField(opt_val.data(), nullptr, ":");
                    if (attr.empty()) {
                        track->_attr.emplace(opt_val, "");
                    } else {
                        track->_attr.emplace(attr, FindField(opt_val.data(), ":", nullptr));
                    }
                    break;
                }
                default: track->_other[opt] = opt_val; break;
            }
        }
    }

    for (auto &track_ptr : _trackn_vec) {
        auto &track = *track_ptr;
        auto it = track._attr.find("range");
        if (it != track._attr.end()) {
            char name[16] = {0}, start[16] = {0}, end[16] = {0};
            int ret = sscanf(it->second.data(), "%15[^=]=%15[^-]-%15s", name, start, end);
            if (3 == ret || 2 == ret) {
                if (strcmp(start, "now") == 0) {
                    strcpy(start, "0");
                }
                track._start = (float) atof(start);
                track._end = (float) atof(end);
                track._duration = track._end - track._start;
            }
        }

        for (it = track._attr.find("rtpmap"); it != track._attr.end() && it->first == "rtpmap";) {
            auto &rtpmap = it->second;
            int pt, samplerate, channel;
            char codec[16] = {0};

            sscanf(rtpmap.data(), "%d", &pt);
            if (track._pt != pt) {
                //pt不匹配
                it = track._attr.erase(it);
                continue;
            }
            if (4 == sscanf(rtpmap.data(), "%d %15[^/]/%d/%d", &pt, codec, &samplerate, &channel)) {
                track._codec = codec;
                track._samplerate = samplerate;
                track._channel = channel;
            } else if (3 == sscanf(rtpmap.data(), "%d %15[^/]/%d", &pt, codec, &samplerate)) {
                track._pt = pt;
                track._codec = codec;
                track._samplerate = samplerate;
            }
            if (!track._samplerate && track._type == TrackVideo) {
                //未设置视频采样率时，赋值为90000
                track._samplerate = 90000;
            }
            ++it;
        }

        for (it = track._attr.find("fmtp"); it != track._attr.end() && it->first == "fmtp"; ) {
            auto &fmtp = it->second;
            int pt;
            sscanf(fmtp.data(), "%d", &pt);
            if (track._pt != pt) {
                //pt不匹配
                it = track._attr.erase(it);
                continue;
            }
            track._fmtp = FindField(fmtp.data(), " ", nullptr);
            ++it;
        }

        it = track._attr.find("control");
        if (it != track._attr.end()) {
            track._control = it->second;
        }
    }
}

1、track包括三部分：session、audio、video。

2、 auto lines = split(sdp, "\n")取出每一行，然后解析。

vector<string> split(const string &s, const char *delim) {
    vector<string> ret;
    size_t last = 0;
    auto index = s.find(delim, last);
    while (index != string::npos) {
        if (index - last > 0) {
            ret.push_back(s.substr(last, index - last));
        }
        last = index + strlen(delim);
        index = s.find(delim, last);
    }
    if (!s.size() || s.size() - last > 0) {
        ret.push_back(s.substr(last));
    }
    return ret;
}

3、track->_t 表示开始和结束的时间。

4、track->_b 表示带宽

5、if (4 == sscanf(opt_val.data (), " %15[^ ] %d %15[^ ] %d", type, &port, rtp, &pt))处理一个媒体信息。m=video 0 RTP/AVP 96

6、'a'的属性值存入到track->_attr。

7、其他的属性值存入到track->_other中。

8、for (it = track._attr.find("rtpmap"); it != track._attr.end() && it->first == "rtpmap";)解析

 

  （1）sscanf(rtpmap.data(), "%d", &pt);可以得到动态负载类型。

    (2) if (4 == sscanf(rtpmap.data(), "%d %15[^/]/%d/%d", &pt, codec, &samplerate, &channel))或else if (3 == sscanf(rtpmap.data(), "%d %15[^/]/%d", &pt, codec, &samplerate)。可以得到动态负载类型、编码名称、时钟频率、及通过号。

9、for (it = track._attr.find("fmtp"); it != track._attr.end() && it->first == "fmtp"; )解fmtp（附加参数），未对附加参数进一步解析。