当RTSPServer收到对某个媒体的DESCRIBE请求时，它会找到对应的ServerMediaSession,调用ServerMediaSession::generateSDPDescription()。generateSDPDescription()中会遍历调用ServerMediaSession中所有的调用ServerMediaSubsession，通过subsession->sdplines()取得每个Subsession的sdp，合并成一个完整的SDP返回之。我们几乎可以断定，文件的打开和分析应该是在每个Subsession的sdplines()函数中完成的，看看这个函数：
1: char const* ondemandServerMediaSubsession::sdplines()2: {3: if (fSdplines == NULL) {4: // We need to construct a set of SDP lines that describe this5: // subsession (as a unicast stream). To do so,we first create6: // dummy (unused) source and "RTPSink" objects,7: // whose parameters we use for the SDP lines:8: unsigned estBitrate;9: FramedSource* inputSource = createNewStreamSource(0,estBitrate);10: if (inputSource == NULL)11: return NULL; // file not found12:13: struct in_addr dummyAddr;14: dummyAddr.s_addr = 0;15: Groupsock dummyGroupsock(envir(),dummyAddr,0);16: unsigned char rtpPayloadType = 96 + trackNumber() - 1; // if dynamic17: RTPSink* dummyRTPSink = createNewRTPSink(&dummyGroupsock,18: rtpPayloadType,inputSource);19:20: setSdplinesFromrTPSink(dummyRTPSink,inputSource,monospace; font-size:12px"> 21: Medium::close(dummyRTPSink);22: closeStreamSource(inputSource);23: }24:25: return fSdplines;26: }27:
Subsession中直接保存了对应媒体文件的SDP，但是在第一次获取时fSdplines为NULL，所以需先获取fSdplines。其做法比较费事，是通过建临时的Source和RTPSink，把它们连接成一个StreamToken，Playing一段时间之后才取得了fSdplines。createNewStreamSource()和createNewRTPSink()都是虚函数，所以此处创建的source和sink都是继承类指定的，我们分析的是H264，也就是H264VideoFileServerMediaSubsession所指定的，来看一下这两个函数：
1: FramedSource* H264VideoFileServerMediaSubsession::createNewStreamSource(
2: /*clientSessionId*/,monospace; font-size:12px"> 3: unsigned& estBitrate)
4: { 5: estBitrate = 500; // kbps,estimate 6:
7: // Create the video source: 8: ByteStreamFileSource* fileSource = ByteStreamFileSource::createNew(envir(),fFileName);
9: if (fileSource == NULL) return NULL;
11: fFileSize = fileSource->fileSize();
13: // Create a framer for the Video Elementary Stream: 14: return H264VideoStreamFramer::createNew(envir(),fileSource); 15: }
16:
17: RTPSink* H264VideoFileServerMediaSubsession::createNewRTPSink(
18: Groupsock* rtpGroupsock,monospace; font-size:12px"> 19: char rtpPayloadTypeIfDynamic,monospace; font-size:12px"> 20: FramedSource* /*inputSource*/)
21: { 22: return H264VideoRTPSink::createNew(envir(),rtpGroupsock,rtpPayloadTypeIfDynamic); 23: }
24:
可以看到，分别创建了H264VideoStreamFramer和H264VideoRTPSink。可以肯定H264VideoStreamFramer也是一个Source，但它内部又利用了另一个source--ByteStreamFileSource。后面会分析为什么要这样做，这里先不要管它。还没有看到真正打开文件的代码，继续探索：
void ondemandServerMediaSubsession::setSdplinesFromrTPSink(
2: RTPSink* rtpSink,monospace; font-size:12px"> 3: FramedSource* inputSource,255)">unsigned estBitrate)
5: { 6: if (rtpSink == NULL) return;
8:
const* mediaType = rtpSink->sdpMediaType();
10: char rtpPayloadType = rtpSink->rtpPayloadType(); 11: struct in_addr serverAddrForSDP; 12: serverAddrForSDP.s_addr = fServerAddressForSDP;
char* const ipAddressstr = strDup(our_inet_ntoa(serverAddrForSDP));
char* rtpmapLine = rtpSink->rtpmapLine();
15: const* rangeLine = rangeSdpline();
16: const* auxSdpline = getAuxSdpline(rtpSink,monospace; font-size:12px"> 17: if (auxSdpline == NULL)
18: auxSdpline = "";
20: const* const sdpFmt = "m=%s %u RTP/AVP %d\r\n"
21: "c=IN IP4 %s\r\n" 22: "b=AS:%u\r\n" 23: "%s" 24: " 25: " 26: "a=control:%s\r\n"; 27: unsigned sdpFmtSize = strlen(sdpFmt) + strlen(mediaType) + 5 /* max short len */
28: + 3 /* max char len */ 29: + strlen(ipAddressstr) + 20 /* max int len */
30: + strlen(rtpmapLine) + strlen(rangeLine) + strlen(auxSdpline)
31: + strlen(trackId());
32: char* sdplines = new char[sdpFmtSize];
33: sprintf(sdplines,sdpFmt,mediaType,// m= <media>
34: fPortNumForSDP,0)">// m= <port>
35: rtpPayloadType,0)">// m= <fmt list>
36: ipAddressstr,0)">// c= address
37: estBitrate,0)">// b=AS:<bandwidth>
38: rtpmapLine,0)">// a=rtpmap:... (if present)
39: rangeLine,0)">// a=range:... (if present)
40: auxSdpline,0)">// optional extra SDP line
41: trackId()); // a=control:<track-id> 42: delete[] (char*) rangeLine;
43: delete[] rtpmapLine; 44: delete[] ipAddressstr;
45:
46: fSdplines = strDup(sdplines);
47: delete[] sdplines;
48: }
49:
此函数中取得Subsession的sdp并保存到fSdplines。打开文件应在rtpSink->rtpmapLine()甚至是Source创建时已经做了。我们不防先把它放一放，而是先把SDP的获取过程搞个通透。所以把焦点集中到getAuxSdpline()上。
const* ondemandServerMediaSubsession::getAuxSdpline(
3: FramedSource* 5: // Default implementation: return rtpSink == NULL ? NULL : rtpSink->auxSdpline();
7: }
8:
很简单，调用了rtpSink->auxSdpline()那么我们要看H264VideoRTPSink::auxSdpline()：不用看了，很简单，取得source 中保存的PPS,SPS等形成a=fmpt行。但事实上并没有这么简单，H264VideoFileServerMediaSubsession重写了getAuxSdpline()！如果不重写，则说明auxSdpline已经在前面分析文件时获得了，那么既然重写，就说明前面没有获取到，只能在这个函数中重写。H264VideoFileServerMediaSubsession中这个函数:
const* H264VideoFileServerMediaSubsession::getAuxSdpline(
3: FramedSource* inputSource)
if (fAuxSdpline != NULL)
return fAuxSdpline; // it's already been set up (for a prevIoUs client)
7:
8: if (fDummyRTPSink == NULL) { // we're not already setting it up for another,concurrent stream
9: // Note: For H264 video files,the 'config' information ("profile-level-id" and "sprop-parameter-sets") isn't kNown // until we start reading the file. This means that "rtpSink"s "auxSdpline()" will be NULL initially,monospace; font-size:12px"> 11: // and we need to start reading data from our file until this changes.
12: fDummyRTPSink = rtpSink;
13:
14: // Start reading the file: 15: fDummyRTPSink->startPlaying(*inputSource,afterPlayingDummy,255)">this);
17: // Check whether the sink's 'auxSdpline()' is ready: 18: checkForAuxSdpline( 19: }
20:
21: envir().taskScheduler().doEventLoop(&fDoneFlag);
22:
23: return fAuxSdpline;
24: }
25:
void H264VideoFileServerMediaSubsession::checkForAuxSdpline1()
const* dasl;
4:
if (fAuxSdpline != NULL) {
// Signal the event loop that we're done:
7: setDoneFlag();
8: } else if (fDummyRTPSink != NULL
9: && (dasl = fDummyRTPSink->auxSdpline()) != NULL) {
10: fAuxSdpline = strDup(dasl);
11: fDummyRTPSink = NULL;
14: setDoneFlag();
15: } else { // try again after a brief delay:
int uSecsToDelay = 100000; // 100 ms 18: nextTask() = envir().taskScheduler().scheduleDelayedTask(uSecsToDelay,monospace; font-size:12px"> 19: (TaskFunc*) checkForAuxSdpline,monospace; font-size:12px"> 20: }
21: }
22:
它检查是否已取得Aux sdp，如果取得了，设置结束标志，直接返回。如果没有，就检查是否sink中已取得了aux sdp，如果是，也设置结束标志，返回。如果还没有取得，则把这个检查函数做为delay task加入计划任务中。每100毫秒检查一次，每检查一次主要就是调用一次fDummyRTPSink->auxSdpline()。大循环在检测到fDoneFlag改变时停止，此时已取得了aux sdp。但是如果直到文件结束也没有得到aux sdp，则afterPlayingDummy( )被执行，在其中停止掉这个大循环。然后在父类Subsession中关掉这些临时的source和sink。在真正播放时重新创建。
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