??????? 在live555学习（七） --DESCRIBE命令处理中，对如何打开文件并获得其SDP信息已做了描述，这里针对H264数据的处理再做进一步分析。
??????? 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 this
5: // subsession (as a unicast stream). To do so,we first create
6: // 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 found
12:
13: struct in_addr dummyAddr;
14: dummyAddr.s_addr = 0;
15: Groupsock dummyGroupsock(envir(),dummyAddr,0);
16: unsigned char rtpPayloadType = 96 + trackNumber() - 1; // if dynamic
17: 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 %drn"
21: "c=IN IP4 %srn"
22: "b=AS:%urn"
23: "%s"
24: " 25: " 26: "a=control:%srn";
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:
??????? 注释里面解释得很清楚，H264不能在文件头中取得PPS/SPS，必须在播放一下后（当然，它是一个原始流文件，没有文件头）才行。也就是说不能从rtpSink中取得了。为了保证在函数退出前能取得AuxSDP，把大循环搬到这里来了。afterPlayingDummy()是在播放结束也就是取得aux sdp之后执行。在大循环之前的checkForAuxSDPLine()做了什么呢？?
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。
http://blog.csdn.net/nkmnkm/article/details/6931400
