Live555源代码解读（2）
三、消息循环
看服端的主体：live555MediaServer.cpp中的main()函数，可见其创建一个RTSPServer类实例后，即进入一个函数env->taskScheduler().doEventLoop()中，看名字很明显是一个消息循坏，执行到里面后不停地转圈，生名不息，转圈不止。那么在这个人生的圈圈中如何实现RTSP服务和RTP传输呢？别想那么远了，还是先看这个圈圈中实现了什么功能吧。
[cpp] view plaincopy
void BasicTaskScheduler0::doEventLoop(char* watchVariable) {
// Repeatedly loop, handling readble sockets and timed events:
while (1) {
if (watchVariable != NULL && *watchVariable != 0)
break;
SingleStep();
}
}
BasicTaskScheduler0从TaskScheduler派生，所以还是一个任务调度对象，所以依然说明任务调度对象是整个程序的发动机。
循环中每次走一步：SingleStep()。这走一步中都做些什么呢？
总结为以下四步：
１为所有需要操作的socket执行select。
２找出第一个应执行的socket任务(handler)并执行之。
３找到第一个应响应的事件，并执行之。
４找到第一个应执行的延迟任务并执行之。
可见，每一步中只执行三个任务队列中的一项。下面详细分析函数SingleStep()：
[cpp] view plaincopy
//循坏中主要执行的函数
void BasicTaskScheduler::SingleStep(unsigned maxDelayTime) {
fd_set readSet = fReadSet; // make a copy for this select() call
fd_set writeSet = fWriteSet; // ditto
fd_set exceptionSet = fExceptionSet; // ditto
//计算select socket们时的超时时间。
DelayInterval const& timeToDelay = fDelayQueue.timeToNextAlarm();
struct timeval tv_timeToDelay;
tv_timeToDelay.tv_sec = timeToDelay.seconds();
tv_timeToDelay.tv_usec = timeToDelay.useconds();
// Very large "tv_sec" values cause select() to fail.
// Don't make it any larger than 1 million seconds (11.5 days)
const long MAX_TV_SEC = MILLION;
if (tv_timeToDelay.tv_sec > MAX_TV_SEC) {
tv_timeToDelay.tv_sec = MAX_TV_SEC;
}
// Also check our "maxDelayTime" parameter (if it's > 0):
if (maxDelayTime > 0
&& (tv_timeToDelay.tv_sec > (long) maxDelayTime / MILLION
|| (tv_timeToDelay.tv_sec == (long) maxDelayTime / MILLION
&& tv_timeToDelay.tv_usec
> (long) maxDelayTime % MILLION))) {
tv_timeToDelay.tv_sec = maxDelayTime / MILLION;
tv_timeToDelay.tv_usec = maxDelayTime % MILLION;
}
//先执行socket的select操作，以确定哪些socket任务（handler）需要执行。
int selectResult = select(fMaxNumSockets,
&readSet, &writeSet,&exceptionSet,
&tv_timeToDelay);
if (selectResult < 0) {
//#if defined(__WIN32__) || defined(_WIN32)
int err = WSAGetLastError();
// For some unknown reason, select() in Windoze sometimes fails with WSAEINVAL if
// it was called with no entries set in "readSet". If this happens, ignore it:
if (err == WSAEINVAL && readSet.fd_count == 0) {
err = EINTR;
// To stop this from happening again, create a dummy socket:
int dummySocketNum = socket(AF_INET, SOCK_DGRAM, 0);
FD_SET((unsigned) dummySocketNum, &fReadSet);
}
if (err != EINTR) {
//#else
// if (errno != EINTR && errno != EAGAIN) {
//#endif
// Unexpected error - treat this as fatal:
//#if !defined(_WIN32_WCE)
// perror("BasicTaskScheduler::SingleStep(): select() fails");
//#endif
internalError();
}
}
// Call the handler function for one readable socket:
HandlerIterator iter(*fHandlers);
HandlerDescriptor* handler;
// To ensure forward progress through the handlers, begin past the last
// socket number that we handled:
if (fLastHandledSocketNum >= 0) {
//找到上次执行的socket handler的下一个
while ((handler = iter.next()) != NULL) {
if (handler->socketNum == fLastHandledSocketNum)
break;
}
if (handler == NULL) {
fLastHandledSocketNum = -1;
iter.reset(); // start from the beginning instead
}
}
//从找到的handler开始，找一个可以执行的handler，不论其状态是可读，可写，还是出错，执行之。
while ((handler = iter.next()) != NULL) {
int sock = handler->socketNum; // alias
int resultConditionSet = 0;
if (FD_ISSET(sock, &readSet)
&& FD_ISSET(sock, &fReadSet)/*sanity check*/)
resultConditionSet |= SOCKET_READABLE;
if (FD_ISSET(sock, &writeSet)
&& FD_ISSET(sock, &fWriteSet)/*sanity check*/)
resultConditionSet |= SOCKET_WRITABLE;
if (FD_ISSET(sock, &exceptionSet)
&& FD_ISSET(sock, &fExceptionSet)/*sanity check*/)
resultConditionSet |= SOCKET_EXCEPTION;
if ((resultConditionSet & handler->conditionSet)
!= 0 && handler->handlerProc != NULL) {
fLastHandledSocketNum = sock;
// Note: we set "fLastHandledSocketNum" before calling the handler,
// in case the handler calls "doEventLoop()" reentrantly.
(*handler->handlerProc)(handler->clientData, resultConditionSet);
break;
}
}
//如果寻找完了依然没有执行任何handle，则从头再找。
if (handler == NULL && fLastHandledSocketNum >= 0) {
// We didn't call a handler, but we didn't get to check all of them,
// so try again from the beginning:
iter.reset();
while ((handler = iter.next()) != NULL) {
int sock = handler->socketNum; // alias
int resultConditionSet = 0;
if (FD_ISSET(sock, &readSet)&& FD_ISSET(sock, &fReadSet)/*sanity check*/)
resultConditionSet |= SOCKET_READABLE;
if (FD_ISSET(sock, &writeSet)&& FD_ISSET(sock, &fWriteSet)/*sanity check*/)
resultConditionSet |= SOCKET_WRITABLE;
if (FD_ISSET(sock, &exceptionSet) && FD_ISSET(sock, &fExceptionSet)/*sanity check*/)
resultConditionSet |= SOCKET_EXCEPTION;
if ((resultConditionSet & handler->conditionSet)
!= 0 && handler->handlerProc != NULL) {
fLastHandledSocketNum = sock;
// Note: we set "fLastHandledSocketNum" before calling the handler,
// in case the handler calls "doEventLoop()" reentrantly.
(*handler->handlerProc)(handler->clientData, resultConditionSet);
break;
}
}
//依然没有找到可执行的handler。
if (handler == NULL)
fLastHandledSocketNum = -1; //because we didn't call a handler
}
//响应事件
// Also handle any newly-triggered event
// (Note that we do this *after* calling a socket handler,
// in case the triggered event handler modifies The set of readable sockets.)
if (fTriggersAwaitingHandling != 0) {
if (fTriggersAwaitingHandling == fLastUsedTriggerMask) {
// Common-case optimization for a single event trigger:
fTriggersAwaitingHandling = 0;
if (fTriggeredEventHandlers[fLastUsedTriggerNum] != NULL) {
//执行一个事件处理函数
(*fTriggeredEventHandlers[fLastUsedTriggerNum])(fTriggeredEventClientDatas[fLastUsedTriggerNum]);
}
} else {
// Look for an event trigger that needs handling
// (making sure that we make forward progress through all possible triggers):
unsigned i = fLastUsedTriggerNum;
EventTriggerId mask = fLastUsedTriggerMask;
do {
i = (i + 1) % MAX_NUM_EVENT_TRIGGERS;
mask >>= 1;
if (mask == 0)
mask = 0x80000000;
if ((fTriggersAwaitingHandling & mask) != 0) {
//执行一个事件响应
fTriggersAwaitingHandling &= ~mask;
if (fTriggeredEventHandlers[i] != NULL) {
(*fTriggeredEventHandlers[i])(fTriggeredEventClientDatas[i]);
}
fLastUsedTriggerMask = mask;
fLastUsedTriggerNum = i;
break;
}
} while (i != fLastUsedTriggerNum);
}
}
//执行一个最迫切的延迟任务。
// Also handle any delayed event that may have come due.
fDelayQueue.handleAlarm();
}