信号量的理解Semaphore s

两个进程共享信号量s,

一旦其中一个进程执行了semwait(s),它将得到信号量--》进入临界区，使s减1；
（To receive a signal via semaphore s,a Process executes the primitive semwait(s)）

而第二个进程被阻止进入临界区，因为当它试图执行semwait(s)时，s 为 0，它会被挂起以等待第一个进程离开临界区并执行semsignal(s)(释放信号量s)
(To transmit a signal via semaphore s, a process executes the primitive semsignal(s))

这个时候第二个进程可以恢复执行。


three operations are defined:

 A Semaphore may be initialized to a nonnegative integer value;

 the Semawait operation decrements the semaphore value. If the value becomes negative ,then the process executing the semWait is blocked.Otherweise,the process continues execution;

The SemSingal operation increments the semaphore value.If the resuling value is less than or equal to zero,then a process blocked by a semWait operation,if any,is unblocked.