说明：  信号量也称为信号锁，主要应用于进程间的同步和互斥，在用于互斥时，通常作为资源锁。信号量通常通过两个原子操作wait(P)和signal(V)来访问。wait操作使信号量的值+1，signal操作使信号量的值-1。本程序模拟了操作系统的pv操作
(Semaphore is also known as signal lock, mainly used in inter-process synchronization and mutual exclusion, are used in mutually exclusive, it is usually as a resource lock. Semaphore usually through two atomic operations wait (P) and signal (V) to access. semaphore wait operation so that the value of+ 1, signal operation of the signal value of the volume of-1. This procedure simulates the operation of the operating system pv)