计数信号量用来控制同时访问某个特定资源的操作数或同时执行某个指定操作的数量
A counting semaphore.Conceptually, a semaphore maintains a set of permits. Each acquire blocks if necessary until a permit is available, and then takes it. Each release adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.A count
