假设系统有n（n>5）个并发进程，它们竞争互斥资源R。若采用PV操作，当有3个 进程同时申请资源R，而系统只能满足其中1个进程的申请时，资源R对应的信号量S 的值应为（ ）。
- A.-1
- B.-2
- C.-3
- D.0
正确答案及解析
正确答案
解析
PV操作：是实现进程同步和互斥的常用方法，P操作和V操作是低级通信原语，在执行期间不可分割；其中P操作表示申请一个资源，V操作表示释放一个资源。
P操作的定义：S：=S-1，若S>=0，则执行P操作的进程继续执行；若S＜0，则将该进程设为阻塞状态（因为无可用资源），并将其插入阻塞队列。
V操作的定义：S：=S+1，若S>0，则执行V操作的进程继续执行；若S＜=0，则从阻塞状态唤醒一个进程，并将其插入就绪队列，然后执行V操作的进程继续。
本题3个进程同时申请，1个进程获得资源，即1个进程运行另2个进程等待；当S值为负值时，S值的绝对值就是等待进程的数量，因此，此时S的绝对值应该为2，所以S=-2。
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