假设系统有n (n
5）个并发进程共享资源R，且资源R的可用数为3。若采用PV操作，则相应的信号量S的取值范围应为（ ）。
- A.-3~n-5
- B.-3~5
- C.-(n-5)~5
- D.-(n-3)~3
正确答案及解析
正确答案
解析
PV操作与信号量的处理相关，P表示通过的意思，V表示释放的意思。一般来说，信号量S大于等于0时，S表示可用资源的数量。执行一次P操作意味着请求分配一个单位资源，因此S的值减1；当S小于0时，表示已经没有可用资源，请求者必须等待别的进程释放该类资源，它才能运行下去。而执行一个V操作意味着释放一个单位资源，因此S的值加1；若S小于0，表示有某些进程正在等待该资源，因此要唤醒一个等待状态的进程，使之运行下去。初始值资源数为3，所以信号量S的最大值是3，n进程申请，则信号量S的最小值为3-n，也就是 -(n-3)。
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