- A.V(S1)和P(S2)
- B.P(S1)和V(S2)
- C.V(S1)和V(S2)
- D.V(S2)和P(S1)
正确答案及解析
正确答案
解析
根据前驱图，P2进程需要等待P1进程的通知，故需要利用P(S1)操作测试P1进程是否运行完，所以空①应填P(S1)，P2进程运行结束需要利用V操作分别通知P3、P4进程，由于P3进程执行前已经用P(S2)，所以空②应填V(S2)。
根据前驱图，P3进程运行结束需要利用两个V操作分别通知P4、P5进程，故空③应为两个V操作；又由于P4执行前需要等待进程P2、P3的结果，需要2个P操作，而P4进程的程序中执行前只有1个P操作，故空④应为1个P操作。采用排除法，对于试题(49)的选项A、选项B、选项C和选项D中，只有选项A满足条件。
根据前驱图P4、P5进程执行完都需要通知P6进程，P6进程运行前执行了P(S6)和P(S7)，而P5进程执行完后执行了V(S7)，故P4进程应该执行V(S6)，即空⑤应填V(S6)。又因为P5进程开始运行前必须等待P3进程的通知，需要用P(S5)操作测试P3进程是否运行完。故空⑥应填写P(S5)。
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