进程Pa不断向管道写数据，进程Pb从管道中读取数据并加工处理，如图2．7所示。如果采用PV操作来实现进程Pa和Pb的管道通信，并且保证这两个进程并发执行的正确性，则至少需要( )。
- A.1个信号量，信号量的初始值为0
- B.2个信号量，信号量的初始值为0、1
- C.3个信号量，信号量的初始值为0、0、1
- D.4个信号量，信号量的初始值为0、0、1、1
正确答案及解析
正确答案
解析
这是一个典型的生产者和消费者问题。其中进程Pa和Pb分别为生产者和消费者，管道为临界区。我们的程序应该设置一个同步信号量，为1时说明管道已满拒绝Pa再写入数据；为0时说明管道为空拒绝Pb再读出数据。管道初始是没有数据的，所以初始值为0(特例情况即管道的大小为1个单位)。程序还需要1个互斥信号量来保证程序只有一个进程访问管道，其初始值为1。
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