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We're covering multithreaded programming in a class I'm taking. The professor offered a bonus question that I have been trying, to no avail, to figure out:

Each of processes P0, P1, P2 and P3 have to wait for the other three to cross or reach a particular synchronization point in their code, and only then may that process cross its own synchronization point.

I already know how to answer the question with four semaphores, the hard part is doing it with only one semaphore.

Any suggestions or hints as to how to proceed?

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原来教授的意思是说你可以使用两个信号量而不是一个。他相信，正如我在考虑了一段时间后所做的那样，使用单个信号量是不可能的。

于 2009-10-27T04:39:30.580 回答
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如果它不是二进制信号量，只需将信号量初始化为 -4。

于 2009-10-26T01:00:09.163 回答
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您对施加于您的解决方案的限制有点了解，但请参阅The Little Book of Semaphores并通读有关障碍的部分。那应该给你一些想法。

于 2009-10-26T02:27:12.403 回答