Multiprocess Synchronization with a Single Semaphore

Question

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?

Answer 1

Just initialize your semaphore at -4 if it's not a binary one.

Answer 2

You are a little light on the constraints imposed on your solution but see The Little Book of Semaphores and read through the sections on barriers. That should give you some ideas.

Answer 3

Turns out the professor had meant to say that you could use two semaphores instead of one. He believes, as I do after having thought about the problem for a while, that it is impossible to do with a single semaphore.