How does semaphore work?

Question

Can the semaphore be lower than 0? I mean, say I have a semaphore with N=3 and I call \"down\" 4 times, then N will remain 0 but one process will be blocked?

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Answer 1

Yes, a negative value means you have processes waiting for the semaphore to be released. A positive value means you can call acquire that many times before the semaphore blocks.

You could think of the value in this way: a positive number means there are that many resources available. A negative value means there are that many entities needing a resource when all resources are taken at the moment. When you acquire a resource you decrement the value, when you release it you increase the value. If the value is still >= 0 after the decrement you get the resource, otherwise your entity is put into a queue.

A nice explanation of semaphores in Wikipedia: http://en.wikipedia.org/wiki/Semaphore_(programming)