Semaphores - General Notes

Semaphores can be used for thread synchronization within the same process.

A semaphore is a synchronization object which has a count associated with it, typically representing the available number of some limited resource. A thread acquires one of these resources by using the acquire call; if none of the resources is available, the call either blocks or returns with an error depending on a "wait" option. If a resource is available (the count is non-zero), the count is decremented and the call returns with no error.

A thread releases a resource using the release call, which increments the count; if other threads are blocked because no resources are available (the count is zero), one of them is released. The others remain blocked because in the process of releasing the first one, the count is decremented back to zero.