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Semaphores are counters that can be either incremented or decremented. They can be used in situations where there is a finite limit to a resource and a mechanism is needed to impose that limit. An example might be a buffer that has a fixed size. Every time an element is added to a buffer, the number of available positions is decreased. Every time an element is removed, the number available is increased.
Semaphores can also be used to mimic mutexes; if there is only one element in the semaphore, then it can be either acquired or available, exactly as a mutex can be either locked or unlocked.
Semaphores will also signal or wake up threads that are waiting on them to use available resources; hence, they can be used for signaling between threads. For example, a thread might set a semaphore once it has completed some initialization. Other threads could wait on the semaphore and be signaled to start work once the initialization is complete.
Depending on the implementation, the method that acquires a semaphore might be called wait, down, or acquire, and the method to release a semaphore might be called post, up, signal, or release. When the semaphore no longer has resources available, the threads requesting resources will block until resources are available.
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