A semaphore is a thread synchronization device that can be used to control access to a shared resource.
Two flavors of semaphores are provided: counting semaphores and binary semaphores.
A counting semaphore is a counter that can be accessed concurrently by several threads. The typical use is to synchronize producers and consumers of a resource by counting how many units of the resource are available.
The two basic operations on semaphores are:
Binary semaphores are a variant of counting semaphores where semaphores can only take two values, 0 and 1.
A binary semaphore can be used to control access to a single shared resource, with value 1 meaning "resource is available" and value 0 meaning "resource is unavailable".
The "release" operation of a binary semaphore sets its value to 1, and "acquire" waits until the value is 1 and sets it to 0.
A binary semaphore can be used instead of a mutex (see module
Mutex) when the mutex discipline (of unlocking the mutex from the
thread that locked it) is too restrictive. The "acquire" operation
corresponds to locking the mutex, and the "release" operation to
unlocking it, but "release" can be performed in a thread different
than the one that performed the "acquire". Likewise, it is safe
to release a binary semaphore that is already available.