Conceptually, a semaphore is a nonnegative integer count. Semaphores are typically used to coordinate access to resources, with the semaphore count initialized to the number of free resources. Threads then atomically increment the count when resources are added and atomically decrement the count when resources are removed.
When the semaphore count becomes zero, no more resources are present. Threads that try to decrement the semaphore when the count is zero block until the count becomes greater than zero.
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Because semaphores need not be acquired and be released by the same thread, semaphores can be used for asynchronous event notification, such as in signal handlers. And, because semaphores contain state, semaphores can be used asynchronously without acquiring a mutex lock as is required by condition variables. However, semaphores are not as efficient as mutex locks.
The scheduling policy determines the order in which blocked threads are awakened. The default scheduling policy, SCHED_OTHER, does not specify the order in which threads are awakened. Under the SCHED_FIFO and SCHED_RR real-time scheduling policies, threads are awakened in priority order.
Semaphores must be initialized before use, however semaphores do not have attributes.