About Semaphores

Semaphores signal the beginning or ending of an operation and provide mutually exclusive ownership of resources. Typically, semaphores are used to prevent more than one process or thread within a process from accessing a resource, such as shared memory, at the same time.

Semaphores are defined by OS/2 and reside in an internal memory buffer. They are divided into three types, according to the functionality they provide:

• Event semaphores enable a thread to notify waiting threads that an event has occurred. The waiting threads then resume execution, performing operations that are dependent on the completion of the signaled event.

• Mutual exclusion (mutex) semaphores enable threads to serialize their access to shared resources. That is, ownership of a mutex semaphore is used by cooperating threads as a prerequisite for performing operations on a resource. (Threads cooperate by using the mutex semaphore functions to ensure that access to the resource is mutually exclusive.)

• Multiple wait (muxwait) semaphores enable threads to wait either for multiple events to occur, or for multiple resources to become available. Alternatively, a flag can be set so that a thread waits for any one of multiple events to occur, or for any one of multiple resources to become available.