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.