A semaphore. The type 'a is only a phantom type specifying if the semaphore is a named or unnamed one. This avoids the possible error of closing/destroying a semaphore with the wrong function.

sem_wait s decrements (locks) the semaphore s. If the value of s is greater than zero, the decrement proceeds and sem_wait returns immediately. Otherwise the call blocks until either is becomes possible due to a concurrent sem_post or a signal handler interrupts the call.

sem_post s increments (unlocks) the semaphore s.

sem_init n creates and initializes a new unnamed semaphore. The initial value of the semaphore is n.

sem_getvalue s returns the current value of the semaphore s.

sem_destroy s destroys the unnamed semaphore s.

sem_close s closes the named semaphore s.

sem_trywait s is the same as sem_wait s, except that if the decrement cannot be performed immediately, sem_trywait returns an error instead of blocking.

sem_timedwait s t is the same as sem_wait s except that t specifies the amount of time the call should block if the decrement cannot be immediately perforned. t is an asbolute time; if it has already expired by the time of the call, and the semaphore s could not be locked immediately, sem_timedwait s t fails with a timeout error.

sem_unlink path removes the named semaphore referenced by path. The semaphore is removed immediately but only destroyed once all other processes that have the semaphore open close it.

sem_open path flags perms n creates a new named semaphore with initial value n.