Conceptually, a semaphore is an non-negative integer count that is atomically incremented and decremented. Typically this represents the number of resources available. The _lwp_sema_init() function initializes the count, _lwp_sema_post() atomically increments the count, and _lwp_sema_wait() waits for the count to become greater than 0 and then atomically decrements it.

LWP semaphores must be initialized before use. The _lwp_sema_init() function initializes the count associated with the LWP semaphore pointed to by sema to count.

The _lwp_sema_wait() function blocks the calling LWP until the semaphore count becomes greater than 0 and then atomically decrements it.

The _lwp_sema_trywait() function atomically decrements the count if it is greater than zero. Otherwise it returns an error.

The _lwp_sema_post() function atomically increments the semaphore count. If there are any LWPs blocked on the semaphore, one is unblocked.

Upon successful completion, 0 is returned. A non-zero value indicates an error.

The _lwp_sema_init(), _lwp_sema_trywait(), _lwp_sema_wait(), and _lwp_sema_post() functions will fail if:

EINVAL

The sema argument points to an invalid semaphore.

EFAULT

The sema argument points to an illegal address.

The _lwp_sema_wait() function will fail if:

EINTR

The function execution was interrupted by a signal or fork(2).

The _lwp_sema_trywait() function will fail if:

EBUSY

The function was called on a semaphore with a zero count.

The _lwp_sema_post() function will fail if:

EOVERFLOW

The value of the sema argument exceeds SEM_VALUE_MAX.

fork(2)