Semaphores

Semaphores are a facility well-suited to managing the allocation and deallocation of identical resources.

• The semaphore can be initialized to the number of resources.

• A thread needing a resource should atomically decrement the associated semaphore with the sema_wait(S) function.

• If the resource is not available (semaphore count non-positive) the caller will block in sema_wait(S) until one becomes available.

     int sema_wait(
     	sema_t *sema
     );
  

• When a resource is no longer in use, the thread releasing the resource should increment the associated semaphore with the sema_post(S) function.

     int sema_post(
     	sema_t *sema
     );
  

• If any threads are blocked on that semaphore, the call to sema_post will make one runnable so that it can (implicitly) decrement the semaphore and return from sema_wait(S).

Additional considerations:

• This mechanism lacks the following features of the IPC style semaphore facility:

  • Increment/decrement by values greater than 1.

  • Operations on semaphore sets.

  • The ability to, ``block while count is non-zero'' instead of the usual rule, ``block only when count is zero.''

  • The ability to automatically release semaphores on termination (SEM_UNDO flag).

• A semaphore initialized to 1 is almost equivalent to a mutex. In such cases, use the mutex facilities; they are more efficient, having been optimized for that case.

• The sema_post(S) function can validly be used to increase a semaphore count above that defined by sema_init(S).