NAME    [Toc]    [Back]

     sema, sema_init, sema_destroy, sema_post, sema_wait, sema_timedwait,
     sema_trywait, sema_value -- kernel counting semaphore

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/lock.h>
     #include <sys/sema.h>

     void
     sema_init(struct sema *sema, int value, const char *description);

     void
     sema_destroy(struct sema *sema);

     void
     sema_post(struct sema *sema);

     void
     sema_wait(struct sema *sema);

     int
     sema_timedwait(struct sema *sema, int timo);

     int
     sema_trywait(struct sema *sema);

     int
     sema_value(struct sema *sema);

DESCRIPTION    [Toc]    [Back]

     Counting semaphores provide a mechanism for synchronizing access to a
     pool of resources.  Unlike mutexes, semaphores do not have the concept of
     an owner, so they can also be useful in situations where one thread needs
     to acquire a resource, and another thread needs to release it.  Each semaphore
 has an integer value associated with it.  Posting (incrementing)
     always succeeds, but waiting (decrementing) can only successfully complete
 if the resulting value of the semaphore is greater than or equal to
     zero.

     Semaphores should not be used where mutexes and condition variables will
     suffice.  Semaphores are a more complex synchronization mechanism than
     mutexes and condition variables, and are not as efficient.

     Semaphores are created with sema_init(), where sema is a pointer to space
     for a struct sema, value is the initial value of the semaphore, and
     description is a pointer to a null-terminated character string that
     describes the semaphore.  Semaphores are destroyed with sema_destroy().
     A semaphore is posted (incremented) with sema_post().  A semaphore is
     waited on (decremented) with sema_wait(), sema_timedwait(), or
     sema_trywait().  The timo argument to sema_timedwait() specifies the minimum
 time in ticks to wait before returning with failure.	sema_value()
     returns the current value of the semaphore.

     sema_timedwait() and sema_trywait() will return 0 if waiting on the semaphore
 failed; otherwise a non-zero value will be returned to indicate
     success.

SEE ALSO    [Toc]    [Back]

     condvar(9), mtx_pool(9), mutex(9), sx(9)


FreeBSD 5.2.1			August 13, 2001 		 FreeBSD 5.2.1