SEMA(9)

NAME

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

SYNOPSIS

     #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

     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 sem-
     aphore has an integer value associated with it.  Posting (incrementing)
     always succeeds, but waiting (decrementing) can only successfully com-
     plete 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 min-
     imum time in ticks to wait before returning with failure.	sema_value()
     is used to read the current value of the semaphore.

RETURN VALUES

     The sema_value() function returns the current value of the semaphore.

     If decrementing the semaphore would result in its value being negative,
     sema_trywait() returns 0 to indicate failure.  Otherwise, a non-zero
     value is returned to indicate success.

     The sema_timedwait() function returns 0 if waiting on the semaphore suc-
     ceeded; otherwise a non-zero error code is returned.

ERRORS

     The sema_timedwait() function will fail if:

     [EWOULDBLOCK]	Timeout expired.

SEE ALSO

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