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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
     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() 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
     succeeded; otherwise a non-zero error code is returned.

ERRORS

     The sema_timedwait() function will fail if:

     [EWOULDBLOCK]      Timeout expired.

SEE ALSO

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