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SEMA(9)			 BSD Kernel Developer's Manual		       SEMA(9)

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

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

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

     sema_destroy(struct sema *sema);

     sema_post(struct sema *sema);

     sema_wait(struct sema *sema);

     sema_timedwait(struct sema *sema, int timo);

     sema_trywait(struct sema *sema);

     sema_value(struct sema *sema);

     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

     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.

     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.

     The sema_timedwait() function will fail if:

     [EWOULDBLOCK]	Timeout expired.

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

BSD			       February 1, 2006				   BSD

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