NAME

     dispatch_semaphore_create, dispatch_semaphore_signal, dispatch_semaphore_wait — synchronized
     counting semaphore

SYNOPSIS

     #include <dispatch/dispatch.h>

     dispatch_semaphore_t
     dispatch_semaphore_create(long count);

     long
     dispatch_semaphore_signal(dispatch_semaphore_t semaphore);

     long
     dispatch_semaphore_wait(dispatch_semaphore_t semaphore, dispatch_time_t timeout);

DESCRIPTION

     Dispatch semaphores are used to synchronize threads.  The timeout parameter is creatable
     with the dispatch_time(3) or dispatch_walltime(3) functions.

COMPLETION SYNCHRONIZATION

     If the count parameter is equal to zero, then the semaphore is useful for synchronizing
     completion of work.  For example:

           sema = dispatch_semaphore_create(0);

           dispatch_async(queue, ^{
                   foo();
                   dispatch_semaphore_signal(sema);
           });

           bar();

           dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);

FINITE RESOURCE POOL

     If the count parameter is greater than zero, then the semaphore is useful for managing a
     finite pool of resources.  For example, a library that wants to limit Unix descriptor usage:

           sema = dispatch_semaphore_create(getdtablesize() / 4);

     At each Unix FD allocation:

           dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
           fd = open("/etc/services", O_RDONLY);

     When each FD is closed:

           close(fd);
           dispatch_semaphore_signal(sema);

RETURN VALUES

     The dispatch_semaphore_create() function returns NULL if no memory is available or if the
     count parameter is less than zero.

     The dispatch_semaphore_signal() function returns non-zero when a thread is woken.
     Otherwise, zero is returned.

     The dispatch_semaphore_wait() function returns zero upon success and non-zero after the
     timeout expires. If the timeout is DISPATCH_TIME_FOREVER, then dispatch_semaphore_wait()
     waits forever and always returns zero.

MEMORY MODEL

     Dispatch semaphores are retained and released via calls to dispatch_retain() and
     dispatch_release().

CAVEATS

     Dispatch semaphores are strict counting semaphores.  In other words, dispatch semaphores do
     not saturate at any particular value.  Saturation can be achieved through atomic compare-
     and-swap logic.  What follows is a saturating binary semaphore:

     void
     saturating_semaphore_signal(dispatch_semaphore_t dsema, int *sent)
     {
             if (__sync_bool_compare_and_swap(sent, 0, 1)) {
                     dispatch_semaphore_signal(dsema);
             }
     }

     void
     saturating_semaphore_wait(dispatch_semaphore_t dsema, int *sent)
     {
             *sent = 0;
             dispatch_semaphore_wait(dsema, DISPATCH_TIME_FOREVER);
     }

SEE ALSO

     dispatch(3), dispatch_object(3)