dispatch_semaphore_create(3)				   BSD Library Functions Manual 			      dispatch_semaphore_create(3)

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 dispatch_semaphore_wait() function decrements the semaphore. If the resulting value is less than zero, it waits for a signal from a
     thread that increments the semaphore by calling dispatch_semaphore_signal() before returning.  The timeout parameter is creatable with the
     dispatch_time(3) or dispatch_walltime(3) functions.

     The dispatch_semaphore_signal() function increments the counting semaphore. If the previous value was less than zero, it wakes one of the
     threads that are waiting in dispatch_semaphore_wait() before returning.

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_FOR-
     EVER, 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

     Unbalanced dispatch semaphores cannot be released.  For a given semaphore, calls to dispatch_semaphore_signal() and dispatch_semaphore_wait()
     must be balanced before dispatch_release() is called on it.

SEE ALSO

     dispatch(3), dispatch_object(3)

Darwin								    May 1, 2009 							    Darwin