dispatch_semaphore_signal man page on Darwin

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dispatch_semaphore_cr... BSD Library Functions Manual dispatch_semaphore_cr...

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 parame‐
     ter is creatable with the dispatch_time(3) or dispatch_walltime(3) func‐
     tions.

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
     Unbalanced dispatch semaphores cannot be released.	 For a given sema‐
     phore, 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
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