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)