dispatch_semaphore_create - Man Page

synchronized counting semaphore


#include <dispatch/dispatch.h>

dispatch_semaphore_create(long count);

dispatch_semaphore_signal(dispatch_semaphore_t semaphore);

dispatch_semaphore_wait(dispatch_semaphore_t semaphore, dispatch_time_t timeout);


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. If the timeout is reached without a signal being received, the semaphore is re-incremented before the function returns.

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, ^{


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:


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().


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)

Referenced By

dispatch(3), dispatch_async(3), dispatch_group_create(3), dispatch_object(3), dispatch_time(3).

May 1, 2009