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dispatch_semaphore_create(3) Library Functions Manuadispatch_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 re- sulting value is less than zero, it waits for a signal from a thread that increments the semaphore by calling dispatch_semaphore_signal() before re- turning. 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 re- turns. 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_FOREVER, then dispatch_semaphore_wait() waits forever and always returns zero. MEMORY MODEL Dispatch semaphores are retained and released via calls to dispatch_re- tain() 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 dispatch_semaphore_create(3)
NAME | SYNOPSIS | DESCRIPTION | COMPLETION SYNCHRONIZATION | FINITE RESOURCE POOL | RETURN VALUES | MEMORY MODEL | CAVEATS | SEE ALSO
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