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 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_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
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)
Darwin May 1, 2009 Darwin