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
Mac OS X 10.8 - Generated Sat Aug 25 17:59:47 CDT 2012