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)
