dispatch_semaphore_create.3   [plain text]

.\" Copyright (c) 2008-2010 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_semaphore_create 3
.Os Darwin
.Sh NAME
.Nm dispatch_semaphore_create ,
.Nm dispatch_semaphore_signal ,
.Nm dispatch_semaphore_wait
.Nd synchronized counting semaphore
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft dispatch_semaphore_t
.Fo dispatch_semaphore_create
.Fa "long count"
.Fc
.Ft long
.Fo dispatch_semaphore_signal
.Fa "dispatch_semaphore_t semaphore"
.Fc
.Ft long
.Fo dispatch_semaphore_wait
.Fa "dispatch_semaphore_t semaphore" "dispatch_time_t timeout"
.Fc
.Sh DESCRIPTION
Dispatch semaphores are used to synchronize threads.
The
.Fa timeout
parameter is creatable with the
.Xr dispatch_time 3
or
.Xr dispatch_walltime 3
functions.
.Sh COMPLETION SYNCHRONIZATION
If the
.Fa count
parameter is equal to zero, then the semaphore is useful for synchronizing
completion of work.
For example:
.Bd -literal -offset indent
sema = dispatch_semaphore_create(0);

dispatch_async(queue, ^{
	foo();
	dispatch_semaphore_signal(sema);
});

bar();

dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
.Ed
.Sh FINITE RESOURCE POOL
If the
.Fa 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:
.Bd -literal -offset indent
sema = dispatch_semaphore_create(getdtablesize() / 4);
.Ed
.Pp
At each Unix FD allocation:
.Bd -literal -offset indent
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
fd = open("/etc/services", O_RDONLY);
.Ed
.Pp
When each FD is closed:
.Bd -literal -offset indent
close(fd);
dispatch_semaphore_signal(sema);
.Ed
.Sh RETURN VALUES
The
.Fn dispatch_semaphore_create
function returns NULL if no memory is available or if the
.Fa count
parameter is less than zero.
.Pp
The
.Fn dispatch_semaphore_signal
function returns non-zero when a thread is woken.
Otherwise, zero is returned.
.Pp
The
.Fn dispatch_semaphore_wait
function returns zero upon success and non-zero after the timeout expires. If
the timeout is DISPATCH_TIME_FOREVER, then
.Fn dispatch_semaphore_wait
waits forever and always returns zero.
.Sh MEMORY MODEL
Dispatch semaphores are retained and released via calls to
.Fn dispatch_retain
and
.Fn dispatch_release .
.Sh CAVEATS
Unbalanced dispatch semaphores cannot be released.
For a given semaphore, calls to
.Fn dispatch_semaphore_signal
and
.Fn dispatch_semaphore_wait
must be balanced before
.Fn dispatch_release
is called on it.
.Pp
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:
.Bd -literal
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);
}
.Ed
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_object 3