.\" $OpenBSD: atomic_cas_uint.9,v 1.6 2014/07/18 10:40:14 dlg Exp $
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.\" Copyright (c) 2014 David Gwynne <dlg@openbsd.org>
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.Dd $Mdocdate: July 18 2014 $
.Dt ATOMIC_CAS_UINT 9
.Os
.Sh NAME
.Nm atomic_cas_uint ,
.Nm atomic_cas_ulong ,
.Nm atomic_cas_ptr
.Nd atomic compare-and-swap operations
.Sh SYNOPSIS
.In sys/atomic.h
.Ft unsigned int
.Fn atomic_cas_uint "volatile unsigned int *p" "unsigned int expected" "unsigned int new"
.Ft unsigned long
.Fn atomic_cas_ulong "volatile unsigned long *p" "unsigned long expected" "unsigned long new"
.Ft void *
.Fn atomic_cas_ptr "volatile void *p" "void *expected" "void *new"
.Sh DESCRIPTION
The atomic_cas set of functions provide an interface for atomically
performing compare-and-swap operations with respect to interrupts
and multiple processors in the system.
.Pp
The value referenced by the pointer
.Fa p
is compared against
.Fa expected .
If these values are equal then
.Fa new
replaces the value stored at
.Fa p .
.Sh CONTEXT
.Fn atomic_cas_uint ,
.Fn atomic_cas_ulong ,
and
.Fn atomic_cas_ptr
can all be called during autoconf, from process context, or from interrupt context.
.Sh RETURN VALUES
These functions return the value at
.Fa p
as it was before the attempt to swap it.
.Sh SEE ALSO
.Xr atomic_swap_uint 9
.Sh HISTORY
The atomic_cas functions first appeared in
.Nx 5.0
and
.Ox 5.5 .