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.\" $NetBSD: atomic_cas.3,v 1.5 2014/02/02 18:06:33 dholland Exp $ .\" .\" Copyright (c) 2007, 2010 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" This code is derived from software contributed to The NetBSD Foundation .\" by Jason R. Thorpe. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED .\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS .\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS .\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN .\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE .\" POSSIBILITY OF SUCH DAMAGE. .\" .Dd February 2, 2014 .Dt ATOMIC_CAS 3 .Os .Sh NAME .Nm atomic_cas , .Nm atomic_cas_32 , .Nm atomic_cas_uint , .Nm atomic_cas_ulong , .Nm atomic_cas_ptr , .Nm atomic_cas_64 , .Nm atomic_cas_32_ni , .Nm atomic_cas_uint_ni , .Nm atomic_cas_ulong_ni , .Nm atomic_cas_ptr_ni , .Nm atomic_cas_64_ni .Nd atomic compare-and-swap operations .\" .Sh LIBRARY .\" .Lb libc .Sh SYNOPSIS .In sys/atomic.h .Ft uint32_t .Fn atomic_cas_32 "volatile uint32_t *ptr" "uint32_t expected" "uint32_t new" .Ft unsigned int .Fn atomic_cas_uint "volatile unsigned int *ptr" "unsigned int expected" \ "unsigned int new" .Ft unsigned long .Fn atomic_cas_ulong "volatile unsigned long *ptr" "unsigned long expected" \ "unsigned long new" .Ft void * .Fn atomic_cas_ptr "volatile void *ptr" "void *expected" "void *new" .Ft uint64_t .Fn atomic_cas_64 "volatile uint64_t *ptr" "uint64_t expected" "uint64_t new" .Ft uint32_t .Fn atomic_cas_32_ni "volatile uint32_t *ptr" "uint32_t expected" \ "uint32_t new" .Ft unsigned int .Fn atomic_cas_uint_ni "volatile unsigned int *ptr" "unsigned int expected" \ "unsigned int new" .Ft unsigned long .Fn atomic_cas_ulong_ni "volatile unsigned long *ptr" \ "unsigned long expected" "unsigned long new" .Ft void * .Fn atomic_cas_ptr_ni "volatile void *ptr" "void *expected" "void *new" .Ft uint64_t .Fn atomic_cas_64_ni "volatile uint64_t *ptr" "uint64_t expected" \ "uint64_t new" .Sh DESCRIPTION The .Nm atomic_cas family of functions perform an atomic conditional assignment. The value .Fa new is assigned to the variable referenced by .Fa ptr . The assignment succeeds if and only if its current value matches the value .Fa expected . If the value is different, the assignment fails and no change is made. This operation is sometimes known as .Dq compare-and-swap . These functions always return the value found via .Fa ptr . Callers test for success by comparing the return value to the value passed as .Fa expected ; if they are equal then the new value was stored; if they are not, the value was not changed. .Pp The non-interlocked variants, .Fn *_ni , guarantee atomicity within the same CPU with respect to interrupts and preemption. They are not atomic with respect to different CPUs. These can be used to avoid interprocessor synchronization overhead in some cases; for example, they are suitable for synchronized operations on a variable shared by a thread and an interrupt that are bound to the same CPU. .Pp The 64-bit variants of these functions are available only on platforms that can support atomic 64-bit memory access. Applications can check for the availability of 64-bit atomic memory operations by testing if the pre-processor macro .Dv __HAVE_ATOMIC64_OPS is defined. .Sh SEE ALSO .Xr atomic_ops 3 .Sh HISTORY The .Nm atomic_cas functions first appeared in .Nx 5.0 . .Sh NOTES On some architectures, a .Fn *_ni variant is merely an alias for the corresponding standard compare-and-swap operation. While the non-interlocked variant behaves correctly on those architectures, it does not avoid the interprocessor synchronization overhead.