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Revision 1.2, Fri Feb 12 22:23:17 2010 UTC (10 years, 3 months ago) by dyoung
With help from rmind@, describe the non-interlocked (*_ni) variants of the standard atomic compare-and-swap operations. Tell some caveats. Manual page links, *_ni.3 -> atomic_cas.3 are coming up after a successful 'build.sh distribution'.
.\" $NetBSD: atomic_cas.3,v 1.2 2010/02/12 22:23:17 dyoung 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 11, 2010 .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 old" "uint32_t new" .Ft unsigned int .Fn atomic_cas_uint "volatile unsigned int *ptr" "unsigned int old" \ "unsigned int new" .Ft unsigned long .Fn atomic_cas_ulong "volatile unsigned long *ptr" "unsigned long old" \ "unsigned long new" .Ft void * .Fn atomic_cas_ptr "volatile void *ptr" "void *old" "void *new" .Ft uint64_t .Fn atomic_cas_64 "volatile uint64_t *ptr" "uint64_t old" "uint64_t new" .Ft uint32_t .Fn atomic_cas_32_ni "volatile uint32_t *ptr" "uint32_t old" "uint32_t new" .Ft unsigned int .Fn atomic_cas_uint_ni "volatile unsigned int *ptr" "unsigned int old" \ "unsigned int new" .Ft unsigned long .Fn atomic_cas_ulong_ni "volatile unsigned long *ptr" "unsigned long old" \ "unsigned long new" .Ft void * .Fn atomic_cas_ptr_ni "volatile void *ptr" "void *old" "void *new" .Ft uint64_t .Fn atomic_cas_64_ni "volatile uint64_t *ptr" "uint64_t old" "uint64_t new" .Sh DESCRIPTION The .Nm atomic_cas family of functions perform a compare-and-swap operation in an atomic fashion. The value of the variable referenced by .Fa ptr is compared against .Fa old . If the values are equal, .Fa new is stored in the variable referenced by .Fa ptr . .Pp The old value of the variable referenced by .Fa ptr is always returned regardless of whether or not the new value was stored. Applications can test for success of the operation by comparing the return value to the value passed as .Fa old ; if they are equal then the new value was stored. .Pp The non-interlocked variants, .Fn *_ni , guarantee atomicity within the same CPU with respect to interrupts and preemption. For example, they are suitable for synchronizing compare-and-swap operations on a variable shared by a thread and an interrupt that are bound to the same CPU. The .Fn *_ni variants are not atomic with respect to different CPUs. .Fn *_ni variants should avoid the interprocessor synchronization overhead of the standard compare-and-swap operations. .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.