ATOMIC_CAS(3) BSD Library Functions Manual ATOMIC_CAS(3)NAME
atomic_cas, atomic_cas_32, atomic_cas_uint, atomic_cas_ulong,
atomic_cas_ptr, atomic_cas_64, atomic_cas_32_ni, atomic_cas_uint_ni,
atomic_cas_ulong_ni, atomic_cas_ptr_ni, atomic_cas_64_ni — atomic com‐
pare-and-swap operations
SYNOPSIS
#include <sys/atomic.h>
uint32_t
atomic_cas_32(volatile uint32_t *ptr, uint32_t old, uint32_t new);
unsigned int
atomic_cas_uint(volatile unsigned int *ptr, unsigned int old,
unsigned int new);
unsigned long
atomic_cas_ulong(volatile unsigned long *ptr, unsigned long old,
unsigned long new);
void *
atomic_cas_ptr(volatile void *ptr, void *old, void *new);
uint64_t
atomic_cas_64(volatile uint64_t *ptr, uint64_t old, uint64_t new);
uint32_t
atomic_cas_32_ni(volatile uint32_t *ptr, uint32_t old, uint32_t new);
unsigned int
atomic_cas_uint_ni(volatile unsigned int *ptr, unsigned int old,
unsigned int new);
unsigned long
atomic_cas_ulong_ni(volatile unsigned long *ptr, unsigned long old,
unsigned long new);
void *
atomic_cas_ptr_ni(volatile void *ptr, void *old, void *new);
uint64_t
atomic_cas_64_ni(volatile uint64_t *ptr, uint64_t old, uint64_t new);
DESCRIPTION
The atomic_cas family of functions perform a compare-and-swap operation
in an atomic fashion. The value of the variable referenced by ptr is
compared against old. If the values are equal, new is stored in the
variable referenced by ptr.
The old value of the variable referenced by 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 old; if they are equal then the new value was stored.
The non-interlocked variants, *_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
*_ni() variants are not atomic with respect to different CPUs. *_ni()
variants should avoid the interprocessor synchronization overhead of the
standard compare-and-swap operations.
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 __HAVE_ATOMIC64_OPS is defined.
SEE ALSOatomic_ops(3)HISTORY
The atomic_cas functions first appeared in NetBSD 5.0.
NOTES
On some architectures, a *_ni() variant is merely an alias for the corre‐
sponding standard compare-and-swap operation. While the non-interlocked
variant behaves correctly on those architectures, it does not avoid the
interprocessor synchronization overhead.
BSD February 11, 2010 BSD
