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 compare-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 ALSO
atomic_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 corresponding standard compare-and-swap operation. While the non-interlocked variant behaves correctly on those architectures, it does not avoid the interprocessor synchronization overhead.