ATOMIC_CAS(3) BSD Library Functions Manual ATOMIC_CAS(3)
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
atomic_cas_32(volatile uint32_t *ptr, uint32_t old, uint32_t new);
atomic_cas_uint(volatile unsigned int *ptr, unsigned int old, unsigned int new);
atomic_cas_ulong(volatile unsigned long *ptr, unsigned long old, unsigned long new);
atomic_cas_ptr(volatile void *ptr, void *old, void *new);
atomic_cas_64(volatile uint64_t *ptr, uint64_t old, uint64_t new);
atomic_cas_32_ni(volatile uint32_t *ptr, uint32_t old, uint32_t new);
atomic_cas_uint_ni(volatile unsigned int *ptr, unsigned int old, unsigned int new);
atomic_cas_ulong_ni(volatile unsigned long *ptr, unsigned long old, unsigned long new);
atomic_cas_ptr_ni(volatile void *ptr, void *old, void *new);
atomic_cas_64_ni(volatile uint64_t *ptr, uint64_t old, uint64_t new);
The atomic_cas family of functions perform a compare-and-swap operation in an atomic fash-
ion. 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 compar-
ing the return value to the value passed as old; if they are equal then the new value was
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
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.
The atomic_cas functions first appeared in NetBSD 5.0.
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.
BSD February 11, 2010 BSD