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
expected, uint32_t
new);
unsigned int
atomic_cas_uint
(volatile
unsigned int *ptr,
unsigned int expected,
unsigned int new);
unsigned long
atomic_cas_ulong
(volatile
unsigned long *ptr,
unsigned long expected,
unsigned long new);
void *
atomic_cas_ptr
(volatile
void *ptr, void
*expected, void
*new);
uint64_t
atomic_cas_64
(volatile
uint64_t *ptr, uint64_t
expected, uint64_t
new);
uint32_t
atomic_cas_32_ni
(volatile
uint32_t *ptr, uint32_t
expected, uint32_t
new);
unsigned int
atomic_cas_uint_ni
(volatile
unsigned int *ptr,
unsigned int expected,
unsigned int new);
unsigned long
atomic_cas_ulong_ni
(volatile
unsigned long *ptr,
unsigned long expected,
unsigned long new);
void *
atomic_cas_ptr_ni
(volatile
void *ptr, void
*expected, void
*new);
uint64_t
atomic_cas_64_ni
(volatile
uint64_t *ptr, uint64_t
expected, uint64_t
new);
DESCRIPTION
Theatomic_cas
family of functions perform an atomic
conditional assignment. The value new is assigned to the
variable referenced by ptr. The assignment succeeds if
and only if its current value matches the value
expected. If the value is different, the assignment
fails and no change is made. This operation is sometimes known as
“compare-and-swap”. These functions always return the value
found via ptr. Callers test for success by comparing the
return value to the value passed as expected; if they
are equal then the new value was stored; if they are not, the value was not
changed.
The non-interlocked variants,
*_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.
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
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.