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ATOMIC_CAS_UINT(9) Kernel Developer's Manual ATOMIC_CAS_UINT(9)

atomic_cas_uint, atomic_cas_ulong, atomic_cas_ptr
atomic compare-and-swap operations

#include <sys/atomic.h>
unsigned int
atomic_cas_uint(volatile unsigned int *p, unsigned int expected, unsigned int new);
unsigned long
atomic_cas_ulong(volatile unsigned long *p, unsigned long expected, unsigned long new);
void *
atomic_cas_ptr(volatile void *p, void *expected, void *new);

The atomic_cas set of functions provide an interface for atomically performing compare-and-swap operations with respect to interrupts and multiple processors in the system.
The value referenced by the pointer p is compared against expected. If these values are equal then new replaces the value stored at p.

atomic_cas_uint(), atomic_cas_ulong(), and atomic_cas_ptr() can all be called during autoconf, from process context, or from interrupt context.

These functions return the value at p as it was before the attempt to swap it.

atomic_swap_uint(9)

The atomic_cas functions first appeared in NetBSD 5.0 and OpenBSD 5.5.
July 18, 2014 OpenBSD-current