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.Dd February 11, 2010
.Dt ATOMIC_CAS 3
.Os
.Sh NAME
.Nm atomic_cas ,
.Nm atomic_cas_32 ,
.Nm atomic_cas_uint ,
.Nm atomic_cas_ulong ,
.Nm atomic_cas_ptr ,
.Nm atomic_cas_64 ,
.Nm atomic_cas_32_ni ,
.Nm atomic_cas_uint_ni ,
.Nm atomic_cas_ulong_ni ,
.Nm atomic_cas_ptr_ni ,
.Nm atomic_cas_64_ni
.Nd atomic compare-and-swap operations
.\" .Sh LIBRARY
.\" .Lb libc
.Sh SYNOPSIS
.In sys/atomic.h
.Ft uint32_t
.Fn atomic_cas_32 "volatile uint32_t *ptr" "uint32_t old" "uint32_t new"
.Ft unsigned int
.Fn atomic_cas_uint "volatile unsigned int *ptr" "unsigned int old" \
    "unsigned int new"
.Ft unsigned long
.Fn atomic_cas_ulong "volatile unsigned long *ptr" "unsigned long old" \
    "unsigned long new"
.Ft void *
.Fn atomic_cas_ptr "volatile void *ptr" "void *old" "void *new"
.Ft uint64_t
.Fn atomic_cas_64 "volatile uint64_t *ptr" "uint64_t old" "uint64_t new"
.Ft uint32_t
.Fn atomic_cas_32_ni "volatile uint32_t *ptr" "uint32_t old" "uint32_t new"
.Ft unsigned int
.Fn atomic_cas_uint_ni "volatile unsigned int *ptr" "unsigned int old" \
    "unsigned int new"
.Ft unsigned long
.Fn atomic_cas_ulong_ni "volatile unsigned long *ptr" "unsigned long old" \
    "unsigned long new"
.Ft void *
.Fn atomic_cas_ptr_ni "volatile void *ptr" "void *old" "void *new"
.Ft uint64_t
.Fn atomic_cas_64_ni "volatile uint64_t *ptr" "uint64_t old" "uint64_t new"
.Sh DESCRIPTION
The
.Nm atomic_cas
family of functions perform a compare-and-swap operation in an atomic fashion.
The value of the variable referenced by
.Fa ptr
is compared against
.Fa old .
If the values are equal,
.Fa new
is stored in the variable referenced by
.Fa ptr .
.Pp
The old value of the variable referenced by
.Fa 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
.Fa old ;
if they are equal then the new value was stored.
.Pp
The non-interlocked variants,
.Fn *_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
.Fn *_ni
variants are not atomic with respect to different CPUs.
.Fn *_ni
variants should avoid the interprocessor synchronization overhead
of the standard compare-and-swap operations.
.Pp
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
.Dv __HAVE_ATOMIC64_OPS
is defined.
.Sh SEE ALSO
.Xr atomic_ops 3
.Sh HISTORY
The
.Nm atomic_cas
functions first appeared in
.Nx 5.0 .
.Sh NOTES
On some architectures, a
.Fn *_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.