April 11, 2007 ATOMIC_CAS 3

### NAME

atomic_cas, atomic_cas_32, atomic_cas_uint, atomic_cas_ulong, atomic_cas_ptr, atomic_cas_64 - atomic compare-and-swap operations

### SYNOPSIS

```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`)

### 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 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 NetBSD5.0.