ATOMIC_OPS(3) BSD Library Functions Manual ATOMIC_OPS(3)
atomic_ops -- atomic memory operations
The atomic_ops family of functions provide atomic memory operations. There are 7 classes of atomic memory operations available:
atomic_add(3) These functions perform atomic addition.
atomic_and(3) These functions perform atomic logical ``and''.
atomic_cas(3) These functions perform atomic compare-and-swap.
atomic_dec(3) These functions perform atomic decrement.
atomic_inc(3) These functions perform atomic increment.
atomic_or(3) These functions perform atomic logical ``or''.
atomic_swap(3) These functions perform atomic swap.
Where the architecture does not provide hardware support for atomic compare and swap (CAS), atomicity is provided by a restartable sequence
or by a spinlock. The chosen method is not ordinarily distinguishable by or visible to users of the interface. The following architectures
can be assumed to provide CAS in hardware: alpha, amd64, i386, powerpc, powerpc64, sparc64.
Scope and Restrictions
If hardware CAS is available, the atomic operations are globally atomic: operations within a memory region shared between processes are guar-
anteed to be performed atomically. If hardware CAS is not available, it may only be assumed that the operations are atomic with respect to
threads in the same process. Additionally, if hardware CAS is not available, the atomic operations must not be used within a signal handler.
Users of atomic memory operations should not make assumptions about how the memory access is performed (specifically, the width of the memory
access). For this reason, applications making use of atomic memory operations should limit their use to regular memory. The results of
using atomic memory operations on anything other than regular memory are undefined.
Users of atomic memory operations should take care to modify any given memory location either entirely with atomic operations or entirely
with some other synchronization mechanism. Intermixing of atomic operations with other synchronization mechanisms for the same memory loca-
tion results in undefined behavior.
Visibility and Ordering of Memory Accesses
If hardware CAS is available, stores to the target memory location by an atomic operation will reach global visibility before the operation
completes. If hardware CAS is not available, the store may not reach global visibility until some time after the atomic operation has com-
pleted. However, in all cases a subsequent atomic operation on the same memory cell will be delayed until the result of any preceeding oper-
ation has reached global visibility.
Atomic operations are strongly ordered with respect to each other. The global visibility of other loads and stores before and after an
atomic operation is undefined. Applications that require synchronization of loads and stores with respect to an atomic operation must use
memory barriers. See membar_ops(3).
Because atomic memory operations require expensive synchronization at the hardware level, applications should take care to minimize their
use. In certain cases, it may be more appropriate to use a mutex, especially if more than one memory location will be modified.
atomic_add(3), atomic_and(3), atomic_cas(3), atomic_dec(3), atomic_inc(3), atomic_or(3), atomic_swap(3), membar_ops(3)
The atomic_ops functions first appeared in NetBSD 5.0.
April 14, 2010 BSD