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ATOMIC_OPS(3)              Library Functions Manual              ATOMIC_OPS(3)

NAME
     atomic_ops - atomic memory operations

SYNOPSIS
     #include <sys/atomic.h>

DESCRIPTION
     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 bitwise "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 bitwise "or".

           atomic_swap(3)  These functions perform atomic swap.

   Synchronization Mechanisms
     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: aarch64, 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 guaranteed
     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 location
     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
     completed.  However, in all cases a subsequent atomic operation on the
     same memory cell will be delayed until the result of any preceding
     operation 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).

   Performance
     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.

SEE ALSO
     atomic_add(3), atomic_and(3), atomic_cas(3), atomic_dec(3),
     atomic_inc(3), atomic_or(3), atomic_swap(3), membar_ops(3)

HISTORY
     The atomic_ops functions first appeared in NetBSD 5.0.

NetBSD 10.99                   September 6, 2023                  NetBSD 10.99