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.Dd September 6, 2023
.Dt ATOMIC_OPS 3
.Os
.Sh NAME
.Nm atomic_ops
.Nd atomic memory operations
.\" .Sh LIBRARY
.\" .Lb libc
.Sh SYNOPSIS
.In sys/atomic.h
.Sh DESCRIPTION
The
.Nm atomic_ops
family of functions provide atomic memory operations.
There are 7 classes of atomic memory operations available:
.Pp
.Bl -tag -width "atomic_swap(3)" -offset indent
.It Xr atomic_add 3
These functions perform atomic addition.
.It Xr atomic_and 3
These functions perform atomic bitwise
.Dq and .
.It Xr atomic_cas 3
These functions perform atomic compare-and-swap.
.It Xr atomic_dec 3
These functions perform atomic decrement.
.It Xr atomic_inc 3
These functions perform atomic increment.
.It Xr atomic_or 3
These functions perform atomic bitwise
.Dq or .
.It Xr atomic_swap 3
These functions perform atomic swap.
.El
.Ss 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.
.Ss 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.
.Pp
Users of atomic memory operations should not make assumptions about how
the memory access is performed
.Pq 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.
.Pp
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.
.Ss 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.
.Pp
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
.Xr membar_ops 3 .
.Ss 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.
.Sh SEE ALSO
.Xr atomic_add 3 ,
.Xr atomic_and 3 ,
.Xr atomic_cas 3 ,
.Xr atomic_dec 3 ,
.Xr atomic_inc 3 ,
.Xr atomic_or 3 ,
.Xr atomic_swap 3 ,
.Xr membar_ops 3
.Sh HISTORY
The
.Nm atomic_ops
functions first appeared in
.Nx 5.0 .