.\" Copyright (c) 2011 Ed Schouten <ed@FreeBSD.org> .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .Dd December 27, 2011 .Dt stdatomic 3 .Os .Sh NAME .Nm ATOMIC_VAR_INIT , .Nm atomic_init , .Nm atomic_load , .Nm atomic_store , .Nm atomic_exchange , .Nm atomic_compare_exchange_strong , .Nm atomic_compare_exchange_weak , .Nm atomic_fetch_add , .Nm atomic_fetch_and , .Nm atomic_fetch_or , .Nm atomic_fetch_sub , .Nm atomic_fetch_xor , .Nm atomic_is_lock_free .Nd type-generic atomic operations .Sh SYNOPSIS .In stdatomic.h .Pp .Vt _Atomic(T) Va v No = \*[Fn-font]ATOMIC_VAR_INIT\*[No-font] Ns Pq Fa c ; .Vt _Atomic T Va v No = \*[Fn-font]ATOMIC_VAR_INIT\*[No-font] Ns Pq Fa c ; .Ft void .Fn atomic_init "_Atomic(T) *object" "T value" .Ft T .Fn atomic_load "_Atomic(T) *object" .Ft T .Fn atomic_load_explicit "_Atomic(T) *object" "memory_order order" .Ft void .Fn atomic_store "_Atomic(T) *object" "T desired" .Ft void .Fn atomic_store_explicit "_Atomic(T) *object" "T desired" "memory_order order" .Ft T .Fn atomic_exchange "_Atomic(T) *object" "T desired" .Ft T .Fn atomic_exchange_explicit "_Atomic(T) *object" "T desired" "memory_order order" .Ft _Bool .Fn atomic_compare_exchange_strong "_Atomic(T) *object" "T *expected" "T desired" .Ft _Bool .Fn atomic_compare_exchange_strong_explicit "_Atomic(T) *object" "T *expected" "T desired" "memory_order success" "memory_order failure" .Ft _Bool .Fn atomic_compare_exchange_weak "_Atomic(T) *object" "T *expected" "T desired" .Ft _Bool .Fn atomic_compare_exchange_weak_explicit "_Atomic(T) *object" "T *expected" "T desired" "memory_order success" "memory_order failure" .Ft T .Fn atomic_fetch_add "_Atomic(T) *object" "T operand" .Ft T .Fn atomic_fetch_add_explicit "_Atomic(T) *object" "T operand" "memory_order order" .Ft T .Fn atomic_fetch_and "_Atomic(T) *object" "T operand" .Ft T .Fn atomic_fetch_and_explicit "_Atomic(T) *object" "T operand" "memory_order order" .Ft T .Fn atomic_fetch_or "_Atomic(T) *object" "T operand" .Ft T .Fn atomic_fetch_or_explicit "_Atomic(T) *object" "T operand" "memory_order order" .Ft T .Fn atomic_fetch_sub "_Atomic(T) *object" "T operand" .Ft T .Fn atomic_fetch_sub_explicit "_Atomic(T) *object" "T operand" "memory_order order" .Ft T .Fn atomic_fetch_xor "_Atomic(T) *object" "T operand" .Ft T .Fn atomic_fetch_xor_explicit "_Atomic(T) *object" "T operand" "memory_order order" .Ft _Bool .Fn atomic_is_lock_free "const _Atomic(T) *object" .Sh DESCRIPTION The header .In stdatomic.h provides type-generic operations on atomic operations. .Pp Atomic variables are declared using the .Vt _Atomic() type specifier or the .Vt _Atomic type qualifier. Such variables are not type-compatible with their non-atomic counterparts and may have different alignment. .Pp Operations on atomic variables that do not use the .Fn atomic_ interfaces, including compound assignment operations, will behave as if the .Pf non- Fn _explicit versions of those interfaces had been used. .Pp The .Fn atomic_init operation initializes the atomic variable .Fa object with .Fa value . Atomic variables can be initialized while being declared using .Fn ATOMIC_VAR_INIT . .Pp The .Fn atomic_load operation returns the value of atomic variable .Fa object . The .Fn atomic_store operation sets the atomic variable .Fa object to the .Fa desired value. .Pp The .Fn atomic_exchange operation combines the behaviour of .Fn atomic_load and .Fn atomic_store . It sets the atomic variable .Fa object to the desired .Fa value and returns the original contents of the atomic variable. .Pp The .Fn atomic_compare_exchange_strong operation stores the .Fa desired value into atomic variable .Fa object , but only if the atomic variable is equal to the .Fa expected value. Upon success, the operation returns .Dv true . Upon failure, the .Fa expected value is overwritten with the contents of the atomic variable and .Dv false is returned. .Pp The .Fn atomic_compare_exchange_weak operation is identical to .Fn atomic_compare_exchange_strong , but is allowed to fail even if atomic variable .Fa object is equal to the .Fa expected value. When an .Fn atomic_compare_exchange operation is in a loop, the weak version will yield better performance on some platforms. When .Fn atomic_compare_exchange_weak would require a loop and .Fn atomic_compare_exchange_strong would not, the strong version is preferable. .Pp The .Fn atomic_fetch_add operation adds the value .Fa operand to atomic variable .Fa object and returns the original contents of the atomic variable. .Pp The .Fn atomic_fetch_and operation applies the .Em and operator to atomic variable .Fa object and value .Fa operand and stores the result into .Fa object , while returning the original contents of the atomic variable. .Pp The .Fn atomic_fetch_or operation applies the .Em or operator to atomic variable .Fa object and value .Fa operand and stores the result into .Fa object , while returning the original contents of the atomic variable. .Pp The .Fn atomic_fetch_sub operation subtracts the value .Fa operand from atomic variable .Fa object and returns the original contents of the atomic variable. .Pp The .Fn atomic_fetch_xor operation applies the .Em xor operator to atomic variable .Fa object and value .Fa operand and stores the result into .Fa object , while returning the original contents of the atomic variable. .Pp The .Fn atomic_is_lock_free operation returns whether atomic variable .Fa object uses locks to implement atomic operations. .Sh MEMORY ORDER C11 defines a memory model that may allow for the reordering of operations in the absence of fences or explicit memory ordering operations. The .Pf non- Fn _explicit interfaces use the strictest available memory order: sequential consistency. The .Fn _explicit interfaces allow for configuration of the memory order operation which is present. The types of available memory order operations are explained in more detail in .St -isoC-2011 . .Pp The .Fa order parameter of the .Fn _explicit interfaces can have one of the following values: .Bl -tag -width memory_order_relaxed .It Dv memory_order_relaxed Operation does not order memory. .It Dv memory_order_consume Performs a consume operation. .It Dv memory_order_acquire Performs an acquire operation. .It Dv memory_order_release Performs a release operation. .It Dv memory_order_acq_rel Performs both an acquire and a release operation. .It Dv memory_order_seq_cst Provides sequential consistency. .El .Sh SEE ALSO .Xr atomic 3 , .Xr pthread 3 .Sh STANDARDS These interfaces conform to .St -isoC-2011 .