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.Dd December 27, 2011
.Dt ATOMIC_VAR_INIT 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
_Atomic(T)
.Fa v
= ATOMIC_VAR_INIT(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 macros for atomic operations.
Atomic operations can be used by multithreaded programs to provide
shared variables between threads that in most cases may be modified
without acquiring locks.
.Pp
Atomic variables are declared using the
.Fn _Atomic
type specifier.
These variables are not type-compatible with their non-atomic
counterparts.
Depending on the compiler used, atomic variables may be opaque and can
therefore only be influenced using the macros described.
.Pp
The
.Fn atomic_init
macro initializes the atomic variable
.Fa object
with a
.Fa value .
Atomic variables can be initialized while being declared using
.Fn ATOMIC_VAR_INIT .
.Pp
The
.Fn atomic_load
macro returns the value of atomic variable
.Fa object .
The
.Fn atomic_store
macro sets the atomic variable
.Fa object
to its
.Fa desired
value.
.Pp
The
.Fn atomic_exchange
macro combines the behaviour of
.Fn atomic_load
and
.Fn atomic_store .
It sets the atomic variable
.Fa object
to its desired
.Fa value
and returns the original contents of the atomic variable.
.Pp
The
.Fn atomic_compare_exchange_strong
macro stores a
.Fa desired
value into atomic variable
.Fa object ,
only if the atomic variable is equal to its
.Fa expected
value.
Upon success, the macro returns
.Dv true .
Upon failure, the
.Fa desired
value is overwritten with the value of the atomic variable and
.Dv false
is returned.
The
.Fn atomic_compare_exchange_weak
macro is identical to
.Fn atomic_compare_exchange_strong ,
but is allowed to fail even if atomic variable
.Fa object
is equal to its
.Fa expected
value.
.Pp
The
.Fn atomic_fetch_add
macro 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
macro applies the
.Em and
operator to atomic variable
.Fa object
and
.Fa operand
and stores the value into
.Fa object ,
while returning the original contents of the atomic variable.
.Pp
The
.Fn atomic_fetch_or
macro applies the
.Em or
operator to atomic variable
.Fa object
and
.Fa operand
and stores the value into
.Fa object ,
while returning the original contents of the atomic variable.
.Pp
The
.Fn atomic_fetch_sub
macro 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
macro applies the
.Em xor
operator to atomic variable
.Fa object
and
.Fa operand
and stores the value into
.Fa object ,
while returning the original contents of the atomic variable.
.Pp
The
.Fn atomic_is_lock_free
macro returns whether atomic variable
.Fa object
uses locks when using atomic operations.
.Sh BARRIERS
The atomic operations described previously are implemented in such a way
that they disallow both the compiler and the executing processor to
re-order any nearby memory operations across the atomic operation.
In certain cases this behaviour may cause suboptimal performance.
To mitigate this, every atomic operation has an
.Fn _explicit
version that allows the re-ordering to be configured.
.Pp
The
.Fa order
parameter of these
.Fn _explicit
macros can have one of the following values.
.Bl -tag -width memory_order_relaxed
.It Dv memory_order_relaxed
No operation orders memory.
.It Dv memory_order_consume
Perform consume operation.
.It Dv memory_order_acquire
Acquire fence.
.It Dv memory_order_release
Release fence.
.It Dv memory_order_acq_rel
Acquire and release fence.
.It Dv memory_order_seq_cst
Sequentially consistent acquire and release fence.
.El
.Pp
The previously described macros are identical to the
.Fn _explicit
macros, when
.Fa order
is
.Dv memory_order_seq_cst .
.Sh COMPILER SUPPORT
These atomic operations are typically implemented by the compiler, as
they must be implemented type-generically and must often use special
hardware instructions.
As this interface has not been adopted by most compilers yet, the
.In stdatomic.h
header implements these macros on top of existing compiler intrinsics to
provide forward compatibility.
.Pp
This means that certain aspects of the interface, such as support for
different barrier types may simply be ignored.
When using GCC, all atomic operations are executed as if they are using
.Dv memory_order_seq_cst .
.Pp
Instead of using the atomic operations provided by this interface,
.St -isoC-2011
allows the atomic variables to be modified directly using built-in
language operators.
This behaviour cannot be emulated for older compilers.
To prevent unintended non-atomic access to these variables, this header
file places the atomic variable in a structure when using an older
compiler.
.Pp
When using GCC on architectures on which it lacks support for built-in
atomic intrinsics, these macros may emit function calls to fallback
routines.
These fallback routines are only implemented for 32-bits and 64-bits
datatypes, if supported by the CPU.
.Sh SEE ALSO
.Xr pthread 3 ,
.Xr atomic 9
.Sh STANDARDS
These macros attempt to conform to
.St -isoC-2011 .
.Sh HISTORY
These macros appeared in
.Fx 10.0 .
.Sh AUTHORS
.An Ed Schouten Aq ed@FreeBSD.org ,
.An David Chisnall Aq theraven@FreeBSD.org