NAME

SYNOPSIS

_Atomic(T) v = ATOMIC_VAR_INIT(c);
void
atomic_init(_Atomic(T) *object, T value);

T
atomic_load(_Atomic(T) *object);

T
atomic_load_explicit(_Atomic(T) *object, memory_order order);

void
atomic_store(_Atomic(T) *object, T desired);

void
atomic_store_explicit(_Atomic(T) *object, T desired, memory_order order);

T
atomic_exchange(_Atomic(T) *object, T desired);

T
atomic_exchange_explicit(_Atomic(T) *object, T desired, memory_order order);

_Bool
atomic_compare_exchange_strong(_Atomic(T) *object, T *expected, T desired);

_Bool
atomic_compare_exchange_strong_explicit(_Atomic(T) *object, T *expected, T desired, memory_order success, memory_order failure);

_Bool
atomic_compare_exchange_weak(_Atomic(T) *object, T *expected, T desired);

_Bool
atomic_compare_exchange_weak_explicit(_Atomic(T) *object, T *expected, T desired, memory_order success, memory_order failure);

T
atomic_fetch_add(_Atomic(T) *object, T operand);

T
atomic_fetch_add_explicit(_Atomic(T) *object, T operand, memory_order order);

T
atomic_fetch_and(_Atomic(T) *object, T operand);

T
atomic_fetch_and_explicit(_Atomic(T) *object, T operand, memory_order order);

T
atomic_fetch_or(_Atomic(T) *object, T operand);

T
atomic_fetch_or_explicit(_Atomic(T) *object, T operand, memory_order order);

T
atomic_fetch_sub(_Atomic(T) *object, T operand);

T
atomic_fetch_sub_explicit(_Atomic(T) *object, T operand, memory_order order);

T
atomic_fetch_xor(_Atomic(T) *object, T operand);

T
atomic_fetch_xor_explicit(_Atomic(T) *object, T operand, memory_order order);

_Bool
atomic_is_lock_free(const _Atomic(T) *object);

DESCRIPTION

Atomic variables are declared using the _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.

The atomic_init() macro initializes the atomic variable object with a value. Atomic variables can be initialized while being declared using ATOMIC_VAR_INIT().

The atomic_load() macro returns the value of atomic variable object. The atomic_store() macro sets the atomic variable object to its desired value.

The atomic_exchange() macro combines the behaviour of atomic_load() and atomic_store(). It sets the atomic variable object to its desired value and returns the original contents of the atomic variable.

The atomic_compare_exchange_strong() macro stores a desired value into atomic variable object, only if the atomic variable is equal to its expected value. Upon success, the macro returns true. Upon failure, the desired value is overwritten with the value of the atomic variable and false is returned. The atomic_compare_exchange_weak() macro is identical to atomic_compare_exchange_strong(), but is allowed to fail even if atomic variable object is equal to its expected value.

The atomic_fetch_add() macro adds the value operand to atomic variable object and returns the original contents of the atomic variable.

The atomic_fetch_and() macro applies the and operator to atomic variable object and operand and stores the value into object, while returning the original contents of the atomic variable.

The atomic_fetch_or() macro applies the or operator to atomic variable object and operand and stores the value into object, while returning the original contents of the atomic variable.

The atomic_fetch_sub() macro subtracts the value operand from atomic variable object and returns the original contents of the atomic variable.

The atomic_fetch_xor() macro applies the xor operator to atomic variable object and operand and stores the value into object, while returning the original contents of the atomic variable.

The atomic_is_lock_free() macro returns whether atomic variable object uses locks when using atomic operations.

BARRIERS

The order parameter of these _explicit() macros can have one of the following values.

memory_order_relaxed

No operation orders memory.

memory_order_consume

Perform consume operation.

memory_order_acquire

Acquire fence.

memory_order_release

Release fence.

memory_order_acq_rel

Acquire and release fence.

memory_order_seq_cst

Sequentially consistent acquire and release fence.

The previously described macros are identical to the _explicit() macros, when order is memory_order_seq_cst.

COMPILER SUPPORT

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 memory_order_seq_cst.

Instead of using the atomic operations provided by this interface, ISO/IEC 9899:2011 (“ISO C11”) 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.

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.

SEE ALSO

STANDARDS

HISTORY

AUTHORS