namespace std {
struct atomic_flag {
bool test_and_set(memory_order = memory_order_seq_cst) volatile noexcept;
bool test_and_set(memory_order = memory_order_seq_cst) noexcept;
void clear(memory_order = memory_order_seq_cst) volatile noexcept;
void clear(memory_order = memory_order_seq_cst) noexcept;
atomic_flag() noexcept = default;
atomic_flag(const atomic_flag&) = delete;
atomic_flag& operator=(const atomic_flag&) = delete;
atomic_flag& operator=(const atomic_flag&) volatile = delete;
};
bool atomic_flag_test_and_set(volatile atomic_flag*) noexcept;
bool atomic_flag_test_and_set(atomic_flag*) noexcept;
bool atomic_flag_test_and_set_explicit(volatile atomic_flag*, memory_order) noexcept;
bool atomic_flag_test_and_set_explicit(atomic_flag*, memory_order) noexcept;
void atomic_flag_clear(volatile atomic_flag*) noexcept;
void atomic_flag_clear(atomic_flag*) noexcept;
void atomic_flag_clear_explicit(volatile atomic_flag*, memory_order) noexcept;
void atomic_flag_clear_explicit(atomic_flag*, memory_order) noexcept;
#define ATOMIC_FLAG_INIT see below
}The atomic_flag type provides the classic test-and-set functionality. It has two states, set and clear.
Operations on an object of type atomic_flag shall be lock-free. [ Note: Hence the operations should also be address-free. — end note ]
The atomic_flag type is a standard-layout struct. It has a trivial default constructor and a trivial destructor.
The macro ATOMIC_FLAG_INIT shall be defined in such a way that it can be used to initialize an object of type atomic_flag to the clear state. The macro can be used in the form:
atomic_flag guard = ATOMIC_FLAG_INIT;
It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static. Unless initialized with ATOMIC_FLAG_INIT, it is unspecified whether an atomic_flag object has an initial state of set or clear.
bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept;
bool atomic_flag_test_and_set(atomic_flag* object) noexcept;
bool atomic_flag_test_and_set_explicit(volatile atomic_flag* object, memory_order order) noexcept;
bool atomic_flag_test_and_set_explicit(atomic_flag* object, memory_order order) noexcept;
bool atomic_flag::test_and_set(memory_order order = memory_order_seq_cst) volatile noexcept;
bool atomic_flag::test_and_set(memory_order order = memory_order_seq_cst) noexcept;
Effects: Atomically sets the value pointed to by object or by this to true. Memory is affected according to the value of order. These operations are atomic read-modify-write operations ([intro.multithread]).
void atomic_flag_clear(volatile atomic_flag* object) noexcept;
void atomic_flag_clear(atomic_flag* object) noexcept;
void atomic_flag_clear_explicit(volatile atomic_flag* object, memory_order order) noexcept;
void atomic_flag_clear_explicit(atomic_flag* object, memory_order order) noexcept;
void atomic_flag::clear(memory_order order = memory_order_seq_cst) volatile noexcept;
void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;
Requires: The order argument shall not be memory_order_consume, memory_order_acquire, nor memory_order_acq_rel.