32 Concurrency support library [thread]

32.5 Atomic operations [atomics]

32.5.10 Flag type and operations [atomics.flag]

namespace std { struct atomic_flag { constexpr atomic_flag() noexcept; atomic_flag(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) = delete; atomic_flag& operator=(const atomic_flag&) volatile = delete; bool test(memory_order = memory_order::seq_cst) const volatile noexcept; bool test(memory_order = memory_order::seq_cst) const noexcept; 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; void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept; void wait(bool, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; void notify_one() noexcept; void notify_all() volatile noexcept; void notify_all() noexcept; }; }

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.

The operations should also be address-free.

The atomic_flag type is a standard-layout struct.

It has a trivial destructor.

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constexpr atomic_flag::atomic_flag() noexcept;

Effects: Initializes *this to the clear state.

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bool atomic_flag_test(const volatile atomic_flag* object) noexcept;
bool atomic_flag_test(const atomic_flag* object) noexcept;
bool atomic_flag_test_explicit(const volatile atomic_flag* object,
                               memory_order order) noexcept;
bool atomic_flag_test_explicit(const atomic_flag* object,
                               memory_order order) noexcept;
bool atomic_flag::test(memory_order order = memory_order::seq_cst) const volatile noexcept;
bool atomic_flag::test(memory_order order = memory_order::seq_cst) const noexcept;

For atomic_flag_test, let order be memory_order::seq_cst.

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by object or this.

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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]).

Returns: Atomically, the value of the object immediately before the effects.

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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;

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically sets the value pointed to by object or by this to false.

Memory is affected according to the value of order.

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void atomic_flag_wait(const volatile atomic_flag* object, bool old) noexcept;
void atomic_flag_wait(const atomic_flag* object, bool old) noexcept;
void atomic_flag_wait_explicit(const volatile atomic_flag* object,
                               bool old, memory_order order) noexcept;
void atomic_flag_wait_explicit(const atomic_flag* object,
                               bool old, memory_order order) noexcept;
void atomic_flag::wait(bool old, memory_order order =
                                   memory_order::seq_cst) const volatile noexcept;
void atomic_flag::wait(bool old, memory_order order =
                                   memory_order::seq_cst) const noexcept;

For atomic_flag_wait, let order be memory_order::seq_cst.

Let flag be object for the non-member functions and this for the member functions.

Preconditions: order is memory_order::relaxed, memory_order::consume, memory_order::ac-
quire, or memory_order::seq_cst.

Effects: Repeatedly performs the following steps, in order:

(15.1)
Evaluates flag->test(order) != old.
(15.2)
If the result of that evaluation is true, returns.
(15.3)
Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: This function is an atomic waiting operation ([atomics.wait]).

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void atomic_flag_notify_one(volatile atomic_flag* object) noexcept;
void atomic_flag_notify_one(atomic_flag* object) noexcept;
void atomic_flag::notify_one() volatile noexcept;
void atomic_flag::notify_one() noexcept;

Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

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void atomic_flag_notify_all(volatile atomic_flag* object) noexcept;
void atomic_flag_notify_all(atomic_flag* object) noexcept;
void atomic_flag::notify_all() volatile noexcept;
void atomic_flag::notify_all() noexcept;

Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.

Remarks: This function is an atomic notifying operation ([atomics.wait]).

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#define ATOMIC_FLAG_INIT see below

Remarks: The macro ATOMIC_FLAG_INIT is 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.