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[atomics.flag]
# 32 Concurrency support library [[thread]](./#thread)
## 32.5 Atomic operations [[atomics]](atomics#flag)
### 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; constexpr bool test(memory_order = memory_order::seq_cst) const noexcept; bool test_and_set(memory_order = memory_order::seq_cst) volatile noexcept; constexpr bool test_and_set(memory_order = memory_order::seq_cst) noexcept; void clear(memory_order = memory_order::seq_cst) volatile noexcept; constexpr void clear(memory_order = memory_order::seq_cst) noexcept; void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept; constexpr void wait(bool, memory_order = memory_order::seq_cst) const noexcept; void notify_one() volatile noexcept; constexpr void notify_one() noexcept; void notify_all() volatile noexcept; constexpr void notify_all() noexcept; };}
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6822)
The atomic_flag type provides the classic test-and-set functionality[.](#1.sentence-1)
It has two states, set and clear[.](#1.sentence-2)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6825)
Operations on an object of type atomic_flag shall be lock-free[.](#2.sentence-1)
The operations should also be address-free[.](#2.sentence-2)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6829)
The atomic_flag type is a standard-layout struct[.](#3.sentence-1)
It has a trivial destructor[.](#3.sentence-2)
[🔗](#lib:atomic_flag,constructor)
`constexpr atomic_flag::atomic_flag() noexcept;
`
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6839)
*Effects*: Initializes *this to the clear state[.](#4.sentence-1)
[🔗](#lib:atomic_flag_test)
`bool atomic_flag_test(const volatile atomic_flag* object) noexcept;
constexpr bool atomic_flag_test(const atomic_flag* object) noexcept;
bool atomic_flag_test_explicit(const volatile atomic_flag* object,
memory_order order) noexcept;
constexpr 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;
constexpr bool atomic_flag::test(memory_order order = memory_order::seq_cst) const noexcept;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6859)
For atomic_flag_test, let order be memory_order​::​seq_cst[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6862)
*Preconditions*: order ismemory_order​::​relaxed,memory_order​::​acquire, ormemory_order​::​seq_cst[.](#6.sentence-1)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6869)
*Effects*: Memory is affected according to the value of order[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6873)
*Returns*: Atomically returns the value pointed to by object or this[.](#8.sentence-1)
[🔗](#lib:atomic_flag_test_and_set)
`bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept;
constexpr 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;
constexpr 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;
constexpr bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) noexcept;
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6891)
*Effects*: Atomically sets the value pointed to by object or by this to true[.](#9.sentence-1)
Memory is affected according to the value oforder[.](#9.sentence-2)
These operations are atomic read-modify-write operations ([[intro.multithread]](intro.multithread "6.10.2 Multi-threaded executions and data races"))[.](#9.sentence-3)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6896)
*Returns*: Atomically, the value of the object immediately before the effects[.](#10.sentence-1)
[🔗](#lib:atomic_flag_clear)
`void atomic_flag_clear(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_clear(atomic_flag* object) noexcept;
void atomic_flag_clear_explicit(volatile atomic_flag* object, memory_order order) noexcept;
constexpr 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;
constexpr void atomic_flag::clear(memory_order order = memory_order::seq_cst) noexcept;
`
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6914)
*Preconditions*: order ismemory_order​::​relaxed,memory_order​::​release, ormemory_order​::​seq_cst[.](#11.sentence-1)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6921)
*Effects*: Atomically sets the value pointed to by object or by this tofalse[.](#12.sentence-1)
Memory is affected according to the value of order[.](#12.sentence-2)
[🔗](#lib:atomic_flag_wait)
`void atomic_flag_wait(const volatile atomic_flag* object, bool old) noexcept;
constexpr 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;
constexpr 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;
constexpr void atomic_flag::wait(bool old, memory_order order =
memory_order::seq_cst) const noexcept;
`
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6944)
For atomic_flag_wait,
let order be memory_order​::​seq_cst[.](#13.sentence-1)
Let flag be object for the non-member functions andthis for the member functions[.](#13.sentence-2)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6950)
*Preconditions*: order ismemory_order​::​relaxed,memory_order​::​acquire, ormemory_order​::​seq_cst[.](#14.sentence-1)
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6957)
*Effects*: Repeatedly performs the following steps, in order:
- [(15.1)](#15.1)
Evaluates flag->test(order) != old[.](#15.1.sentence-1)
- [(15.2)](#15.2)
If the result of that evaluation is true, returns[.](#15.2.sentence-1)
- [(15.3)](#15.3)
Blocks until it
is unblocked by an atomic notifying operation or is unblocked spuriously[.](#15.3.sentence-1)
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6970)
*Remarks*: This function is an atomic waiting operation ([[atomics.wait]](atomics.wait "32.5.6 Waiting and notifying"))[.](#16.sentence-1)
[🔗](#itemdecl:6)
`void atomic_flag_notify_one(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_notify_one(atomic_flag* object) noexcept;
void atomic_flag::notify_one() volatile noexcept;
constexpr void atomic_flag::notify_one() noexcept;
`
[17](#17)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6983)
*Effects*: Unblocks the execution of at least one atomic waiting operation
that is eligible to be unblocked ([[atomics.wait]](atomics.wait "32.5.6 Waiting and notifying")) by this call,
if any such atomic waiting operations exist[.](#17.sentence-1)
[18](#18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L6989)
*Remarks*: This function is an atomic notifying operation ([[atomics.wait]](atomics.wait "32.5.6 Waiting and notifying"))[.](#18.sentence-1)
[🔗](#itemdecl:7)
`void atomic_flag_notify_all(volatile atomic_flag* object) noexcept;
constexpr void atomic_flag_notify_all(atomic_flag* object) noexcept;
void atomic_flag::notify_all() volatile noexcept;
constexpr void atomic_flag::notify_all() noexcept;
`
[19](#19)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7002)
*Effects*: Unblocks the execution of all atomic waiting operations
that are eligible to be unblocked ([[atomics.wait]](atomics.wait "32.5.6 Waiting and notifying")) by this call[.](#19.sentence-1)
[20](#20)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7007)
*Remarks*: This function is an atomic notifying operation ([[atomics.wait]](atomics.wait "32.5.6 Waiting and notifying"))[.](#20.sentence-1)
[🔗](#itemdecl:8)
`#define ATOMIC_FLAG_INIT see below
`
[21](#21)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L7017)
*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[.](#21.sentence-1)
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[.](#21.sentence-3)
For a complete static-duration object, that initialization shall be static[.](#21.sentence-4)