[atomics.wait]

# 32 Concurrency support library [[thread]](./#thread)

## 32.5 Atomic operations [[atomics]](atomics#wait)

### 32.5.6 Waiting and notifying [atomics.wait]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L3144)

[*Atomic waiting operations*](#def:atomic,waiting_operation "32.5.6 Waiting and notifying [atomics.wait]") and [*atomic notifying operations*](#def:atomic,notifying_operation "32.5.6 Waiting and notifying [atomics.wait]") provide a mechanism to wait for the value of an atomic object to change
more efficiently than can be achieved with polling[.](#1.sentence-1)

An atomic waiting operation may block until it is unblocked
by an atomic notifying operation, according to each function's effects[.](#1.sentence-2)

[*Note [1](#note-1)*:

Programs are not guaranteed to observe transient atomic values,
an issue known as the A-B-A problem,
resulting in continued blocking if a condition is only temporarily met[.](#1.sentence-3)

— *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L3157)

[*Note [2](#note-2)*:

The following functions are atomic waiting operations:

- [(2.1)](#2.1)

atomic<T>​::​wait,

- [(2.2)](#2.2)

atomic_flag​::​wait,

- [(2.3)](#2.3)

atomic_wait and atomic_wait_explicit,

- [(2.4)](#2.4)

atomic_flag_wait and atomic_flag_wait_explicit, and

- [(2.5)](#2.5)

atomic_ref<T>​::​wait[.](#2.sentence-1)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L3169)

[*Note [3](#note-3)*:

The following functions are atomic notifying operations:

- [(3.1)](#3.1)

atomic<T>​::​notify_one and atomic<T>​::​notify_all,

- [(3.2)](#3.2)

atomic_flag​::​notify_one and atomic_flag​::​notify_all,

- [(3.3)](#3.3)

atomic_notify_one and atomic_notify_all,

- [(3.4)](#3.4)

atomic_flag_notify_one and atomic_flag_notify_all, and

- [(3.5)](#3.5)

atomic_ref<T>​::​notify_one and atomic_ref<T>​::​notify_all[.](#3.sentence-1)

— *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L3182)

A call to an atomic waiting operation on an atomic object M is [*eligible to be unblocked*](#def:eligible_to_be_unblocked "32.5.6 Waiting and notifying [atomics.wait]") by a call to an atomic notifying operation on M if there exist side effects X and Y on M such that:

- [(4.1)](#4.1)

the atomic waiting operation has blocked after observing the result of X,

- [(4.2)](#4.2)

X precedes Y in the modification order of M, and

- [(4.3)](#4.3)

Y happens before the call to the atomic notifying operation[.](#4.sentence-1)