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cppdraft/atomics/wait.md

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+[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)