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Difference between revisions of "cpp/atomic/atomic wait"

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< cpp‎ | atomic
m
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{{cpp/thread/navbar}}
 
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{{dcl begin}}
 
{{dcl begin}}
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{{dcl header | atomic }}
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{{dcl header|atomic}}
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{{dcl rev begin | num=1 | since=c++20}}
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{{dcl|num=1|since=c++20|
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{{dcl |  
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template< class T >
 
template< class T >
 
void atomic_wait( const std::atomic<T>* object,
 
void atomic_wait( const std::atomic<T>* object,
 
                   typename std::atomic<T>::value_type old );
 
                   typename std::atomic<T>::value_type old );
 
}}
 
}}
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{{dcl |
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{{dcl|num=2|since=c++20|
 
template< class T >
 
template< class T >
 
void atomic_wait( const volatile std::atomic<T>* object,
 
void atomic_wait( const volatile std::atomic<T>* object,
 
                   typename std::atomic<T>::value_type old );
 
                   typename std::atomic<T>::value_type old );
 
}}
 
}}
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{{dcl rev end}}
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{{dcl|num=3|since=c++20|
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{{dcl rev begin | num=2 | since=c++20}}
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{{dcl |  
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template< class T >
 
template< class T >
 
void atomic_wait_explicit( const std::atomic<T>* object,  
 
void atomic_wait_explicit( const std::atomic<T>* object,  
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                           std::memory_order order );
 
                           std::memory_order order );
 
}}
 
}}
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{{dcl |  
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{{dcl|num=4|since=c++20|
 
template< class T >
 
template< class T >
 
void atomic_wait_explicit( const volatile std::atomic<T>* object,
 
void atomic_wait_explicit( const volatile std::atomic<T>* object,
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                           std::memory_order order );
 
                           std::memory_order order );
 
}}
 
}}
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{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
 
Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:  
 
Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:  
−
 
+
* Compare the [[cpp/language/object|value representation]] of {{c|object->load()}} (for overloads {{v|1,2}}) or {{c|object->load(order)}} (for overloads {{v|3,4}}) with that of {{c|old}}.
−
* Compare the [[cpp/language/object|value representation]] of {{c|object->load(std::memory_order_seq_cst)}} or {{c|object->load(order)}} with that of {{tt|old}}.
+
 
** If those are bitwise equal, then blocks until {{c|*object}} is notified by {{lc|std::atomic::notify_one()}} or {{lc|std::atomic::notify_all()}}, or the thread is unblocked spuriously.
 
** If those are bitwise equal, then blocks until {{c|*object}} is notified by {{lc|std::atomic::notify_one()}} or {{lc|std::atomic::notify_all()}}, or the thread is unblocked spuriously.
 
** Otherwise, returns.
 
** Otherwise, returns.
  
 
These functions are guaranteed to return only if value has changed, even if the underlying implementation unblocks spuriously.
 
These functions are guaranteed to return only if value has changed, even if the underlying implementation unblocks spuriously.
−
 
+
@1,2@ Equivalent to {{c|object->wait(old)}}.
−
@1@ Equivalent to {{c|object->wait(old, std::memory_order_seq_cst)}}.
+
@3,4@ Equivalent to {{c|object->wait(old, order)}}.
−
 
+
@@ If {{c|order}} is one of {{c|std::memory_order::release}} and {{c|std::memory_order::acq_rel}}, the behavior is undefined.
−
@2@ Equivalent to {{c|object->wait(old, order)}}.
+
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | object | pointer to the atomic object to check and wait on}}
+
{{par|object|pointer to the atomic object to check and wait on}}
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{{par | old | the value to check the atomic object no longer contains}}
+
{{par|old|the value to check the atomic object no longer contains}}
−
{{par | order | the memory synchronization ordering for this operation: must not be {{lc|std::memory_order::release}} or {{lc|std::memory_order::acq_rel}} }}
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{{par|order|the memory synchronization ordering}}
 
{{par end}}  
 
{{par end}}  
  
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This form of change-detection is often more efficient than simple polling or pure spinlocks.
 
This form of change-detection is often more efficient than simple polling or pure spinlocks.
  
−
Due to the [https://en.wikipedia.org/wiki/ABA_problem ABA problem], transient changes from {{tt|old}} to another value and back to {{tt|old}} might be missed, and not unblock.
+
Due to the {{enwiki|ABA problem}}, transient changes from {{c|old}} to another value and back to {{c|old}} might be missed, and not unblock.
  
 
The comparison is bitwise (similar to {{lc|std::memcmp}}); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.
 
The comparison is bitwise (similar to {{lc|std::memcmp}}); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.
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===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/atomic/atomic/dsc notify_one | atomic}}
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{{dsc inc|cpp/atomic/atomic/dsc wait|atomic}}
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{{dsc inc | cpp/atomic/atomic/dsc notify_all | atomic}}
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{{dsc inc|cpp/atomic/atomic/dsc notify_one|atomic}}
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{{dsc inc | cpp/atomic/dsc atomic_notify_one}}
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{{dsc inc|cpp/atomic/atomic/dsc notify_all|atomic}}
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{{dsc inc | cpp/atomic/dsc atomic_notify_all}}
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{{dsc inc|cpp/atomic/dsc atomic_notify_one}}
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{{dsc inc|cpp/atomic/dsc atomic_notify_all}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 03:35, 25 October 2024

 
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Defined in header <atomic>
template< class T >

void atomic_wait( const std::atomic<T>* object,

                  typename std::atomic<T>::value_type old );
 (1) (since C++20)
template< class T >

void atomic_wait( const volatile std::atomic<T>* object,

                  typename std::atomic<T>::value_type old );
 (2) (since C++20)
template< class T >

void atomic_wait_explicit( const std::atomic<T>* object,
                           typename std::atomic<T>::value_type old,

                           std::memory_order order );
 (3) (since C++20)
template< class T >

void atomic_wait_explicit( const volatile std::atomic<T>* object,
                           typename std::atomic<T>::value_type old,

                           std::memory_order order );
 (4) (since C++20)

Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:

  • Compare the value representation of object->load() (for overloads (1,2)) or object->load(order) (for overloads (3,4)) with that of old.
    • If those are bitwise equal, then blocks until *object is notified by std::atomic::notify_one() or std::atomic::notify_all(), or the thread is unblocked spuriously.
    • Otherwise, returns.

These functions are guaranteed to return only if value has changed, even if the underlying implementation unblocks spuriously.

1,2) Equivalent to object->wait(old).
3,4) Equivalent to object->wait(old, order).
If order is one of std::memory_order::release and std::memory_order::acq_rel, the behavior is undefined.

Contents

[edit] Parameters

object - pointer to the atomic object to check and wait on
old - the value to check the atomic object no longer contains
order - the memory synchronization ordering

[edit] Return value

(none)

[edit] Notes

This form of change-detection is often more efficient than simple polling or pure spinlocks.

Due to the ABA problem, transient changes from old to another value and back to old might be missed, and not unblock.

The comparison is bitwise (similar to std::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.

[edit] Example

This section is incomplete
Reason: no example

[edit] See also

(C++20)
 blocks the thread until notified and the atomic value changes
(public member function of std::atomic<T>) [edit]
(C++20)
 notifies at least one thread waiting on the atomic object
(public member function of std::atomic<T>) [edit]
(C++20)
 notifies all threads blocked waiting on the atomic object
(public member function of std::atomic<T>) [edit]
(C++20)
 notifies a thread blocked in atomic_wait
(function template) [edit]
(C++20)
 notifies all threads blocked in atomic_wait
(function template) [edit]
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