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

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{{dcl|num=1|since=c++20|1=
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{{dcl | 1=
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void wait( {{#ifeq:{{{1|}}}|atomic_flag|bool|T}} old, std::memory_order order =
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void wait( {{{2|}}} old, std::memory_order {{=}} std::memory_order::seq_cst ) const noexcept;
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          {{space as|{{#ifeq:{{{1|}}}|atomic_flag|bool|T}}}}         std::memory_order::seq_cst ) const noexcept;
 
}}
 
}}
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{{dcl | 1=
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{{dcl|num=2|since=c++20|1=
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void wait( {{{2|}}} old, std::memory_order {{=}} std::memory_order::seq_cst ) const volatile noexcept;
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void wait( {{#ifeq:{{{1|}}}|atomic_flag|bool|T}} old, std::memory_order order =
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          {{space as|{{#ifeq:{{{1|}}}|atomic_flag|bool|T}}}}         std::memory_order::seq_cst ) const volatile noexcept;
 
}}
 
}}
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{{dcl rev end}}
 
 
{{dcl end}}
 
{{dcl end}}
  
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Provides a means to check if the {{tt|{{{1|}}}}} object's value is not the ''old'' one, and if it is the ''old'' one then blocks until its value changes to something else. This form of change-detection is often more efficient than simple polling or pure spinlocks.
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Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:
 +
* Compare {{#ifeq:{{{1|}}}|atomic_flag|{{c|this->test(order)}}|the [[cpp/language/object|value representation]] of {{c|this->load(order)}}}} with that of {{c|old}}.
 +
** If those are equal, then blocks until {{c|*this}} is notified by {{lc|notify_one()}} or {{lc|notify_all()}}, or the thread is unblocked spuriously.
 +
** Otherwise, returns.
  
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Performs the following logical pseudo-code: {{source|1=
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These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.
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while(true) {
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    {{#ifeq:{{{2|}}} | bool |
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    if (test(order) !{{=}} old) return;
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    |
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    if (load(order) !{{=}} old) return;
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    }}
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    block_until_modified(this); // exposition only - implementation defined
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}
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}}
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The correlary functions to unblock are {{c|notify_one()}} or {{c|notify_all()}}, but see Notes section.
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If {{c|order}} is one of {{c|std::memory_order::release}} and {{c|std::memory_order::acq_rel}}, the behavior is undefined.
−
 
+
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===Preconditions===
+
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''order'' is neither std::memory_order::release nor std::memory_order::acq_rel.
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===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
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{{par | old | the value to check the {{{1|}}}'s object no longer contains}}
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{{par|old|the value to check the {{tt|{{{1|}}}}}'s object no longer contains}}
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{{par | order | the memory synchronization ordering for this operation: must not be std::memory_order::release nor std::memory_order::acq_rel}}
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{{par|order|memory order constraints to enforce}}
 
{{par end}}  
 
{{par end}}  
  
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===Notes===
 
===Notes===
−
Due to the [https://en.wikipedia.org/wiki/ABA_problem ABA problem], transient changes from ''old'' to something else and back to ''old'' might be missed, and not unblock.
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This form of change-detection is often more efficient than simple polling or pure spinlocks.
  
−
Because the logical {{c|block_until_modified()}} pseudo-code is implementation defined, the functions may unblock and return due to reasons other than a notifying function being invoked.
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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.
 +
 
 +
{{#ifeq:{{{1|}}}|atomic_flag||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.}}
  
 
===Example===
 
===Example===
−
{{todo}}
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{{example
 +
|code=
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{{#ifeq:{{{1|atomic}}}|atomic|#include <atomic>
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#include <chrono>
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#include <future>
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#include <iostream>
 +
#include <thread>
 +
 
 +
using namespace std::literals;
 +
 
 +
int main()
 +
{
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    std::atomic<bool> all_tasks_completed{false};
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    std::atomic<unsigned> completion_count{};
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    std::future<void> task_futures[16];
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    std::atomic<unsigned> outstanding_task_count{16};
 +
   
 +
    // Spawn several tasks which take different amounts of
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    // time, then decrement the outstanding task count.
 +
    for (std::future<void>& task_future : task_futures)
 +
        task_future = std::async([&]
 +
        {
 +
            // This sleep represents doing real work...
 +
            std::this_thread::sleep_for(50ms);
 +
           
 +
            ++completion_count;
 +
            --outstanding_task_count;
 +
           
 +
            // When the task count falls to zero, notify
 +
            // the waiter (main thread in this case).
 +
            if (outstanding_task_count.load() == 0)
 +
            {
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                all_tasks_completed = true;
 +
                all_tasks_completed.notify_one();
 +
            }
 +
        });
 +
   
 +
    all_tasks_completed.wait(false);
 +
   
 +
    std::cout << "Tasks completed = " << completion_count.load() << '\n';
 +
}|}}
 +
|output=
 +
Tasks completed = 16
 +
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/atomic/{{{1|}}}/dsc notify_one}}
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{{dsc inc|cpp/atomic/atomic/dsc notify_one|{{{1|}}}}}
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{{dsc inc | cpp/atomic/{{{1|}}}/dsc notify_all}}
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{{dsc inc|cpp/atomic/atomic/dsc notify_all|{{{1|}}}}}
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{{#ifeq:{{{2|}}} | bool |
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{{#ifeq:{{{1|}}}|atomic_flag|
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{{dsc inc | cpp/atomic/dsc atomic_flag_notify_one}}
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{{dsc inc|cpp/atomic/dsc atomic_flag_notify_one}}
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{{dsc inc | cpp/atomic/dsc atomic_flag_notify_all}}
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{{dsc inc|cpp/atomic/dsc atomic_flag_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_one}}
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{{dsc inc | cpp/atomic/dsc atomic_notify_all}}
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{{dsc inc|cpp/atomic/dsc atomic_notify_all}}
 
}}
 
}}
 
{{dsc end}}
 
{{dsc end}}
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−
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 

Latest revision as of 06:13, 19 November 2023

Template:cpp/atomic//title Template:cpp/atomic//navbar

void wait( T old, std::memory_order order =
                      std::memory_order::seq_cst ) const noexcept;
 (1) (since C++20)
void wait( T old, std::memory_order order =
                      std::memory_order::seq_cst ) const volatile noexcept;
 (2) (since C++20)

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

  • Compare the value representation of this->load(order) with that of old.
    • If those are equal, then blocks until *this is notified by notify_one() or notify_all(), or the thread is unblocked spuriously.
    • Otherwise, returns.

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

If order is one of std::memory_order::release and std::memory_order::acq_rel, the behavior is undefined.

Contents

[edit] Parameters

old - the value to check the 's object no longer contains
order - memory order constraints to enforce

[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

Run this code
#include <atomic>
#include <chrono>
#include <future>
#include <iostream>
#include <thread>
 
using namespace std::literals;
 
int main()
{
    std::atomic<bool> all_tasks_completed{false};
    std::atomic<unsigned> completion_count{};
    std::future<void> task_futures[16];
    std::atomic<unsigned> outstanding_task_count{16};
 
    // Spawn several tasks which take different amounts of
    // time, then decrement the outstanding task count.
    for (std::future<void>& task_future : task_futures)
        task_future = std::async([&]
        {
            // This sleep represents doing real work...
            std::this_thread::sleep_for(50ms);
 
            ++completion_count;
            --outstanding_task_count;
 
            // When the task count falls to zero, notify
            // the waiter (main thread in this case).
            if (outstanding_task_count.load() == 0)
            {
                all_tasks_completed = true;
                all_tasks_completed.notify_one();
            }
        });
 
    all_tasks_completed.wait(false);
 
    std::cout << "Tasks completed = " << completion_count.load() << '\n';
}

Output: 

Tasks completed = 16

[edit] See also

(C++20)
 notifies at least one thread waiting on the atomic object
(public member function of Template:cpp/atomic//title) [edit]
(C++20)
 notifies all threads blocked waiting on the atomic object
(public member function of Template:cpp/atomic//title) [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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