Difference between revisions of "cpp/atomic/atomic wait"
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* 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}}. | * 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 | + | ** 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. | ||
Revision as of 08:45, 26 February 2020
| Defined in header <atomic>
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| (1) | (since C++20) | |
| template< class T > void atomic_wait( const std::atomic<T>* object, |
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| template< class T > void atomic_wait( const volatile std::atomic<T>* object, |
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| (2) | (since C++20) | |
| template< class T > void atomic_wait_explicit( const std::atomic<T>* object, |
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| template< class T > void atomic_wait_explicit( const volatile std::atomic<T>* object, |
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Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:
- Compare the value representation of object->load(std::memory_order_seq_cst) or object->load(order) 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 underlying implementation unblocks spuriously.
Contents |
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 for this operation: must not be std::memory_order::release or std::memory_order::acq_rel |
Return value
(none)
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::memcpy); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.
Example
| This section is incomplete Reason: no example |
See also
| (C++20) |
notifies at least one thread waiting on the atomic object (public member function of std::atomic<T>)
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| (C++20) |
notifies all threads blocked waiting on the atomic object (public member function of std::atomic<T>)
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| (C++20) |
notifies a thread blocked in atomic_wait (function template) |
| (C++20) |
notifies all threads blocked in atomic_wait (function template) |
