NAME¶

std::atomic_wait,std::atomic_wait_explicit - std::atomic_wait,std::atomic_wait_explicit

Synopsis¶

template< class T >

void atomic_wait( const std::atomic<T>* object, (1) (since C++20)

typename std::atomic<T>::value_type old );
template< class T >

void atomic_wait( const volatile std::atomic<T>* object, (2) (since C++20)

typename std::atomic<T>::value_type old );
template< class T >

void atomic_wait_explicit( const std::atomic<T>* object,
typename std::atomic<T>::value_type (3) (since C++20)
old,

std::memory_order order );
template< class T >

void atomic_wait_explicit( const volatile std::atomic<T>* object,
typename std::atomic<T>::value_type (4) (since C++20)
old,

std::memory_order order );

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.

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

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::memcmp); 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¶

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

Category:¶

* Todo no example