NAME

std::atomic_flag_wait,std::atomic_flag_wait_explicit - std::atomic_flag_wait,std::atomic_flag_wait_explicit

Synopsis

Defined in header <atomic>
void atomic_flag_wait( const atomic_flag*
object,
bool old ) noexcept;
void atomic_flag_wait( const volatile
atomic_flag* object,
bool old ) noexcept;
void atomic_flag_wait_explicit( const
atomic_flag* object,
(1) (since C++20)
bool old,

std::memory_order order ) noexcept; (2) (since C++20)
void atomic_flag_wait_explicit( const volatile
atomic_flag* object,

bool old,

std::memory_order order ) noexcept;

Performs atomic waiting operations.

Compares object->test(std::memory_order_seq_cst) or object->test(order) with old,
and if they are equal then blocks until *object is notified by
std::atomic_flag::notify_one() or std::atomic_flag::notify_all() (or the thread is
unblocked spuriously). This is repeated until the values compare unequal.

1) Equivalent to object->wait(old, std::memory_order_seq_cst).
2) Equivalent to object->wait(old, order).

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

Parameters

object - pointer to the atomic flag to check and wait on
old - the value to check the atomic flag 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.

Example

This section is incomplete
Reason: no example

See also

notify_one notifies at least one thread waiting on the atomic object
(C++20) (public member function of std::atomic_flag)
notify_all notifies all threads blocked waiting on the atomic object
(C++20) (public member function of std::atomic_flag)
atomic_flag_notify_one notifies a thread blocked in atomic_flag_wait
(C++20) (function)
atomic_flag_notify_all notifies all threads blocked in atomic_flag_wait
(C++20) (function)