From cppreference.com
Defined in header
voidatomic_flag_wait(constatomic_flag*object,boolold)noexcept; (1) (since C++20)voidatomic_flag_wait(constvolatileatomic_flag*object,boolold)noexcept; (2) (since C++20)voidatomic_flag_wait_explicit(constatomic_flag*object,boolold,std::memory_orderorder)noexcept; (3) (since C++20)voidatomic_flag_wait_explicit(constvolatileatomic_flag*object,boolold,std::memory_orderorder)noexcept; (4) (since C++20)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.
1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.
If order is one of std::memory_order_release and std::memory_order_acq_rel, the behavior is undefined.
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 Return value
(none)
Notes
This form of change-detection is often more efficient than simple polling or pure spinlocks.
Due to the
, transient changes from old to another value and back to old might be missed, and not unblock.
Example
See also
(C++20)
notifies at least one thread waiting on the atomic object
(public member function of std::atomic_flag)
(C++20)
notifies all threads blocked waiting on the atomic object
(public member function of std::atomic_flag)
(C++20)
notifies a thread blocked in atomic_flag_wait
(function)
(C++20)
notifies all threads blocked in atomic_flag_wait
(function)