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std::atomic_flag::wait(3)     C++ Standard Libary    std::atomic_flag::wait(3)

NAME
       std::atomic_flag::wait -	std::atomic_flag::wait

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

	  Performs atomic waiting operations. Behaves as if it repeatedly per-
       forms the
	  following steps:

	    * Compare this->test(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.

Parameters
	  old	 -  the	value to check the atomic_flag's object	no longer con-
       tains
	  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)
	  notify_all		  notifies  all	threads	blocked	waiting	on the
       atomic object
	  (C++20)		 (public member	function)
	  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)

http://cppreference.com		  2022.07.31	     std::atomic_flag::wait(3)

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