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std::condit...::wait_until(3)  C++ Standard Libary std::condit...::wait_until(3)

NAME
     std::condition_variable_any::wait_until	    -	    std::condition_vari-
     able_any::wait_until

Synopsis
	template< class Lock, class Clock, class Duration >

	std::cv_status
	    wait_until( Lock&  lock,					     (1)
     (since C++11)

			const std::chrono::time_point<Clock, Duration>&
	abs_time );
	template< class Lock, class Clock, class Duration, class Predicate
	>

	bool  wait_until(  Lock& lock,					     (2)
     (since C++11)
			 const std::chrono::time_point<Clock, Duration>&
	abs_time,

			 Predicate pred );
	template< class Lock, class Clock, class Duration, class Predicate
	>

	bool wait_until( Lock& lock, std::stop_token  stoken,		     (3)
     (since C++20)
			 const std::chrono::time_point<Clock, Duration>&
	abs_time,

			 Predicate pred );

	wait_until  causes the current thread to block until the condition vari-
     able is
	notified, the given duration has been elapsed, or a spurious wakeup  oc-
     curs. pred can
	be optionally provided to detect spurious wakeup.

	1) Atomically calls lock.unlock() and blocks on *this.
	The  thread  will be unblocked when notify_all() or notify_one() is exe-
     cuted, or
	abs_time is reached. It may also be unblocked spuriously.
	When unblocked, calls lock.lock() (possibly blocking on the lock),  then
     returns.
	2,3) Waiting for a specific condition to become true, can be used to ig-
     nore spurious
	awakenings.
	2) Equivalent to while (!pred())
	    if (wait_until(lock, abs_time) == std::cv_status::timeout)
		return pred();
	return true;.
	3)  Registers  *this  for the duration of this call, to be notified if a
     stop request
	is made on stoken's associated stop-state;  it	is  then  equivalent  to
     while
	(!stoken.stop_requested())
	{
	    if (pred())
		return true;
	    if (wait_until(lock, abs_time) == std::cv_status::timeout)
		return pred();
	}
	return pred();.

	Right after wait_until returns, lock is locked by the calling thread. If
     this
	postcondition cannot be satisfied^[1], calls std::terminate.

	 1.  a	This  can happen if the re-locking of the mutex throws an excep-
     tion.

Parameters
	lock	 - an lock which must be locked by the calling thread
	stoken	 - a stop token to register interruption for
	abs_time - the time point where waiting expires
	pred	 - the predicate to check whether the waiting can be completed

Type requirements
	-
	Lock must meet the requirements of BasicLockable.
	-
	Predicate must meet the requirements of FunctionObject.

Return value
	1) std::cv_status::timeout if abs_time has been reached, otherwise
	std::cv_status::no_timeout.
	2,3) The latest result of pred() before returning to the caller.

Exceptions
	1) Timeout-related exceptions.
	2,3) Timeout-related exceptions, and any exception thrown by pred.

Notes
	The standard recommends that the clock tied to abs_time be used to  mea-
     sure time;
	that clock is not required to be a monotonic clock. There are no guaran-
     tees
	regarding the behavior of this function if the clock is adjusted discon-
     tinuously,
	but the existing implementations convert abs_time from Clock to
	std::chrono::system_clock  and	delegate to POSIX pthread_cond_timedwait
     so that the
	wait honors adjustments to the system clock, but not  to  the  user-pro-
     vided Clock. In
	any  case,  the  function  also  may  wait  for  longer than until after
     abs_time has been
	reached due to scheduling or resource contention delays.

	Even if the clock in use is std::chrono::steady_clock or another monoto-
     nic clock, a
	system clock adjustment may induce a spurious wakeup.

	The effects of notify_one()/notify_all() and each of  the  three  atomic
     parts of
	wait()/wait_for()/wait_until()	(unlock+wait,  wakeup,	and  lock)  take
     place in a
	single total order that can be viewed as modification order of an atomic
     variable:
	the order is specific to this individual condition variable. This  makes
     it
	impossible  for  notify_one()  to, for example, be delayed and unblock a
     thread that
	started waiting just after the call to notify_one() was made.

Example
     // Run this code

      #include <chrono>
      #include <condition_variable>
      #include <iostream>
      #include <thread>

      std::condition_variable_any cv;
      std::mutex cv_m; // This mutex is used for three purposes:
		       // 1) to synchronize accesses to i
		       // 2) to synchronize accesses to std::cerr
		       // 3) for the condition variable cv
      int i = 0;

      void waits()
      {
	  std::unique_lock<std::mutex> lk(cv_m);
	  std::cerr << "Waiting... \n";
	  cv.wait(lk, []{ return i == 1; });
	  std::cerr << "...finished waiting. i == 1\n";
      }

      void signals()
      {
	  std::this_thread::sleep_for(std::chrono::seconds(1));
	  {
	      std::lock_guard<std::mutex> lk(cv_m);
	      std::cerr << "Notifying...\n";
	  }
	  cv.notify_all();

	  std::this_thread::sleep_for(std::chrono::seconds(1));

	  {
	      std::lock_guard<std::mutex> lk(cv_m);
	      i = 1;
	      std::cerr << "Notifying again...\n";
	  }
	  cv.notify_all();
      }

      int main()
      {
	  std::thread t1(waits), t2(waits), t3(waits), t4(signals);
	  t1.join();
	  t2.join();
	  t3.join();
	  t4.join();
      }

Possible output:
      Waiting...
      Waiting...
      Waiting...
      Notifying...
      Notifying again...
      ...finished waiting. i == 1
      ...finished waiting. i == 1
      ...finished waiting. i == 1

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied  to	      Behavior as published		 Correct
     behavior
	LWG 2093 C++11	    timeout-related exceptions were	  mentions these
     exceptions
			    missing in the specification
	LWG  2135  C++11       the  behavior  was  unclear  if		   calls
     std::terminate in
			    lock.lock() throws an exception	  this case

See also
	wait	    blocks  the  current  thread until the condition variable is
     awakened
		   (public member function)
		   blocks the current thread until  the  condition  variable  is
     awakened or
	wait_until until specified time point has been reached
		   (public member function)

Hidden category:
	  * Pages with unreviewed LWG DR marker

http://cppreference.com 	   2024.06.10	   std::condit...::wait_until(3)
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