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std::condit...ny::wait_for(3)  C++ Standard Libary std::condit...ny::wait_for(3)

NAME
     std::condition_variable_any::wait_for	   -	    std::condition_vari-
     able_any::wait_for

Synopsis
	template< class Lock, class Rep, class Period >

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

				 const std::chrono::duration<Rep, Period>&
	rel_time );
	template< class Lock, class Rep, class Period, class Predicate >

	bool  wait_for(  Lock& lock, const std::chrono::duration<Rep,	     (2)
     (since C++11)
	Period>& rel_time,

		       Predicate pred );
	template< class Lock, class Rep, class Period, class Predicate >

	bool wait_for( Lock& lock, std::stop_token  stoken,		     (3)
     (since C++20)
		       const std::chrono::duration<Rep, Period>& rel_time,

		       Predicate pred );

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

	1)	   Equivalent	      to	 return 	wait_until(lock,
     std::chrono::steady_clock::now() +
	rel_time);.
	2,3) Waiting for a specific condition to become true, can be used to ig-
     nore spurious
	awakenings.
	2)	   Equivalent	      to	 return 	wait_until(lock,
     std::chrono::steady_clock::now() +
	rel_time, std::move(pred));.
	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 re-
     turn
	wait_until(lock, std::move(stoken),
			  std::chrono::steady_clock::now()	+      rel_time,
     std::move(pred));.

	Right  after  wait_for 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
	rel_time - the maximum duration to wait
	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 rel_time has been elapsed since the begin-
     ning of this
	call, 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
	Even  if notified under lock, overload (1) makes no guarantees about the
     state of the
	associated predicate when returning due to timeout.

	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...ny::wait_for(3)
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