wait_until causes the current thread to block until the condition variable is notified, the given time point has been reached, or a spurious wakeup occurs.pred can be optionally provided to detect spurious wakeup.

Right afterwait_until returns,lock.owns_lock() istrue, andlock.mutex() is locked by the calling thread. If these postconditions cannot be satisfied^[1], callsstd::terminate.

If any of the following conditions is satisfied, the behavior is undefined:

• lock.owns_lock() isfalse.
• lock.mutex() is not locked by the calling thread.
• If some other threads are also waiting on*this,lock.mutex() is different from the mutex unlocked by the waiting functions (wait,wait_for andwait_until) called on*this by those threads.

1. ↑This can happen if the re-locking of the mutex throws an exception.

[edit]Parameters

[edit]Return value

[edit]Exceptions

[edit]Notes

The standard recommends that the clock tied toabs_time be used to measure time; that clock is not required to be a monotonic clock. There are no guarantees regarding the behavior of this function if the clock is adjusted discontinuously, but the existing implementations convertabs_time fromClock tostd::chrono::system_clock and delegate to POSIXpthread_cond_timedwait so that the wait honors adjustments to the system clock, but not to the user-providedClock. In any case, the function also may wait for longer than until afterabs_time has been reached due to scheduling or resource contention delays.

Even if the clock in use isstd::chrono::steady_clock or another monotonic clock, a system clock adjustment may induce a spurious wakeup.

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

[edit]Example

#include <chrono>#include <condition_variable>#include <iostream>#include <thread> std::condition_variable 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 cvint 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();}

Waiting...Waiting...Waiting...Notifying...Notifying again......finished waiting. i == 1...finished waiting. i == 1...finished waiting. i == 1

[edit]Defect reports

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

[edit]See also

• In other languages

• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文