# std::condition_variable::wait_until

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< cpp‎ | thread‎ | condition variable

template< class Clock, class Duration > std::cv_status wait_until( std::unique_lock<std::mutex>& lock, const std::chrono::time_point<Clock, Duration>& timeout_time ); | (1) | (since C++11)
template< class Clock, class Duration, class Predicate > bool wait_until( std::unique_lock<std::mutex>& lock, const std::chrono::time_point<Clock, Duration>& timeout_time, Predicate stop_waiting ); | (2) | (since C++11)

`wait_until` causes the current thread to block until the condition variable is notified, a specific time is reached, or a spurious wakeup occurs, optionally looping until some predicate is satisfied (bool(stop_waiting()) == true).

1) Atomically releases `lock`, blocks the current executing thread, and adds it to the list of threads waiting on *this. The thread will be unblocked when notify_all() or notify_one() is executed, or when the absolute time point `timeout_time` is reached. It may also be unblocked spuriously. When unblocked, regardless of the reason, `lock` is reacquired and `wait_until` exits.
2) Equivalent to

    while (!stop_waiting()) {
        if (wait_until(lock, timeout_time) == std::cv_status::timeout) {
            return stop_waiting();
        }
    }
    return true;

This overload may be used to ignore spurious wakeups.

Calling this function if `lock.mutex()` is not locked by the current thread is undefined behavior.

Calling this function if `lock.mutex()` is not the same mutex as the one used by all other threads that are currently waiting on the same condition variable is undefined behavior.

If these functions fail to meet the postcondition (lock.owns_lock()==true and lock.mutex() is locked by the calling thread), std::terminate is called. For example, this could happen if relocking the mutex throws an exception.

`Clock` must meet the Clock requirements. The program is ill-formed if std::chrono::is_clock_v<Clock> is false. (since C++20)

### Parameters

lock | - | an object of type std::unique_lock<std::mutex>, which must be locked by the current thread
timeout_time | - | an object of type std::chrono::time_point representing the time when to stop waiting
stop_waiting | - | predicate which returns ​false if the waiting should be continued (bool(stop_waiting()) == false). The signature of the predicate function should be equivalent to the following: bool pred();​

### Return value

1) std::cv_status::timeout if the absolute timeout specified by `timeout_time` was reached, std::cv_status::no_timeout otherwise.
2) false if the predicate `stop_waiting` still evaluates to false after the `timeout_time` timeout expired, otherwise true. If the timeout had already expired, evaluates and returns the result of `stop_waiting`.

### Exceptions

1) Any exception thrown by clock, time point, or duration during the execution (clocks, time points, and durations provided by the standard library never throw)
2) Same as (1) but may also propagate exceptions thrown by `stop_waiting`

### Notes

The standard recommends that the clock tied to `timeout_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 convert `timeout_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-provided `Clock`. In any case, the function also may wait for longer than until after `timeout_time` has been reached due to scheduling or resource contention delays.

Even if the clock in use is std::chrono::steady_clock or another monotonic 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 <iostream>
    #include <atomic>
    #include <condition_variable>
    #include <thread>
    #include <chrono>
    using namespace std::chrono_literals;

    std::condition_variable cv;
    std::mutex cv_m;
    std::atomic<int> i{0};

    void waits(int idx)
    {
        std::unique_lock<std::mutex> lk(cv_m);
        auto now = std::chrono::system_clock::now();
        if(cv.wait_until(lk, now + idx*100ms, [](){return i == 1;}))
            std::cerr << "Thread " << idx << " finished waiting. i == " << i << '\n';
        else
            std::cerr << "Thread " << idx << " timed out. i == " << i << '\n';
    }

    void signals()
    {
        std::this_thread::sleep_for(120ms);
        std::cerr << "Notifying...\n";
        cv.notify_all();
        std::this_thread::sleep_for(100ms);
        i = 1;
        std::cerr << "Notifying again...\n";
        cv.notify_all();
    }

    int main()
    {
        std::thread t1(waits, 1), t2(waits, 2), t3(waits, 3), t4(signals);
        t1.join();
        t2.join();
        t3.join();
        t4.join();
    }

Possible output:

    Thread 1 timed out. i == 0
    Notifying...
    Thread 2 timed out. i == 0
    Notifying again...
    Thread 3 finished waiting. i == 1

### Defect reports

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

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2093 | C++11 | timeout-related exceptions were missing in the specification | mentioned
LWG 2135 | C++11 | `wait_until` threw an exception on unlocking/relocking failure | calls std::terminate

### See also

wait | blocks the current thread until the condition variable is awakened (public member function)
wait_for | blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function)

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