# std::condition_variable_any::wait_until

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< cpp‎ | thread‎ | condition variable any

template< class Lock, class Clock, class Duration > std::cv_status wait_until( Lock& lock, const std::chrono::time_point <Clock, Duration>& abs_time ); | (1) | (since C++11)
template< class Lock, class Clock, class Duration, class Predicate > bool wait_until( Lock& lock, const std::chrono::time_point <Clock, Duration>& abs_time, Predicate pred ); | (2) | (since C++11)
template< class Lock, class Clock, class Duration, class Predicate > bool wait_until( Lock& lock, std::stop_token stoken, const std::chrono::time_point <Clock, Duration>& abs_time, Predicate pred ); | (3) | (since C++20)

`wait_until` causes the current thread to block until the condition variable is notified, the given duration has been elapsed, or a spurious wakeup occurs. 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 executed, 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 ignore 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. ↑ This can happen if the re-locking of the mutex throws an exception.

### 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.
- pred() must be a valid expression, and its type and value category must meet the BooleanTestable requirements.

### 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 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 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-provided `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 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 <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 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 | mentions these exceptions
LWG 2114 (P2167R3) | C++11 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened
LWG 2135 | C++11 | the behavior was unclear if lock.lock() throws an exception | calls std::terminate in this case

### See also

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

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