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# std::condition_variable::wait_until

From cppreference.com
< cpp‎ | thread‎ | condition variable

[edit template]

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<mutex>& lock, const std::chrono::time_point<Clock, Duration>& timeout_time, Predicate pred ); | (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.

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. If this function exits via exception, `lock` is also reacquired.
2) Equivalent to

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

This overload may be used to ignore spurious wakeups.

The clock tied to `timeout_time` is used, which means that adjustments of the clock are taken into account. Thus, the maximum duration of the block might, but might not, be less or more than timeout_time - Clock::now() at the time of the call, depending on the direction of the adjustment. The function also may block for longer than until after `timeout_time` has been reached due to scheduling or resource contention delays.

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.

### [edit] 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
pred | - | predicate which returns ​false if the waiting should be continued. The signature of the predicate function should be equivalent to the following: bool pred();​

### [edit] Return value

1) std::cv_status::timeout if the absolute timeout specified by `abs_time` was reached, std::cv_status::no_timeout overwise.
2) false if the predicate `pred` still evaluates to false after the `abs_time` timeout expired, otherwise true.

### [edit] Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().

### [edit] Example

Run this code

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

    std::condition_variable cv;
    std::mutex cv_m;
    std::atomic<int> i = ATOMIC_VAR_INIT(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 + std::chrono::milliseconds(idx*100), [](){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(std::chrono::milliseconds(120));
        std::cerr << "Notifying...\n";
        cv.notify_all();
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
        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

### [edit] See also

wait | blocks the current thread until the condition variable is woken up (public member function) [edit]
wait_for | blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function) [edit]

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