# std::condition_variable::wait_for

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< cpp‎ | thread‎ | condition variable

template< class Rep, class Period > std::cv_status wait_for( std::unique_lock<std::mutex>& lock, const std::chrono::duration<Rep, Period>& rel_time); | (1) | (since C++11)
template< class Rep, class Period, class Predicate > bool wait_for( std::unique_lock<std::mutex>& lock, const std::chrono::duration<Rep, Period>& rel_time, Predicate stop_waiting); | (2) | (since C++11)

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 relative timeout `rel_time` expires. It may also be unblocked spuriously. When unblocked, regardless of the reason, `lock` is reacquired and `wait_for()` exits.
2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() + rel_time, std::move(stop_waiting));. This overload may be used to ignore spurious awakenings by looping until some predicate is satisfied (bool(stop_waiting()) == true).

The standard recommends that a steady clock be used to measure the duration. This function may block for longer than `timeout_duration` 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.

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.

### Parameters

lock | - | an object of type std::unique_lock<std::mutex>, which must be locked by the current thread
rel_time | - | an object of type std::chrono::duration representing the maximum time to spend waiting. Note that rel_time must be small enough not to overflow when added to std::chrono::steady_clock::now().
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 relative timeout specified by `rel_time` expired, std::cv_status::no_timeout otherwise.
2) false if the predicate `stop_waiting` still evaluates to false after the `rel_time` timeout expired, otherwise true.

### 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

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 <iostream>
    #include <atomic>
    #include <condition_variable>
    #include <thread>
    #include <chrono>
    using namespace std::chrono_literals;

    std::condition_variable cv;
    std::mutex cv_m;
    int i;

    void waits(int idx)
    {
        std::unique_lock<std::mutex> lk(cv_m);
        if(cv.wait_for(lk, 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);
        {
            std::lock_guard<std::mutex> lk(cv_m);
            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();
    }

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_for` 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_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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