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# std::condition_variable::wait_for

From cppreference.com
< cpp‎ | thread‎ | condition variable

[edit template]

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

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

This overload may be used to ignore spurious awakenings.

A steady clock is 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.

### [edit] 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
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 relative timeout specified by `rel_time` expired, std::cv_status::no_timeout overwise.
2) false if the predicate `pred` still evaluates to false after the `rel_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{0};

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

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_until | blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function) [edit]

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