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# std::condition_variable::wait

From cppreference.com
< cpp‎ | thread‎ | condition variable

[edit template]

void wait( std::unique_lock<std::mutex>& lock ); | (1) | (since C++11)
template< class Predicate > void wait( std::unique_lock<std::mutex>& lock, Predicate pred ); | (2) | (since C++11)

`wait` causes the current thread to block until the condition variable is notified 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. It may also be unblocked spuriously. When unblocked, regardless of the reason, `lock` is reacquired and `wait` exits. If this function exits via exception, `lock` is also reacquired.

2) Equivalent to

    while (!pred()) {
        wait(lock);
    }

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true. Note that before enter to this method `lock` must be acquired, after `wait(lock)` exits it is also reacquired, i.e. `lock` can be used as a guard to `pred()` access.

### [edit] Parameters

lock | - | an object of type std::unique_lock<std::mutex>, which must be locked by the current thread
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

(none)

### [edit] Exceptions

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

### [edit] Notes

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] Example

[edit]
Run this code

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

    std::condition_variable 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

### [edit] See also

wait_for | blocks the current thread until the condition variable is woken up or after the specified timeout duration (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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