# std::condition_variable::notify_all

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< cpp‎ | thread‎ | condition variable

void notify_all() noexcept; |  | (since C++11)

Unblocks all threads currently waiting for *this.

### Parameters

(none)

### Return value

(none)

### Notes

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.

The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s); in fact doing so is a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock.

### Example

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

### See also

notify_one | notifies one waiting thread (public member function)

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