[c] notify_all

 #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();

}

Waiting... 

Waiting... 

Waiting... 

Notifying...

Notifying again...

...finished waiting. i == 1

...finished waiting. i == 1

...finished waiting. i == 1