Wait_for c++

#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
#include <condition_variable>
 
std::mutex mtx;
std::condition_variable cnd;
 
void fu_th1()
{
    static int flag = 1;
    while (flag)
    {
        std::unique_lock<std::mutex> lck(mtx);
        std::cout << "a";
        auto b = cnd.wait_for(lck, std::chrono::milliseconds(1000));
        if (b==std::cv_status::timeout) flag = 0;
    };
};
 
int main()
{
    std::thread f1(fu_th1);
 
    for (int i = 0; i < 40; ++i)
    {
        cnd.notify_all();
    };
 
    f1.join();
 
    std::cout << '\n';
 
    system("pause");
    return 0;
}

cnd.wait_for(lck, std::chrono::milliseconds(1000));

for (int i = 0; i < 40; ++i)
    {
        cnd.notify_all();
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    };