如何使用条件变量唤醒多个线程

Question

I want to run multiple threads every 100ms. In order to achieve that I thought of introducing std::mutex and std::condition_variable . The problem I'm facing is on what basis the threads should go into waiting state. This is my current code

std::mutex m;
std::condition_variable cv;

Timer_Thread.cpp

while (true) {
    std::lock_guard<std::mutex> LG(m);
    cv.notify_all(); // notifies every 100ms
}

Thread1.cpp

// multiple threads should run every 100ms
while (true) {
    std::unique_lock<std::mutex> UL(m);
    cv.wait(UL);
    UL.unlock();

    // do rest of the work
}

As you can see the threads are waiting without checking any predicate. Can somebody suggest any alternatives to achieve the same. All I want is to notify multiple threads every 100ms simultaneously.

Answer 1

As you mentioned about spurious wake-ups, thus my solution is to use another variable flag to let the waked thread can distinguish the correct notified case and spurious waked up case. So actually, what comes to my mind is semaphore. The counting_semaphore needs c++20, but i think writing a similar naive semaphore in pre C++20 version by using condition_variable and mutex is not a hard thing to you.

std::counting_semaphore<MAX_THREAD_NUM> semaphore;

// Timer_Thread.cpp
while (true) {
    semaphore.release(thread_num); // notifies every 100ms
}

// Thread1.cpp
// multiple threads should run every 100ms
while (true) {
    semaphore.acquire();
    // do rest of the work
}