C++中的条件变量wait_for

Question

I am working with condition_variable on Visual studio 2019. The condition_variable.wait_for() function returns std::cv_status::no_timeout without any notification.

#include <iostream>
#include <thread>
#include <chrono>
#include <mutex>

std::condition_variable cv;
std::mutex mtx;
bool called = false;

void printThread()
{
    std::unique_lock<std::mutex> lck(mtx);
    while (std::cv_status::timeout == cv.wait_for(lck, std::chrono::seconds(1)))
    {
        std::cout << "*";
    }
    std::cout << "thread exits" << std::endl;
}

int main()
{
    std::thread th(printThread);
    th.join();
    std::cout << "program exits" << std::endl;
}

I think the code will never exit and keep printing * , but it exits after printing some * .

Here is the output:

********************************************************************thread exits
program exits

Why does this happen? Is it the so-called "spurious wakeups"？

Answer 1

Yes, it's a "spurious wakeup". This is explained on cppreference.com's reference page for wait_for :

It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait_for() exits.

Translation: there are gremlins in your computer. They get grumpy, occasionally. And if they do get grumpy, wait_for returns before the requested timeout expires. And when that happens:

Return value

1. std::cv_status::timeout if the relative timeout specified by rel_time expired, std::cv_status::no_timeout otherwise.

And that seems to be exactly what you're seeing. The C++ standard permits a C++ implementation to return from wait_for prematurely, for arbitrary reasons, and unless you do return from wait_for when the timeout expires, no_timeout is what you get.

You might be wondering why wait_for (and several other similar functions) may decide to throw up their hands and return "spuriously". But that would be a different question...

Answer 2

As already explained, it is waking up due spurious wakeup . Such thing make the function wait_for completely useless. The solution is to use the wait_until saving the current time before entering the wait loop:

int count = 1;
std::mutex mutex;
std::condition_variable condition_variable;

void wait() {
    std::unique_lock<std::mutex> lock(mutex);
    count--;

    int timeout = 1000; // 1 second
    std::chrono::time_point<std::chrono::system_clock> timenow = 
        std::chrono::system_clock::now();

    while(count < 0) {
        std::cv_status status = condition_variable.wait_until(
            lock, 
            timenow + std::chrono::duration<double,std::ratio<1,1000>>(timeout));

        if ( std::cv_status::timeout == status) {
            count++;
            break;
        }
    }
}