C++ condition variable with no time out

Question

Recently, I met a problem which is related with condition variable in C++. The code is shown below :

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

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

int main(){

std::unique_lock<std::mutex> uniqueLock(mutex);

while (true)
{

    if(cv.wait_for(uniqueLock, std::chrono::milliseconds(1000)) == std::cv_status::no_timeout)
    {
      std::cout << "has image" << std::endl;

    }
    else
    {
        std::cout<< "time out " << std::endl;
    }

}

return 0;
}

The goal of this code is that : each time when condition variable is notified in another thread (cv.notify()), it show "has image " in the console, and if it can not be notified more than 1000 milliseconds, it shows "time out".

So the theoretical output of the above code is (because the condition variable is not notified) :

time out
time out
time out
time out

But when i execute this code in the Vs2015, I found that the output is strange:

has image
time out
has image
time out
time out
time out
has image
has image
time out
time out
time out
time out
time out
has image
has image

I would like to know why i have this output and how can i achieve my goal

Thanks !

Answer 1

I don't know what the cause of your error is (but there are some plausible explanations in the comments). However, one way to fix your issue is to use the other overload of wait_for, which includes a predicate.

It could look something like this (hasImage is just a bool here, replace it with something that makes sense for your needs - !imageStorage.empty() or similar):

while (true)
{

    if (cv.wait_for(uniqueLock, std::chrono::milliseconds(1000), []() {return hasImage;}))
    {
        std::cout << "has image" << std::endl;
        hasImage = false;
    }
    else
    {
        std::cout << "time out " << std::endl;
    }

}

The pertinent point is that the predicate checks if there actually is a new image, and if there isn't then it should continue to wait.

One limitation with this method is that, if the predicate returns false (no image), then you don't know if the condition variable woke due to a spurious wakeup, a timeout, or if there actually was an image but another thread just took it away before this one woke up. But if that is something your design can handle, then this variation works very well.