Using pthread to create a clock

Question

I would like to create a new thread that remains in a loop. Something like this:

void Clock() {
    double Time = 0;
    while (1) {
        usleep(1000);
        Time = (Time+0.1);
    }
    return;
}

I would then like my other existing threads to be able to access the latest clock value and use it. How do I declare the "Time" variable so that it can be accessed by all threads and how would I alter the code above to accommodate that? And also what would i need to do to access it?

@JoachimPileborg I have many different source files, can i declare a global variable in a header file to ensure its able to be used in all of my source files? or is there a different solution — Mst137, Aug 25 '16 at 14:31
Just making it a global variable would be begging for a race condition. Make it a static with an accessor function that locks a mutex. Lock the same mutex in your `Clock()` function (just while updating, not while sleeping!). — Fred Larson, Aug 25 '16 at 14:32
@FredLarson: I would not recommend using a mutex. That way, reading the clock in another thread can greatly increase clock-drift. I'd *at least* use a lock-free atomic. But obviously using a proper timer would be preferable anyway. — EOF, Aug 25 '16 at 14:35
@EOF: That's great. Just as long as the race condition is prevented somehow. And yes, this isn't a wheel that should be reinvented. — Fred Larson, Aug 25 '16 at 14:38

score 1 · Answer 1 · edited May 23 '17 at 10:33

If you just want to obtain the current time, you can do so without a thread. Have a look at Get the current time in C for example.

If you use a thread, please be aware that threads run independently. Have a look at Mutex lock threads for a simple example. You will need to use pthread_create to create the thread, and pthread_mutex to protect the Time variable.

I would recommend you start with that first, and leave atomics to the next time.

Using pthread to create a clock

1 Answers1