linux - C: Different implementation of clock() in Windows and other OS?

Question

I had to write a very simple console program for university that had to measure the time required to make an input.

Therefor I used clock() in front and after an fgets()call. When running on my Windows computer it worked perfectly. However when running on my friends Mac-Book and Linux-PC it gave extremely small results (a few micro seconds of time only).

I tried the following code on all 3 OS:

#include <stdio.h>
#include <time.h>
#include <unistd.h>

void main()
{
    clock_t t;

    printf("Sleeping for a bit
");

    t = clock();

    // Alternatively some fgets(...)
    usleep(999999);

    t = clock() - t;

    printf("Processor time spent: %lf", ((double)t) / CLOCKS_PER_SEC);
}

On windows the output shows 1 second (or the amount of time you took to type when using fgets), on the other two OS not much more than 0 seconds.

Now my question is why there is such a difference in implementation of clock() on these OS. For windows it seems like the clock keeps ticking while the thread is sleeping/waiting but for Linux and Mac isn't?

Edit: Thank you for the answers so far guys, so it's just Microsoft's faulty implementation really.

Could anyone please answer my last question:

Also is there a way to measure what I wanted do measure on all 3 systems using C-standard libraries since clock() only seems to work this way on Windows?

Answer 1

You're encountering a known bug in Microsoft's C Runtime. Even though the behavior is not conforming to any ISO C standard, it won't be fixed. From the bug report:

However, we have opted to avoid reimplementing clock() in such a way that it might return time values advancing faster than one second per physical second, as this change would silently break programs depending on the previous behavior (and we expect there are many such programs).