why C clock() returns 0

I've got something like this:

clock_t start, end;
start=clock();

something_else();

end=clock();
printf("\nClock cycles are: %d - %d\n",start,end);

and I always get as an output "Clock cycles are: 0 - 0"

Any idea why this happens?

(Just to give little detail, the something_else() function performs a left-to-right exponentiation using montgomery representation, moreover I don't know for certain that the something_else() function does indeed take some not negligible time.)

This is on Linux. The result of uname -a is:

Linux snowy.*****.ac.uk 2.6.32-71.el6.x86_64 #1 SMP Fri May 20 03:51:51 BST 2011 x86_64 x86_64 x86_64 GNU/Linux

Solution 1:

clock function does not measure CPU clock cycles.

C says clock "returns the implementation’s best approximation to the processor time used by the program since the beginning of an implementation-defined era related only to the program invocation."

If between two successive clock calls you program takes less time than one unity of the clock function, you could get 0.

POSIX clock defines the unity with CLOCKS_PER_SEC as 1000000 (unity is then 1 microsecond).

http://pubs.opengroup.org/onlinepubs/009604499/functions/clock.html

To measure clock cycles in x86/x64 you can use inline assembly to retreive the clock count of the CPU Time Stamp Counter register rdtsc.

Solution 2:

I guess the reason is that your something_else() consumes so little time that exceed the precision of clock(). I tried calling clock() twice consequently and both start and end is zero, but result is reasonable when I do some time-consuming stuff between.

Here is my test code snippet:

int main(void) {   
    clock_t start, end;
    start = clock();
    int c;
    for (int i = 0; i < 100; i++) {
        for (int j = 0; j < (1<<30); j++) {
            c++;
        }
    }
    end = clock();
    printf("start = %d, end = %d\n", start, end);
    return 0;
}

And the result on my computer is:

start = 0, end = 27700000

Also, two tips：

1. When testing, do not use any compiler optimization. You may think your something_else() is time-consuming but the compiler may just ignore those operations (especially loops) since it think them as meaningless.
2. Use sizeof(clock_t) on your platform to see the size of clock_t.

Solution 3:

Well, do you want the time something_else() takes? Try this:

#include <sys/time.h>
#include <stdio.h>  
#include <unistd.h>
int main(void) {
    struct timeval start, end;
    long mtime, secs, usecs;    

    gettimeofday(&start, NULL);
    something_else();
    gettimeofday(&end, NULL);
    secs  = end.tv_sec  - start.tv_sec;
    usecs = end.tv_usec - start.tv_usec;
    mtime = ((secs) * 1000 + usecs/1000.0) + 0.5;
    printf("Elapsed time: %ld millisecs\n", mtime);
    return 0;
}

Solution 4:

The right way of using clock() to measure time would be:

printf("\nTime elapsed: %.2f\n",1.0*(end-start)/CLOCKS_PER_SEC);

This is because clock_t isn't guaranteed to be an int, or any other type for that matter.