Clock_t returns big negative numbers

Question

I am trying to measure the running time in nanoseconds of a function by using clock() function, like this:

clock_t start_t = clock();  
random_function();  
clock_t end_t = clock();
printf("Elapsed time: %f\n", (double)(end_t - start_t) / CLOCKS_PER_SEC);

When I run the program I sometimes get very big negative numbers as a result. Why does this happen?

This would mean that it's not advisable to use this function (clock()) to measure the time at all, since it gives random results? — AN00, Aug 10 '16 at 19:30
Okay, I understand. But it still gives negative results sometimes, so it troubles the execution of the program. — AN00, Aug 10 '16 at 19:32
Did you try to use a debugger to see values of start_t and end_t when this happens? — Marian Spanik, Aug 10 '16 at 20:01
Yes, I printed their values and were also big negative numbers. — AN00, Aug 10 '16 at 20:02

score 0 · Answer 1 · answered Aug 10 '16 at 19:54

0

If your platform us using unsigned for clock_t you can use modulo std::numeric_limits<clock_t>::max() subtraction, instead of the absolute one

answered Aug 10 '16 at 19:54

bobah

1 Answers1