clock() just returns 0

Question

The C `clock()` function just returns a zero

clock() function always returning 0

why C clock() returns 0

I looked up all these questions and answers

And I learned that clock() returns clock ticks per some constant which differs per systems

And time() returns the number of seconds.

First, I was trying to measure the execution time of my sorting algorithm using clock() like this:

#include <iostream>
#include <ctime>

... Some other headers and codes

a = clock();
exchange_sort();
a = clock() - a;

... Rest of the code

I tried many different data types with a like int, clock_t, long, float. And I sorted a pretty big size array int arr[1000] with already increased order. But the value of a was always 0, so I tried to find the reason using gdb and I set a breakpoint to the line where the sorting algorithm is located so that I can check the value of a = clock(); and there has to be some number inside the variable but there was only 0.

So after that, I tried to check whether the function was the problem itself or something else like this:

#include <iostream>
#include <iostream>

int main()
{
    int a;
    clock_t b;
    float c;
    long d;
    a = clock();
    b = clock();
    c = clock();
    d = clock();
    return 0;
}

And I checked the value of each variable through gdb and there were just garbage numbers before I put the return value of clock() but after I put there were only 0s inside the variables. So apparently clock() just returns 0 all the time in my conclusion

I really don't know how can I fix this

My g++ version is 4.4.7. I ran this in Linux My processor is x86_64-redhat-linux

Answer 1

The clock() function is a coarse measure of CPU time used. Your code doesn't use enough CPU time to measure with such a coarse measure. You should probably switch to something like getrusage instead.

Answer 2

My code used enough CPU time. But it seems the clock is only ticking with a step of 15625ms.

My advice is to use <chrono> instead.