why C clock() returns 0

Question

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);


        

Answer 1

I encountered this same issue while trying to time the difference between a generic class and a non-generic class both using a vector, on Red Hat Linux with C++ and g++ compiler. It appears if your program runs slower than a single clock, the clock() reading will always be zero (0).

This code will always return 0

#include <iostream>
#include <ctime>

using namespace std;

int main() {

    cout << clock() << endl;

    return 0;
}

When I added a for loop with an index up to ten million, to slow the program down, then I got a number 20000 as the result from clock()

#include <iostream>
#include <ctime>

using namespace std;

int main() {

    for (int i = 0; i < 10000000; i++) {}
    cout << clock() << endl;

    return 0;
}

Certainly depending on the stats of your box, the results will vary, I am running this code with multi-processor Xeon CPU's and also a huge amount of RAM.