Why is CLOCKS_PER_SEC not the actual number of clocks per second?

Question

I have just written this short C++ program to approximate the actual number of clock ticks per second.

#include <iostream> #include <time.h>  using namespace std;  int main () {      for(int i = 0; i < 10 ; i++) {          int first_clock = clock();         int first_time = time(NULL);          while(time(NULL) <= first_time) {}          int second_time = time(NULL);         int second_clock = clock();          cout << "Actual clocks per second = " << (second_clock - first_clock)/(second_time - first_time) << "\n";          cout << "CLOCKS_PER_SEC = " << CLOCKS_PER_SEC << "\n";      }      return 0;  } 

When I run the program, I get output that looks like this.

Actual clocks per second = 199139 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 638164 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 610735 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 614835 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 642327 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 562068 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 605767 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 619543 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 650243 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 639128 CLOCKS_PER_SEC = 1000000 

Why doesn't the actual number of clock ticks per second match up with CLOCKS_PER_SEC? They're not even approximately equal. What's going on here?

Answer 1

clock returns the amount of time spent in your program. There are 1,000,000 clock ticks per second total^*. It appears that your program consumed 60% of them.

Something else used the other 40%.

^*_{Okay, there are virtually 1,000,000 clock ticks per second. The actual number is normalized so your program perceives 1,000,000 ticks.}