C in a Nutshell

Name

clock

Synopsis

Obtains the CPU time used by the process

#include <time.h>
clock_tclock( void );

If you want to know how much CPU time your program has used, call the clock() function. The function’s return type, clock_t, is defined in time.h as long. If the function returns -1, then the CPU time is not available. Note that the value of clock() does not reflect actual elapsed time, as it doesn’t include any time the system may have spent on other tasks.

The basic unit of CPU time, called a “tick,” varies from one system to another. To convert the result of the clock() call into seconds, divide it by the constant CLOCKS_PER_SEC, which is also defined in time.h.

Example

#include <stdio.h>
#include <time.h>
time_t start, stop;
clock_t ticks; long count;

int main()
{time(&start);
  for (count = 0; count <= 50000000; ++count)
  {
    if (count % 1000000 != 0) continue;   /* measure only full millions */
    ticks = clock();
    printf("Performed %ld million integer divisions; "
           "used %0.2f seconds of CPU time.\n",  count / 1000000,
           (double)ticks/CLOCKS_PER_SEC);
  }
  time(&stop);
  printf("Finished in about %.0f seconds.\n", difftime(stop, start));
  return 0;
}

This program produces 51 lines of output, ending with something like this:

Performed 50 million integer divisions; used 2.51 seconds of CPU time.
Finished in about 6 seconds.

See Also

time(), difftime()