clock

Description

The clock function determines the amount of processor time used since the invocation of the calling process, measured in CLOCKS_PER_SEC of a second.

The code below counts the number of seconds that passed while running some for loop.

#include <stdio.h>
#include <time.h>
#include <math.h>
int main()
{
  clock_t start = clock();
  for (long i = 0; i < 100000000; ++i)
    exp(log((double)i));
  clock_t finish = clock();
  printf("It took %d seconds to execute the for loop.\n",
  (finish - start) / CLOCKS_PER_SEC);
  return 0;
}

It took 23 seconds to execute the for loop.

Return Values

The clock function returns the amount of time used unless an error occurs, in which case the return value is -1.

Standards

The clock function conforms to ISO/IEC 9899:1990 ("ISO C90"). However, version 2 of the Single UNIX Specification ('SUSv2') CLOCKS_PER_SEC to be defined as one million. FreeBSD does not conform to this requirement; changing the value would introduce binary incompatibility and one million is still inadequate on modern processors.