How do I measure time in C?

You can use the time.h library, specifically the time and difftime functions:

/* difftime example */
#include <stdio.h>
#include <time.h>

int main ()
{
  time_t start,end;
  double dif;

  time (&start);
  // Do some calculation.
  time (&end);
  dif = difftime (end,start);
  printf ("Your calculations took %.2lf seconds to run.\n", dif );

  return 0;
}

(Example adapted from the difftime webpage linked above.)

Please note that this method can only give seconds worth of accuracy - time_t records the seconds since the UNIX epoch (Jan 1st, 1970).

You can use the clock method in time.h

Example:

clock_t start = clock();
/*Do something*/
clock_t end = clock();
float seconds = (float)(end - start) / CLOCKS_PER_SEC;