How do I measure time in C?

Question

I want to find out for how long (approximately) some block of code executes. Something like this:

startStopwatch();
// do some calculations
stopStopwatch();
printf("%lf", timeMesuredInSeconds);

How?

Answer 1

You can use the clock method in ctime.h

Example:

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

Answer 2

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).

Answer 3

If you don't need fantastic resolution, you could use GetTickCount(): http://msdn.microsoft.com/en-us/library/ms724408(VS.85).aspx (If it's for something other than your own simple diagnostics, then note that this number can wrap around, so you'll need to handle that with a little arithmetic).

QueryPerformanceCounter is another reasonable option. (It's also described on MSDN)

Answer 4

GetTickCount().

#include <windows.h>
void MeasureIt()
{
    DWORD dwStartTime = GetTickCount();
    DWORD dwElapsed;

    DoSomethingThatYouWantToTime();

    dwElapsed = GetTickCount() - dwStartTime;

    printf("It took %d.%3d seconds to complete\n", dwElapsed/1000, dwElapsed - dwElapsed/1000);
}