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 time.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

Sometime it's needed to measure astronomical time rather than CPU time (especially this applicable on Linux):

#include <time.h>

double what_time_is_it()
{
    struct timespec now;
    clock_gettime(CLOCK_REALTIME, &now);
    return now.tv_sec + now.tv_nsec*1e-9;
}

int main() {
    double time = what_time_is_it();
    printf("time taken %.6lf\n", what_time_is_it() - time);
    return 0;
}

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);
}

Answer 5

I would use the QueryPerformanceCounter and QueryPerformanceFrequency functions of the Windows API. Call the former before and after the block and subtract (current − old) to get the number of "ticks" between the instances. Divide this by the value obtained by the latter function to get the duration in seconds.

Answer 6

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 7

The standard C library provides the time function and it is useful if you only need to compare seconds. If you need millisecond precision, though, the most portable way is to call timespec_get. It can tell time up to nanosecond precision, if the system supports. Calling it, however, takes a bit more effort because it involves a struct. Here's a function that just converts the struct to a simple 64-bit integer.

#include <stdio.h>
#include <inttypes.h>
#include <time.h>

int64_t millis()
{
    struct timespec now;
    timespec_get(&now, TIME_UTC);
    return ((int64_t) now.tv_sec) * 1000 + ((int64_t) now.tv_nsec) / 1000000;
}

int main(void)
{
    printf("Unix timestamp with millisecond precision: %" PRId64 "\n", millis());
}

Unlike clock, this function returns a Unix timestamp so it will correctly account for the time spent in blocking functions, such as sleep. This is a useful property for benchmarking and implementing delays that take running time into account.