Question

I need to time my C program inside the program itself (as I am timing multiple subsections of the code and outputting their time). It must be done with (at least) millisecond accuracy. My understanding is that “time.h” functions only work to the second. Is there another way to do this?

Answer 1

You can use the function clock() from <time.h>:

The clock() function returns an approximation of processor time used by the program.

You divide the return value by the macro CLOCKS_PER_SEC to get a time amount in seconds, and if you wanted the time in milliseconds you could multiply the value by 1000, ie:

double get_time_as_ms(void) {
   return ((double)(clock() * 1000) / CLOCKS_PER_SEC);
}

Or more accurately, have a start and end clock_t and then calculate the difference:

double duration_as_ms(clock_t start, clock_t end) {
    return ((double)(end - start) * 1000) / CLOCKS_PER_SEC;

}

clock_t start = clock(); /* start of program */
/* ... */
clock_t end = clock(); /* end of program */
/* ... */
printf("Duration: %fms\m", duration_as_ms(start, end));

EDIT: Just thought I’d add, a common value for CLOCKS_PER_SEC is 1000000, meaning that the value returned by clock() would be a one-hundred-thousandth of a second. Therefore it’s accurate to one one-thousandth of a millisecond (a microsecond).