How to print time difference in accuracy of milliseconds and nanoseconds from C in Linux?

Question

I have this program which prints the time difference between 2 different instances, but it prints in accuracy of seconds. I want to print it in milliseconds and another in nanoseconds difference.

//Prints in accuracy of seconds

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

int main(void)
{
    time_t now, later;
    double seconds;

    time(&now);
    sleep(2);

    time(&later);
    seconds = difftime(later, now);

    printf("%.f seconds difference", seconds);
}

How can I accomplish that?

Answer 1

Read first the time(7) man page.

Then, you can use clock_gettime(2) syscall (you may need to link -lrt to get it).

So you could try

    struct timespec tstart={0,0}, tend={0,0};
    clock_gettime(CLOCK_MONOTONIC, &tstart);
    some_long_computation();
    clock_gettime(CLOCK_MONOTONIC, &tend);
    printf("some_long_computation took about %.5f seconds
",
           ((double)tend.tv_sec + 1.0e-9*tend.tv_nsec) - 
           ((double)tstart.tv_sec + 1.0e-9*tstart.tv_nsec));

Don't expect the hardware timers to have a nanosecond accuracy, even if they give a nanosecond resolution. And don't try to measure time durations less than several milliseconds: the hardware is not faithful enough. You may also want to use clock_getres to query the resolution of some clock.