Question

I am trying to measure CPU time using following code.

timespec time1, time2, temp_time;

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1);
 int i;
 int cpu_sum = 0;


 for (i = 0; i < nelements; i++) {

  cpu_sum += array[i];

 }    
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2);
temp_time.tv_sec = time2.tv_sec - time1.tv_sec;
printf( sum: %d using CPU in %lf ms \n",cpu_sum, temp_time.tv_sec);

But I always get time as 0.000ms
Any idea what is wrong here.

Any help would be appreciated.

Thanks

Answer 1

1. You’re invoking undefined behavior by passing the wrong argument types to printf (time_t, which is probably long, instead of double).

2. tv_sec just contains the whole seconds part of the time. You need to also use tv_nsec to get the nanoseconds part.

Try something like:

temp_time.tv_sec = time2.tv_sec - time1.tv_sec;
temp_time.tv_nsec = time2.tv_nsec - time2.tv_nsec;
if (temp_time.tv_nsec < 0) {
    temp_time.tv_nsec += 1000000000;
    temp_time.tv_sec--;
}
printf("%lld.%.9ld\n", (long long)temp_time.tv_sec, (long)temp_time.tv_nsec);