C - gettimeofday for computing time?

Question

do you know how to use gettimeofday for measuring computing time? I can measure one time by this code:

  char buffer[30];   struct timeval tv;    time_t curtime;     gettimeofday(&tv, NULL);   curtime=tv.tv_sec;   strftime(buffer,30,"%m-%d-%Y  %T.",localtime(&curtime));  printf("%s%ld\n",buffer,tv.tv_usec); 

This one is made before computing, second one after. But do you know how to subtracts it?

I need result in miliseconds

Answer 1

To subtract timevals:

gettimeofday(&t0, 0); /* ... */ gettimeofday(&t1, 0); long elapsed = (t1.tv_sec-t0.tv_sec)*1000000 + t1.tv_usec-t0.tv_usec; 

This is assuming you'll be working with intervals shorter than ~2000 seconds, at which point the arithmetic may overflow depending on the types used. If you need to work with longer intervals just change the last line to:

long long elapsed = (t1.tv_sec-t0.tv_sec)*1000000LL + t1.tv_usec-t0.tv_usec; 

Answer 2

If you want to measure code efficiency, or in any other way measure time intervals, the following will be easier:

#include <time.h>  int main() {    clock_t start = clock();    //... do work here    clock_t end = clock();    double time_elapsed_in_seconds = (end - start)/(double)CLOCKS_PER_SEC;    return 0; } 

hth