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Calculate run time in milliseconds

I am running a C++ program that takes an NxN array and reads a
vertexCover. One requirement of this project is that it returns an
elapsed time of each function so the function efficiency can be
calculated based upon an input.

I am attempting to implement the time.h
header file and use 2 clock_t variables in order to calculate the
difference between the 2 times. Using for example t2-t1. When I return
this value I receive 0 seconds run time continually for each function.

I am looking for some way to output the run time of each function in
milliseconds to be able to calculate the fast run time without having to
input a large graph (1000+ nodes). Is there a multi-platform way to do
1 Solution

 struct timeval tv1;
 struct timeval tv2;


 fprintf(stderr," time needed %i %i \n",tv2.tv_sec-otv1.tv_sec,tv2.tv_usec-otv1.tv_usec);

for windows check:

tv.tv_usec is in micro seconds,if you want to have mili, you must divide by 1000:

 fprintf(stderr," time needed %i %f \n",tv2.tv_sec-otv1.tv_sec,(tv2.tv_usec-otv1.tv_usec)/1000);
For Windows, you can use the GetTickCount() function to get how many milliseconds have elasped since the system was started. You can call it multiple times and then subtract the values to get the elapsed time. (Note that in my experience, the difference between 2 successive calls will always by in multiples of 15 milliseconds or so, that is, for very short times you will get 0, and then the next lowest value you get will be 15 milliseconds. So it is not too precise for really small time intervals.

A little more work for Windows is to use the high-resolution timer (QueryPerformanceFrequency and QueryPerformanceCounter

LARGE_INTEGER time1, time2;



double elapsed = (double)(time2.QuadPart-time1.QuadPart)/(double)freq.QuadPart;
int ielapsed = elapsed * 1000000.0 + 0.5;  // elapsed time in microseconds.

>  fprintf(stderr," time needed %i %f \n",tv2.tv_sec-otv1.tv_sec,(tv2.tv_usec-otv1.tv_usec)/1000);
is a bit of a problem if you don't do the carry properly.. there is a timeval_subtract function floating around which I am not sure who wrote it (I couldn't put the proper credit here) but I have a copy of it in my library.  So to borrow _iskywalker_ 's code:

#include <iostream>
#include <cstdlib>
#include <sys/resource.h>
using namespace std;

int timeval_subtract(struct timeval *result, struct timeval *x, struct timeval *y)
  if (x->tv_usec < y->tv_usec) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y ->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  if (x->tv_usec - y->tv_usec > 1000000) {
    int nsec = (x->tv_usec - y->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;

  return x->tv_sec < y->tv_sec;

struct timeval tv1, tv2, elapse;

timeval_subtract(&elapse, &(usage[1].ru_utime), &(usage[0].ru_utime)); // this is user time...
cout << "user\t" << double(elapse.tv_sec) + double(elapse.tv_usec)/1000000.0 << "sec" << endl;
timeval_subtract(&elapse, &(usage[1].ru_stime), &(usage[0].ru_stime)); // this is system time...
cout << "sys\t" << double(elapse.tv_sec) + double(elapse.tv_usec)/1000000.0 << "sec" << endl;
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