how to calculate time difference in milliseconds in C/C++

Biddle · 03-17-2009, 01:17 PM

Well I suppose the question is:
How do you suppose we represent a negative number in an unsigned int?
In reflection both codes return a value which indicates if the value is positive or negative, which I failed to notice in your code sample.
I stand corrected.

paulsm4 · 03-17-2009, 01:33 PM

Hi, waqasdaar -

Now that you've got these cool routines (and they *are* cool!) ... please be sure to take the results with a pound of salt (or better, to quote Andrew Tanenbaum, with a "metric ton of salt").

Results are seldom accurate to-the-millisecond, much less to the microsecond. At the very least, you'll definitely want to run multiple tests, and average your results...

wje_lq · 03-17-2009, 01:45 PM

Quote:

Originally Posted by paulsm4

Now that you've got these cool routines (and they *are* cool!) ... please be sure to take the results with a pound of salt (or better, to quote Andrew Tanenbaum, with a "metric ton of salt").

Your, um, timing, sir, is impeccable.

ta0kira · 03-17-2009, 02:24 PM

I don't understand why the other functions posted are so complicated. It can be done in a single long line.

Code:

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <sys/timex.h>


static int difference_micro(struct ntptimeval *bBefore,
struct ntptimeval *aAfter)
{
	return (signed long long) aAfter->time.tv_sec * 1000000ll +
	       (signed long long) aAfter->time.tv_usec -
	       (signed long long) bBefore->time.tv_sec * 1000000ll -
	       (signed long long) bBefore->time.tv_usec;
}

int main()
{
	srand(time(NULL));
	struct ntptimeval start, elapsed;

	struct timespec timeout = { rand() % 2, rand() % 1000000000 };

	ntp_gettime(&start);
	nanosleep(&timeout, NULL);
	ntp_gettime(&elapsed);

	fprintf(stdout, "{ %u, %.9u } -> %ims\n", timeout.tv_sec, timeout.tv_nsec,
	  difference_micro(&start, &elapsed) / 1000);
}

Kevin Barry

wje_lq · 03-17-2009, 04:02 PM

Quote:

Originally Posted by ta0kira

I don't understand why the other functions posted are so complicated. It can be done in a single long line.

You're right, it can. But the reason the other functions are that way is not because waqasdaar needed the extra complication (he didn't), but because of the history of the thread. jiml8 posted something that worked for him, which returned the difference as a struct timeval. I changed it somewhat so it returned both a struct timeval and a long long.

The code kinda changed incrementally. Or, as one of my past cow-orkers was known to say, "excrementally".