Re: gettimeofday problem

Richard B. Johnson
2002-06-24T19:41:07Z

On Mon, 24 Jun 2002, Salvatore D'Angelo wrote:

> In this piece of code I convert seconds and microseconds in
> milliseconds. I think the problem is not in my code, in fact I wrote the
> following piece of code in Java, and it does not work too. In the for
> loop the 90% of times b > a while for 10% of times not.
>
> class Prova {
> public static void main(....) {
> for (;;) {
> long a = System.currentTimeMillis();
> long b = System.currentTimeMillis();
>
> if (a > b) {
> System.out.println("Wrong!!!!!!!!!!!!!");
> }
> }
> }
> }
>
>

This has been running since I first read your mail about 10:00 this
morning. The kernel is 2.4.18

#include <stdio.h>
#include <sys/time.h>
#define MICRO 1000000
#define ULL unsigned long long
int main(void);
static ULL tim(void);

static ULL tim()
{
struct timeval t;
(void)gettimeofday(&t, NULL);
return (ULL)t.tv_sec * MICRO + (ULL)t.tv_usec;
}
int main()
{
ULL a, b, cnt;
for(cnt=0;;cnt++)
{
a = tim();
b = tim();
if(b < a)
break;
}
printf("Failed after %llu\n", cnt);
printf("a = %llu, b = %llu\n", a, b);
return 0;
}

It seems to work fine. That said, I didn't use your code or check
for the possibility of a sign-change miscompare. I just made sure
I don't have any by using unsigned stuff. You may want to try
this code to see if you have a compiler (or coding) problem.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

Windows-2000/Professional isn't.