Re: gettimeofday problem

[Richard B. Johnson]

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.