From: Martin Ambuhl <mambuhl AT earthlink DOT net>
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Subject: Re: integer overflow
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Date: Mon, 28 Jul 2003 00:28:41 GMT
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Paul Cousoulis wrote:
> Why does this code result in an integer overflow?
> 
> unsigned long res;
> 
> res = (1<<31)-1;

Because (1 << 31) is a signed expression. Never use signed integers for 
bit twiddling. Here's an example without such overflow.


#include <stdio.h>

int main(void)
{
     unsigned long res;
     res = 1LU << 31;
     printf("1LU << 31     : %lu\n", res);
     res = (1LU << 31)-1;
     printf("(1LU << 31)-1 : %lu\n", res);
     return 0;
}

1LU << 31     : 2147483648
(1LU << 31)-1 : 2147483647