Subj : possible overflow?
To   : comp.programming
From : Rob Somers
Date : Sun Aug 21 2005 08:51 am

I am still fiddling with bitwise operations - this time I have been working
on a program that accepts input (an unsigned integral) from the user, and
then after some 'conversion' outputs that number in binary format.  I have
one problem that is vexing me though:

#include <stdio.h>

int main(void)
{
        unsigned int input = 8;
        unsigned long int i;
        unsigned long int j = 0;
        size_t number_of_bits = sizeof(j) * 8;


        j = (1 << (number_of_bits - 1));
        for (i = 1; i != 0; i <<= 1) {
                if (input & j) {
                       (void) putchar('1');
                } else {
                       (void) putchar('0');
                }
                j >>= 1;
        }
        puts("");
        return 0;
}

That is  a working program that gives an example of my problem - its output
is:

0000000000000000000000000000111111111111111111111111111111111000

instead of:

0000000000000000000000000000000000000000000000000000000000001000

If I declare i and j as unsigned char, short or int, I get results as I
would expect, output of 8, 16, or 32 bits respectively.  But long is not
working.  I have sent it through the debugger, to no avail.  I am stumped.

My machine is a 64 bit Athlon64.  Any help appreciated.

.