function to count number of set bits

From: G Patel (gaya.patel_at_gmail.com)
Date: 01/29/05 

Date: 29 Jan 2005 14:06:16 -0800

Hi, I'm wondering if anyone knows if the following function will
function properly as a set-bit counter on non 2s complement machines
(as K&R2 implies).

| int bitcount(unsigned x)
| {
| int count;
|
| for(count = 0; x != 0; count++, x &= (x-1))
| ;
|
| return count;
| }

I can't think of a reason why this would fail on a 1s complement or
sign-mag machine (and can't find a non 2s compliment machine to try it
on). Is it portable as far as C is concerned?

And also, if I declare a signed int in the main program and want to set
the msb to 1, can I do this (32bit ints)?:

int b = 0x8000000;
/* is the 0x80000000 taken as an unsigned long constant or a signed
int? */

int count = bitcount(b);
/* is this undefined- trying to send a negative int to bitcount
function? */

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