Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!mcvax!ukc!etive!hwcs!zen!frank
From: frank@zen.co.uk (Frank Wales)
Newsgroups: comp.unix.questions
Subject: Re: Re^2: Bit Switching - How?
Summary: works for me
Message-ID: <1558@zen.UUCP>
Date: 10 Apr 89 10:48:08 GMT
References: <1138@uvm-gen.UUCP> <9977@smoke.BRL.MIL> <2345@van-bc.UUCP> <2255@solo5.cs.vu.nl> <1574@lznv.ATT.COM>
Reply-To: frank@zen.co.uk (Frank Wales)
Organization: Zengrange Limited, Leeds, England
Lines: 46

In article <1574@lznv.ATT.COM> jlw@lznv.ATT.COM (J.L.WOOD) writes:
>> sl@van-bc.UUCP (pri=-10 Stuart Lynne) writes:
>> \For fast generic swapping I use:
>> 
>> \#define	swap(a,b) {a^=b;b^=a;a^= b;}
>> 
>
>Also be careful how you use this algorithm if you are using pointers.
>I once used this method in a sorting routine as my basic exchange and
>got badly burned.
>
>If a and b point to the same location, you get zero as in 0, nada, goose egg.

Well, this surprised me, since blurfl^blurfl does give 0, but blurfl^0
gives blurfl again, so you should get it back, unless some pretty funky
pointer-to-int conversion is taking place.  I tried the following
myself (admittedly on a fairly well-behaved machine from HP),

#include<stdio.h>
#define ptr_swap(a,b)  (a=(void *)((unsigned long)a^(unsigned long)b),\
			b=(void *)((unsigned long)a^(unsigned long)b),\
			a=(void *)((unsigned long)a^(unsigned long)b))

main(argc,argv)
char *argv[];
{
  char *a=argv[0],*b=argv[0];

  (void)printf("a=%lx, b=%lx\n",(unsigned long)a,(unsigned long)b);
  ptr_swap(a,b);
  (void)printf("a=%lx, b=%lx\n",(unsigned long)a,(unsigned long)b);
  return 0;
}

and got:

a=68023000, b=68023000
a=68023000, b=68023000

I'd be interested to find out the machine on which this XOR trick
fails to swap integral values correctly.  [I hardly ever use it, but
I'd still expect it to work.]  Or is there some portability problem
lurking here that my Monday morning mind has missed?
--
Frank Wales, Systems Manager,        [frank@zen.co.uk<->mcvax!zen.co.uk!frank]
Zengrange Ltd., Greenfield Rd., Leeds, ENGLAND, LS9 8DB. (+44) 532 489048 x217