Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!quintus!ok
From: ok@quintus.UUCP (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <564@cresswell.quintus.UUCP>
Date: 23 Jan 88 03:45:24 GMT
References: <3819@sigi.Colorado.EDU> <5080013@hpfcdc.HP.COM> <7120@brl-smoke.ARPA> <3476@ihlpf.ATT.COM>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 14

In article <3476@ihlpf.ATT.COM>, nevin1@ihlpf.ATT.COM (00704A-Liber) writes:
> In article <2382@ihuxv.ATT.COM> rck@ihuxv.ATT.COM (R. C. Kukuk) writes:
> >> In article <3887@sigi.Colorado.EDU> swarbric@tramp.Colorado.EDU (SWARBRICK FRANCIS JOHN) writes:
> >> >#define swap(a,b) ((a) = ((b) = ((a) = (a) ^ (b)) ^ (b)) ^ (a))
> >> >He says it will work for anything (int, char *, etc.).
> >
> >This oldie fails when a == b.  Why?  Think about it.
> 
> I did.  I still don't see why it fails.
> 
	swap(a, a);
sets a to 0, whatever its previous value was.  If you do
	swap(*p, *q);
it can easily happen that p==q, in which case you get this problem.