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From: throopw@xyzzy.UUCP
Newsgroups: comp.lang.c
Subject: Re: typeof isn't enough to define swap correctly
Message-ID: <254@xyzzy.UUCP>
Date: Sat, 12-Sep-87 15:29:32 EDT
Article-I.D.: xyzzy.254
Posted: Sat Sep 12 15:29:32 1987
Date-Received: Sun, 13-Sep-87 08:40:57 EDT
References: <557@rocky.STANFORD.EDU> <1880@sol.ARPA> <109@umigw.MIAMI.EDU> <2019@sfsup.UUCP>
Organization: Data General, RTP NC.
Lines: 36

> mpl@sfsup.UUCP (M.P.Lindner)
> #define swap(a, b) { \
> 	register int	i; \
> 	for (i = 0; i < sizeof(a); i++) { \
> 		((char *) &a)[i] ^= ((char *) &b)[i]; \
> 		((char *) &b)[i] ^= ((char *) &a)[i]; \
> 		((char *) &a)[i] ^= ((char *) &b)[i]; \
> 	} \
> }
> OK, so it's not as pretty or efficient as everyone would like, but it's
> completely general purpose and doesn't need any new features.

No, I'm not about to pick the nit that everybody else does at this
point, which is that it doesn't work for register variables.  No, I'm
going to pick the nit that the typebreaking done when copying things of
arbitrary type as characters is first of all not guaranteed to work
correctly by draft X3J11 (nor by K&R), and second of all I'm familiar
with a machine on which it would have failed.

In particular, imagine a type with a data format that depends upon where
in memory it is located.  A common example of this is executable code.
To copy executable code from place to place, it is often not sufficent
to treat it as an array of bytes and copy those bytes.

Before you dismiss this as frivolous, there is at least one architecture
where *pointers* have this property... that is, the format of the
pointer depends upon where in memory it is stored.  Thus, versions of
"swap" that do type breaking of this kind are vulnerable to failure when
fed structures containing pointers to swap, for example.

Just thought y'all might be interested in this little-known bit of trivia.

--
To understand a program you must become both the machine and the program.
                                        --- Alan J. Perlis
-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw