Xref: utzoo comp.lang.c:12583 comp.std.c:371
Path: utzoo!utgpu!water!watmath!clyde!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c,comp.std.c
Subject: Re: memcpy vs. memmove
Message-ID: <7448@haddock.ima.isc.com>
Date: 14 Sep 88 16:21:12 GMT
References: <8389@smoke.ARPA> <7039@bloom-beacon.MIT.EDU> <2063@cuuxb.ATT.COM>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Followup-To: comp.std.c
Organization: Interactive Systems, Boston
Lines: 19

In article <2063@cuuxb.ATT.COM> mmengel@cuuxb.ATT.COM (Marc Mengel) writes:
>To check whether the source and destination overlap, you have to compare two
>pointer values.  You can only do this legally ($3.3.8) if the two pointers
>point into the same array/aggregate.

Watch it -- the first `you' refers to the implementation; the second refers to
the user.  Certainly the implementation is free to generate any code that
happens to work.

>Hence you need memmove() for shifting items in an array, and memcpy() for
>moving items between arrays.  Using memmove() on items not in the same array
>is therefore not guaranteed to work, as the comparison of pointers not in the
>same array/aggregate is undefined.

No, memmove() is guaranteed to work on *any* valid pointers.  What you've
really proved is that memmove() cannot be portably implemented in C.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
Followups to comp.std.c.