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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <26843@sun.uucp>
Date: Sat, 29-Aug-87 03:16:30 EDT
Article-I.D.: sun.26843
Posted: Sat Aug 29 03:16:30 1987
Date-Received: Sun, 30-Aug-87 08:47:07 EDT
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Xref: mnetor comp.lang.c:4024 comp.unix.wizards:3969

> How does anything know if s & c are pointing to members of the same array?
> If s isn't 0 terminated, c could end up anywhere...

If "s" isn't 0 terminated, the result returned from "strlen" isn't meaningful
anyway!  As such, the fact that "c" might not be in the same array is hardly
relevant.

The rules don't say that the implementation MUST detect whether the two
pointers belong to the same array, and slap your wrists if they aren't; they
say that the behavior is *undefined* if the pointers aren't members of the same
array!  As such, nobody *has* to know if "s" and "c" are pointing to members of
the same array.  In any *valid* call to "strlen", the pointers will be members
of the same array:

	1) It could be a string constant, which is an array;

	2) It could be an object declared as an array;

	3) It could be an array allocated by "malloc", or an array that is a
	   component of an object allocated by "malloc".

In all *these* cases, if the array contains a valid string, the call to
"strlen" must return a meaningful result, and your sample code for "strlen"
will subtract two pointers that point to members of the same array, or a
pointer that points to a member of an array from a pointer one past the end of
that array, both of which are valid.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com