Path: csiph.com!news.swapon.de!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.std.c
Subject: Re: May a string span multiple, independent objects?
Date: Fri, 5 Jul 2024 07:14:43 -0000 (UTC)
Organization: A noiseless patient Spider
Lines: 67
Message-ID: <20240705000419.170@kylheku.com>
References: <20240703141500$00ed@vinc17.org>
Injection-Date: Fri, 05 Jul 2024 09:14:43 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="0e37005f4851073825761cec33a52e11"; logging-data="3358387"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19E9ZlS7QcRvAtpXXSFt5rleXferwdmVx4="
User-Agent: slrn/pre1.0.4-9 (Linux)
Cancel-Lock: sha1:QtYtBqWPfmbykcioDtqhFecE0LI=
Xref: csiph.com comp.std.c:6656

On 2024-07-03, Vincent Lefevre <vincent-news@vinc17.net> wrote:
> ISO C17 (and C23 draft) 7.1.1 defines a string as follows: "A string
> is a contiguous sequence of characters terminated by and including
> the first null character."
>
> But may a string span multiple, independent objects that happens
> to be contiguous in memory?

It is undefined behavior. Implementations are allowed to track the
provenance of a displaced pointer, and diagnose when it is out of bounds
even if the displaced value points into a valid object, and even if th
eprogram validates that via a well-defined equality test.

> For instance, is the following program valid and what does the ISO C
> standard say about that?
>
> #include <stdio.h>
> #include <string.h>
>
> typedef char *volatile vp;
>
> int main (void)
> {
>   char a = '\0', b = '\0';
>   vp p = &a, q = &b;
>
>   printf ("%p\n", (void *) p);
>   printf ("%p\n", (void *) q);
>   if (p + 1 == q)
>     {
>       a = 'x';
>       printf ("%zd\n", strlen (p));
>     }

In this situation, the p + 1 expression is well-defined as well
the p + 1 == q test.

However, while *q is a valid expression that evaluates to zero,
*(p + 1) isn't valid. The one byte past the object pointer value
may not be dereferenced.

The equivalence p + 1 == q doesn't save it; p + 1 is displaced from p,
unrelated to q.

>   if (q + 1 == p)
>     {
>       b = 'x';
>       printf ("%zd\n", strlen (q));
>     }
>   return 0;
> }
>
> If such a program is valid, would there be issues by working with
> pointers on such a string, say, dereferencing p[1] in the first "if"
> (which is normally UB)?

An issue could be that the implementation's optimizer assumes that
p + 1 and q are poiners to distinct objects, even in the middle
of a block of code that is conditional on p + 1 == q.

If the code executes *(p + 1) = 'a', a subsequent evaluation of
*q or b can still produce 0.

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca