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Re: May a string span multiple, independent objects?

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James Kuyper <jameskuyper@alumni.caltech.edu> writes:

> On 7/3/24 10:31, Vincent Lefevre 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?
...
>> 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';
>
> a and b are not guaranteed to be contiguous.
>
>> vp p = &a, q = &b;
>>
>> printf ("%p\n", (void *) p);
>> printf ("%p\n", (void *) q);
>> if (p + 1 == q)
>> {
>
> That comparison is legal, and has well-defined behavior. It will be true
> only if they are in fact contiguous.
>
>> a = 'x';
>> printf ("%zd\n", strlen (p));
>
> Because strlen() must take a pointer to 'a' (which is treated, for these
> purposes, as a array of char of length 1), and increment it one past the
> end of that array, and then dereference that pointer to check whether it
> points as a null character, the behavior is undefined.

I think this is slightly misleading.  It suggests that the UB comes from
something strlen /must/ do, but strlen must be thought of as a black
box.  We can't base anyhting on a assumed implementation.

But our conclusion is correct because there is explicit wording covering
this case.  The section on "String function conventions" (7.24.1)
states:

  "If an array is accessed beyond the end of an object, the behavior is
  undefined."

-- 
Ben.

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Thread

May a string span multiple, independent objects? Vincent Lefevre <vincent-news@vinc17.net> - 2024-07-03 14:31 +0000
  Re: May a string span multiple, independent objects? Hans-Bernhard Bröker <HBBroeker@gmail.com> - 2024-07-03 17:23 +0200
    Re: May a string span multiple, independent objects? Vincent Lefevre <vincent-news@vinc17.net> - 2024-07-03 15:37 +0000
    Re: May a string span multiple, independent objects? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-03 12:11 -0400
    Re: May a string span multiple, independent objects? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-08 08:51 -0700
  Re: May a string span multiple, independent objects? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-03 11:59 -0400
    Re: May a string span multiple, independent objects? Ben Bacarisse <ben@bsb.me.uk> - 2024-07-03 22:08 +0100
      Re: May a string span multiple, independent objects? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-03 17:36 -0400
      Re: May a string span multiple, independent objects? Vincent Lefevre <vincent-news@vinc17.net> - 2024-07-04 13:22 +0000
        Re: May a string span multiple, independent objects? Ben Bacarisse <ben@bsb.me.uk> - 2024-07-05 05:14 +0100
        Re: May a string span multiple, independent objects? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-07-05 01:37 -0400
        Re: May a string span multiple, independent objects? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-08-08 08:35 -0700
  Re: May a string span multiple, independent objects? Kaz Kylheku <643-408-1753@kylheku.com> - 2024-07-05 07:14 +0000

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