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Re: May a string span multiple, independent objects?

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Vincent Lefevre <vincent-news@vinc17.net> writes:

> In article <87zfqy6v54.fsf@bsb.me.uk>,
>   Ben Bacarisse <ben@bsb.me.uk> wrote:
>
>> 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.
>
> I agree (and note that strlen is not necessarily written in C).
>
>> 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."
>
> Arguments of these functions are either arrays and strings, where a
> string is not defined as being an array (or a part of an array). So
> I don't see why this text, as written, would apply to strings.

I'd say because an object like a (or b) is considered to be an array of
length one.

> BTW, the definition of an object is rather vague: "region of data
> storage in the execution environment, the contents of which can
> represent values". But it is not excluded that contiguous areas
> can form an object.

Indeed.  In fact an array is an object made up of contiguous objects,
but &a points to an array of length one.

> Similarly, malloc() is specified as allocating space for an object,
> but this does not mean that one initially has an object in the
> allocated space, though with the above restriction, this would
> be important to be able to use memset() on this storage area.

I think you have an object as soon all the storage is allocated.

-- 
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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