Re: Loops (was Re: do { quit; } else { })

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From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Loops (was Re: do { quit; } else { })
Date	Tue, 13 May 2025 19:37:53 -0700
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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> [...]
>>
>>>> My personal interpretation is that this:
>>>>
>>>> void func(int arr[static 5]) {
>>>> }
>>>>
>>>> int main(void) {
>>>>     int arr[10];
>>>>     func(arr+5);    // OK
>>>>     // func(arr+6); // UB
>>>> }
>>>>
>>>> is valid, because, for example, the last 5 elements of a 10-element
>>>> array object can be treated as a 5-element array object.  gcc seems
>>>> to agree, based on the fact that it warns about func(arr+6) but
>>>> not about func(arr+5).
>>>>
>>>> This is a fundamental part of my mental model of C, but in a few
>>>> minutes of searching I wasn't able to find explicit wording in the
>>>> standard that supports it.
>>>
>>> In N1570, 6.7.6.3 p7.
>>
>> Did you mean to imply that that paragraph supports (or refutes) my
>> statement?  [...]
>
> No.  I posted the reference to say that the cited paragraph supports
> the conclusion that 'func(arr+6)' is undefined behavior.

I wish you had said so in the first place.  Of course func(arr+6) has
undefined behavior.  Did anyone in this thread say or imply otherwise?

>> """
>> A declaration of a parameter as ??array of _type_?? shall
>> be adjusted to ??qualified pointer to _type_??, where the
>> type qualifiers (if any) are those specified within the [ and ]
>> of the array type derivation.  If the keyword static also appears
>> within the [ and ] of the array type derivation, then for each call
>> to the function, the value of the corresponding actual argument
>> shall provide access to the first element of an array with at least
>> as many elements as specified by the size expression.
>> """
>>
>> The question is whether, for example, the last 5 elements of a
>> 10-element array object can be treated as a 5-element array object.
>> If someone can cite wording in the standard that answers that
>> question, I'd appreciate it.  (I'll be happier if the answer is yes.)
>
> To me it seems obvious that 6.7.6.3 p7 is meant to cover the
> case of 'func(arr+6)' as being undefined behavior.

But that's not the question I was addressing.  My question is whether
func(arr+5) has defined behavior, based on whether or not a+5 points to
the *first element* of an array.

> Note that 6.7.6.3 p7 doesn't say "array object", it says just
> "array".  I believe the choice of wording is neither an accident nor
> an oversight.

Then please explain what you see as the difference.  Wording in the
standard to support the distinction would be welcome.

Given `int arr[10];`, do the last 5 elements of arr constitute an
"array"?   Do they constitute an "array object"?  And the same
questions for arr as a whole.

>> Looking into this a bit more, I realize that the question doesn't
>> matter if there's no "static" keyword between the [ and ].  In that
>> case, the parameter is of pointer type, and the description of
>> pointer arithmetic (N1570 6.5.6p8) explicitly allows the pointer
>> to point to the i-th element of an array object.  The wording for
>> [static N] is the only place I've seen (so far) that specifically
>> refers to the *first* element of an array object, raising the
>> question of whether a subobject of an array object is itself an
>> array object.
>
> Again, not an array object, just an array.
>
>> This might just be some slightly sloppy wording that was
>> introduced in C99 and never corrected.
>
> I draw the opposite conclusion.  The wording of 6.7.6.3 p7 was
> carefully chosen so that it would cover cases like 'func(arr+6)'.

But func(arr+5) is the case I was wondering about.  (That's why I
commented out the func(arr+6) call.)

[snip]

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-04 07:31 -0700
  Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-05-04 18:08 +0300
    Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-05-05 10:42 +0300
    Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-10 06:43 -0700
      Re: Loops (was Re: do { quit; } else { }) Muttley@dastardlyhq.com - 2025-05-10 15:56 +0000
        Re: Loops (was Re: do { quit; } else { }) scott@slp53.sl.home (Scott Lurndal) - 2025-05-10 17:48 +0000
          Re: Loops (was Re: do { quit; } else { }) Muttley@dastardlyhq.com - 2025-05-11 08:20 +0000
        Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-10 14:29 -0700
          Re: Loops (was Re: do { quit; } else { }) Muttley@dastardlyhq.com - 2025-05-11 08:21 +0000
            Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-11 12:02 +0200
              Re: Loops (was Re: do { quit; } else { }) Muttley@dastardlyhq.com - 2025-05-11 15:30 +0000
                Re: Loops (was Re: do { quit; } else { }) scott@slp53.sl.home (Scott Lurndal) - 2025-05-11 16:29 +0000
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-11 18:49 +0200
              Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-11 14:41 -0700
              Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-11 17:43 -0400
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-11 15:06 -0700
                Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-11 18:30 -0400
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-11 18:15 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-11 19:09 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 00:16 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-12 02:23 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 07:19 -0700
                Re: Loops (was Re: do { quit; } else { }) Richard Heathfield <rjh@cpax.org.uk> - 2025-05-12 15:34 +0100
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 22:42 -0700
                Re: Loops (was Re: do { quit; } else { }) Richard Heathfield <rjh@cpax.org.uk> - 2025-05-13 07:31 +0100
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-14 21:12 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-13 09:30 -0700
                Re: Loops (was Re: do { quit; } else { }) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-13 22:28 +0200
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-12 13:31 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-14 20:44 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-14 21:45 -0700
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-12 17:24 +0200
                Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-12 00:07 -0400
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 00:43 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-12 02:27 -0700
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-12 17:18 +0200
                Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-12 19:53 -0400
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 23:03 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-12 19:04 -0700
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-12 17:08 +0200
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-12 13:38 -0700
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-13 12:41 +0200
                Re: Loops (was Re: do { quit; } else { }) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-13 23:16 +0200
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-13 14:35 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-13 15:10 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-13 15:41 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-13 18:38 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-13 19:37 -0700
                Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-13 23:54 -0400
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-13 21:19 -0700
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-13 21:12 -0700
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-13 22:38 -0700
                Re: Loops (was Re: do { quit; } else { }) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-14 03:35 +0000
                Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-13 21:54 -0700
                Re: Loops (was Re: do { quit; } else { }) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-05-14 06:31 +0000
                Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-06-10 06:01 -0700
                Re: Loops (was Re: do { quit; } else { }) Bonita Montero <Bonita.Montero@gmail.com> - 2025-06-14 12:24 +0200
                Re: Loops (was Re: do { quit; } else { }) scott@slp53.sl.home (Scott Lurndal) - 2025-06-14 13:57 +0000
                Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-06-14 22:27 +0300
                Re: Loops (was Re: do { quit; } else { }) Bonita Montero <Bonita.Montero@gmail.com> - 2025-06-15 09:32 +0200
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-14 13:00 +0200
                Re: Loops (was Re: do { quit; } else { }) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-14 13:20 +0200
                Re: Loops (was Re: do { quit; } else { }) James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-05-14 23:20 -0400
                Re: Loops (was Re: do { quit; } else { }) David Brown <david.brown@hesbynett.no> - 2025-05-15 11:23 +0200
            Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-11 17:59 -0700
              Re: Loops (was Re: do { quit; } else { }) Muttley@DastardlyHQ.org - 2025-05-12 10:11 +0000
                Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-05-12 17:09 +0300
      Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-05-11 01:09 +0300
        Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-11 17:30 -0700
          Re: Loops (was Re: do { quit; } else { }) Michael S <already5chosen@yahoo.com> - 2025-05-12 16:18 +0300
            Re: Loops (was Re: do { quit; } else { }) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-05-14 11:09 -0700
          Re: Loops (was Re: do { quit; } else { }) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-05-13 15:57 +0200
  Re: Loops (was Re: do { quit; } else { }) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-05-04 13:52 -0700

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