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Groups > comp.lang.c > #126963
| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Opinion sought: Is this strictly conforming C? |
| Date | 2018-02-21 15:51 -0800 |
| Organization | None to speak of |
| Message-ID | <lnmv01lx50.fsf@kst-u.example.com> (permalink) |
| References | (15 earlier) <p6jqts$1fak$1@gioia.aioe.org> <lnzi42kz9r.fsf@kst-u.example.com> <95ef2dbd-24aa-415b-ada7-6e933b27cf2b@googlegroups.com> <lnr2pekq2p.fsf@kst-u.example.com> <5b4dbe5b-b7b0-48f4-8511-d443695f0e5b@googlegroups.com> |
supercat@casperkitty.com writes:
> On Wednesday, February 21, 2018 at 3:09:31 PM UTC-6, Keith Thompson wrote:
[...]
>> Given a declaration
>> int a[5][5];
>> wouldn't evaluating
>> a[1][7]
>> require aliasing an array object as a different array type?
>>
>> If a C90 implementation forced a run-time trap on an attempt to evaluate
>> a[1][7], what (normative) clause of C90 would it violate?
>
> The code above would create an array object sized to hold a total of 25
> values of type "int". Assume "int" is 4 bytes; the size of the object
> would be 100 bytes.
Yes, it would create an array object that contains 25 int objects, but
not an array object of type int[25]. Each of the 5 elements of a is of
type int[5].
> According to C89 3.3.6, the subexpression a[1] would multiply the value in
> the brackets (i.e. 1) by the size of *a [i.e. 20], and add that result to
> the address of the aforementioned array object. Then applying [7] to that
> would multiply the value 7 by the size of **a [i.e. 4] and that to the
> just-computed address, yielding an address 48 bytes past the start of the
> 100-byte array object.
Yes, the C89 definition of pointer arithmetic refers to multiplication.
(In my opinion the current definition, which does not, is better.)
C89 3.3.6:
When an expression that has integral type is added to
or subtracted from a pointer, the integral value is first
multiplied by the size of the object pointed to. The result
has the type of the pointer operand. If the pointer operand
points to a member of an array object, and the array object is
large enough, the result points to a member of the same array
object, appropriately offset from the original member.
For a[1][7] to work the way you want it to, there would have be an array
object of type int[N], with N > 7. (The most straightforward way to
have this would be to treat all of a as an array object of type
int[25].) The elements of a are not int objects; they're array objects
of type int[5].
C89 3.3 ("An object shall have its stored value accessed only by an
lvalue that has one of the following types:") does not permit accessing
a, or part of a, with an lvalue of type int[N] with N > 7?
What object of type int[N] do you believe exists?
> I would expect that quality compilers will typically have a variety of
> conforming and non-conforming modes, some of which will reject some
> programs that are unambiguously Strictly Conforming. The usefulness of
> having a compiler accept or squawk at or trap certain constructs is largely
> orthogonal to the question of whether those constructs are defined or
> forbidden by the Standard.
I didn't ask whether a such an implementation would be useful. I asked
what (normative) clause of C90 it would violate.
I suppose I should clarify that I'm not commenting on whether the
behavior of a[1][7] *should* be defined. I'm discussing whether the C89
standard actually does define its behavior.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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Opinion sought: Is this strictly conforming C? lee@ansync.com - 2018-02-15 17:20 -0800
Re: Opinion sought: Is this strictly conforming C? James Kuyper <jameskuyper@verizon.net> - 2018-02-15 22:15 -0500
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-16 08:44 -0800
Re: Opinion sought: Is this strictly conforming C? jameskuyper@verizon.net - 2018-02-16 11:47 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-16 13:00 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 12:25 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 13:21 -0800
Re: Opinion sought: Is this strictly conforming C? Randy Howard <rhoward.mx@EverybodyUsesIt.com> - 2018-02-19 15:26 -0600
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 13:59 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 14:38 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 15:21 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 15:59 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 17:11 -0800
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-20 09:17 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 07:58 -0800
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-20 18:21 +0100
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-20 09:25 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 11:52 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-20 22:27 -0500
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 23:07 -0800
Re: Opinion sought: Is this strictly conforming C? Manfred <noname@invalid.add> - 2018-02-21 14:07 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 09:02 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 09:50 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 12:46 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 13:09 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 15:15 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 15:51 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 17:06 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 17:46 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:18 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 22:14 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 11:39 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 14:59 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-21 22:14 -0500
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:50 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-22 07:22 -0500
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 12:40 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-22 13:00 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 13:48 -0800
Re: Opinion sought: Is this strictly conforming C? scott@slp53.sl.home (Scott Lurndal) - 2018-02-22 22:14 +0000
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:05 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:39 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-22 10:39 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 13:20 -0800
Re: Opinion sought: Is this strictly conforming C? asetofsymbols@gmail.com - 2018-02-23 00:30 -0800
Re: Opinion sought: Is this strictly conforming C? Manfred <noname@invalid.add> - 2018-02-16 20:05 +0100
Re: Opinion sought: Is this strictly conforming C? lee@ansync.com - 2018-02-16 11:14 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 11:30 -0800
Re: Opinion sought: Is this strictly conforming C? Steve Carroll <frelwizzen@gmail.com> - 2018-03-01 09:01 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 11:08 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 17:16 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:16 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:25 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-22 07:41 -0500
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-24 13:24 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-25 12:07 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-25 18:34 -0500
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-26 08:21 -0800
Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-26 22:22 -0500
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-27 09:16 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-27 08:48 -0800
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-28 10:24 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-28 09:03 -0800
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-28 22:31 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-28 15:19 -0800
Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-03-01 12:37 +0100
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-03-01 11:03 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-03-01 11:22 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 11:48 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 15:11 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 16:14 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 19:53 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 21:27 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 00:52 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 01:54 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 06:11 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-03-01 12:31 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-03-01 13:15 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 14:55 -0800
Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-03 02:19 -0800
Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 03:03 -0800
Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-25 15:36 -0800
Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-26 10:17 -0800
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