Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.std.c > #6375

Re: contradiction about the INFINITY macro

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.std.c
Subject Re: contradiction about the INFINITY macro
Date 2021-11-10 08:02 -0800
Organization A noiseless patient Spider
Message-ID <86wnlg9ey7.fsf@linuxsc.com> (permalink)
References (2 earlier) <20210930105413$d6e8@zira.vinc17.org> <86wnmoov7c.fsf@linuxsc.com> <20211009201151$a68b@zira.vinc17.org> <861r3pbbwh.fsf@linuxsc.com> <20211110124948$eea4@zira.vinc17.org>

Show all headers | View raw

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

> In article <861r3pbbwh.fsf@linuxsc.com>,
>   Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
>
>> Vincent Lefevre <vincent-news@vinc17.net> writes:
>>
>>> In article <86wnmoov7c.fsf@linuxsc.com>,
>>>   Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
>>>
>>>> What occurs is defined behavior and (for implementations that do
>>>> not have the needed value for infinity) violates a constraint.
>>>> A diagnostic must be produced.
>>>
>>> If this is defined behavior, where is the result of an overflow
>>> defined by the standard?  (I can see only 7.12.1p5, but this is
>>> for math functions;  here, this is a constant that overflows.)
>>
>> I'm wondering if you have resolved your original uncertainty
>> about the behavior of INFINITY in an implementation that does
>> not support infinities?
>
> I suspect that by saying "overflow", the standard actually meant that
> the result is not in the range of representable values.  This is the
> only way the footnote "In this case, using INFINITY will violate the
> constraint in 6.4.4 and thus require a diagnostic." can make sense
> (the constraint in 6.4.4 is about the range, not overflow).  But IMHO,
> the failing constraint makes the behavior undefined, actually makes
> the program erroneous.

Suppose we have an implementation that does not support
infinities, a range of double and long double up to about ten to
the 99999, and ask it to translate the following .c file

    double way_too_big = 1.e1000000;

This constant value violates the constraint in 6.4.4.  Do you
think this .c file (and any program it is part of) has undefined
behavior?  If so, do you think any constraint violation implies
undefined behavior, or just some of them?

> Similarly, on
>
> static int i = 1 / 0;
> int main (void)
> {
>   return 0;
> }
>
> GCC fails to translate the program due to the failing constraint:
>
> tst.c:1:16: error: initializer element is not constant
>     1 | static int i = 1 / 0;
>       |                ^
>
> (this is not just a diagnostic, GCC does not generate an
> executable).

Note that the C standard does not distinguish between "errors"
and "warnings" in diagnostic messages.  Either is allowed
regardless of whether undefined behavior is present.

> Ditto with Clang:
>
> tst.c:1:18: error: initializer element is not a compile-time constant
> static int i = 1 / 0;
>                ~~^~~

The messages indicate that the failure is not about exceeding the
range of a type, but rather about satisfying the constraints for
constant expressions, in particular 6.6 p4, which says in part

    Each constant expression shall evaluate to a constant [...]

The problem here is that 1/0 doesn't evaluate to anything,
because division by 0 is not defined.  Any question of range of
representable values doesn't enter into it.

Back to comp.std.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-30 01:47 +0000
  Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-29 19:05 -0700
    Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-30 11:24 +0000
      Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-30 08:38 -0700
        Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-01 09:05 +0000
          Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 12:20 -0700
            Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-04 09:26 +0000
              Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-04 10:34 -0700
                Re: contradiction about the INFINITY macro Geoff Clare <geoff@clare.See-My-Signature.invalid> - 2021-10-05 13:53 +0100
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-06 00:12 +0000
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-07 07:05 -0700
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-07 07:51 -0700
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-08 11:41 -0700
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-09 19:49 +0000
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-09 14:28 -0700
          Re: contradiction about the INFINITY macro Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2021-10-01 22:55 +0200
            Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 14:26 -0700
          Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-08 08:30 -0700
            Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-08 11:40 -0700
              Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-17 21:00 -0800
            Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-09 20:05 +0000
              Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-11 12:40 -0400
              Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-17 21:02 -0800
      Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-08 00:02 -0700
        Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-09 20:17 +0000
          Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-11 12:40 -0400
            Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-11 12:39 -0700
              Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-11 21:04 -0400
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-11 18:33 -0700
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-03 12:03 -0800
                Re: contradiction about the INFINITY macro Richard Damon <Richard@Damon-Family.org> - 2022-01-03 16:45 -0500
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-03 14:36 -0800
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-04 02:10 -0500
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-17 10:09 -0800
              Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-03 11:55 -0800
            Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-26 10:01 +0000
              Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-26 12:53 -0400
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-28 09:38 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-28 11:23 -0400
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-10-29 12:12 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-30 02:08 -0400
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-08 02:44 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-08 01:46 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-08 10:56 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-08 13:50 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-09 02:48 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-09 00:50 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-09 10:12 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-09 12:51 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-10 12:48 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-10 12:03 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-12 23:17 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-12 21:03 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-15 09:18 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-15 14:25 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-16 01:17 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-16 10:29 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-12-08 10:09 +0000
                Re: contradiction about the INFINITY macro Derek Jones <derek@NOSPAM-knosof.co.uk> - 2021-11-16 11:32 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-16 10:35 -0500
          Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-09 07:13 -0800
            Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-10 13:16 +0000
              Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-10 08:02 -0800
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-10 15:01 -0800
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-13 00:30 +0000
                Re: contradiction about the INFINITY macro Thomas Koenig <tkoenig@netcologne.de> - 2021-12-02 22:14 +0000
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-03 12:48 -0800
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-12 23:55 +0000
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-15 07:59 -0800
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-15 23:39 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-15 20:00 -0500
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-16 01:28 +0000
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-11-16 01:57 +0000
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-11-16 09:52 -0500
                Re: contradiction about the INFINITY macro Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-16 19:00 -0800
                Re: contradiction about the INFINITY macro Vincent Lefevre <vincent-news@vinc17.net> - 2021-12-08 10:56 +0000
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-03 12:56 -0800
                Re: contradiction about the INFINITY macro James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-03 22:45 -0800
                Re: contradiction about the INFINITY macro Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-17 05:35 -0800
  Re: contradiction about the INFINITY macro Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-09-30 03:20 +0100

csiph-web