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Groups > comp.std.c > #6408

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 2022-01-03 12:48 -0800
Organization A noiseless patient Spider
Message-ID <86k0fgy287.fsf@linuxsc.com> (permalink)
References (4 earlier) <20211009201151$a68b@zira.vinc17.org> <861r3pbbwh.fsf@linuxsc.com> <20211110124948$eea4@zira.vinc17.org> <86wnlg9ey7.fsf@linuxsc.com> <87fss3wr6t.fsf@nosuchdomain.example.com>

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> 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?
>
> (Jumping in though the question was addressed to someone else.)
>
> I think it's a tricky question.  I think the language would be
> cleaner if the standard explicitly stated that violating a
> constraint always results in undefined behavior -- or if it
> explicitly stated that it doesn't.  (The former is my personal
> preference.)

Here are some statements that I believe are true:

1. The C standard has no statement that says directly that
   constraint violations result in undefined behavior.

2. The definition of "constraint" in the C standard is ambiguous
   and does not have a single objective meaning.

3. There are no indications (at least none that I am aware of) in
   the C standard, or any other official writing of the WG14 group,
   of what meaning is intended by the ISO C group for the question
   in question.

> Clearly a compiler is allowed (but not required) to reject a program
> that violates a constraint.  If it does so, there is no behavior.

Same comment as I gave in my other recent posting - programs have
behavior, in the sense used in the C standard, regardless of
whether any implementation accepts or rejects them.  (The behavior
may be "undefined behavior".)

> So the question is whether the behavior is undefined if the
> implementation chooses not to reject it.  (I personally don't see a
> whole lot of value in defining the behavior of code that could have
> been rejected outright.

Again, there are lots of constructs that clearly have defined
behavior, and yet implementations can choose to reject them.

> I'm also not a big fan of the fact that
> required diagnostics don't have to be fatal, but that's not likely
> to change.)

IMO this view is short sighted.  Implementations are allowed to
define extensions as long as they are documented and don't change
the meaning of any strictly conforming program.  If any required
diagnostic has to be fatal, that would disallow all kinds of
useful extensions.

Speaking just for myself, I would like implementations to provide
an option under which any required diagnostic would result in the
program being rejected.  But only an option, and in any case that
is in the area of QOI issues, which the C standard has explicitly
chosen not to address.

> [analysis of possible interpretations of the above code fragment]

My question was only about whether there is undefined behavior.

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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

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