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Re: contradiction about the INFINITY macro

From Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups comp.std.c
Subject Re: contradiction about the INFINITY macro
Date 2021-10-01 14:26 -0700
Organization None to speak of
Message-ID <87tui0h1xq.fsf@nosuchdomain.example.com> (permalink)
References (1 earlier) <87pmsqizrh.fsf@nosuchdomain.example.com> <20210930105413$d6e8@zira.vinc17.org> <87lf3ehy4v.fsf@nosuchdomain.example.com> <20211001083755$efb3@zira.vinc17.org> <58udnbOX8PAx6Mr8nZ2dnUU78aHNnZ2d@giganews.com>

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Jakob Bohm <jb-usenet@wisemo.com.invalid> writes:
> On 2021-10-01 11:05, Vincent Lefevre wrote:
>> In article <87lf3ehy4v.fsf@nosuchdomain.example.com>,
>>    Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>> 
>>> No, it doesn't force the implementation to violate a constraint.  It
>>> says that a *program* that uses the INFINITY macro violates a constraint
>>> (if the implementation doesn't support infinities).
>> Then this means that the C standard defines something the user must
>> not use (except when __STDC_IEC_559__ is defined, as in this case,
>> INFINITY is guaranteed to expand to the true infinity).
>> 
>>> Constraints apply to programs, not to implementations.
>> This is related, as programs will be transformed by an
>> implementation.
>> 
>>> It means that if a program assumes that INFINITY is meaningful, and it's
>>> compiled for a target system where it isn't, a diagnostic is guaranteed.
>>> And again, it might have made more sense to say that INFINITY is not
>>> defined for such implementations (as is done for the NAN macro), but
>>> perhaps there was existing practice.
>> Yes, currently there is no way of fallback (without things like
>> autoconf tests).
>> Shouldn't the standard by changed to make INFINITY conditionally
>> defined (if not required to expand to a true infinity)?
>> This should not break existing programs.
>
> The fallback is to test for defined(FP_INFINITE), see below.
>
>> 
>>> Here's what the C99 Rationale says:
>> 
>>>      What is INFINITY on machines that do not support infinity? It should
>>>      be defined along the lines of: #define INFINITY 9e99999f, where
>>>      there are enough 9s in the exponent so that the value is too large
>>>      to represent as a float, hence, violates the constraint of 6.4.4
>>>      Constants.  In addition, the number classification macro FP_INFINITE
>>>      should not be defined. That allows an application to test for the
>>>      existance of FP_INFINITE as a safe way to determine if infinity is
>>>      supported; this is the feature test macro for support for infinity.
>> 
>>> The problem with this is that the standard itself doesn't say that
>>> FP_INFINITE is defined conditionally.
>> Even if FP_INFINITE could be defined conditionally, this would not
>> imply that INFINITY is usable, since for instance, long double may
>> have an infinity but not float.
>> 
>
> I don't know if there is a set of similar macros for double and long
> double types buried somewhere in the standard.

The INFINITY and NAN macros are defined only for float.  I think the
assumption is that either all floating-point types support infinities,
or none do, and likewise for NaNs.  On the other hand, fpclassify() is a
macro that can be applied to an expression of any floating-point type.

The problem, as I said, is that the standard doesn't say that
FP_INFINITE is conditionally defined.  Since they're specified in the
same section that explicitly says that NAN is conditionally defined, I
think the only reasonable reading of the standard's wording is that
FP_INFINITE is defined whether infinities are supported or not.
If they're not, it just means that fpclassify() will never return
FP_INFINITE and isinf() always returns 0.

The author(s) of the Rationale obviously *thought* that FP_INFINITE is
conditionally defined.  They were mistaken.  The Rationale itself makes
it clear that the Standard, not the Rationale, is what defines the
language.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

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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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