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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.std.c
Subject: Re: contradiction about the INFINITY macro
Date: Thu, 07 Oct 2021 07:05:35 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Vincent Lefevre <vincent-news@vinc17.net> writes:
>
>> In article <877dewimc9.fsf@nosuchdomain.example.com>,
>>   Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>>
>>> Vincent Lefevre <vincent-news@vinc17.net> writes:
>>>
>>>> In article <87lf3ehy4v.fsf@nosuchdomain.example.com>,
>>>> 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.
>>>
>>> The standard only defines INFINITY and NAN for type float.  I
>>> think the implication is that it assumes either all floating types
>>> have NaNs and/or infinities, or none do.  That might be a valid
>>> assumption.
>>
>> But the standard doesn't say that explicitly.  It even just says
>> "if and only if the implementation supports quiet NaNs for the
>> float type".  If the intent were to have NaN support for all the
>> FP types or none, why doesn't it say "... for the floating types"
>> instead of "... for the float type"?
>
> Since the NAN macro is of type float (if it's define), it only makes
> sense to define it that way.  Presumably if an implementation had
> NaN for float but not for double, it would define NAN.

If float has a NaN then so do double and long double, because of
6.2.5 paragraph 10.  Similarly for infinity (or infinities).