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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.std.c
Subject: Re: contradiction about the INFINITY macro
Date: Thu, 30 Sep 2021 03:20:23 +0100
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Vincent Lefevre <vincent-news@vinc17.net> writes:

> In ISO C99:TC3 to C17, 7.12p4:
>
>   The macro INFINITY expands to a constant expression of type float
>   representing positive or unsigned infinity, if available; else to a
>   positive constant of type float that overflows at translation time.
>
> Consider the "else" case. It is said that INFINITY expands to a
> constant and that it overflows, so that it is not in the range of
> representable values of float.
>
> But in 6.4.4p2:
>
>   Each constant shall have a type and the value of a constant shall
>   be in the range of representable values for its type.
>
> which would imply that INFINITY expands to a value in the range of
> representable values of float, contradicted by 7.12p4.

Right.  But there is footnote that clarifies matters:

  "In this case, using INFINITY will violate the constraint in 6.4.4 and
  thus require a diagnostic."

so any program using INFINITY has undefined behaviour because the intent
is to violate 6.4.4.  I agree that there should be a better way to
specify it, but expanding to a constant that violates the constraints on
such constants is clumsy but reasonably clear.

-- 
Ben.