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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.std.c
Subject: Re: contradiction about the INFINITY macro
Date: Mon, 11 Oct 2021 12:39:50 -0700
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On 10/9/21 4:17 PM, Vincent Lefevre wrote:
>> 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.)
>
> "For decimal floating constants, and also for hexadecimal floating
> constants when FLT_RADIX is not a power of 2, the result is either
> the nearest representable value, or the larger or smaller representable
> value immediately adjacent to the nearest representable value, chosen in
> an implementation-defined manner.
> For hexadecimal floating constants when FLT_RADIX is a power of 2, the
> result is correctly rounded." (6.4.4.2p3)
>
> In the case of overflow, for a type that cannot represent infinity,
> there is only one "nearest representable value", which is DBL_MAX.

But does that apply when a constraint is violated?

6.4.4p2, a constraint, says:

    Each constant shall have a type and the value of a constant shall be
    in the range of representable values for its type.

A "constraint", aside from triggering a required diagnostic, is a
"restriction, either syntactic or semantic, by which the exposition of
language elements is to be interpreted", which is IMHO a bit vague.

My mental model is that if a program violates a constraint and the
implementation still accepts it (i.e., the required diagnostic is a
non-fatal warning) the program's behavior is undefined -- but the
standard doesn't say that.  Of course if the implementation rejects the
program, it has no behavior.

For what it's worth, given this:

    double too_big = 1e1000;

gcc, clang, and tcc all print a warning and set too_big to infinity.
That's obviously valid if the behavior is undefined.  I think it's also
valid if the behavior is defined; the nearest representable value is
DBL_MAX, and the larger representable value immediately adjacent to
DBL_MAX is infinity.

It doesn't seem to me to be particularly useful to say that a program
can be rejected, but its behavior is defined if the implementation
chooses not to reject it.

-- 
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 */