Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: NaN's (was Re: FLOATING NULL?)
Message-ID: <1991Jun2.005549.18303@zoo.toronto.edu>
Date: Sun, 2 Jun 1991 00:55:49 GMT
References: <13223@uhccux.uhcc.Hawaii.Edu> <1991May28.153655.24199@zoo.toronto.edu> <2726@root44.co.uk> <14@tdatirv.UUCP> <1991Jun1.214347.11696@zoo.toronto.edu> <24890@lanl.gov>
Organization: U of Toronto Zoology

In article <24890@lanl.gov> jlg@cochiti.lanl.gov (Jim Giles) writes:
>|> [...]           It is simply a fact that in IEEE floating point, x!=x
>|> is sometimes true, and optimizing it to `false' is a compiler bug.
>
>I depends on how the `!=' operator is defined in C...

Hmm, good point.

>I can't find any specific reference in the ANSI C document which defines
>which behaviour is appropriate.  The ANSI/IEEE document Std 754-1985 
>(IEEE Standard for Binary Floating-Point Numbers) _recommends_ that the
>_Fortran_ operator .NE. should be defined as `?<>'...

ANSI C doesn't touch the issue because C exists on many non-IEEE platforms
and there didn't seem to be much prior art on which to base even an optional
specification for an IEEEFP interface.  (X3J11, unlike a lot of other
standards committees, usually took "standardize existing practice, don't
try to invent it" pretty seriously.)  However, in the absence of anything
more precise, I think we could take 754's advice on FORTRAN as relevant
to C as well.

The NCEG is attempting to come up with a consensus C->754 interface,
I believe.
-- 
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  henry@zoo.toronto.edu  utzoo!henry