Re: strtod ("nan") returns negative NaN

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Aug 14 21:17, Masamichi Hosoda wrote:
> > On Aug 14 11:56, Corinna Vinschen wrote:
> >> On Aug 14 13:45, Masamichi Hosoda wrote:
> >> > >From a50ee5a4747a99c70469a53fe959f3dc22d3b79a Mon Sep 17 00:00:00 2001
> >> > From: Masamichi Hosoda <trueroad@trueroad.jp>
> >> > Date: Tue, 14 Aug 2018 12:50:32 +0900
> >> > Subject: [PATCH] Fix strtod ("nan") returns qNaN
> >> > 
> >> > The definition of qNaN for x86_64 and x86 was wrong.
> >> > So strtod ("nan") returned sNaN instead of qNaN.
> >> > 
> >> > Furthermore, it was inverted the sign bit with the presence of `-` character.
> >> > So strtod ("-nan") returned qNaN.
> >> > 
> >> > This commit fixes definition of qNaN
> >> > and removes the sign bit inversion when evaluating "nan".
> >> > ---
> >> >  newlib/libc/stdlib/gd_qnan.h | 8 ++++----
> >> >  newlib/libc/stdlib/strtod.c  | 1 +
> >> >  2 files changed, 5 insertions(+), 4 deletions(-)
> >> [...]
> > With your patch, strtold looks more correct, but it still prints the
> > sign of NaN:
> > 
> >   strtod ("nan", NULL) = nan
> >   strtod ("-nan", NULL) = nan
> >   strtold ("nan", NULL) = nan
> >   strtold ("-nan", NULL) = -nan
> >   nan ("") = nan
> > 
> > Question: What's wrong with that?  Wouldn't it be more correct if
> > strtod returns -NaN for "-nan" as well?
> 
> In my investigate,
> strtold sets sign bit when parameter has '-' character.
> The wrong long double NaN definition is negative NaN that is set sign bit.
> So without my patch, both strtold ("nan") and
> strtold ("-nan") return negative NaN.
> 
> On the other hand, strtod inverts the sign when parameter has '-' character.
> The wrong double NaN definition is negative NaN.
> So without my patch, strtod ("nan") returns negative NaN
> and strtod ("-nan") returns positive NaN.

Your patch improves the situation, that's a sure thing and I did not
question that.

I just wonder why returning -NaN when the input is "-nan" isn't the
better approach.  After all:

  printf ("nan (\"\") = %f\n", nan (""));
  printf ("-nan (\"\") = %f\n", -nan (""));

==>

  nan ("") = nan
  -nan ("") = -nan

So, shouldn't the ideal outcome be this:

  strtod ("nan", NULL) = nan
  strtod ("-nan", NULL) = -nan
  strtold ("nan", NULL) = nan
  strtold ("-nan", NULL) = -nan

?

Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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