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From: Achim Gratz <Stromeko@nexgo.de>
To: cygwin@cygwin.com
Subject: Re: strtod ("nan") returns negative NaN
References: <CANnLRdii8mX71D936AhhEYm+S0s4gS5RGgpPhV_rB6=kzmXKsw@mail.gmail.com>	<20180814.100952.1684125661222835312.trueroad@trueroad.jp>	<20180814.113135.1571893395887584078.trueroad@trueroad.jp>	<20180814.134527.917341694729989717.trueroad@trueroad.jp>	<20180814095618.GT3747@calimero.vinschen.de>	<20180814103900.GU3747@calimero.vinschen.de>
Date: Tue, 14 Aug 2018 21:24:05 +0200
In-Reply-To: <20180814103900.GU3747@calimero.vinschen.de> (Corinna Vinschen's	message of "Tue, 14 Aug 2018 12:39:00 +0200")
Message-ID: <87lg98eoq2.fsf@Rainer.invalid>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
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Corinna Vinschen writes:
> 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?

That's iffy, the treatment of sign bits for NaN is quite different from
the usual arithmetic rules.  A NaN is literally "not a number", i.e. the
computation has left the domain of representable FP numbers, so it
really doesn't have a sign.  That doesn't stop folks from using the sign
bit on its representation, but that's a different story.  The sign
properly belong to what is called the "payload", which is usally
ignored.  So converting "-nan" (or NaN multiplied by -1) really ought to
be just plain NaN.


Regards,
Achim.
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