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Date: Thu, 25 Nov 2021 13:45:06 +0100
To: cygwin@cygwin.com
Subject: Re: possible snprintf() regression in 3.3.2
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From: Corinna Vinschen via Cygwin <cygwin@cygwin.com>
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Cc: Corinna Vinschen <corinna-cygwin@cygwin.com>
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On Nov 25 21:02, Takashi Yano via Cygwin wrote:
> On Wed, 24 Nov 2021 10:28:13 +0100
> Corinna Vinschen wrote:
> > On Nov 24 18:14, Takashi Yano via Cygwin wrote:
> > > On Wed, 24 Nov 2021 17:52:04 +0900
> > > Takashi Yano wrote:
> > > > The printed value is still something wrong...
> > > > sqrt(2)*1e70 should be an integer value.
> > > 
> > > I mean...
> > > 
> > > sqrt(2)*1e70 is actually not an integer, however, double has mantissa
> > > of only 52 bit. So, (double value)*(5^70*2^70) should be an integer.
> > 
> > The conversion is a bit inexact, I guess, but that's another problem
> > of this old ldto, right?
> 
> I looked into this problem and found that:
> 
> This problem is in principle unavoidable with current algorithms.
> This is because the current algorithm uses a value of 10^n for
> the conversion. When n>62, the value does not fit into the 144
> bits of the mantissa part of the internal representation in ldtoa.
> This degrades the precision.

Thanks for checking.

Corinna

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