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Date: Wed, 17 Nov 2021 13:27:55 +0100
To: cygwin AT cygwin DOT com
Subject: Re: possible snprintf() regression in 3.3.2
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Nov 17 18:21, Takashi Yano via Cygwin wrote:
> On Wed, 17 Nov 2021 11:37:18 +1100
> Tony Cook wrote:
> > This came up from regression testing perl.
> > 
> > Regression testing of perl @4a1b9dd524007193213d3919d6a331109608b90c
> > used (from uname):
> > [...]
> I found the caused by the commit:
> commit 4d90e5335914551862831de3e02f6c102b78435b
> Author: Corinna Vinschen <corinna AT vinschen DOT de>
> Date:   Thu Nov 4 11:30:44 2021 +0100
> 
>     ldtoa: fix dropping too many digits from output
> 
>     ldtoa cuts the number of digits it returns based on a computation of
>     number of supported bits (144) divide by log10(2).  Not only is the
>     integer approximation of log10(2) ~= 8/27 missing a digit here, it
>     also fails to take really small double and long double values into
>     account.
> 
>     Allow for the full potential precision of long double values.  At the
>     same time, change the local string array allocation to request only as
>     much bytes as necessary to support the caller-requested number of
>     digits, to keep the stack size low on small targets.
> 
>     In the long run a better fix would be to switch to gdtoa, as the BSD
>     variants, as well as Mingw64 do.
> 
>     Signed-off-by: Corinna Vinschen <corinna AT vinschen DOT de>
> 
> Reverting this commit solves the problem.
> 
> Corinna, could you please have a look?

I don't have a good solution.  The old ldtoa code is lacking, for
switching newlib to gdtoa I simply don't have the time.  On the newlib
list was a short discussion starting at
https://sourceware.org/pipermail/newlib/2021/018626.html but nothing
came out of it yet.

Patches gratefully accepted (except just reverting the above change).


Corinna

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