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Date: Fri, 4 Apr 2014 10:12:59 -0400
From: Rich Felker <>
Subject: Re: printf issues

On Fri, Apr 04, 2014 at 09:32:18AM -0400, Morten Welinder wrote:
> I am seeing differences in how printf("%.200Lg",val) works between musl code
> and glibc.
> Here are some samples.  The top line is how musl prints, the bottom is glibc.
> -65878995336522048.000000000
> -65878995336522048
> 1954675876964773.500000000
> 1954675876964773.5
> 3953605802361882.000000000
> 3953605802361882

Confirmed that this bug exists. It seems to be an error in how the
trailing zero removal logic works in certain cases, but I didn't
figure out the pattern immediately except that it seems to be
associated with large-magnitude numbers like yours. I'll run some
better tests in a little bit.

> Unrelatedly, from function fmt_fp:
> #define CONCAT2(x,y) x ## y
> #define CONCAT(x,y) CONCAT2(x,y)
> [...]
>             long double round = CONCAT(0x1p,LDBL_MANT_DIG);
> That code is cute as a Hello Kitty door knocker, but really?  Let's hope nobody
> gets the urge to define LDBL_MANT_DIG as 0100 or (80-16) or some such.
> The first case will still compile, but get the wrong result.

Yes, this assumes our headers which define it as an unadorned decimal
constant. Do you have any ideas for a clean way to avoid this
assumption without having to compute the value at runtime?


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