Bug in Cygwin strtod()

[Corinna Vinschen]

Here's an error report w/ replies on the Cygwin list:

On Dec 19 08:03, marco atzeri wrote:
> On 12/19/2012 2:30 AM, KHMan wrote:
> >On 12/19/2012 8:54 AM, Cary R. wrote:
> >>The following code demonstrates a subtle bug in the Cygwin version of
> >>strtod(). The value it generates is slightly different than the value
> >>when using the math header files. This used to work correctly some
> >>time ago (months). I just took the time to track the problem down in
> >>our regression suite.
> >>
> >>#include<math.h>
> >>#include<stdio.h>
> >>#include<stdlib.h>
> >>
> >>int main()
> >>{
> >>     double value = M_LN10;
> >>
> >>     printf("generate const. - value: %0.15f.\n", value);
> >>     printf("expected const. - value: 2.302585092994046.\n");
> >>
> >>     printf("\n");
> >>     value = strtod("2.30258509299404568402", 0);
> >>
> >>     /* Note: the last digit is incorrect. */
> >>     printf("generate strtod - value: %0.15f.\n", value);
> >>     printf("expected strtod - value: 2.302585092994046.\n");
> >>
> >>     return 0;
> >>}
> >
> >Yep, looks like the significand misses its mark by 1.
> >
> 
> strtod is from newlib.
> the last change in the code is one year and half ago
> 
> http://sourceware.org/cgi-bin/cvsweb.cgi/src/newlib/libc/stdlib/strtod.c.diff?r1=1.17&r2=1.18&cvsroot=src

In fact, the aforementioned change is the culprit of the wrong
result.  The discussion about the patch starts here:
http://sourceware.org/ml/newlib/2011/msg00178.html

For now I have reverted the patch.  It might have fixed some problem,
but it introduced a new one, so it was apparently not correct, and I
was not able to reproduce the described problem in the first place.


Corinna

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