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Date: Fri, 03 Mar 2006 22:43:34 +0100
From: Roberto Bagnara <bagnara AT cs DOT unipr DOT it>
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CC: "The Parma Polyhedra Library developers' list" <ppl-devel AT cs DOT unipr DOT it>
Subject: Precision of doubles and stdio
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Hi there,

the following little program

#include <stdio.h>

int main() {
   double d;
   scanf("%lf", &d);
   printf("%.1000g\n", d);
   return 0;
}

does this on Linux/i686

$ gcc -W -Wall in.c
$ a.out
70.9
70.900000000000005684341886080801486968994140625

and does the following under Cygwin on the same machine:

roberto AT quark /tmp
$ gcc -W -Wall in.c

roberto AT quark /tmp
$ ./a.exe
70.9
70.90000000000000568434188608080148696899414

Why?  Is there a way to reconcile the two behaviors?
Notice that I know about the x87 and its vaguaries:
nonetheless I wonder why such a scanf immediately
followed by a printf shows a difference between
Cygwin and Linux.
All the best,

     Roberto

-- 
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara AT cs DOT unipr DOT it

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