[EMAIL PROTECTED] wrote:
Roberto Bagnara wrote:
Tim Prince wrote:
Roberto Bagnara wrote:
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:
[EMAIL PROTECTED] /tmp
$ gcc -W -Wall in.c
[EMAIL PROTECTED] /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.
If you haven't gone out of your way to install similar printf()
supportlibraries on cygwin and linux, they will definitely not be the
same. Mypast reading of various relevant documents convinced me that
digitsbeyond the 17th in formatting of doubles are not required by
anystandard to be consistent between implementations. They have no
usefulfunction, as 17 digits are sufficient to determine uniquely
thecorresponding binary value in IEEE 754 format.
Thank you Tim. We were unaware of this giant bug in the C standard.
Bug in the C standard??? May I suggest the study of
http://docs.sun.com/source/806-3568/ncg_goldberg.html
I know it very well, thanks. But you are missing the point.
There is no exact binary representation for
powers of ten.
So what? You are again missing the point completely:
you are talking about binary representations for powers
of ten, while the point here is decimal representations
for numbers that are given by an integer times a powers of two.
A binary value in IEEE 754 format is either a special value
or corresponds to one real number. This real number always
has a finite decimal representation.
BTW: this is 100% off-topic for the Cygwin mailing list.
All the best,
Roberto
--
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:[EMAIL PROTECTED]
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