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Re: Precision of doubles and stdio


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:

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

roberto@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.
If you haven't gone out of your way to install similar printf() support libraries on cygwin and linux, they will definitely not be the same. My past reading of various relevant documents convinced me that digits beyond the 17th in formatting of doubles are not required by any standard to be consistent between implementations. They have no useful function, as 17 digits are sufficient to determine uniquely the corresponding binary value in IEEE 754 format.

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