Precision of doubles and stdio

[Tim Prince]

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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