Re: 16 byte long double with only 10 byte precision?

[Michael Mair <Michael.Mair@xxxxxxxxxxxxxxx>]

Larry I Smith wrote:
> Pavel Pokorny wrote:
>
> >   Dear gcc friends,
> >
> > can you, please, help me 
> > to use a 16 byte long double precision (35 decimal digits)?
> > It looks like a 10 byte (18 digits) precision 
> > on my AMD Opteron hp xw9300 workstation
> > although sizeof reports 16 bytes!
> >
> > #include<stdio.h>
> > int main()
> > {
> > int i;
> > long double x,dx=0.1,x0=2;
> >
> > for (i=1;i<20;i++){
> >   dx = dx/10;
> >   x = x0 + dx;
> >   x = x - x0;
> >   (void) printf ("i=%d x=%LG\n",i,x);
> > };
> > (void) printf ("        sizeof(x) = %d \n", (int)sizeof(x));
> > }
> >
> > i=1 x=0.01
> > i=2 x=0.001
> > i=3 x=0.0001
> > i=4 x=1E-05
> > i=5 x=1E-06
> > i=6 x=1E-07
> > i=7 x=1E-08
> > i=8 x=1E-09
> > i=9 x=1E-10
> > i=10 x=1E-11
> > i=11 x=1E-12
> > i=12 x=1E-13
> > i=13 x=1E-14
> > i=14 x=1.00007E-15
> > i=15 x=9.99634E-17
> > i=16 x=9.97466E-18
> > i=17 x=1.0842E-18
> > i=18 x=0
> > i=19 x=0
> >        sizeof(x) = 16 
> >
> > gcc (GCC) 3.4.4 20050721 (Red Hat 3.4.4-2)
> > Linux 2.6.9-22.EL #1 Mon Sep 19 17:49:49 EDT 2005 x86_64 x86_64 x86_64
> >
> > Thanks for any advice.
>
>
> If the precision is not specified in the printf format string,
> then it defaults to 6; see 'man -S3 printf' for details.
> For example, to get a precision of 30 use "%.30LG", e.g.:
>
>     printf ("i=%d x=%.30LG\n",i,x);

Apart from that: In <float.h>, the numerical limits introduced
by the C Standard are specified. So, you can ask for the number
of mantissa bits, effective decimal digits etc.

Cheers
 Michael
--
E-Mail: Mine is an   /at/ gmx /dot/ de   address.