[Bug libc/5044] printf doesn't take the rounding mode into account

["khalil dot ghorbal at cea dot fr" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From khalil dot ghorbal at cea dot fr  2008-06-05 17:52 -------
Hi all,

I have noticed the same problem with libc-2.3.6 (I'm using a precompiled libc6
debian(etch) package).

In the example submitted by Vincent, maybe we should calculate x after choosing
the rounding mode: since x
is evaluated first, the default rounding mode (toward +oo) is used. Altering the
rounding mode later won't
change the internal binary representation for x (the type qualifier volatile
here is without effect).

But even doing this don't change the output of printf !

In fact, to get a correct (with respect to IEEE 754) result, I used an
intermediate variable, say "b", set
to 3.0, we get (see bottom for modified source code):
(using %a)
1/3 rounded downward: 0x1.5555555555555p-2
1/3 rounded   upward: 0x1.5555555555556p-2
(using %.17f)
1/3 rounded downward: 0.33333333333333331
1/3 rounded   upward: 0.33333333333333337

A height precision (at least .17) is needed to see the difference using %f
instead of %a (and this is not a normal behaviour, as one should see directly
the difference, since we print two different numbers !).

To summarize :
- fesetround don't influence the internal representation of constants such as
1./3. (or 1./10.)
- using %f seems to have some troubles ...


Hope this helps.

Regards,

<<<<<< source modified >>>>>>>>
#include <stdio.h>
#include <fenv.h>

static volatile double x;

void out (const char *s, int r)
{
        double b = 3.0;
        if (fesetround (r))
                fprintf (stderr, "fesetround error\n");
        else
                x = 1.0/b;
        printf ("1/3 rounded %s: %a\n", s, x);
}

int main (void)
{
        out ("downward", FE_DOWNWARD);
        out ("  upward", FE_UPWARD);
        return 0;
}

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=5044

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