Ludovic Courtès wrote:
> > And finally, what's the hexdump of that DQNAN constant?
> >
> > #include <math.h>
> > #include <stdio.h>
> > int main()
> > {
> > printf("%04X%04X\n", DQNAN[1], DQNAN[0]);
> > return 0;
> > }
>
> FFF800000000
I meant %08X instead of %04X. DQNAN therefore likely is
FFF8000000000000
which decomposes into
sign = 1 (irrelevant for NaNs),
exponent = 0x7FF (highest possible value)
mantissa = 0x8000000000000
and this is precisely the common encoding of a quiet NaN
(see <http://en.wikipedia.org/wiki/Not_a_number>).
> > What's the result for a program that uses a quiet NaN, computed
> > differently?
> >
> > #include <math.h>
> > double zero = 0.0;
> > int main()
> > {
> > return isnan (zero / zero);
> > };
>
> SIGFPE with both `cc' and GCC 4.0.2.
Oh, right, division by floating-point zero and overflow exceptions lead
to SIGFPE by default on OSF/1 and Linux/Alpha. That's why we have the module
'fpieee' in gnulib.
> > What's the result when you compile it with cc?
> > $ cc foo.c -lm
> > $ ./a.out
> > $ echo $?
>
> 1
>
> That's with "Compaq C V6.5-303 (dtk) on Compaq Tru64 UNIX V5.1B
> (Rev. 2650)".
>
> Should we conclude that this is a GCC bug?
Possibly. And what result do you get with "gcc -mieee" ?
Bruno
