Date: Thu, 11 Mar 1999 09:48:40 -0800 (PST) From: John Polstra <jdp@polstra.com> To: cjclark@home.com Cc: hackers@freebsd.org Subject: Re: FP Math Problem Message-ID: <199903111748.JAA57916@vashon.polstra.com> In-Reply-To: <199903042248.RAA08114@cc942873-a.ewndsr1.nj.home.com>
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In article <199903042248.RAA08114@cc942873-a.ewndsr1.nj.home.com>,
Crist J. Clark <cjclark@home.com> wrote:
> I am trying to figure out why the following program produces a
> floating point exception and core dumps.
>
> #include <stdio.h>
> #include <floatingpoint.h>
> #include <float.h>
>
> int main()
> {
>
> double a = 1e200;
> double b;
>
> printf("fpgetmask: %o\n",fpgetmask());
>
> printf("a: %e\n",a);
>
> b = a*a;
>
> printf("isinf: %d\n",isinf(b));
>
> printf("isnan: %d\n",isnan(b));
>
> printf("b > DBL_MAX: %d\n",b>DBL_MAX);
>
> printf("b: %e\n",b);
>
> return 0;
> }
>
> My output is,
>
> [112:~/tmp] ./dtest
> fpgetmask: 15
> a: 1.000000e+200
> isinf: 0
> isnan: 0
> Floating exception (core dumped)
>
> I started with this on -questions and (although the thread got
> sidetracked by a bad test program I presented at the beginning) other
> FreeBSDers have not reproduced the error, but get what I would expect,
>
> % ./dtest
> fpgetmask: 15
> a: 1.000000e+200
> isinf: 1
> isnan: 0
> b > DBL_MAX: 1
> b: inf
>
> However, I get the FPE and core dump on my three FreeBSD machines
> (different hardware on each).
I played around with this a little bit on a Pentium-II. I also got
the floating exception. There _should_ be a floating exception, but
it should happen at the "b = a*a" statement, I believe. If you add
a second "b = a*a" after it, then you do get the FPE.
As the code is written, for some reason nothing is stored into b at
all. That's why some people get the core dump later on and others
don't. (It might also depend on the specific FP hardware.) You can
confirm this by initializing b to 1.0.
Bruce Evans <bde@freebsd.org> could probably explain this. You
might wish to ask him about it.
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Nobody ever went broke underestimating the taste of the American public."
-- H. L. Mencken
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