Date: Wed, 03 Mar 1999 17:06:26 +1100 From: Andrew Johns <A_Johns@TurnAround.com.au> To: cjclark@home.com Cc: Greg Black <gjb@comkey.com.au>, freebsd-questions@FreeBSD.ORG Subject: Re: FP Math Message-ID: <36DCD162.AC62A10E@TurnAround.com.au> References: <199903030335.WAA03732@cc942873-a.ewndsr1.nj.home.com>
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"Crist J. Clark" wrote:
> Greg Black wrote,
> > > Example: Here are two quick C program that I think show why I am
> > > confused. The first produces an exception and core dumps (for me
> > > anyway). In this one, I have set floats to values outside of their
> > > range. It dumps at the point marked,
> >
> > This question has nothing at all to do with FreeBSD (or any OS)
> > and everything to do with a failure to understand the rules of
> > the C language, so I'll keep this very short.
>
> I asked on FreeBSD because I do not get these type of errors when I
> run on one of my IRIXes.
>
> > > #include <stdio.h>
> > >
> > > int main()
> > > {
> > > float a = -1.0e-41;
> >
> > At this point your program has entered the domain of undefined
> > behaviour and from here on the compiler is free to do absolutely
> > anything it likes, including generating the code represented by
> > the following fragment:
> >
> > execl("/bin/rm", "rm", "-rf", "/", 0);
> >
> > If you assign a double constant to a float and that constant is
> > outside the range of the float (as defined precisely in the
> > header <float.h>), then the behaviour is undefined.
>
> OK, your sarcasm aside, now that you mention it, I see how assigning a
> constant outside of the float range is different than running into
> overflows during processing.
>
> > This also means that the program is allowed to appear to get
> > some things right, but gives you no right to expect that. As
> > for why anybody would deliberately do something like this, given
> > its complete uselessness, that's another question.
>
> To simulate where a more complex and useful program is failing.
>
> > In short, the answer is: "you can't do that."
>
> I won't.
>
> > > Could someone explain this?
> >
> > Done.
>
> OK, but why does this give me a floating point exception,
>
> #include <stdio.h>
>
> int main()
> {
>
> double a = 1.0e41;
> float b;
>
> b = (float)a;
>
> printf("%f\n",b);
>
> return 0;
> }
>
> This time I skipped any problems of assigning a out-of-bounds value in
> a constant expression, but get the same result. This is the question I
> intended to ask.
>
> > Anybody who wants or needs further illumination should either
> > get and read the C Standard or visit comp.lang.c (if it still
> > exists).
>
> I have found no further illumination in my K&R. As far as I can tell
> from it, all of the above should give defined results. I was under the
> impression a cast would produce the under/overflow conditions outlined
> in 'man math.'
>
> If my motivation is of any help, I have computational programs that
> I sometimes run on my FreeBSD machine and sometimes on IRIX. The main
> reason I sometimes resort to the IRIX is because I do not get the
> pesky FP exceptions there. Same source code, same input, same
> compiler (gcc), but different results. My best guess is the math
> libraries, and I am trying to figure out where the difference is and
> how to fix it.
>
> For example, the above program returns,
>
> inf
>
> On my SGI.
>
inf = infinity??
ie: overflow
ie: exception if overflow is not trapped
=> SGI is merely trapping the oveflow.
--
Regards | _/\_/\
Andrew Johns BSc (Comp Sci) | / \
TurnAround Solutions Pty Ltd | \_...__/
http://www.turnaround.com.au/ | \/
"The box said 'Requires Windows 95, NT, or better,' so I installed
FreeBSD."
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