Date: Tue, 2 Mar 1999 22:35:53 -0500 (EST) From: "Crist J. Clark" <cjc@cc942873-a.ewndsr1.nj.home.com> To: gjb@comkey.com.au (Greg Black) Cc: cjclark@home.com, freebsd-questions@FreeBSD.ORG Subject: Re: FP Math Message-ID: <199903030335.WAA03732@cc942873-a.ewndsr1.nj.home.com> In-Reply-To: <19990303022902.23520.qmail@alpha.comkey.com.au> from Greg Black at "Mar 3, 99 12:29:02 pm"
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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.
--
Crist J. Clark cjclark@home.com
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