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Date: Wed, 26 Aug 1998 15:13:52 +0200
From: Martin Cracauer <cracauer@cons.org>
To: Stephen Hocking-Senior Programmer PGS Tensor Perth <shocking@prth.pgs.com>,
        current@FreeBSD.ORG
Subject: Re: Floating Point Exceptions, signal handlers & subsequent ops
References: <199808250753.PAA29567@ariadne.tensor.pgs.com> <19980825180559.A9890@cons.org> <19980826144302.C20511@cons.org>
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In <19980826144302.C20511@cons.org>, Martin Cracauer wrote: 
> In <19980825180559.A9890@cons.org>, Martin Cracauer wrote: 
> > In <199808250753.PAA29567@ariadne.tensor.pgs.com>, Stephen Hocking-Senior Programmer PGS Tensor Perth wrote: 
> > > 
> > > I've noticed in one of my applications that the first FP operation after 
> > > return from a caught SIGFPE is invalid. I have a signal handler installed that 
> > > just prints out some basic info (like "SIGFPE caught"). The first FP op after 
> > > this (in my case, converting a long to a double) just gives garbage. Repeat 
> > > the same statement and it gives a sensible result. Has anyone else seen this 
> > > before I file a PR with code to reproduce the problem?
> > 
> > I just did a short test and for me the next FP operation after a
> > caught SIGFPE (division by zero) is still sane (without
> > setjump/jumpjump).
> 
> I'm afraid I was talking nonsense here :-( This test program shows the
> problem. Now the question, why did my other test work well...?

Arg, this code doesn't work because it is missing includes, please
ignore it. Give me a break, I'll follow up with a useful example
later.
 
> #include <stdio.h>
> #include <stdlib.h>
> #include <signal.h>
> 
> void
> handler(int s)
> {
>   write(1, "sig\n", 4);
> }
> 
> int
> main(void)
> {
>   double f0;
>   double f2;
>   double f4;
>   double f8;
>   double res;
> 
>   /* Prevent optimization */
>   f0 = atof("0.0");
>   f2 = atof("2.0");
>   f4 = atof("4.0");
>   f8 = atof("8.0");
> 
>   signal (SIGFPE, handler);
> 
>   res = f2 / f0;
> 
>   printf("After division by zero: %g\n", res);
> 
>   res = f4 + f8;
> 
>   if (res != 12.0) {
>     printf("Next operation didn't work: %g\n", res);
>   }
>   else {
>     printf("Next operation worked: %g\n", res);
>   }
> 
>   return 0;
> }

-- 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Martin Cracauer <cracauer@cons.org> http://www.cons.org/cracauer
BSD User Group Hamburg, Germany     http://www.bsdhh.org/

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