[ ghc-Bugs-537295 ] div 0 0 give exception / endless loop

Bugs item #537295, was opened at 2002-03-31 01:36
Message generated for change (Comment added) made by simonmar
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Category: Runtime System
Group: 5.02
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: geoff (snailtalk)
Assigned to: Simon Marlow (simonmar)
Summary: div 0 0 give exception / endless loop

Initial Comment:
There is a problem with the div in ghc, to reproduce:

start ghci and type div n 0 where n is any integer (so
div 3 0 would work)

on a linux box it gives a floating point exception
while on a BSD 4.4 box it gives and endless loop using
about 87 % of cpu.

I have verified this on both the latest release
(5.02.2) and 5.02.1.



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>Comment By: Simon Marlow (simonmar)
Date: 2003-05-06 08:47

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No.  When the bug was originally closed, the reason given 
was "Fixed by NOT ignoring SIGFPE."  (my emphasis).

The divide-by-zero exception was introduced in GHC 5.04.3.


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Comment By: geoff (snailtalk)
Date: 2003-05-03 21:37

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Yes.  Because when this bug was closed, it was stated that
it was fixed by ignoring SIGFPE (20020702).  Therefore, I
could only assume that was the fix (sorry I forgot to to
strace).  Anyway since the signal has nothing to with ghc,
let us not worry about it.

Is this exception behavior for div new in ghc 5.04.3? 
Because while this was marked as fixed in the bug tracking
system fixed in July '02, and since then 5.04.2 has been
released, and 5.04.2 (what I am using now) does not exhibit
the same behavior (for both div 0 0 and mod 0 0 case).

Loading package haskell98 ... linking ... done.
Prelude> div 0 0
zsh: floating point exception  ghci

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 5.04.2
$ 


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Comment By: Simon Marlow (simonmar)
Date: 2003-04-28 13:01

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Could you elaborate?  I can't see a bug.

On my FreeBSD box I just tried:

Prelude> :m +GHC.Base
Prelude GHC.Base> I# (0# `divInt#` 0#)
zsh: 487 floating point exception (core dumped)  ghci

I'm not sure what your 'signal' program is supposed to be 
illustrating: GHC doesn't catch SIGFPE at all so I don't think 
any behaviour of the SIGFPE handler could possibly be 
affecting us.

Furthermore, GHC 5.04.3 turns normal Int division-by-zero 
into an exception:

Prelude> div 0 0
*** Exception: divide by zero

(which is why I had to call the primitive directly above).

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Comment By: geoff (snailtalk)
Date: 2003-04-23 19:07

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It has come to my attention that it is still not fixed in
5.04.2 (my fault for not reading the exact nature of the
fix).  I assume because for SIGFPE in Unix, the result is
undefined after return from the signal handler.

Here is some sample code to demonstrate:

usage: signal (+|-)u (signal|sigaction)

options:
+u  : uninstall signal in sighandler
-u   : don't uninstall signal in sighandler
signal:  install using signal()
sigaction:  install using sigaction()
 
so for example, signal +u sigaction would install the
handler using sigaction and uninstall the signal in the
signal handler.  The signal is hardcoded to SIGFPE.


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Comment By: Simon Marlow (simonmar)
Date: 2002-07-02 11:30

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Fixed by not ignoring SIGFPE.

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