Re: [Caml-list] Catching exceptions into strings

[Michael Vanier <mvanier@cs.caltech.edu>]

Xavier,

Thanks for the clear explanation of a topic that always puzzled me.
Personally, I could live with SIGFPE just aborting the program.  I'm also
curious: is it possible to catch floating point errors in ocaml's bytecode?
Or is this, too, dependent on the ansi C/Posix standards?  I tried this
with java and was interested to find that integer division by zero is
caught but floating point division by zero isn't.  

<off-topic> 
Given all this, I wonder how C# handles "checked arithmetic"?  Are the
rules different on Windows?
</off-topic>

Mike

> Date: Wed, 12 Jun 2002 10:53:08 +0200
> From: Xavier Leroy <xavier.leroy@inria.fr>
> 
> > Hmm.  I didn't think it was possible to catch floating point exceptions in
> > ocaml (something I'd very much like to be able to do).  Why doesn't ocaml
> > support this?  Or does it?
> 
> Your question requires a bit of decoding.
> 
> Floating-point operations in OCaml follow the IEEE-standard usage of
> never causing an exception, but return infinities or (silent) NaNs
> when something is weird.
> 
> The "Floating point exception" you saw in Gaurav's message is a
> misnomer: *integer* division by zero generates a SIGFPE signal
> under Unix, which is printed as "Floating-point exception" but
> really means "Integer arithmetic exception".
> 
> Most processors (and I believe the IEEE standard) offer the option
> to raise exceptions instead of returning NaNs, infinities or denormals.
> This facility is not available from OCaml because it's not provided
> by ANSI C nor by POSIX, which are essentially what the OCaml runtime
> system is building upon.
> 
> Even then, you'd get a SIGFPE when a floating point operation goes bad,
> and we're back to the issue of mapping a Unix *synchronous* signal to
> a Caml exception.  (Asynchronous signals are already dealt with.)
> It can be done, although not trivially.  But the main issue here is to
> get enough context about the signal.  Raising Division_by_zero every
> time SIGFPE is received is not correct: maybe someone did "kill -FPE <pid>"
> from the outside; maybe the SIGFPE was generated inside a C library
> linked with the Caml code, and which has its own rules for dealing
> with it.
> 
> In other terms, what is needed is to collect enough context on the
> SIGFPE occurrence to 1) make sure it's an integer division operation
> that failed, and 2) make sure this happened in OCaml code; only then
> we could generate a Division_by_zero exception.  But of course
> there is no portable way to do all this under Unix.
> 
> In summary: you can get SIGFPE, but they can only stop your program,
> handling them as exceptions is just not possible.
> 
> - Xavier Leroy
> 
> 
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