Re: [Caml-list] Initializing CAMLlocalX values to Val_unit

["Richard W.M. Jones" <rich@annexia.org>]

On Fri, Feb 24, 2017 at 08:43:38AM -0500, Arlen Cox wrote:
> > ?  Interestingly none of this code has actually crashed in production
> > as far as I'm aware.
> 
> I'm fairly certain this is because the OCaml runtime checks pointers
> to ensure that they're in the OCaml heap before tracing them.

OK I see - I knew this but didn't know if it applied to
NULL pointers too.

> I don't
> know how this holds up historically, but last time I looked (4.03)
> when tracing, OCaml runs Classify_addr(a) on addresses to determine if
> they should be traced into.  Because of this, NULL pointers will not
> be followed (and thus not crash) and perhaps more interestingly raw
> (naked) pointers that don't point to anything in an OCaml-registered
> page will also not be traced.
> 
> There is a define related to NAKED_POINTERS that if defined will
> improve performance by disabling the check for Is_in_heap(a) (macro
> for Classify_addr).  Naked pointer support can be disabled during
> configure (--no-naked-pointers) and that may make code that does not
> initialize things correctly crash.  Naked pointers are checked in the
> default configuration.

Thanks (and also to Gerd's response).

Rich.

-- 
Richard Jones