Re: [Caml-list] va_arg values

[Bob Matcuk <Hamartiology@squeg.net>]

Thank you for your long reply! This is exactly the information I was
looking for. Cleared up the role of the GC for me. I'm not used to
working with any kind of automatic GC; I'm somewhat of a hardcore C
fanatic. It didn't occur to me that the GC might move things around,
though I feel I should have known! Doh!

On Mon, 15 Jan 2007 23:18:51 -0500
"Chris King" <colanderman@gmail.com> wrote:

> True, false.  caml_callbackN executes arbitrary code, which may or may
> not trip the GC.  hash_variant and caml_get_public_method are
> questionable also (since they return values), but looking at the Caml
> source code, it seems that they are safe (but I don't think the docs
> guarantee this).

Indeed. I was a bit weary of that myself.

> (BTW you should use caml_hash_variant rather than hash_variant; the
> comment for caml_get_public_method in caml/mlvalues.h should probably
> be updated to this effect also.)

Aye - I grabbed that from the documentation. I've noticed there are a
couple places where the documentation is missing the "caml_" but for
some reason, I didn't even think twice about that one.

> You could do this with caml_stat_alloc and caml_stat_free (in
> caml/memory.h).  These are equivalent to malloc/free but throw Caml's
> out-of-memory exception if they fail.  However in this case, I would
> simply declare args as an array.  Otherwise, if the callback throws an
> exception, args will not be freed unless you explicitly catch
> exceptions via caml_callbackN_exn, free it, and then re-raise the
> exception.

I hadn't even thought of that! Thanks! Not used to functions that don't
return other than the exec's and exit.

> Note that if you have no control over the C functions higher up the
> call chain (say an external library which calls your function), they
> could exhibit similar problems if they are unaware of the possibility
> of your function raising an exception.  The best thing to do in such a
> case would be to return an error condition if possible, or at the very
> least, print a warning and return or exit gracefully (the functions in
> caml/printexc.h help here).

Excellent advice; thanks again.

> K&R C doesn't, but GCC does.  If you're using another compiler or some
> compatibility flag, then the alloca function (usually found in
> alloca.h) should do the trick.  It allocates space on the stack
> exactly like an array declaration does, so the guts of CAMLlocalN
> should apply to it.

The problem with alloca is that it is not as portable (though, I can't
see what the problem is - I believe most, if not all architectures
could implement it as a single instruction). Still, given your previous
comment about the callback throwing an exception, perhaps it is the
best way to go...

Thank you again for your clarifications.

--
Bob Matcuk
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