From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: Markus Mottl <mottl@miss.wu-wien.ac.at>
Cc: OCAML <caml-list@inria.fr>
Subject: Re: fancy GC question
Date: Tue, 12 Dec 2000 16:38:39 +0100 [thread overview]
Message-ID: <20001212163839.B26381@pauillac.inria.fr> (raw)
In-Reply-To: <20001212040549.A5381@miss.wu-wien.ac.at>; from mottl@miss.wu-wien.ac.at on Tue, Dec 12, 2000 at 04:05:50AM +0100
> When I allocate an integer array in OCaml, which is always boxed, both
> the pointers to and the elements are obviously contiguous in memory.
> One could exploit this in C-interfaces under the restriction that the
> array is never changed by the OCaml-runtime, e.g.:
>
> int *ar = (int *) &Field(v_ar, 0);
>
> And then one can read/write directly into the integer array without
> having to follow an indirection (an intermediate pointer) by treating
> "ar" as a normal C array.
>
> But is this really always safe if only C writes to the array?
It is safe if the C code does not perform any allocation in the Caml
heap while it is using the "ar" pointer. An allocation can trigger a
garbage collection, which can move the Caml block denoted by "v_ar";
after this, the "ar" pointer no longer points inside the block!
(If "v_ar" is registered as a local root with the garbage collector,
its value will be updated after the GC to reflect the new address of
the block; however, the GC has no mechanism for updating derived
pointers such as "ar" in your example.)
But, yes, this is a safe trick to use in e.g. a tight loop that does
not allocate in the Caml heap.
> Can other effects mess up the
> fact that the pointers map continuously on a contiguous chunk of memory
> (of integers)?
I'm not sure I understand the question, but the various "fields" of a
Caml block (as accessed with the Field macro) are always contiguous in
memory.
- Xavier Leroy
next prev parent reply other threads:[~2000-12-14 18:12 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-12-12 3:05 Markus Mottl
2000-12-12 15:38 ` Xavier Leroy [this message]
2000-12-13 23:06 ` Markus Mottl
2000-12-12 12:18 Damien Doligez
2000-12-14 22:10 Markus Mottl
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