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Date: Thu, 29 Jan 2004 15:02:09 +0000
To: ronniec95@lineone.net
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] CamlDL/Abstract pointers problem
Message-ID: <20040129150209.GB30750@redhat.com>
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From: Richard Jones <rich@annexia.org>
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On Thu, Jan 29, 2004 at 01:06:39PM +0000, ronniec95@lineone.net wrote:
> All I want to do is just give Ocaml a pointer to something I've created
> in C and pass it back to other C functions later; don't want it to do anything
> with it (including moving it around the ocaml heap).\

It's my understanding that you can't just pass a pointer back to OCaml
and cast it to a value.  OCaml will think that the pointer points to
an OCaml heap object, and hence try to examine / move / delete it in
the garbage collector.  Instead you need to do one of two things:
either set the bottom bit to 1, in which case OCaml will treat it as a
plain integer, or else wrap it up in a box, as in the code below.  The
Abstract_tag tells the garbage collector not to look inside the box.

static value
Val_voidptr (void *ptr)
{
  CAMLparam0 ();
  CAMLlocal1 (rv);
  rv = alloc (1, Abstract_tag);
  Field(rv, 0) = (value) ptr;
  CAMLreturn (rv);
}

#define Voidptr_val(type,rv) ((type *) Field ((rv), 0))

You need to call Val_voidptr to wrap up your C void* pointer into an
OCaml box, and Voidptr_val to unwrap and get your pointer back again.

In any case you need to also add lots of calls to Gc.full_major () to
your code, which will trigger an early core dump if there is some
memory corruption.

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
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