From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <edwin+ml-ocaml@etorok.net>
X-Original-To: caml-list@sympa.inria.fr
Delivered-To: caml-list@sympa.inria.fr
Received: from mail1-relais-roc.national.inria.fr (mail1-relais-roc.national.inria.fr [192.134.164.82])
	by sympa.inria.fr (Postfix) with ESMTPS id ACF9E7EE94
	for <caml-list@sympa.inria.fr>; Sun, 30 Dec 2012 15:01:10 +0100 (CET)
Received-SPF: None (mail1-smtp-roc.national.inria.fr: no sender
  authenticity information available from domain of
  edwin+ml-ocaml@etorok.net) identity=pra;
  client-ip=176.9.138.55;
  receiver=mail1-smtp-roc.national.inria.fr;
  envelope-from="edwin+ml-ocaml@etorok.net";
  x-sender="edwin+ml-ocaml@etorok.net";
  x-conformance=sidf_compatible
Received-SPF: Pass (mail1-smtp-roc.national.inria.fr: domain of
  edwin+ml-ocaml@etorok.net designates 176.9.138.55 as
  permitted sender) identity=mailfrom; client-ip=176.9.138.55;
  receiver=mail1-smtp-roc.national.inria.fr;
  envelope-from="edwin+ml-ocaml@etorok.net";
  x-sender="edwin+ml-ocaml@etorok.net";
  x-conformance=sidf_compatible; x-record-type="v=spf1"
Received-SPF: Pass (mail1-smtp-roc.national.inria.fr: domain of
  postmaster@mail.etorok.net designates 176.9.138.55 as
  permitted sender) identity=helo; client-ip=176.9.138.55;
  receiver=mail1-smtp-roc.national.inria.fr;
  envelope-from="edwin+ml-ocaml@etorok.net";
  x-sender="postmaster@mail.etorok.net";
  x-conformance=sidf_compatible; x-record-type="v=spf1"
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: Ao8JAJZI4FCwCYo3/2dsb2JhbABEg0i6FRZzgh4BAQVAAQE4DwsYCSUPAkYTCAKIE6UDhDoBBYx+BoxXgRqDKZYPhWuKXYJ2
X-IronPort-AV: E=Sophos;i="4.84,381,1355094000"; 
   d="scan'208";a="187986536"
Received: from mail.etorok.net ([176.9.138.55])
  by mail1-smtp-roc.national.inria.fr with ESMTP; 30 Dec 2012 15:01:10 +0100
Received: from [172.30.42.2] (unknown [79.114.35.44])
	by mail.etorok.net (Postfix) with ESMTPSA id 258B746B5
	for <caml-list@inria.fr>; Sun, 30 Dec 2012 15:01:09 +0100 (CET)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=etorok.net;
	s=MAILOUT; t=1356876069;
	bh=tZIQvSB+qFvplTChpurtZxTU2d0/RMTh+QoZ3abaKww=;
	h=Message-ID:Date:From:MIME-Version:To:Subject:References:
	 In-Reply-To:Content-Type:Content-Transfer-Encoding;
	b=Rduv0U8ROUV1dLl1NiRm7zFw2zL0W6TOIe7N0cfNgZ0I2FxFj0lFlzaXVJBC38kka
	 2OBPjpwDsHPkyGLaKVFFyAM/EACDRQIohCvR/f03cvh4olvuhisvriiBH7VgKxQUXd
	 YZS5eDcDbskmGQ7bnjOhoZRxb4opLvGytDgbcnRg=
Message-ID: <50E04922.207@etorok.net>
Date: Sun, 30 Dec 2012 16:01:06 +0200
From: =?ISO-8859-1?Q?T=F6r=F6k_Edwin?= <edwin+ml-ocaml@etorok.net>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.11) Gecko/20121122 Icedove/10.0.11
MIME-Version: 1.0
To: caml-list@inria.fr
References: <20121230140823.20f39bcd@xivilization.net>
In-Reply-To: <20121230140823.20f39bcd@xivilization.net>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-Virus-Scanned: clamav-milter 0.97.6 at mail
X-Virus-Status: Clean
Subject: Re: [Caml-list] C interop: Return values in parameters

On 12/30/2012 03:08 PM, Marek Kubica wrote:
> Hi,
> 
> I am trying to wrap a C library in OCaml but I don't know how to do
> this particular thing:
> 
> I have a library that looks roughly like this:
> 
> int function(void** ptr);
> 
> So I get an int as return value to show whether the function succeeded
> and it *sets* the ptr.
> 
> void* ptr;
> function(&ptr);
> // ptr is different now
> 
> How can I wrap such a function in OCaml?
> When I call my OCaml wrapper
> 
> let retval = function ptr in
>     ...
> 
> the pointer does not get updated. Is there a function in the C API to
> force OCaml to update the values?

Use a '<yourtype> ref' for the parameter (or a record with a mutable field) on the OCaml side,
and you can update the field on the C side then.

Or if your C type is not actually void*, and your C function doesn't have side-effects (besides updating ptr)
you can also make the OCaml function return the actual value, and raise an exception if the function failed.

Best regards,
--Edwin