From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: ludo@gnu.org (Ludovic =?iso-8859-1?Q?Court=E8s?=) Newsgroups: gmane.lisp.guile.devel Subject: Re: modifying pointers Date: Fri, 09 Apr 2010 18:30:07 +0200 Message-ID: <87k4sgpp0w.fsf@gnu.org> References: NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Trace: dough.gmane.org 1270830792 20412 80.91.229.12 (9 Apr 2010 16:33:12 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Fri, 9 Apr 2010 16:33:12 +0000 (UTC) Cc: guile-devel To: Andy Wingo Original-X-From: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Fri Apr 09 18:33:10 2010 Return-path: Envelope-to: guile-devel@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1O0H8n-00074o-5x for guile-devel@m.gmane.org; Fri, 09 Apr 2010 18:33:09 +0200 Original-Received: from localhost ([127.0.0.1]:47221 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O0H8l-0004Hc-RY for guile-devel@m.gmane.org; Fri, 09 Apr 2010 12:33:07 -0400 Original-Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1O0H62-0003Bp-1D for guile-devel@gnu.org; Fri, 09 Apr 2010 12:30:18 -0400 Original-Received: from [140.186.70.92] (port=41527 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1O0H60-0003BV-E6 for guile-devel@gnu.org; Fri, 09 Apr 2010 12:30:17 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1O0H5v-0001Up-N3 for guile-devel@gnu.org; Fri, 09 Apr 2010 12:30:16 -0400 Original-Received: from mail2-relais-roc.national.inria.fr ([192.134.164.83]:40315) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1O0H5v-0001Ui-IL for guile-devel@gnu.org; Fri, 09 Apr 2010 12:30:11 -0400 X-IronPort-AV: E=Sophos;i="4.52,178,1270418400"; d="scan'208";a="48646827" Original-Received: from reverse-83.fdn.fr (HELO nixey) ([80.67.176.83]) by mail2-relais-roc.national.inria.fr with ESMTP/TLS/DHE-RSA-AES128-SHA; 09 Apr 2010 18:30:10 +0200 X-URL: http://www.fdn.fr/~lcourtes/ X-Revolutionary-Date: 20 Germinal an 218 de la =?iso-8859-1?Q?R=E9volution?= X-PGP-Key-ID: 0xEA52ECF4 X-PGP-Key: http://www.fdn.fr/~lcourtes/ludovic.asc X-PGP-Fingerprint: 83C4 F8E5 10A3 3B4C 5BEA D15D 77DD 95E2 EA52 ECF4 X-OS: x86_64-unknown-linux-gnu In-Reply-To: (Andy Wingo's message of "Fri, 09 Apr 2010 11:05:21 +0200") User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-BeenThere: guile-devel@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "Developers list for Guile, the GNU extensibility library" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Errors-To: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Xref: news.gmane.org gmane.lisp.guile.devel:10180 Archived-At: Hi, Andy Wingo writes: > You added a change to foreign-set! for %null-pointer. Would it not make > sense instead to make foreign pointers of type "void" unsettable? Not necessarily. See the test that was added: --8<---------------cut here---------------start------------->8--- (pass-if "foreign-set! other-null-pointer" (let ((f (bytevector->foreign (make-bytevector 2)))) (and (not (=3D 0 (foreign-ref f))) (begin (foreign-set! f 0) (=3D 0 (foreign-ref f))) (begin ;; Here changing the pointer value of F is perfectly valid. (foreign-set! f 777) (=3D 777 (foreign-ref f)))))) --8<---------------cut here---------------end--------------->8--- Here a ((void *) 777) pointer is created. However, this is not allowed for =E2=80=98%null-pointer=E2=80=99 because (1= ) that would change its semantics, and (2) it=E2=80=99s defined as =E2=80=98const=E2=80= =99 anyway, so modifying it leads to a segfault. Thanks, Ludo=E2=80=99.