From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 24 Jan 2001 08:47:03 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 24 Jan 2001 08:46:54 -0500 Received: from tomcat.admin.navo.hpc.mil ([204.222.179.33]:29567 "EHLO tomcat.admin.navo.hpc.mil") by vger.kernel.org with ESMTP id ; Wed, 24 Jan 2001 08:46:47 -0500 Date: Wed, 24 Jan 2001 07:46:20 -0600 (CST) From: Jesse Pollard Message-Id: <200101241346.HAA51300@tomcat.admin.navo.hpc.mil> To: andrea@suse.de, Richard Henderson Subject: Re: Compatibility issue with 2.2.19pre7 Cc: Russell King , Hubert Mantel , Linux Kernel Mailing List , Alan Cox X-Mailer: [XMailTool v3.1.2b] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Andrea Arcangeli : > On Tue, Jan 23, 2001 at 11:51:15PM -0800, Richard Henderson wrote: > > On Thu, Jan 11, 2001 at 12:59:24AM +0100, Andrea Arcangeli wrote: > > > What I said is that I can write this C code: > > > > > > int x[2], * p = (int *) (((char *) &x)+1); > > > main() > > > { > > > *p = 0; > > > } > > > > > > This is legal C code. > > > > Err, no. This is not "legal" by any stretch of the imagination. > > This code has undefined behaviour. > > I know this code has undefined behaviour at _runtime_. But I thought > you were obliged to allow it to compile. That was my only point. > > > We aren't even obliged to allow this to compile. > > I'd love if you could forbid it to compile. It would have to be implementation dependant/architecture dependant- The only way to know is to realize that *p initializer value is invalid is to know: 1. integer addresses MUST be aligned (say 4 or 8 byte boundaries) 2. the computation of (int *) (((char *) &x)+1) is a constant at compile time, and not link time (some linkers can do address offset calculations if "x" happens to be an external - ie. x +/- offset and offset is a constant) Other systems would have to allow it to compile (and link). It still has undefined behaviour, and is non-portable. ------------------------------------------------------------------------- Jesse I Pollard, II Email: pollard@navo.hpc.mil Any opinions expressed are solely my own. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/