From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965154AbXCLFwz (ORCPT ); Mon, 12 Mar 2007 01:52:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965148AbXCLFwz (ORCPT ); Mon, 12 Mar 2007 01:52:55 -0400 Received: from sccrmhc11.comcast.net ([204.127.200.81]:38238 "EHLO sccrmhc11.comcast.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965154AbXCLFwy (ORCPT ); Mon, 12 Mar 2007 01:52:54 -0400 Subject: Re: Style Question From: Nicholas Miell To: Jan Engelhardt Cc: Cong WANG , linux-kernel@vger.kernel.org In-Reply-To: References: <2375c9f90703110715v4278f83flbc9fce6c542472a5@mail.gmail.com> <2375c9f90703112237u4b27c530x34e310a93fd04ef0@mail.gmail.com> Content-Type: text/plain Date: Sun, 11 Mar 2007 22:52:51 -0700 Message-Id: <1173678771.2964.18.camel@entropy> Mime-Version: 1.0 X-Mailer: Evolution 2.8.3 (2.8.3-1.0.njm.1) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2007-03-12 at 06:40 +0100, Jan Engelhardt wrote: > On Mar 12 2007 13:37, Cong WANG wrote: > > > > The following code is picked from drivers/kvm/kvm_main.c: > > > > static struct kvm_vcpu *vcpu_load(struct kvm *kvm, int vcpu_slot) > > { > > struct kvm_vcpu *vcpu = &kvm->vcpus[vcpu_slot]; > > > > mutex_lock(&vcpu->mutex); > > if (unlikely(!vcpu->vmcs)) { > > mutex_unlock(&vcpu->mutex); > > return 0; > > } > > return kvm_arch_ops->vcpu_load(vcpu); > > } > > > > Obviously, it used 0 rather than NULL when returning a pointer to > > indicate an error. Should we fix such issue? > > Indeed. If it was for me, something like that should throw a compile error. > > >>[...] > > I think it's more clear to indicate we are using a pointer rather than > > an integer when we use NULL in kernel. But in userspace, using NULL is > > for portbility of the program, although most (*just* most, NOT all) of > > NULL's defination is ((void*)0). ;-) > > NULL has the same bit pattern as the number zero. (I'm not saying the bit > pattern is all zeroes. And I am not even sure if NULL ought to have the same > pattern as zero.) So C++ could use (void *)0, if it would let itself :p Not necessarily. You can use 0 at the source level, but the compiler has to convert it to the actual NULL pointer bit pattern, whatever it may be. In C++, NULL is typically defined to 0 (with no void* cast) by most compilers because 0 (and only 0) can be implicitly converted to to null pointer of any ponter type without a cast. GCC introduced the __null extension so that NULL still works correctly in C++ when passed to a varargs function on 64-bit platforms. (This just works in C because C makes NULL ((void*)0) is thus is the right size. In C++, the 0 ends up being an int instead of a pointer when passed to a varargs function, and things tend to blow up when they read the garbage high bits. Of course, nobody else does this, so you still have to use (void*)NULL to be portable.) -- Nicholas Miell