Date: Wed, 15 Mar 2017 22:10:42 -0700 From: Conrad Meyer <cem@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: src-committers <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r315360 - head/lib/libkvm Message-ID: <CAG6CVpVbtX4s2gOv_zAzn2fSXcYtLU_xe-obfnU6i03sHS8ECg@mail.gmail.com> In-Reply-To: <CANCZdfrV1XezigONuLb1gYOCPzJL_UiT=mb1gDRH%2BdcMofEwPA@mail.gmail.com> References: <201703160231.v2G2VgxK082641@repo.freebsd.org> <CANCZdfqgU8DJTdp4HkVxTU0PNpSGn45wJ0S1su=y2Td_uiVncA@mail.gmail.com> <58A53702-FFF6-45E7-ACCD-9B776530064E@gmail.com> <CANCZdfrV1XezigONuLb1gYOCPzJL_UiT=mb1gDRH%2BdcMofEwPA@mail.gmail.com>
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I don't have much to add. Warner is totally correct here. It is a (good) style cleanup with no functional change. Let's leave it alone. Thanks, Conrad On Wed, Mar 15, 2017 at 9:53 PM, Warner Losh <imp@bsdimp.com> wrote: > On Wed, Mar 15, 2017 at 10:44 PM, Ngie Cooper (yaneurabeya) > <yaneurabeya@gmail.com> wrote: >> >>> On Mar 15, 2017, at 21:32, Warner Losh <imp@bsdimp.com> wrote: >>> >>> On Wed, Mar 15, 2017 at 8:31 PM, Ngie Cooper <ngie@freebsd.org> wrote: >>>> Author: ngie >>>> Date: Thu Mar 16 02:31:42 2017 >>>> New Revision: 315360 >>>> URL: https://svnweb.freebsd.org/changeset/base/315360 >>>> >>>> Log: >>>> Return NULL instead of 0 on failure in _kvm_open, kvm_open{,2,files} >>>> >>>> This is being done for the following reasons: >>>> - kvm_open(3), etc says they will return NULL. >>>> - NULL by definition is (void*)0 per POSIX, but can be redefined, >>>> depending on the compiler, etc. >>> >>> No, it can't. The C language requires all integral expressions that >>> evaluate to zero to convert to the NULL pointer. This is independent >>> of the internal representation of the NULL pointer. >>> >>> So this change is an NOP for all compilers. It's a good STYLE change. >> >> Someone made an argument a few weeks ago about NULL being definable as a non-zero value on some esoteric architectures or OSes. > > No. That's confused. NULL must always be 0. A conversion between 0 and > a pointer always must give a null-pointer. Always. You can't defined > NULL to -1 ever. Even if that happens to be the binary representation > of a NULL pointer, it must be 0. > >> I agree though, this is largely stylistic/pedantic for a good cause. If someone set NULL to something non-zero in value, they would be looking for pain :). > > You can never set NULL to non-zero integral value (possibly with a > cast). You can have the internal representation have non-zero bits, > but the compiler must hide that. > > This does mean that M_ZERO and calloc() won't set pointers to null > pointers on such architectures, but this 0 that you replaced is > completely safe. > > I can provide references to the appropriate standards. I made the same > point when someone made that (incorrect) argument. >
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