Date: Wed, 15 Mar 2017 22:53:47 -0600 From: Warner Losh <imp@bsdimp.com> To: "Ngie Cooper (yaneurabeya)" <yaneurabeya@gmail.com> Cc: Ngie Cooper <ngie@freebsd.org>, 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: <CANCZdfrV1XezigONuLb1gYOCPzJL_UiT=mb1gDRH%2BdcMofEwPA@mail.gmail.com> In-Reply-To: <58A53702-FFF6-45E7-ACCD-9B776530064E@gmail.com> References: <201703160231.v2G2VgxK082641@repo.freebsd.org> <CANCZdfqgU8DJTdp4HkVxTU0PNpSGn45wJ0S1su=y2Td_uiVncA@mail.gmail.com> <58A53702-FFF6-45E7-ACCD-9B776530064E@gmail.com>
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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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