Date: Mon, 22 Feb 2016 11:57:14 -0800 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Conrad Meyer <cem@FreeBSD.org> Cc: Benjamin Kaduk <bjkfbsd@gmail.com>, Alan Somers <asomers@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, Sergey Kandaurov <pluknet@gmail.com>, "src-committers@freebsd.org" <src-committers@freebsd.org> Subject: Re: svn commit: r295768 - head/usr.sbin/iostat Message-ID: <20160222195713.GC1945@FreeBSD.org> In-Reply-To: <CAG6CVpUDWjSF6ht1hHKXp0PPh-H6zg51tace8=m0DEqdOrWGSA@mail.gmail.com> References: <201602182008.u1IK81vg092127@repo.freebsd.org> <CAE-mSO%2B7p=Equq81PPQjfZv1piPydBr4Mnk363CEs3w9EXRi9w@mail.gmail.com> <CAOtMX2hyZ=GFgp8F6t0dMDYF5FSPdoP9KdMU7V5rmscpaPUnsw@mail.gmail.com> <20160219230612.GR1945@FreeBSD.org> <CAJ5_RoCbWLUkwdRv4FeFVGpzCENooJbokH0SNXtiCeoRSeW%2BfQ@mail.gmail.com> <CAG6CVpUDWjSF6ht1hHKXp0PPh-H6zg51tace8=m0DEqdOrWGSA@mail.gmail.com>
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On Fri, Feb 19, 2016 at 04:50:40PM -0800, Conrad Meyer wrote: C> > It is not quite as simple as this would make it sound. The elements or C> > members of an aggregate (e.g.) structure type are initialized as if it were C> > an object of static storage duration (i.e., to zero) if the initializer list C> > does not contain enough initializers for all members of the aggregate type, C> > per item 21 of section 6.7.8 of n1256.pdf. However, such initialization C> > does not necessarily need to zero any padding bytes that are present, which C> > may take unspecified values. Personally, I think this particular clang C> > warning can be too aggressive, especially for complex structs, but on the C> > other hand given the indeterminateness of padding, bzero/memset are often a C> > better choice anyway. C> C> By definition, padding byte contents are unused. There is no reason C> their values matter one way or another, so why do we care about the C> distinction between bzero and member zero initialization? Well, one can run bcmp() or a hashing function over entire structure, in that case contents of padding matter. But I still don't want to see this warning in -Wall, since it kills very useful C99 initializers. The code, that runs bcmp() or hashes over structs, should take care, and we shouldn't pessimize the entire build for its sake. -- Totus tuus, Glebius.
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