Date: Fri, 19 Feb 2016 17:14:44 -0600 From: Benjamin Kaduk <bjkfbsd@gmail.com> To: Gleb Smirnoff <glebius@freebsd.org> Cc: 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: <CAJ5_RoCbWLUkwdRv4FeFVGpzCENooJbokH0SNXtiCeoRSeW%2BfQ@mail.gmail.com> In-Reply-To: <20160219230612.GR1945@FreeBSD.org> References: <201602182008.u1IK81vg092127@repo.freebsd.org> <CAE-mSO%2B7p=Equq81PPQjfZv1piPydBr4Mnk363CEs3w9EXRi9w@mail.gmail.com> <CAOtMX2hyZ=GFgp8F6t0dMDYF5FSPdoP9KdMU7V5rmscpaPUnsw@mail.gmail.com> <20160219230612.GR1945@FreeBSD.org>
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On Fri, Feb 19, 2016 at 5:06 PM, Gleb Smirnoff <glebius@freebsd.org> wrote: > On Fri, Feb 19, 2016 at 08:49:43AM -0700, Alan Somers wrote: > A> On Fri, Feb 19, 2016 at 5:24 AM, Sergey Kandaurov <pluknet@gmail.com> > wrote: > A> Yeah, it was being implicitly zeroized before. But Clang complained > A> about the structures being only partially initialized. Since the > A> whole point of my commit was to increase the WARNS level, I explicitly > A> zeroed the zero fields to silence Clang. > > Isn't zero filling part of the standard? I don't see why lack of > explicit zeroing is a warning? Looks a false warning to me. > It is not quite as simple as this would make it sound. The elements or members of an aggregate (e.g.) structure type are initialized as if it were an object of static storage duration (i.e., to zero) if the initializer list does not contain enough initializers for all members of the aggregate type, per item 21 of section 6.7.8 of n1256.pdf. However, such initialization does not necessarily need to zero any padding bytes that are present, which may take unspecified values. Personally, I think this particular clang warning can be too aggressive, especially for complex structs, but on the other hand given the indeterminateness of padding, bzero/memset are often a better choice anyway. -Ben
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