Date: Fri, 30 Jan 2015 19:40:33 +0000 From: "Robert N. M. Watson" <rwatson@FreeBSD.org> To: Joerg Sonnenberger <joerg@britannica.bec.de> Cc: svn-src-all@freebsd.org Subject: Re: svn commit: r277203 - in head/sys: kern sys Message-ID: <AAB6DA30-A9BA-43A6-9624-2231FBAB596C@FreeBSD.org> In-Reply-To: <20150115123557.GC7669@britannica.bec.de> References: <201501142344.t0ENi0tI088747@svn.freebsd.org> <20150115123557.GC7669@britannica.bec.de>
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> On 15 Jan 2015, at 12:35, Joerg Sonnenberger <joerg@britannica.bec.de> wrote: > > On Wed, Jan 14, 2015 at 11:44:00PM +0000, Robert Watson wrote: >> - As we anticipate embedding mbufs headers within variable-size regions of >> memory in the future, change the definitions of byte arrays embedded in >> mbufs to be of size [0] rather than [MLEN] and [MHLEN]. > > This is not valid C. You may be able to use flexible array members ([]) > instead. Life is not that simple -- see the Phabricator review discussion which considers this point in detail. The short version is: using [] for the last entry in a union within a structure is explicitly disallowed in the C spec, actually works fine with [0] in gcc + extensions, which are a feature used in the kernel already. If in doubt, try replacing [0] with [] and recompiling, it's an instructive and morally improving process. We will want to feed this back to the C standardisation folk at some point. Robert
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