Date: Mon, 13 Feb 2006 10:21:29 +0200 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: "M. Warner Losh" <imp@bsdimp.com> Cc: arch@freebsd.org, stefanf@freebsd.org, des@des.no Subject: Re: [releng_6 tinderbox] failure on sparc64/sparc64 Message-ID: <20060213082129.GA13997@flame.pc> In-Reply-To: <20060213.002310.125802352.imp@bsdimp.com> References: <20060205084813.GN21806@wombat.fafoe.narf.at> <867j89n71d.fsf@xps.des.no> <20060205220211.GA5151@falcon.midgard.homeip.net> <20060213.002310.125802352.imp@bsdimp.com>
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On 2006-02-13 00:23, "M. Warner Losh" <imp@bsdimp.com> wrote: > In message: <20060205220211.GA5151@falcon.midgard.homeip.net> > Erik Trulsson <ertr1013@student.uu.se> writes: > : On Sun, Feb 05, 2006 at 04:45:34PM +0100, Dag-Erling Sm?rgrav wrote: > : > Stefan Farfeleder <stefanf@FreeBSD.org> writes: > : > > On Sat, Feb 04, 2006 at 03:58:56PM -0700, M. Warner Losh wrote: > : > > > This driver wants to access these structures as arrays of uint32_t. > : > > > It used to cast directly, but that isn't allowed. So, I've passed > : > > > the cast through a (void *). Is that allowed? Eg: > : > > > > : > > > struct foo foo; > : > > > ((uint32_t *)(void *)&foo)[3] = 12; > : > > > > : > > > is that OK? > : > > I'm afraid that only silences the warning without solving the underlying > : > > problem. I don't think there's a Standard conforming way to treat a > : > > struct foo as an uint32_t array. > : > > : > A union should do the trick. > : > : No, it will not. If you have a struct foo and try to access it as an array > : of int, the program will have unspecified (and maybe even undefined) > : behaviour. It does not matter if you do it with a union or by casting > : pointers. > : > : In general, if you have an object of type X, then the only ways it can be > : accessed is either as an object of type X, or as an array of [unsigned] > : char. > > So the proper fix for the above code is: > > struct foo foo; > uint32_t value[sizeof(foo) / sizeof(uint32_t)]; > > memcpy(value, &foo); > // write out value one 32-bit word at a time > > Is that right? Or at least 'proper' here means defined. AFAIK, yes.
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