Date: Mon, 13 Feb 2006 00:23:10 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: ertr1013@student.uu.se Cc: des@des.no, stefanf@freebsd.org, arch@freebsd.org Subject: Re: [releng_6 tinderbox] failure on sparc64/sparc64 Message-ID: <20060213.002310.125802352.imp@bsdimp.com> In-Reply-To: <20060205220211.GA5151@falcon.midgard.homeip.net> References: <20060205084813.GN21806@wombat.fafoe.narf.at> <867j89n71d.fsf@xps.des.no> <20060205220211.GA5151@falcon.midgard.homeip.net>
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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=F8rgrav 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 uint3= 2_t. : > > > It used to cast directly, but that isn't allowed. So, I've pas= sed : > > > the cast through a (void *). Is that allowed? Eg: : > > > = : > > > struct foo foo; : > > > ((uint32_t *)(void *)&foo)[3] =3D 12; : > > > = : > > > is that OK? : > > I'm afraid that only silences the warning without solving the und= erlying : > > problem. I don't think there's a Standard conforming way to trea= t 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 castin= g : pointers. : = : In general, if you have an object of type X, then the only ways it ca= n be : accessed is either as an object of type X, or as an array of [unsigne= d] : 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. Warner = =
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