Date: Fri, 10 Jun 2005 14:02:55 +0300 From: Ruslan Ermilov <ru@freebsd.org> To: Giorgos Keramidas <keramida@freebsd.org> Cc: Dag-Erling Sm?rgrav <des@des.no>, freebsd-current@freebsd.org Subject: Re: [current tinderbox] failure on ...all... Message-ID: <20050610110254.GB79872@ip.net.ua> In-Reply-To: <20050610102813.GA81548@orion.daedalusnetworks.priv> References: <p0621025fbeceac0673f8@128.113.24.47> <84dead720506091950779d1661@mail.gmail.com> <86oeae3d8f.fsf@xps.des.no> <84dead72050610001675a32c19@mail.gmail.com> <863brq3bbz.fsf@xps.des.no> <84dead7205061001534b9385b3@mail.gmail.com> <863brqy41j.fsf@xps.des.no> <20050610091624.GA35628@wombat.fafoe.narf.at> <20050610094615.GC79474@ip.net.ua> <20050610102813.GA81548@orion.daedalusnetworks.priv>
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On Fri, Jun 10, 2005 at 01:28:13PM +0300, Giorgos Keramidas wrote:
> On 2005-06-10 12:46, Ruslan Ermilov <ru@freebsd.org> wrote:
> >On Fri, Jun 10, 2005 at 11:16:27AM +0200, Stefan Farfeleder wrote:
> >>On Fri, Jun 10, 2005 at 11:06:16AM +0200, Dag-Erling Sm?rgrav wrote:
> >>>Joseph Koshy <joseph.koshy@gmail.com> writes:
> >>>> Dag-Erling Sm?rgrav <des@des.no> writes:
> >>>> > It also seems strange to me that you on the one hand introduce a
> >>>> > new struct to separate MD and MI interfaces, and on the other hand
> >>>> > continue to assume that they are assignment-compatible.
> >>>> I'd be very surprised if two C structures with identical definitions
> >>>> were not assignment compatible.
> >>>
> >>> I wouldn't be surprised if the standard says they aren't.
> >>> Unfortunately, my copy is at home.
> >>
> >> Do you mean the following?
> >>
> >> struct t1 { int a; } x;
> >> struct t2 { int a; } y =3D { 42 };
> >> x =3D y;
> >>
> >> The types `struct t1' and `struct t2' are not compatible and thus not
> >> assignable. See 6.2.7 and 6.5.16.1.
> >
> > If you're to byte-copy say t1 to t2, is it guaranteed to work? That
> > is, do both types are guaranteed to have the same size and alignment
> > of their structure members? I'm pretty sure this is guaranteed, as
> > lot of code assumes this, for example, the sockaddr* structures.
>=20
> That would be very hard to guarantee if two different modules that use
> the types are compiled with different alignment options, right?
>=20
> /* header1.h */
> struct t1 {short t1s; int t1a;};
>=20
> /* header2.h */
> struct t2 {short t2s; int t2a;};
>=20
> /* module1.c */
> #include "header1.h"
> struct t1 x;
>=20
> /* module2.c */
> #include "header1.h"
> #include "header2.h"
> extern struct t1 x;
> struct t2 y;
>=20
> If the two modules are compiled with different options that may affect
> struct member alignment, how would one ensure that it is correct to
> use code like this in module.c?
>=20
> y.t2s =3D 10;
> y.t2a =3D 100;
> memcpy(&x, &y, sizeof(x));
>=20
> Even the use of sizeof(x) is tricky here, since there is no guarantee
> that sizeof(x) < sizeof(y).
>=20
And if both modules use the same t1 but compiled with different alignment
options, the memcpy() in module.c won't work either, even a simple assignme=
nt
could break. :-)
Cheers,
--=20
Ruslan Ermilov
ru@FreeBSD.org
FreeBSD committer
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