Date: Fri, 10 Jun 2005 13:28:13 +0300 From: Giorgos Keramidas <keramida@freebsd.org> To: Ruslan Ermilov <ru@freebsd.org> Cc: Dag-Erling Sm?rgrav <des@des.no>, freebsd-current@freebsd.org Subject: Re: [current tinderbox] failure on ...all... Message-ID: <20050610102813.GA81548@orion.daedalusnetworks.priv> In-Reply-To: <20050610094615.GC79474@ip.net.ua> References: <20050609234619.AD1F67306E@freebsd-current.sentex.ca> <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>
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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 = { 42 };
>> x = 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.
That would be very hard to guarantee if two different modules that use
the types are compiled with different alignment options, right?
/* header1.h */
struct t1 {short t1s; int t1a;};
/* header2.h */
struct t2 {short t2s; int t2a;};
/* module1.c */
#include "header1.h"
struct t1 x;
/* module2.c */
#include "header1.h"
#include "header2.h"
extern struct t1 x;
struct t2 y;
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?
y.t2s = 10;
y.t2a = 100;
memcpy(&x, &y, sizeof(x));
Even the use of sizeof(x) is tricky here, since there is no guarantee
that sizeof(x) < sizeof(y).
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