Date: Fri, 10 Jun 2005 14:53:40 +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: <20050610115340.GA81777@orion.daedalusnetworks.priv> In-Reply-To: <20050610110254.GB79872@ip.net.ua> References: <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> <20050610110254.GB79872@ip.net.ua>
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On 2005-06-10 14:02, Ruslan Ermilov <ru@freebsd.org> wrote:
>On Fri, Jun 10, 2005 at 01:28:13PM +0300, Giorgos Keramidas wrote:
>> 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).
>
> 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 assignment
> could break. :-)
Probably no. Two struct t1 objects have the same type and assignment
between the two may be implemented as "element-at-a-time memcpy)", as
mentioned in $6.2.6.1(6) and footnote (42) of the C99 standard. This
may or may not break though, but I couldn't find any good reference
to it.
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