cvs commit: src/include ar.h
yar at comp.chem.msu.su
Fri Nov 17 06:56:08 UTC 2006
On Thu, Nov 16, 2006 at 04:52:07PM -0700, M. Warner Losh wrote:
> In message: <20061116090412.GB37133 at comp.chem.msu.su>
> Yar Tikhiy <yar at comp.chem.msu.su> writes:
> : On Mon, Nov 13, 2006 at 10:19:58AM -0700, M. Warner Losh wrote:
> : > :
> : > : BTW, you are responsible for the __packed in <netinet/ip.h>. Please remove
> : > : it. The __CTASSERT() is enough to detect if heroic packing is ever needed.
> : > : The only danger is if something has grown to depend on __packed reducing
> : > : alignment as a side effect. E.g., suppose we had a byte string containing
> : > : a bytewise copy of a struct ip. If the copy might be misaligned, then it
> : > : should be copied to an actual struct ip before accessing it as a struct,
> : > : but code that accesses it directly using ((struct ip *)&bs[N]) would work
> : > : now due to the reduced alignment. Places that really need __packed should
> : > : probably use __aligned() to restore the natural alignment.
> : >
> : > DO NOT REMOVE IT. IT IS ABSOLUTELY REQUIRED FOR ARM TO WORK RIGHT.
> : > If you want to remove it, then you must make sure arm works right
> : > after it because I'll add it back.
> : Many years ago I was taught that comments in code could help to
> : avoid such clashes in software development. Is this true no more? ;-)
> You don't add comments like:
> i++; // Add one to i.
> This is a similar class. It is for any compiler that has differing
> alignment requirements than i386.
This is an oversimplification of the case. If it were so simple,
no doubts about it would be raised. That's why I suggested adding
a comment explaining that historically struct ip was lucky to be
packed/aligned properly, but that wasn't backed by the C standard
in fact, and eventually architectures appeared, e.g., ARM, which
broke the false assumption. It's a rather edifying case. Then
you'll have a smaller chance of having to yell in capital letters
again, "DO NOT REMOVE IT. IT IS ABSOLUTELY REQUIRED FOR ARM TO
WORK RIGHT." -- hopefully, not only regarding struct ip.
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