Re: alignment

[Guy Harris <guy () netapp com>]

On Sat, Nov 09, 2002 at 10:34:50AM +0900, Jun-ichiro itojun Hagino wrote:
> > >    on alignment - what was the reason why we can't do
> > >    __alignment__((__packed__))?  is it GCC specific?
> > Yes, it is GCC-specific.
>
>       is there any portable way of doing this? (other than byte-by-byte
>       fetching)

None that I know of.  Some compilers have similar features, I think
(MSVC++ has something similar).  I don't know whether all compilers that
have it generate "safe" code on strict-alignment processors (although it
seems stupid *not* to).  I suspect not all compilers people would use to
compile tcpdump or libpcap have it.

>       if not, is it acceptable if we test the feature in
>       configure.in and use it if available?

I'd say so (as long as this doesn't involve peppering the code with
#ifdefs - if the feature *isn't* available, you still have to use the
EXTRACT macros), although note that "__attribute((packed))", at least
according to the GCC 3.2 manual:

        http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Variable-Attributes.html#Variable%20Attributes

changes the layout of the structure, so you'd have to add padding to
structures that lack it.
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