Luigi Rizzo wrote:
> sorry but all this just does not make sense to me.
> sizeof(foo) should give the same result irrespective of
> where you use it.
> Perhaps the best thing would be to put a
>         printf("struct ip_fw has size %d\n", sizeof(struct ip_fw));
> both in ipfw2.c and somewhere in ip_fw2.c and see if there is
> a mismatch between the two numbers.

I have to assume that what didn't make sense was that his patch
worked?  8-).

He's making the valid point that for:

        struct foo *fee;

It's possible that:

        sizeof(struct foo) != (((char *)&fee[1]) - ((char *)&fee[0]))

because of end-padding, which is not accounted for in arrays,
and that inter-structure padding depends on ordering of elements
(for a good example of this, see the struct direct name element
reference macro, which is also padding independent).

Basically, end-padding happens because arrays of structures need to
have their first element properly aligned, so there is a pad added
after each element to ensure that the following element starts on
an alignment boundary.

I still say that on 486 and higher, the "disallow unaligned access"
bit in the processor control register should be enabled, so your
kernel will panic if you try this.  8-).

-- Terry

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