netdev - Re: [GIT PULL] [IPV6] COMPAT: Fix SSM applications on 64bit kernels.

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Date:	Sat, 26 Apr 2008 00:09:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dlstevens@...ibm.com
Cc:	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [GIT PULL] [IPV6] COMPAT: Fix SSM applications on 64bit
 kernels.

From: David Stevens <dlstevens@...ibm.com>
Date: Sat, 26 Apr 2008 00:00:49 -0700

>         I did see that plain old structure field __attribute((4))__
> wasn't correct (still padded), and gcc docs use the same language
> to define the behavior for structure field attribute "aligned" and
> type attribute "aligned" (at least with my limited research). So,
> it surprised me that the field attribute results in pad and the
> type attribute doesn't, and I wonder if it's good to rely on that
> difference given the same documentation for both as "minimum alignment".
> But if the compiler changes its notion of when to pad this in a way
> that breaks it, we can always revisit it later. :-)

You're right about this point of course:

--------------------
struct foo {
	int a;
	unsigned long b;
};

struct foo_align4 {
	int a;
	unsigned long b;
} __attribute__((aligned(4)));

int main(void)
{
	printf("Normal: 'b' offset is %Zu\n",
	       __builtin_offsetof(struct foo, b));
	printf("Align4: 'b' offset is %Zu\n",
	       __builtin_offsetof(struct foo_align4, b));
	return 0;
}
--------------------

gives:

--------------------
Normal: 'b' offset is 8
Align4: 'b' offset is 8
--------------------

on sparc64.

So if we need to use packed because of that specific problem here,
fine.
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