linux-kernel - Re: Subtleties of __attribute_

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Message-Id: <20061206.100528.75760584.davem@davemloft.net>
Date:	Wed, 06 Dec 2006 10:05:28 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rmk+lkml@....linux.org.uk
Cc:	jblunck@...e.de, phil_arcwk_endecott@...zphil.org,
	linux-kernel@...r.kernel.org
Subject: Re: Subtleties of __attribute__((packed))

From: Russell King <rmk+lkml@....linux.org.uk>
Date: Wed, 6 Dec 2006 17:54:23 +0000

> It does not say "and as such the struct may be aligned to any alignment".

Consider the implication for arrays and pointer arithmetic, it's just
a logical consequence, that's all.  It's why the alignment cannot be
assumed for packed structures.

If you have, for example:

struct example {
	char b;
	short c;
} __attribute__((packed));

And I give you:

extern void foo(struct example *p);

and go:

	foo(p + 1);

It is clear that the compiler must assume that all instances
of a packed structure are not necessarily aligned properly.

Even if "p" is aligned, "p + 1" definitely won't be.  And this
goes for any array indexing of the given packed structure.

That's why every pointer to such a struct must be assumed to be
unaligned in these cases.

So even though the documentation may not say this explicitly, it's an
implicit logical side effect of packed structures.
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