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From: David Miller <>
Subject: Re: Subtleties of __attribute__((packed))
Date: Wed, 06 Dec 2006 10:05:28 -0800 (PST)	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Russell King <>
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.

  reply	other threads:[~2006-12-06 18:05 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2006-12-06 13:20 Subtleties of __attribute__((packed)) Phil Endecott
2006-12-06 14:01 ` Frederik Deweerdt
2006-12-06 14:24   ` Phil Endecott
2006-12-06 15:04 ` Jan Blunck
2006-12-06 15:22   ` Phil Endecott
2006-12-06 15:54     ` Jan Blunck
2006-12-06 16:02       ` Andreas Schwab
2006-12-06 16:41         ` Jan Blunck
2006-12-06 17:28           ` Andreas Schwab
2006-12-06 16:13       ` Phil Endecott
2006-12-06 16:26         ` Jan Blunck
2006-12-06 17:54       ` Russell King
2006-12-06 18:05         ` David Miller [this message]
2006-12-07  9:48         ` Jan Blunck
2006-12-07 10:30           ` Andreas Schwab

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