Messages in this thread | | | Date | Wed, 06 Dec 2006 10:05:28 -0800 (PST) | Subject | Re: Subtleties of __attribute__((packed)) | From | David Miller <> |
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From: Russell King <rmk+lkml@arm.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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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