linux-kernel - alignment and packing of struct types (was Re: [PATCH 2/3] Import fw-ohci driver.)

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Message-ID: <457C91A5.9040002@s5r6.in-berlin.de>
Date:	Mon, 11 Dec 2006 00:00:53 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	linux-kernel@...r.kernel.org
CC:	Kristian Høgsberg <krh@...hat.com>,
	Jeff Garzik <jeff@...zik.org>
Subject: alignment and packing of struct types (was Re: [PATCH 2/3] Import
 fw-ohci driver.)

I wrote:
> Kristian Høgsberg wrote:
>> Stefan Richter wrote:
>>> Kristian Høgsberg wrote:
>>>> Yup, I've done away with the bitfields and switched to a mix of __le16
>>>> and __le32 struct fields.
>>> I suppose the struct should get __attribute__((packed)) then.
>> I guess it wouldn't harm, but is it really necessary?  Would gcc ever
>> insert padding here, all the 32 bit fields a 32 bit aligned, and so are
>> the 16 bit fields.
> 
> Is 2-byte alignment of 16bit struct members guaranteed on all platforms?
> 
> A related question:
> If I specify a struct which, among else, contains 32bit quantities, then
> any variable of this struct type is supposed to be at least 4-byte-aligned.
> No if I specifiy this struct as packed, will variables of this type still
> be aligned on 4 byte boundaries or will the compiler assume no alignment?
> In other words, should it be __attribute__((packed,aligned(4))) then?
> I'm speaking about situations where I not only wish to avoid unnecessarily
> bad machine code due to unaligned access but where the device requires
> 4-byte alignment too.

After sending this, I realized I should have changed the subject. :-)
-- 
Stefan Richter
-=====-=-==- ==-- -=-=-
http://arcgraph.de/sr/
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