linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Message-Id: <200711231402.48763.vlobanov@speakeasy.net>
Date:	Fri, 23 Nov 2007 14:02:48 -0800
From:	Vadim Lobanov <vlobanov@...akeasy.net>
To:	Daniel Drake <dsd@...too.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Documentation about unaligned memory access

On Thursday 22 November 2007 04:15:53 pm Daniel Drake wrote:
> Fortunately things are not too complex, as in most cases, the compiler
> ensures that things will work for you. For example, take the following
> structure:
>
> 	struct foo {
> 		u16 field1;
> 		u32 field2;
> 		u8 field3;
> 	};
>
> Fortunately, the compiler understands the alignment constraints, so in the
> above case it would insert 2 bytes of padding inbetween field1 and field2.
> Therefore, for standard structure types you can always rely on the compiler
> to pad structures so that accesses to fields are suitably aligned (assuming
> you do not cast the field to a type of different length).

It would also insert 3 bytes of padding after field3, in order to satisfy 
alignment constraints for arrays of these structures.

> Sidenote: in the above example, you may wish to reorder the fields in the
> above structure so that the overall structure uses less memory. For
> example, moving field3 to sit inbetween field1 and field2 (where the
> padding is inserted) would shrink the overall structure by 1 byte:
>
> 	struct foo {
> 		u16 field1;
> 		u8 field3;
> 		u32 field2;
> 	};

It will actually shrink it by 4 bytes, for the very same reason.

-- Vadim Lobanov


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