Re: [Xen-devel] [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/06/2016 21:30, Daniel Kiper wrote:
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> ---
>  doc/multiboot.texi |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/doc/multiboot.texi b/doc/multiboot.texi
> index c81b2ea..bf02a1b 100644
> --- a/doc/multiboot.texi
> +++ b/doc/multiboot.texi
> @@ -1384,6 +1384,7 @@ document, but are included for prospective operating 
> system and boot
>  loader writers.
>  
>  @menu
> +* C structure alignment and padding consideration::
>  * Notes on PC::                 
>  * BIOS device mapping techniques::  
>  * Example OS code::             
> @@ -1391,6 +1392,22 @@ loader writers.
>  @end menu
>  
>  
> +@node C structure alignment and padding consideration
> +@section C structure alignment and padding consideration
> +
> +Many C compilers try to optimize memory accesses aligning structure

"by aligning"

> +members properly. Usually they reach the goal by adding some padding.

What does "properly" mean here?  The default padding will be specified
by the default ABI the compiler conforms to.

> +This is very useful thing in general. However, if you try to mix assembler
> +with C or use C to implement structure low level access this behavior
> +may lead, at least, to quite surprising results. Hence, compiler should
> +be instructed to not optimize such accesses. Usually it is done by special
> +attribute added to structure definition, e.g. GCC compatible sources use
> +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not
> +required if it is known that its members are properly aligned and compiler
> +does not do any optimization. Very good example of this is shown below in
> +@file{multiboot2.h} file.

I am not sure what you are trying to say.

~Andrew

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