Re: [Sbcl-devel] [PATCH] optional explicit sb-alien struct alignment

[Thiemo S. <th...@ne...>]

Cyrus Harmon wrote:
> Dear sbcl-devel,
> 
> The following patch allows for explicitly setting an :alignment  
> keyword option in sb-alien struct definitions such as the following:
> 
> (sb-alien:define-alien-type align-test-struct
>     (sb-alien:struct nil
>                      (s1 sb-alien:unsigned-char)
>                      (c1 sb-alien:unsigned-char :alignment 16)
>                      (c2 sb-alien:unsigned-char :alignment 32)
>                      (c3 sb-alien:unsigned-char :alignment 32)
>                      (c4 sb-alien:unsigned-char :alignment 8)))
> 
> This may seem like a rather silly thing to do, but certain compilers  
> support a #pragma pack that packs struct fields more tightly together  
> and this can be used to ensure that the sb-alien struct alignment  
> matches the compilers.

As a sidenote, a plain "#pragma pack" usually means byte-packed to the
minimal size, which is different to what is wanted here.

Sidenote 2, such pragmas break the C ABI guarantees about struct
layout, they usually shouldn't show up in exported interfaces.

> My actual motivation for this, although I  
> would request that you not dismiss this out of hand just because this  
> happens to be the motivating reason, is to make sb-alien struct  
> definitions for various Mac OS toolbox structures that, for 68k  
> compatibility reasons, are compiled with an Apple specific #pragma  
> align=m68k directive that causes things to be 16-bit aligned instead  
> of 32-bit aligned.

Since compiler writers don't use completely random alignment rules,
it might be better to have one :maximum-alignment key for the whole
struct/union.

(Probably I underestimate the insanity of Apple's compiler guys, the
struct you are describing usually has no padding whatsoever.)


Thiemo