Storable tuples and what is 'alignment'?

Simon Marlow simonmar at
Tue Aug 6 04:47:45 EDT 2002

[ moved to ffi at ]

> You also need to allow for any padding which is required to satisfy
> the alignment constraints of b, e.g.
> 	sizeOf (a,b) = sizeOf a + padding (a,b) + sizeOf b
> 		where padding (a,b) = (alignment b - sizeOf a 
> `mod` alignment b) `mod` alignment b
> >     alignment (a,b) = alignment a + alignment b
> The alignment of the overall structure is determined by the alignment
> of the first element. Subsequent elements are aligned (if necessary)
> by the addition of padding between elements.
> 	alignment (a,_) = alignment a

I think in fact that the aligment of the whole structure should be

	alignment (a,b) = alignment a `max` aligment b

which would mean that sizeOf should be defined as

	sizeOf (a,b) = sizeOf a + padding + sizeOf b
		where b_align = alignment b
			padding = case sizeOf a `mod` b_align of
					 0 -> 0
					 n -> b_align - n

consider for example

	struct { int a; double b }

on a 32-bit architecture where doubles must be aligned on an 8-byte
boundary (eg. Sparc), gcc will always align the structure on an 8-byte
boundary and put 4 bytes of padding between the elements.


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