[Vm-dev] Re: FFI crashes VM on Linux (was: Z3950 on OSX: module yaz3 not found)

[Andreas Raab]

On 12/24/2010 5:27 PM, Igor Stasenko wrote:
> If you using structs, one of the things you need to be aware is struct
> fields memory alignment.
>
> for instance:
>
> struct foo {
>    char x;
>    char y;
> } bar;
>
> try compile and print:
>
> printf ('%d" ,&bar.y -&bar.x );
>
> depending on compiler's default alignment, it could be 1 or 4 or
> anything else. :)
> This is of course may vary from platform to platform and from compiler
> to compiler.
>
> Not sure if you can specify the struct alignment using FFI package.

You can. The default i.e.,

fields
	^#(
		(x	'char')
		(y	'char')
	)

is 1-aligned, to change it to be 4-aligned you can tell the FFI how much 
bytes each element in the spec takes, i.e.,

fields
	^#(
		(x	'char'	4)
		(y	'char'	4)
	)

More commonly, you would have alignments like this:

fields
	^#(
		(r	'byte')
		(g	'byte')
		(b	'byte'	2) "<- align next element on word boundary"
		(a	'ulong')
	)


Cheers,
   - Andreas