There is currently no way to declare 2D arrays.

One work around for a small struct like that is to declare 4 arrays.
e.g.

class ALLEGRO_TRANSFORM < FFI::Struct
   layout :m0, [:float, 4], :m1, [:float, 4], :m2, [:float, 4], :m3, [:float, 4]
end

Then instead of accessing the second element of the first array as
[:m][0][1], it would be [:m0][1]

Kinda messy, but its what you will have to do for now.

Please file an issue at http://github.com/ffi/ffi/issues so this will
can get implemented at some stage.

2009/12/11 galdor <khaelin@gmail.com>:

> Hi,
>
> I'm trying to bind the following structure:
>
> struct ALLEGRO_TRANSFORM {
>   float m[4][4];
> };
>
> But with the following FFI declaration:
>
> class ALLEGRO_TRANSFORM < FFI::Struct
>    layout :m, [[:float, 4], 4]
> end
>
> I get the following error:
>
> /usr/lib/ruby/gems/1.9.1/gems/ffi-0.6.0pre/lib/ffi/struct.rb:177:in
> `add_array': undefined method `alignment' for [:float, 4]:Array
> (NoMethodError)
>        from /usr/lib/ruby/gems/1.9.1/gems/ffi-0.6.0pre/lib/ffi/
> struct.rb:177:in `array_layout'
>        from /usr/lib/ruby/gems/1.9.1/gems/ffi-0.6.0pre/lib/ffi/
> struct.rb:106:in `layout'
>        from lib/allegro/ffi_allegro.rb:558:in
> `<class:ALLEGRO_TRANSFORM>'
>        from lib/allegro/ffi_allegro.rb:557:in `<module:Allegro>'
>        from lib/allegro/ffi_allegro.rb:4:in `<main>'
>
> Is there a specific syntax for 2D arrays ?
>
> Regards,
>
> Nicolas Martyanoff
>