[Haskell-cafe] FFI and struct arguments

[Bayley, Alistair]

> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Duncan Coutts
> Sent: 17 July 2008 12:46
> 
> On Wed, 2008-07-16 at 22:45 -0300, Felipe Lessa wrote:
> > Hi,
> > 
> > I tried googling and searching the haskellwiki about this but wasn't
> > lucky enough. My question is: is there a way to send struct 
> arguments
> > to C functions via the FFI or do I need to create a C 
> wrapper? I guess
> > there isn't, and while I can live without it, I'd like to leave no
> > doubt.
> 
> If the struct is passed by reference of course then you're 
> fine, but if
> it's by value then you need a C wrapper. The reason is because it's
> beyond the scope of the FFI to know the structure layout and 
> how to map
> that to haskell types. That's the domain of FFI 
> pre-processors. However
> I don't know of any FFI pre-processors that help in this 
> case. Passing C
> structs by value seems to be pretty rare in exported C APIs.


hsc2hs can help a bit (I haven't used the other FFI tools, so don't take
this as an endorsement of hsc2hs over them). You could create a wrapper
that marshals your Vector to a vector struct like this:

---- .hsc file:

#include <vect.h>  -- whatever header contains your C vector struct

data VectorC = Ptr ()  -- opaque data type, like void*

-- your Haskell vector
data Vector = Vector Float Float

vector2cvect :: Vector -> IO VectorC
vector2cvect (Vector x y) = do
  ptr <- mallocBytes #{size vect}
  pokeByteOff #{offset vect, x} x
  pokeByteOff #{offset vect, x} x
  return ptr

cvect2Vector :: VectorC -> IO Vector
cvect2Vector ptr = do
  x <- peekByteOff ptr #{offset vect, x}
  y <- peekByteOff ptr #{offset vect, y}
  return (Vector x y)

-- inline your *vect->vect wrapper (ends up in generated .c file)
#def void funcWrapper(vect *v) { func(*v); }

foreign import stdcall funcWr unsafe "funcWrapper" :: VectorC -> IO ()

main = do
  vc <- vector2cvect (Vector 3 4)
  funcWr vc
  free vc
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