[Haskell-beginners] help with FFI: passing char* to dll function + pointer freeing

20 Sep 2013

      Please, I just can not figure this out:

I have a method in my C dll:
int somemethod(char *param, int length, double array[], int UB1, int UB2,
bool isValid);

I would like to use this method in Haskell, so I have defined it like:

{-# OPTIONS_GHC -fglasgow-exts #-}
import Foreign
import Foreign.C

foreign import stdcall unsafe "somemethod"  c_somemethod :: Ptr CString
                                                         -> Int
                                                         -> Ptr (Double)
                                                         -> Int
                                                         -> Int
                                                         -> Bool
                                                         -> IO Int
main = do
  let param = ["input"]
  let paramLength = length param
      realTable = [ 1.0, 2.0, 3.0 ] :: [Double]
      ub1 = 0
      ub2 = 2
      isValid = False
  realTablePtr <- newArray realTable
  x <- c_somemethod param paramLength realTablePtr ub1 ub2 isValid
  free realTablePtr
  putStrLn $ "c_somemethod output: " ++ show x
  putStrLn "Done"

When I try to compile this I get error regarding the param string. I have
no idea how to define it. Additionally to this, is the freeing of realTablePtr
correct?

compile error:
 Couldn't match expected type `Ptr CString' with actual type `[t0]'
 In the first argument of `c_somemethod', namely `param'

Please, how can I ix this?

cheers,
m.

[Haskell-beginners] help with FFI: passing char* to dll function + pointer freeing

Miro Karpis