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

21 Sep 2013


      Thanks so far, but I'm still having troubles with converting String to
CString.... The error I'm getting is:

 Couldn't match type `IO CString' with `Ptr CChar'
 Expected type: CString
   Actual type: IO CString


code:

-------
import Foreign
import Foreign.C

--int somemethod(char *param, int length, double array[], int UB1, int UB2,
bool isValid);
foreign import stdcall unsafe "setmoduletable"  c_setmoduletable ::
   CString -> CInt -> Ptr CDouble -> CInt -> CInt -> CInt -> IO CInt

main = do
  let param = newCString "someString"
  --paramLength = length param
  let realTable = [ 0.0111125, 0.0111125, 0.009525] :: [CDouble]
      ub1 = 0::CInt
      ub2 = 2::CInt
      --isValid = False
  realTablePtr <- newArray realTable
  x <- c_setmoduletable param 14 realTablePtr ub1 ub2 0
  free realTablePtr
  putStrLn $ "c_setmoduletable output: " ++ show x
  putStrLn "Done"


cheers,
m.

On Sat, Sep 21, 2013 at 4:21 PM, Edward Z. Yang  wrote:
...
You should use the C variants of all the types, so
type CBool = CInt
foreign import stdcall unsafe "setmoduletable"  c_setmoduletable ::
    CString -> CInt -> Ptr CDouble -> CInt -> CInt -> CBool -> IO CInt
Edward
Excerpts from Miro Karpis's message of Sat Sep 21 08:55:58 -0400 2013:
...
thanks,.. so far I came to this. I can compile it but not getting the
right
values from the method. Most probably some problem with the pointers...
----------------
--int setmoduletable(char *param, int length, double array[], int UB1,
int
UB2, bool isValid);
foreign import stdcall unsafe "setmoduletable"  c_setmoduletable :: Ptr
Char
-> Int
-> Ptr (Double)
-> Int
-> Int
-> Bool
-> IO Int
main = do
  let param = "someString"
  let paramLength = length param
      realTable = [ 0.0111125, 0.0111125, 0.009525] :: [Double]
      ub1 = 0
      ub2 = 2
      isValid = False
  realTablePtr <- newArray realTable
  paramPtr <- newArray param
  x <- c_setmoduletable paramPtr paramLength realTablePtr ub1 ub2 isValid
  free realTablePtr
  free paramPtr
  putStrLn $ "c_setmoduletable output: " ++ show x
  putStrLn "Done"
On Sat, Sep 21, 2013 at 1:06 AM, Brandon Allbery 
...
On Fri, Sep 20, 2013 at 6:53 PM, Miro Karpis <
miroslav.karpis@gmail.com>wrote:
...
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:
foreign import stdcall unsafe "somemethod"  c_somemethod :: Ptr
CString
You don't want Ptr CString.  See:
Prelude> :m +Foreign.C.String
    Prelude Foreign.C.String> :i CString
    type CString = GHC.Ptr.Ptr Foreign.C.Types.CChar
  -- Defined in `Foreign.C.String'
In other words, CString is an alias for Ptr CChar. Ptr CString
corresponds
to (char **), not (char *).
--
brandon s allbery kf8nh                               sine nomine
associates
allbery.b@gmail.com
ballbery@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad
http://sinenomine.net