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

[Miro Karpis]

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 <ezyang at mit.edu> 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 <allbery.b at gmail.com
> >wrote:
> >
> > > On Fri, Sep 20, 2013 at 6:53 PM, Miro Karpis <
> miroslav.karpis at 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 at gmail.com
> > > ballbery at sinenomine.net
> > > unix, openafs, kerberos, infrastructure, xmonad
> > > http://sinenomine.net
> > >
>
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