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

[Edward Z. Yang]

Hello Miro,

The immediate problem is that you need to use the bind
notation (param <- newCString "foo") rather than let
notation, but if the string will not be used beyond the
function call, it is preferable to use withCString

Edward

Excerpts from Miro Karpis's message of Sat Sep 21 15:25:58 -0400 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 <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
> > > >
> >