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

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 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