I do not think i can help you, but my FFi to functions with similar signature looks like this:

C definition:

PDFLIB_API int PDFLIB_CALL
PDF_begin_document(PDF *p, const char *filename, int len, const char *optlist);

haskell FFI:

c_beginDocument :: Pdf -> String -> String -> IO Int
c_beginDocument pdf a2 a3 =
  withCString a2 $ \a2' ->
  withCString a3 $ c_beginDocument'_ pdf a2' 0

foreign import ccall safe "Pdflib.chs.h PDF_begin_document" c_beginDocument'_ :: Pdf -> Ptr CChar -> CInt -> Ptr CChar -> IO Int

The difference as you see, i'm using withCString to convert from ahskell to C and i'm passing 0 as the string size.

On Friday, March 14, 2014 3:24:23 PM UTC-7, Miroslav Karpis wrote:

Hi, please can you help me with following?

I have a call to an external dll (via ffi) which if executed from ghci works fine. 

If I compile my code with 'ghc -o fm Mycode.hs -L. -lextdll' and run it I get  'Segmentation fault/access violation in generated code'.

main = do

let param = "FilePath"

let value = "C:/dev/misc/haskell/services/FM"

result <- liftIO $ FM.setmodulestring param value

return "done"

setmodulestring :: String -> String -> IO CInt

setmodulestring param value = do

let cParamLength = fromIntegral $ length param ::CInt

    cValueLength = fromIntegral $ length value ::CInt

    setVarInArray = (-1)::CInt

alloca $ \cParam -> do

alloca $ \cValue -> do

result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray

return result

If I try also with following, the behaviour is the same:

setmodulestring2 :: String -> String -> IO CInt

setmodulestring2 param value = do

cParam <- newCWString param

cValue <- newCWString value

let cParamLength = fromIntegral $ length param ::CInt

   cValueLength = fromIntegral $ length value ::CInt

   setVarInArray = (-1)::CInt 

result <- c_setmodulestring cParam cParamLength cValue cValueLength setVarInArray

free cParam

free cValue

return res

Any comments/ideas more than appreciated.

Cheers,

Miro