FFI, export a ByteString

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FFI, export a ByteString

Kees Bleijenberg
My function in Haskell takes a CString and returns a ByteString (of Word8).

I tried to put this function in a Haskell made Windows dll.  I want to use
this function in a program written in C.


The most simple program  that compiled was:


Import qualified B.ByteString as B

import Foreign.StablePtr

import Foreign.C.String

import Foreign.Ptr


foreign export stdcall parseRuitType :: CString -> IO (Ptr ())

parseRuitType s = do

                            p <- (newStablePtr ( B.pack [10,11,12]))

                            return $ castStablePtrToPtr p


For testing I ignore s.


In the C-program :


res = parseRuitType ("5" )

No errors, but the returned value res (or the dereferenced res) makes no
sense (not 10,11,12).


My idea was: the result of the function(a ByteString of Word8) must be
locked. No garbage collection. But newStablePtr does not return a pointer.
So I have to convert the newStablePtr to a real pointer and then coerce this
in the C program to a pointer to  a array of word8.?

I've tried to return the ByteString  in the Haskell function. But now the
compiler complains: Unacceptable result type in foreign declaration :


I'am missing the big picture.  Any ideas?









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