The function 'castStablePtrToPtr' does not actually return a pointer you can usefully de-reference in C - it just gives you a value that will fit into a 'void*' in C-land. A StablePtr is, conceptually, just a pointer-sized index into an array maintained by the Haskell runtime, you can only de-reference it in Haskell. But you can pass it around in C because it fits into a pointer-sized variable or struct member. On Thu, Apr 11, 2013 at 10:59 AM, Kees Bleijenberg wrote:

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 :

hs_init(NULL,NULL)

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 : B.ByteString

I’am missing the big picture. Any ideas?

Greetings

Kees

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