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
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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