struct-string

See the corresponding SO question!

Suppose you have a C-struct

typedef struct {
  uint32_t num;
  char*    str;
} MyStruct;

and a function f that does some operation on it. The char* num has to be allocated a sufficient buffer before calling f:

char buf[64];	//somehow you know 64 is enough
MyStruct s = {1, buf};
f(buf);

The question is how to write a Haskell FFI for this kind of C API. Start with

data MyStruct = MyStruct {
    num :: Word32,
    str :: String
} deriving Show

and write a Storable instance. My idea is to allocate 64 bytes at the end which will serve as buffer for the string:

instance Storable MyStruct where
    sizeOf _ = 8{-alignment!-} + sizeOf (nullPtr :: CString) + 64{-buffer-}
    alignment _ = 8

poke has to change the pointer in str to point to the allocated buffer and then the Haskell string has to be copied into it:

    poke p x = do
        pokeByteOff p 0 (num x)
        poke strPtr bufPtr
        withCStringLen (str x) $ \(p',l) -> copyBytes bufPtr p' (l+1) -- +1? not sure
        where strPtr = castPtr $ plusPtr p 8 :: Ptr CString
              bufPtr = castPtr $ plusPtr p 16 :: CString
              

peek is then straightforward:

    peek p = MyStruct 
         <$> peek (castPtr p)
         <*> peekCAString (castPtr $ plusPtr p 16)