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 Showand 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 _ = 8poke 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)
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