c - Haskell FFI 关于双指针参数的问题

Question

我正在尝试绑定(bind)到外部 C 库中的 C 函数，该函数返回指向内存缓冲区的指针和缓冲区的长度:

extern int myfunc(int arg1, unsigned char **buffer, size_t *buffer_len);

我尝试了以下代码，但结果出现段错误。

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign
import Foreign.C.Types (CInt(..), CUChar(..), CSize(..))
import Foreign.Ptr (Ptr)
import Foreign.Marshal.Array (peekArray)

foreign import ccall unsafe "myfunc" c_myfunc :: CInt -> Ptr (Ptr CUChar) -> Ptr CSize -> IO (CInt)

getBuffer :: Int -> IO [CUChar]
getBuffer arg1 = do
    alloca $ \buffer -> do
        alloca $ \buflen -> do
            res <- c_myfunc (CInt arg1) buffer buflen

            buflen' <- fromIntegral <$> peek buflen

            cs  <- peekArray buflen' buffer  -- return a [Ptr CUChar] of size buflen
            cs' <- mapM peek cs  -- return a [CUChar] of size buflen

            return cs'

我假设 peekArray 确实分配了足够的内存，所以我不确定它哪里出了问题。欢迎任何帮助。

Answer 1

愚蠢的错误，我错过了取消引用步骤。此代码现在可以运行:

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign
import Foreign.C.Types (CInt(..), CUChar(..), CSize(..))
import Foreign.Ptr (Ptr)
import Foreign.Marshal.Array (peekArray)

foreign import ccall unsafe "myfunc" c_myfunc :: CInt -> Ptr (Ptr CUChar) -> Ptr CSize -> IO (CInt)

getBuffer :: Int -> IO [CUChar]
getBuffer arg1 = do
    alloca $ \ ptrbuf -> do
        alloca $ \buflen -> do
            res <- c_myfunc (CInt arg1) ptrBuf buflen

            buflen' <- fromIntegral <$> peek buflen

            buffer  <- peek ptrBuf    -- I MISSED THIS ONE. Extra dereferencing needed.
            peekArray buflen' buffer  -- return a [CUChar] of size buflen'