Haskell FFI Issue With Double Pointer Argument

Question

I am trying to bind to a C function from an external C library which returns a pointer to a memory buffer and the length of the buffer :

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

I tried the following code but got a segmentation fault as a result.

{-# 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'

I assume that peekArray did allocate enough memory, so I am not sure where it did go wrong. Any help welcome.

Janthelme · Accepted Answer · 2015-12-29 10:56:34Z

2

Stupid mistake, I missed a dereferencing step. This code now works :

{-# 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'

edited Dec 29, 2015 at 10:56

answered Dec 29, 2015 at 10:38

Janthelme

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2 Comments

chi Over a year ago

If you want, you can simplify it noting thatcs <- blah ; return cs is equivalent to blah.

Janthelme Over a year ago

@ Chi. You are right, I did not sanitize completely before posting. I'll edit. Thanks.

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