I'm calling a function by FFI that returns two strings by writing into output buffers that you provide. The code I currently have looks like this (minus some error handling):
import qualified Data.ByteString as S import qualified Data.ByteString.Unsafe as SU
import Foreign.C (CChar, CInt(..), CSize(..)) import Foreign.ForeignPtr (mallocForeignPtrBytes, withForeignPtr) import Foreign.Ptr (Ptr) import System.IO.Unsafe (unsafePerformIO)
seed_keypair :: S.ByteString -> (S.ByteString, S.ByteString) seed_keypair seed | S.length seed /= signSeed = error "seed has incorrect length" | otherwise = unsafePerformIO $ do pk <- mallocForeignPtrBytes signPK sk <- mallocForeignPtrBytes signSK SU.unsafeUseAsCString seed $ \pseed -> withForeignPtr pk $ \ppk -> withForeignPtr sk $ \psk -> do 0 <- c_sign_seed_keypair ppk psk pseed bpk <- S.packCStringLen (ppk, signPK) bsk <- S.packCStringLen (psk, signSK) return (bpk, bsk)
foreign import ccall "crypto_sign_seed_keypair" c_sign_seed_keypair :: Ptr CChar -> Ptr CChar -> Ptr CChar -> IO CInt
However, this needlessly makes a copy of the output buffers to create the final result. What I really want to do is just write the string directly into a buffer allocated and used by a new ByteString; is there some way to accomplish this? (Any other comments about what I'm doing would also be appreciated, this happens to be my first attempt at using FFI!) -- mithrandi, i Ainil en-Balandor, a faer Ambar