Looks like you need 'unsafePackCStringFinalizer' (or one of it's variants) from Data.ByteString.Unsafe module. It lets you reuse the buffer and deallocate it in finalizer. Something similar is used e.g. in 'kyotocabinet' package. 23 Дек 2014 г. 1:48 пользователь "Tristan Seligmann" < mithrandi@mithrandi.net> написал:
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe