[Haskell-cafe] Allocating a new ByteString

[Yuras Shumovich]

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 at 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
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