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Add peekCString, withCString for converting NUL terminated C strings #254

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Add peekCString, withCString for converting NUL terminated C strings #254

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36 changes: 31 additions & 5 deletions src/Data/Text/Foreign.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,9 @@ module Data.Text.Foreign
, useAsPtr
, asForeignPtr
-- ** Encoding as UTF-8
, peekCString
, peekCStringLen
, withCString
, withCStringLen
-- * Unsafe conversion code
, lengthWord16
Expand All @@ -39,12 +41,13 @@ import Control.Monad.ST.Unsafe (unsafeIOToST)
#else
import Control.Monad.ST (unsafeIOToST)
#endif
import Data.ByteString.Unsafe (unsafePackCStringLen, unsafeUseAsCStringLen)
import Data.ByteString (useAsCString)
import Data.ByteString.Unsafe (unsafePackCString, unsafePackCStringLen, unsafeUseAsCStringLen)
import Data.Text.Encoding (decodeUtf8, encodeUtf8)
import Data.Text.Internal (Text(..), empty)
import Data.Text.Unsafe (lengthWord16)
import Data.Word (Word16)
import Foreign.C.String (CStringLen)
import Foreign.C.String (CString, CStringLen)
import Foreign.ForeignPtr (ForeignPtr, mallocForeignPtrArray, withForeignPtr)
import Foreign.Marshal.Alloc (allocaBytes)
import Foreign.Ptr (Ptr, castPtr, plusPtr)
Expand Down Expand Up @@ -153,6 +156,16 @@ asForeignPtr t@(Text _arr _off len) = do
withForeignPtr fp $ unsafeCopyToPtr t
return (fp, I16 len)

-- | /O(n)/ Decode a NUL terminated C string, which is assumed to have
-- been encoded as UTF-8. If decoding fails, a 'UnicodeException' is
-- thrown.
--
-- @since 1.2.5.0
peekCString :: CString -> IO Text
peekCString cs = do
bs <- unsafePackCString cs
return $! decodeUtf8 bs

-- | /O(n)/ Decode a C string with explicit length, which is assumed
-- to have been encoded as UTF-8. If decoding fails, a
-- 'UnicodeException' is thrown.
Expand All @@ -163,9 +176,22 @@ peekCStringLen cs = do
bs <- unsafePackCStringLen cs
return $! decodeUtf8 bs

-- | Marshal a 'Text' into a C string encoded as UTF-8 in temporary
-- storage, with explicit length information. The encoded string may
-- contain NUL bytes, and is not followed by a trailing NUL byte.
-- | /O(n)/ Marshal a 'Text' into a NUL terminated C string encoded as
-- UTF-8 in temporary storage. The 'Text' must not contain any NUL
-- characters.
--
-- The temporary storage is freed when the subcomputation terminates
-- (either normally or via an exception), so the pointer to the
-- temporary storage must /not/ be used after this function returns.
--
-- @since 1.2.5.0
withCString :: Text -> (CString -> IO a) -> IO a
withCString t act = useAsCString (encodeUtf8 t) act
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Since text-2 this does redundant copying.

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Since text-2 this does redundant copying.

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It's redundant in both. encodeUtf8 creates a bytestring, which can be used unsafely, as nothing else refers to it.

In text-2.0 one could have unsafe withCString (or just withCStringLen, as there is no NUL at the end) variants though, which would just pass the Ptr to ByteArray contents.

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Scrap that. text ByteArray's are not (always) pinned, so sometimes copying is unavoidable.

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At least one round of memcpy is unavoidable unless the byte array in Text has that final null.


-- | /O(n)/ Marshal a 'Text' into a C string encoded as UTF-8 in
-- temporary storage, with explicit length information. The encoded
-- string may contain NUL bytes, and is not followed by a trailing NUL
-- byte.
--
-- The temporary storage is freed when the subcomputation terminates
-- (either normally or via an exception), so the pointer to the
Expand Down