Handle.hs 26.2 KB
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{-# LANGUAGE Trustworthy #-}
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{-# LANGUAGE CPP
           , NoImplicitPrelude
           , RecordWildCards
           , NondecreasingIndentation
  #-}
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{-# OPTIONS_GHC -Wno-unused-matches #-}
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-----------------------------------------------------------------------------
-- |
-- Module      :  GHC.IO.Handle
-- Copyright   :  (c) The University of Glasgow, 1994-2009
-- License     :  see libraries/base/LICENSE
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--
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-- Maintainer  :  libraries@haskell.org
-- Stability   :  provisional
-- Portability :  non-portable
--
-- External API for GHC's Handle implementation
--
-----------------------------------------------------------------------------

module GHC.IO.Handle (
   Handle,
   BufferMode(..),
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   mkFileHandle, mkDuplexHandle,
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   hFileSize, hSetFileSize, hIsEOF, isEOF, hLookAhead,
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   hSetBuffering, hSetBinaryMode, hSetEncoding, hGetEncoding,
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   hFlush, hFlushAll, hDuplicate, hDuplicateTo,
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   hClose, hClose_help,
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   LockMode(..), hLock, hTryLock,

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   HandlePosition, HandlePosn(..), hGetPosn, hSetPosn,
   SeekMode(..), hSeek, hTell,
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   hIsOpen, hIsClosed, hIsReadable, hIsWritable, hGetBuffering, hIsSeekable,
   hSetEcho, hGetEcho, hIsTerminalDevice,
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   hSetNewlineMode, Newline(..), NewlineMode(..), nativeNewline,
   noNewlineTranslation, universalNewlineMode, nativeNewlineMode,

   hShow,

   hWaitForInput, hGetChar, hGetLine, hGetContents, hPutChar, hPutStr,

   hGetBuf, hGetBufNonBlocking, hPutBuf, hPutBufNonBlocking
 ) where

import GHC.IO
import GHC.IO.Exception
import GHC.IO.Encoding
import GHC.IO.Buffer
import GHC.IO.BufferedIO ( BufferedIO )
import GHC.IO.Device as IODevice
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import GHC.IO.Handle.FD
import GHC.IO.Handle.Lock
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import GHC.IO.Handle.Types
import GHC.IO.Handle.Internals
import GHC.IO.Handle.Text
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import qualified GHC.IO.BufferedIO as Buffered
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import GHC.Base
import GHC.Exception
import GHC.MVar
import GHC.IORef
import GHC.Show
import GHC.Num
import GHC.Real
import Data.Maybe
import Data.Typeable

-- ---------------------------------------------------------------------------
-- Closing a handle

-- | Computation 'hClose' @hdl@ makes handle @hdl@ closed.  Before the
-- computation finishes, if @hdl@ is writable its buffer is flushed as
-- for 'hFlush'.
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-- Performing 'hClose' on a handle that has already been closed has no effect;
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-- doing so is not an error.  All other operations on a closed handle will fail.
-- If 'hClose' fails for any reason, any further operations (apart from
-- 'hClose') on the handle will still fail as if @hdl@ had been successfully
-- closed.

hClose :: Handle -> IO ()
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hClose h@(FileHandle _ m)     = do
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  mb_exc <- hClose' h m
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  hClose_maybethrow mb_exc h
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hClose h@(DuplexHandle _ r w) = do
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  excs <- mapM (hClose' h) [r,w]
  hClose_maybethrow (listToMaybe (catMaybes excs)) h
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hClose_maybethrow :: Maybe SomeException -> Handle -> IO ()
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hClose_maybethrow Nothing  h = return ()
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hClose_maybethrow (Just e) h = hClose_rethrow e h
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hClose_rethrow :: SomeException -> Handle -> IO ()
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hClose_rethrow e h =
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  case fromException e of
    Just ioe -> ioError (augmentIOError ioe "hClose" h)
    Nothing  -> throwIO e

hClose' :: Handle -> MVar Handle__ -> IO (Maybe SomeException)
hClose' h m = withHandle' "hClose" h m $ hClose_help

-----------------------------------------------------------------------------
-- Detecting and changing the size of a file

-- | For a handle @hdl@ which attached to a physical file,
-- 'hFileSize' @hdl@ returns the size of that file in 8-bit bytes.

hFileSize :: Handle -> IO Integer
hFileSize handle =
    withHandle_ "hFileSize" handle $ \ handle_@Handle__{haDevice=dev} -> do
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    case haType handle_ of
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      ClosedHandle              -> ioe_closedHandle
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      SemiClosedHandle          -> ioe_semiclosedHandle
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      _ -> do flushWriteBuffer handle_
              r <- IODevice.getSize dev
              if r /= -1
                 then return r
                 else ioException (IOError Nothing InappropriateType "hFileSize"
                                   "not a regular file" Nothing Nothing)


-- | 'hSetFileSize' @hdl@ @size@ truncates the physical file with handle @hdl@ to @size@ bytes.

hSetFileSize :: Handle -> Integer -> IO ()
hSetFileSize handle size =
    withHandle_ "hSetFileSize" handle $ \ handle_@Handle__{haDevice=dev} -> do
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    case haType handle_ of
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      ClosedHandle              -> ioe_closedHandle
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      SemiClosedHandle          -> ioe_semiclosedHandle
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      _ -> do flushWriteBuffer handle_
              IODevice.setSize dev size
              return ()

-- ---------------------------------------------------------------------------
-- Detecting the End of Input

-- | For a readable handle @hdl@, 'hIsEOF' @hdl@ returns
-- 'True' if no further input can be taken from @hdl@ or for a
-- physical file, if the current I\/O position is equal to the length of
-- the file.  Otherwise, it returns 'False'.
--
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-- NOTE: 'hIsEOF' may block, because it has to attempt to read from
-- the stream to determine whether there is any more data to be read.
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hIsEOF :: Handle -> IO Bool
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hIsEOF handle = wantReadableHandle_ "hIsEOF" handle $ \Handle__{..} -> do

  cbuf <- readIORef haCharBuffer
  if not (isEmptyBuffer cbuf) then return False else do

  bbuf <- readIORef haByteBuffer
  if not (isEmptyBuffer bbuf) then return False else do

  -- NB. do no decoding, just fill the byte buffer; see #3808
  (r,bbuf') <- Buffered.fillReadBuffer haDevice bbuf
  if r == 0
     then return True
     else do writeIORef haByteBuffer bbuf'
             return False
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-- ---------------------------------------------------------------------------
-- isEOF

-- | The computation 'isEOF' is identical to 'hIsEOF',
-- except that it works only on 'stdin'.

isEOF :: IO Bool
isEOF = hIsEOF stdin

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-- ---------------------------------------------------------------------------
-- Looking ahead

-- | Computation 'hLookAhead' returns the next character from the handle
-- without removing it from the input buffer, blocking until a character
-- is available.
--
-- This operation may fail with:
--
--  * 'isEOFError' if the end of file has been reached.

hLookAhead :: Handle -> IO Char
hLookAhead handle =
  wantReadableHandle_ "hLookAhead"  handle hLookAhead_

-- ---------------------------------------------------------------------------
-- Buffering Operations

-- Three kinds of buffering are supported: line-buffering,
-- block-buffering or no-buffering.  See GHC.IO.Handle for definition and
-- further explanation of what the type represent.

-- | Computation 'hSetBuffering' @hdl mode@ sets the mode of buffering for
-- handle @hdl@ on subsequent reads and writes.
--
-- If the buffer mode is changed from 'BlockBuffering' or
-- 'LineBuffering' to 'NoBuffering', then
--
--  * if @hdl@ is writable, the buffer is flushed as for 'hFlush';
--
--  * if @hdl@ is not writable, the contents of the buffer is discarded.
--
-- This operation may fail with:
--
--  * 'isPermissionError' if the handle has already been used for reading
--    or writing and the implementation does not allow the buffering mode
--    to be changed.

hSetBuffering :: Handle -> BufferMode -> IO ()
hSetBuffering handle mode =
  withAllHandles__ "hSetBuffering" handle $ \ handle_@Handle__{..} -> do
  case haType of
    ClosedHandle -> ioe_closedHandle
    _ -> do
         if mode == haBufferMode then return handle_ else do

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         -- See [note Buffer Sizing] in GHC.IO.Handle.Types
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          -- check for errors:
          case mode of
              BlockBuffering (Just n) | n <= 0    -> ioe_bufsiz n
              _ -> return ()
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          -- for input terminals we need to put the terminal into
          -- cooked or raw mode depending on the type of buffering.
          is_tty <- IODevice.isTerminal haDevice
          when (is_tty && isReadableHandleType haType) $
                case mode of
#ifndef mingw32_HOST_OS
        -- 'raw' mode under win32 is a bit too specialised (and troublesome
        -- for most common uses), so simply disable its use here.
                  NoBuffering -> IODevice.setRaw haDevice True
#else
                  NoBuffering -> return ()
#endif
                  _           -> IODevice.setRaw haDevice False

          -- throw away spare buffers, they might be the wrong size
          writeIORef haBuffers BufferListNil

          return Handle__{ haBufferMode = mode,.. }

-- -----------------------------------------------------------------------------
-- hSetEncoding

-- | The action 'hSetEncoding' @hdl@ @encoding@ changes the text encoding
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-- for the handle @hdl@ to @encoding@.  The default encoding when a 'Handle' is
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-- created is 'localeEncoding', namely the default encoding for the current
-- locale.
--
-- To create a 'Handle' with no encoding at all, use 'openBinaryFile'.  To
-- stop further encoding or decoding on an existing 'Handle', use
-- 'hSetBinaryMode'.
--
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-- 'hSetEncoding' may need to flush buffered data in order to change
-- the encoding.
--
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hSetEncoding :: Handle -> TextEncoding -> IO ()
hSetEncoding hdl encoding = do
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  withAllHandles__ "hSetEncoding" hdl $ \h_@Handle__{..} -> do
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    flushCharBuffer h_
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    closeTextCodecs h_
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    openTextEncoding (Just encoding) haType $ \ mb_encoder mb_decoder -> do
    bbuf <- readIORef haByteBuffer
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    ref <- newIORef (errorWithoutStackTrace "last_decode")
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    return (Handle__{ haLastDecode = ref,
                      haDecoder = mb_decoder,
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                      haEncoder = mb_encoder,
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                      haCodec   = Just encoding, .. })
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-- | Return the current 'TextEncoding' for the specified 'Handle', or
-- 'Nothing' if the 'Handle' is in binary mode.
--
-- Note that the 'TextEncoding' remembers nothing about the state of
-- the encoder/decoder in use on this 'Handle'.  For example, if the
-- encoding in use is UTF-16, then using 'hGetEncoding' and
-- 'hSetEncoding' to save and restore the encoding may result in an
-- extra byte-order-mark being written to the file.
--
hGetEncoding :: Handle -> IO (Maybe TextEncoding)
hGetEncoding hdl =
  withHandle_ "hGetEncoding" hdl $ \h_@Handle__{..} -> return haCodec

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-- -----------------------------------------------------------------------------
-- hFlush

-- | The action 'hFlush' @hdl@ causes any items buffered for output
-- in handle @hdl@ to be sent immediately to the operating system.
--
-- This operation may fail with:
--
--  * 'isFullError' if the device is full;
--
--  * 'isPermissionError' if a system resource limit would be exceeded.
--    It is unspecified whether the characters in the buffer are discarded
--    or retained under these circumstances.

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hFlush :: Handle -> IO ()
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hFlush handle = wantWritableHandle "hFlush" handle flushWriteBuffer

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-- | The action 'hFlushAll' @hdl@ flushes all buffered data in @hdl@,
-- including any buffered read data.  Buffered read data is flushed
-- by seeking the file position back to the point before the bufferred
-- data was read, and hence only works if @hdl@ is seekable (see
-- 'hIsSeekable').
--
-- This operation may fail with:
--
--  * 'isFullError' if the device is full;
--
--  * 'isPermissionError' if a system resource limit would be exceeded.
--    It is unspecified whether the characters in the buffer are discarded
--    or retained under these circumstances;
--
--  * 'isIllegalOperation' if @hdl@ has buffered read data, and is not
--    seekable.

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hFlushAll :: Handle -> IO ()
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hFlushAll handle = withHandle_ "hFlushAll" handle flushBuffer

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-- -----------------------------------------------------------------------------
-- Repositioning Handles

data HandlePosn = HandlePosn Handle HandlePosition

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-- | @since 4.1.0.0
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instance Eq HandlePosn where
    (HandlePosn h1 p1) == (HandlePosn h2 p2) = p1==p2 && h1==h2

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-- | @since 4.1.0.0
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instance Show HandlePosn where
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   showsPrec p (HandlePosn h pos) =
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        showsPrec p h . showString " at position " . shows pos

  -- HandlePosition is the Haskell equivalent of POSIX' off_t.
  -- We represent it as an Integer on the Haskell side, but
  -- cheat slightly in that hGetPosn calls upon a C helper
  -- that reports the position back via (merely) an Int.
type HandlePosition = Integer

-- | Computation 'hGetPosn' @hdl@ returns the current I\/O position of
-- @hdl@ as a value of the abstract type 'HandlePosn'.

hGetPosn :: Handle -> IO HandlePosn
hGetPosn handle = do
    posn <- hTell handle
    return (HandlePosn handle posn)

-- | If a call to 'hGetPosn' @hdl@ returns a position @p@,
-- then computation 'hSetPosn' @p@ sets the position of @hdl@
-- to the position it held at the time of the call to 'hGetPosn'.
--
-- This operation may fail with:
--
--  * 'isPermissionError' if a system resource limit would be exceeded.

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hSetPosn :: HandlePosn -> IO ()
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hSetPosn (HandlePosn h i) = hSeek h AbsoluteSeek i

-- ---------------------------------------------------------------------------
-- hSeek

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{- Note:
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 - when seeking using `SeekFromEnd', positive offsets (>=0) means
   seeking at or past EOF.

 - we possibly deviate from the report on the issue of seeking within
   the buffer and whether to flush it or not.  The report isn't exactly
   clear here.
-}

-- | Computation 'hSeek' @hdl mode i@ sets the position of handle
-- @hdl@ depending on @mode@.
-- The offset @i@ is given in terms of 8-bit bytes.
--
-- If @hdl@ is block- or line-buffered, then seeking to a position which is not
-- in the current buffer will first cause any items in the output buffer to be
-- written to the device, and then cause the input buffer to be discarded.
-- Some handles may not be seekable (see 'hIsSeekable'), or only support a
-- subset of the possible positioning operations (for instance, it may only
-- be possible to seek to the end of a tape, or to a positive offset from
-- the beginning or current position).
-- It is not possible to set a negative I\/O position, or for
-- a physical file, an I\/O position beyond the current end-of-file.
--
-- This operation may fail with:
--
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--  * 'isIllegalOperationError' if the Handle is not seekable, or does
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--     not support the requested seek mode.
--
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--  * 'isPermissionError' if a system resource limit would be exceeded.

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hSeek :: Handle -> SeekMode -> Integer -> IO ()
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hSeek handle mode offset =
    wantSeekableHandle "hSeek" handle $ \ handle_@Handle__{..} -> do
    debugIO ("hSeek " ++ show (mode,offset))
    buf <- readIORef haCharBuffer

    if isWriteBuffer buf
        then do flushWriteBuffer handle_
                IODevice.seek haDevice mode offset
        else do

    let r = bufL buf; w = bufR buf
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    if mode == RelativeSeek && isNothing haDecoder &&
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       offset >= 0 && offset < fromIntegral (w - r)
        then writeIORef haCharBuffer buf{ bufL = r + fromIntegral offset }
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        else do
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    flushCharReadBuffer handle_
    flushByteReadBuffer handle_
    IODevice.seek haDevice mode offset


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-- | Computation 'hTell' @hdl@ returns the current position of the
-- handle @hdl@, as the number of bytes from the beginning of
-- the file.  The value returned may be subsequently passed to
-- 'hSeek' to reposition the handle to the current position.
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--
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-- This operation may fail with:
--
--  * 'isIllegalOperationError' if the Handle is not seekable.
--
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hTell :: Handle -> IO Integer
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hTell handle =
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    wantSeekableHandle "hGetPosn" handle $ \ handle_@Handle__{..} -> do

      posn <- IODevice.tell haDevice

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      -- we can't tell the real byte offset if there are buffered
      -- Chars, so must flush first:
      flushCharBuffer handle_

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      bbuf <- readIORef haByteBuffer

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      let real_posn
           | isWriteBuffer bbuf = posn + fromIntegral (bufferElems bbuf)
           | otherwise          = posn - fromIntegral (bufferElems bbuf)
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      cbuf <- readIORef haCharBuffer
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      debugIO ("\nhGetPosn: (posn, real_posn) = " ++ show (posn, real_posn))
      debugIO ("   cbuf: " ++ summaryBuffer cbuf ++
            "   bbuf: " ++ summaryBuffer bbuf)

      return real_posn

-- -----------------------------------------------------------------------------
-- Handle Properties

-- A number of operations return information about the properties of a
-- handle.  Each of these operations returns `True' if the handle has
-- the specified property, and `False' otherwise.

hIsOpen :: Handle -> IO Bool
hIsOpen handle =
    withHandle_ "hIsOpen" handle $ \ handle_ -> do
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    case haType handle_ of
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      ClosedHandle         -> return False
      SemiClosedHandle     -> return False
      _                    -> return True

hIsClosed :: Handle -> IO Bool
hIsClosed handle =
    withHandle_ "hIsClosed" handle $ \ handle_ -> do
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    case haType handle_ of
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      ClosedHandle         -> return True
      _                    -> return False

{- not defined, nor exported, but mentioned
   here for documentation purposes:

    hSemiClosed :: Handle -> IO Bool
    hSemiClosed h = do
       ho <- hIsOpen h
       hc <- hIsClosed h
       return (not (ho || hc))
-}

hIsReadable :: Handle -> IO Bool
hIsReadable (DuplexHandle _ _ _) = return True
hIsReadable handle =
    withHandle_ "hIsReadable" handle $ \ handle_ -> do
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    case haType handle_ of
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      ClosedHandle         -> ioe_closedHandle
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      SemiClosedHandle     -> ioe_semiclosedHandle
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      htype                -> return (isReadableHandleType htype)

hIsWritable :: Handle -> IO Bool
hIsWritable (DuplexHandle _ _ _) = return True
hIsWritable handle =
    withHandle_ "hIsWritable" handle $ \ handle_ -> do
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    case haType handle_ of
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      ClosedHandle         -> ioe_closedHandle
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      SemiClosedHandle     -> ioe_semiclosedHandle
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      htype                -> return (isWritableHandleType htype)

-- | Computation 'hGetBuffering' @hdl@ returns the current buffering mode
-- for @hdl@.

hGetBuffering :: Handle -> IO BufferMode
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hGetBuffering handle =
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    withHandle_ "hGetBuffering" handle $ \ handle_ -> do
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    case haType handle_ of
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      ClosedHandle         -> ioe_closedHandle
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      _ ->
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           -- We're being non-standard here, and allow the buffering
           -- of a semi-closed handle to be queried.   -- sof 6/98
          return (haBufferMode handle_)  -- could be stricter..

hIsSeekable :: Handle -> IO Bool
hIsSeekable handle =
    withHandle_ "hIsSeekable" handle $ \ handle_@Handle__{..} -> do
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    case haType of
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      ClosedHandle         -> ioe_closedHandle
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      SemiClosedHandle     -> ioe_semiclosedHandle
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      AppendHandle         -> return False
      _                    -> IODevice.isSeekable haDevice

-- -----------------------------------------------------------------------------
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-- Changing echo status
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-- | Set the echoing status of a handle connected to a terminal.

hSetEcho :: Handle -> Bool -> IO ()
hSetEcho handle on = do
    isT   <- hIsTerminalDevice handle
    if not isT
     then return ()
     else
      withHandle_ "hSetEcho" handle $ \ Handle__{..} -> do
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      case haType of
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         ClosedHandle -> ioe_closedHandle
         _            -> IODevice.setEcho haDevice on

-- | Get the echoing status of a handle connected to a terminal.

hGetEcho :: Handle -> IO Bool
hGetEcho handle = do
    isT   <- hIsTerminalDevice handle
    if not isT
     then return False
     else
       withHandle_ "hGetEcho" handle $ \ Handle__{..} -> do
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       case haType of
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         ClosedHandle -> ioe_closedHandle
         _            -> IODevice.getEcho haDevice

-- | Is the handle connected to a terminal?

hIsTerminalDevice :: Handle -> IO Bool
hIsTerminalDevice handle = do
    withHandle_ "hIsTerminalDevice" handle $ \ Handle__{..} -> do
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     case haType of
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       ClosedHandle -> ioe_closedHandle
       _            -> IODevice.isTerminal haDevice

-- -----------------------------------------------------------------------------
-- hSetBinaryMode

-- | Select binary mode ('True') or text mode ('False') on a open handle.
-- (See also 'openBinaryFile'.)
--
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-- This has the same effect as calling 'hSetEncoding' with 'char8', together
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-- with 'hSetNewlineMode' with 'noNewlineTranslation'.
--
hSetBinaryMode :: Handle -> Bool -> IO ()
hSetBinaryMode handle bin =
  withAllHandles__ "hSetBinaryMode" handle $ \ h_@Handle__{..} ->
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    do
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         flushCharBuffer h_
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         closeTextCodecs h_
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         mb_te <- if bin then return Nothing
                         else fmap Just getLocaleEncoding
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         openTextEncoding mb_te haType $ \ mb_encoder mb_decoder -> do

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         -- should match the default newline mode, whatever that is
         let nl    | bin       = noNewlineTranslation
                   | otherwise = nativeNewlineMode

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         bbuf <- readIORef haByteBuffer
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         ref <- newIORef (errorWithoutStackTrace "codec_state", bbuf)
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         return Handle__{ haLastDecode = ref,
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                          haEncoder  = mb_encoder,
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                          haDecoder  = mb_decoder,
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                          haCodec    = mb_te,
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                          haInputNL  = inputNL nl,
                          haOutputNL = outputNL nl, .. }
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-- -----------------------------------------------------------------------------
-- hSetNewlineMode

-- | Set the 'NewlineMode' on the specified 'Handle'.  All buffered
-- data is flushed first.
hSetNewlineMode :: Handle -> NewlineMode -> IO ()
hSetNewlineMode handle NewlineMode{ inputNL=i, outputNL=o } =
  withAllHandles__ "hSetNewlineMode" handle $ \h_@Handle__{..} ->
    do
         flushBuffer h_
         return h_{ haInputNL=i, haOutputNL=o }

-- -----------------------------------------------------------------------------
-- Duplicating a Handle

-- | Returns a duplicate of the original handle, with its own buffer.
-- The two Handles will share a file pointer, however.  The original
-- handle's buffer is flushed, including discarding any input data,
-- before the handle is duplicated.

hDuplicate :: Handle -> IO Handle
hDuplicate h@(FileHandle path m) = do
  withHandle_' "hDuplicate" h m $ \h_ ->
      dupHandle path h Nothing h_ (Just handleFinalizer)
hDuplicate h@(DuplexHandle path r w) = do
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  write_side@(FileHandle _ write_m) <-
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     withHandle_' "hDuplicate" h w $ \h_ ->
        dupHandle path h Nothing h_ (Just handleFinalizer)
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  read_side@(FileHandle _ read_m) <-
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    withHandle_' "hDuplicate" h r $ \h_ ->
        dupHandle path h (Just write_m) h_  Nothing
  return (DuplexHandle path read_m write_m)

dupHandle :: FilePath
          -> Handle
          -> Maybe (MVar Handle__)
          -> Handle__
          -> Maybe HandleFinalizer
          -> IO Handle
dupHandle filepath h other_side h_@Handle__{..} mb_finalizer = do
  -- flush the buffer first, so we don't have to copy its contents
  flushBuffer h_
  case other_side of
    Nothing -> do
       new_dev <- IODevice.dup haDevice
       dupHandle_ new_dev filepath other_side h_ mb_finalizer
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    Just r  ->
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       withHandle_' "dupHandle" h r $ \Handle__{haDevice=dev} -> do
         dupHandle_ dev filepath other_side h_ mb_finalizer

dupHandle_ :: (IODevice dev, BufferedIO dev, Typeable dev) => dev
           -> FilePath
           -> Maybe (MVar Handle__)
           -> Handle__
           -> Maybe HandleFinalizer
           -> IO Handle
dupHandle_ new_dev filepath other_side h_@Handle__{..} mb_finalizer = do
   -- XXX wrong!
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  mb_codec <- if isJust haEncoder then fmap Just getLocaleEncoding else return Nothing
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  mkHandle new_dev filepath haType True{-buffered-} mb_codec
      NewlineMode { inputNL = haInputNL, outputNL = haOutputNL }
      mb_finalizer other_side

-- -----------------------------------------------------------------------------
-- Replacing a Handle

{- |
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Makes the second handle a duplicate of the first handle.  The second
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handle will be closed first, if it is not already.

This can be used to retarget the standard Handles, for example:

> do h <- openFile "mystdout" WriteMode
>    hDuplicateTo h stdout
-}

hDuplicateTo :: Handle -> Handle -> IO ()
hDuplicateTo h1@(FileHandle path m1) h2@(FileHandle _ m2)  = do
 withHandle__' "hDuplicateTo" h2 m2 $ \h2_ -> do
   _ <- hClose_help h2_
   withHandle_' "hDuplicateTo" h1 m1 $ \h1_ -> do
     dupHandleTo path h1 Nothing h2_ h1_ (Just handleFinalizer)
hDuplicateTo h1@(DuplexHandle path r1 w1) h2@(DuplexHandle _ r2 w2)  = do
 withHandle__' "hDuplicateTo" h2 w2  $ \w2_ -> do
   _ <- hClose_help w2_
   withHandle_' "hDuplicateTo" h1 w1 $ \w1_ -> do
     dupHandleTo path h1 Nothing w2_ w1_ (Just handleFinalizer)
 withHandle__' "hDuplicateTo" h2 r2  $ \r2_ -> do
   _ <- hClose_help r2_
   withHandle_' "hDuplicateTo" h1 r1 $ \r1_ -> do
     dupHandleTo path h1 (Just w1) r2_ r1_ Nothing
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hDuplicateTo h1 _ =
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  ioe_dupHandlesNotCompatible h1


ioe_dupHandlesNotCompatible :: Handle -> IO a
ioe_dupHandlesNotCompatible h =
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   ioException (IOError (Just h) IllegalOperation "hDuplicateTo"
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                "handles are incompatible" Nothing Nothing)

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dupHandleTo :: FilePath
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            -> Handle
            -> Maybe (MVar Handle__)
            -> Handle__
            -> Handle__
            -> Maybe HandleFinalizer
            -> IO Handle__
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dupHandleTo filepath h other_side
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            hto_@Handle__{haDevice=devTo,..}
            h_@Handle__{haDevice=dev} mb_finalizer = do
  flushBuffer h_
  case cast devTo of
    Nothing   -> ioe_dupHandlesNotCompatible h
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    Just dev' -> do
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      _ <- IODevice.dup2 dev dev'
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      FileHandle _ m <- dupHandle_ dev' filepath other_side h_ mb_finalizer
      takeMVar m

-- ---------------------------------------------------------------------------
-- showing Handles.
--
-- | 'hShow' is in the 'IO' monad, and gives more comprehensive output
-- than the (pure) instance of 'Show' for 'Handle'.

hShow :: Handle -> IO String
hShow h@(FileHandle path _) = showHandle' path False h
hShow h@(DuplexHandle path _ _) = showHandle' path True h

showHandle' :: String -> Bool -> Handle -> IO String
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showHandle' filepath is_duplex h =
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  withHandle_ "showHandle" h $ \hdl_ ->
    let
     showType | is_duplex = showString "duplex (read-write)"
              | otherwise = shows (haType hdl_)
    in
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    return
      (( showChar '{' .
        showHdl (haType hdl_)
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            (showString "loc=" . showString filepath . showChar ',' .
             showString "type=" . showType . showChar ',' .
             showString "buffering=" . showBufMode (unsafePerformIO (readIORef (haCharBuffer hdl_))) (haBufferMode hdl_) . showString "}" )
      ) "")
   where

    showHdl :: HandleType -> ShowS -> ShowS
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    showHdl ht cont =
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       case ht of
        ClosedHandle  -> shows ht . showString "}"
        _ -> cont

    showBufMode :: Buffer e -> BufferMode -> ShowS
    showBufMode buf bmo =
      case bmo of
        NoBuffering   -> showString "none"
        LineBuffering -> showString "line"
        BlockBuffering (Just n) -> showString "block " . showParen True (shows n)
        BlockBuffering Nothing  -> showString "block " . showParen True (shows def)
      where
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       def :: Int
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       def = bufSize buf
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