System/IO.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE CPP, NoImplicitPrelude, CApiFFI #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  System.IO
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- The standard IO library.
    --
    -----------------------------------------------------------------------------

    module System.IO (
        -- * The IO monad

        IO,
        fixIO,

        -- * Files and handles

        FilePath,

        Handle,             -- abstract, instance of: Eq, Show.

        -- | GHC note: a 'Handle' will be automatically closed when the garbage
        -- collector detects that it has become unreferenced by the program.
        -- However, relying on this behaviour is not generally recommended:
        -- the garbage collector is unpredictable.  If possible, use
        -- an explicit 'hClose' to close 'Handle's when they are no longer
        -- required.  GHC does not currently attempt to free up file
        -- descriptors when they have run out, it is your responsibility to
        -- ensure that this doesn't happen.

        -- ** Standard handles

        -- | Three handles are allocated during program initialisation,
        -- and are initially open.

        stdin, stdout, stderr,

        -- * Opening and closing files

        -- ** Opening files

        withFile,
        openFile,
        IOMode(ReadMode,WriteMode,AppendMode,ReadWriteMode),

        -- ** Closing files

        hClose,

        -- ** Special cases

        -- | These functions are also exported by the "Prelude".

        readFile,
        writeFile,
        appendFile,

        -- ** File locking

        -- $locking

        -- * Operations on handles

        -- ** Determining and changing the size of a file

        hFileSize,
        hSetFileSize,

        -- ** Detecting the end of input

        hIsEOF,
        isEOF,

        -- ** Buffering operations

        BufferMode(NoBuffering,LineBuffering,BlockBuffering),
        hSetBuffering,
        hGetBuffering,
        hFlush,

        -- ** Repositioning handles

        hGetPosn,
        hSetPosn,
        HandlePosn,                -- abstract, instance of: Eq, Show.

        hSeek,
        SeekMode(AbsoluteSeek,RelativeSeek,SeekFromEnd),
        hTell,

        -- ** Handle properties

        hIsOpen, hIsClosed,
        hIsReadable, hIsWritable,
        hIsSeekable,

        -- ** Terminal operations (not portable: GHC only)

        hIsTerminalDevice,

        hSetEcho,
        hGetEcho,

        -- ** Showing handle state (not portable: GHC only)

        hShow,

        -- * Text input and output

        -- ** Text input

        hWaitForInput,
        hReady,
        hGetChar,
        hGetLine,
        hLookAhead,
        hGetContents,

        -- ** Text output

        hPutChar,
        hPutStr,
        hPutStrLn,
        hPrint,

        -- ** Special cases for standard input and output

        -- | These functions are also exported by the "Prelude".

        interact,
        putChar,
        putStr,
        putStrLn,
        print,
        getChar,
        getLine,
        getContents,
        readIO,
        readLn,

        -- * Binary input and output

        withBinaryFile,
        openBinaryFile,
        hSetBinaryMode,
        hPutBuf,
        hGetBuf,
        hGetBufSome,
        hPutBufNonBlocking,
        hGetBufNonBlocking,

        -- * Temporary files

        openTempFile,
        openBinaryTempFile,
        openTempFileWithDefaultPermissions,
        openBinaryTempFileWithDefaultPermissions,

        -- * Unicode encoding\/decoding

        -- | A text-mode 'Handle' has an associated 'TextEncoding', which
        -- is used to decode bytes into Unicode characters when reading,
        -- and encode Unicode characters into bytes when writing.
        --
        -- The default 'TextEncoding' is the same as the default encoding
        -- on your system, which is also available as 'localeEncoding'.
        -- (GHC note: on Windows, we currently do not support double-byte
        -- encodings; if the console\'s code page is unsupported, then
        -- 'localeEncoding' will be 'latin1'.)
        --
        -- Encoding and decoding errors are always detected and reported,
        -- except during lazy I/O ('hGetContents', 'getContents', and
        -- 'readFile'), where a decoding error merely results in
        -- termination of the character stream, as with other I/O errors.

        hSetEncoding,
        hGetEncoding,

        -- ** Unicode encodings
        TextEncoding,
        latin1,
        utf8, utf8_bom,
        utf16, utf16le, utf16be,
        utf32, utf32le, utf32be,
        localeEncoding,
        char8,
        mkTextEncoding,

        -- * Newline conversion

        -- | In Haskell, a newline is always represented by the character
        -- '\n'.  However, in files and external character streams, a
        -- newline may be represented by another character sequence, such
        -- as '\r\n'.
        --
        -- A text-mode 'Handle' has an associated 'NewlineMode' that
        -- specifies how to transate newline characters.  The
        -- 'NewlineMode' specifies the input and output translation
        -- separately, so that for instance you can translate '\r\n'
        -- to '\n' on input, but leave newlines as '\n' on output.
        --
        -- The default 'NewlineMode' for a 'Handle' is
        -- 'nativeNewlineMode', which does no translation on Unix systems,
        -- but translates '\r\n' to '\n' and back on Windows.
        --
        -- Binary-mode 'Handle's do no newline translation at all.
        --
        hSetNewlineMode,
        Newline(..), nativeNewline,
        NewlineMode(..),
        noNewlineTranslation, universalNewlineMode, nativeNewlineMode,
      ) where

    import Control.Exception.Base

    import Data.Bits
    import Data.Maybe
    import Foreign.C.Error
    #if defined(mingw32_HOST_OS)
    import Foreign.C.String
    #endif
    import Foreign.C.Types
    import System.Posix.Internals
    import System.Posix.Types

    import GHC.Base
    import GHC.List
    import GHC.IORef
    import GHC.Num
    import GHC.IO hiding ( bracket, onException )
    import GHC.IO.IOMode
    import GHC.IO.Handle.FD
    import qualified GHC.IO.FD as FD
    import GHC.IO.Handle
    import GHC.IO.Handle.Text ( hGetBufSome, hPutStrLn )
    import GHC.IO.Exception ( userError )
    import GHC.IO.Encoding
    import Text.Read
    import GHC.Show
    import GHC.MVar

    -- -----------------------------------------------------------------------------
    -- Standard IO

    -- | Write a character to the standard output device
    -- (same as 'hPutChar' 'stdout').

    putChar         :: Char -> IO ()
    putChar c       =  hPutChar stdout c

    -- | Write a string to the standard output device
    -- (same as 'hPutStr' 'stdout').

    putStr          :: String -> IO ()
    putStr s        =  hPutStr stdout s

    -- | The same as 'putStr', but adds a newline character.

    putStrLn        :: String -> IO ()
    putStrLn s      =  hPutStrLn stdout s

    -- | The 'print' function outputs a value of any printable type to the
    -- standard output device.
    -- Printable types are those that are instances of class 'Show'; 'print'
    -- converts values to strings for output using the 'show' operation and
    -- adds a newline.
    --
    -- For example, a program to print the first 20 integers and their
    -- powers of 2 could be written as:
    --
    -- > main = print ([(n, 2^n) | n <- [0..19]])

    print           :: Show a => a -> IO ()
    print x         =  putStrLn (show x)

    -- | Read a character from the standard input device
    -- (same as 'hGetChar' 'stdin').

    getChar         :: IO Char
    getChar         =  hGetChar stdin

    -- | Read a line from the standard input device
    -- (same as 'hGetLine' 'stdin').

    getLine         :: IO String
    getLine         =  hGetLine stdin

    -- | The 'getContents' operation returns all user input as a single string,
    -- which is read lazily as it is needed
    -- (same as 'hGetContents' 'stdin').

    getContents     :: IO String
    getContents     =  hGetContents stdin

    -- | The 'interact' function takes a function of type @String->String@
    -- as its argument.  The entire input from the standard input device is
    -- passed to this function as its argument, and the resulting string is
    -- output on the standard output device.

    interact        ::  (String -> String) -> IO ()
    interact f      =   do s <- getContents
                           putStr (f s)

    -- | The 'readFile' function reads a file and
    -- returns the contents of the file as a string.
    -- The file is read lazily, on demand, as with 'getContents'.

    readFile        :: FilePath -> IO String
    readFile name   =  openFile name ReadMode >>= hGetContents

    -- | The computation 'writeFile' @file str@ function writes the string @str@,
    -- to the file @file@.
    writeFile :: FilePath -> String -> IO ()
    writeFile f txt = withFile f WriteMode (\ hdl -> hPutStr hdl txt)

    -- | The computation 'appendFile' @file str@ function appends the string @str@,
    -- to the file @file@.
    --
    -- Note that 'writeFile' and 'appendFile' write a literal string
    -- to a file.  To write a value of any printable type, as with 'print',
    -- use the 'show' function to convert the value to a string first.
    --
    -- > main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])

    appendFile      :: FilePath -> String -> IO ()
    appendFile f txt = withFile f AppendMode (\ hdl -> hPutStr hdl txt)

    -- | The 'readLn' function combines 'getLine' and 'readIO'.

    readLn          :: Read a => IO a
    readLn          =  do l <- getLine
                          r <- readIO l
                          return r

    -- | The 'readIO' function is similar to 'read' except that it signals
    -- parse failure to the 'IO' monad instead of terminating the program.

    readIO          :: Read a => String -> IO a
    readIO s        =  case (do { (x,t) <- reads s ;
                                  ("","") <- lex t ;
                                  return x }) of
                            [x]    -> return x
                            []     -> ioError (userError "Prelude.readIO: no parse")
                            _      -> ioError (userError "Prelude.readIO: ambiguous parse")

    -- | The Unicode encoding of the current locale
    --
    -- This is the initial locale encoding: if it has been subsequently changed by
    -- 'GHC.IO.Encoding.setLocaleEncoding' this value will not reflect that change.
    localeEncoding :: TextEncoding
    localeEncoding = initLocaleEncoding

    -- | Computation 'hReady' @hdl@ indicates whether at least one item is
    -- available for input from handle @hdl@.
    --
    -- This operation may fail with:
    --
    --  * 'System.IO.Error.isEOFError' if the end of file has been reached.

    hReady          :: Handle -> IO Bool
    hReady h        =  hWaitForInput h 0

    -- | Computation 'hPrint' @hdl t@ writes the string representation of @t@
    -- given by the 'shows' function to the file or channel managed by @hdl@
    -- and appends a newline.
    --
    -- This operation may fail with:
    --
    --  * 'System.IO.Error.isFullError' if the device is full; or
    --
    --  * 'System.IO.Error.isPermissionError' if another system resource limit would be exceeded.

    hPrint          :: Show a => Handle -> a -> IO ()
    hPrint hdl      =  hPutStrLn hdl . show

    -- | @'withFile' name mode act@ opens a file using 'openFile' and passes
    -- the resulting handle to the computation @act@.  The handle will be
    -- closed on exit from 'withFile', whether by normal termination or by
    -- raising an exception.  If closing the handle raises an exception, then
    -- this exception will be raised by 'withFile' rather than any exception
    -- raised by 'act'.
    withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
    withFile name mode = bracket (openFile name mode) hClose

    -- | @'withBinaryFile' name mode act@ opens a file using 'openBinaryFile'
    -- and passes the resulting handle to the computation @act@.  The handle
    -- will be closed on exit from 'withBinaryFile', whether by normal
    -- termination or by raising an exception.
    withBinaryFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
    withBinaryFile name mode = bracket (openBinaryFile name mode) hClose

    -- ---------------------------------------------------------------------------
    -- fixIO

    -- | The implementation of 'mfix' for 'IO'. If the function passed
    -- to 'fixIO' inspects its argument, the resulting action will throw
    -- 'FixIOException'.
    fixIO :: (a -> IO a) -> IO a
    fixIO k = do
        m <- newEmptyMVar
        ans <- unsafeDupableInterleaveIO
                 (readMVar m `catch` \BlockedIndefinitelyOnMVar ->
                                        throwIO FixIOException)
        result <- k ans
        putMVar m result
        return result

    -- NOTE: we do our own explicit black holing here, because GHC's lazy
    -- blackholing isn't enough.  In an infinite loop, GHC may run the IO
    -- computation a few times before it notices the loop, which is wrong.
    --
    -- NOTE2: the explicit black-holing with an IORef ran into trouble
    -- with multiple threads (see #5421), so now we use an MVar. We used
    -- to use takeMVar with unsafeInterleaveIO. This, however, uses noDuplicate#,
    -- which is not particularly cheap. Better to use readMVar, which can be
    -- performed in multiple threads safely, and to use unsafeDupableInterleaveIO
    -- to avoid the noDuplicate cost.
    --
    -- What we'd ideally want is probably an IVar, but we don't quite have those.
    -- STM TVars look like an option at first, but I don't think they are:
    -- we'd need to be able to write to the variable in an IO context, which can
    -- only be done using 'atomically', and 'atomically' is not allowed within
    -- unsafePerformIO. We can't know if someone will try to use the result
    -- of fixIO with unsafePerformIO!
    --
    -- See also System.IO.Unsafe.unsafeFixIO.
    --

    -- | The function creates a temporary file in ReadWrite mode.
    -- The created file isn\'t deleted automatically, so you need to delete it manually.
    --
    -- The file is created with permissions such that only the current
    -- user can read\/write it.
    --
    -- With some exceptions (see below), the file will be created securely
    -- in the sense that an attacker should not be able to cause
    -- openTempFile to overwrite another file on the filesystem using your
    -- credentials, by putting symbolic links (on Unix) in the place where
    -- the temporary file is to be created.  On Unix the @O_CREAT@ and
    -- @O_EXCL@ flags are used to prevent this attack, but note that
    -- @O_EXCL@ is sometimes not supported on NFS filesystems, so if you
    -- rely on this behaviour it is best to use local filesystems only.
    --
    openTempFile :: FilePath   -- ^ Directory in which to create the file
                 -> String     -- ^ File name template. If the template is \"foo.ext\" then
                               -- the created file will be \"fooXXX.ext\" where XXX is some
                               -- random number. Note that this should not contain any path
                               -- separator characters.
                 -> IO (FilePath, Handle)
    openTempFile tmp_dir template
        = openTempFile' "openTempFile" tmp_dir template False 0o600

    -- | Like 'openTempFile', but opens the file in binary mode. See 'openBinaryFile' for more comments.
    openBinaryTempFile :: FilePath -> String -> IO (FilePath, Handle)
    openBinaryTempFile tmp_dir template
        = openTempFile' "openBinaryTempFile" tmp_dir template True 0o600

    -- | Like 'openTempFile', but uses the default file permissions
    openTempFileWithDefaultPermissions :: FilePath -> String
                                       -> IO (FilePath, Handle)
    openTempFileWithDefaultPermissions tmp_dir template
        = openTempFile' "openTempFileWithDefaultPermissions" tmp_dir template False 0o666

    -- | Like 'openBinaryTempFile', but uses the default file permissions
    openBinaryTempFileWithDefaultPermissions :: FilePath -> String
                                             -> IO (FilePath, Handle)
    openBinaryTempFileWithDefaultPermissions tmp_dir template
        = openTempFile' "openBinaryTempFileWithDefaultPermissions" tmp_dir template True 0o666

    openTempFile' :: String -> FilePath -> String -> Bool -> CMode
                  -> IO (FilePath, Handle)
    openTempFile' loc tmp_dir template binary mode
        | pathSeparator `elem` template
        = fail $ "openTempFile': Template string must not contain path separator characters: "++template
        | otherwise = findTempName
      where
        -- We split off the last extension, so we can use .foo.ext files
        -- for temporary files (hidden on Unix OSes). Unfortunately we're
        -- below filepath in the hierarchy here.
        (prefix,suffix) =
           case break (== '.') $ reverse template of
             -- First case: template contains no '.'s. Just re-reverse it.
             (rev_suffix, "")       -> (reverse rev_suffix, "")
             -- Second case: template contains at least one '.'. Strip the
             -- dot from the prefix and prepend it to the suffix (if we don't
             -- do this, the unique number will get added after the '.' and
             -- thus be part of the extension, which is wrong.)
             (rev_suffix, '.':rest) -> (reverse rest, '.':reverse rev_suffix)
             -- Otherwise, something is wrong, because (break (== '.')) should
             -- always return a pair with either the empty string or a string
             -- beginning with '.' as the second component.
             _                      -> errorWithoutStackTrace "bug in System.IO.openTempFile"

        findTempName = do
          rs <- rand_string
          let filename = prefix ++ rs ++ suffix
              filepath = tmp_dir `combine` filename
          r <- openNewFile filepath binary mode
          case r of
            FileExists -> findTempName
            OpenNewError errno -> ioError (errnoToIOError loc errno Nothing (Just tmp_dir))
            NewFileCreated fd -> do
              (fD,fd_type) <- FD.mkFD fd ReadWriteMode Nothing{-no stat-}
                                   False{-is_socket-}
                                   True{-is_nonblock-}

              enc <- getLocaleEncoding
              h <- mkHandleFromFD fD fd_type filepath ReadWriteMode False{-set non-block-} (Just enc)

              return (filepath, h)

          where
            -- XXX bits copied from System.FilePath, since that's not available here
            combine a b
                      | null b = a
                      | null a = b
                      | last a == pathSeparator = a ++ b
                      | otherwise = a ++ [pathSeparator] ++ b

    tempCounter :: IORef Int
    tempCounter = unsafePerformIO $ newIORef 0
    {-# NOINLINE tempCounter #-}

    -- build large digit-alike number
    rand_string :: IO String
    rand_string = do
      r1 <- c_getpid
      r2 <- atomicModifyIORef tempCounter (\n -> (n+1, n))
      return $ show r1 ++ "-" ++ show r2

    data OpenNewFileResult
      = NewFileCreated CInt
      | FileExists
      | OpenNewError Errno

    openNewFile :: FilePath -> Bool -> CMode -> IO OpenNewFileResult
    openNewFile filepath binary mode = do
      let oflags1 = rw_flags .|. o_EXCL

          binary_flags
            | binary    = o_BINARY
            | otherwise = 0

          oflags = oflags1 .|. binary_flags
      fd <- withFilePath filepath $ \ f ->
              c_open f oflags mode
      if fd < 0
        then do
          errno <- getErrno
          case errno of
            _ | errno == eEXIST -> return FileExists
    #if defined(mingw32_HOST_OS)
            -- If c_open throws EACCES on windows, it could mean that filepath is a
            -- directory. In this case, we want to return FileExists so that the
            -- enclosing openTempFile can try again instead of failing outright.
            -- See bug #4968.
            _ | errno == eACCES -> do
              withCString filepath $ \path -> do
                -- There is a race here: the directory might have been moved or
                -- deleted between the c_open call and the next line, but there
                -- doesn't seem to be any direct way to detect that the c_open call
                -- failed because of an existing directory.
                exists <- c_fileExists path
                return $ if exists
                  then FileExists
                  else OpenNewError errno
    #endif
            _ -> return (OpenNewError errno)
        else return (NewFileCreated fd)

    #if defined(mingw32_HOST_OS)
    foreign import ccall "file_exists" c_fileExists :: CString -> IO Bool
    #endif

    -- XXX Should use filepath library
    pathSeparator :: Char
    #if defined(mingw32_HOST_OS)
    pathSeparator = '\\'
    #else
    pathSeparator = '/'
    #endif

    -- XXX Copied from GHC.Handle
    std_flags, output_flags, rw_flags :: CInt
    std_flags    = o_NONBLOCK   .|. o_NOCTTY
    output_flags = std_flags    .|. o_CREAT
    rw_flags     = output_flags .|. o_RDWR

    -- $locking
    -- Implementations should enforce as far as possible, at least locally to the
    -- Haskell process, multiple-reader single-writer locking on files.
    -- That is, /there may either be many handles on the same file which manage input, or just one handle on the file which manages output/.  If any
    -- open or semi-closed handle is managing a file for output, no new
    -- handle can be allocated for that file.  If any open or semi-closed
    -- handle is managing a file for input, new handles can only be allocated
    -- if they do not manage output.  Whether two files are the same is
    -- implementation-dependent, but they should normally be the same if they
    -- have the same absolute path name and neither has been renamed, for
    -- example.
    --
    -- /Warning/: the 'readFile' operation holds a semi-closed handle on
    -- the file until the entire contents of the file have been consumed.
    -- It follows that an attempt to write to a file (using 'writeFile', for
    -- example) that was earlier opened by 'readFile' will usually result in
    -- failure with 'System.IO.Error.isAlreadyInUseError'.