System/Posix/Internals.hs

{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE CPP, NoImplicitPrelude, CApiFFI #-}
{-# OPTIONS_HADDOCK not-home #-}

-----------------------------------------------------------------------------
-- |
-- Module : System.Posix.Internals
-- Copyright : (c) The University of Glasgow, 1992-2002
-- License : see libraries/base/LICENSE
--
-- Maintainer : cvs-ghc@haskell.org
-- Stability : internal
-- Portability : non-portable (requires POSIX)
--
-- POSIX support layer for the standard libraries.
-- This library is built on *every* platform, including Win32.
--
-- Non-posix compliant in order to support the following features:
-- * S_ISSOCK (no sockets in POSIX)
--
-----------------------------------------------------------------------------

module System.Posix.Internals where

#include "HsBaseConfig.h"

import System.Posix.Types

import Foreign
import Foreign.C

-- import Data.Bits
import Data.Maybe

#if !defined(HTYPE_TCFLAG_T)
import System.IO.Error
#endif

import GHC.Base
import GHC.Num
import GHC.Real
import GHC.IO
import GHC.IO.IOMode
import GHC.IO.Exception
import GHC.IO.Device
#if !defined(mingw32_HOST_OS)
import {-# SOURCE #-} GHC.IO.Encoding (getFileSystemEncoding)
import qualified GHC.Foreign as GHC
#endif

-- ---------------------------------------------------------------------------
-- Debugging the base package

puts :: String -> IO ()
puts s = withCAStringLen (s ++ "\n") $ \(p, len) -> do
 -- In reality should be withCString, but assume ASCII to avoid loop
 -- if this is called by GHC.Foreign
 _ <- c_write 1 (castPtr p) (fromIntegral len)
 return ()


-- ---------------------------------------------------------------------------
-- Types

data {-# CTYPE "struct flock" #-} CFLock
data {-# CTYPE "struct group" #-} CGroup
data {-# CTYPE "struct lconv" #-} CLconv
data {-# CTYPE "struct passwd" #-} CPasswd
data {-# CTYPE "struct sigaction" #-} CSigaction
data {-# CTYPE "sigset_t" #-} CSigset
data {-# CTYPE "struct stat" #-} CStat
data {-# CTYPE "struct termios" #-} CTermios
data {-# CTYPE "struct tm" #-} CTm
data {-# CTYPE "struct tms" #-} CTms
data {-# CTYPE "struct utimbuf" #-} CUtimbuf
data {-# CTYPE "struct utsname" #-} CUtsname

type FD = CInt

-- ---------------------------------------------------------------------------
-- stat()-related stuff

fdFileSize :: FD -> IO Integer
fdFileSize fd =
 allocaBytes sizeof_stat $ \ p_stat -> do
 throwErrnoIfMinus1Retry_ "fileSize" $
 c_fstat fd p_stat
 c_mode <- st_mode p_stat :: IO CMode
 if not (s_isreg c_mode)
 then return (-1)
 else do
 c_size <- st_size p_stat
 return (fromIntegral c_size)

fileType :: FilePath -> IO IODeviceType
fileType file =
 allocaBytes sizeof_stat $ \ p_stat -> do
 withFilePath file $ \p_file -> do
 throwErrnoIfMinus1Retry_ "fileType" $
 c_stat p_file p_stat
 statGetType p_stat

-- NOTE: On Win32 platforms, this will only work with file descriptors
-- referring to file handles. i.e., it'll fail for socket FDs.
fdStat :: FD -> IO (IODeviceType, CDev, CIno)
fdStat fd =
 allocaBytes sizeof_stat $ \ p_stat -> do
 throwErrnoIfMinus1Retry_ "fdType" $
 c_fstat fd p_stat
 ty <- statGetType p_stat
 dev <- st_dev p_stat
 ino <- st_ino p_stat
 return (ty,dev,ino)

fdType :: FD -> IO IODeviceType
fdType fd = do (ty,_,_) <- fdStat fd; return ty

statGetType :: Ptr CStat -> IO IODeviceType
statGetType p_stat = do
 c_mode <- st_mode p_stat :: IO CMode
 case () of
 _ | s_isdir c_mode -> return Directory
 | s_isfifo c_mode || s_issock c_mode || s_ischr c_mode
 -> return Stream
 | s_isreg c_mode -> return RegularFile
 -- Q: map char devices to RawDevice too?
 | s_isblk c_mode -> return RawDevice
 | otherwise -> ioError ioe_unknownfiletype

ioe_unknownfiletype :: IOException
ioe_unknownfiletype = IOError Nothing UnsupportedOperation "fdType"
 "unknown file type"
 Nothing
 Nothing

fdGetMode :: FD -> IO IOMode
#if defined(mingw32_HOST_OS)
fdGetMode _ = do
 -- We don't have a way of finding out which flags are set on FDs
 -- on Windows, so make a handle that thinks that anything goes.
 let flags = o_RDWR
#else
fdGetMode fd = do
 flags <- throwErrnoIfMinus1Retry "fdGetMode"
 (c_fcntl_read fd const_f_getfl)
#endif
 let
 wH = (flags .&. o_WRONLY) /= 0
 aH = (flags .&. o_APPEND) /= 0
 rwH = (flags .&. o_RDWR) /= 0

 mode
 | wH && aH = AppendMode
 | wH = WriteMode
 | rwH = ReadWriteMode
 | otherwise = ReadMode

 return mode

#if defined(mingw32_HOST_OS)
withFilePath :: FilePath -> (CWString -> IO a) -> IO a
withFilePath = withCWString

newFilePath :: FilePath -> IO CWString
newFilePath = newCWString

peekFilePath :: CWString -> IO FilePath
peekFilePath = peekCWString
#else

withFilePath :: FilePath -> (CString -> IO a) -> IO a
newFilePath :: FilePath -> IO CString
peekFilePath :: CString -> IO FilePath
peekFilePathLen :: CStringLen -> IO FilePath

withFilePath fp f = getFileSystemEncoding >>= \enc -> GHC.withCString enc fp f
newFilePath fp = getFileSystemEncoding >>= \enc -> GHC.newCString enc fp
peekFilePath fp = getFileSystemEncoding >>= \enc -> GHC.peekCString enc fp
peekFilePathLen fp = getFileSystemEncoding >>= \enc -> GHC.peekCStringLen enc fp

#endif

-- ---------------------------------------------------------------------------
-- Terminal-related stuff

#if defined(HTYPE_TCFLAG_T)

setEcho :: FD -> Bool -> IO ()
setEcho fd on = do
 tcSetAttr fd $ \ p_tios -> do
 lflag <- c_lflag p_tios :: IO CTcflag
 let new_lflag
 | on = lflag .|. fromIntegral const_echo
 | otherwise = lflag .&. complement (fromIntegral const_echo)
 poke_c_lflag p_tios (new_lflag :: CTcflag)

getEcho :: FD -> IO Bool
getEcho fd = do
 tcSetAttr fd $ \ p_tios -> do
 lflag <- c_lflag p_tios :: IO CTcflag
 return ((lflag .&. fromIntegral const_echo) /= 0)

setCooked :: FD -> Bool -> IO ()
setCooked fd cooked =
 tcSetAttr fd $ \ p_tios -> do

 -- turn on/off ICANON
 lflag <- c_lflag p_tios :: IO CTcflag
 let new_lflag | cooked = lflag .|. (fromIntegral const_icanon)
 | otherwise = lflag .&. complement (fromIntegral const_icanon)
 poke_c_lflag p_tios (new_lflag :: CTcflag)

 -- set VMIN & VTIME to 1/0 respectively
 when (not cooked) $ do
 c_cc <- ptr_c_cc p_tios
 let vmin = (c_cc `plusPtr` (fromIntegral const_vmin)) :: Ptr Word8
 vtime = (c_cc `plusPtr` (fromIntegral const_vtime)) :: Ptr Word8
 poke vmin 1
 poke vtime 0

tcSetAttr :: FD -> (Ptr CTermios -> IO a) -> IO a
tcSetAttr fd fun = do
 allocaBytes sizeof_termios $ \p_tios -> do
 throwErrnoIfMinus1Retry_ "tcSetAttr"
 (c_tcgetattr fd p_tios)

 -- Save a copy of termios, if this is a standard file descriptor.
 -- These terminal settings are restored in hs_exit().
 when (fd <= 2) $ do
 p <- get_saved_termios fd
 when (p == nullPtr) $ do
 saved_tios <- mallocBytes sizeof_termios
 copyBytes saved_tios p_tios sizeof_termios
 set_saved_termios fd saved_tios

 -- tcsetattr() when invoked by a background process causes the process
 -- to be sent SIGTTOU regardless of whether the process has TOSTOP set
 -- in its terminal flags (try it...). This function provides a
 -- wrapper which temporarily blocks SIGTTOU around the call, making it
 -- transparent.
 allocaBytes sizeof_sigset_t $ \ p_sigset -> do
 allocaBytes sizeof_sigset_t $ \ p_old_sigset -> do
 throwErrnoIfMinus1_ "sigemptyset" $
 c_sigemptyset p_sigset
 throwErrnoIfMinus1_ "sigaddset" $
 c_sigaddset p_sigset const_sigttou
 throwErrnoIfMinus1_ "sigprocmask" $
 c_sigprocmask const_sig_block p_sigset p_old_sigset
 r <- fun p_tios -- do the business
 throwErrnoIfMinus1Retry_ "tcSetAttr" $
 c_tcsetattr fd const_tcsanow p_tios
 throwErrnoIfMinus1_ "sigprocmask" $
 c_sigprocmask const_sig_setmask p_old_sigset nullPtr
 return r

foreign import ccall unsafe "HsBase.h __hscore_get_saved_termios"
 get_saved_termios :: CInt -> IO (Ptr CTermios)

foreign import ccall unsafe "HsBase.h __hscore_set_saved_termios"
 set_saved_termios :: CInt -> (Ptr CTermios) -> IO ()

#else

-- 'raw' mode for Win32 means turn off 'line input' (=> buffering and
-- character translation for the console.) The Win32 API for doing
-- this is GetConsoleMode(), which also requires echoing to be disabled
-- when turning off 'line input' processing. Notice that turning off
-- 'line input' implies enter/return is reported as '\r' (and it won't
-- report that character until another character is input..odd.) This
-- latter feature doesn't sit too well with IO actions like IO.hGetLine..
-- consider yourself warned.
setCooked :: FD -> Bool -> IO ()
setCooked fd cooked = do
 x <- set_console_buffering fd (if cooked then 1 else 0)
 if (x /= 0)
 then ioError (ioe_unk_error "setCooked" "failed to set buffering")
 else return ()

ioe_unk_error :: String -> String -> IOException
ioe_unk_error loc msg
 = ioeSetErrorString (mkIOError OtherError loc Nothing Nothing) msg

-- Note: echoing goes hand in hand with enabling 'line input' / raw-ness
-- for Win32 consoles, hence setEcho ends up being the inverse of setCooked.
setEcho :: FD -> Bool -> IO ()
setEcho fd on = do
 x <- set_console_echo fd (if on then 1 else 0)
 if (x /= 0)
 then ioError (ioe_unk_error "setEcho" "failed to set echoing")
 else return ()

getEcho :: FD -> IO Bool
getEcho fd = do
 r <- get_console_echo fd
 if (r == (-1))
 then ioError (ioe_unk_error "getEcho" "failed to get echoing")
 else return (r == 1)

foreign import ccall unsafe "consUtils.h set_console_buffering__"
 set_console_buffering :: CInt -> CInt -> IO CInt

foreign import ccall unsafe "consUtils.h set_console_echo__"
 set_console_echo :: CInt -> CInt -> IO CInt

foreign import ccall unsafe "consUtils.h get_console_echo__"
 get_console_echo :: CInt -> IO CInt

foreign import ccall unsafe "consUtils.h is_console__"
 is_console :: CInt -> IO CInt

#endif

-- ---------------------------------------------------------------------------
-- Turning on non-blocking for a file descriptor

setNonBlockingFD :: FD -> Bool -> IO ()
#if !defined(mingw32_HOST_OS)
setNonBlockingFD fd set = do
 flags <- throwErrnoIfMinus1Retry "setNonBlockingFD"
 (c_fcntl_read fd const_f_getfl)
 let flags' | set = flags .|. o_NONBLOCK
 | otherwise = flags .&. complement o_NONBLOCK
 when (flags /= flags') $ do
 -- An error when setting O_NONBLOCK isn't fatal: on some systems
 -- there are certain file handles on which this will fail (eg. /dev/null
 -- on FreeBSD) so we throw away the return code from fcntl_write.
 _ <- c_fcntl_write fd const_f_setfl (fromIntegral flags')
 return ()
#else

-- bogus defns for win32
setNonBlockingFD _ _ = return ()

#endif

-- -----------------------------------------------------------------------------
-- Set close-on-exec for a file descriptor

#if !defined(mingw32_HOST_OS)
setCloseOnExec :: FD -> IO ()
setCloseOnExec fd = do
 throwErrnoIfMinus1_ "setCloseOnExec" $
 c_fcntl_write fd const_f_setfd const_fd_cloexec
#endif

-- -----------------------------------------------------------------------------
-- foreign imports

#if !defined(mingw32_HOST_OS)
type CFilePath = CString
#else
type CFilePath = CWString
#endif

foreign import ccall unsafe "HsBase.h __hscore_open"
 c_open :: CFilePath -> CInt -> CMode -> IO CInt

foreign import ccall safe "HsBase.h __hscore_open"
 c_safe_open :: CFilePath -> CInt -> CMode -> IO CInt

foreign import ccall unsafe "HsBase.h __hscore_fstat"
 c_fstat :: CInt -> Ptr CStat -> IO CInt

foreign import ccall unsafe "HsBase.h __hscore_lstat"
 lstat :: CFilePath -> Ptr CStat -> IO CInt

{- Note: Win32 POSIX functions
Functions that are not part of the POSIX standards were
at some point deprecated by Microsoft. This deprecation
was performed by renaming the functions according to the
C++ ABI Section 17.6.4.3.2b. This was done to free up the
namespace of normal Windows programs since Windows isn't
POSIX compliant anyway.

These were working before since the RTS was re-exporting
these symbols under the undeprecated names. This is no longer
being done. See #11223

See https://msdn.microsoft.com/en-us/library/ms235384.aspx
for more.

However since we can't hope to get people to support Windows
packages we should support the deprecated names. See #12497
-}
foreign import capi unsafe "unistd.h lseek"
 c_lseek :: CInt -> COff -> CInt -> IO COff

foreign import ccall unsafe "HsBase.h access"
 c_access :: CString -> CInt -> IO CInt

foreign import ccall unsafe "HsBase.h chmod"
 c_chmod :: CString -> CMode -> IO CInt

foreign import ccall unsafe "HsBase.h close"
 c_close :: CInt -> IO CInt

foreign import ccall unsafe "HsBase.h creat"
 c_creat :: CString -> CMode -> IO CInt

foreign import ccall unsafe "HsBase.h dup"
 c_dup :: CInt -> IO CInt

foreign import ccall unsafe "HsBase.h dup2"
 c_dup2 :: CInt -> CInt -> IO CInt

foreign import ccall unsafe "HsBase.h isatty"
 c_isatty :: CInt -> IO CInt

#if defined(mingw32_HOST_OS)
-- See Note: Windows types
foreign import capi unsafe "HsBase.h _read"
 c_read :: CInt -> Ptr Word8 -> CUInt -> IO CInt

-- See Note: Windows types
foreign import capi safe "HsBase.h _read"
 c_safe_read :: CInt -> Ptr Word8 -> CUInt -> IO CInt

foreign import ccall unsafe "HsBase.h _umask"
 c_umask :: CMode -> IO CMode

-- See Note: Windows types
foreign import capi unsafe "HsBase.h _write"
 c_write :: CInt -> Ptr Word8 -> CUInt -> IO CInt

-- See Note: Windows types
foreign import capi safe "HsBase.h _write"
 c_safe_write :: CInt -> Ptr Word8 -> CUInt -> IO CInt

foreign import ccall unsafe "HsBase.h _pipe"
 c_pipe :: Ptr CInt -> IO CInt
#else
-- We use CAPI as on some OSs (eg. Linux) this is wrapped by a macro
-- which redirects to the 64-bit-off_t versions when large file
-- support is enabled.

-- See Note: Windows types
foreign import capi unsafe "HsBase.h read"
 c_read :: CInt -> Ptr Word8 -> CSize -> IO CSsize

-- See Note: Windows types
foreign import capi safe "HsBase.h read"
 c_safe_read :: CInt -> Ptr Word8 -> CSize -> IO CSsize

foreign import ccall unsafe "HsBase.h umask"
 c_umask :: CMode -> IO CMode

-- See Note: Windows types
foreign import capi unsafe "HsBase.h write"
 c_write :: CInt -> Ptr Word8 -> CSize -> IO CSsize

-- See Note: Windows types
foreign import capi safe "HsBase.h write"
 c_safe_write :: CInt -> Ptr Word8 -> CSize -> IO CSsize

foreign import ccall unsafe "HsBase.h pipe"
 c_pipe :: Ptr CInt -> IO CInt
#endif

foreign import ccall unsafe "HsBase.h unlink"
 c_unlink :: CString -> IO CInt

foreign import capi unsafe "HsBase.h utime"
 c_utime :: CString -> Ptr CUtimbuf -> IO CInt

foreign import ccall unsafe "HsBase.h getpid"
 c_getpid :: IO CPid

foreign import ccall unsafe "HsBase.h __hscore_stat"
 c_stat :: CFilePath -> Ptr CStat -> IO CInt

foreign import ccall unsafe "HsBase.h __hscore_ftruncate"
 c_ftruncate :: CInt -> COff -> IO CInt

#if !defined(mingw32_HOST_OS)
foreign import capi unsafe "HsBase.h fcntl"
 c_fcntl_read :: CInt -> CInt -> IO CInt

foreign import capi unsafe "HsBase.h fcntl"
 c_fcntl_write :: CInt -> CInt -> CLong -> IO CInt

foreign import capi unsafe "HsBase.h fcntl"
 c_fcntl_lock :: CInt -> CInt -> Ptr CFLock -> IO CInt

foreign import ccall unsafe "HsBase.h fork"
 c_fork :: IO CPid

foreign import ccall unsafe "HsBase.h link"
 c_link :: CString -> CString -> IO CInt

-- capi is required at least on Android
foreign import capi unsafe "HsBase.h mkfifo"
 c_mkfifo :: CString -> CMode -> IO CInt

foreign import capi unsafe "signal.h sigemptyset"
 c_sigemptyset :: Ptr CSigset -> IO CInt

foreign import capi unsafe "signal.h sigaddset"
 c_sigaddset :: Ptr CSigset -> CInt -> IO CInt

foreign import capi unsafe "signal.h sigprocmask"
 c_sigprocmask :: CInt -> Ptr CSigset -> Ptr CSigset -> IO CInt

-- capi is required at least on Android
foreign import capi unsafe "HsBase.h tcgetattr"
 c_tcgetattr :: CInt -> Ptr CTermios -> IO CInt

-- capi is required at least on Android
foreign import capi unsafe "HsBase.h tcsetattr"
 c_tcsetattr :: CInt -> CInt -> Ptr CTermios -> IO CInt

foreign import ccall unsafe "HsBase.h waitpid"
 c_waitpid :: CPid -> Ptr CInt -> CInt -> IO CPid
#endif

-- POSIX flags only:
foreign import ccall unsafe "HsBase.h __hscore_o_rdonly" o_RDONLY :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_wronly" o_WRONLY :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_rdwr" o_RDWR :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_append" o_APPEND :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_creat" o_CREAT :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_excl" o_EXCL :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_trunc" o_TRUNC :: CInt

-- non-POSIX flags.
foreign import ccall unsafe "HsBase.h __hscore_o_noctty" o_NOCTTY :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_nonblock" o_NONBLOCK :: CInt
foreign import ccall unsafe "HsBase.h __hscore_o_binary" o_BINARY :: CInt

foreign import capi unsafe "sys/stat.h S_ISREG" c_s_isreg :: CMode -> CInt
foreign import capi unsafe "sys/stat.h S_ISCHR" c_s_ischr :: CMode -> CInt
foreign import capi unsafe "sys/stat.h S_ISBLK" c_s_isblk :: CMode -> CInt
foreign import capi unsafe "sys/stat.h S_ISDIR" c_s_isdir :: CMode -> CInt
foreign import capi unsafe "sys/stat.h S_ISFIFO" c_s_isfifo :: CMode -> CInt

s_isreg :: CMode -> Bool
s_isreg cm = c_s_isreg cm /= 0
s_ischr :: CMode -> Bool
s_ischr cm = c_s_ischr cm /= 0
s_isblk :: CMode -> Bool
s_isblk cm = c_s_isblk cm /= 0
s_isdir :: CMode -> Bool
s_isdir cm = c_s_isdir cm /= 0
s_isfifo :: CMode -> Bool
s_isfifo cm = c_s_isfifo cm /= 0

foreign import ccall unsafe "HsBase.h __hscore_sizeof_stat" sizeof_stat :: Int
foreign import ccall unsafe "HsBase.h __hscore_st_mtime" st_mtime :: Ptr CStat -> IO CTime
#if defined(mingw32_HOST_OS)
foreign import ccall unsafe "HsBase.h __hscore_st_size" st_size :: Ptr CStat -> IO Int64
#else
foreign import ccall unsafe "HsBase.h __hscore_st_size" st_size :: Ptr CStat -> IO COff
#endif
foreign import ccall unsafe "HsBase.h __hscore_st_mode" st_mode :: Ptr CStat -> IO CMode
foreign import ccall unsafe "HsBase.h __hscore_st_dev" st_dev :: Ptr CStat -> IO CDev
foreign import ccall unsafe "HsBase.h __hscore_st_ino" st_ino :: Ptr CStat -> IO CIno

foreign import ccall unsafe "HsBase.h __hscore_echo" const_echo :: CInt
foreign import ccall unsafe "HsBase.h __hscore_tcsanow" const_tcsanow :: CInt
foreign import ccall unsafe "HsBase.h __hscore_icanon" const_icanon :: CInt
foreign import ccall unsafe "HsBase.h __hscore_vmin" const_vmin :: CInt
foreign import ccall unsafe "HsBase.h __hscore_vtime" const_vtime :: CInt
foreign import ccall unsafe "HsBase.h __hscore_sigttou" const_sigttou :: CInt
foreign import ccall unsafe "HsBase.h __hscore_sig_block" const_sig_block :: CInt
foreign import ccall unsafe "HsBase.h __hscore_sig_setmask" const_sig_setmask :: CInt
foreign import ccall unsafe "HsBase.h __hscore_f_getfl" const_f_getfl :: CInt
foreign import ccall unsafe "HsBase.h __hscore_f_setfl" const_f_setfl :: CInt
foreign import ccall unsafe "HsBase.h __hscore_f_setfd" const_f_setfd :: CInt
foreign import ccall unsafe "HsBase.h __hscore_fd_cloexec" const_fd_cloexec :: CLong

#if defined(HTYPE_TCFLAG_T)
foreign import ccall unsafe "HsBase.h __hscore_sizeof_termios" sizeof_termios :: Int
foreign import ccall unsafe "HsBase.h __hscore_sizeof_sigset_t" sizeof_sigset_t :: Int

foreign import ccall unsafe "HsBase.h __hscore_lflag" c_lflag :: Ptr CTermios -> IO CTcflag
foreign import ccall unsafe "HsBase.h __hscore_poke_lflag" poke_c_lflag :: Ptr CTermios -> CTcflag -> IO ()
foreign import ccall unsafe "HsBase.h __hscore_ptr_c_cc" ptr_c_cc :: Ptr CTermios -> IO (Ptr Word8)
#endif

s_issock :: CMode -> Bool
#if !defined(mingw32_HOST_OS)
s_issock cmode = c_s_issock cmode /= 0
foreign import capi unsafe "sys/stat.h S_ISSOCK" c_s_issock :: CMode -> CInt
#else
s_issock _ = False
#endif

foreign import ccall unsafe "__hscore_bufsiz" dEFAULT_BUFFER_SIZE :: Int
foreign import capi unsafe "stdio.h value SEEK_CUR" sEEK_CUR :: CInt
foreign import capi unsafe "stdio.h value SEEK_SET" sEEK_SET :: CInt
foreign import capi unsafe "stdio.h value SEEK_END" sEEK_END :: CInt

{-
Note: Windows types

Windows' _read and _write have types that differ from POSIX. They take an
unsigned int for lengh and return a signed int where POSIX uses size_t and
ssize_t. Those are different on x86_64 and equivalent on x86. We import them
with the types in Microsoft's documentation which means that c_read,
c_safe_read, c_write and c_safe_write have different Haskell types depending on
the OS.
-}