Quoth Einar Karttunen :
| It seems that opening the same file multiple times (one writer
| and multiple readers) is not supported at least on *nix with
| GHC. I want to use one Handle to use append data till the
| end of the file while other Handles perform random access
| IO with seeks on the file.
How is it not supported? What happens with something like this
module Main (main) where
import IO (openFile, IOMode(..), SeekMode(..), hPutStr, hSeek, hGetLine)
import System (getArgs)
main = do
af <- openFile "z" AppendMode
sf <- openFile "z" ReadMode
hPutStr af "append this line\n"
hSeek sf AbsoluteSeek 40
hGetLine sf >>= print
hPutStr af "append this line\n"
hSeek sf AbsoluteSeek 60
hGetLine sf >>= print
(It works for me, with hugs on a more or less UNIX-like platform.)
Donn Cave, donn@drizzle.com
| Sharing the same Handle for all the threads is not possible
| since they perform seeks and may thus mess each other up.
| Hiding the Handle behind a mutex would limit concurrency
| more than I like.
|
| Thus I wanted to open multiple Handles to the file, but
| this seems quite hard. My best guess is to create a function
| like:
|
| #ifdef mingw32_HOST_OS
| openUnlocked fn mode = openBinaryFile fn mode
| #else
| openUnlocked fn mode = withMVar mutex $ do
| h <- openBinaryFile fn mode
| fd <- handleToFd h
| unlockFile $ fromIntegral fd
| return h
|
| {-# NOINLINE mutes #-}
| mutex = unsafePerformIO $ newMVar ()
| #endif
|
| Is there really no simpler solution?
