[Haskell-beginners] Re: how to read file with locking

[Joey Hess]

Ertugrul Soeylemez wrote:
>   readFileLocked :: FilePath -> IO B.ByteString
>   readFileLocked fn =
>     withFile fn ReadMode $ \h -> do
>       lockFile h  -- for a suitable function lockFile
>       B.hGetContents h
> 
> It is, BTW, always preferable to use withFile over openFile, if you can.
> This makes your code cleaner and also exception-safe.

Unless there is a better locking primative than waitToSetLock available,
I don't know how to build your lockFile function. It seems that it would
have a side effect of closing the handle. It could return a new handle
like this, but then withFile's automatic close of the file would be
defeated.

lockFile h = do
	lockfd <- handleToFd h -- closes h
	waitToSetLock lockfd (lockType mode, AbsoluteSeek, 0, 0)
	newh <- fdToHandle lockfd
	return newh

Here's what I'm using instead.

withFileLocked file mode action = do
	-- TODO: find a way to use bracket here
        handle <- openFile file mode
        lockfd <- handleToFd handle -- closes handle
        waitToSetLock lockfd (lockType mode, AbsoluteSeek, 0, 0)
        handle' <- fdToHandle lockfd
        ret <- action handle'
        hClose handle'
        return ret
                where
                        lockType ReadMode = ReadLock
                        lockType _ = WriteLock

BTW, thanks for the hint that ByteString has a strict getContents! I was
prototyping my code with String and thought I'd worry about optimisation
later, but that is a good reason to use ByteString up front.

Jimmy Wylie wrote:
> Implementations should enforce as far as possible, at least locally to the
> Haskell process, multiple-reader single-writer locking on files.

According to strace, this does not involve any system-level locking with
flock/fcntl/lockf. It is done internally to the ghc process.

-- 
see shy jo
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 828 bytes
Desc: Digital signature
Url : http://www.haskell.org/pipermail/beginners/attachments/20101010/300232c5/attachment.bin