> 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
>
I was looking here:
http://www.haskell.org/ghc/docs/6.12.2/html/libraries/unix-2.4.0.1/System-Posix-IO.html
Instead of creating the handle, then converting to an fd, only to return
a new handle, why don't you start with the file descriptor and convert
at the end of the process. I don't have time for a full piece of code,
but maybe something like this:
lockFile file = do
fd <- openFd ReadOnly Nothing defaultFileFlags
waitToSetLock fd (lockType mode, AbsoluteSeek, 0, 0)
handle <- fdToHandle fd
return handle
You could also use the same sort of code in your withFileLocked function.
> According to strace, this does not involve any system-level locking with
> flock/fcntl/lockf. It is done internally to the ghc process.
>
Thanks for testing that out. I appreciate the information.
Ciao,
Jimmy