System.IO.Lock

System.IO.Lock provides thread-friendly file locks. The locking functions in System.Posix.IO (actually, it's just System.Posix.IO.waitToSetLock) will block the entire program, not just the calling thread (even with the threaded runtime). This module avoids the problem by spawning a new process for each lock and communicating with it over pipes.

Advantages:

• Only blocks the calling thread
• Works both with and without -threaded

Disadvantages:

• Forks one new process per lock
• Consumes one file descriptor per lock

Oddities:

• Closing the file descriptor doesn't affect the lock (because it's really in a separate process); you must call unLock instead.

Documentation

data LockMode

Constructors

LockRead

LockWrite

Instances

Bounded LockMode

Enum LockMode

Eq LockMode

Ord LockMode

Read LockMode

Show LockMode

Typeable LockMode

data LockDescriptor

Instances

Show LockDescriptor

Typeable LockDescriptor

setLock :: Fd -> (LockMode, SeekMode, FileOffset, FileOffset) -> IO LockDescriptor

setLock locks the specified region of the file. It blocks the calling thread until the lock is granted.

setLockAll :: Fd -> LockMode -> IO LockDescriptor

setLockAll fd lm is equivalent to setLock fd (lm, AbsoluteSeek, 0, 0). It locks the entire file, no matter how big it is.

unLock :: LockDescriptor -> IO ()

unLock destroys the given lock.