System.FileLock

This module provides a portable interface to file locks as a mechanism for inter-process synchronization.

Each file lock is associated with a file. When taking a lock, the assiciated file is created if it's not present, then the file is locked in an OS-dependent way. While the lock is being held, no other process or thread can take it, unless the specified SharedExclusive values allow it.

All locks held by a process are released when the process exits. They can also be explicitly released using unlockFile.

It is not recommended to open or otherwise use lock files for other purposes, because it tends to expose differences between operating systems. For example, on Windows openFile for a lock file will fail when the lock is held, but on Unix it won't.

Note on the implementation: currently the module uses flock(2) on non-Windows platforms, and LockFileEx on Windows.

On non-Windows platforms, InterruptibleFFI is used in the implementation to ensures that blocking lock calls can be correctly interrupted by async exceptions (e.g. functions like timeout). This has been tested on Linux.

Documentation

data FileLock

A token that represents ownership of a lock.

Instances

Eq FileLock

Defined in System.FileLock

Methods

(==) :: FileLock -> FileLock -> Bool

(/=) :: FileLock -> FileLock -> Bool

data SharedExclusive

A type of lock to be taken.

Constructors

Shared

Other process can hold a shared lock at the same time.

Exclusive

No other process can hold a lock, shared or exclusive.

Instances

Show SharedExclusive

Defined in System.FileLock

Methods

showsPrec :: Int -> SharedExclusive -> ShowS

show :: SharedExclusive -> String

showList :: [SharedExclusive] -> ShowS

Eq SharedExclusive

Defined in System.FileLock

Methods

(==) :: SharedExclusive -> SharedExclusive -> Bool

(/=) :: SharedExclusive -> SharedExclusive -> Bool

lockFile :: FilePath -> SharedExclusive -> IO FileLock

Take a lock. This function blocks until the lock is available.

tryLockFile :: FilePath -> SharedExclusive -> IO (Maybe FileLock)

Try to take a lock. This function does not block. If the lock is not immediately available, it returns Nothing.

unlockFile :: FileLock -> IO ()

Release the lock.

withFileLock :: FilePath -> SharedExclusive -> (FileLock -> IO a) -> IO a

Perform some action with a lock held. Blocks until the lock is available.

withTryFileLock :: FilePath -> SharedExclusive -> (FileLock -> IO a) -> IO (Maybe a)

Perform sme action with a lock held. Non-blocking.