Safe Haskell	Safe-Inferred
Language	Haskell2010

System.FileLock

Description

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.

Synopsis

data FileLock
data SharedExclusive
- = Shared
- | Exclusive
lockFile :: FilePath -> SharedExclusive -> IO FileLock
tryLockFile :: FilePath -> SharedExclusive -> IO (Maybe FileLock)
unlockFile :: FileLock -> IO ()
withFileLock :: FilePath -> SharedExclusive -> (FileLock -> IO a) -> IO a
withTryFileLock :: FilePath -> SharedExclusive -> (FileLock -> IO a) -> IO (Maybe a)

Documentation

data FileLock #

A token that represents ownership of a lock.

Instances

Instances details

Eq FileLock #
Instance details 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

Instances details

Eq SharedExclusive #
Instance details Defined in System.FileLock Methods (==) :: SharedExclusive -> SharedExclusive -> Bool # (/=) :: SharedExclusive -> SharedExclusive -> Bool #
Show SharedExclusive #
Instance details Defined in System.FileLock Methods showsPrec :: Int -> SharedExclusive -> ShowS # show :: SharedExclusive -> String # showList :: [SharedExclusive] -> ShowS #

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.