System.IO.Lock

Description

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.

Synopsis

Documentation

data LockMode Source

Constructors

LockRead
LockWrite

Instances

Bounded LockMode
Enum LockMode
Eq LockMode
Ord LockMode
Read LockMode
Show LockMode
Typeable LockMode

data LockDescriptor Source

Instances

Show LockDescriptor
Typeable LockDescriptor

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

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

setLockAll :: Fd -> LockMode -> IO LockDescriptor Source

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 ()Source

unLock destroys the given lock.