Control.Concurrent.ReadWriteLock

Multiple-reader, single-writer locks. Used to protect shared resources which may be concurrently read, but only sequentially written.

All functions are exception safe. Throwing asynchronous exceptions will not compromise the internal state of an RWLock. This means it is perfectly safe to kill a thread that is blocking on, for example, acquireRead.

See also Java's version: http://java.sun.com/javase/7/docs/api/java/util/concurrent/locks/ReadWriteLock.html

This module is designed to be imported qualified. We suggest importing it like:

import           Control.Concurrent.ReadWriteLock        ( RWLock )
import qualified Control.Concurrent.ReadWriteLock as RWL ( ... )

Documentation

data RWLock

Multiple-reader, single-writer lock. Is in one of three states:

• "Free": Read or write access can be acquired without blocking.
• "Read": One or more threads have acquired read access. Blocks write access.
• "Write": A single thread has acquired write access. Blocks other threads from acquiring both read and write access.

Instances

Eq RWLock

Methods

(==) :: RWLock -> RWLock -> Bool

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

Creating Read-Write Locks

new :: IO RWLock

Create a new RWLock in the "free" state; either read or write access can be acquired without blocking.

newAcquiredRead :: IO RWLock

Create a new RWLock in the "read" state; only read can be acquired without blocking.

newAcquiredWrite :: IO RWLock

Create a new RWLock in the "write" state; either acquiring read or write will block.

Read access

Blocking

acquireRead :: RWLock -> IO ()

Acquire the read lock.

Blocks if another thread has acquired write access. If acquireRead terminates without throwing an exception the state of the RWLock will be "read".

Implementation note: Throws an exception when more than (maxBound :: Int) simultaneous threads acquire the read lock. But that is unlikely.

releaseRead :: RWLock -> IO ()

Release the read lock.

If the calling thread was the last one to relinquish read access the state will revert to "free".

It is an error to release read access to an RWLock which is not in the "read" state.

withRead :: RWLock -> IO a -> IO a

A convenience function wich first acquires read access and then performs the computation. When the computation terminates, whether normally or by raising an exception, the read lock is released.

waitRead :: RWLock -> IO ()

• When the state is "write", waitRead blocks until a call to releaseWrite in another thread changes the state to "free".
• When the state is "free" or "read" waitRead returns immediately.

waitRead does not alter the state of the lock.

Note that waitRead is just a convenience function defined as:

waitRead l = mask_ $ acquireRead l >> releaseRead l

Non-blocking

tryAcquireRead :: RWLock -> IO Bool

Try to acquire the read lock; non blocking.

Like acquireRead, but doesn't block. Returns True if the resulting state is "read", False otherwise.

tryWithRead :: RWLock -> IO a -> IO (Maybe a)

A non-blocking withRead. First tries to acquire the lock. If that fails, Nothing is returned. If it succeeds, the computation is performed. When the computation terminates, whether normally or by raising an exception, the lock is released and Just the result of the computation is returned.

Write access

Blocking

acquireWrite :: RWLock -> IO ()

Acquire the write lock.

Blocks if another thread has acquired either read or write access. If acquireWrite terminates without throwing an exception the state of the RWLock will be "write".

releaseWrite :: RWLock -> IO ()

Release the write lock.

If releaseWrite terminates without throwing an exception the state will be "free".

It is an error to release write access to an RWLock which is not in the "write" state.

withWrite :: RWLock -> IO a -> IO a

A convenience function wich first acquires write access and then performs the computation. When the computation terminates, whether normally or by raising an exception, the write lock is released.

waitWrite :: RWLock -> IO ()

• When the state is "write" or "read" waitWrite blocks until a call to releaseWrite or releaseRead in another thread changes the state to "free".
• When the state is "free" waitWrite returns immediately.

waitWrite does not alter the state of the lock.

Note that waitWrite is just a convenience function defined as:

waitWrite l = mask_ $ acquireWrite l >> releaseWrite l

Non-blocking

tryAcquireWrite :: RWLock -> IO Bool

Try to acquire the write lock; non blocking.

Like acquireWrite, but doesn't block. Returns True if the resulting state is "write", False otherwise.

tryWithWrite :: RWLock -> IO a -> IO (Maybe a)

A non-blocking withWrite. First tries to acquire the lock. If that fails, Nothing is returned. If it succeeds, the computation is performed. When the computation terminates, whether normally or by raising an exception, the lock is released and Just the result of the computation is returned.