The QReadWriteLock class provides read and write locking. More...

Public Types
enum	RecursionMode

Public Methods
	QReadWriteLock (RecursionMode recursionMode=NonRecursive)

	~QReadWriteLock ()

void	lockForRead ()

void	lockForWrite ()

bool	tryLockForRead ()

bool	tryLockForRead (int timeout)

bool	tryLockForWrite ()

bool	tryLockForWrite (int timeout)

void	unlock ()

Friends
class	QWaitCondition

Detailed Description

The QReadWriteLock class provides read andwrite locking. A read write lock is a synchronization tool for protecting resources that can be accessed for reading and writing. This type of lock is useful if you want to allow multiple threads to have simultaneous read only access, but as soon as one thread wants to write to the resource, all other threads must be blocked until the writing is complete.

In many cases, QReadWriteLock is a direct competitor to QMutex. QReadWriteLock is a good choice if there are many concurrent reads and writing occurs infrequently.

QReadWriteLock lock;
 
void ReaderThread::run()
{
   lock.lockForRead();
   read_file();
   lock.unlock();
 
   // do something
}
 
void WriterThread::run()
{
   lock.lockForWrite();
   write_file();
   lock.unlock();
 
   // do something
}

To ensure that writers are not blocked forever by readers, readers attempting to obtain a lock will not succeed if there is a blocked writer waiting for access, even if the lock is currently only accessed by other readers. Also, if the lock is accessed by a writer and another writer comes in, that writer will have priority over any readers that might also be waiting.

Like QMutex, a QReadWriteLock can be recursively locked by the same thread when constructed in QReadWriteLock::RecursionMode. In such cases, unlock() must be called the same number of times lockForWrite() or lockForRead() was called. The lock type can not be changed when trying to lock recursively, i.e. it is not possible to lock for reading in a thread that already has locked for writing (and vice versa).

See also: QReadLocker, QWriteLocker, QMutex, QSemaphore

Member Enumeration Documentation

enum QReadWriteLock::RecursionMode

Constant	Value	Description
`QReadWriteLock::Recursive`	`1`	In this mode, a thread can lock the same QReadWriteLock multiple times and the mutex will not be unlocked until a corresponding number of unlock() calls have been made.
`QReadWriteLock::NonRecursive`	`0`	In this mode, a thread may only lock a QReadWriteLock once.

See also: QReadWriteLock()

Constructor & Destructor Documentation

QReadWriteLock::QReadWriteLock ( RecursionMode recursionMode = NonRecursive )

explicit

Constructs a QReadWriteLock object in the given recursionMode.

See also: lockForRead(), lockForWrite(), RecursionMode

QReadWriteLock::~QReadWriteLock ( )

Destroys the QReadWriteLock object.

Warning: Destroying a read-write lock that is in use may result in undefined behavior.

Method Documentation

void QReadWriteLock::lockForRead ( )

Locks the lock for reading. This method will block the current thread if any thread (including the current) has locked for writing.

See also: unlock(), lockForWrite(), tryLockForRead()

void QReadWriteLock::lockForWrite ( )

Locks the lock for writing. This method will block the current thread if another thread has locked for reading or writing.

See also: unlock(), lockForRead(), tryLockForWrite()

bool QReadWriteLock::tryLockForRead ( )

Attempts to lock for reading. If the lock was obtained, this function returns true, otherwise it returns false instead of waiting for the lock to become available, i.e. it does not block.

The lock attempt will fail if another thread has locked for writing. If the lock was obtained, the lock must be unlocked with unlock() before another thread can successfully lock it.

See also: unlock(), lockForRead()

bool QReadWriteLock::tryLockForRead ( int timeout )

Attempts to lock for reading. This method returns true if the lock was obtained, otherwise it returns false. If another thread has locked for writing, this function will wait for at most timeout milliseconds for the lock to become available.

Passing a negative number as the timeout is equivalent to calling lockForRead(). This method will wait forever until lock can be locked for reading when timeout is negative. If the lock was obtained, the lock must be unlocked with unlock() before another thread can successfully lock it.

See also: unlock(), lockForRead()

bool QReadWriteLock::tryLockForWrite ( )

Attempts to lock for writing. If the lock was obtained, this method returns true, otherwise it returns false immediately. The lock attempt will fail if another thread has locked for reading or writing. If the lock was obtained, the lock must be unlocked with unlock() before another thread can successfully lock it.

See also: unlock(), lockForWrite()

bool QReadWriteLock::tryLockForWrite ( int timeout )

Attempts to lock for writing. This method returns true if the lock was obtained, otherwise it returns false. If another thread has locked for reading or writing, this function will wait for at most timeout milliseconds for the lock to become available.

Passing a negative number as the timeout is equivalent to calling lockForWrite(). This method will wait forever until lock can be locked for writing when timeout is negative. If the lock was obtained, the lock must be unlocked with unlock() before another thread can successfully lock it.

See also: unlock(), lockForWrite()

void QReadWriteLock::unlock ( )

Unlocks the lock. Attempting to unlock a lock that is not locked is an error, and will result in program termination.

See also: lockForRead(), lockForWrite(), tryLockForRead(), tryLockForWrite()

Public Types

Public Methods

Friends

Detailed Description

Member Enumeration Documentation

Constructor & Destructor Documentation

Method Documentation