- Qt 5.15
- Qt Core
- C++ Classes
- QReadWriteLock
- Qt 5.15.15 Reference Documentation

# QReadWriteLock Class

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

Header: | #include <QReadWriteLock>
qmake: | QT += core

- List of all members, including inherited members

Note: All functions in this class are thread-safe.

## Public Types

enum | RecursionMode { Recursive, NonRecursive }

## Public Functions

| QReadWriteLock(RecursionMode recursionMode = NonRecursive)
| ~QReadWriteLock()
void | lockForRead()
void | lockForWrite()
bool | tryLockForRead()
bool | tryLockForRead(int timeout)
bool | tryLockForWrite()
bool | tryLockForWrite(int timeout)
void | unlock()

## Detailed Description

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.

Example:

     QReadWriteLock lock;

     void ReaderThread::run()
     {
         ...
         lock.lockForRead();
         read_file();
         lock.unlock();
         ...
     }

     void WriterThread::run()
     {
         ...
         lock.lockForWrite();
         write_file();
         lock.unlock();
         ...
     }

To ensure that writers aren't 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 with QReadWriteLock::Recursive as QReadWriteLock::RecursionMode. In such cases, unlock() must be called the same number of times lockForWrite() or lockForRead() was called. Note that the lock type cannot 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, and QSemaphore.

## Member Type Documentation

### enum QReadWriteLock::RecursionMode

Constant | Value | Description
--- | --- | ---
`QReadWriteLock::Recursive` | `1` | In this mode, a thread can lock the same QReadWriteLock multiple times. The QReadWriteLock won't 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.

This enum was introduced or modified in Qt 4.4.

See also QReadWriteLock().

## Member Function Documentation

### QReadWriteLock::QReadWriteLock(RecursionMode recursionMode = NonRecursive)

Constructs a QReadWriteLock object in the given recursionMode.

The default recursion mode is NonRecursive.

This function was introduced in Qt 4.4.

See also lockForRead(), lockForWrite(), and RecursionMode.

### QReadWriteLock::~QReadWriteLock()

Destroys the QReadWriteLock object.

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

### void QReadWriteLock::lockForRead()

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

It is not possible to lock for read if the thread already has locked for write.

See also unlock(), lockForWrite(), and tryLockForRead().

### void QReadWriteLock::lockForWrite()

Locks the lock for writing. This function will block the current thread if another thread (including the current) has locked for reading or writing (unless the lock has been created using the QReadWriteLock::Recursive mode).

It is not possible to lock for write if the thread already has locked for read.

See also unlock(), lockForRead(), and 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 for writing.

It is not possible to lock for read if the thread already has locked for write.

See also unlock() and lockForRead().

### bool QReadWriteLock::tryLockForRead(int timeout)

This is an overloaded function.

Attempts to lock for reading. This function 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.

Note: Passing a negative number as the timeout is equivalent to calling lockForRead(), i.e. this function 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 for writing.

It is not possible to lock for read if the thread already has locked for write.

See also unlock() and lockForRead().

### bool QReadWriteLock::tryLockForWrite()

Attempts to lock for writing. If the lock was obtained, this function 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.

It is not possible to lock for write if the thread already has locked for read.

See also unlock() and lockForWrite().

### bool QReadWriteLock::tryLockForWrite(int timeout)

This is an overloaded function.

Attempts to lock for writing. This function 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.

Note: Passing a negative number as the timeout is equivalent to calling lockForWrite(), i.e. this function 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.

It is not possible to lock for write if the thread already has locked for read.

See also unlock() and 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(), and tryLockForWrite().