QMutex Class

The QMutex class provides access serialization between threads. More...

Header: #include <QMutex>
qmake: QT += core
Inherits: QBasicMutex
Inherited By:

QRecursiveMutex

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

Public Types

enum RecursionMode { Recursive, NonRecursive }

Public Functions

QMutex(QMutex::RecursionMode mode)
QMutex()
~QMutex()
bool isRecursive() const
void lock()
bool tryLock(int timeout = 0)
bool try_lock()
bool try_lock_for(std::chrono::duration<Rep, Period> duration)
bool try_lock_until(std::chrono::time_point<Clock, Duration> timePoint)
void unlock()

Detailed Description

The purpose of a QMutex is to protect an object, data structure or section of code so that only one thread can access it at a time (this is similar to the Java synchronized keyword). It is usually best to use a mutex with a QMutexLocker since this makes it easy to ensure that locking and unlocking are performed consistently.

For example, say there is a method that prints a message to the user on two lines:

int number = 6;

void method1()
{
    number *= 5;
    number /= 4;
}

void method2()
{
    number *= 3;
    number /= 2;
}

If these two methods are called in succession, the following happens:

// method1()
number *= 5;        // number is now 30
number /= 4;        // number is now 7

// method2()
number *= 3;        // number is now 21
number /= 2;        // number is now 10

If these two methods are called simultaneously from two threads then the following sequence could result:

// Thread 1 calls method1()
number *= 5;        // number is now 30

// Thread 2 calls method2().
//
// Most likely Thread 1 has been put to sleep by the operating
// system to allow Thread 2 to run.
number *= 3;        // number is now 90
number /= 2;        // number is now 45

// Thread 1 finishes executing.
number /= 4;        // number is now 11, instead of 10

If we add a mutex, we should get the result we want:

QMutex mutex;
int number = 6;

void method1()
{
    mutex.lock();
    number *= 5;
    number /= 4;
    mutex.unlock();
}

void method2()
{
    mutex.lock();
    number *= 3;
    number /= 2;
    mutex.unlock();
}

Then only one thread can modify number at any given time and the result is correct. This is a trivial example, of course, but applies to any other case where things need to happen in a particular sequence.

When you call lock() in a thread, other threads that try to call lock() in the same place will block until the thread that got the lock calls unlock(). A non-blocking alternative to lock() is tryLock().

QMutex is optimized to be fast in the non-contended case. A non-recursive QMutex will not allocate memory if there is no contention on that mutex. It is constructed and destroyed with almost no overhead, which means it is fine to have many mutexes as part of other classes.

See also QRecursiveMutex, QMutexLocker, QReadWriteLock, QSemaphore, and QWaitCondition.

Member Type Documentation

enum QMutex::RecursionMode

ConstantValueDescription
QMutex::Recursive1In this mode, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. You should use QRecursiveMutex for this use-case.
QMutex::NonRecursive0In this mode, a thread may only lock a mutex once.

See also QMutex() and QRecursiveMutex.

Member Function Documentation

QMutex::QMutex(QMutex::RecursionMode mode)

Constructs a new mutex. The mutex is created in an unlocked state.

If mode is QMutex::Recursive, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. Otherwise a thread may only lock a mutex once. The default is QMutex::NonRecursive.

Recursive mutexes are slower and take more memory than non-recursive ones.

See also lock() and unlock().

QMutex::QMutex()

Constructs a new mutex. The mutex is created in an unlocked state.

QMutex::~QMutex()

Destroys the mutex.

Warning: Destroying a locked mutex may result in undefined behavior.

bool QMutex::isRecursive() const

Returns true if the mutex is recursive.

This function was introduced in Qt 5.7.

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