int number = 6;

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

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

// 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

// 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

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();
}

Member Function Documentation

void QMutex::unlock()

Unlocks the mutex. Attempting to unlock a mutex in a different thread to the one that locked it results in an error. Unlocking a mutex that is not locked results in undefined behavior.

See also lock().

bool QMutex::try_lock_until(std::chrono::time_point<Clock, Duration> timePoint)

Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false. If another thread has locked the mutex, this function will wait at least until timePoint for the mutex to become available.

Note: Passing a timePoint which has already passed is equivalent to calling try_lock(). This behavior differs from tryLock().

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will always return false when attempting to lock the mutex recursively.

This documentation was introduced in Qt 5.8.

See also lock() and unlock().

bool QMutex::try_lock_for(std::chrono::duration<Rep, Period> duration)

Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false. If another thread has locked the mutex, this function will wait for at least duration for the mutex to become available.

Note: Passing a negative duration as the duration is equivalent to calling try_lock(). This behavior differs from tryLock().

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will always return false when attempting to lock the mutex recursively.

This documentation was introduced in Qt 5.8.

See also lock() and unlock().

bool QMutex::try_lock()

Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false.

This function is provided for compatibility with the Standard Library concept Lockable. It is equivalent to tryLock().

The function returns true if the lock was obtained; otherwise it returns false

This documentation was introduced in Qt 5.8.

bool QMutex::tryLock(int timeout = 0)

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

Note: Passing a negative number as the timeout is equivalent to calling lock(), i.e. this function will wait forever until mutex can be locked if timeout is negative.

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will always return false when attempting to lock the mutex recursively.

See also lock() and unlock().

void QMutex::lock()

Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will dead-lock when the mutex is locked recursively.

See also unlock().

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.