QMutex Class Reference

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). For example, say there is a method which 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;        // nubmer 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().

See also Environment Classes and Threading.

Member Function Documentation

QMutex::QMutex ( bool recursive = FALSE )

QMutex::~QMutex () [virtual]

Warning: If you destroy a mutex that still holds a lock the resultant behavior is undefined.

void QMutex::lock ()

See also unlock() and locked().

bool QMutex::locked ()

Warning: Due to differing implementations of recursive mutexes on various platforms, calling this function from the same thread that previously locked the mutex will return undefined results.

See also lock() and unlock().

bool QMutex::tryLock ()

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

See also lock(), unlock(), and locked().

void QMutex::unlock ()

See also lock() and locked().

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