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 (In Java terms, this is similar to the synchronized keyword). For example, say there is a method which prints a message to the user on two lines:

    void someMethod()
    {
        qDebug("Hello");
        qDebug("World");
    }
    

If this method is called simultaneously from two threads then the following sequence could result:

    Hello
    Hello
    World
    World
    

If we add a mutex:

    QMutex mutex;

    void someMethod()
    {
        mutex.lock();
        qDebug("Hello");
        qDebug("World");
        mutex.unlock();
    }
    

In Java terms this would be:

    void someMethod()
    {
        synchronized {
        qDebug("Hello");
        qDebug("World");
        }
    }
    

Then only one thread can execute someMethod() at a time and the order of messages is always 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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