QMutex(3qt)                                                        QMutex(3qt)



NAME
       QMutex - Access serialization between threads

SYNOPSIS
       All the functions in this class are thread-safe when Qt is built with
       thread support.</p>

       #include <qmutex.h>

   Public Members
       QMutex ( bool recursive = FALSE )
       virtual ~QMutex ()
       void lock ()
       void unlock ()
       bool locked ()
       bool tryLock ()

DESCRIPTION
       The QMutex class provides access serialization between threads.

       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 )
       Constructs a new mutex. The mutex is created in an unlocked state. A
       recursive mutex is created if recursive is TRUE; a normal mutex is
       created if recursive is FALSE (the default). With a recursive mutex, a
       thread can lock the same mutex multiple times and it will not be
       unlocked until a corresponding number of unlock() calls have been made.

QMutex::~QMutex () [virtual]
       Destroys the mutex.

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

void QMutex::lock ()
       Attempt to lock the mutex. If another thread has locked the mutex then
       this call will block until that thread has unlocked it.

       See also unlock() and locked().

bool QMutex::locked ()
       Returns TRUE if the mutex is locked by another thread; otherwise
       returns FALSE.

       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 ()
       Attempt to lock the mutex. If the lock was obtained, this function
       returns TRUE. If another thread has locked the mutex, this function
       returns FALSE, instead of waiting for the mutex to become available,
       i.e. it does not block.

       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 ()
       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 behaviour (varies between different
       Operating Systems' thread implementations).

       See also lock() and locked().


SEE ALSO
       http://doc.trolltech.com/qmutex.html
       http://www.trolltech.com/faq/tech.html

COPYRIGHT
       Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com.  See the
       license file included in the distribution for a complete license
       statement.

AUTHOR
       Generated automatically from the source code.

BUGS
       If you find a bug in Qt, please report it as described in
       http://doc.trolltech.com/bughowto.html.  Good bug reports help us to
       help you. Thank you.

       The definitive Qt documentation is provided in HTML format; it is
       located at $QTDIR/doc/html and can be read using Qt Assistant or with a
       web browser. This man page is provided as a convenience for those users
       who prefer man pages, although this format is not officially supported
       by Trolltech.

       If you find errors in this manual page, please report them to qt-
       bugs@trolltech.com.  Please include the name of the manual page
       (qmutex.3) and the Qt version (3.3.8).



Trolltech AS                    2 February 2007                    QMutex(3qt)
