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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 method1()
	   {
	       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 1992-2001 Trolltech AS, 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.3qt) and the Qt version (3.1.1).

Trolltech AS				 9 December 2002			      QMutex(3qt)
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