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RedHat 9 (Linux i386) - man page for qmutexlocker (redhat section 3qt)

QMutexLocker(3qt)								QMutexLocker(3qt)

NAME
       QMutexLocker - Simplifies locking and unlocking QMutexes

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

       #include <qmutex.h>

   Public Members
       QMutexLocker ( QMutex * mutex )
       ~QMutexLocker ()
       QMutex * mutex () const

DESCRIPTION
       The QMutexLocker class simplifies locking and unlocking QMutexes.

       The purpose of QMutexLocker is to simplify QMutex locking and unlocking. Locking and
       unlocking a QMutex in complex functions and statements or in exception handling code is
       error prone and difficult to debug. QMutexLocker should be used in such situations to
       ensure that the state of the mutex is well defined and always locked and unlocked
       properly.

       QMutexLocker should be created within a function where a QMutex needs to be locked. The
       mutex is locked when QMutexLocker is created, and unlocked when QMutexLocker is destroyed.

       For example, this complex function locks a QMutex upon entering the function and unlocks
       the mutex at all the exit points:

	   int complexFunction( int flag )
	   {
	       mutex.lock();
	       int return_value = 0;
	       switch ( flag ) {
	       case 0:
	       case 1:
		   {
		       mutex.unlock();
		       return moreComplexFunction( flag );
		   }
	       case 2:
		   {
		       int status = anotherFunction();
		       if ( status < 0 ) {
			   mutex.unlock();
			   return -2;
		       }
		       return_value = status + flag;
		       break;
		   }
	       default:
		   {
		       if ( flag > 10 ) {
			   mutex.unlock();
			   return -1;
		       }
		       break;
		   }
	       }
	       mutex.unlock();
	       return return_value;
	   }

       This example function will get more complicated as it is developed, which increases the
       likelihood that errors will occur.

       Using QMutexLocker greatly simplifies the code, and makes it more readable:

	   int complexFunction( int flag )
	   {
	       QMutexLocker locker( &mutex );
	       int return_value = 0;
	       switch ( flag ) {
	       case 0:
	       case 1:
		   {
		       return moreComplexFunction( flag );
		   }
	       case 2:
		   {
		       int status = anotherFunction();
		       if ( status < 0 )
			   return -2;
		       return_value = status + flag;
		       break;
		   }
	       default:
		   {
		       if ( flag > 10 )
			   return -1;
		       break;
		   }
	       }
	       return return_value;
	   }

       Now, the mutex will always be unlocked when the QMutexLocker object is destroyed (when the
       function returns since locker is an auto variable).

       The same principle applies to code that throws and catches exceptions. An exception that
       is not caught in the function that has locked the mutex has no way of unlocking the mutex
       before the exception is passed up the stack to the calling function.

       QMutexLocker also provides a mutex() member function that returns the mutex on which the
       QMutexLocker is operating. This is useful for code that needs access to the mutex, such as
       QWaitCondition::wait(). For example:

	   class SignalWaiter
	   {
	   private:
	       QMutexLocker locker;
	   public:
	       SignalWaiter( QMutex *mutex )
		   : locker( mutex )
	       {
	       }
	       void waitForSignal()
	       {
		   ...
		   ...
		   ...
		   while ( ! signalled )
		       waitcondition.wait( locker.mutex() );
		   ...
		   ...
		   ...
	       }
	   };

       See also QMutex, QWaitCondition, Environment Classes, and Threading.

MEMBER FUNCTION DOCUMENTATION
QMutexLocker::QMutexLocker ( QMutex * mutex )
       Constructs a QMutexLocker and locks mutex. The mutex will be unlocked when the
       QMutexLocker is destroyed. If mutex is zero, QMutexLocker does nothing.

       See also QMutex::lock().

QMutexLocker::~QMutexLocker ()
       Destroys the QMutexLocker and unlocks the mutex which was locked in the constructor.

       See also QMutexLocker::QMutexLocker() and QMutex::unlock().

QMutex * QMutexLocker::mutex () const
       Returns a pointer to the mutex which was locked in the constructor.

       See also QMutexLocker::QMutexLocker().

SEE ALSO
       http://doc.trolltech.com/qmutexlocker.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 (qmutexlocker.3qt) and the Qt version (3.1.1).

Trolltech AS				 9 December 2002			QMutexLocker(3qt)


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