1QMutexLocker(3qt) QMutexLocker(3qt)
2
6 QMutexLocker - Simplifies locking and unlocking QMutexes
7
9 All the functions in this class are thread-safe when Qt is built with
10 thread support.</p>
11 #include <qmutex.h>
13 Public Members
15 QMutexLocker ( QMutex * mutex )
16 ~QMutexLocker ()
17 QMutex * mutex () const
18
20 The QMutexLocker class simplifies locking and unlocking QMutexes.
21 The purpose of QMutexLocker is to simplify QMutex locking and
23 unlocking. Locking and unlocking a QMutex in complex functions and
24 statements or in exception handling code is error prone and difficult
25 to debug. QMutexLocker should be used in such situations to ensure that
26 the state of the mutex is well defined and always locked and unlocked
27 properly.
28 QMutexLocker should be created within a function where a QMutex needs
30 to be locked. The mutex is locked when QMutexLocker is created, and
31 unlocked when QMutexLocker is destroyed.
32 For example, this complex function locks a QMutex upon entering the
34 function and unlocks the mutex at all the exit points:
35 int complexFunction( int flag )
37 {
38 mutex.lock();
39 int return_value = 0;
40 switch ( flag ) {
41 case 0:
42 case 1:
43 {
44 mutex.unlock();
45 return moreComplexFunction( flag );
46 }
47 case 2:
48 {
49 int status = anotherFunction();
50 if ( status < 0 ) {
51 mutex.unlock();
52 return -2;
53 }
54 return_value = status + flag;
55 break;
56 }
57 default:
58 {
59 if ( flag > 10 ) {
60 mutex.unlock();
61 return -1;
62 }
63 break;
64 }
65 }
66 mutex.unlock();
67 return return_value;
68 }
69 This example function will get more complicated as it is developed,
71 which increases the likelihood that errors will occur.
72 Using QMutexLocker greatly simplifies the code, and makes it more
74 readable:
75 int complexFunction( int flag )
77 {
78 QMutexLocker locker( &mutex );
79 int return_value = 0;
80 switch ( flag ) {
81 case 0:
82 case 1:
83 {
84 return moreComplexFunction( flag );
85 }
86 case 2:
87 {
88 int status = anotherFunction();
89 if ( status < 0 )
90 return -2;
91 return_value = status + flag;
92 break;
93 }
94 default:
95 {
96 if ( flag > 10 )
97 return -1;
98 break;
99 }
100 }
101 return return_value;
102 }
103 Now, the mutex will always be unlocked when the QMutexLocker object is
105 destroyed (when the function returns since locker is an auto variable).
106 Note that the mutex will be unlocked after the call to
107 moreComplexFunction() in this example, avoiding possible bugs caused by
108 unlocking the mutex too early, as in the first example.
109 The same principle applies to code that throws and catches exceptions.
111 An exception that is not caught in the function that has locked the
112 mutex has no way of unlocking the mutex before the exception is passed
113 up the stack to the calling function.
114 QMutexLocker also provides a mutex() member function that returns the
116 mutex on which the QMutexLocker is operating. This is useful for code
117 that needs access to the mutex, such as QWaitCondition::wait(). For
118 example:
119 class SignalWaiter
121 {
122 private:
123 QMutexLocker locker;
124 public:
125 SignalWaiter( QMutex *mutex )
126 : locker( mutex )
127 {
128 }
129 void waitForSignal()
130 {
131 ...
132 ...
133 ...
134 while ( ! signalled )
135 waitcondition.wait( locker.mutex() );
136 ...
137 ...
138 ...
139 }
140 };
141 See also QMutex, QWaitCondition, Environment Classes, and Threading.
143
146 Constructs a QMutexLocker and locks mutex. The mutex will be unlocked
147 when the QMutexLocker is destroyed. If mutex is zero, QMutexLocker does
148 nothing.
149 See also QMutex::lock().
151
153 Destroys the QMutexLocker and unlocks the mutex which was locked in the
154 constructor.
155 See also QMutexLocker::QMutexLocker() and QMutex::unlock().
157
159 Returns a pointer to the mutex which was locked in the constructor.
160 See also QMutexLocker::QMutexLocker().
162
165 http://doc.trolltech.com/qmutexlocker.html
166 http://www.trolltech.com/faq/tech.html
167
169 Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com. See the
170 license file included in the distribution for a complete license
171 statement.
172
174 Generated automatically from the source code.
175
177 If you find a bug in Qt, please report it as described in
178 http://doc.trolltech.com/bughowto.html. Good bug reports help us to
179 help you. Thank you.
180 The definitive Qt documentation is provided in HTML format; it is
182 located at $QTDIR/doc/html and can be read using Qt Assistant or with a
183 web browser. This man page is provided as a convenience for those users
184 who prefer man pages, although this format is not officially supported
185 by Trolltech.
186 If you find errors in this manual page, please report them to qt-
188 bugs@trolltech.com. Please include the name of the manual page
189 (qmutexlocker.3qt) and the Qt version (3.3.8).
190Trolltech AS 2 February 2007 QMutexLocker(3qt)