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<?xml version="1.0" encoding="iso-8859-1"?>
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<html><head><title>QMutex Class Reference</title><style>h3.fn,span.fn { margin-left: 1cm; text-indent: -1cm }
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</style></head><body><table border="0" cellpadding="0" cellspacing="0" width="100%"><tr /><td align="left" valign="top" width="32"><img align="left" border="0" height="32" src="images/rb-logo.png" width="32" /></td><td width="1">  </td><td class="postheader" valign="center"><a href="../pyqt4ref.html"><font color="#004faf">Home</font></a> · <a href="classes.html"><font color="#004faf">All Classes</font></a> · <a href="modules.html"><font color="#004faf">Modules</font></a></td></table><h1 align="center">QMutex Class Reference<br /><sup><sup>[<a href="qtcore.html">QtCore</a> module]</sup></sup></h1><p>The QMutex class provides access serialization between threads. <a href="#details">More...</a></p>
<h3>Types</h3><ul><li><div class="fn" />enum <b><a href="qmutex.html#RecursionMode-enum">RecursionMode</a></b> { NonRecursive, Recursive }</li></ul><h3>Methods</h3><ul><li><div class="fn" /><b><a href="qmutex.html#QMutex">__init__</a></b> (<i>self</i>, RecursionMode <i>mode</i> = QMutex.NonRecursive)</li><li><div class="fn" /><b><a href="qmutex.html#lock">lock</a></b> (<i>self</i>)</li><li><div class="fn" />bool <b><a href="qmutex.html#tryLock">tryLock</a></b> (<i>self</i>)</li><li><div class="fn" /><b><a href="qmutex.html#unlock">unlock</a></b> (<i>self</i>)</li></ul><a name="details" /><hr /><h2>Detailed Description</h2><p>The QMutex class provides access serialization between threads.</p>
<p>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 <tt>synchronized</tt> keyword). It is usually best to use a mutex with a <a href="qmutexlocker.html">QMutexLocker</a> since this makes it easy to ensure that locking and unlocking are performed consistently.</p>
<p>For example, say there is a method that prints a message to the user on two lines:</p>
<pre>
int number = 6;
void method1()
{
number *= 5;
number /= 4;
}
void method2()
{
number *= 3;
number /= 2;
}
</pre>
<p>If these two methods are called in succession, the following happens:</p>
<pre>
// method1()
number *= 5; // number is now 30
number /= 4; // number is now 7
// method2()
number *= 3; // number is now 21
number /= 2; // number is now 10
</pre>
<p>If these two methods are called simultaneously from two threads then the following sequence could result:</p>
<pre>
// 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
</pre>
<p>If we add a mutex, we should get the result we want:</p>
<pre>
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();
}
</pre>
<p>Then only one thread can modify <tt>number</tt> 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.</p>
<p>When you call <a href="qmutex.html#lock">lock</a>() in a thread, other threads that try to call <a href="qmutex.html#lock">lock</a>() in the same place will block until the thread that got the lock calls <a href="qmutex.html#unlock">unlock</a>(). A non-blocking alternative to <a href="qmutex.html#lock">lock</a>() is <a href="qmutex.html#tryLock">tryLock</a>().</p>
<p>See also <a href="qmutexlocker.html">QMutexLocker</a>, <a href="qreadwritelock.html">QReadWriteLock</a>, <a href="qsemaphore.html">QSemaphore</a>, and <a href="qwaitcondition.html">QWaitCondition</a>.</p>
<hr /><h2>Type Documentation</h2><h3 class="fn"><a name="RecursionMode-enum" />QMutex.RecursionMode</h3><table border="1" cellpadding="2" cellspacing="1" width="100%">
<tr>
<th width="25%">Constant</th>
<th width="15%">Value</th>
<th width="60%">Description</th>
</tr>
<tr>
<td valign="top"><tt>QMutex.Recursive</tt></td>
<td align="center" valign="top"><tt>1</tt></td>
<td valign="top">In this mode, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of <a href="qmutex.html#unlock">unlock</a>() calls have been made.</td>
</tr>
<tr>
<td valign="top"><tt>QMutex.NonRecursive</tt></td>
<td align="center" valign="top"><tt>0</tt></td>
<td valign="top">In this mode, a thread may only lock a mutex once.</td>
</tr>
</table>
<br />
<br />
<p>See also <a href="qmutex.html#QMutex">QMutex</a>().</p>
<hr /><h2>Method Documentation</h2><h3 class="fn"><a name="QMutex" />QMutex.__init__ (<i>self</i>, <a href="qmutex.html#RecursionMode-enum">RecursionMode</a> <i>mode</i> = QMutex.NonRecursive)</h3><p>Constructs a new mutex. The mutex is created in an unlocked state.</p>
<p>If <i>mode</i> is <a href="qmutex.html#RecursionMode-enum">QMutex.Recursive</a>, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of <a href="qmutex.html#unlock">unlock</a>() calls have been made. The default is <a href="qmutex.html#RecursionMode-enum">QMutex.NonRecursive</a>.</p>
<p>See also <a href="qmutex.html#lock">lock</a>() and <a href="qmutex.html#unlock">unlock</a>().</p>
<h3 class="fn"><a name="lock" />QMutex.lock (<i>self</i>)</h3><p>Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it.</p>
<p>See also <a href="qmutex.html#unlock">unlock</a>().</p>
<h3 class="fn"><a name="tryLock" />bool QMutex.tryLock (<i>self</i>)</h3><p>Attempts to lock the mutex. If the lock was obtained, this function returns true. If another thread has locked the mutex, this function returns false immediately.</p>
<p>If the lock was obtained, the mutex must be unlocked with <a href="qmutex.html#unlock">unlock</a>() before another thread can successfully lock it.</p>
<p>See also <a href="qmutex.html#lock">lock</a>() and <a href="qmutex.html#unlock">unlock</a>().</p>
<h3 class="fn"><a name="unlock" />QMutex.unlock (<i>self</i>)</h3><p>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 behavior.</p>
<p>See also <a href="qmutex.html#lock">lock</a>().</p>
<address><hr /><div align="center"><table border="0" cellspacing="0" width="100%"><tr class="address"><td width="25%">PyQt 4.0.1 for X11</td><td align="center" width="50%">Copyright © <a href="http://www.riverbankcomputing.com">Riverbank Computing Ltd</a> and <a href="http://www.trolltech.com">Trolltech AS</a> 2006</td><td align="right" width="25%">Qt 4.1.4</td></tr></table></div></address></body></html>
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