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<td align="right" valign="top" width="230"><a href="http://www.trolltech.com"><img src="images/trolltech-logo.png" align="right" width="203" height="32" border="0" /></a></td></tr></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>
<pre> #include <QMutex></pre><p><b>Note:</b> All the functions in this class are <a href="threads.html#thread-safe">thread-safe</a>.</p>
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<li><a href="qmutex-members.html">List of all members, including inherited members</a></li>
<li><a href="qmutex-qt3.html">Qt 3 support members</a></li>
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<a name="public-types"></a>
<h3>Public Types</h3>
<ul>
<li><div class="fn"/>enum <b><a href="qmutex.html#RecursionMode-enum">RecursionMode</a></b> { Recursive, NonRecursive }</li>
</ul>
<a name="public-functions"></a>
<h3>Public Functions</h3>
<ul>
<li><div class="fn"/><b><a href="qmutex.html#QMutex">QMutex</a></b> ( RecursionMode <i>mode</i> = NonRecursive )</li>
<li><div class="fn"/><b><a href="qmutex.html#dtor.QMutex">~QMutex</a></b> ()</li>
<li><div class="fn"/>void <b><a href="qmutex.html#lock">lock</a></b> ()</li>
<li><div class="fn"/>bool <b><a href="qmutex.html#tryLock">tryLock</a></b> ()</li>
<li><div class="fn"/>void <b><a href="qmutex.html#unlock">unlock</a></b> ()</li>
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<a name="details"></a>
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<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><span class="comment"> // method1()</span>
number *= 5; <span class="comment">// number is now 30</span>
number /= 4; <span class="comment">// number is now 7</span>
<span class="comment"> // method2()</span>
number *= 3; <span class="comment">// number is now 21</span>
number /= 2; <span class="comment">// number is now 10</span></pre>
<p>If these two methods are called simultaneously from two threads then the following sequence could result:</p>
<pre><span class="comment"> // Thread 1 calls method1()</span>
number *= 5; <span class="comment">// number is now 30</span>
<span class="comment"> // Thread 2 calls method2().</span>
<span class="comment"> //</span>
<span class="comment"> // Most likely Thread 1 has been put to sleep by the operating</span>
<span class="comment"> // system to allow Thread 2 to run.</span>
number *= 3; <span class="comment">// number is now 90</span>
number /= 2; <span class="comment">// number is now 45</span>
<span class="comment"> // Thread 1 finishes executing.</span>
number /= 4; <span class="comment">// number is now 11, instead of 10</span></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>Member Type Documentation</h2>
<h3 class="fn"><a name="RecursionMode-enum"></a>enum QMutex::RecursionMode</h3>
<p><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></p>
<p>See also <a href="qmutex.html#QMutex">QMutex</a>().</p>
<hr />
<h2>Member Function Documentation</h2>
<h3 class="fn"><a name="QMutex"></a>QMutex::QMutex ( <a href="qmutex.html#RecursionMode-enum">RecursionMode</a> <i>mode</i> = 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="dtor.QMutex"></a>QMutex::~QMutex ()</h3>
<p>Destroys the mutex.</p>
<p><b>Warning:</b> Destroying a locked mutex may result in undefined behavior.</p>
<h3 class="fn"><a name="lock"></a>void QMutex::lock ()</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"></a>bool QMutex::tryLock ()</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"></a>void QMutex::unlock ()</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>
<hr />
<h2>Member Function Documentation</h2>
<h3 class="fn"><a name="QMutex-2"></a>QMutex::QMutex ( bool <i>recursive</i> )</h3>
<p>Use the constructor that takes a <a href="qmutex.html#RecursionMode-enum">RecursionMode</a> parameter instead.</p>
<h3 class="fn"><a name="locked"></a>bool QMutex::locked ()</h3>
<p>Returns true if the mutex is locked by another thread; otherwise returns false.</p>
<p>It is generally a bad idea to use this function, because code that uses it has a race condition. Use <a href="qmutex.html#tryLock">tryLock</a>() and <a href="qmutex.html#unlock">unlock</a>() instead.</p>
<p>For example, if you have code like</p>
<pre><font color="#404040"> bool isLocked = mutex.locked();</font></pre>
<p>you can rewrite it as</p>
<pre> bool isLocked = true;
if (mutex.tryLock()) {
mutex.unlock();
isLocked = false;
}</pre>
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