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<h1 class="title">QMutex Class Reference</h1>
<!-- $$$QMutex-brief -->
<p>The QMutex class provides access serialization between threads. <a href="#details">More...</a></p>
<!-- @@@QMutex -->
<pre class="cpp"> <span class="preprocessor">#include <QMutex></span></pre><p><b>Note:</b> All functions in this class are <a href="threads-reentrancy.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>
<h2>Public Types</h2>
<table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> enum </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#RecursionMode-enum">RecursionMode</a></b> { Recursive, NonRecursive }</td></tr>
</table>
<a name="public-functions"></a>
<h2>Public Functions</h2>
<table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#QMutex">QMutex</a></b> ( RecursionMode <i>mode</i> = NonRecursive )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#dtor.QMutex">~QMutex</a></b> ()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#lock">lock</a></b> ()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#tryLock">tryLock</a></b> ()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#tryLock-2">tryLock</a></b> ( int <i>timeout</i> )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qmutex.html#unlock">unlock</a></b> ()</td></tr>
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<a name="details"></a>
<!-- $$$QMutex-description -->
<div class="descr">
<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 class="cpp"> <span class="type">int</span> number <span class="operator">=</span> <span class="number">6</span>;
<span class="type">void</span> method1()
{
number <span class="operator">*</span><span class="operator">=</span> <span class="number">5</span>;
number <span class="operator">/</span><span class="operator">=</span> <span class="number">4</span>;
}
<span class="type">void</span> method2()
{
number <span class="operator">*</span><span class="operator">=</span> <span class="number">3</span>;
number <span class="operator">/</span><span class="operator">=</span> <span class="number">2</span>;
}</pre>
<p>If these two methods are called in succession, the following happens:</p>
<pre class="cpp"> <span class="comment">// method1()</span>
number <span class="operator">*</span><span class="operator">=</span> <span class="number">5</span>; <span class="comment">// number is now 30</span>
number <span class="operator">/</span><span class="operator">=</span> <span class="number">4</span>; <span class="comment">// number is now 7</span>
<span class="comment">// method2()</span>
number <span class="operator">*</span><span class="operator">=</span> <span class="number">3</span>; <span class="comment">// number is now 21</span>
number <span class="operator">/</span><span class="operator">=</span> <span class="number">2</span>; <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 class="cpp"> <span class="comment">// Thread 1 calls method1()</span>
number <span class="operator">*</span><span class="operator">=</span> <span class="number">5</span>; <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 <span class="operator">*</span><span class="operator">=</span> <span class="number">3</span>; <span class="comment">// number is now 90</span>
number <span class="operator">/</span><span class="operator">=</span> <span class="number">2</span>; <span class="comment">// number is now 45</span>
<span class="comment">// Thread 1 finishes executing.</span>
number <span class="operator">/</span><span class="operator">=</span> <span class="number">4</span>; <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 class="cpp"> <span class="type">QMutex</span> mutex;
<span class="type">int</span> number <span class="operator">=</span> <span class="number">6</span>;
<span class="type">void</span> method1()
{
mutex<span class="operator">.</span>lock();
number <span class="operator">*</span><span class="operator">=</span> <span class="number">5</span>;
number <span class="operator">/</span><span class="operator">=</span> <span class="number">4</span>;
mutex<span class="operator">.</span>unlock();
}
<span class="type">void</span> method2()
{
mutex<span class="operator">.</span>lock();
number <span class="operator">*</span><span class="operator">=</span> <span class="number">3</span>;
number <span class="operator">/</span><span class="operator">=</span> <span class="number">2</span>;
mutex<span class="operator">.</span>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>
</div>
<p><b>See also </b><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>
<!-- @@@QMutex -->
<div class="types">
<h2>Member Type Documentation</h2>
<!-- $$$RecursionMode$$$NonRecursive$$$Recursive -->
<h3 class="fn"><a name="RecursionMode-enum"></a>enum QMutex::<span class="name">RecursionMode</span></h3>
<table class="valuelist"><tr valign="top" class="odd"><th class="tblConst">Constant</th><th class="tblval">Value</th><th class="tbldscr">Description</th></tr>
<tr><td class="topAlign"><tt>QMutex::Recursive</tt></td><td class="topAlign"><tt>1</tt></td><td class="topAlign">In this mode, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of <a href="qmutex.html#unlock">unlock</a>() calls have been made.</td></tr>
<tr><td class="topAlign"><tt>QMutex::NonRecursive</tt></td><td class="topAlign"><tt>0</tt></td><td class="topAlign">In this mode, a thread may only lock a mutex once.</td></tr>
</table>
<p><b>See also </b><a href="qmutex.html#QMutex">QMutex</a>().</p>
<!-- @@@RecursionMode -->
</div>
<div class="func">
<h2>Member Function Documentation</h2>
<!-- $$$QMutex[overload1]$$$QMutexRecursionMode -->
<h3 class="fn"><a name="QMutex"></a>QMutex::<span class="name">QMutex</span> ( <span class="type"><a href="qmutex.html#RecursionMode-enum">RecursionMode</a></span> <i>mode</i> = NonRecursive )</h3>
<p>Constructs a new mutex. The mutex is created in an unlocked state.</p>
<p>If <i>mode</i> is <a href="qmutex.html#RecursionMode-enum">QMutex::Recursive</a>, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of <a href="qmutex.html#unlock">unlock</a>() calls have been made. The default is <a href="qmutex.html#RecursionMode-enum">QMutex::NonRecursive</a>.</p>
<p><b>See also </b><a href="qmutex.html#lock">lock</a>() and <a href="qmutex.html#unlock">unlock</a>().</p>
<!-- @@@QMutex -->
<!-- $$$~QMutex[overload1]$$$~QMutex -->
<h3 class="fn"><a name="dtor.QMutex"></a>QMutex::<span class="name">~QMutex</span> ()</h3>
<p>Destroys the mutex.</p>
<p><b>Warning:</b> Destroying a locked mutex may result in undefined behavior.</p>
<!-- @@@~QMutex -->
<!-- $$$lock[overload1]$$$lock -->
<h3 class="fn"><a name="lock"></a><span class="type">void</span> QMutex::<span class="name">lock</span> ()</h3>
<p>Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it.</p>
<p>Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a <a href="qmutex.html#RecursionMode-enum">recursive mutex</a>. If this mutex is a <a href="qmutex.html#RecursionMode-enum">non-recursive mutex</a>, this function will <i>dead-lock</i> when the mutex is locked recursively.</p>
<p><b>See also </b><a href="qmutex.html#unlock">unlock</a>().</p>
<!-- @@@lock -->
<!-- $$$tryLock[overload1]$$$tryLock -->
<h3 class="fn"><a name="tryLock"></a><span class="type">bool</span> QMutex::<span class="name">tryLock</span> ()</h3>
<p>Attempts to lock the mutex. If the lock was obtained, this function returns true. If another thread has locked the mutex, this function returns false immediately.</p>
<p>If the lock was obtained, the mutex must be unlocked with <a href="qmutex.html#unlock">unlock</a>() before another thread can successfully lock it.</p>
<p>Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a <a href="qmutex.html#RecursionMode-enum">recursive mutex</a>. If this mutex is a <a href="qmutex.html#RecursionMode-enum">non-recursive mutex</a>, this function will <i>always</i> return false when attempting to lock the mutex recursively.</p>
<p><b>See also </b><a href="qmutex.html#lock">lock</a>() and <a href="qmutex.html#unlock">unlock</a>().</p>
<!-- @@@tryLock -->
<!-- $$$tryLock$$$tryLockint -->
<h3 class="fn"><a name="tryLock-2"></a><span class="type">bool</span> QMutex::<span class="name">tryLock</span> ( <span class="type">int</span> <i>timeout</i> )</h3>
<p>This is an overloaded function.</p>
<p>Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false. If another thread has locked the mutex, this function will wait for at most <i>timeout</i> milliseconds for the mutex to become available.</p>
<p>Note: Passing a negative number as the <i>timeout</i> is equivalent to calling <a href="qmutex.html#lock">lock</a>(), i.e. this function will wait forever until mutex can be locked if <i>timeout</i> is negative.</p>
<p>If the lock was obtained, the mutex must be unlocked with <a href="qmutex.html#unlock">unlock</a>() before another thread can successfully lock it.</p>
<p>Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a <a href="qmutex.html#RecursionMode-enum">recursive mutex</a>. If this mutex is a <a href="qmutex.html#RecursionMode-enum">non-recursive mutex</a>, this function will <i>always</i> return false when attempting to lock the mutex recursively.</p>
<p><b>See also </b><a href="qmutex.html#lock">lock</a>() and <a href="qmutex.html#unlock">unlock</a>().</p>
<!-- @@@tryLock -->
<!-- $$$unlock[overload1]$$$unlock -->
<h3 class="fn"><a name="unlock"></a><span class="type">void</span> QMutex::<span class="name">unlock</span> ()</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><b>See also </b><a href="qmutex.html#lock">lock</a>().</p>
<!-- @@@unlock -->
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