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<td align="right" valign="top" width="230"><img src="images/trolltech-logo.png" align="right" width="203" height="32" border="0" /></td></tr></table><h1 align="center">QSemaphore Class Reference</h1>

<p>The QSemaphore class provides a general counting semaphore. <a href="#details">More...</a></p>

<pre>#include <QSemaphore></pre><p>Part of the <a href="qtcore.html">QtCore</a> module.</p>

<p><b>Note:</b> All the functions in this class are <a href="threads.html#thread-safe">thread-safe</a>.</p>

<ul>

<li><a href="qsemaphore-members.html">List of all members, including inherited members</a></li>

</ul>

<h3>Public Functions</h3>

<ul>

<li><div class="fn"/><b><a href="qsemaphore.html#QSemaphore">QSemaphore</a></b> ( int <i>n</i> = 0 )</li>

<li><div class="fn"/><b><a href="qsemaphore.html#dtor.QSemaphore">~QSemaphore</a></b> ()</li>

<li><div class="fn"/>void <b><a href="qsemaphore.html#acquire">acquire</a></b> ( int <i>n</i> = 1 )</li>

<li><div class="fn"/>int <b><a href="qsemaphore.html#available">available</a></b> () const</li>

<li><div class="fn"/>void <b><a href="qsemaphore.html#release">release</a></b> ( int <i>n</i> = 1 )</li>

<li><div class="fn"/>bool <b><a href="qsemaphore.html#tryAcquire">tryAcquire</a></b> ( int <i>n</i> = 1 )</li>

</ul>

<hr />

<h2>Detailed Description</h2>

<p>The QSemaphore class provides a general counting semaphore.</p>

<p>A semaphore is a generalization of a mutex. While a mutex can only be locked once, it's possible to acquire a semaphore multiple times. Semaphores are typically used to protect a certain number of identical resources.</p>

<p>Semaphores support two fundamental operations, <a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#release">release</a>():</p>

<ul>

<li>acquire(<i>n</i>) tries to acquire <i>n</i> resources. If there aren't that many resources available, the call will block until this is the case.</li>

<li>release(<i>n</i>) releases <i>n</i> resources.</li>

</ul>

<p>There's also a <a href="qsemaphore.html#tryAcquire">tryAcquire</a>() function that returns immediately if it cannot acquire the resources, and an <a href="qsemaphore.html#available">available</a>() function that returns the number of available resources at any time.</p>

<p>Example:</p>

<pre>  QSemaphore sem(5); // sem.available() == 5

sem.acquire(3); // sem.available() == 2

sem.acquire(2); // sem.available() == 0

sem.release(5); // sem.available() == 5

sem.release(5); // sem.available() == 10

sem.tryAcquire(1); // sem.available() == 9, returns true

sem.tryAcquire(250); // sem.available() == 9, returns false</pre>

<p>A typical application of semaphores is for controlling access to a circular buffer shared by a producer thread and a consumer thread. The <a href="threads-semaphores.html">Semaphores</a> example shows how to use QSemaphore to solve that problem.</p>

<p>A non-computing example of a semaphore would be dining at a restaurant. A semaphore is initialized with the number of chairs in the restaurant. As people arrive, they want a seat. As seats are filled, <a href="qsemaphore.html#available">available</a>() is decremented. As people leave, the <a href="qsemaphore.html#available">available</a>() is incremented, allowing more people to enter. If a party of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but a party of 4 people would be seated (taking the available seats to 5, making the party of 10 people wait longer).</p>

<p>See also <a href="qmutex.html">QMutex</a>, <a href="qwaitcondition.html">QWaitCondition</a>, and <a href="qthread.html">QThread</a>.</p>

<hr />

<h2>Member Function Documentation</h2>

<h3 class="fn"><a name="QSemaphore"></a>QSemaphore::QSemaphore ( int <i>n</i> = 0 )</h3>

<p>Creates a new semaphore and initializes the number of resources it guards to <i>n</i> (by default, 0).</p>

<p>See also <a href="qsemaphore.html#release">release</a>() and <a href="qsemaphore.html#available">available</a>().</p>

<h3 class="fn"><a name="dtor.QSemaphore"></a>QSemaphore::~QSemaphore ()</h3>

<p>Destroys the semaphore.</p>

<p><b>Warning:</b> Destroying a semaphore that is in use may result in undefined behavior.</p>

<h3 class="fn"><a name="acquire"></a>void QSemaphore::acquire ( int <i>n</i> = 1 )</h3>

<p>Tries to acquire <tt>n</tt> resources guarded by the semaphore. If <i>n</i> > <a href="qsemaphore.html#available">available</a>(), this call will block until enough resources are available.</p>

<p>See also <a href="qsemaphore.html#release">release</a>(), <a href="qsemaphore.html#available">available</a>(), and <a href="qsemaphore.html#tryAcquire">tryAcquire</a>().</p>

<h3 class="fn"><a name="available"></a>int QSemaphore::available () const</h3>

<p>Returns the number of resources currently available to the semaphore. This number can never be negative.</p>

<p>See also <a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#release">release</a>().</p>

<h3 class="fn"><a name="release"></a>void QSemaphore::release ( int <i>n</i> = 1 )</h3>

<p>Releases <i>n</i> resources guarded by the semaphore.</p>

<p>This function can be used to "create" resources as well. For example:</p>

<pre>  QSemaphore sem(5); // a semaphore that guards 5 resources

sem.acquire(5); // acquire all 5 resources

sem.release(5); // release the 5 resources

sem.release(10); // "create" 10 new resources</pre>

<p>See also <a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#available">available</a>().</p>

<h3 class="fn"><a name="tryAcquire"></a>bool QSemaphore::tryAcquire ( int <i>n</i> = 1 )</h3>

<p>Tries to acquire <tt>n</tt> resources guarded by the semaphore and returns true on success. If <a href="qsemaphore.html#available">available</a>() < <i>n</i>, this call immediately returns false without acquiring any resources.</p>

<p>Example:</p>

<pre>  QSemaphore sem(5); // sem.available() == 5

sem.tryAcquire(250); // sem.available() == 5, returns false

sem.tryAcquire(3); // sem.available() == 2, returns true</pre>

<p>See also <a href="qsemaphore.html#acquire">acquire</a>().</p>

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