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<td width="1"> </td><td class="postheader" valign="center"><a href="index.html"><font color="#004faf">Home</font></a> · <a href="classes.html"><font color="#004faf">All Classes</font></a> · <a href="functions.html"><font color="#004faf">All Functions</font></a> · <a href="overviews.html"><font color="#004faf">Overviews</font></a></td></tr></table><h1 class="title">QSemaphore Class Reference<br /><span class="small-subtitle">[<a href="qtcore.html">QtCore</a> module]</span>
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<p>The QSemaphore class provides a general counting semaphore. <a href="#details">More...</a></p>
<pre> #include <QSemaphore></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="qsemaphore-members.html">List of all members, including inherited members</a></li>
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<a name="public-functions"></a>
<h2>Public Functions</h2>
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<tr><td class="memItemLeft" align="right" valign="top"></td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#QSemaphore">QSemaphore</a></b> ( int <i>n</i> = 0 )</td></tr>
<tr><td class="memItemLeft" align="right" valign="top"></td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#dtor.QSemaphore">~QSemaphore</a></b> ()</td></tr>
<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#acquire">acquire</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft" align="right" valign="top">int </td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#available">available</a></b> () const</td></tr>
<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#release">release</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#tryAcquire">tryAcquire</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><b><a href="qsemaphore.html#tryAcquire-2">tryAcquire</a></b> ( int <i>n</i>, int <i>timeout</i> )</td></tr>
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<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); <span class="comment">// sem.available() == 5</span>
sem.acquire(3); <span class="comment">// sem.available() == 2</span>
sem.acquire(2); <span class="comment">// sem.available() == 0</span>
sem.release(5); <span class="comment">// sem.available() == 5</span>
sem.release(5); <span class="comment">// sem.available() == 10</span>
sem.tryAcquire(1); <span class="comment">// sem.available() == 9, returns true</span>
sem.tryAcquire(250); <span class="comment">// sem.available() == 9, returns false</span></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>, <a href="qthread.html">QThread</a>, and <a href="threads-semaphores.html">Semaphores Example</a>.</p>
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<h2>Member Function Documentation</h2>
<a name="//apple_ref/cpp/instm/QSemaphore/QSemaphore"></a>
<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>
<a name="//apple_ref/cpp/instm/QSemaphore/~QSemaphore"></a>
<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>
<a name="//apple_ref/cpp/instm/QSemaphore/acquire"></a>
<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>
<a name="//apple_ref/cpp/instm/QSemaphore/available"></a>
<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>
<a name="//apple_ref/cpp/instm/QSemaphore/release"></a>
<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); <span class="comment">// a semaphore that guards 5 resources</span>
sem.acquire(5); <span class="comment">// acquire all 5 resources</span>
sem.release(5); <span class="comment">// release the 5 resources</span>
sem.release(10); <span class="comment">// "create" 10 new resources</span></pre>
<p>See also <a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#available">available</a>().</p>
<a name="//apple_ref/cpp/instm/QSemaphore/tryAcquire"></a>
<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); <span class="comment">// sem.available() == 5</span>
sem.tryAcquire(250); <span class="comment">// sem.available() == 5, returns false</span>
sem.tryAcquire(3); <span class="comment">// sem.available() == 2, returns true</span></pre>
<p>See also <a href="qsemaphore.html#acquire">acquire</a>().</p>
<h3 class="fn"><a name="tryAcquire-2"></a>bool QSemaphore::tryAcquire ( int <i>n</i>, int <i>timeout</i> )</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 will wait for at most <i>timeout</i> milliseconds for resources to become available.</p>
<p>Note: Passing a negative number as the <i>timeout</i> is equivalent to calling <a href="qsemaphore.html#acquire">acquire</a>(), i.e. this function will wait forever for resources to become available if <i>timeout</i> is negative.</p>
<p>Example:</p>
<pre> QSemaphore sem(5); <span class="comment">// sem.available() == 5</span>
sem.tryAcquire(250, 1000); <span class="comment">// sem.available() == 5, waits 1000 milliseconds and returns false</span>
sem.tryAcquire(3, 30000); <span class="comment">// sem.available() == 2, returns true without waiting</span></pre>
<p>See also <a href="qsemaphore.html#acquire">acquire</a>().</p>
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