<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en_US" lang="en_US">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<!-- qsemaphore.cpp -->
<title>Qt 4.8: QSemaphore Class Reference</title>
<link rel="stylesheet" type="text/css" href="style/style.css" />
<script src="scripts/jquery.js" type="text/javascript"></script>
<script src="scripts/functions.js" type="text/javascript"></script>
<link rel="stylesheet" type="text/css" href="style/superfish.css" />
<link rel="stylesheet" type="text/css" href="style/narrow.css" />
<!--[if IE]>
<meta name="MSSmartTagsPreventParsing" content="true">
<meta http-equiv="imagetoolbar" content="no">
<![endif]-->
<!--[if lt IE 7]>
<link rel="stylesheet" type="text/css" href="style/style_ie6.css">
<![endif]-->
<!--[if IE 7]>
<link rel="stylesheet" type="text/css" href="style/style_ie7.css">
<![endif]-->
<!--[if IE 8]>
<link rel="stylesheet" type="text/css" href="style/style_ie8.css">
<![endif]-->
<script src="scripts/superfish.js" type="text/javascript"></script>
<script src="scripts/narrow.js" type="text/javascript"></script>
</head>
<body class="" onload="CheckEmptyAndLoadList();">
<div class="header" id="qtdocheader">
<div class="content">
<div id="nav-logo">
<a href="index.html">Home</a></div>
<a href="index.html" class="qtref"><span>Qt Reference Documentation</span></a>
<div id="narrowsearch"></div>
<div id="nav-topright">
<ul>
<li class="nav-topright-home"><a href="http://qt.digia.com/">Qt HOME</a></li>
<li class="nav-topright-dev"><a href="http://qt-project.org/">DEV</a></li>
<li class="nav-topright-doc nav-topright-doc-active"><a href="http://qt-project.org/doc/">
DOC</a></li>
<li class="nav-topright-blog"><a href="http://blog.qt.digia.com/">BLOG</a></li>
</ul>
</div>
<div id="shortCut">
<ul>
<li class="shortCut-topleft-inactive"><span><a href="index.html">Qt 4.8</a></span></li>
<li class="shortCut-topleft-active"><a href="http://qt-project.org/doc/">ALL VERSIONS </a></li>
</ul>
</div>
<ul class="sf-menu" id="narrowmenu">
<li><a href="#">API Lookup</a>
<ul>
<li><a href="classes.html">Class index</a></li>
<li><a href="functions.html">Function index</a></li>
<li><a href="modules.html">Modules</a></li>
<li><a href="namespaces.html">Namespaces</a></li>
<li><a href="qtglobal.html">Global Declarations</a></li>
<li><a href="qdeclarativeelements.html">QML elements</a></li>
</ul>
</li>
<li><a href="#">Qt Topics</a>
<ul>
<li><a href="qt-basic-concepts.html">Programming with Qt</a></li>
<li><a href="qtquick.html">Device UIs & Qt Quick</a></li>
<li><a href="qt-gui-concepts.html">UI Design with Qt</a></li>
<li><a href="supported-platforms.html">Supported Platforms</a></li>
<li><a href="technology-apis.html">Qt and Key Technologies</a></li>
<li><a href="best-practices.html">How-To's and Best Practices</a></li>
</ul>
</li>
<li><a href="#">Examples</a>
<ul>
<li><a href="all-examples.html">Examples</a></li>
<li><a href="tutorials.html">Tutorials</a></li>
<li><a href="demos.html">Demos</a></li>
<li><a href="qdeclarativeexamples.html">QML Examples</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div class="wrapper">
<div class="hd">
<span></span>
</div>
<div class="bd group">
<div class="sidebar">
<div class="searchlabel">
Search index:</div>
<div class="search" id="sidebarsearch">
<form id="qtdocsearch" action="" onsubmit="return false;">
<fieldset>
<input type="text" name="searchstring" id="pageType" value="" />
<div id="resultdialog">
<a href="#" id="resultclose">Close</a>
<p id="resultlinks" class="all"><a href="#" id="showallresults">All</a> | <a href="#" id="showapiresults">API</a> | <a href="#" id="showarticleresults">Articles</a> | <a href="#" id="showexampleresults">Examples</a></p>
<p id="searchcount" class="all"><span id="resultcount"></span><span id="apicount"></span><span id="articlecount"></span><span id="examplecount"></span> results:</p>
<ul id="resultlist" class="all">
</ul>
</div>
</fieldset>
</form>
</div>
<div class="box first bottombar" id="lookup">
<h2 title="API Lookup"><span></span>
API Lookup</h2>
<div id="list001" class="list">
<ul id="ul001" >
<li class="defaultLink"><a href="classes.html">Class index</a></li>
<li class="defaultLink"><a href="functions.html">Function index</a></li>
<li class="defaultLink"><a href="modules.html">Modules</a></li>
<li class="defaultLink"><a href="namespaces.html">Namespaces</a></li>
<li class="defaultLink"><a href="qtglobal.html">Global Declarations</a></li>
<li class="defaultLink"><a href="qdeclarativeelements.html">QML elements</a></li>
</ul>
</div>
</div>
<div class="box bottombar" id="topics">
<h2 title="Qt Topics"><span></span>
Qt Topics</h2>
<div id="list002" class="list">
<ul id="ul002" >
<li class="defaultLink"><a href="qt-basic-concepts.html">Programming with Qt</a></li>
<li class="defaultLink"><a href="qtquick.html">Device UIs & Qt Quick</a></li>
<li class="defaultLink"><a href="qt-gui-concepts.html">UI Design with Qt</a></li>
<li class="defaultLink"><a href="supported-platforms.html">Supported Platforms</a></li>
<li class="defaultLink"><a href="technology-apis.html">Qt and Key Technologies</a></li>
<li class="defaultLink"><a href="best-practices.html">How-To's and Best Practices</a></li>
</ul>
</div>
</div>
<div class="box" id="examples">
<h2 title="Examples"><span></span>
Examples</h2>
<div id="list003" class="list">
<ul id="ul003">
<li class="defaultLink"><a href="all-examples.html">Examples</a></li>
<li class="defaultLink"><a href="tutorials.html">Tutorials</a></li>
<li class="defaultLink"><a href="demos.html">Demos</a></li>
<li class="defaultLink"><a href="qdeclarativeexamples.html">QML Examples</a></li>
</ul>
</div>
</div>
</div>
<div class="wrap">
<div class="toolbar">
<div class="breadcrumb toolblock">
<ul>
<li class="first"><a href="index.html">Home</a></li>
<!-- Breadcrumbs go here -->
<li><a href="modules.html">Modules</a></li>
<li><a href="qtcore.html">QtCore</a></li>
<li>QSemaphore</li>
</ul>
</div>
<div class="toolbuttons toolblock">
<ul>
<li id="smallA" class="t_button">A</li>
<li id="medA" class="t_button active">A</li>
<li id="bigA" class="t_button">A</li>
<li id="print" class="t_button"><a href="javascript:this.print();">
<span>Print</span></a></li>
</ul>
</div>
</div>
<div class="content mainContent">
<div class="toc">
<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#public-functions">Public Functions</a></li>
<li class="level1"><a href="#details">Detailed Description</a></li>
</ul>
</div>
<h1 class="title">QSemaphore Class Reference</h1>
<!-- $$$QSemaphore-brief -->
<p>The QSemaphore class provides a general counting semaphore. <a href="#details">More...</a></p>
<!-- @@@QSemaphore -->
<pre class="cpp"> <span class="preprocessor">#include <QSemaphore></span></pre><p><b>Note:</b> All functions in this class are <a href="threads-reentrancy.html#thread-safe">thread-safe</a>.</p>
<ul>
<li><a href="qsemaphore-members.html">List of all members, including inherited members</a></li>
</ul>
<a name="public-functions"></a>
<h2>Public Functions</h2>
<table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#QSemaphore">QSemaphore</a></b> ( int <i>n</i> = 0 )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#dtor.QSemaphore">~QSemaphore</a></b> ()</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#acquire">acquire</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> int </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#available">available</a></b> () const</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> void </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#release">release</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#tryAcquire">tryAcquire</a></b> ( int <i>n</i> = 1 )</td></tr>
<tr><td class="memItemLeft rightAlign topAlign"> bool </td><td class="memItemRight bottomAlign"><b><a href="qsemaphore.html#tryAcquire-2">tryAcquire</a></b> ( int <i>n</i>, int <i>timeout</i> )</td></tr>
</table>
<a name="details"></a>
<!-- $$$QSemaphore-description -->
<div class="descr">
<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 class="cpp"> <span class="type">QSemaphore</span> sem(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span><a href="qsemaphore.html#acquire">acquire</a>(<span class="number">3</span>); <span class="comment">// sem.available() == 2</span>
sem<span class="operator">.</span><a href="qsemaphore.html#acquire">acquire</a>(<span class="number">2</span>); <span class="comment">// sem.available() == 0</span>
sem<span class="operator">.</span><a href="qsemaphore.html#release">release</a>(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span><a href="qsemaphore.html#release">release</a>(<span class="number">5</span>); <span class="comment">// sem.available() == 10</span>
sem<span class="operator">.</span><a href="qsemaphore.html#tryAcquire">tryAcquire</a>(<span class="number">1</span>); <span class="comment">// sem.available() == 9, returns true</span>
sem<span class="operator">.</span><a href="qsemaphore.html#tryAcquire">tryAcquire</a>(<span class="number">250</span>); <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>
</div>
<p><b>See also </b><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>
<!-- @@@QSemaphore -->
<div class="func">
<h2>Member Function Documentation</h2>
<!-- $$$QSemaphore[overload1]$$$QSemaphoreint -->
<h3 class="fn"><a name="QSemaphore"></a>QSemaphore::<span class="name">QSemaphore</span> ( <span class="type">int</span> <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><b>See also </b><a href="qsemaphore.html#release">release</a>() and <a href="qsemaphore.html#available">available</a>().</p>
<!-- @@@QSemaphore -->
<!-- $$$~QSemaphore[overload1]$$$~QSemaphore -->
<h3 class="fn"><a name="dtor.QSemaphore"></a>QSemaphore::<span class="name">~QSemaphore</span> ()</h3>
<p>Destroys the semaphore.</p>
<p><b>Warning:</b> Destroying a semaphore that is in use may result in undefined behavior.</p>
<!-- @@@~QSemaphore -->
<!-- $$$acquire[overload1]$$$acquireint -->
<h3 class="fn"><a name="acquire"></a><span class="type">void</span> QSemaphore::<span class="name">acquire</span> ( <span class="type">int</span> <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><b>See also </b><a href="qsemaphore.html#release">release</a>(), <a href="qsemaphore.html#available">available</a>(), and <a href="qsemaphore.html#tryAcquire">tryAcquire</a>().</p>
<!-- @@@acquire -->
<!-- $$$available[overload1]$$$available -->
<h3 class="fn"><a name="available"></a><span class="type">int</span> QSemaphore::<span class="name">available</span> () const</h3>
<p>Returns the number of resources currently available to the semaphore. This number can never be negative.</p>
<p><b>See also </b><a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#release">release</a>().</p>
<!-- @@@available -->
<!-- $$$release[overload1]$$$releaseint -->
<h3 class="fn"><a name="release"></a><span class="type">void</span> QSemaphore::<span class="name">release</span> ( <span class="type">int</span> <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 class="cpp"> <span class="type"><a href="qsemaphore.html">QSemaphore</a></span> sem(<span class="number">5</span>); <span class="comment">// a semaphore that guards 5 resources</span>
sem<span class="operator">.</span><a href="qsemaphore.html#acquire">acquire</a>(<span class="number">5</span>); <span class="comment">// acquire all 5 resources</span>
sem<span class="operator">.</span>release(<span class="number">5</span>); <span class="comment">// release the 5 resources</span>
sem<span class="operator">.</span>release(<span class="number">10</span>); <span class="comment">// "create" 10 new resources</span></pre>
<p><b>See also </b><a href="qsemaphore.html#acquire">acquire</a>() and <a href="qsemaphore.html#available">available</a>().</p>
<!-- @@@release -->
<!-- $$$tryAcquire[overload1]$$$tryAcquireint -->
<h3 class="fn"><a name="tryAcquire"></a><span class="type">bool</span> QSemaphore::<span class="name">tryAcquire</span> ( <span class="type">int</span> <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 class="cpp"> <span class="type"><a href="qsemaphore.html">QSemaphore</a></span> sem(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span>tryAcquire(<span class="number">250</span>); <span class="comment">// sem.available() == 5, returns false</span>
sem<span class="operator">.</span>tryAcquire(<span class="number">3</span>); <span class="comment">// sem.available() == 2, returns true</span></pre>
<p><b>See also </b><a href="qsemaphore.html#acquire">acquire</a>().</p>
<!-- @@@tryAcquire -->
<!-- $$$tryAcquire$$$tryAcquireintint -->
<h3 class="fn"><a name="tryAcquire-2"></a><span class="type">bool</span> QSemaphore::<span class="name">tryAcquire</span> ( <span class="type">int</span> <i>n</i>, <span class="type">int</span> <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 class="cpp"> <span class="type"><a href="qsemaphore.html">QSemaphore</a></span> sem(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span><a href="qsemaphore.html#tryAcquire">tryAcquire</a>(<span class="number">250</span><span class="operator">,</span> <span class="number">1000</span>); <span class="comment">// sem.available() == 5, waits 1000 milliseconds and returns false</span>
sem<span class="operator">.</span><a href="qsemaphore.html#tryAcquire">tryAcquire</a>(<span class="number">3</span><span class="operator">,</span> <span class="number">30000</span>); <span class="comment">// sem.available() == 2, returns true without waiting</span></pre>
<p><b>See also </b><a href="qsemaphore.html#acquire">acquire</a>().</p>
<!-- @@@tryAcquire -->
</div>
</div>
</div>
</div>
<div class="ft">
<span></span>
</div>
</div>
<div class="footer">
<p>
<acronym title="Copyright">©</acronym> 2013 Digia Plc and/or its
subsidiaries. Documentation contributions included herein are the copyrights of
their respective owners.</p>
<br />
<p>
The documentation provided herein is licensed under the terms of the
<a href="http://www.gnu.org/licenses/fdl.html">GNU Free Documentation
License version 1.3</a> as published by the Free Software Foundation.</p>
<p>
Documentation sources may be obtained from <a href="http://www.qt-project.org">
www.qt-project.org</a>.</p>
<br />
<p>
Digia, Qt and their respective logos are trademarks of Digia Plc
in Finland and/or other countries worldwide. All other trademarks are property
of their respective owners. <a title="Privacy Policy"
href="http://en.gitorious.org/privacy_policy/">Privacy Policy</a></p>
</div>
<script src="scripts/functions.js" type="text/javascript"></script>
</body>
</html>
