<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<!-- qsemaphore.cpp -->
<title>QSemaphore Class | Qt Core 5.9</title>
<link rel="stylesheet" type="text/css" href="style/offline-simple.css" />
<script type="text/javascript">
document.getElementsByTagName("link").item(0).setAttribute("href", "style/offline.css");
// loading style sheet breaks anchors that were jumped to before
// so force jumping to anchor again
setTimeout(function() {
var anchor = location.hash;
// need to jump to different anchor first (e.g. none)
location.hash = "#";
setTimeout(function() {
location.hash = anchor;
}, 0);
}, 0);
</script>
</head>
<body>
<div class="header" id="qtdocheader">
<div class="main">
<div class="main-rounded">
<div class="navigationbar">
<table><tr>
<td >Qt 5.9</td><td ><a href="qtcore-index.html">Qt Core</a></td><td ><a href="qtcore-module.html">C++ Classes</a></td><td >QSemaphore</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
</tr></table>
</div>
</div>
<div class="content">
<div class="line">
<div class="content mainContent">
<div class="sidebar">
<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>
<div class="sidebar-content" id="sidebar-content"></div></div>
<h1 class="title">QSemaphore Class</h1>
<!-- $$$QSemaphore-brief -->
<p>The <a href="qsemaphore.html">QSemaphore</a> class provides a general counting semaphore. <a href="#details">More...</a></p>
<!-- @@@QSemaphore -->
<div class="table"><table class="alignedsummary">
<tr><td class="memItemLeft rightAlign topAlign"> Header:</td><td class="memItemRight bottomAlign"> <span class="preprocessor">#include <QSemaphore></span>
</td></tr><tr><td class="memItemLeft rightAlign topAlign"> qmake:</td><td class="memItemRight bottomAlign"> QT += core</td></tr></table></div><ul>
<li><a href="qsemaphore-members.html">List of all members, including inherited members</a></li>
</ul>
<p><b>Note:</b> All functions in this class are thread-safe.</p>
<a name="public-functions"></a>
<h2 id="public-functions">Public Functions</h2>
<div class="table"><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-1">tryAcquire</a></b>(int <i>n</i>, int <i>timeout</i>)</td></tr>
</table></div>
<a name="details"></a>
<!-- $$$QSemaphore-description -->
<div class="descr">
<h2 id="details">Detailed Description</h2>
<p>The <a href="qsemaphore.html">QSemaphore</a> 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"><a href="qsemaphore.html#QSemaphore">QSemaphore</a></span> sem(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span>acquire(<span class="number">3</span>); <span class="comment">// sem.available() == 2</span>
sem<span class="operator">.</span>acquire(<span class="number">2</span>); <span class="comment">// sem.available() == 0</span>
sem<span class="operator">.</span>release(<span class="number">5</span>); <span class="comment">// sem.available() == 5</span>
sem<span class="operator">.</span>release(<span class="number">5</span>); <span class="comment">// sem.available() == 10</span>
sem<span class="operator">.</span>tryAcquire(<span class="number">1</span>); <span class="comment">// sem.available() == 9, returns true</span>
sem<span class="operator">.</span>tryAcquire(<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="qtcore-threads-semaphores-example.html">Semaphores Example</a> shows how to use <a href="qsemaphore.html">QSemaphore</a> 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="qtcore-threads-semaphores-example.html">Semaphores Example</a>.</p>
<!-- @@@QSemaphore -->
<div class="func">
<h2>Member Function Documentation</h2>
<!-- $$$QSemaphore[overload1]$$$QSemaphoreint -->
<h3 class="fn" id="QSemaphore"><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" id="dtor.QSemaphore"><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" id="acquire"><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 <code>n</code> 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" id="available"><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" id="release"><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">QSemaphore</a></span> sem(<span class="number">5</span>); <span class="comment">// a semaphore that guards 5 resources</span>
sem<span class="operator">.</span>acquire(<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" id="tryAcquire"><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 <code>n</code> resources guarded by the semaphore and returns <code>true</code> on success. If <a href="qsemaphore.html#available">available</a>() < <i>n</i>, this call immediately returns <code>false</code> without acquiring any resources.</p>
<p>Example:</p>
<pre class="cpp">
<span class="type"><a href="qsemaphore.html#QSemaphore">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" id="tryAcquire-1"><a name="tryAcquire-1"></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 <code>n</code> resources guarded by the semaphore and returns <code>true</code> 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">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="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>tryAcquire(<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>
</div>
<div class="footer">
<p>
<acronym title="Copyright">©</acronym> 2017 The Qt Company Ltd.
Documentation contributions included herein are the copyrights of
their respective owners.<br> 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.<br> Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property
of their respective owners. </p>
</div>
</body>
</html>
distrib
>
Mageia
>
6
>
armv5tl
>
media
>
core-updates
>
by-pkgid
>
768f7d9f703884aa2562bf0a651086df
>
files
>
619
