/usr/share/qt3/doc/html/qmutex.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/src/tools/qmutex_unix.cpp:333 -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>QMutex Class</title>
<style type="text/css"><!--
fn { margin-left: 1cm; text-indent: -1cm; }
a:link { color: #004faf; text-decoration: none }
a:visited { color: #672967; text-decoration: none }
body { background: #ffffff; color: black; }
--></style>
</head>
<body>

<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tr bgcolor="#E5E5E5">
<td valign=center>
 <a href="index.html">
<font color="#004faf">Home</font></a>
 | <a href="classes.html">
<font color="#004faf">All&nbsp;Classes</font></a>
 | <a href="mainclasses.html">
<font color="#004faf">Main&nbsp;Classes</font></a>
 | <a href="annotated.html">
<font color="#004faf">Annotated</font></a>
 | <a href="groups.html">
<font color="#004faf">Grouped&nbsp;Classes</font></a>
 | <a href="functions.html">
<font color="#004faf">Functions</font></a>
</td>
<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>QMutex Class Reference</h1>

<p>The QMutex class provides access serialization between threads.
<a href="#details">More...</a>
<p>All the functions in this class are <a href="threads.html#threadsafe">thread-safe</a> when Qt is built with thread support.</p>
<p><tt>#include &lt;<a href="qmutex-h.html">qmutex.h</a>&gt;</tt>
<p><a href="qmutex-members.html">List of all member functions.</a>
<h2>Public Members</h2>
<ul>
<li class=fn><a href="#QMutex"><b>QMutex</b></a> ( bool&nbsp;recursive = FALSE )</li>
<li class=fn>virtual <a href="#~QMutex"><b>~QMutex</b></a> ()</li>
<li class=fn>void <a href="#lock"><b>lock</b></a> ()</li>
<li class=fn>void <a href="#unlock"><b>unlock</b></a> ()</li>
<li class=fn>bool <a href="#locked"><b>locked</b></a> ()</li>
<li class=fn>bool <a href="#tryLock"><b>tryLock</b></a> ()</li>
</ul>
<hr><a name="details"></a><h2>Detailed Description</h2>



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). For
example, say there is a method which prints a message to the user
on two lines:
<p> <pre>
    int number = 6;

    void method1()
    {
        number *= 5;
        number /= 4;
    }

    void method2()
    {
        number *= 3;
        number /= 2;
    }
    </pre>
 
<p> If these two methods are called in succession, the following happens:
<p> <pre>
    // method1()
    number *= 5;        // number is now 30
    number /= 4;        // number is now 7

    // method2()
    number *= 3;        // nubmer is now 21
    number /= 2;        // number is now 10
    </pre>
 
<p> If these two methods are called simultaneously from two threads then the
following sequence could result:
<p> <pre>
    // Thread 1 calls method1()
    number *= 5;        // number is now 30

    // Thread 2 calls method2().
    //
    // Most likely Thread 1 has been put to sleep by the operating
    // system to allow Thread 2 to run.
    number *= 3;        // number is now 90
    number /= 2;        // number is now 45

    // Thread 1 finishes executing.
    number /= 4;        // number is now 11, instead of 10
    </pre>
 
<p> If we add a mutex, we should get the result we want:
<p> <pre>
    QMutex mutex;
    int number = 6;

    void method1()
    {
        mutex.<a href="#lock">lock</a>();
        number *= 5;
        number /= 4;
        mutex.<a href="#unlock">unlock</a>();
    }

    void method2()
    {
        mutex.<a href="#lock">lock</a>();
        number *= 3;
        number /= 2;
        mutex.<a href="#unlock">unlock</a>();
    }
    </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> When you call <a href="#lock">lock</a>() in a thread, other threads that try to call
lock() in the same place will block until the thread that got the
lock calls <a href="#unlock">unlock</a>(). A non-blocking alternative to lock() is
<a href="#tryLock">tryLock</a>().
<p>See also <a href="environment.html">Environment Classes</a> and <a href="thread.html">Threading</a>.

<hr><h2>Member Function Documentation</h2>
<h3 class=fn><a name="QMutex"></a>QMutex::QMutex ( bool&nbsp;recursive = FALSE )
</h3>
Constructs a new mutex. The mutex is created in an unlocked state.
A recursive mutex is created if <em>recursive</em> is TRUE; a normal
mutex is created if <em>recursive</em> is FALSE (the default). With a
recursive mutex, a thread can lock the same mutex multiple times
and it will not be unlocked until a corresponding number of
<a href="#unlock">unlock</a>() calls have been made.

<h3 class=fn><a name="~QMutex"></a>QMutex::~QMutex ()<tt> [virtual]</tt>
</h3>
Destroys the mutex.
<p> <b>Warning:</b> If you destroy a mutex that still holds a lock the
resultant behavior is undefined.

<h3 class=fn>void <a name="lock"></a>QMutex::lock ()
</h3>
Attempt to lock the mutex. If another thread has locked the mutex
then this call will <em>block</em> until that thread has unlocked it.
<p> <p>See also <a href="#unlock">unlock</a>() and <a href="#locked">locked</a>().

<h3 class=fn>bool <a name="locked"></a>QMutex::locked ()
</h3>
Returns TRUE if the mutex is locked by another thread; otherwise
returns FALSE.
<p> <b>Warning:</b> Due to differing implementations of recursive mutexes on
various platforms, calling this function from the same thread that
previously locked the mutex will return undefined results.
<p> <p>See also <a href="#lock">lock</a>() and <a href="#unlock">unlock</a>().

<h3 class=fn>bool <a name="tryLock"></a>QMutex::tryLock ()
</h3>
Attempt to lock the mutex. If the lock was obtained, this function
returns TRUE. If another thread has locked the mutex, this
function returns FALSE, instead of waiting for the mutex to become
available, i.e. it does not block.
<p> If the lock was obtained, the mutex must be unlocked with <a href="#unlock">unlock</a>()
before another thread can successfully lock it.
<p> <p>See also <a href="#lock">lock</a>(), <a href="#unlock">unlock</a>(), and <a href="#locked">locked</a>().

<h3 class=fn>void <a name="unlock"></a>QMutex::unlock ()
</h3>
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 behaviour (varies
between different Operating Systems' thread implementations).
<p> <p>See also <a href="#lock">lock</a>() and <a href="#locked">locked</a>().

<!-- eof -->
<hr><p>
This file is part of the <a href="index.html">Qt toolkit</a>.
Copyright &copy; 1995-2007
<a href="http://www.trolltech.com/">Trolltech</a>. All Rights Reserved.<p><address><hr><div align=center>
<table width=100% cellspacing=0 border=0><tr>
<td>Copyright &copy; 2007
<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
<td align=right><div align=right>Qt 3.3.8</div>
</table></div></address></body>
</html>
qt3-doc 3:3.3.8-b-8ubuntu3 / usr / share / qt3 / doc / html / qmutex.html
