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150 lines
6.8 KiB
150 lines
6.8 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/src/tools/qsemaphore.cpp:45 -->
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<head>
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<title>QSemaphore Class</title>
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>QSemaphore Class Reference</h1>
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<p>The QSemaphore class provides a robust integer semaphore.
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<a href="#details">More...</a>
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<p>All the functions in this class are <a href="threads.html#threadsafe">thread-safe</a> when Qt is built with thread support.</p>
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<p><tt>#include <<a href="qsemaphore-h.html">qsemaphore.h</a>></tt>
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<p><a href="qsemaphore-members.html">List of all member functions.</a>
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<h2>Public Members</h2>
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<ul>
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<li class=fn><a href="#QSemaphore"><b>QSemaphore</b></a> ( int maxcount )</li>
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<li class=fn>virtual <a href="#~QSemaphore"><b>~QSemaphore</b></a> ()</li>
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<li class=fn>int <a href="#available"><b>available</b></a> () const</li>
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<li class=fn>int <a href="#total"><b>total</b></a> () const</li>
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<li class=fn>int <a href="#operator++"><b>operator++</b></a> ( int )</li>
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<li class=fn>int <a href="#operator--"><b>operator--</b></a> ( int )</li>
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<li class=fn>int <a href="#operator+-eq"><b>operator+=</b></a> ( int n )</li>
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<li class=fn>int <a href="#operator--eq"><b>operator-=</b></a> ( int n )</li>
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<li class=fn>bool <a href="#tryAccess"><b>tryAccess</b></a> ( int n )</li>
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</ul>
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<hr><a name="details"></a><h2>Detailed Description</h2>
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The QSemaphore class provides a robust integer semaphore.
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<p>
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<p> A QSemaphore can be used to serialize thread execution, in a
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similar way to a <a href="qmutex.html">QMutex</a>. A semaphore differs from a mutex, in
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that a semaphore can be accessed by more than one thread at a
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time.
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<p> For example, suppose we have an application that stores data in a
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large tree structure. The application creates 10 threads
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(commonly called a thread pool) to perform searches on the tree.
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When the application searches the tree for some piece of data, it
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uses one thread per base node to do the searching. A semaphore
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could be used to make sure that two threads don't try to search
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the same branch of the tree at the same time.
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<p> A non-computing example of a semaphore would be dining at a
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restuarant. A semaphore is initialized to have a maximum count
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equal to the number of chairs in the restuarant. As people
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arrive, they want a seat. As seats are filled, the semaphore is
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accessed, once per person. As people leave, the access is
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released, allowing more people to enter. If a party of 10 people
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want to be seated, but there are only 9 seats, those 10 people
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will wait, but a party of 4 people would be seated (taking the
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available seats to 5, making the party of 10 people wait longer).
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<p> When a semaphore is created it is given a number which is the
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maximum number of concurrent accesses it will permit. Accesses to
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the sempahore are gained using <a href="#operator++">operator++</a>() or <a href="#operator+-eq">operator+=</a>(), and
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released with <a href="#operator--">operator--</a>() or <a href="#operator--eq">operator-=</a>(). The number of
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accesses allowed is retrieved with <a href="#available">available</a>(), and the total
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number with <a href="#total">total</a>(). Note that the incrementing functions will
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block if there aren't enough available accesses. Use <a href="#tryAccess">tryAccess</a>()
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if you want to acquire accesses without blocking.
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<p>See also <a href="environment.html">Environment Classes</a> and <a href="thread.html">Threading</a>.
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<hr><h2>Member Function Documentation</h2>
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<h3 class=fn><a name="QSemaphore"></a>QSemaphore::QSemaphore ( int maxcount )
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</h3>
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Creates a new semaphore. The semaphore can be concurrently
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accessed at most <em>maxcount</em> times.
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<h3 class=fn><a name="~QSemaphore"></a>QSemaphore::~QSemaphore ()<tt> [virtual]</tt>
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</h3>
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Destroys the semaphore.
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<p> <b>Warning:</b> If you destroy a semaphore that has accesses in use the
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resultant behavior is undefined.
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<h3 class=fn>int <a name="available"></a>QSemaphore::available () const
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</h3>
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Returns the number of accesses currently available to the
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semaphore.
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<h3 class=fn>int <a name="operator++"></a>QSemaphore::operator++ ( int )
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</h3>
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Postfix ++ operator.
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<p> Try to get access to the semaphore. If <a href="#available">available</a>() == 0, this
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call will block until it can get access, i.e. until <a href="#available">available</a>() >
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0.
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<h3 class=fn>int <a name="operator+-eq"></a>QSemaphore::operator+= ( int n )
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</h3>
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Try to get access to the semaphore. If <a href="#available">available</a>() < <em>n</em>, this
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call will block until it can get all the accesses it wants, i.e.
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until <a href="#available">available</a>() >= <em>n</em>.
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<h3 class=fn>int <a name="operator--"></a>QSemaphore::operator-- ( int )
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</h3>
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Postfix -- operator.
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<p> Release access of the semaphore. This wakes all threads waiting
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for access to the semaphore.
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<h3 class=fn>int <a name="operator--eq"></a>QSemaphore::operator-= ( int n )
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</h3>
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Release <em>n</em> accesses to the semaphore.
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<h3 class=fn>int <a name="total"></a>QSemaphore::total () const
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</h3>
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Returns the total number of accesses to the semaphore.
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<h3 class=fn>bool <a name="tryAccess"></a>QSemaphore::tryAccess ( int n )
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</h3>
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Try to get access to the semaphore. If <a href="#available">available</a>() < <em>n</em>, this
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function will return FALSE immediately. If <a href="#available">available</a>() >= <em>n</em>,
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this function will take <em>n</em> accesses and return TRUE. This
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function does <em>not</em> block.
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<!-- eof -->
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<hr><p>
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This file is part of the <a href="index.html">Qt toolkit</a>.
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Copyright © 1995-2007
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<a href="http://www.trolltech.com/">Trolltech</a>. All Rights Reserved.<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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</html>
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