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<div class="navpath"><a class="el" href="namespaceost.html">ost</a>::<a class="el" href="classost_1_1_semaphore.html">Semaphore</a>
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<h1>ost::Semaphore Class Reference</h1><!-- doxytag: class="ost::Semaphore" -->
<p>A semaphore is generally used as a synchronization object between multiple threads or to protect a limited and finite resource such as a memory or thread pool.
<a href="#_details">More...</a></p>
<p><code>#include <<a class="el" href="thread_8h_source.html">thread.h</a>></code></p>
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Inheritance diagram for ost::Semaphore:</div>
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<img src="classost_1_1_semaphore.png" usemap="#ost::Semaphore_map" alt=""/>
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<area href="classost_1_1_thread_queue.html" alt="ost::ThreadQueue" shape="rect" coords="0,56,112,80"/>
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<p><a href="classost_1_1_semaphore-members.html">List of all members.</a></p>
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<tr><td colspan="2"><h2>Public Member Functions</h2></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"> </td><td class="memItemRight" valign="bottom"><a class="el" href="classost_1_1_semaphore.html#a0a6f933da386bbe01b2b0b6b266bcc85">Semaphore</a> (unsigned resource=0)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">The initial value of the semaphore can be specified. <a href="#a0a6f933da386bbe01b2b0b6b266bcc85"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">virtual </td><td class="memItemRight" valign="bottom"><a class="el" href="classost_1_1_semaphore.html#a138de594478446aa07f3bad15dce3846">~Semaphore</a> ()</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Destroying a semaphore also removes any system resources associated with it. <a href="#a138de594478446aa07f3bad15dce3846"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="classost_1_1_semaphore.html#a1f2526be16a463d2392a74f668a91d80">wait</a> (<a class="el" href="thread_8h.html#af412159e5cef839836a5e7b19ee75d1c">timeout_t</a> timeout=0)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Wait is used to keep a thread held until the semaphore counter is greater than 0. <a href="#a1f2526be16a463d2392a74f668a91d80"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="classost_1_1_semaphore.html#a7535c9c3f3c60596b21b5275342cbe1e">post</a> (void)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Posting to a semaphore increments its current value and releases the first thread waiting for the semaphore if it is currently at 0. <a href="#a7535c9c3f3c60596b21b5275342cbe1e"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="classost_1_1_semaphore.html#a23c458d249a3f68b83deaa5c7986dfca">force_unlock_after_cancellation</a> ()</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Call it after a deferred cancellation to avoid deadlocks. <a href="#a23c458d249a3f68b83deaa5c7986dfca"></a><br/></td></tr>
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<hr/><a name="_details"></a><h2>Detailed Description</h2>
<p>A semaphore is generally used as a synchronization object between multiple threads or to protect a limited and finite resource such as a memory or thread pool. </p>
<p>The semaphore has a counter which only permits access by one or more threads when the value of the semaphore is non-zero. Each access reduces the current value of the semaphore by 1. One or more threads can wait on a semaphore until it is no longer 0, and hence the semaphore can be used as a simple thread synchronization object to enable one thread to pause others until the thread is ready or has provided data for them. Semaphores are typically used as a counter for protecting or limiting concurrent access to a given resource, such as to permitting at most "x" number of threads to use resource "y", for example.</p>
<dl class="author"><dt><b>Author:</b></dt><dd>David Sugar <<a href="mailto:dyfet@ostel.com">dyfet@ostel.com</a>> <a class="el" href="classost_1_1_semaphore.html" title="A semaphore is generally used as a synchronization object between multiple threads...">Semaphore</a> counter for thread synchronization. </dd></dl>
<dl><dt><b>Examples: </b></dt><dd>
<p><a class="el" href="tcpservice_8cpp-example.html#_a15">tcpservice.cpp</a>.</p>
</dd>
</dl><hr/><h2>Constructor & Destructor Documentation</h2>
<a class="anchor" id="a0a6f933da386bbe01b2b0b6b266bcc85"></a><!-- doxytag: member="ost::Semaphore::Semaphore" ref="a0a6f933da386bbe01b2b0b6b266bcc85" args="(unsigned resource=0)" -->
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<td class="memname">ost::Semaphore::Semaphore </td>
<td>(</td>
<td class="paramtype">unsigned </td>
<td class="paramname"> <em>resource</em> = <code>0</code></td>
<td> ) </td>
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<p>The initial value of the semaphore can be specified. </p>
<p>An initial value is often used When used to lock a finite resource or to specify the maximum number of thread instances that can access a specified resource.</p>
<dl><dt><b>Parameters:</b></dt><dd>
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<tr><td valign="top"></td><td valign="top"><em>resource</em> </td><td>specify initial resource count or 0 default. </td></tr>
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</dd>
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<a class="anchor" id="a138de594478446aa07f3bad15dce3846"></a><!-- doxytag: member="ost::Semaphore::~Semaphore" ref="a138de594478446aa07f3bad15dce3846" args="()" -->
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<td class="memname">virtual ost::Semaphore::~Semaphore </td>
<td>(</td>
<td class="paramname"></td>
<td> ) </td>
<td><code> [virtual]</code></td>
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<p>Destroying a semaphore also removes any system resources associated with it. </p>
<p>If a semaphore has threads currently waiting on it, those threads will all continue when a semaphore is destroyed. </p>
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<hr/><h2>Member Function Documentation</h2>
<a class="anchor" id="a23c458d249a3f68b83deaa5c7986dfca"></a><!-- doxytag: member="ost::Semaphore::force_unlock_after_cancellation" ref="a23c458d249a3f68b83deaa5c7986dfca" args="()" -->
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<td class="memname">void ost::Semaphore::force_unlock_after_cancellation </td>
<td>(</td>
<td class="paramname"></td>
<td> ) </td>
<td></td>
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<p>Call it after a deferred cancellation to avoid deadlocks. </p>
<p>From PTHREAD_COND_TIMEDWAIT(3P): A condition wait (whether timed or not) is a cancellation point. When the cancelability enable state of a thread is set to PTHREAD_CANCEL_DEFERRED, a side effect of acting upon a cancellation request while in a condition wait is that the mutex is (in effect) re-acquired before calling the first cancellation cleanup handler. </p>
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<a class="anchor" id="a7535c9c3f3c60596b21b5275342cbe1e"></a><!-- doxytag: member="ost::Semaphore::post" ref="a7535c9c3f3c60596b21b5275342cbe1e" args="(void)" -->
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<td class="memname">void ost::Semaphore::post </td>
<td>(</td>
<td class="paramtype">void </td>
<td class="paramname"></td>
<td> ) </td>
<td></td>
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<p>Posting to a semaphore increments its current value and releases the first thread waiting for the semaphore if it is currently at 0. </p>
<p>Interestingly, there is no support to increment a semaphore by any value greater than 1 to release multiple waiting threads in either pthread or the win32 API. Hence, if one wants to release a semaphore to enable multiple threads to execute, one must perform multiple post operations.</p>
<dl class="see"><dt><b>See also:</b></dt><dd><a class="el" href="classost_1_1_semaphore.html#a1f2526be16a463d2392a74f668a91d80" title="Wait is used to keep a thread held until the semaphore counter is greater than 0...">wait</a> </dd></dl>
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<a class="anchor" id="a1f2526be16a463d2392a74f668a91d80"></a><!-- doxytag: member="ost::Semaphore::wait" ref="a1f2526be16a463d2392a74f668a91d80" args="(timeout_t timeout=0)" -->
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<td class="memname">bool ost::Semaphore::wait </td>
<td>(</td>
<td class="paramtype"><a class="el" href="thread_8h.html#af412159e5cef839836a5e7b19ee75d1c">timeout_t</a> </td>
<td class="paramname"> <em>timeout</em> = <code>0</code></td>
<td> ) </td>
<td></td>
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<p>Wait is used to keep a thread held until the semaphore counter is greater than 0. </p>
<p>If the current thread is held, then another thread must increment the semaphore. Once the thread is accepted, the semaphore is automatically decremented, and the thread continues execution.</p>
<p>The pthread semaphore object does not support a timed "wait", and hence to maintain consistancy, neither the posix nor win32 source trees support "timed" semaphore objects.</p>
<dl class="return"><dt><b>Returns:</b></dt><dd>false if timed out </dd></dl>
<dl><dt><b>Parameters:</b></dt><dd>
<table border="0" cellspacing="2" cellpadding="0">
<tr><td valign="top"></td><td valign="top"><em>timeout</em> </td><td>period in milliseconds to wait </td></tr>
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</dd>
</dl>
<dl class="see"><dt><b>See also:</b></dt><dd><a class="el" href="classost_1_1_semaphore.html#a7535c9c3f3c60596b21b5275342cbe1e" title="Posting to a semaphore increments its current value and releases the first thread...">post</a> </dd></dl>
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<hr/>The documentation for this class was generated from the following file:<ul>
<li><a class="el" href="thread_8h_source.html">thread.h</a></li>
</ul>
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<hr class="footer"/><address style="text-align: right;"><small>Generated on Fri Nov 12 09:55:47 2010 for GNU CommonC++ by
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