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25<div class="refentry" lang="en">
26<a name="boost.barrier"></a><div class="titlepage"></div>
27<div class="refnamediv">
28<h2><span class="refentrytitle">Class barrier</span></h2>
29<p>boost::barrier &#8212; <p>An object of class <a href="barrier.html" title="Class barrier">barrier</a> is a synchronization
30        primitive used  to cause a set of threads to wait until they each perform a
31        certain function or each reach a particular point in their execution.</p></p>
32</div>
33<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
34<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis">
35<span class="bold"><strong>class</strong></span> barrier : <span class="bold"><strong>private</strong></span> boost::noncopyable   <span class="emphasis"><em>// Exposition only</em></span>
36{
37<span class="bold"><strong>public</strong></span>:
38  <span class="emphasis"><em>// <a href="barrier.html#boost.barrierconstruct-copy-destruct">construct/copy/destruct</a></em></span>
39  <a href="barrier.html#id751828-bb">barrier</a>(size_t);
40  <a href="barrier.html#id825827-bb">~barrier</a>();
41
42  <span class="emphasis"><em>// <a href="barrier.html#id768369-bb">waiting</a></em></span>
43  <span class="type"><span class="bold"><strong>bool</strong></span></span> <a href="barrier.html#id768373-bb">wait</a>();
44};</pre></div>
45<div class="refsect1" lang="en">
46<a name="id1736550"></a><h2>Description</h2>
47<p>When a barrier is created, it is initialized with a thread count N.
48                The first N-1 calls to <code class="computeroutput">wait()</code> will all cause their threads to be blocked.
49                The Nth call to <code class="computeroutput">wait()</code> will allow all  of the waiting threads, including
50                the Nth thread, to be placed in a ready state.  The Nth call will also "reset"
51                the barrier such that, if an additional N+1th call is made to <code class="computeroutput">wait()</code>,
52                it will be as though this were the first call to <code class="computeroutput">wait()</code>; in other
53                words, the N+1th to 2N-1th calls to <code class="computeroutput">wait()</code> will cause their
54                threads to be blocked, and the 2Nth call to <code class="computeroutput">wait()</code> will allow all of
55                the waiting threads, including the 2Nth thread, to be placed in a ready state
56                and reset the barrier. This functionality allows the same set of N threads to re-use
57                a barrier object to  synchronize their execution at multiple points during their
58                execution.</p>
59<p>See <a href="../thread.html#thread.glossary" title="Glossary">Glossary</a> for definitions of thread
60                states <a href="../thread.html#thread.glossary.thread-state">blocked</a>
61                and <a href="../thread.html#thread.glossary.thread-state">ready</a>.
62                Note that "waiting" is a synonym for blocked.</p>
63<div class="refsect2" lang="en">
64<a name="id1736632"></a><h3>
65<a name="boost.barrierconstruct-copy-destruct"></a><code class="computeroutput">barrier</code> construct/copy/destruct</h3>
66<div class="orderedlist"><ol type="1">
67<li>
68<pre class="literallayout"><a name="id751828-bb"></a>barrier(size_t count);</pre>
69<div class="variablelist"><table border="0">
70<col align="left" valign="top">
71<tbody><tr>
72<td>
73<span class="term">Effects:</span></td>
74<td>Constructs a <a href="barrier.html" title="Class barrier">barrier</a> object that
75        will cause <code class="computeroutput">count</code> threads to block on a call to <code class="computeroutput">wait()</code>.
76        </td>
77</tr></tbody>
78</table></div>
79</li>
80<li>
81<pre class="literallayout"><a name="id825827-bb"></a>~barrier();</pre>
82<div class="variablelist"><table border="0">
83<col align="left" valign="top">
84<tbody><tr>
85<td>
86<span class="term">Effects:</span></td>
87<td>Destroys <code class="computeroutput">*this</code>. If threads are still executing
88                their <code class="computeroutput">wait()</code> operations, the behavior for these threads is undefined.
89                </td>
90</tr></tbody>
91</table></div>
92</li>
93</ol></div>
94</div>
95<div class="refsect2" lang="en">
96<a name="id1736735"></a><h3>
97<a name="id768369-bb"></a><code class="computeroutput">barrier</code> waiting</h3>
98<div class="orderedlist"><ol type="1"><li>
99<pre class="literallayout"><span class="type"><span class="bold"><strong>bool</strong></span></span> <a name="id768373-bb"></a>wait();</pre>
100<div class="variablelist"><table border="0">
101<col align="left" valign="top">
102<tbody>
103<tr>
104<td>
105<span class="term">Effects:</span></td>
106<td>
107<p>Wait until N threads call <code class="computeroutput">wait()</code>, where
108          N equals the <code class="computeroutput">count</code> provided to the constructor for the
109          barrier object.</p>
110<p><span class="bold"><strong>Note</strong></span> that if the barrier is
111          destroyed before <code class="computeroutput">wait()</code> can return, the behavior is
112          undefined.</p>
113</td>
114</tr>
115<tr>
116<td>
117<span class="term">Returns:</span></td>
118<td>Exactly one of the N threads will receive a return value
119                  of <code class="computeroutput">true</code>, the others will receive a value of <code class="computeroutput">false</code>.
120                  Precisely which thread receives the return value of <code class="computeroutput">true</code> will
121                  be implementation-defined. Applications can use this value to designate one
122                  thread as a leader that will take a certain action, and the other threads
123                  emerging from the barrier can wait for that action to take place.</td>
124</tr>
125</tbody>
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127</li></ol></div>
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133<td align="right"><small>Copyright © 2001-2003 William E. Kempf</small></td>
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