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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="fiber.synchronization.barriers"></a><a class="link" href="barriers.html" title="Barriers">Barriers</a>
  28 </h3></div></div></div>
  29 <p>
  30         A barrier is a concept also known as a <span class="emphasis"><em>rendezvous</em></span>, it
  31         is a synchronization point between multiple contexts of execution (fibers).
  32         The barrier is configured for a particular number of fibers (<code class="computeroutput"><span class="identifier">n</span></code>), and as fibers reach the barrier they
  33         must wait until all <code class="computeroutput"><span class="identifier">n</span></code> fibers
  34         have arrived. Once the <code class="computeroutput"><span class="identifier">n</span></code>-th
  35         fiber has reached the barrier, all the waiting fibers can proceed, and the
  36         barrier is reset.
  37       </p>
  38 <p>
  39         The fact that the barrier automatically resets is significant. Consider a
  40         case in which you launch some number of fibers and want to wait only until
  41         the first of them has completed. You might be tempted to use a <code class="computeroutput"><span class="identifier">barrier</span><span class="special">(</span><span class="number">2</span><span class="special">)</span></code> as the synchronization
  42         mechanism, making each new fiber call its <a class="link" href="barriers.html#barrier_wait"><code class="computeroutput">barrier::wait()</code></a> method,
  43         then calling <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code>
  44         in the launching fiber to wait until the first other fiber completes.
  45       </p>
  46 <p>
  47         That will in fact unblock the launching fiber. The unfortunate part is that
  48         it will continue blocking the <span class="emphasis"><em>remaining</em></span> fibers.
  49       </p>
  50 <p>
  51         Consider the following scenario:
  52       </p>
  53 <div class="orderedlist"><ol class="orderedlist" type="1">
  54 <li class="listitem">
  55             Fiber <span class="quote">&#8220;<span class="quote">main</span>&#8221;</span> launches fibers A, B, C and D, then calls
  56             <code class="computeroutput"><span class="identifier">barrier</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>.
  57           </li>
  58 <li class="listitem">
  59             Fiber C finishes first and likewise calls <code class="computeroutput"><span class="identifier">barrier</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>.
  60           </li>
  61 <li class="listitem">
  62             Fiber <span class="quote">&#8220;<span class="quote">main</span>&#8221;</span> is unblocked, as desired.
  63           </li>
  64 <li class="listitem">
  65             Fiber B calls <code class="computeroutput"><span class="identifier">barrier</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>. Fiber B is <span class="emphasis"><em>blocked!</em></span>
  66           </li>
  67 <li class="listitem">
  68             Fiber A calls <code class="computeroutput"><span class="identifier">barrier</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>. Fibers A and B are unblocked.
  69           </li>
  70 <li class="listitem">
  71             Fiber D calls <code class="computeroutput"><span class="identifier">barrier</span><span class="special">::</span><span class="identifier">wait</span><span class="special">()</span></code>. Fiber D is blocked indefinitely.
  72           </li>
  73 </ol></div>
  74 <p>
  75         (See also <a class="link" href="../when_any/when_any/when_any__simple_completion.html#wait_first_simple_section">when_any, simple completion</a>.)
  76       </p>
  77 <div class="note"><table border="0" summary="Note">
  78 <tr>
  79 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  80 <th align="left">Note</th>
  81 </tr>
  82 <tr><td align="left" valign="top"><p>
  83           It is unwise to tie the lifespan of a barrier to any one of its participating
  84           fibers. Although conceptually all waiting fibers awaken <span class="quote">&#8220;<span class="quote">simultaneously,</span>&#8221;</span>
  85           because of the nature of fibers, in practice they will awaken one by one
  86           in indeterminate order.<sup>[<a name="fiber.synchronization.barriers.f0" href="#ftn.fiber.synchronization.barriers.f0" class="footnote">1</a>]</sup> The rest of the waiting fibers will still be blocked in <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code>,
  87           which must, before returning, access data members in the barrier object.
  88         </p></td></tr>
  89 </table></div>
  90 <p>
  91         </p>
  92 <h5>
  93 <a name="class_barrier_bridgehead"></a>
  94   <span><a name="class_barrier"></a></span>
  95   <a class="link" href="barriers.html#class_barrier">Class <code class="computeroutput">barrier</code></a>
  96 </h5>
  97 <p>
  98       </p>
  99 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fiber</span><span class="special">/</span><span class="identifier">barrier</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 100
 101 <span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
 102 <span class="keyword">namespace</span> <span class="identifier">fibers</span> <span class="special">{</span>
 103
 104 <span class="keyword">class</span> <span class="identifier">barrier</span> <span class="special">{</span>
 105 <span class="keyword">public</span><span class="special">:</span>
 106     <span class="keyword">explicit</span> <span class="identifier">barrier</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">);</span>
 107
 108     <span class="identifier">barrier</span><span class="special">(</span> <span class="identifier">barrier</span> <span class="keyword">const</span><span class="special">&amp;)</span> <span class="special">=</span> <span class="keyword">delete</span><span class="special">;</span>
 109     <span class="identifier">barrier</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">barrier</span> <span class="keyword">const</span><span class="special">&amp;)</span> <span class="special">=</span> <span class="keyword">delete</span><span class="special">;</span>
 110
 111     <span class="keyword">bool</span> <span class="identifier">wait</span><span class="special">();</span>
 112 <span class="special">};</span>
 113
 114 <span class="special">}}</span>
 115 </pre>
 116 <p>
 117         Instances of <a class="link" href="barriers.html#class_barrier"><code class="computeroutput">barrier</code></a> are not copyable or movable.
 118       </p>
 119 <h5>
 120 <a name="fiber.synchronization.barriers.h0"></a>
 121         <span><a name="fiber.synchronization.barriers.constructor"></a></span><a class="link" href="barriers.html#fiber.synchronization.barriers.constructor">Constructor</a>
 122       </h5>
 123 <pre class="programlisting"><span class="keyword">explicit</span> <span class="identifier">barrier</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">initial</span><span class="special">);</span>
 124 </pre>
 125 <div class="variablelist">
 126 <p class="title"><b></b></p>
 127 <dl>
 128 <dt><span class="term">Effects:</span></dt>
 129 <dd><p>
 130               Construct a barrier for <code class="computeroutput"><span class="identifier">initial</span></code>
 131               fibers.
 132             </p></dd>
 133 <dt><span class="term">Throws:</span></dt>
 134 <dd><p>
 135               <code class="computeroutput"><span class="identifier">fiber_error</span></code>
 136             </p></dd>
 137 <dt><span class="term">Error Conditions:</span></dt>
 138 <dd><p>
 139               <span class="bold"><strong>invalid_argument</strong></span>: if <code class="computeroutput"><span class="identifier">initial</span></code> is zero.
 140             </p></dd>
 141 </dl>
 142 </div>
 143 <p>
 144         </p>
 145 <h5>
 146 <a name="barrier_wait_bridgehead"></a>
 147   <span><a name="barrier_wait"></a></span>
 148   <a class="link" href="barriers.html#barrier_wait">Member function <code class="computeroutput">wait</code>()</a>
 149 </h5>
 150 <p>
 151       </p>
 152 <pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">wait</span><span class="special">();</span>
 153 </pre>
 154 <div class="variablelist">
 155 <p class="title"><b></b></p>
 156 <dl>
 157 <dt><span class="term">Effects:</span></dt>
 158 <dd><p>
 159               Block until <code class="computeroutput"><span class="identifier">initial</span></code>
 160               fibers have called <code class="computeroutput"><span class="identifier">wait</span></code>
 161               on <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
 162               When the <code class="computeroutput"><span class="identifier">initial</span></code>-th
 163               fiber calls <code class="computeroutput"><span class="identifier">wait</span></code>, all
 164               waiting fibers are unblocked, and the barrier is reset.
 165             </p></dd>
 166 <dt><span class="term">Returns:</span></dt>
 167 <dd><p>
 168               <code class="computeroutput"><span class="keyword">true</span></code> for exactly one fiber
 169               from each batch of waiting fibers, <code class="computeroutput"><span class="keyword">false</span></code>
 170               otherwise.
 171             </p></dd>
 172 <dt><span class="term">Throws:</span></dt>
 173 <dd><p>
 174               <code class="computeroutput"><span class="identifier">fiber_error</span></code>
 175             </p></dd>
 176 </dl>
 177 </div>
 178 <div class="footnotes">
 179 <br><hr width="100" align="left">
 180 <div class="footnote"><p><sup>[<a name="ftn.fiber.synchronization.barriers.f0" href="#fiber.synchronization.barriers.f0" class="para">1</a>] </sup>
 181             The current implementation wakes fibers in FIFO order: the first to call
 182             <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code>
 183             wakes first, and so forth. But it is perilous to rely on the order in
 184             which the various fibers will reach the <code class="computeroutput"><span class="identifier">wait</span><span class="special">()</span></code> call.
 185           </p></div>
 186 </div>
 187 </div>
 188 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 189 <td align="left"></td>
 190 <td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<p>
 191         Distributed under the Boost Software License, Version 1.0. (See accompanying
 192         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
 193       </p>
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