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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>)
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