1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Barriers</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Fiber"> 8<link rel="up" href="../synchronization.html" title="Synchronization"> 9<link rel="prev" href="conditions.html" title="Condition Variables"> 10<link rel="next" href="channels.html" title="Channels"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="conditions.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../synchronization.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="channels.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 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">“<span class="quote">main</span>”</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">“<span class="quote">main</span>”</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">“<span class="quote">simultaneously,</span>”</span> 85 because of the nature of fibers, in practice they will awaken one by one 86 in indeterminate order.<a href="#ftn.fiber.synchronization.barriers.f0" class="footnote" name="fiber.synchronization.barriers.f0"><sup class="footnote">[1]</sup></a> 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 class="phrase"><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"><</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">></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">&)</span> <span class="special">=</span> <span class="keyword">delete</span><span class="special">;</span> 109 <span class="identifier">barrier</span> <span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">barrier</span> <span class="keyword">const</span><span class="special">&)</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 class="phrase"><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 class="variablelist"> 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 class="phrase"><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 class="variablelist"> 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 style="width:100; text-align:left;margin-left: 0"> 180<div id="ftn.fiber.synchronization.barriers.f0" class="footnote"><p><a href="#fiber.synchronization.barriers.f0" class="para"><sup class="para">[1] </sup></a> 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 © 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> 194</div></td> 195</tr></table> 196<hr> 197<div class="spirit-nav"> 198<a accesskey="p" href="conditions.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../synchronization.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="channels.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 199</div> 200</body> 201</html> 202
