1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Threads and Boost.Asio</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="../../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../core.html" title="Core Concepts and Functionality"> 9<link rel="prev" href="async.html" title="The Proactor Design Pattern: Concurrency Without Threads"> 10<link rel="next" href="strands.html" title="Strands: Use Threads Without Explicit Locking"> 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="async.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="strands.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="boost_asio.overview.core.threads"></a><a class="link" href="threads.html" title="Threads and Boost.Asio">Threads and Boost.Asio</a> 28</h4></div></div></div> 29<h6> 30<a name="boost_asio.overview.core.threads.h0"></a> 31 <span class="phrase"><a name="boost_asio.overview.core.threads.thread_safety"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.thread_safety">Thread 32 Safety</a> 33 </h6> 34<p> 35 In general, it is safe to make concurrent use of distinct objects, but 36 unsafe to make concurrent use of a single object. However, types such as 37 <code class="computeroutput"><span class="identifier">io_context</span></code> provide a stronger 38 guarantee that it is safe to use a single object concurrently. 39 </p> 40<h6> 41<a name="boost_asio.overview.core.threads.h1"></a> 42 <span class="phrase"><a name="boost_asio.overview.core.threads.thread_pools"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.thread_pools">Thread 43 Pools</a> 44 </h6> 45<p> 46 Multiple threads may call <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> to set up a pool of threads from which 47 completion handlers may be invoked. This approach may also be used with 48 <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code> 49 as a means to perform arbitrary computational tasks across a thread pool. 50 </p> 51<p> 52 Note that all threads that have joined an <code class="computeroutput"><span class="identifier">io_context</span></code>'s 53 pool are considered equivalent, and the <code class="computeroutput"><span class="identifier">io_context</span></code> 54 may distribute work across them in an arbitrary fashion. 55 </p> 56<h6> 57<a name="boost_asio.overview.core.threads.h2"></a> 58 <span class="phrase"><a name="boost_asio.overview.core.threads.internal_threads"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.internal_threads">Internal 59 Threads</a> 60 </h6> 61<p> 62 The implementation of this library for a particular platform may make use 63 of one or more internal threads to emulate asynchronicity. As far as possible, 64 these threads must be invisible to the library user. In particular, the 65 threads: 66 </p> 67<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 68<li class="listitem"> 69 must not call the user's code directly; and 70 </li> 71<li class="listitem"> 72 must block all signals. 73 </li> 74</ul></div> 75<p> 76 This approach is complemented by the following guarantee: 77 </p> 78<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 79 Asynchronous completion handlers will only be called from threads that 80 are currently calling <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>. 81 </li></ul></div> 82<p> 83 Consequently, it is the library user's responsibility to create and manage 84 all threads to which the notifications will be delivered. 85 </p> 86<p> 87 The reasons for this approach include: 88 </p> 89<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 90<li class="listitem"> 91 By only calling <code class="computeroutput"><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code> from a single thread, the user's 92 code can avoid the development complexity associated with synchronisation. 93 For example, a library user can implement scalable servers that are 94 single-threaded (from the user's point of view). 95 </li> 96<li class="listitem"> 97 A library user may need to perform initialisation in a thread shortly 98 after the thread starts and before any other application code is executed. 99 For example, users of Microsoft's COM must call <code class="computeroutput"><span class="identifier">CoInitializeEx</span></code> 100 before any other COM operations can be called from that thread. 101 </li> 102<li class="listitem"> 103 The library interface is decoupled from interfaces for thread creation 104 and management, and permits implementations on platforms where threads 105 are not available. 106 </li> 107</ul></div> 108<h6> 109<a name="boost_asio.overview.core.threads.h3"></a> 110 <span class="phrase"><a name="boost_asio.overview.core.threads.see_also"></a></span><a class="link" href="threads.html#boost_asio.overview.core.threads.see_also">See 111 Also</a> 112 </h6> 113<p> 114 <a class="link" href="../../reference/io_context.html" title="io_context">io_context</a>, <a class="link" href="../../reference/post.html" title="post">post</a>. 115 </p> 116</div> 117<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 118<td align="left"></td> 119<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 120 Kohlhoff<p> 121 Distributed under the Boost Software License, Version 1.0. (See accompanying 122 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>) 123 </p> 124</div></td> 125</tr></table> 126<hr> 127<div class="spirit-nav"> 128<a accesskey="p" href="async.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="strands.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 129</div> 130</body> 131</html> 132
