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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.threads"></a><a class="link" href="threads.html" title="Threads and Boost.Asio">Threads and Boost.Asio</a>
</h4></div></div></div>
<h6>
<a name="boost_asio.overview.core.threads.h0"></a>
<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
Safety</a>
</h6>
<p>
In general, it is safe to make concurrent use of distinct objects, but
unsafe to make concurrent use of a single object. However, types such as
<code class="computeroutput"><span class="identifier">io_context</span></code> provide a stronger
guarantee that it is safe to use a single object concurrently.
</p>
<h6>
<a name="boost_asio.overview.core.threads.h1"></a>
<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
Pools</a>
</h6>
<p>
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
completion handlers may be invoked. This approach may also be used with
<code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
as a means to perform arbitrary computational tasks across a thread pool.
</p>
<p>
Note that all threads that have joined an <code class="computeroutput"><span class="identifier">io_context</span></code>'s
pool are considered equivalent, and the <code class="computeroutput"><span class="identifier">io_context</span></code>
may distribute work across them in an arbitrary fashion.
</p>
<h6>
<a name="boost_asio.overview.core.threads.h2"></a>
<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
Threads</a>
</h6>
<p>
The implementation of this library for a particular platform may make use
of one or more internal threads to emulate asynchronicity. As far as possible,
these threads must be invisible to the library user. In particular, the
threads:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
must not call the user's code directly; and
</li>
<li class="listitem">
must block all signals.
</li>
</ul></div>
<p>
This approach is complemented by the following guarantee:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
Asynchronous completion handlers will only be called from threads that
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>.
</li></ul></div>
<p>
Consequently, it is the library user's responsibility to create and manage
all threads to which the notifications will be delivered.
</p>
<p>
The reasons for this approach include:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
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
code can avoid the development complexity associated with synchronisation.
For example, a library user can implement scalable servers that are
single-threaded (from the user's point of view).
</li>
<li class="listitem">
A library user may need to perform initialisation in a thread shortly
after the thread starts and before any other application code is executed.
For example, users of Microsoft's COM must call <code class="computeroutput"><span class="identifier">CoInitializeEx</span></code>
before any other COM operations can be called from that thread.
</li>
<li class="listitem">
The library interface is decoupled from interfaces for thread creation
and management, and permits implementations on platforms where threads
are not available.
</li>
</ul></div>
<h6>
<a name="boost_asio.overview.core.threads.h3"></a>
<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
Also</a>
</h6>
<p>
<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>.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2018 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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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