ceph/src/boost/libs/context/doc/html/context/overview.html

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  27 <a name="context.overview"></a><a class="link" href="overview.html" title="Overview">Overview</a>
  28 </h2></div></div></div>
  29 <p>
  30       <span class="bold"><strong>Boost.Context</strong></span> is a foundational library that
  31       provides a sort of cooperative multitasking on a single thread. By providing
  32       an abstraction of the current execution state in the current thread, including
  33       the stack (with local variables) and stack pointer, all registers and CPU flags,
  34       and the instruction pointer, a <span class="emphasis"><em>execution_context</em></span> represents
  35       a specific point in the application's execution path. This is useful for building
  36       higher-level abstractions, like <span class="emphasis"><em>coroutines</em></span>, <span class="emphasis"><em>cooperative
  37       threads (userland threads)</em></span> or an equivalent to <a href="http://msdn.microsoft.com/en-us/library/9k7k7cf0%28v=vs.80%29.aspx" target="_top">C#
  38       keyword <span class="emphasis"><em>yield</em></span></a> in C++.
  39     </p>
  40 <p>
  41       <span class="emphasis"><em>execution_context</em></span> provides the means to suspend the current
  42       execution path and to transfer execution control, thereby permitting another
  43       context to run on the current thread. This state full transfer mechanism enables
  44       a context to suspend execution from within nested functions and, later, to
  45       resume from where it was suspended. While the execution path represented by
  46       a <span class="emphasis"><em>execution_context</em></span> only runs on a single thread, it can
  47       be migrated to another thread at any given time.
  48     </p>
  49 <p>
  50       A context switch between threads requires system calls (involving the OS kernel),
  51       which can cost more than thousand CPU cycles on x86 CPUs. By contrast, transferring
  52       control among them requires only few CPU cycles because it does not involve
  53       system calls as it is done within a single thread.
  54     </p>
  55 <p>
  56       In order to use the classes and functions described here, you can either include
  57       the specific headers specified by the descriptions of each class or function,
  58       or include the master library header:
  59     </p>
  60 <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">context</span><span class="special">/</span><span class="identifier">all</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  61 </pre>
  62 <p>
  63       which includes all the other headers in turn.
  64     </p>
  65 <p>
  66       All functions and classes are contained in the namespace <span class="emphasis"><em>boost::context</em></span>.
  67     </p>
  68 <div class="important"><table border="0" summary="Important">
  69 <tr>
  70 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
  71 <th align="left">Important</th>
  72 </tr>
  73 <tr><td align="left" valign="top"><p>
  74         <span class="emphasis"><em>execution_context</em></span> requires C++11!
  75       </p></td></tr>
  76 </table></div>
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  79 <td align="left"></td>
  80 <td align="right"><div class="copyright-footer">Copyright &#169; 2014 Oliver Kowalke<p>
  81         Distributed under the Boost Software License, Version 1.0. (See accompanying
  82         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>)
  83       </p>
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