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<link rel="home" href="../../index.html" title="Chapter 1. Context">
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<a accesskey="p" href="../rationale.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../rationale.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="x86_and_floating_point_env.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="context.rationale.other_apis_"></a><a class="link" href="other_apis_.html" title="Other APIs">Other APIs </a>
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</h3></div></div></div>
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<h5>
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<a name="context.rationale.other_apis_.h0"></a>
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<span><a name="context.rationale.other_apis_.setjmp___longjmp__"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.setjmp___longjmp__">setjmp()/longjmp()</a>
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</h5>
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<p>
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C99 defines <code class="computeroutput"><span class="identifier">setjmp</span><span class="special">()</span></code>/<code class="computeroutput"><span class="identifier">longjmp</span><span class="special">()</span></code>
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to provide non-local jumps but it does not require that <span class="emphasis"><em>longjmp()</em></span>
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preserves the current stack frame. Therefore, jumping into a function which
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was exited via a call to <span class="emphasis"><em>longjmp()</em></span> is undefined <sup>[<a name="context.rationale.other_apis_.f0" href="#ftn.context.rationale.other_apis_.f0" class="footnote">2</a>]</sup>.
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</p>
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<a name="ucontext"></a><h5>
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<a name="context.rationale.other_apis_.h1"></a>
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<span><a name="context.rationale.other_apis_.ucontext_t"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.ucontext_t">ucontext_t</a>
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</h5>
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<p>
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Since POSIX.1-2004 <code class="computeroutput"><span class="identifier">ucontext_t</span></code>
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is deprecated and was removed in POSIX.1-2008! The function signature of
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<code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code>
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is:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">makecontext</span><span class="special">(</span><span class="identifier">ucontext_t</span> <span class="special">*</span><span class="identifier">ucp</span><span class="special">,</span> <span class="keyword">void</span> <span class="special">(*</span><span class="identifier">func</span><span class="special">)(),</span> <span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="special">...);</span>
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</pre>
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<p>
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The third argument of <code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code> specifies the number of integer arguments
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that follow which will require function pointer cast if <code class="computeroutput"><span class="identifier">func</span></code>
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will accept those arguments which is undefined in C99 <sup>[<a name="context.rationale.other_apis_.f1" href="#ftn.context.rationale.other_apis_.f1" class="footnote">3</a>]</sup>.
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</p>
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<p>
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The arguments in the var-arg list are required to be integers, passing pointers
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in var-arg list is not guaranteed to work, especially it will fail for architectures
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where pointers are larger than integers.
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</p>
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<p>
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<code class="computeroutput"><span class="identifier">ucontext_t</span></code> preserves signal
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mask between context switches which involves system calls consuming a lot
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of CPU cycles (ucontext_t is slower; a context switch takes <a class="link" href="../performance.html#performance"><span class="emphasis"><em>two
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magnitutes of order more CPU cycles</em></span></a> more than <span class="emphasis"><em>fcontext_t</em></span>).
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</p>
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<h5>
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<a name="context.rationale.other_apis_.h2"></a>
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<span><a name="context.rationale.other_apis_.windows_fibers"></a></span><a class="link" href="other_apis_.html#context.rationale.other_apis_.windows_fibers">Windows
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fibers</a>
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</h5>
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<p>
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A drawback of Windows Fiber API is that <code class="computeroutput"><span class="identifier">CreateFiber</span><span class="special">()</span></code> does not accept a pointer to user allocated
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stack space preventing the reuse of stacks for other context instances. Because
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the Windows Fiber API requires to call <code class="computeroutput"><span class="identifier">ConvertThreadToFiber</span><span class="special">()</span></code> if <code class="computeroutput"><span class="identifier">SwitchFiber</span><span class="special">()</span></code> is called for a thread which has not been
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converted to a fiber. For the same reason <code class="computeroutput"><span class="identifier">ConvertFiberToThread</span><span class="special">()</span></code> must be called after return from <code class="computeroutput"><span class="identifier">SwitchFiber</span><span class="special">()</span></code>
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if the thread was forced to be converted to a fiber before (which is inefficient).
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</p>
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<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span> <span class="identifier">is_a_fiber</span><span class="special">()</span> <span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">ConvertThreadToFiber</span><span class="special">(</span> <span class="number">0</span><span class="special">);</span>
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<span class="identifier">SwitchToFiber</span><span class="special">(</span> <span class="identifier">ctx</span><span class="special">);</span>
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<span class="identifier">ConvertFiberToThread</span><span class="special">();</span>
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<span class="special">}</span>
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</pre>
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<p>
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If the condition <code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">>=</span> <span class="identifier">_WIN32_WINNT_VISTA</span></code>
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is met function <code class="computeroutput"><span class="identifier">IsThreadAFiber</span><span class="special">()</span></code> is provided in order to detect if the current
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thread was already converted. Unfortunately Windows XP + SP 2/3 defines
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<code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">>=</span>
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<span class="identifier">_WIN32_WINNT_VISTA</span></code> without providing
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<code class="computeroutput"><span class="identifier">IsThreadAFiber</span><span class="special">()</span></code>.
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</p>
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<div class="footnotes">
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<br><hr width="100" align="left">
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<div class="footnote"><p><sup>[<a name="ftn.context.rationale.other_apis_.f0" href="#context.rationale.other_apis_.f0" class="para">2</a>] </sup>
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ISO/IEC 9899:1999, 2005, 7.13.2.1:2
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</p></div>
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<div class="footnote"><p><sup>[<a name="ftn.context.rationale.other_apis_.f1" href="#context.rationale.other_apis_.f1" class="para">3</a>] </sup>
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ISO/IEC 9899:1999, 2005, J.2
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</p></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2014 Oliver Kowalke<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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