ceph/src/boost/libs/context/doc/html/context/rationale/other_apis_.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="context.rationale.other_apis_"></a><a class="link" href="other_apis_.html" title="Other APIs">Other APIs </a>
  28 </h3></div></div></div>
  29 <h5>
  30 <a name="context.rationale.other_apis_.h0"></a>
  31         <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>
  32       </h5>
  33 <p>
  34         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>
  35         to provide non-local jumps but it does not require that <span class="emphasis"><em>longjmp()</em></span>
  36         preserves the current stack frame. Therefore, jumping into a function which
  37         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>.
  38       </p>
  39 <h5>
  40 <a name="context.rationale.other_apis_.h1"></a>
  41         <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>
  42       </h5>
  43 <p>
  44         Since POSIX.1-2003 <code class="computeroutput"><span class="identifier">ucontext_t</span></code>
  45         is deprecated and was removed in POSIX.1-2008! The function signature of
  46         <code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code>
  47         is:
  48       </p>
  49 <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>
  50 </pre>
  51 <p>
  52         The third argument of <code class="computeroutput"><span class="identifier">makecontext</span><span class="special">()</span></code> specifies the number of integer arguments
  53         that follow which will require function pointer cast if <code class="computeroutput"><span class="identifier">func</span></code>
  54         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>.
  55       </p>
  56 <p>
  57         The arguments in the var-arg list are required to be integers, passing pointers
  58         in var-arg list is not guaranteed to work, especially it will fail for architectures
  59         where pointers are larger than integers.
  60       </p>
  61 <p>
  62         <code class="computeroutput"><span class="identifier">ucontext_t</span></code> preserves signal
  63         mask between context switches which involves system calls consuming a lot
  64         of CPU cycles (ucontext_t is slower by perfomance_link[factor 13x] relative
  65         to <code class="computeroutput"><span class="identifier">fcontext_t</span></code>).
  66       </p>
  67 <h5>
  68 <a name="context.rationale.other_apis_.h2"></a>
  69         <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
  70         fibers</a>
  71       </h5>
  72 <p>
  73         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
  74         stack space preventing the reuse of stacks for other context instances. Because
  75         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
  76         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>
  77         if the thread was forced to be converted to a fiber before (which is inefficient).
  78       </p>
  79 <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>
  80 <span class="special">{</span>
  81     <span class="identifier">ConvertThreadToFiber</span><span class="special">(</span> <span class="number">0</span><span class="special">);</span>
  82     <span class="identifier">SwitchToFiber</span><span class="special">(</span> <span class="identifier">ctx</span><span class="special">);</span>
  83     <span class="identifier">ConvertFiberToThread</span><span class="special">();</span>
  84 <span class="special">}</span>
  85 </pre>
  86 <p>
  87         If the condition <code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">&gt;=</span> <span class="identifier">_WIN32_WINNT_VISTA</span></code>
  88         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
  89         thread was already converted. Unfortunately Windows XP + SP 2/3 defines
  90         <code class="computeroutput"><span class="identifier">_WIN32_WINNT</span> <span class="special">&gt;=</span>
  91         <span class="identifier">_WIN32_WINNT_VISTA</span></code> without providing
  92         <code class="computeroutput"><span class="identifier">IsThreadAFiber</span><span class="special">()</span></code>.
  93       </p>
  94 <div class="footnotes">
  95 <br><hr width="100" align="left">
  96 <div class="footnote"><p><sup>[<a name="ftn.context.rationale.other_apis_.f0" href="#context.rationale.other_apis_.f0" class="para">2</a>] </sup>
  97           ISO/IEC 9899:1999, 2005, 7.13.2.1:2
  98         </p></div>
  99 <div class="footnote"><p><sup>[<a name="ftn.context.rationale.other_apis_.f1" href="#context.rationale.other_apis_.f1" class="para">3</a>] </sup>
 100           ISO/IEC 9899:1999, 2005, J.2
 101         </p></div>
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 104 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 105 <td align="left"></td>
 106 <td align="right"><div class="copyright-footer">Copyright &#169; 2014 Oliver Kowalke<p>
 107         Distributed under the Boost Software License, Version 1.0. (See accompanying
 108         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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