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  26 <div class="section">
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  28 <a name="boost_process.faq"></a><a class="link" href="faq.html" title="Frequently Asked Questions">Frequently Asked Questions</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="faq.html#boost_process.faq.dead_lock">Why does this produce a
  32       deadlock?</a></span></dt>
  33 <dt><span class="section"><a href="faq.html#boost_process.faq.closep">Why does the pipe not close?</a></span></dt>
  34 <dt><span class="section"><a href="faq.html#boost_process.faq.wchar_t">When will the codecvt be used?</a></span></dt>
  35 </dl></div>
  36 <div class="section">
  37 <div class="titlepage"><div><div><h3 class="title">
  38 <a name="boost_process.faq.dead_lock"></a><a class="link" href="faq.html#boost_process.faq.dead_lock" title="Why does this produce a deadlock?">Why does this produce a
  39       deadlock?</a>
  40 </h3></div></div></div>
  41 <p>
  42         Now let's revisit our c++filt example and we will put in an obvious mistake.
  43         This might however be not as obvious for more complex applications.
  44       </p>
  45 <p>
  46 </p>
  47 <pre class="programlisting"><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">demangle</span><span class="special">(</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">in</span><span class="special">)</span>
  48 <span class="special">{</span>
  49
  50     <span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
  51     <span class="identifier">opstream</span> <span class="identifier">os</span><span class="special">;</span>
  52     <span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"c++filt"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">,</span> <span class="identifier">std_in</span> <span class="special">&lt;</span> <span class="identifier">os</span><span class="special">);</span>
  53
  54     <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">data</span><span class="special">;</span>
  55     <span class="keyword">for</span> <span class="special">(</span><span class="keyword">auto</span> <span class="special">&amp;</span> <span class="identifier">elem</span> <span class="special">:</span> <span class="identifier">data</span><span class="special">)</span>
  56     <span class="special">{</span>
  57         <span class="identifier">string</span> <span class="identifier">line</span><span class="special">;</span>
  58         <span class="identifier">getline</span><span class="special">(</span><span class="identifier">is</span><span class="special">,</span> <span class="identifier">line</span><span class="special">);</span>
  59         <span class="identifier">os</span> <span class="special">&lt;&lt;</span> <span class="identifier">elem</span><span class="special">;</span>
  60     <span class="special">}</span>
  61 <span class="special">}</span>
  62 </pre>
  63 <p>
  64         We switched the read and write operation up, so that's causing a dead-lock.
  65         This locks immediately. This is because <code class="computeroutput"><span class="identifier">c</span><span class="special">++</span><span class="identifier">filt</span></code>
  66         expects input, before outputting any data. The launching process on the other
  67         hand wait's for it's output.
  68       </p>
  69 </div>
  70 <div class="section">
  71 <div class="titlepage"><div><div><h3 class="title">
  72 <a name="boost_process.faq.closep"></a><a class="link" href="faq.html#boost_process.faq.closep" title="Why does the pipe not close?">Why does the pipe not close?</a>
  73 </h3></div></div></div>
  74 <p>
  75         Now for another example, which might look correct, let's consider you want
  76         to use <code class="computeroutput"><span class="identifier">ls</span></code> to read the current
  77         directory.
  78       </p>
  79 <p>
  80 </p>
  81 <pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
  82 <span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"ls"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">);</span>
  83
  84 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
  85 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">is</span> <span class="special">&gt;&gt;</span> <span class="identifier">file</span><span class="special">)</span>
  86     <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"File: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">file</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
  87 </pre>
  88 <p>
  89       </p>
  90 <p>
  91         This will also deadlock, because the pipe does not close when the subprocess
  92         exits. So the <code class="computeroutput"><span class="identifier">ipstream</span></code> will
  93         still look for data even though the process has ended.
  94       </p>
  95 <div class="note"><table border="0" summary="Note">
  96 <tr>
  97 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
  98 <th align="left">Note</th>
  99 </tr>
 100 <tr><td align="left" valign="top"><p>
 101           It is not possible to use automatically pipe-closing in this library, because
 102           a pipe might be a file-handle (as for async pipes on windows).
 103         </p></td></tr>
 104 </table></div>
 105 <p>
 106         But, since pipes are buffered, you might get incomplete data if you do this:
 107       </p>
 108 <p>
 109 </p>
 110 <pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
 111 <span class="identifier">child</span> <span class="identifier">c</span><span class="special">(</span><span class="string">"ls"</span><span class="special">,</span> <span class="identifier">std_out</span> <span class="special">&gt;</span> <span class="identifier">is</span><span class="special">);</span>
 112
 113 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
 114 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">running</span><span class="special">())</span>
 115 <span class="special">{</span>
 116     <span class="identifier">is</span> <span class="special">&gt;&gt;</span> <span class="identifier">file</span><span class="special">;</span>
 117     <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"File: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">file</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 118 <span class="special">}</span>
 119 </pre>
 120 <p>
 121       </p>
 122 <p>
 123         It is therefore highly recommended that you use the asynchronous api if you
 124         are not absolutely sure how the output will look.
 125       </p>
 126 </div>
 127 <div class="section">
 128 <div class="titlepage"><div><div><h3 class="title">
 129 <a name="boost_process.faq.wchar_t"></a><a class="link" href="faq.html#boost_process.faq.wchar_t" title="When will the codecvt be used?">When will the codecvt be used?</a>
 130 </h3></div></div></div>
 131 <p>
 132         Since windows does not use UTF-8 it is sometimes unavoidable to use the
 133         <code class="computeroutput"><span class="keyword">wchar_t</span></code> version of the WinApi.
 134         To keep this library consistent it provides <code class="computeroutput"><span class="keyword">wchar_t</span></code>
 135         support on posix also.
 136       </p>
 137 <p>
 138         Since the posix api is purely <code class="computeroutput"><span class="keyword">char</span></code>
 139         every <code class="computeroutput"><span class="keyword">wchar_t</span></code> based type will
 140         be converted into <code class="computeroutput"><span class="keyword">char</span></code>.
 141       </p>
 142 <p>
 143         Windows on the other hand is more selective; the default is to use <code class="computeroutput"><span class="keyword">char</span></code>, but if any parameter requires <code class="computeroutput"><span class="keyword">wchar_t</span></code>, everything will be converted to
 144         <code class="computeroutput"><span class="keyword">wchar_t</span></code>. This also includes
 145         <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span></code>. Additionally, if the system does
 146         not provide the <code class="computeroutput"><span class="keyword">char</span></code> api (as
 147         is the case with Windows CE) everything will also be converted.
 148       </p>
 149 </div>
 150 </div>
 151 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 152 <td align="left"></td>
 153 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2012 Julio M. Merino Vidal, Ilya Sokolov,
 154       Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright &#169; 2016 Klemens D. Morgenstern<p>
 155         Distributed under the Boost Software License, Version 1.0. (See accompanying
 156         file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 157       </p>
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