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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_process.faq"></a><a class="link" href="faq.html" title="Frequently Asked Questions">Frequently Asked Questions</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="faq.html#boost_process.faq.dead_lock">Why does this produce a
deadlock?</a></span></dt>
<dt><span class="section"><a href="faq.html#boost_process.faq.closep">Why does the pipe not close?</a></span></dt>
<dt><span class="section"><a href="faq.html#boost_process.faq.wchar_t">When will the codecvt be used?</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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
deadlock?</a>
</h3></div></div></div>
<p>
Now let's revisit our c++filt example and we will put in an obvious mistake.
This might however be not as obvious for more complex applications.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">vector</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">demangle</span><span class="special">(</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">in</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
<span class="identifier">opstream</span> <span class="identifier">os</span><span class="special">;</span>
<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">></span> <span class="identifier">is</span><span class="special">,</span> <span class="identifier">std_in</span> <span class="special"><</span> <span class="identifier">os</span><span class="special">);</span>
<span class="identifier">vector</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">data</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">auto</span> <span class="special">&</span> <span class="identifier">elem</span> <span class="special">:</span> <span class="identifier">data</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">string</span> <span class="identifier">line</span><span class="special">;</span>
<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>
<span class="identifier">os</span> <span class="special"><<</span> <span class="identifier">elem</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
We switched the read and write operation up, so that's causing a dead-lock.
This locks immediately. This is because <code class="computeroutput"><span class="identifier">c</span><span class="special">++</span><span class="identifier">filt</span></code>
expects input, before outputting any data. The launching process on the other
hand waits for its output.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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>
</h3></div></div></div>
<p>
Now for another example, which might look correct, let's consider you want
to use <code class="computeroutput"><span class="identifier">ls</span></code> to read the current
directory.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
<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">></span> <span class="identifier">is</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
<span class="keyword">while</span> <span class="special">(</span><span class="identifier">is</span> <span class="special">>></span> <span class="identifier">file</span><span class="special">)</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"File: "</span> <span class="special"><<</span> <span class="identifier">file</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
This will also deadlock, because the pipe does not close when the subprocess
exits. So the <code class="computeroutput"><span class="identifier">ipstream</span></code> will
still look for data even though the process has ended.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
It is not possible to use automatically pipe-closing in this library, because
a pipe might be a file-handle (as for async pipes on windows).
</p></td></tr>
</table></div>
<p>
But, since pipes are buffered, you might get incomplete data if you do this:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">ipstream</span> <span class="identifier">is</span><span class="special">;</span>
<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">></span> <span class="identifier">is</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">file</span><span class="special">;</span>
<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>
<span class="special">{</span>
<span class="identifier">is</span> <span class="special">>></span> <span class="identifier">file</span><span class="special">;</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"File: "</span> <span class="special"><<</span> <span class="identifier">file</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
It is therefore highly recommended that you use the asynchronous api if you
are not absolutely sure how the output will look.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<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>
</h3></div></div></div>
<p>
Since windows does not use UTF-8 it is sometimes unavoidable to use the
<code class="computeroutput"><span class="keyword">wchar_t</span></code> version of the WinApi.
To keep this library consistent it provides <code class="computeroutput"><span class="keyword">wchar_t</span></code>
support on posix also.
</p>
<p>
Since the posix api is purely <code class="computeroutput"><span class="keyword">char</span></code>
every <code class="computeroutput"><span class="keyword">wchar_t</span></code> based type will
be converted into <code class="computeroutput"><span class="keyword">char</span></code>.
</p>
<p>
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
<code class="computeroutput"><span class="keyword">wchar_t</span></code>. This also includes
<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
not provide the <code class="computeroutput"><span class="keyword">char</span></code> api (as
is the case with Windows CE) everything will also be converted.
</p>
</div>
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Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright © 2016 Klemens D. Morgenstern<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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