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19 дек 2024, 19:11
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Frequently Asked Questions</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../process.html" title="Chapter 29. Boost.Process"> <link rel="prev" href="extend.html" title="Extensions"> <link rel="next" href="../process/reference.html" title="Reference"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="extend.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../process.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="../process/reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <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 automatic 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> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2006-2012 Julio M. Merino Vidal, Ilya Sokolov, Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright © 2016 Klemens D. Morgenstern<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="extend.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../process.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="../process/reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>