1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Frequently Asked Questions</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../process.html" title="Chapter 29. Boost.Process"> 10<link rel="prev" href="extend.html" title="Extensions"> 11<link rel="next" href="../process/reference.html" title="Reference"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 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"><</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> 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">></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> 53 54 <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> 55 <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> 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"><<</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 waits for its 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">></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">>></span> <span class="identifier">file</span><span class="special">)</span> 86 <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> 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 automatic 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">></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">>></span> <span class="identifier">file</span><span class="special">;</span> 117 <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> 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 © 2006-2012 Julio M. Merino Vidal, Ilya Sokolov, 154 Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright © 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> 158</div></td> 159</tr></table> 160<hr> 161<div class="spirit-nav"> 162<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> 163</div> 164</body> 165</html> 166
