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  26 <div class="section">
  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="boost_process.concepts"></a><a class="link" href="concepts.html" title="Concepts">Concepts</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="concepts.html#boost_process.concepts.pipes">Pipes</a></span></dt>
  32 <dt><span class="section"><a href="concepts.html#boost_process.concepts.process">Processes</a></span></dt>
  33 <dt><span class="section"><a href="concepts.html#boost_process.concepts.env">Environment</a></span></dt>
  34 </dl></div>
  35 <p>
  36       In this section, some of the underlying concepts of the operating system used
  37       in this library, will be explained. In the following chapters we will presume
  38       knowledge of that. Though please note, that this is a short summary and not
  39       conclusive of everything that can be done.
  40     </p>
  41 <p>
  42       The goal of this library is to implement a portable wrapper, so that we will
  43       explain mostly what windows and posix have in common.
  44     </p>
  45 <div class="section">
  46 <div class="titlepage"><div><div><h3 class="title">
  47 <a name="boost_process.concepts.pipes"></a><a class="link" href="concepts.html#boost_process.concepts.pipes" title="Pipes">Pipes</a>
  48 </h3></div></div></div>
  49 <div class="toc"><dl class="toc">
  50 <dt><span class="section"><a href="concepts.html#boost_process.concepts.pipes.anonymous">Anonymous Pipes</a></span></dt>
  51 <dt><span class="section"><a href="concepts.html#boost_process.concepts.pipes.named">Named Pipes</a></span></dt>
  52 </dl></div>
  53 <p>
  54         Pipes are a facility for communication between different threads, processes
  55         and in some cases machines, the operating system provides.
  56       </p>
  57 <p>
  58         The typical feature of a pipe is, that it is one channel, to which two handles
  59         are given, one for reading (source), one for writing (sink). In that it is
  60         different than other facilities (like sockets) and provides another way to
  61         manage the connectivity: if one side of the pipe is closed (i.e. the pipe
  62         is broken), the other is notified.
  63       </p>
  64 <p>
  65         Pipes are typically used for interprocess communication. The main reason
  66         is, that pipes can be directly assigned to the process stdio, i.e. stderr,
  67         stdin and stdout. Additionally, half of the pipe can be inherited to the
  68         child process and closed in the father process. This will cause the pipe
  69         to be broken when the child process exits.
  70       </p>
  71 <p>
  72         Though please note, that if the the same thread reads and write to a pipe,
  73         it will only talk to itself.
  74       </p>
  75 <div class="section">
  76 <div class="titlepage"><div><div><h4 class="title">
  77 <a name="boost_process.concepts.pipes.anonymous"></a><a class="link" href="concepts.html#boost_process.concepts.pipes.anonymous" title="Anonymous Pipes">Anonymous Pipes</a>
  78 </h4></div></div></div>
  79 <p>
  80           The usual type of pipes, are the anonymous ones. Since the have no name,
  81           a handle to them can only be obtained from duplicating either handle.
  82         </p>
  83 <p>
  84           In this library the following functions are used for the creation of unnamed
  85           pipes:
  86         </p>
  87 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  88 <li class="listitem">
  89               <a href="http://pubs.opengroup.org/onlinepubs/7908799/xsh/pipe.html" target="_top">posix</a>
  90             </li>
  91 <li class="listitem">
  92               <a href="https://msdn.microsoft.com/de-de/library/windows/desktop/aa365152.aspx" target="_top">windows</a>
  93             </li>
  94 </ul></div>
  95 </div>
  96 <div class="section">
  97 <div class="titlepage"><div><div><h4 class="title">
  98 <a name="boost_process.concepts.pipes.named"></a><a class="link" href="concepts.html#boost_process.concepts.pipes.named" title="Named Pipes">Named Pipes</a>
  99 </h4></div></div></div>
 100 <p>
 101           As the name suggests, named pipes have a string identifier. This means
 102           that a handle to them can be obtained with the identifier, too.
 103         </p>
 104 <p>
 105           The implementation on posix uses <a href="(http://pubs.opengroup.org/onlinepubs/009695399/functions/mkfifo.html" target="_top">fifos</a>,
 106           which means, that the named pipe behaves like a file.
 107         </p>
 108 <p>
 109           Windows does provide a facility called <a href="https://msdn.microsoft.com/en-us/library/windows/desktop/aa365150(v=vs.85).aspx" target="_top">named
 110           pipes</a>, which also have file-like names, but are in a different
 111           scope than the actual file system.
 112         </p>
 113 <div class="note"><table border="0" summary="Note">
 114 <tr>
 115 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 116 <th align="left">Note</th>
 117 </tr>
 118 <tr><td align="left" valign="top"><p>
 119             The main reason named pipes are part of this library, is because they
 120             need to be internally used for asynchrounous communication on windows.
 121           </p></td></tr>
 122 </table></div>
 123 </div>
 124 </div>
 125 <div class="section">
 126 <div class="titlepage"><div><div><h3 class="title">
 127 <a name="boost_process.concepts.process"></a><a class="link" href="concepts.html#boost_process.concepts.process" title="Processes">Processes</a>
 128 </h3></div></div></div>
 129 <div class="toc"><dl class="toc">
 130 <dt><span class="section"><a href="concepts.html#boost_process.concepts.process.exit_code">Exit code</a></span></dt>
 131 <dt><span class="section"><a href="concepts.html#boost_process.concepts.process.termination">Termination</a></span></dt>
 132 </dl></div>
 133 <p>
 134         A process is an independently executable entity, which is different from
 135         a thread, in that it has its own resources. Those include memory and hardware
 136         resources.
 137       </p>
 138 <p>
 139         Every process is identified by a unique number<a href="#ftn.boost_process.concepts.process.f0" class="footnote" name="boost_process.concepts.process.f0"><sup class="footnote">[28]</sup></a>, called the process identification digit, <code class="computeroutput"><span class="identifier">pid</span></code>.
 140       </p>
 141 <div class="section">
 142 <div class="titlepage"><div><div><h4 class="title">
 143 <a name="boost_process.concepts.process.exit_code"></a><a class="link" href="concepts.html#boost_process.concepts.process.exit_code" title="Exit code">Exit code</a>
 144 </h4></div></div></div>
 145 <p>
 146           A process will return an integer value indicating whether it was successful.
 147           On posix there are more codes associated with that, but not so on windows.
 148           Therefore there is not such encoding currently in the library. However
 149           an exit code of zero means the process was successful, while one different
 150           than zero indicates an error.
 151         </p>
 152 </div>
 153 <div class="section">
 154 <div class="titlepage"><div><div><h4 class="title">
 155 <a name="boost_process.concepts.process.termination"></a><a class="link" href="concepts.html#boost_process.concepts.process.termination" title="Termination">Termination</a>
 156 </h4></div></div></div>
 157 <p>
 158           Processes can also be forced to exit. There are two ways to do this, signal
 159           the process to so and wait, and just terminate the process without conditions.
 160         </p>
 161 <p>
 162           Usually the first approach is to signal an exit request, but windows -
 163           unlike posix - does not provide a consistent way to do this. Hence this
 164           is not part of the library and only the hard terminate is.
 165         </p>
 166 </div>
 167 </div>
 168 <div class="section">
 169 <div class="titlepage"><div><div><h3 class="title">
 170 <a name="boost_process.concepts.env"></a><a class="link" href="concepts.html#boost_process.concepts.env" title="Environment">Environment</a>
 171 </h3></div></div></div>
 172 <p>
 173         The environment is a map of variables local to every process. The most significant
 174         one for this library is the <code class="computeroutput"><span class="identifier">PATH</span></code>
 175         variable, which contains a list of paths, that ought to be searched for executables.
 176         A shell will do this automatically, while this library provides a function
 177         for that.
 178       </p>
 179 </div>
 180 <div class="footnotes">
 181 <br><hr style="width:100; text-align:left;margin-left: 0">
 182 <div id="ftn.boost_process.concepts.process.f0" class="footnote"><p><a href="#boost_process.concepts.process.f0" class="para"><sup class="para">[28] </sup></a>
 183           it is unique as long as the process is active
 184         </p></div>
 185 </div>
 186 </div>
 187 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 188 <td align="left"></td>
 189 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2012 Julio M. Merino Vidal, Ilya Sokolov,
 190       Felipe Tanus, Jeff Flinn, Boris Schaeling<br>Copyright &#169; 2016 Klemens D. Morgenstern<p>
 191         Distributed under the Boost Software License, Version 1.0. (See accompanying
 192         file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 193       </p>
 194 </div></td>
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