1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Integrating Fibers with Nonblocking I/O</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Chapter 1. Fiber"> 8<link rel="up" href="../index.html" title="Chapter 1. Fiber"> 9<link rel="prev" href="callbacks/then_there_s____boost_asio__.html" title="Then There’s Boost.Asio"> 10<link rel="next" href="when_any.html" title="when_any / when_all functionality"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="callbacks/then_there_s____boost_asio__.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="when_any.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="fiber.nonblocking"></a><a name="nonblocking"></a><a class="link" href="nonblocking.html" title="Integrating Fibers with Nonblocking I/O">Integrating 28 Fibers with Nonblocking I/O</a> 29</h2></div></div></div> 30<h4> 31<a name="fiber.nonblocking.h0"></a> 32 <span class="phrase"><a name="fiber.nonblocking.overview"></a></span><a class="link" href="nonblocking.html#fiber.nonblocking.overview">Overview</a> 33 </h4> 34<p> 35 <span class="emphasis"><em>Nonblocking</em></span> I/O is distinct from <span class="emphasis"><em>asynchronous</em></span> 36 I/O. A true async I/O operation promises to initiate the operation and notify 37 the caller on completion, usually via some sort of callback (as described in 38 <a class="link" href="callbacks.html#callbacks">Integrating Fibers with Asynchronous Callbacks</a>). 39 </p> 40<p> 41 In contrast, a nonblocking I/O operation refuses to start at all if it would 42 be necessary to block, returning an error code such as <a href="http://man7.org/linux/man-pages/man3/errno.3.html" target="_top"><code class="computeroutput"><span class="identifier">EWOULDBLOCK</span></code></a>. The operation is performed 43 only when it can complete immediately. In effect, the caller must repeatedly 44 retry the operation until it stops returning <code class="computeroutput"><span class="identifier">EWOULDBLOCK</span></code>. 45 </p> 46<p> 47 In a classic event-driven program, it can be something of a headache to use 48 nonblocking I/O. At the point where the nonblocking I/O is attempted, a return 49 value of <code class="computeroutput"><span class="identifier">EWOULDBLOCK</span></code> requires 50 the caller to pass control back to the main event loop, arranging to retry 51 again on the next iteration. 52 </p> 53<p> 54 Worse, a nonblocking I/O operation might <span class="emphasis"><em>partially</em></span> succeed. 55 That means that the relevant business logic must continue receiving control 56 on every main loop iteration until all required data have been processed: a 57 doubly-nested loop, implemented as a callback-driven state machine. 58 </p> 59<p> 60 <span class="bold"><strong>Boost.Fiber</strong></span> can simplify this problem immensely. 61 Once you have integrated with the application's main loop as described in 62 <a class="link" href="integration.html#integration">Sharing a Thread with Another Main Loop</a>, 63 waiting for the next main-loop iteration is as simple as calling <a class="link" href="fiber_mgmt/this_fiber.html#this_fiber_yield"><code class="computeroutput">this_fiber::yield()</code></a>. 64 </p> 65<h4> 66<a name="fiber.nonblocking.h1"></a> 67 <span class="phrase"><a name="fiber.nonblocking.example_nonblocking_api"></a></span><a class="link" href="nonblocking.html#fiber.nonblocking.example_nonblocking_api">Example 68 Nonblocking API</a> 69 </h4> 70<p> 71 For purposes of illustration, consider this API: 72 </p> 73<p> 74</p> 75<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">NonblockingAPI</span> <span class="special">{</span> 76<span class="keyword">public</span><span class="special">:</span> 77 <span class="identifier">NonblockingAPI</span><span class="special">();</span> 78 79 <span class="comment">// nonblocking operation: may return EWOULDBLOCK</span> 80 <span class="keyword">int</span> <span class="identifier">read</span><span class="special">(</span> <span class="identifier">std</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="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">desired</span><span class="special">);</span> 81 82 <span class="special">...</span> 83<span class="special">};</span> 84</pre> 85<p> 86 </p> 87<h4> 88<a name="fiber.nonblocking.h2"></a> 89 <span class="phrase"><a name="fiber.nonblocking.polling_for_completion"></a></span><a class="link" href="nonblocking.html#fiber.nonblocking.polling_for_completion">Polling 90 for Completion</a> 91 </h4> 92<p> 93 We can build a low-level wrapper around <code class="computeroutput"><span class="identifier">NonblockingAPI</span><span class="special">::</span><span class="identifier">read</span><span class="special">()</span></code> 94 that shields its caller from ever having to deal with <code class="computeroutput"><span class="identifier">EWOULDBLOCK</span></code>: 95 </p> 96<p> 97</p> 98<pre class="programlisting"><span class="comment">// guaranteed not to return EWOULDBLOCK</span> 99<span class="keyword">int</span> <span class="identifier">read_chunk</span><span class="special">(</span> <span class="identifier">NonblockingAPI</span> <span class="special">&</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">std</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="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">desired</span><span class="special">)</span> <span class="special">{</span> 100 <span class="keyword">int</span> <span class="identifier">error</span><span class="special">;</span> 101 <span class="keyword">while</span> <span class="special">(</span> <span class="identifier">EWOULDBLOCK</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">error</span> <span class="special">=</span> <span class="identifier">api</span><span class="special">.</span><span class="identifier">read</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">desired</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">{</span> 102 <span class="comment">// not ready yet -- try again on the next iteration of the</span> 103 <span class="comment">// application's main loop</span> 104 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">this_fiber</span><span class="special">::</span><span class="identifier">yield</span><span class="special">();</span> 105 <span class="special">}</span> 106 <span class="keyword">return</span> <span class="identifier">error</span><span class="special">;</span> 107<span class="special">}</span> 108</pre> 109<p> 110 </p> 111<h4> 112<a name="fiber.nonblocking.h3"></a> 113 <span class="phrase"><a name="fiber.nonblocking.filling_all_desired_data"></a></span><a class="link" href="nonblocking.html#fiber.nonblocking.filling_all_desired_data">Filling 114 All Desired Data</a> 115 </h4> 116<p> 117 Given <code class="computeroutput"><span class="identifier">read_chunk</span><span class="special">()</span></code>, 118 we can straightforwardly iterate until we have all desired data: 119 </p> 120<p> 121</p> 122<pre class="programlisting"><span class="comment">// keep reading until desired length, EOF or error</span> 123<span class="comment">// may return both partial data and nonzero error</span> 124<span class="keyword">int</span> <span class="identifier">read_desired</span><span class="special">(</span> <span class="identifier">NonblockingAPI</span> <span class="special">&</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">std</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="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">desired</span><span class="special">)</span> <span class="special">{</span> 125 <span class="comment">// we're going to accumulate results into 'data'</span> 126 <span class="identifier">data</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span> 127 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">chunk</span><span class="special">;</span> 128 <span class="keyword">int</span> <span class="identifier">error</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 129 <span class="keyword">while</span> <span class="special">(</span> <span class="identifier">data</span><span class="special">.</span><span class="identifier">length</span><span class="special">()</span> <span class="special"><</span> <span class="identifier">desired</span> <span class="special">&&</span> 130 <span class="special">(</span> <span class="identifier">error</span> <span class="special">=</span> <span class="identifier">read_chunk</span><span class="special">(</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">chunk</span><span class="special">,</span> <span class="identifier">desired</span> <span class="special">-</span> <span class="identifier">data</span><span class="special">.</span><span class="identifier">length</span><span class="special">()</span> <span class="special">)</span> <span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span> 131 <span class="identifier">data</span><span class="special">.</span><span class="identifier">append</span><span class="special">(</span> <span class="identifier">chunk</span><span class="special">);</span> 132 <span class="special">}</span> 133 <span class="keyword">return</span> <span class="identifier">error</span><span class="special">;</span> 134<span class="special">}</span> 135</pre> 136<p> 137 </p> 138<p> 139 (Of <span class="emphasis"><em>course</em></span> there are more efficient ways to accumulate 140 string data. That's not the point of this example.) 141 </p> 142<h4> 143<a name="fiber.nonblocking.h4"></a> 144 <span class="phrase"><a name="fiber.nonblocking.wrapping_it_up"></a></span><a class="link" href="nonblocking.html#fiber.nonblocking.wrapping_it_up">Wrapping 145 it Up</a> 146 </h4> 147<p> 148 Finally, we can define a relevant exception: 149 </p> 150<p> 151</p> 152<pre class="programlisting"><span class="comment">// exception class augmented with both partially-read data and errorcode</span> 153<span class="keyword">class</span> <span class="identifier">IncompleteRead</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span> <span class="special">{</span> 154<span class="keyword">public</span><span class="special">:</span> 155 <span class="identifier">IncompleteRead</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">what</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">partial</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">:</span> 156 <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span> <span class="identifier">what</span><span class="special">),</span> 157 <span class="identifier">partial_</span><span class="special">(</span> <span class="identifier">partial</span><span class="special">),</span> 158 <span class="identifier">ec_</span><span class="special">(</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">{</span> 159 <span class="special">}</span> 160 161 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">get_partial</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> 162 <span class="keyword">return</span> <span class="identifier">partial_</span><span class="special">;</span> 163 <span class="special">}</span> 164 165 <span class="keyword">int</span> <span class="identifier">get_errorcode</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> 166 <span class="keyword">return</span> <span class="identifier">ec_</span><span class="special">;</span> 167 <span class="special">}</span> 168 169<span class="keyword">private</span><span class="special">:</span> 170 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">partial_</span><span class="special">;</span> 171 <span class="keyword">int</span> <span class="identifier">ec_</span><span class="special">;</span> 172<span class="special">};</span> 173</pre> 174<p> 175 </p> 176<p> 177 and write a simple <code class="computeroutput"><span class="identifier">read</span><span class="special">()</span></code> 178 function that either returns all desired data or throws <code class="computeroutput"><span class="identifier">IncompleteRead</span></code>: 179 </p> 180<p> 181</p> 182<pre class="programlisting"><span class="comment">// read all desired data or throw IncompleteRead</span> 183<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">read</span><span class="special">(</span> <span class="identifier">NonblockingAPI</span> <span class="special">&</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">desired</span><span class="special">)</span> <span class="special">{</span> 184 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">data</span><span class="special">;</span> 185 <span class="keyword">int</span> <span class="identifier">ec</span><span class="special">(</span> <span class="identifier">read_desired</span><span class="special">(</span> <span class="identifier">api</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">desired</span><span class="special">)</span> <span class="special">);</span> 186 187 <span class="comment">// for present purposes, EOF isn't a failure</span> 188 <span class="keyword">if</span> <span class="special">(</span> <span class="number">0</span> <span class="special">==</span> <span class="identifier">ec</span> <span class="special">||</span> <span class="identifier">EOF</span> <span class="special">==</span> <span class="identifier">ec</span><span class="special">)</span> <span class="special">{</span> 189 <span class="keyword">return</span> <span class="identifier">data</span><span class="special">;</span> 190 <span class="special">}</span> 191 192 <span class="comment">// oh oh, partial read</span> 193 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostringstream</span> <span class="identifier">msg</span><span class="special">;</span> 194 <span class="identifier">msg</span> <span class="special"><<</span> <span class="string">"NonblockingAPI::read() error "</span> <span class="special"><<</span> <span class="identifier">ec</span> <span class="special"><<</span> <span class="string">" after "</span> 195 <span class="special"><<</span> <span class="identifier">data</span><span class="special">.</span><span class="identifier">length</span><span class="special">()</span> <span class="special"><<</span> <span class="string">" of "</span> <span class="special"><<</span> <span class="identifier">desired</span> <span class="special"><<</span> <span class="string">" characters"</span><span class="special">;</span> 196 <span class="keyword">throw</span> <span class="identifier">IncompleteRead</span><span class="special">(</span> <span class="identifier">msg</span><span class="special">.</span><span class="identifier">str</span><span class="special">(),</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">ec</span><span class="special">);</span> 197<span class="special">}</span> 198</pre> 199<p> 200 </p> 201<p> 202 Once we can transparently wait for the next main-loop iteration using <a class="link" href="fiber_mgmt/this_fiber.html#this_fiber_yield"><code class="computeroutput">this_fiber::yield()</code></a>, 203 ordinary encapsulation Just Works. 204 </p> 205<p> 206 The source code above is found in <a href="../../../examples/adapt_nonblocking.cpp" target="_top">adapt_nonblocking.cpp</a>. 207 </p> 208</div> 209<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 210<td align="left"></td> 211<td align="right"><div class="copyright-footer">Copyright © 2013 Oliver Kowalke<p> 212 Distributed under the Boost Software License, Version 1.0. (See accompanying 213 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 214 </p> 215</div></td> 216</tr></table> 217<hr> 218<div class="spirit-nav"> 219<a accesskey="p" href="callbacks/then_there_s____boost_asio__.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="when_any.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 220</div> 221</body> 222</html> 223
