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| 4 | <title>Integrating Fibers with Nonblocking I/O</title> | |
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| 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><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><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><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><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><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> | |
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