1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>errno - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../motivation/exceptions.html"><img src="../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../motivation.html"><img src="../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../motivation/error_codes.html"><img src="../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">errno</h1></div></div></div> 14 15 16<p>The idiom of returning, upon failure, a special value and storing an error code 17(an <code>int</code>) inside a global (or thread-local) object <code>errno</code> is inherited from C, 18and used in its Standard Library:</p> 19<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">int</span> <span class="nf">readValue</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span> <span class="n">filename</span><span class="p">)</span> 20<span class="p">{</span> 21 <span class="n">FILE</span><span class="o">*</span> <span class="n">f</span> <span class="o">=</span> <span class="n">fopen</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s">"r"</span><span class="p">);</span> 22 <span class="k">if</span> <span class="p">(</span><span class="n">f</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> 23 <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="c1">// special value indicating failure 24</span><span class="c1"></span> <span class="c1">// keep errno value set by fopen() 25</span><span class="c1"></span> 26 <span class="kt">int</span> <span class="n">i</span><span class="p">;</span> 27 <span class="kt">int</span> <span class="n">r</span> <span class="o">=</span> <span class="n">fscanf</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="s">"%d"</span><span class="p">,</span> <span class="o">&</span><span class="n">i</span><span class="p">);</span> 28 <span class="k">if</span> <span class="p">(</span><span class="n">r</span> <span class="o">==</span> <span class="mi">0</span> <span class="o">||</span> <span class="n">r</span> <span class="o">==</span> <span class="n">EOF</span><span class="p">)</span> <span class="p">{</span> <span class="c1">// special values: i not read 29</span><span class="c1"></span> <span class="n">errno</span> <span class="o">=</span> <span class="n">ENODATA</span><span class="p">;</span> <span class="c1">// choose error value to return 30</span><span class="c1"></span> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> 31 32 <span class="n">fclose</span><span class="p">(</span><span class="n">f</span><span class="p">);</span> 33 <span class="n">errno</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="c1">// clear error info (success) 34</span><span class="c1"></span> <span class="k">return</span> <span class="n">i</span><span class="p">;</span> 35<span class="p">}</span> 36</code></pre></div> 37<p>One advantage (to some, and a disadvantage to others) of this technique is that it 38uses familiar control statements (<code>if</code> and <code>return</code>) to indicate all execution 39paths that handle failures. When we read this code we know when and under what 40conditions it can exit without producing the expected result.</p> 41 42<h3 id="downsides">Downsides</h3> 43 44<p>Because on failure, as well as success, we write into a global (or thread-local) 45object, our functions are not <em>pure</em>: they have <em>side effects</em>. This means many 46useful compiler optimizations (like common subexpression elimination) cannot be 47applied. This shows that it is not only C++ that chooses suboptimal solutions 48for reporting failures.</p> 49 50<p>Whatever type we return, we always need a special value to spare, which is 51sometimes troublesome. In the above example, if the successfully read value of 52<code>i</code> is <code>0</code>, and we return it, our callers will think it is a failure even though 53it is not.</p> 54 55<p>Error propagation using <code>if</code> statements and early <code>return</code>s is manual. We can easily 56forget to check for the failure, and incorrectly let the subsequent operations 57execute, potentially causing damage to the program state.</p> 58 59<p>Upon nearly each function call layer we may have to change error code value 60so that it reflects the error condition adequate to the current layer. If we 61do so, the original error code is gone.</p> 62 63 64 </div><p><small>Last revised: January 16, 2019 at 01:05:39 +0100</small></p> 65<hr> 66<div class="spirit-nav"> 67<a accesskey="p" href="../motivation/exceptions.html"><img src="../images/prev.png" alt="Prev"></a> 68 <a accesskey="u" href="../motivation.html"><img src="../images/up.png" alt="Up"></a> 69 <a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../motivation/error_codes.html"><img src="../images/next.png" alt="Next"></a></div></body> 70</html> 71
