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<title>Calling it from C - Boost.Outcome documentation</title>
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<body><div id="boost-common-heading-doc" style="background: #574D74 url(/boost-outcome/images/header-bg.png) repeat-x top left;">
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<h1 class="heading-title">
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<a href="/">
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<img src="/boost-outcome/images/space.png" alt="Boost C++ Libraries" class="heading-logo" />
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<span class="heading-boost">Boost</span>
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<span class="heading-cpplibraries">C++ Libraries</span>
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<p class="heading-quote">
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<q>...one of the most highly
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regarded and expertly designed C++ library projects in the
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world.</q> <span class="heading-attribution">— <a href=
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"http://www.gotw.ca/" class="external">Herb Sutter</a> and <a href=
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"http://en.wikipedia.org/wiki/Andrei_Alexandrescu" class="external">Andrei
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Alexandrescu</a>, <a href=
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"http://safari.awprofessional.com/?XmlId=0321113586" class="external">C++
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Coding Standards</a></span></p>
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</div><div id="content">
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<p>Now let us call our <code>result</code> returning C++ function from C:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp">
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</span><span class="cp">#include</span> <span class="cpf"><string.h> // for strerror</span><span class="cp">
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</span><span class="cp"></span><span class="c1">// This header in Outcome is pure C, it provides a suite of C helper macros
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</span><span class="c1"></span><span class="cp">#include</span> <span class="cpf">"../../../include/outcome/result.h"</span><span class="cp">
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</span><span class="cp"></span>
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<span class="c1">// Declare our C++ function's returning result type. Only needs to be done once.
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</span><span class="c1"></span><span class="n">CXX_DECLARE_RESULT_EC</span><span class="p">(</span><span class="n">size_t</span><span class="p">,</span> <span class="n">size_t</span><span class="p">);</span>
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<span class="c1">// Tell C about our C++ function
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</span><span class="c1"></span><span class="k">extern</span> <span class="nf">CXX_RESULT_EC</span><span class="p">(</span><span class="n">size_t</span><span class="p">)</span> <span class="n">to_string</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">buffer</span><span class="p">,</span> <span class="n">size_t</span> <span class="n">bufferlen</span><span class="p">,</span> <span class="kt">int</span> <span class="n">v</span><span class="p">);</span>
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<span class="kt">void</span> <span class="nf">print</span><span class="p">(</span><span class="kt">int</span> <span class="n">v</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="kt">char</span> <span class="n">buffer</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>
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<span class="n">CXX_RESULT_EC</span><span class="p">(</span><span class="n">size_t</span><span class="p">)</span> <span class="n">res</span><span class="p">;</span>
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<span class="n">res</span> <span class="o">=</span> <span class="n">to_string</span><span class="p">(</span><span class="n">buffer</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="n">buffer</span><span class="p">),</span> <span class="n">v</span><span class="p">);</span>
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<span class="k">if</span><span class="p">(</span><span class="n">CXX_RESULT_HAS_VALUE</span><span class="p">(</span><span class="n">res</span><span class="p">))</span>
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<span class="p">{</span>
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<span class="n">printf</span><span class="p">(</span><span class="s">"to_string(%d) fills buffer with '%s' of %zu characters</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">v</span><span class="p">,</span> <span class="n">buffer</span><span class="p">,</span> <span class="n">res</span><span class="p">.</span><span class="n">value</span><span class="p">);</span>
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<span class="k">return</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="c1">// Is the error returned in the std::generic_category domain and thus an errno?
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</span><span class="c1"></span> <span class="k">if</span><span class="p">(</span><span class="n">CXX_RESULT_ERROR_IS_ERRNO</span><span class="p">(</span><span class="n">res</span><span class="p">))</span>
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<span class="p">{</span>
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<span class="c1">// If you get a weird compile error here, note that CXX_RESULT_ERROR()
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</span><span class="c1"></span> <span class="c1">// uses C11 generics, you need a C11 compiler for it to work. If you don't
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</span><span class="c1"></span> <span class="c1">// have a C11 compiler, res.error or res.error.code can be used directly.
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</span><span class="c1"></span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"to_string(%d) failed with error code %d (%s)</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">v</span><span class="p">,</span> <span class="n">CXX_RESULT_ERROR</span><span class="p">(</span><span class="n">res</span><span class="p">),</span> <span class="n">strerror</span><span class="p">(</span><span class="n">CXX_RESULT_ERROR</span><span class="p">(</span><span class="n">res</span><span class="p">)));</span>
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<span class="k">return</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"to_string(%d) failed with unknown error code %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">v</span><span class="p">,</span> <span class="n">CXX_RESULT_ERROR</span><span class="p">(</span><span class="n">res</span><span class="p">));</span>
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<span class="p">}</span>
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<span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">print</span><span class="p">(</span><span class="mi">9</span><span class="p">);</span>
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<span class="n">print</span><span class="p">(</span><span class="mi">99</span><span class="p">);</span>
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<span class="n">print</span><span class="p">(</span><span class="mi">999</span><span class="p">);</span>
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<span class="n">print</span><span class="p">(</span><span class="mi">9999</span><span class="p">);</span>
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<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
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<span class="p">}</span>
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</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/c_api.c#L31" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>Running this C program yields:</p>
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<pre><code>to_string(9) fills buffer with '9' of 1 characters
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to_string(99) fills buffer with '99' of 2 characters
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to_string(999) fills buffer with '999' of 3 characters
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to_string(9999) failed with error code 105 (No buffer space available)
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</code></pre>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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