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  3. <title>Calling it from C - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Calling it from C</h1></div></div></div>
    14. 

  14.   <p>Firstly we need to declare to C our <code>result</code> returning C++ function:</p>
    15. 

  15. 

  16. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// Declare our C++ function&#39;s returning result type. Only needs to be done once.
    17. 

  17. </span><span class="c1">// This declares an `status_result&lt;size_t, system_code&gt;` which is an alias to
    18. 

  18. </span><span class="c1">// `basic_result&lt;size_t, status_code&lt;erased&lt;intptr_t&gt;&gt;&gt;`.
    19. 

  19. </span><span class="c1">//
    20. 

  20. </span><span class="c1">// The first parameter is some unique identifier for this type which will be used
    21. 

  21. </span><span class="c1">// whenever we reference this type in the future.
    22. 

  22. </span><span class="c1"></span><span class="n">CXX_DECLARE_RESULT_SYSTEM</span><span class="p">(</span><span class="n">to_string_rettype</span><span class="p">,</span> <span class="n">size_t</span><span class="p">);</span>
    23. 

  23. 

  24. <span class="c1">// Tell C about our extern C++ function `to_string()`
    25. 

  25. </span><span class="c1"></span><span class="k">extern</span> <span class="nf">CXX_RESULT_SYSTEM</span><span class="p">(</span><span class="n">to_string_rettype</span><span class="p">)</span> <span class="n">_Z9to_stringPcmi</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>
    26. 

  26. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/c_api.c#L40" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    27. 

  27. 

  28. 

  29. <p>Now let&rsquo;s call the C++ function from C:</p>
    30. 

  30. 

  31. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><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>
    32. 

  32. <span class="p">{</span>
    33. 

  33.   <span class="kt">char</span> <span class="n">buffer</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>
    34. 

  34.   <span class="n">CXX_RESULT_SYSTEM</span><span class="p">(</span><span class="n">to_string_rettype</span><span class="p">)</span> <span class="n">res</span><span class="p">;</span>
    35. 

  35. 

  36.   <span class="n">res</span> <span class="o">=</span> <span class="n">_Z9to_stringPcmi</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>
    37. 

  37.   <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>
    38. 

  38.   <span class="p">{</span>
    39. 

  39.     <span class="n">printf</span><span class="p">(</span><span class="s">&#34;to_string(%d) fills buffer with &#39;%s&#39; of %zu characters</span><span class="se">\n</span><span class="s">&#34;</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>
    40. 

  40.     <span class="k">return</span><span class="p">;</span>
    41. 

  41.   <span class="p">}</span>
    42. 

  42.   <span class="c1">// Is the error returned in the POSIX domain and thus an errno?
    43. 

  43. </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>
    44. 

  44.   <span class="p">{</span>
    45. 

  45.     <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">&#34;to_string(%d) failed with error code %d (%s)</span><span class="se">\n</span><span class="s">&#34;</span><span class="p">,</span> <span class="n">v</span><span class="p">,</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">res</span><span class="p">.</span><span class="n">error</span><span class="p">.</span><span class="n">value</span><span class="p">,</span> <span class="n">strerror</span><span class="p">((</span><span class="kt">int</span><span class="p">)</span> <span class="n">res</span><span class="p">.</span><span class="n">error</span><span class="p">.</span><span class="n">value</span><span class="p">));</span>
    46. 

  46.     <span class="k">return</span><span class="p">;</span>
    47. 

  47.   <span class="p">}</span>
    48. 

  48.   <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">&#34;to_string(%d) failed with unknown error code %lld</span><span class="se">\n</span><span class="s">&#34;</span><span class="p">,</span> <span class="n">v</span><span class="p">,</span> <span class="p">(</span><span class="kt">long</span> <span class="kt">long</span><span class="p">)</span> <span class="n">res</span><span class="p">.</span><span class="n">error</span><span class="p">.</span><span class="n">value</span><span class="p">);</span>
    49. 

  49. <span class="p">}</span>
    50. 

  50. 

  51. <span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span>
    52. 

  52. <span class="p">{</span>
    53. 

  53.   <span class="n">print</span><span class="p">(</span><span class="mi">9</span><span class="p">);</span>
    54. 

  54.   <span class="n">print</span><span class="p">(</span><span class="mi">99</span><span class="p">);</span>
    55. 

  55.   <span class="n">print</span><span class="p">(</span><span class="mi">999</span><span class="p">);</span>
    56. 

  56.   <span class="n">print</span><span class="p">(</span><span class="mi">9999</span><span class="p">);</span>
    57. 

  57.   <span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
    58. 

  58. <span class="p">}</span>
    59. 

  59. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/c_api.c#L53" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    60. 

  60. 

  61. 

  62. <p>Running this C program yields:</p>
    63. 

  63. 

  64. <pre><code>to_string(9) fills buffer with '9' of 1 characters
    65. 

  65. to_string(99) fills buffer with '99' of 2 characters
    66. 

  66. to_string(999) fills buffer with '999' of 3 characters
    67. 

  67. to_string(9999) failed with error code 105 (No buffer space available)
    68. 

  68. </code></pre>
    69. 

  69. 

  70. 

  71.         </div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
    72. 

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