doc/guide/localhost/CallingFromSMLToC

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>CallingFromSMLToC</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>MLton&#8217;s <a href="ForeignFunctionInterface">ForeignFunctionInterface</a> allows an SML program to <em>import</em>
  32 C functions.  Suppose you would like to import from C a function with
  33 the following prototype:</p></div>
  34 <div class="listingblock">
  35 <div class="content"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">foo</span> <span class="p">(</span><span class="kt">double</span> <span class="n">d</span><span class="p">,</span> <span class="kt">char</span> <span class="n">c</span><span class="p">);</span>
  36 </pre></div></div></div>
  37 <div class="paragraph"><p>MLton extends the syntax of SML to allow expressions like the following:</p></div>
  38 <div class="listingblock">
  39 <div class="content monospaced">
  40 <pre>_import "foo": real * char -&gt; int;</pre>
  41 </div></div>
  42 <div class="paragraph"><p>This expression denotes a function of type <span class="monospaced">real * char -&gt; int</span> whose
  43 behavior is implemented by calling the C function whose name is <span class="monospaced">foo</span>.
  44 Thinking in terms of C, imagine that there are C variables <span class="monospaced">d</span> of type
  45 <span class="monospaced">double</span>, <span class="monospaced">c</span> of type <span class="monospaced">unsigned char</span>, and <span class="monospaced">i</span> of type <span class="monospaced">int</span>.  Then,
  46 the C statement <span class="monospaced">i = foo (d, c)</span> is executed and <span class="monospaced">i</span> is returned.</p></div>
  47 <div class="paragraph"><p>The general form of an <span class="monospaced">_import</span> expression is:</p></div>
  48 <div class="listingblock">
  49 <div class="content monospaced">
  50 <pre>_import "C function name" attr... : cFuncTy;</pre>
  51 </div></div>
  52 <div class="paragraph"><p>The type and the semicolon are not optional.</p></div>
  53 <div class="paragraph"><p>The function name is followed by a (possibly empty) sequence of
  54 attributes, analogous to C <span class="monospaced">__attribute__</span> specifiers.</p></div>
  55 </div>
  56 </div>
  57 <div class="sect1">
  58 <h2 id="_example">Example</h2>
  59 <div class="sectionbody">
  60 <div class="paragraph"><p><span class="monospaced">import.sml</span> imports the C function <span class="monospaced">ffi</span> and the C variable <span class="monospaced">FFI_INT</span>
  61 as follows.</p></div>
  62 <div class="listingblock">
  63 <div class="content"><div class="highlight"><pre><span class="cm">(* main.sml *)</span><span class="w"></span>
  64
  65 <span class="cm">(* Declare ffi to be implemented by calling the C function ffi. *)</span><span class="w"></span>
  66 <span class="k">val</span><span class="w"> </span><span class="n">ffi</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">_</span><span class="n">import</span><span class="w"> </span><span class="s">&quot;ffi&quot;</span><span class="w"> </span><span class="n">public</span><span class="p">:</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="n">array</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">char</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">char</span><span class="p">;</span><span class="w"></span>
  67 <span class="k">open</span><span class="w"> </span><span class="n">Array</span><span class="w"></span>
  68
  69 <span class="k">val</span><span class="w"> </span><span class="n">size</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">10</span><span class="w"></span>
  70 <span class="k">val</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">tabulate</span><span class="w"> </span><span class="p">(</span><span class="n">size</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="n">i</span><span class="p">)</span><span class="w"></span>
  71 <span class="k">val</span><span class="w"> </span><span class="n">ri</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="mi">0</span><span class="w"></span>
  72 <span class="k">val</span><span class="w"> </span><span class="n">rc</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="p">#</span><span class="s">&quot;0&quot;</span><span class="w"></span>
  73 <span class="k">val</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">17</span><span class="w"></span>
  74
  75 <span class="cm">(* Call the C function *)</span><span class="w"></span>
  76 <span class="k">val</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ffi</span><span class="w"> </span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">Array</span><span class="p">.</span><span class="n">length</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">ri</span><span class="p">,</span><span class="w"> </span><span class="n">rc</span><span class="p">,</span><span class="w"> </span><span class="n">n</span><span class="p">)</span><span class="w"></span>
  77
  78 <span class="cm">(* FFI_INT is declared as public in ffi-import.c *)</span><span class="w"></span>
  79 <span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">nGet</span><span class="p">,</span><span class="w"> </span><span class="n">nSet</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">_</span><span class="n">symbol</span><span class="w"> </span><span class="s">&quot;FFI_INT&quot;</span><span class="w"> </span><span class="n">public</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">int</span><span class="p">)</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="p">(</span><span class="n">int</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="p">);</span><span class="w"></span>
  80
  81 <span class="k">val</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="n">concat</span><span class="w"> </span><span class="p">[</span><span class="n">Int</span><span class="p">.</span><span class="n">toString</span><span class="w"> </span><span class="p">(</span><span class="n">nGet</span><span class="w"> </span><span class="p">()),</span><span class="w"> </span><span class="s">&quot;</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">])</span><span class="w"></span>
  82
  83 <span class="k">val</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  84 <span class="w">   </span><span class="n">print</span><span class="w"> </span><span class="p">(</span><span class="k">if</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">#</span><span class="s">&quot;c&quot;</span><span class="w"> </span><span class="k">andalso</span><span class="w"> </span><span class="n">!ri</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">45</span><span class="w"> </span><span class="k">andalso</span><span class="w"> </span><span class="n">!rc</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">c</span><span class="w"></span>
  85 <span class="w">             </span><span class="k">then</span><span class="w"> </span><span class="s">&quot;success</span><span class="se">\n</span><span class="s">&quot;</span><span class="w"></span>
  86 <span class="w">          </span><span class="k">else</span><span class="w"> </span><span class="s">&quot;fail</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">)</span><span class="w"></span>
  87 </pre></div></div></div>
  88 <div class="paragraph"><p><span class="monospaced">ffi-import.c</span> is</p></div>
  89 <div class="listingblock">
  90 <div class="content"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&quot;export.h&quot;</span><span class="cp"></span>
  91
  92 <span class="n">Int32</span> <span class="n">FFI_INT</span> <span class="o">=</span> <span class="mi">13</span><span class="p">;</span>
  93 <span class="n">Word32</span> <span class="n">FFI_WORD</span> <span class="o">=</span> <span class="mh">0xFF</span><span class="p">;</span>
  94 <span class="n">Bool</span> <span class="n">FFI_BOOL</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  95 <span class="n">Real64</span> <span class="n">FFI_REAL</span> <span class="o">=</span> <span class="mf">3.14159</span><span class="p">;</span>
  96
  97 <span class="n">Char8</span> <span class="nf">ffi</span> <span class="p">(</span><span class="n">Pointer</span> <span class="n">a1</span><span class="p">,</span> <span class="n">Int32</span> <span class="n">a1len</span><span class="p">,</span> <span class="n">Pointer</span> <span class="n">a2</span><span class="p">,</span> <span class="n">Pointer</span> <span class="n">a3</span><span class="p">,</span> <span class="n">Int32</span> <span class="n">n</span><span class="p">)</span> <span class="p">{</span>
  98         <span class="kt">double</span> <span class="o">*</span><span class="n">ds</span> <span class="o">=</span> <span class="p">(</span><span class="kt">double</span><span class="o">*</span><span class="p">)</span><span class="n">a1</span><span class="p">;</span>
  99         <span class="kt">int</span> <span class="o">*</span><span class="n">pi</span> <span class="o">=</span> <span class="p">(</span><span class="kt">int</span><span class="o">*</span><span class="p">)</span><span class="n">a2</span><span class="p">;</span>
 100         <span class="kt">char</span> <span class="o">*</span><span class="n">pc</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span><span class="o">*</span><span class="p">)</span><span class="n">a3</span><span class="p">;</span>
 101         <span class="kt">int</span> <span class="n">i</span><span class="p">;</span>
 102         <span class="kt">double</span> <span class="n">sum</span><span class="p">;</span>
 103
 104         <span class="n">sum</span> <span class="o">=</span> <span class="mf">0.0</span><span class="p">;</span>
 105         <span class="k">for</span> <span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="n">a1len</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
 106                 <span class="n">sum</span> <span class="o">+=</span> <span class="n">ds</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
 107                 <span class="n">ds</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">+=</span> <span class="n">n</span><span class="p">;</span>
 108         <span class="p">}</span>
 109         <span class="o">*</span><span class="n">pi</span> <span class="o">=</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="n">sum</span><span class="p">;</span>
 110         <span class="o">*</span><span class="n">pc</span> <span class="o">=</span> <span class="sc">&#39;c&#39;</span><span class="p">;</span>
 111         <span class="k">return</span> <span class="sc">&#39;c&#39;</span><span class="p">;</span>
 112 <span class="p">}</span>
 113 </pre></div></div></div>
 114 <div class="paragraph"><p>Compile and run the program.</p></div>
 115 <div class="listingblock">
 116 <div class="content monospaced">
 117 <pre>% mlton -default-ann 'allowFFI true' -export-header export.h  import.sml ffi-import.c
 118 % ./import
 119 13
 120 success</pre>
 121 </div></div>
 122 </div>
 123 </div>
 124 <div class="sect1">
 125 <h2 id="_download">Download</h2>
 126 <div class="sectionbody">
 127 <div class="ulist"><ul>
 128 <li>
 129 <p>
 130 <a href="https://raw.github.com/MLton/mlton/master/doc/examples/ffi/import.sml"><span class="monospaced">import.sml</span></a>
 131 </p>
 132 </li>
 133 <li>
 134 <p>
 135 <a href="https://raw.github.com/MLton/mlton/master/doc/examples/ffi/ffi-import.c"><span class="monospaced">ffi-import.c</span></a>
 136 </p>
 137 </li>
 138 </ul></div>
 139 </div>
 140 </div>
 141 <div class="sect1">
 142 <h2 id="_next_steps">Next Steps</h2>
 143 <div class="sectionbody">
 144 <div class="ulist"><ul>
 145 <li>
 146 <p>
 147 <a href="CallingFromSMLToCFunctionPointer">CallingFromSMLToCFunctionPointer</a>
 148 </p>
 149 </li>
 150 </ul></div>
 151 </div>
 152 </div>
 153 </div>
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