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  3. <title>SWIG:Examples:go:simple</title>
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  8. 

  9. <tt>SWIG/Examples/go/simple/</tt>
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  10. <hr>
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  11. 

  12. <H2>Simple Go Example</H2>
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  13. 

  14. <p>
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  15. This example illustrates how you can hook Go to a very simple C program containing
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  16. a function and a global variable.
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  17. 

  18. <h2>The C Code</h2>
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  19. 

  20. Suppose you have the following C code:
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  21. 

  22. <blockquote>
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  23. <pre>
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  24. /* File : example.c */
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  25. 

  26. /* A global variable */
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  27. double Foo = 3.0;
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  28. 

  29. /* Compute the greatest common divisor of positive integers */
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  30. int gcd(int x, int y) {
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  31.   int g;
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  32.   g = y;
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  33.   while (x &gt; 0) {
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  34.     g = x;
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  35.     x = y % x;
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  36.     y = g;
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  37.   }
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  38.   return g;
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  39. }
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  40. </pre>
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  41. </blockquote>
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  42. 

  43. <h2>The SWIG interface</h2>
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  44. 

  45. Here is a simple SWIG interface file:
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  46. 

  47. <blockquote>
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  48. <pre>
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  49. /* File: example.i */
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  50. %module example
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  51. 

  52. extern int gcd(int x, int y);
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  53. extern double Foo;
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  54. </pre>
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  55. </blockquote>
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  56. 

  57. <h2>Compilation</h2>
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  58. 

  59. These are the instructions if you are using <tt>6g</tt>/<tt>8g</tt>
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  60. rather than <tt>gccgo</tt>.
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  61. 

  62. <ol>
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  63. <li>Run <tt>swig -go <a href="example.i">example.i</a></tt>.  This
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  64.   will create the three
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  65.   files <tt>example.go</tt>, <tt>example_gc.c</tt>,
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  66.   and <tt>example_wrap.c</tt>.
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  67. <li>Compile <tt><a href="example.go">example.go</a></tt>
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  68.   using <tt>6g</tt> or <tt>8g</tt>; e.g., <tt>6g example.go</tt>.
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  69. <li>Compile <tt><a href="example_gc.c">example_gc.c</a></tt>
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  70.   using <tt>6c</tt> or <tt>8c</tt>; e.g., <tt>6c example_gc.c</tt>.
    71. 

  71. <li>Put the two object files together into an archive
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  72.   named <tt>example.a</tt>; e.g., <tt>gopack grc example.a example.6
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  73.   example_gc.6</tt>.
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  74. <li>Compile the <tt><a href="example_wrap.c">example_wrap.c</a></tt>
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  75.   file using your standard C compiler with the <tt>-fpic</tt> option;
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  76.   e.g., <tt>gcc -c -O -fpic example_wrap.c</tt>.
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  77. <li>Also compile the actual code, not generated by SWIG; e.g., <tt>gcc
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  78.     -c -O -fpic example.c</tt>.
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  79. <li>Put the gcc compiled object files into a shared library;
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  80.   e.g., <tt>gcc -shared -o example.so example_wrap.o example.o</tt>.
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  81. <li>Compile the program which demonstrates how to use the library;
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  82.   e.g., <tt>6g runme.go</tt>.
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  83. <li>Link the program; e.g., <tt>6l -o runme runme.6</tt>.
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  84. <li>Now you should have a program <tt>runme</tt>.
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  85. </ol>
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  86. 

  87. <h2>Using the extension</h2>
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  88. 

  89. The Go program which demonstrates calling the C functions from Go
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  90. is <a href="runme.go">runme.go</a>.
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  91. 

  92. <h2>Key points</h2>
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  93. 

  94. <ul>
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  95. <li>Use the <tt>import</tt> statement to load your extension module from Go. For example:
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  96. <blockquote>
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  97. <pre>
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  98. import "example"
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  99. </pre>
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  100. </blockquote>
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  101. 

  102. <li>C functions work just like Go functions.  However, the function
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  103.   names are automatically capitalized in order to make the names
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  104.   visible from other Go packages.  For example:
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  105. <blockquote>
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  106. <pre>
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  107. g := example.Gcd(42,105)
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  108. </pre>
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  109. </blockquote>
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  110. 

  111. (If there are name conflicts, you can use the <tt>%rename</tt>
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  112. directive in the .i file or the <tt>-rename</tt> option to Go to
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  113. rename one or the other symbol).
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  114. 

  115. <li>C global variables are accessed using getter and setter
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  116.   functions.  The getter function is named <tt>Get</tt> followed by
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  117.   the capitalized name of the C variable.  The Setter function
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  118.   uses <tt>Set</tt> instead of <tt>Get</tt>.
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  119. <blockquote>
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  120. <pre>
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  121. a = example.GetFoo()
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  122. </pre>
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  123. </blockquote>
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  124. </ul>
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  125. 

  126. <hr>
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  127. </body>
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  128. </html>
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