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| 22 | <title>Packing With Unions (GNU C Language Manual)</title>
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| 58 | <span id="Packing-With-Unions"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Cast-to-Union.html" accesskey="n" rel="next">Cast to Union</a>, Previous: <a href="Unions.html" accesskey="p" rel="prev">Unions</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Packing-With-Unions-1"></span><h3 class="section">15.14 Packing With Unions</h3>
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| 64 |
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| 65 | <p>Sometimes we design a union with the intention of packing various
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| 66 | kinds of objects into a certain amount of memory space. For example.
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| 67 | </p>
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| 68 | <div class="example">
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| 69 | <pre class="example">union bytes8
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| 70 | {
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| 71 | long long big_int_elt;
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| 72 | double double_elt;
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| 73 | struct { int first, second; } two_ints;
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| 74 | struct { void *first, *second; } two_ptrs;
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| 75 | };
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| 76 |
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| 77 | union bytes8 *p;
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>This union makes it possible to look at 8 bytes of data that <code>p</code>
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| 81 | points to as a single 8-byte integer (<code>p->big_int_elt</code>), as a
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| 82 | single floating-point number (<code>p->double_elt</code>), as a pair of
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| 83 | integers (<code>p->two_ints.first</code> and <code>p->two_ints.second</code>), or
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| 84 | as a pair of pointers (<code>p->two_ptrs.first</code> and
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| 85 | <code>p->two_ptrs.second</code>).
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| 86 | </p>
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| 87 | <p>To pack storage with such a union makes assumptions about the sizes of
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| 88 | all the types involved. This particular union was written expecting a
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| 89 | pointer to have the same size as <code>int</code>. On a machine where one
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| 90 | pointer takes 8 bytes, the code using this union probably won’t work
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| 91 | as expected. The union, as such, will function correctly—if you
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| 92 | store two values through <code>two_ints</code> and extract them through
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| 93 | <code>two_ints</code>, you will get the same integers back—but the part of
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| 94 | the program that expects the union to be 8 bytes long could
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| 95 | malfunction, or at least use too much space.
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| 96 | </p>
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| 97 | <p>The above example shows one case where a <code>struct</code> type with no
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| 98 | tag can be useful. Another way to get effectively the same result
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| 99 | is with arrays as members of the union:
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">union eight_bytes
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| 103 | {
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| 104 | long long big_int_elt;
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| 105 | double double_elt;
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| 106 | int two_ints[2];
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| 107 | void *two_ptrs[2];
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| 108 | };
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| 109 | </pre></div>
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