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| 22 | <title>Cast to Union (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Structures.html" rel="up" title="Structures">
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| 33 | <link href="Structure-Constructors.html" rel="next" title="Structure Constructors">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Cast-to-Union"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Structure-Constructors.html" accesskey="n" rel="next">Structure Constructors</a>, Previous: <a href="Packing-With-Unions.html" accesskey="p" rel="prev">Packing With 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="Cast-to-a-Union-Type"></span><h3 class="section">15.15 Cast to a Union Type</h3>
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| 64 | <span id="index-cast-to-a-union"></span>
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| 65 | <span id="index-union_002c-casting-to-a"></span>
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| 66 |
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| 67 | <p>In GNU C, you can explicitly cast any of the alternative types to the
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| 68 | union type; for instance,
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| 69 | </p>
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| 70 | <div class="example">
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| 71 | <pre class="example">(union eight_bytes) (long long) 5
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| 72 | </pre></div>
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| 73 |
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| 74 | <p>makes a value of type <code>union eight_bytes</code> which gets its contents
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| 75 | through the alternative named <code>big_int_elt</code>.
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| 76 | </p>
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| 77 | <p>The value being cast must exactly match the type of the alternative,
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| 78 | so this is not valid:
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| 79 | </p>
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| 80 | <div class="example">
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| 81 | <pre class="example">(union eight_bytes) 5 /* <span class="roman">Error! 5 is <code>int</code>.</span> */
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| 82 | </pre></div>
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| 83 |
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| 84 | <p>A cast to union type looks like any other cast, except that the type
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| 85 | specified is a union type. You can specify the type either with
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| 86 | <code>union <var>tag</var></code> or with a typedef name (see <a href="Defining-Typedef-Names.html">Defining Typedef Names</a>).
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| 87 | </p>
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| 88 | <p>Using the cast as the right-hand side of an assignment to a variable of
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| 89 | union type is equivalent to storing in an alternative of the union:
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| 90 | </p>
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| 91 | <div class="example">
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| 92 | <pre class="example">union foo u;
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| 93 |
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| 94 | u = (union foo) x <span class="roman">means</span> u.i = x
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| 95 |
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| 96 | u = (union foo) y <span class="roman">means</span> u.d = y
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| 97 | </pre></div>
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| 98 |
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| 99 | <p>You can also use the union cast as a function argument:
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">void hack (union foo);
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| 103 | <span class="roman">…</span>
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| 104 | hack ((union foo) x);
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| 105 | </pre></div>
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| 106 |
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| 107 |
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| 108 |
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| 109 |
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