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| 22 | <title>Structs as Parameters (GNU C Language Manual)</title>
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| 23 |
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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="Function-Definitions.html" rel="up" title="Function Definitions">
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| 33 | <link href="Function-Declarations.html" rel="next" title="Function Declarations">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Structs-as-Parameters"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Arrays-as-Parameters.html" accesskey="p" rel="prev">Arrays as Parameters</a>, Up: <a href="Function-Definitions.html" accesskey="u" rel="up">Function Definitions</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="Functions-That-Accept-Structure-Arguments"></span><h4 class="subsection">22.1.5 Functions That Accept Structure Arguments</h4>
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| 64 |
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| 65 | <p>Structures in GNU C are first-class objects, so using them as function
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| 66 | parameters and arguments works in the natural way. This function
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| 67 | <code>swapfoo</code> takes a <code>struct foo</code> with two fields as argument,
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| 68 | and returns a structure of the same type but with the fields
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| 69 | exchanged.
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">struct foo { int a, b; };
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| 73 |
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| 74 | struct foo x;
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| 75 |
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| 76 | struct foo
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| 77 | swapfoo (struct foo inval)
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| 78 | {
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| 79 | struct foo outval;
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| 80 | outval.a = inval.b;
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| 81 | outval.b = inval.a;
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| 82 | return outval;
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| 83 | }
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>This simpler definition of <code>swapfoo</code> avoids using a local
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| 87 | variable to hold the result about to be return, by using a structure
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| 88 | constructor (see <a href="Structure-Constructors.html">Structure Constructors</a>), like this:
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">struct foo
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| 92 | swapfoo (struct foo inval)
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| 93 | {
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| 94 | return (struct foo) { inval.b, inval.a };
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| 95 | }
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>It is valid to define a structure type in a function’s parameter list,
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| 99 | as in
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">int
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| 103 | frob_bar (struct bar { int a, b; } inval)
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| 104 | {
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| 105 | <var>body</var>
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| 106 | }
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| 107 | </pre></div>
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| 108 |
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| 109 | <p>and <var>body</var> can access the fields of <var>inval</var> since the
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| 110 | structure type <code>struct bar</code> is defined for the whole function
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| 111 | body. However, there is no way to create a <code>struct bar</code> argument
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| 112 | to pass to <code>frob_bar</code>, except with kludges. As a result,
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| 113 | defining a structure type in a parameter list is useless in practice.
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| 114 | </p>
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| 115 |
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| 116 |
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| 117 |
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| 118 | </body>
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| 119 | </html>
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