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| 22 | <title>Function Parameter Variables (GNU C Language Manual)</title>
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| 32 | <link href="Function-Definitions.html" rel="up" title="Function Definitions">
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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="Function-Parameter-Variables"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Forward-Function-Declarations.html" accesskey="n" rel="next">Forward Function Declarations</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="Function-Parameter-Variables-1"></span><h4 class="subsection">22.1.1 Function Parameter Variables</h4>
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| 64 | <span id="index-function-parameter-variables"></span>
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| 65 | <span id="index-parameter-variables-in-functions"></span>
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| 66 | <span id="index-parameter-list"></span>
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| 67 |
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| 68 | <p>A function parameter variable is a local variable (see <a href="Local-Variables.html">Local Variables</a>) used within the function to store the value passed as an
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| 69 | argument in a call to the function. Usually we say “function
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| 70 | parameter” or “parameter” for short, not mentioning the fact that
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| 71 | it’s a variable.
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| 72 | </p>
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| 73 | <p>We declare these variables in the beginning of the function
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| 74 | definition, in the <em>parameter list</em>. For example,
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">fib (int n)
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>has a parameter list with one function parameter <code>n</code>, which has
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| 81 | type <code>int</code>.
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| 82 | </p>
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| 83 | <p>Function parameter declarations differ from ordinary variable
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| 84 | declarations in several ways:
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| 85 | </p>
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| 86 | <ul>
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| 87 | <li> Inside the function definition header, commas separate parameter
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| 88 | declarations, and each parameter needs a complete declaration
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| 89 | including the type. For instance, if a function <code>foo</code> has two
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| 90 | <code>int</code> parameters, write this:
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| 91 |
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| 92 | <div class="example">
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| 93 | <pre class="example">foo (int a, int b)
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| 94 | </pre></div>
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| 95 |
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| 96 | <p>You can’t share the common <code>int</code> between the two declarations:
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| 97 | </p>
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| 98 | <div class="example">
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| 99 | <pre class="example">foo (int a, b) /* <span class="roman">Invalid!</span> */
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| 100 | </pre></div>
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| 101 |
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| 102 | </li><li> A function parameter variable is initialized to whatever value is
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| 103 | passed in the function call, so its declaration cannot specify an
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| 104 | initial value.
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| 105 |
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| 106 | </li><li> Writing an array type in a function parameter declaration has the
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| 107 | effect of declaring it as a pointer. The size specified for the array
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| 108 | has no effect at all, and we normally omit the size. Thus,
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| 109 |
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| 110 | <div class="example">
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| 111 | <pre class="example">foo (int a[5])
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| 112 | foo (int a[])
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| 113 | foo (int *a)
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| 114 | </pre></div>
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| 115 |
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| 116 | <p>are equivalent.
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| 117 | </p>
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| 118 | </li><li> The scope of the parameter variables is the entire function body,
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| 119 | notwithstanding the fact that they are written in the function header,
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| 120 | which is just outside the function body.
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| 121 | </li></ul>
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| 122 |
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| 123 | <p>If a function has no parameters, it would be most natural for the
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| 124 | list of parameters in its definition to be empty. But that, in C, has
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| 125 | a special meaning for historical reasons: “Do not check that calls to
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| 126 | this function have the right number of arguments.” Thus,
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| 127 | </p>
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| 128 | <div class="example">
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| 129 | <pre class="example">int
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| 130 | foo ()
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| 131 | {
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| 132 | return 5;
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| 133 | }
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| 134 |
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| 135 | int
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| 136 | bar (int x)
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| 137 | {
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| 138 | return foo (x);
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| 139 | }
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| 140 | </pre></div>
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| 141 |
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| 142 | <p>would not report a compilation error in passing <code>x</code> as an
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| 143 | argument to <code>foo</code>. By contrast,
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| 144 | </p>
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| 145 | <div class="example">
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| 146 | <pre class="example">int
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| 147 | foo (void)
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| 148 | {
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| 149 | return 5;
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| 150 | }
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| 151 |
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| 152 | int
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| 153 | bar (int x)
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| 154 | {
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| 155 | return foo (x);
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| 156 | }
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| 157 | </pre></div>
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| 158 |
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| 159 | <p>would report an error because <code>foo</code> is supposed to receive
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| 160 | no arguments.
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| 161 | </p>
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| 162 | <hr>
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| 163 | <div class="header">
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| 164 | <p>
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| 165 | Next: <a href="Forward-Function-Declarations.html" accesskey="n" rel="next">Forward Function Declarations</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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| 166 | </div>
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