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| 22 | <title>Variable-Length Array Parameters (GNU C Language Manual)</title>
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| 24 | <meta name="description" content="Variable-Length Array Parameters (GNU C Language Manual)">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Advanced-Definitions.html" rel="up" title="Advanced Definitions">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Variable_002dLength-Array-Parameters"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Variable-Number-of-Arguments.html" accesskey="n" rel="next">Variable Number of Arguments</a>, Up: <a href="Advanced-Definitions.html" accesskey="u" rel="up">Advanced 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="Variable_002dLength-Array-Parameters-1"></span><h4 class="subsection">22.7.1 Variable-Length Array Parameters</h4>
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| 64 | <span id="index-variable_002dlength-array-parameters"></span>
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| 65 | <span id="index-array-parameters_002c-variable_002dlength"></span>
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| 66 | <span id="index-functions-that-accept-variable_002dlength-arrays"></span>
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| 67 |
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| 68 | <p>An array parameter can have variable length: simply declare the array
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| 69 | type with a size that isn’t constant. In a nested function, the
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| 70 | length can refer to a variable defined in a containing scope. In any
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| 71 | function, it can refer to a previous parameter, like this:
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">struct entry
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| 75 | tester (int len, char data[len][len])
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| 76 | {
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| 77 | <span class="roman">…</span>
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| 78 | }
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| 79 | </pre></div>
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| 80 |
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| 81 | <p>Alternatively, in function declarations (but not in function
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| 82 | definitions), you can use <code>[*]</code> to denote that the array
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| 83 | parameter is of a variable length, such that these two declarations
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| 84 | mean the same thing:
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| 85 | </p>
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| 86 | <div class="example">
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| 87 | <pre class="example">struct entry
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| 88 | tester (int len, char data[len][len]);
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| 89 | </pre></div>
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| 90 |
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| 91 | <div class="example">
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| 92 | <pre class="example">struct entry
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| 93 | tester (int len, char data[*][*]);
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| 94 | </pre></div>
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| 95 |
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| 96 | <p>The two forms of input are equivalent in GNU C, but emphasizing that
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| 97 | the array parameter is variable-length may be helpful to those
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| 98 | studying the code.
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| 99 | </p>
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| 100 | <p>You can also omit the length parameter, and instead use some other
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| 101 | in-scope variable for the length in the function definition:
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| 102 | </p>
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| 103 | <div class="example">
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| 104 | <pre class="example">struct entry
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| 105 | tester (char data[*][*]);
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| 106 | <span class="roman">…</span>
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| 107 | int dataLength = 20;
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| 108 | <span class="roman">…</span>
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| 109 | struct entry
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| 110 | tester (char data[dataLength][dataLength])
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| 111 | {
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| 112 | <span class="roman">…</span>
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| 113 | }
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| 114 | </pre></div>
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| 115 |
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| 116 |
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| 117 | <span id="index-parameter-forward-declaration"></span>
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| 118 | <p>In GNU C, to pass the array first and the length afterward, you can
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| 119 | use a <em>parameter forward declaration</em>, like this:
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| 120 | </p>
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| 121 | <div class="example">
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| 122 | <pre class="example">struct entry
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| 123 | tester (int len; char data[len][len], int len)
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| 124 | {
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| 125 | <span class="roman">…</span>
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| 126 | }
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| 127 | </pre></div>
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| 128 |
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| 129 | <p>The ‘<samp>int len</samp>’ before the semicolon is the parameter forward
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| 130 | declaration; it serves the purpose of making the name <code>len</code> known
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| 131 | when the declaration of <code>data</code> is parsed.
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| 132 | </p>
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| 133 | <p>You can write any number of such parameter forward declarations in the
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| 134 | parameter list. They can be separated by commas or semicolons, but
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| 135 | the last one must end with a semicolon, which is followed by the
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| 136 | “real” parameter declarations. Each forward declaration must match
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| 137 | a subsequent “real” declaration in parameter name and data type.
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| 138 | </p>
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| 139 | <p>Standard C does not support parameter forward declarations.
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| 140 | </p>
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| 141 | <hr>
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| 142 | <div class="header">
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| 143 | <p>
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| 144 | Next: <a href="Variable-Number-of-Arguments.html" accesskey="n" rel="next">Variable Number of Arguments</a>, Up: <a href="Advanced-Definitions.html" accesskey="u" rel="up">Advanced 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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| 145 | </div>
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