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| 22 | <title>Incomplete Array Types (GNU C Language Manual)</title>
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| 58 | <span id="Incomplete-Array-Types"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Limitations-of-C-Arrays.html" accesskey="n" rel="next">Limitations of C Arrays</a>, Previous: <a href="Array-Type-Designators.html" accesskey="p" rel="prev">Array Type Designators</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</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="Incomplete-Array-Types-1"></span><h3 class="section">16.5 Incomplete Array Types</h3>
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| 64 | <span id="index-incomplete-array-types"></span>
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| 65 | <span id="index-array-types_002c-incomplete"></span>
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| 66 |
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| 67 | <p>An array is equivalent, for most purposes, to a pointer to its zeroth
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| 68 | element. When that is true, the length of the array is irrelevant.
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| 69 | The length needs to be known only for allocating space for the array, or
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| 70 | for <code>sizeof</code> and <code>typeof</code> (see <a href="Auto-Type.html">Auto Type</a>). Thus, in some
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| 71 | contexts C allows
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| 72 | </p>
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| 73 | <ul>
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| 74 | <li> An <code>extern</code> declaration says how to refer to a variable allocated
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| 75 | elsewhere. It does not need to allocate space for the variable,
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| 76 | so if it is an array, you can omit the length. For example,
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| 77 |
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| 78 | <div class="example">
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| 79 | <pre class="example">extern int foo[];
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| 80 | </pre></div>
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| 81 |
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| 82 | </li><li> When declaring a function parameter as an array, the argument value
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| 83 | passed to the function is really a pointer to the array’s zeroth
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| 84 | element. This value does not say how long the array really is, there
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| 85 | is no need to declare it. For example,
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| 86 |
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| 87 | <div class="example">
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| 88 | <pre class="example">int
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| 89 | func (int foo[])
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| 90 | </pre></div>
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| 91 | </li></ul>
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| 92 |
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| 93 | <p>These declarations are examples of <em>incomplete</em> array types, types
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| 94 | that are not fully specified. The incompleteness makes no difference
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| 95 | for accessing elements of the array, but it matters for some other
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| 96 | things. For instance, <code>sizeof</code> is not allowed on an incomplete
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| 97 | type.
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| 98 | </p>
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| 99 | <p>With multidimensional arrays, only the first dimension can be omitted:
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">extern struct chesspiece *funnyboard foo[][8];
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| 103 | </pre></div>
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| 104 |
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| 105 | <p>In other words, the code doesn’t have to say how many rows there are,
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| 106 | but it must state how big each row is.
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| 107 | </p>
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| 108 |
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