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| 22 | <title>Array Parm Qualifiers (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Arrays-as-Parameters.html" rel="up" title="Arrays as Parameters">
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| 57 | <body lang="en">
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| 58 | <span id="Array-Parm-Qualifiers"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Passing-Array-Args.html" accesskey="p" rel="prev">Passing Array Args</a>, Up: <a href="Arrays-as-Parameters.html" accesskey="u" rel="up">Arrays as Parameters</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="Type-qualifiers-on-array-parameters"></span><h4 class="subsubsection">22.1.4.3 Type qualifiers on array parameters</h4>
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| 64 |
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| 65 | <p>You can use the type qualifiers <code>const</code>, <code>restrict</code>, and
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| 66 | <code>volatile</code> with array parameters; for example:
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| 67 | </p>
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| 68 | <div class="example">
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| 69 | <pre class="example">void
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| 70 | clobber4 (volatile int array[20])
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| 71 | <span class="roman">…</span>
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| 72 | </pre></div>
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| 73 |
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| 74 | <p>denotes that <code>array</code> is equivalent to a pointer to a volatile
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| 75 | <code>int</code>. Alternatively:
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| 76 | </p>
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| 77 | <div class="example">
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| 78 | <pre class="example">void
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| 79 | clobber4 (int array[const 20])
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| 80 | <span class="roman">…</span>
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>makes the array parameter equivalent to a constant pointer to an
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| 84 | <code>int</code>. If we want the <code>clobber4</code> function to succeed, it
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| 85 | would not make sense to write
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">void
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| 89 | clobber4 (const int array[20])
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| 90 | <span class="roman">…</span>
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| 91 | </pre></div>
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| 92 |
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| 93 | <p>as this would tell the compiler that the parameter should point to an
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| 94 | array of constant <code>int</code> values, and then we would not be able to
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| 95 | store zeros in them.
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| 96 | </p>
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| 97 | <p>In a function with multiple array parameters, you can use <code>restrict</code>
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| 98 | to tell the compiler that each array parameter passed in will be distinct:
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| 99 | </p>
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| 100 | <div class="example">
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| 101 | <pre class="example">void
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| 102 | foo (int array1[restrict 10], int array2[restrict 10])
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| 103 | <span class="roman">…</span>
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| 104 | </pre></div>
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| 105 |
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| 106 | <p>Using <code>restrict</code> promises the compiler that callers will
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| 107 | not pass in the same array for more than one <code>restrict</code> array
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| 108 | parameter. Knowing this enables the compiler to perform better code
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| 109 | optimization. This is the same effect as using <code>restrict</code>
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| 110 | pointers (see <a href="restrict-Pointers.html">restrict Pointers</a>), but makes it clear when reading
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| 111 | the code that an array of a specific size is expected.
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| 112 | </p>
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| 115 |
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