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| 22 | <title>Passing Array Args (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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| 33 | <link href="Array-Parm-Qualifiers.html" rel="next" title="Array Parm Qualifiers">
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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="Passing-Array-Args"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Array-Parm-Qualifiers.html" accesskey="n" rel="next">Array Parm Qualifiers</a>, Previous: <a href="Array-Parm-Pointer.html" accesskey="p" rel="prev">Array Parm Pointer</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="Passing-array-arguments"></span><h4 class="subsubsection">22.1.4.2 Passing array arguments</h4>
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| 64 |
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| 65 | <p>The function call passes this pointer by
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| 66 | value, like all argument values in C. However, the result is
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| 67 | paradoxical in that the array itself is passed by reference: its
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| 68 | contents are treated as shared memory—shared between the caller and
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| 69 | the called function, that is. When <code>clobber4</code> assigns to element
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| 70 | 4 of <code>array</code>, the effect is to alter element 4 of the array
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| 71 | specified in the call.
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">#include <stddef.h> /* <span class="roman">Defines <code>NULL</code>.</span> */
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| 75 | #include <stdlib.h> /* <span class="roman">Declares <code>malloc</code>,</span> */
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| 76 | /* <span class="roman">Defines <code>EXIT_SUCCESS</code>.</span> */
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| 77 |
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| 78 | int
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| 79 | main (void)
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| 80 | {
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| 81 | int data[] = {1, 2, 3, 4, 5, 6};
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| 82 | int i;
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| 83 |
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| 84 | /* <span class="roman">Show the initial value of element 4.</span> */
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| 85 | for (i = 0; i < 6; i++)
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| 86 | printf ("data[%d] = %d\n", i, data[i]);
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| 87 |
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| 88 | printf ("\n");
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| 89 |
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| 90 | clobber4 (data);
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| 91 |
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| 92 | /* <span class="roman">Show that element 4 has been changed.</span> */
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| 93 | for (i = 0; i < 6; i++)
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| 94 | printf ("data[%d] = %d\n", i, data[i]);
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| 95 |
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| 96 | printf ("\n");
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| 97 |
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| 98 | return EXIT_SUCCESS;
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| 99 | }
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| 100 | </pre></div>
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| 101 |
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| 102 | <p>shows that <code>data[4]</code> has become zero after the call to
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| 103 | <code>clobber4</code>.
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| 104 | </p>
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| 105 | <p>The array <code>data</code> has 6 elements, but passing it to a function
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| 106 | whose argument type is written as <code>int [20]</code> is not an error,
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| 107 | because that really stands for <code>int *</code>. The pointer that is the
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| 108 | real argument carries no indication of the length of the array it
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| 109 | points into. It is not required to point to the beginning of the
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| 110 | array, either. For instance,
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| 111 | </p>
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| 112 | <div class="example">
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| 113 | <pre class="example">clobber4 (data+1);
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| 114 | </pre></div>
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| 115 |
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| 116 | <p>passes an “array” that starts at element 1 of <code>data</code>, and the
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| 117 | effect is to zero <code>data[5]</code> instead of <code>data[4]</code>.
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| 118 | </p>
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| 119 | <p>If all calls to the function will provide an array of a particular
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| 120 | size, you can specify the size of the array to be <code>static</code>:
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| 121 | </p>
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| 122 | <div class="example">
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| 123 | <pre class="example">void
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| 124 | clobber4 (int array[static 20])
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| 125 | <span class="roman">…</span>
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| 126 | </pre></div>
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| 127 |
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| 128 | <p>This is a promise to the compiler that the function will always be
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| 129 | called with an array of 20 elements, so that the compiler can optimize
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| 130 | code accordingly. If the code breaks this promise and calls the
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| 131 | function with, for example, a shorter array, unpredictable things may
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| 132 | happen.
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| 133 | </p>
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| 134 | <hr>
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| 135 | <div class="header">
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| 136 | <p>
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| 137 | Next: <a href="Array-Parm-Qualifiers.html" accesskey="n" rel="next">Array Parm Qualifiers</a>, Previous: <a href="Array-Parm-Pointer.html" accesskey="p" rel="prev">Array Parm Pointer</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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| 138 | </div>
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