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| 22 | <title>Arrays of Variable Length (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Arrays of Variable Length (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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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.html" rel="up" title="Arrays">
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| 33 | <link href="Enumeration-Types.html" rel="next" title="Enumeration Types">
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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="Arrays-of-Variable-Length"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Constructing-Array-Values.html" accesskey="p" rel="prev">Constructing Array Values</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="Arrays-of-Variable-Length-1"></span><h3 class="section">16.9 Arrays of Variable Length</h3>
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| 64 | <span id="index-array-of-variable-length"></span>
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| 65 | <span id="index-variable_002dlength-arrays"></span>
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| 66 |
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| 67 | <p>In GNU C, you can declare variable-length arrays like any other
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| 68 | arrays, but with a length that is not a constant expression. The
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| 69 | storage is allocated at the point of declaration and deallocated when
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| 70 | the block scope containing the declaration exits. For example:
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">#include <stdio.h> /* <span class="roman">Defines <code>FILE</code>.</span> */
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| 74 | #include <string.h> /* <span class="roman">Declares <code>str</code>.</span> */
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| 75 |
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| 76 | FILE *
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| 77 | concat_fopen (char *s1, char *s2, char *mode)
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| 78 | {
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| 79 | char str[strlen (s1) + strlen (s2) + 1];
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| 80 | strcpy (str, s1);
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| 81 | strcat (str, s2);
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| 82 | return fopen (str, mode);
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| 83 | }
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>(This uses some standard library functions; see <a href="https://www.gnu.org/software/libc/manual/html_node/String-and-Array-Utilities.html#String-and-Array-Utilities">String and Array
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| 87 | Utilities</a> in <cite>The GNU C Library Reference Manual</cite>.)
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| 88 | </p>
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| 89 | <p>The length of an array is computed once when the storage is allocated
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| 90 | and is remembered for the scope of the array in case it is used in
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| 91 | <code>sizeof</code>.
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| 92 | </p>
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| 93 | <p><strong>Warning:</strong> don’t allocate a variable-length array if the size
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| 94 | might be very large (more than 100,000), or in a recursive function,
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| 95 | because that is likely to cause stack overflow. Allocate the array
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| 96 | dynamically instead (see <a href="Dynamic-Memory-Allocation.html">Dynamic Memory Allocation</a>).
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| 97 | </p>
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| 98 | <p>Jumping or breaking out of the scope of the array name deallocates the
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| 99 | storage. Jumping into the scope is not allowed; that gives an error
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| 100 | message.
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| 101 | </p>
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| 102 | <p>You can also use variable-length arrays as arguments to functions:
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example">struct entry
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| 106 | tester (int len, char data[len][len])
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| 107 | {
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| 108 | <span class="roman">…</span>
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| 109 | }
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| 110 | </pre></div>
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| 111 |
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| 112 | <p>As usual, a function argument declared with an array type
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| 113 | is really a pointer to an array that already exists.
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| 114 | Calling the function does not allocate the array, so there’s no
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| 115 | particular danger of stack overflow in using this construct.
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| 116 | </p>
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| 117 | <p>To pass the array first and the length afterward, use a forward
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| 118 | declaration in the function’s parameter list (another GNU extension).
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| 119 | For example,
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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 <code>int len</code> before the semicolon is a <em>parameter forward
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| 130 | declaration</em>, and it serves the purpose of making the name <code>len</code>
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| 131 | known 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 “real” declaration in parameter name and data type. ISO C11 does
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| 138 | not support parameter forward declarations.
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| 139 | </p>
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| 140 | <hr>
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| 141 | <div class="header">
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| 142 | <p>
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| 143 | Previous: <a href="Constructing-Array-Values.html" accesskey="p" rel="prev">Constructing Array Values</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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| 144 | </div>
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