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134 lines
5.9 KiB
HTML
134 lines
5.9 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Variable Length - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Empty-Structures.html#Empty-Structures" title="Empty Structures">
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<link rel="next" href="Variadic-Macros.html#Variadic-Macros" title="Variadic Macros">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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</head>
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<body>
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<div class="node">
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<a name="Variable-Length"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>,
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Previous: <a rel="previous" accesskey="p" href="Empty-Structures.html#Empty-Structures">Empty Structures</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.19 Arrays of Variable Length</h3>
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<p><a name="index-variable_002dlength-arrays-2989"></a><a name="index-arrays-of-variable-length-2990"></a><a name="index-VLAs-2991"></a>
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Variable-length automatic arrays are allowed in ISO C99, and as an
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extension GCC accepts them in C90 mode and in C++. These arrays are
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declared like any other automatic arrays, but with a length that is not
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a constant expression. The storage is allocated at the point of
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declaration and deallocated when the block scope containing the declaration
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exits. For
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example:
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<pre class="smallexample"> FILE *
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concat_fopen (char *s1, char *s2, char *mode)
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{
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char str[strlen (s1) + strlen (s2) + 1];
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strcpy (str, s1);
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strcat (str, s2);
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return fopen (str, mode);
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}
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</pre>
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<p><a name="index-scope-of-a-variable-length-array-2992"></a><a name="index-variable_002dlength-array-scope-2993"></a><a name="index-deallocating-variable-length-arrays-2994"></a>Jumping or breaking out of the scope of the array name deallocates the
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storage. Jumping into the scope is not allowed; you get an error
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message for it.
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<p><a name="index-variable_002dlength-array-in-a-structure-2995"></a>As an extension, GCC accepts variable-length arrays as a member of
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a structure or a union. For example:
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<pre class="smallexample"> void
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foo (int n)
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{
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struct S { int x[n]; };
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}
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</pre>
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<p><a name="index-g_t_0040code_007balloca_007d-vs-variable_002dlength-arrays-2996"></a>You can use the function <code>alloca</code> to get an effect much like
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variable-length arrays. The function <code>alloca</code> is available in
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many other C implementations (but not in all). On the other hand,
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variable-length arrays are more elegant.
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<p>There are other differences between these two methods. Space allocated
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with <code>alloca</code> exists until the containing <em>function</em> returns.
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The space for a variable-length array is deallocated as soon as the array
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name's scope ends. (If you use both variable-length arrays and
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<code>alloca</code> in the same function, deallocation of a variable-length array
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also deallocates anything more recently allocated with <code>alloca</code>.)
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<p>You can also use variable-length arrays as arguments to functions:
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<pre class="smallexample"> struct entry
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tester (int len, char data[len][len])
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{
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/* <span class="roman">...</span> */
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}
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</pre>
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<p>The length of an array is computed once when the storage is allocated
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and is remembered for the scope of the array in case you access it with
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<code>sizeof</code>.
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<p>If you want to pass the array first and the length afterward, you can
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use a forward declaration in the parameter list—another GNU extension.
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<pre class="smallexample"> struct entry
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tester (int len; char data[len][len], int len)
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{
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/* <span class="roman">...</span> */
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}
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</pre>
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<p><a name="index-parameter-forward-declaration-2997"></a>The ‘<samp><span class="samp">int len</span></samp>’ before the semicolon is a <dfn>parameter forward
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declaration</dfn>, and it serves the purpose of making the name <code>len</code>
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known when the declaration of <code>data</code> is parsed.
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<p>You can write any number of such parameter forward declarations in the
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parameter list. They can be separated by commas or semicolons, but the
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last one must end with a semicolon, which is followed by the “real”
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parameter declarations. Each forward declaration must match a “real”
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declaration in parameter name and data type. ISO C99 does not support
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parameter forward declarations.
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</body></html>
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