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<title>Using the GNU Compiler Collection (GCC): Variable Length</title>
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<a name="Variable-Length"></a>
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<div class="header">
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<p>
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Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Empty-Structures.html#Empty-Structures" accesskey="p" rel="prev">Empty Structures</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Arrays-of-Variable-Length"></a>
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<h3 class="section">6.19 Arrays of Variable Length</h3>
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<a name="index-variable_002dlength-arrays"></a>
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<a name="index-arrays-of-variable-length"></a>
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<a name="index-VLAs"></a>
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<p>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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</p>
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<div class="smallexample">
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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></div>
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<a name="index-scope-of-a-variable-length-array"></a>
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<a name="index-variable_002dlength-array-scope"></a>
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<a name="index-deallocating-variable-length-arrays"></a>
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<p>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>
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<a name="index-variable_002dlength-array-in-a-structure"></a>
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<p>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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</p>
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<div class="smallexample">
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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></div>
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<a name="index-alloca-vs-variable_002dlength-arrays"></a>
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<p>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>
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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, unless you also use <code>alloca</code> in this scope.
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</p>
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<p>You can also use variable-length arrays as arguments to functions:
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</p>
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<div class="smallexample">
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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></div>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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<a name="index-parameter-forward-declaration"></a>
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<p>The ‘<samp>int len</samp>’ before the semicolon is a <em>parameter forward
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declaration</em>, 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>
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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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Empty-Structures.html#Empty-Structures" accesskey="p" rel="prev">Empty Structures</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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