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| <title>Using the GNU Compiler Collection (GCC): Variable Length</title> |
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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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| <hr> |
| <a name="Arrays-of-Variable-Length"></a> |
| <h3 class="section">6.19 Arrays of Variable Length</h3> |
| <a name="index-variable_002dlength-arrays"></a> |
| <a name="index-arrays-of-variable-length"></a> |
| <a name="index-VLAs"></a> |
| |
| <p>Variable-length automatic arrays are allowed in ISO C99, and as an |
| extension GCC accepts them in C90 mode and in C++. These arrays are |
| declared like any other automatic arrays, but with a length that is not |
| a constant expression. The storage is allocated at the point of |
| declaration and deallocated when the block scope containing the declaration |
| exits. For |
| example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">FILE * |
| concat_fopen (char *s1, char *s2, char *mode) |
| { |
| char str[strlen (s1) + strlen (s2) + 1]; |
| strcpy (str, s1); |
| strcat (str, s2); |
| return fopen (str, mode); |
| } |
| </pre></div> |
| |
| <a name="index-scope-of-a-variable-length-array"></a> |
| <a name="index-variable_002dlength-array-scope"></a> |
| <a name="index-deallocating-variable-length-arrays"></a> |
| <p>Jumping or breaking out of the scope of the array name deallocates the |
| storage. Jumping into the scope is not allowed; you get an error |
| message for it. |
| </p> |
| <a name="index-variable_002dlength-array-in-a-structure"></a> |
| <p>As an extension, GCC accepts variable-length arrays as a member of |
| a structure or a union. For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">void |
| foo (int n) |
| { |
| struct S { int x[n]; }; |
| } |
| </pre></div> |
| |
| <a name="index-alloca-vs-variable_002dlength-arrays"></a> |
| <p>You can use the function <code>alloca</code> to get an effect much like |
| variable-length arrays. The function <code>alloca</code> is available in |
| many other C implementations (but not in all). On the other hand, |
| variable-length arrays are more elegant. |
| </p> |
| <p>There are other differences between these two methods. Space allocated |
| with <code>alloca</code> exists until the containing <em>function</em> returns. |
| The space for a variable-length array is deallocated as soon as the array |
| name’s scope ends. (If you use both variable-length arrays and |
| <code>alloca</code> in the same function, deallocation of a variable-length array |
| also deallocates anything more recently allocated with <code>alloca</code>.) |
| </p> |
| <p>You can also use variable-length arrays as arguments to functions: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct entry |
| tester (int len, char data[len][len]) |
| { |
| /* <span class="roman">…</span> */ |
| } |
| </pre></div> |
| |
| <p>The length of an array is computed once when the storage is allocated |
| and is remembered for the scope of the array in case you access it with |
| <code>sizeof</code>. |
| </p> |
| <p>If you want to pass the array first and the length afterward, you can |
| use a forward declaration in the parameter list—another GNU extension. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct entry |
| tester (int len; char data[len][len], int len) |
| { |
| /* <span class="roman">…</span> */ |
| } |
| </pre></div> |
| |
| <a name="index-parameter-forward-declaration"></a> |
| <p>The ‘<samp>int len</samp>’ before the semicolon is a <em>parameter forward |
| declaration</em>, and it serves the purpose of making the name <code>len</code> |
| known when the declaration of <code>data</code> is parsed. |
| </p> |
| <p>You can write any number of such parameter forward declarations in the |
| parameter list. They can be separated by commas or semicolons, but the |
| last one must end with a semicolon, which is followed by the “real” |
| parameter declarations. Each forward declaration must match a “real” |
| declaration in parameter name and data type. ISO C99 does not support |
| parameter forward declarations. |
| </p> |
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