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| <h3 class="section">6.18 Arrays of Variable Length</h3> |
| |
| <p><a name="index-variable_002dlength-arrays-2290"></a><a name="index-arrays-of-variable-length-2291"></a><a name="index-VLAs-2292"></a> |
| 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 brace-level is exited. For |
| example: |
| |
| <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> |
| <p><a name="index-scope-of-a-variable-length-array-2293"></a><a name="index-variable_002dlength-array-scope-2294"></a><a name="index-deallocating-variable-length-arrays-2295"></a>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-g_t_0040code_007balloca_007d-vs-variable_002dlength-arrays-2296"></a>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>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 |
| will also deallocate anything more recently allocated with <code>alloca</code>.) |
| |
| <p>You can also use variable-length arrays as arguments to functions: |
| |
| <pre class="smallexample"> struct entry |
| tester (int len, char data[len][len]) |
| { |
| /* <span class="roman">...</span> */ |
| } |
| </pre> |
| <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>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. |
| |
| <pre class="smallexample"> struct entry |
| tester (int len; char data[len][len], int len) |
| { |
| /* <span class="roman">...</span> */ |
| } |
| </pre> |
| <p><a name="index-parameter-forward-declaration-2297"></a>The ‘<samp><span class="samp">int len</span></samp>’ before the semicolon is a <dfn>parameter forward |
| declaration</dfn>, and it serves the purpose of making the name <code>len</code> |
| known when the declaration of <code>data</code> is parsed. |
| |
| <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. |
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