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<h3 class="section">Arrays of Variable Length</h3>
<p>Variable-length automatic arrays are allowed in ISO C99, and as an
extension GCC accepts them in C89 mode and in C++. (However, GCC's
implementation of variable-length arrays does not yet conform in detail
to the ISO C99 standard.) 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="example"> 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>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>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="example"> struct entry
tester (int len, char data[len][len])
{
/* <small class="dots">...</small> */
}
</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="example"> struct entry
tester (int len; char data[len][len], int len)
{
/* <small class="dots">...</small> */
}
</pre>
<p>The <code>int len</code> 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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