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<a name="Variable-Length"></a>
<div class="header">
<p>
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<hr>
<a name="Arrays-of-Variable-Length"></a>
<h3 class="section">6.20 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&rsquo;s scope ends, unless you also use <code>alloca</code> in this scope.
</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">&hellip;</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&mdash;another GNU extension.
</p>
<div class="smallexample">
<pre class="smallexample">struct entry
tester (int len; char data[len][len], int len)
{
  /* <span class="roman">&hellip;</span> */
}
</pre></div>

<a name="index-parameter-forward-declaration"></a>
<p>The &lsquo;<samp>int len</samp>&rsquo; 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 &ldquo;real&rdquo;
parameter declarations.  Each forward declaration must match a &ldquo;real&rdquo;
declaration in parameter name and data type.  ISO C99 does not support
parameter forward declarations.
</p>
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