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<a name="Nested-Functions"></a>
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<a name="Nested-Functions-1"></a>
<h3 class="section">6.4 Nested Functions</h3>
<a name="index-nested-functions"></a>
<a name="index-downward-funargs"></a>
<a name="index-thunks"></a>

<p>A <em>nested function</em> is a function defined inside another function.
Nested functions are supported as an extension in GNU C, but are not
supported by GNU C++.
</p>
<p>The nested function&rsquo;s name is local to the block where it is defined.
For example, here we define a nested function named <code>square</code>, and
call it twice:
</p>
<div class="smallexample">
<pre class="smallexample">foo (double a, double b)
{
  double square (double z) { return z * z; }

  return square (a) + square (b);
}
</pre></div>

<p>The nested function can access all the variables of the containing
function that are visible at the point of its definition.  This is
called <em>lexical scoping</em>.  For example, here we show a nested
function which uses an inherited variable named <code>offset</code>:
</p>
<div class="smallexample">
<pre class="smallexample">bar (int *array, int offset, int size)
{
  int access (int *array, int index)
    { return array[index + offset]; }
  int i;
  /* <span class="roman">&hellip;</span> */
  for (i = 0; i &lt; size; i++)
    /* <span class="roman">&hellip;</span> */ access (array, i) /* <span class="roman">&hellip;</span> */
}
</pre></div>

<p>Nested function definitions are permitted within functions in the places
where variable definitions are allowed; that is, in any block, mixed
with the other declarations and statements in the block.
</p>
<p>It is possible to call the nested function from outside the scope of its
name by storing its address or passing the address to another function:
</p>
<div class="smallexample">
<pre class="smallexample">hack (int *array, int size)
{
  void store (int index, int value)
    { array[index] = value; }

  intermediate (store, size);
}
</pre></div>

<p>Here, the function <code>intermediate</code> receives the address of
<code>store</code> as an argument.  If <code>intermediate</code> calls <code>store</code>,
the arguments given to <code>store</code> are used to store into <code>array</code>.
But this technique works only so long as the containing function
(<code>hack</code>, in this example) does not exit.
</p>
<p>If you try to call the nested function through its address after the
containing function exits, all hell breaks loose.  If you try
to call it after a containing scope level exits, and if it refers
to some of the variables that are no longer in scope, you may be lucky,
but it&rsquo;s not wise to take the risk.  If, however, the nested function
does not refer to anything that has gone out of scope, you should be
safe.
</p>
<p>GCC implements taking the address of a nested function using a technique
called <em>trampolines</em>.  This technique was described in
<cite>Lexical Closures for C++</cite> (Thomas M. Breuel, USENIX
C++ Conference Proceedings, October 17-21, 1988).
</p>
<p>A nested function can jump to a label inherited from a containing
function, provided the label is explicitly declared in the containing
function (see <a href="Local-Labels.html#Local-Labels">Local Labels</a>).  Such a jump returns instantly to the
containing function, exiting the nested function that did the
<code>goto</code> and any intermediate functions as well.  Here is an example:
</p>
<div class="smallexample">
<pre class="smallexample">bar (int *array, int offset, int size)
{
  __label__ failure;
  int access (int *array, int index)
    {
      if (index &gt; size)
        goto failure;
      return array[index + offset];
    }
  int i;
  /* <span class="roman">&hellip;</span> */
  for (i = 0; i &lt; size; i++)
    /* <span class="roman">&hellip;</span> */ access (array, i) /* <span class="roman">&hellip;</span> */
  /* <span class="roman">&hellip;</span> */
  return 0;

 /* <span class="roman">Control comes here from <code>access</code>
    if it detects an error.</span>  */
 failure:
  return -1;
}
</pre></div>

<p>A nested function always has no linkage.  Declaring one with
<code>extern</code> or <code>static</code> is erroneous.  If you need to declare the nested function
before its definition, use <code>auto</code> (which is otherwise meaningless
for function declarations).
</p>
<div class="smallexample">
<pre class="smallexample">bar (int *array, int offset, int size)
{
  __label__ failure;
  auto int access (int *, int);
  /* <span class="roman">&hellip;</span> */
  int access (int *array, int index)
    {
      if (index &gt; size)
        goto failure;
      return array[index + offset];
    }
  /* <span class="roman">&hellip;</span> */
}
</pre></div>

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