214 lines
8.6 KiB
HTML
214 lines
8.6 KiB
HTML
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2016 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
||
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
||
|
|
(see below). A copy of the license is included in the section entitled
|
||
|
|
"GNU Free Documentation License".
|
||
|
|
|
||
|
|
(a) The FSF's Front-Cover Text is:
|
||
|
|
|
||
|
|
A GNU Manual
|
||
|
|
|
||
|
|
(b) The FSF's Back-Cover Text is:
|
||
|
|
|
||
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
||
|
|
software. Copies published by the Free Software Foundation raise
|
||
|
|
funds for GNU development. -->
|
||
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Using the GNU Compiler Collection (GCC): Nested Functions</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Using the GNU Compiler Collection (GCC): Nested Functions">
|
||
|
|
<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Nested Functions">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
|
||
|
|
<link href="Constructing-Calls.html#Constructing-Calls" rel="next" title="Constructing Calls">
|
||
|
|
<link href="Labels-as-Values.html#Labels-as-Values" rel="prev" title="Labels as Values">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.indentedblock {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style:oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nocodebreak {white-space:nowrap}
|
||
|
|
span.nolinebreak {white-space:nowrap}
|
||
|
|
span.roman {font-family:serif; font-weight:normal}
|
||
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
||
|
|
<a name="Nested-Functions"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="n" rel="next">Constructing Calls</a>, Previous: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="p" rel="prev">Labels as Values</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<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’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">…</span> */
|
||
|
|
for (i = 0; i < size; i++)
|
||
|
|
/* <span class="roman">…</span> */ access (array, i) /* <span class="roman">…</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’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 > size)
|
||
|
|
goto failure;
|
||
|
|
return array[index + offset];
|
||
|
|
}
|
||
|
|
int i;
|
||
|
|
/* <span class="roman">…</span> */
|
||
|
|
for (i = 0; i < size; i++)
|
||
|
|
/* <span class="roman">…</span> */ access (array, i) /* <span class="roman">…</span> */
|
||
|
|
/* <span class="roman">…</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">…</span> */
|
||
|
|
int access (int *array, int index)
|
||
|
|
{
|
||
|
|
if (index > size)
|
||
|
|
goto failure;
|
||
|
|
return array[index + offset];
|
||
|
|
}
|
||
|
|
/* <span class="roman">…</span> */
|
||
|
|
}
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Constructing-Calls.html#Constructing-Calls" accesskey="n" rel="next">Constructing Calls</a>, Previous: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="p" rel="prev">Labels as Values</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|