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171 lines
7.0 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Nested Functions - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Labels-as-Values.html#Labels-as-Values" title="Labels as Values">
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<link rel="next" href="Constructing-Calls.html#Constructing-Calls" title="Constructing Calls">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Nested-Functions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Constructing-Calls.html#Constructing-Calls">Constructing Calls</a>,
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Previous: <a rel="previous" accesskey="p" href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.4 Nested Functions</h3>
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<p><a name="index-nested-functions-2879"></a><a name="index-downward-funargs-2880"></a><a name="index-thunks-2881"></a>
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A <dfn>nested function</dfn> is a function defined inside another function.
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Nested functions are supported as an extension in GNU C, but are not
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supported by GNU C++.
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<p>The nested function's name is local to the block where it is defined.
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For example, here we define a nested function named <code>square</code>, and
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call it twice:
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<pre class="smallexample"> foo (double a, double b)
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{
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double square (double z) { return z * z; }
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return square (a) + square (b);
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}
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</pre>
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<p>The nested function can access all the variables of the containing
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function that are visible at the point of its definition. This is
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called <dfn>lexical scoping</dfn>. For example, here we show a nested
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function which uses an inherited variable named <code>offset</code>:
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<pre class="smallexample"> bar (int *array, int offset, int size)
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{
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int access (int *array, int index)
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{ return array[index + offset]; }
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int i;
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/* <span class="roman">...</span> */
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for (i = 0; i < size; i++)
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/* <span class="roman">...</span> */ access (array, i) /* <span class="roman">...</span> */
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}
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</pre>
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<p>Nested function definitions are permitted within functions in the places
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where variable definitions are allowed; that is, in any block, mixed
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with the other declarations and statements in the block.
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<p>It is possible to call the nested function from outside the scope of its
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name by storing its address or passing the address to another function:
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<pre class="smallexample"> hack (int *array, int size)
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{
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void store (int index, int value)
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{ array[index] = value; }
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intermediate (store, size);
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}
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</pre>
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<p>Here, the function <code>intermediate</code> receives the address of
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<code>store</code> as an argument. If <code>intermediate</code> calls <code>store</code>,
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the arguments given to <code>store</code> are used to store into <code>array</code>.
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But this technique works only so long as the containing function
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(<code>hack</code>, in this example) does not exit.
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<p>If you try to call the nested function through its address after the
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containing function exits, all hell breaks loose. If you try
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to call it after a containing scope level exits, and if it refers
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to some of the variables that are no longer in scope, you may be lucky,
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but it's not wise to take the risk. If, however, the nested function
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does not refer to anything that has gone out of scope, you should be
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safe.
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<p>GCC implements taking the address of a nested function using a technique
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called <dfn>trampolines</dfn>. This technique was described in
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<cite>Lexical Closures for C++</cite> (Thomas M. Breuel, USENIX
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C++ Conference Proceedings, October 17-21, 1988).
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<p>A nested function can jump to a label inherited from a containing
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function, provided the label is explicitly declared in the containing
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function (see <a href="Local-Labels.html#Local-Labels">Local Labels</a>). Such a jump returns instantly to the
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containing function, exiting the nested function that did the
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<code>goto</code> and any intermediate functions as well. Here is an example:
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<pre class="smallexample"> bar (int *array, int offset, int size)
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{
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__label__ failure;
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int access (int *array, int index)
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{
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if (index > size)
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goto failure;
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return array[index + offset];
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}
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int i;
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/* <span class="roman">...</span> */
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for (i = 0; i < size; i++)
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/* <span class="roman">...</span> */ access (array, i) /* <span class="roman">...</span> */
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/* <span class="roman">...</span> */
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return 0;
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/* <span class="roman">Control comes here from </span><code>access</code><span class="roman">
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if it detects an error.</span> */
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failure:
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return -1;
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}
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</pre>
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<p>A nested function always has no linkage. Declaring one with
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<code>extern</code> or <code>static</code> is erroneous. If you need to declare the nested function
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before its definition, use <code>auto</code> (which is otherwise meaningless
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for function declarations).
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<pre class="smallexample"> bar (int *array, int offset, int size)
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{
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__label__ failure;
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auto int access (int *, int);
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/* <span class="roman">...</span> */
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int access (int *array, int index)
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{
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if (index > size)
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goto failure;
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return array[index + offset];
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}
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/* <span class="roman">...</span> */
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}
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</pre>
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</body></html>
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