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| 22 | <title>Nested Functions (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Advanced-Definitions.html" rel="up" title="Advanced Definitions">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Nested-Functions"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Inline-Function-Definitions.html" accesskey="n" rel="next">Inline Function Definitions</a>, Previous: <a href="Variable-Number-of-Arguments.html" accesskey="p" rel="prev">Variable Number of Arguments</a>, Up: <a href="Advanced-Definitions.html" accesskey="u" rel="up">Advanced Definitions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Nested-Functions-1"></span><h4 class="subsection">22.7.3 Nested Functions</h4>
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| 64 | <span id="index-nested-functions"></span>
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| 65 | <span id="index-functions_002c-nested"></span>
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| 66 | <span id="index-downward-funargs"></span>
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| 67 | <span id="index-thunks"></span>
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| 68 |
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| 69 | <p>A <em>nested function</em> is a function defined inside another function.
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| 70 | The nested function’s name is local to the block where it is defined.
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| 71 | For example, here we define a nested function named <code>square</code>, and
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| 72 | call it twice:
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| 73 | </p>
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| 74 | <div class="example">
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| 75 | <pre class="example">foo (double a, double b)
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| 76 | {
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| 77 | double square (double z) { return z * z; }
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| 78 |
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| 79 | return square (a) + square (b);
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| 80 | }
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>The nested function can access all the variables of the containing
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| 84 | function that are visible at the point of its definition. This is
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| 85 | called <em>lexical scoping</em>. For example, here we show a nested
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| 86 | function that uses an inherited variable named <code>offset</code>:
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| 87 | </p>
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| 88 | <div class="example">
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| 89 | <pre class="example">bar (int *array, int offset, int size)
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| 90 | {
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| 91 | int access (int *array, int index)
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| 92 | { return array[index + offset]; }
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| 93 | int i;
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| 94 | <span class="roman">…</span>
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| 95 | for (i = 0; i < size; i++)
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| 96 | <span class="roman">…</span> access (array, i) <span class="roman">…</span>
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| 97 | }
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| 98 | </pre></div>
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| 99 |
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| 100 | <p>Nested function definitions can appear wherever automatic variable
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| 101 | declarations are allowed; that is, in any block, interspersed with the
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| 102 | other declarations and statements in the block.
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| 103 | </p>
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| 104 | <p>The nested function’s name is visible only within the parent block;
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| 105 | the name’s scope starts from its definition and continues to the end
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| 106 | of the containing block. If the nested function’s name
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| 107 | is the same as the parent function’s name, there wil be
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| 108 | no way to refer to the parent function inside the scope of the
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| 109 | name of the nested function.
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| 110 | </p>
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| 111 | <p>Using <code>extern</code> or <code>static</code> on a nested function definition
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| 112 | is an error.
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| 113 | </p>
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| 114 | <p>It is possible to call the nested function from outside the scope of its
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| 115 | name by storing its address or passing the address to another function.
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| 116 | You can do this safely, but you must be careful:
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| 117 | </p>
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| 118 | <div class="example">
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| 119 | <pre class="example">hack (int *array, int size, int addition)
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| 120 | {
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| 121 | void store (int index, int value)
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| 122 | { array[index] = value + addition; }
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| 123 |
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| 124 | intermediate (store, size);
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| 125 | }
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| 126 | </pre></div>
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| 127 |
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| 128 | <p>Here, the function <code>intermediate</code> receives the address of
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| 129 | <code>store</code> as an argument. If <code>intermediate</code> calls <code>store</code>,
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| 130 | the arguments given to <code>store</code> are used to store into <code>array</code>.
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| 131 | <code>store</code> also accesses <code>hack</code>’s local variable <code>addition</code>.
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| 132 | </p>
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| 133 | <p>It is safe for <code>intermediate</code> to call <code>store</code> because
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| 134 | <code>hack</code>’s stack frame, with its arguments and local variables,
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| 135 | continues to exist during the call to <code>intermediate</code>.
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| 136 | </p>
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| 137 | <p>Calling the nested function through its address after the containing
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| 138 | function has exited is asking for trouble. If it is called after a
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| 139 | containing scope level has exited, and if it refers to some of the
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| 140 | variables that are no longer in scope, it will refer to memory
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| 141 | containing junk or other data. It’s not wise to take the risk.
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| 142 | </p>
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| 143 | <p>The GNU C Compiler implements taking the address of a nested function
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| 144 | using a technique called <em>trampolines</em>. This technique was
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| 145 | described in <cite>Lexical Closures for C<tt>++</tt></cite> (Thomas M. Breuel,
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| 146 | USENIX C<tt>++</tt> Conference Proceedings, October 17–21, 1988).
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| 147 | </p>
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| 148 | <p>A nested function can jump to a label inherited from a containing
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| 149 | function, provided the label was explicitly declared in the containing
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| 150 | function (see <a href="Local-Labels.html">Local Labels</a>). Such a jump returns instantly to the
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| 151 | containing function, exiting the nested function that did the
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| 152 | <code>goto</code> and any intermediate function invocations as well. Here
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| 153 | is an example:
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| 154 | </p>
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| 155 | <div class="example">
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| 156 | <pre class="example">bar (int *array, int offset, int size)
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| 157 | {
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| 158 | /* <span class="roman">Explicitly declare the label <code>failure</code>.</span> */
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| 159 | __label__ failure;
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| 160 | int access (int *array, int index)
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| 161 | {
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| 162 | if (index > size)
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| 163 | /* <span class="roman">Exit this function,</span>
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| 164 | <span class="roman">and return to <code>bar</code>.</span> */
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| 165 | goto failure;
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| 166 | return array[index + offset];
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| 167 | }
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| 168 | </pre><pre class="example">
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| 169 |
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| 170 | </pre><pre class="example"> int i;
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| 171 | <span class="roman">…</span>
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| 172 | for (i = 0; i < size; i++)
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| 173 | <span class="roman">…</span> access (array, i) <span class="roman">…</span>
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| 174 | <span class="roman">…</span>
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| 175 | return 0;
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| 176 |
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| 177 | /* <span class="roman">Control comes here from <code>access</code>
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| 178 | if it does the <code>goto</code>.</span> */
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| 179 | failure:
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| 180 | return -1;
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| 181 | }
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| 182 | </pre></div>
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| 183 |
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| 184 | <p>To declare the nested function before its definition, use
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| 185 | <code>auto</code> (which is otherwise meaningless for function declarations;
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| 186 | see <a href="auto-and-register.html">auto and register</a>). For example,
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| 187 | </p>
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| 188 | <div class="example">
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| 189 | <pre class="example">bar (int *array, int offset, int size)
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| 190 | {
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| 191 | auto int access (int *, int);
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| 192 | <span class="roman">…</span>
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| 193 | <span class="roman">…</span> access (array, i) <span class="roman">…</span>
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| 194 | <span class="roman">…</span>
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| 195 | int access (int *array, int index)
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| 196 | {
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| 197 | <span class="roman">…</span>
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| 198 | }
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| 199 | <span class="roman">…</span>
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| 200 | }
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| 201 | </pre></div>
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| 202 |
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| 203 | <hr>
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| 204 | <div class="header">
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| 205 | <p>
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| 206 | Next: <a href="Inline-Function-Definitions.html" accesskey="n" rel="next">Inline Function Definitions</a>, Previous: <a href="Variable-Number-of-Arguments.html" accesskey="p" rel="prev">Variable Number of Arguments</a>, Up: <a href="Advanced-Definitions.html" accesskey="u" rel="up">Advanced Definitions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 207 | </div>
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| 208 |
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| 209 |
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