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| 22 | <title>Inline Function Definitions (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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| 33 | <link href="Obsolete-Definitions.html" rel="next" title="Obsolete 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="Inline-Function-Definitions"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Nested-Functions.html" accesskey="p" rel="prev">Nested Functions</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="Inline-Function-Definitions-1"></span><h4 class="subsection">22.7.4 Inline Function Definitions</h4>
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| 64 | <span id="index-inline-function-definitions"></span>
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| 65 | <span id="index-function-definitions_002c-inline"></span>
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| 66 | <span id="index-inline"></span>
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| 67 |
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| 68 | <p>To declare a function inline, use the <code>inline</code> keyword in its
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| 69 | definition. Here’s a simple function that takes a pointer-to-<code>int</code>
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| 70 | and increments the integer stored there—declared inline.
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">struct list
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| 74 | {
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| 75 | struct list *first, *second;
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| 76 | };
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| 77 |
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| 78 | inline struct list *
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| 79 | list_first (struct list *p)
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| 80 | {
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| 81 | return p->first;
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| 82 | }
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| 83 |
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| 84 | inline struct list *
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| 85 | list_second (struct list *p)
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| 86 | {
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| 87 | return p->second;
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| 88 | }
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| 89 | </pre></div>
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| 90 |
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| 91 | <p>optimized compilation can substitute the inline function’s body for
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| 92 | any call to it. This is called <em>inlining</em> the function. It
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| 93 | makes the code that contains the call run faster, significantly so if
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| 94 | the inline function is small.
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| 95 | </p>
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| 96 | <p>Here’s a function that uses <code>pair_second</code>:
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| 97 | </p>
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| 98 | <div class="example">
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| 99 | <pre class="example">int
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| 100 | pairlist_length (struct list *l)
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| 101 | {
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| 102 | int length = 0;
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| 103 | while (l)
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| 104 | {
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| 105 | length++;
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| 106 | l = pair_second (l);
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| 107 | }
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| 108 | return length;
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| 109 | }
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| 110 | </pre></div>
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| 111 |
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| 112 | <p>Substituting the code of <code>pair_second</code> into the definition of
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| 113 | <code>pairlist_length</code> results in this code, in effect:
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| 114 | </p>
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| 115 | <div class="example">
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| 116 | <pre class="example">int
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| 117 | pairlist_length (struct list *l)
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| 118 | {
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| 119 | int length = 0;
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| 120 | while (l)
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| 121 | {
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| 122 | length++;
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| 123 | l = l->second;
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| 124 | }
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| 125 | return length;
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| 126 | }
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| 127 | </pre></div>
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| 128 |
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| 129 | <p>Since the definition of <code>pair_second</code> does not say <code>extern</code>
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| 130 | or <code>static</code>, that definition is used only for inlining. It
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| 131 | doesn’t generate code that can be called at run time. If not all the
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| 132 | calls to the function are inlined, there must be a definition of the
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| 133 | same function name in another module for them to call.
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| 134 | </p>
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| 135 | <span id="index-inline-functions_002c-omission-of"></span>
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| 136 | <p>Adding <code>static</code> to an inline function definition means the
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| 137 | function definition is limited to this compilation module. Also, it
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| 138 | generates run-time code if necessary for the sake of any calls that
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| 139 | were not inlined. If all calls are inlined then the function
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| 140 | definition does not generate run-time code, but you can force
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| 141 | generation of run-time code with the option
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| 142 | <samp>-fkeep-inline-functions</samp>.
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| 143 | </p>
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| 144 | <span id="index-extern-inline-function"></span>
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| 145 | <p>Specifying <code>extern</code> along with <code>inline</code> means the function is
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| 146 | external and generates run-time code to be called from other
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| 147 | separately compiled modules, as well as inlined. You can define the
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| 148 | function as <code>inline</code> without <code>extern</code> in other modules so as
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| 149 | to inline calls to the same function in those modules.
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| 150 | </p>
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| 151 | <p>Why are some calls not inlined? First of all, inlining is an
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| 152 | optimization, so non-optimized compilation does not inline.
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| 153 | </p>
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| 154 | <p>Some calls cannot be inlined for technical reasons. Also, certain
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| 155 | usages in a function definition can make it unsuitable for inline
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| 156 | substitution. Among these usages are: variadic functions, use of
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| 157 | <code>alloca</code>, use of computed goto (see <a href="Labels-as-Values.html">Labels as Values</a>), and
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| 158 | use of nonlocal goto. The option <samp>-Winline</samp> requests a warning
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| 159 | when a function marked <code>inline</code> is unsuitable to be inlined. The
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| 160 | warning explains what obstacle makes it unsuitable.
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| 161 | </p>
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| 162 | <p>Just because a call <em>can</em> be inlined does not mean it
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| 163 | <em>should</em> be inlined. The GNU C compiler weighs costs and
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| 164 | benefits to decide whether inlining a particular call is advantageous.
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| 165 | </p>
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| 166 | <p>You can force inlining of all calls to a given function that can be
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| 167 | inlined, even in a non-optimized compilation. by specifying the
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| 168 | ‘<samp>always_inline</samp>’ attribute for the function, like this:
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| 169 | </p>
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| 170 | <div class="example">
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| 171 | <pre class="example">/* <span class="roman">Prototype.</span> */
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| 172 | inline void foo (const char) __attribute__((always_inline));
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| 173 | </pre></div>
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| 174 |
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| 175 | <p>This is a GNU C extension. See <a href="Attributes.html">Attributes</a>.
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| 176 | </p>
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| 177 | <p>A function call may be inlined even if not declared <code>inline</code> in
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| 178 | special cases where the compiler can determine this is correct and
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| 179 | desirable. For instance, when a static function is called only once,
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| 180 | it will very likely be inlined. With <samp>-flto</samp>, link-time
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| 181 | optimization, any function might be inlined. To absolutely prevent
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| 182 | inlining of a specific function, specify
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| 183 | <code>__attribute__((__noinline__))</code> in the function’s definition.
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| 184 | </p>
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| 185 | <hr>
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| 186 | <div class="header">
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| 187 | <p>
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| 188 | Previous: <a href="Nested-Functions.html" accesskey="p" rel="prev">Nested Functions</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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| 189 | </div>
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| 190 |
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| 191 |
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| 192 |
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| 193 | </body>
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