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163 lines
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<title>Inline - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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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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<body>
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<div class="node">
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<a name="Inline"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Volatiles.html#Volatiles">Volatiles</a>,
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Previous: <a rel="previous" accesskey="p" href="Alignment.html#Alignment">Alignment</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">6.41 An Inline Function is As Fast As a Macro</h3>
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<p><a name="index-inline-functions-3400"></a><a name="index-integrating-function-code-3401"></a><a name="index-open-coding-3402"></a><a name="index-macros_002c-inline-alternative-3403"></a>
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By declaring a function inline, you can direct GCC to make
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calls to that function faster. One way GCC can achieve this is to
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integrate that function's code into the code for its callers. This
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makes execution faster by eliminating the function-call overhead; in
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addition, if any of the actual argument values are constant, their
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known values may permit simplifications at compile time so that not
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all of the inline function's code needs to be included. The effect on
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code size is less predictable; object code may be larger or smaller
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with function inlining, depending on the particular case. You can
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also direct GCC to try to integrate all “simple enough” functions
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into their callers with the option <samp><span class="option">-finline-functions</span></samp>.
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<p>GCC implements three different semantics of declaring a function
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inline. One is available with <samp><span class="option">-std=gnu89</span></samp> or
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<samp><span class="option">-fgnu89-inline</span></samp> or when <code>gnu_inline</code> attribute is present
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on all inline declarations, another when
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<samp><span class="option">-std=c99</span></samp>, <samp><span class="option">-std=c11</span></samp>,
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<samp><span class="option">-std=gnu99</span></samp> or <samp><span class="option">-std=gnu11</span></samp>
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(without <samp><span class="option">-fgnu89-inline</span></samp>), and the third
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is used when compiling C++.
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<p>To declare a function inline, use the <code>inline</code> keyword in its
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declaration, like this:
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<pre class="smallexample"> static inline int
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inc (int *a)
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{
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return (*a)++;
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}
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</pre>
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<p>If you are writing a header file to be included in ISO C90 programs, write
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<code>__inline__</code> instead of <code>inline</code>. See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>.
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<p>The three types of inlining behave similarly in two important cases:
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when the <code>inline</code> keyword is used on a <code>static</code> function,
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like the example above, and when a function is first declared without
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using the <code>inline</code> keyword and then is defined with
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<code>inline</code>, like this:
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<pre class="smallexample"> extern int inc (int *a);
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inline int
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inc (int *a)
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{
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return (*a)++;
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}
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</pre>
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<p>In both of these common cases, the program behaves the same as if you
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had not used the <code>inline</code> keyword, except for its speed.
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<p><a name="index-inline-functions_002c-omission-of-3404"></a><a name="index-fkeep_002dinline_002dfunctions-3405"></a>When a function is both inline and <code>static</code>, if all calls to the
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function are integrated into the caller, and the function's address is
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never used, then the function's own assembler code is never referenced.
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In this case, GCC does not actually output assembler code for the
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function, unless you specify the option <samp><span class="option">-fkeep-inline-functions</span></samp>.
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Some calls cannot be integrated for various reasons (in particular,
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calls that precede the function's definition cannot be integrated, and
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neither can recursive calls within the definition). If there is a
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nonintegrated call, then the function is compiled to assembler code as
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usual. The function must also be compiled as usual if the program
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refers to its address, because that can't be inlined.
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<p><a name="index-Winline-3406"></a>Note that certain usages in a function definition can make it unsuitable
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for inline substitution. Among these usages are: variadic functions, use of
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<code>alloca</code>, use of variable-length data types (see <a href="Variable-Length.html#Variable-Length">Variable Length</a>),
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use of computed goto (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>), use of nonlocal goto,
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and nested functions (see <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>). Using <samp><span class="option">-Winline</span></samp>
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warns when a function marked <code>inline</code> could not be substituted,
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and gives the reason for the failure.
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<p><a name="index-automatic-_0040code_007binline_007d-for-C_002b_002b-member-fns-3407"></a><a name="index-g_t_0040code_007binline_007d-automatic-for-C_002b_002b-member-fns-3408"></a><a name="index-member-fns_002c-automatically-_0040code_007binline_007d-3409"></a><a name="index-C_002b_002b-member-fns_002c-automatically-_0040code_007binline_007d-3410"></a><a name="index-fno_002ddefault_002dinline-3411"></a>As required by ISO C++, GCC considers member functions defined within
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the body of a class to be marked inline even if they are
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not explicitly declared with the <code>inline</code> keyword. You can
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override this with <samp><span class="option">-fno-default-inline</span></samp>; see <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">Options Controlling C++ Dialect</a>.
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<p>GCC does not inline any functions when not optimizing unless you specify
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the ‘<samp><span class="samp">always_inline</span></samp>’ attribute for the function, like this:
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<pre class="smallexample"> /* <span class="roman">Prototype.</span> */
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inline void foo (const char) __attribute__((always_inline));
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</pre>
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<p>The remainder of this section is specific to GNU C90 inlining.
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<p><a name="index-non_002dstatic-inline-function-3412"></a>When an inline function is not <code>static</code>, then the compiler must assume
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that there may be calls from other source files; since a global symbol can
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be defined only once in any program, the function must not be defined in
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the other source files, so the calls therein cannot be integrated.
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Therefore, a non-<code>static</code> inline function is always compiled on its
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own in the usual fashion.
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<p>If you specify both <code>inline</code> and <code>extern</code> in the function
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definition, then the definition is used only for inlining. In no case
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is the function compiled on its own, not even if you refer to its
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address explicitly. Such an address becomes an external reference, as
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if you had only declared the function, and had not defined it.
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<p>This combination of <code>inline</code> and <code>extern</code> has almost the
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effect of a macro. The way to use it is to put a function definition in
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a header file with these keywords, and put another copy of the
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definition (lacking <code>inline</code> and <code>extern</code>) in a library file.
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The definition in the header file causes most calls to the function
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to be inlined. If any uses of the function remain, they refer to
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the single copy in the library.
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</body></html>
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