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<title>Using the GNU Compiler Collection (GCC): Inline</title>
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<a name="Inline"></a>
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<div class="header">
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<p>
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Next: <a href="Volatiles.html#Volatiles" accesskey="n" rel="next">Volatiles</a>, Previous: <a href="Alignment.html#Alignment" accesskey="p" rel="prev">Alignment</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>
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</div>
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<hr>
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<a name="An-Inline-Function-is-As-Fast-As-a-Macro"></a>
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<h3 class="section">6.42 An Inline Function is As Fast As a Macro</h3>
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<a name="index-inline-functions"></a>
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<a name="index-integrating-function-code"></a>
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<a name="index-open-coding"></a>
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<a name="index-macros_002c-inline-alternative"></a>
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<p>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>-finline-functions</samp>.
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</p>
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<p>GCC implements three different semantics of declaring a function
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inline. One is available with <samp>-std=gnu89</samp> or
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<samp>-fgnu89-inline</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>-std=c99</samp>, <samp>-std=c11</samp>,
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<samp>-std=gnu99</samp> or <samp>-std=gnu11</samp>
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(without <samp>-fgnu89-inline</samp>), and the third
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is used when compiling C++.
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</p>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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<a name="index-inline-functions_002c-omission-of"></a>
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<a name="index-fkeep_002dinline_002dfunctions-1"></a>
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<p>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>-fkeep-inline-functions</samp>.
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If there is a nonintegrated call, then the function is compiled to
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assembler code as usual. The function must also be compiled as usual if
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the program refers to its address, because that can’t be inlined.
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</p>
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<a name="index-Winline-1"></a>
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<p>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,
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use of <code>alloca</code>, use of computed goto (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>),
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use of nonlocal goto, use of nested functions, use of <code>setjmp</code>, use
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of <code>__builtin_longjmp</code> and use of <code>__builtin_return</code> or
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<code>__builtin_apply_args</code>. Using <samp>-Winline</samp> warns when a
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function marked <code>inline</code> could not be substituted, and gives the
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reason for the failure.
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</p>
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<a name="index-automatic-inline-for-C_002b_002b-member-fns"></a>
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<a name="index-inline-automatic-for-C_002b_002b-member-fns"></a>
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<a name="index-member-fns_002c-automatically-inline"></a>
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<a name="index-C_002b_002b-member-fns_002c-automatically-inline"></a>
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<a name="index-fno_002ddefault_002dinline"></a>
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<p>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>-fno-default-inline</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>
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<p>GCC does not inline any functions when not optimizing unless you specify
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the ‘<samp>always_inline</samp>’ attribute for the function, like this:
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</p>
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<div class="smallexample">
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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></div>
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<p>The remainder of this section is specific to GNU C90 inlining.
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</p>
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<a name="index-non_002dstatic-inline-function"></a>
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<p>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>
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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>
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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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Volatiles.html#Volatiles" accesskey="n" rel="next">Volatiles</a>, Previous: <a href="Alignment.html#Alignment" accesskey="p" rel="prev">Alignment</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>
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