| <html lang="en"> |
| <head> |
| <title>Inline - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> |
| <link rel="prev" href="Alignment.html#Alignment" title="Alignment"> |
| <link rel="next" href="Extended-Asm.html#Extended-Asm" title="Extended Asm"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Inline"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Extended-Asm.html#Extended-Asm">Extended Asm</a>, |
| Previous: <a rel="previous" accesskey="p" href="Alignment.html#Alignment">Alignment</a>, |
| Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">6.38 An Inline Function is As Fast As a Macro</h3> |
| |
| <p><a name="index-inline-functions-2513"></a><a name="index-integrating-function-code-2514"></a><a name="index-open-coding-2515"></a><a name="index-macros_002c-inline-alternative-2516"></a> |
| By declaring a function inline, you can direct GCC to make |
| calls to that function faster. One way GCC can achieve this is to |
| integrate that function's code into the code for its callers. This |
| makes execution faster by eliminating the function-call overhead; in |
| addition, if any of the actual argument values are constant, their |
| known values may permit simplifications at compile time so that not |
| all of the inline function's code needs to be included. The effect on |
| code size is less predictable; object code may be larger or smaller |
| with function inlining, depending on the particular case. You can |
| also direct GCC to try to integrate all “simple enough” functions |
| into their callers with the option <samp><span class="option">-finline-functions</span></samp>. |
| |
| <p>GCC implements three different semantics of declaring a function |
| inline. One is available with <samp><span class="option">-std=gnu89</span></samp> or |
| <samp><span class="option">-fgnu89-inline</span></samp> or when <code>gnu_inline</code> attribute is present |
| on all inline declarations, another when <samp><span class="option">-std=c99</span></samp> or |
| <samp><span class="option">-std=gnu99</span></samp> (without <samp><span class="option">-fgnu89-inline</span></samp>), and the third |
| is used when compiling C++. |
| |
| <p>To declare a function inline, use the <code>inline</code> keyword in its |
| declaration, like this: |
| |
| <pre class="smallexample"> static inline int |
| inc (int *a) |
| { |
| (*a)++; |
| } |
| </pre> |
| <p>If you are writing a header file to be included in ISO C90 programs, write |
| <code>__inline__</code> instead of <code>inline</code>. See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>. |
| |
| <p>The three types of inlining behave similarly in two important cases: |
| when the <code>inline</code> keyword is used on a <code>static</code> function, |
| like the example above, and when a function is first declared without |
| using the <code>inline</code> keyword and then is defined with |
| <code>inline</code>, like this: |
| |
| <pre class="smallexample"> extern int inc (int *a); |
| inline int |
| inc (int *a) |
| { |
| (*a)++; |
| } |
| </pre> |
| <p>In both of these common cases, the program behaves the same as if you |
| had not used the <code>inline</code> keyword, except for its speed. |
| |
| <p><a name="index-inline-functions_002c-omission-of-2517"></a><a name="index-fkeep_002dinline_002dfunctions-2518"></a>When a function is both inline and <code>static</code>, if all calls to the |
| function are integrated into the caller, and the function's address is |
| never used, then the function's own assembler code is never referenced. |
| In this case, GCC does not actually output assembler code for the |
| function, unless you specify the option <samp><span class="option">-fkeep-inline-functions</span></samp>. |
| Some calls cannot be integrated for various reasons (in particular, |
| calls that precede the function's definition cannot be integrated, and |
| neither can recursive calls within the definition). If there is a |
| nonintegrated call, then the function is compiled to assembler code as |
| usual. The function must also be compiled as usual if the program |
| refers to its address, because that can't be inlined. |
| |
| <p><a name="index-Winline-2519"></a>Note that certain usages in a function definition can make it unsuitable |
| for inline substitution. Among these usages are: use of varargs, use of |
| alloca, use of variable sized data types (see <a href="Variable-Length.html#Variable-Length">Variable Length</a>), |
| use of computed goto (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>), use of nonlocal goto, |
| and nested functions (see <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>). Using <samp><span class="option">-Winline</span></samp> |
| will warn when a function marked <code>inline</code> could not be substituted, |
| and will give the reason for the failure. |
| |
| <p><a name="index-automatic-_0040code_007binline_007d-for-C_002b_002b-member-fns-2520"></a><a name="index-g_t_0040code_007binline_007d-automatic-for-C_002b_002b-member-fns-2521"></a><a name="index-member-fns_002c-automatically-_0040code_007binline_007d-2522"></a><a name="index-C_002b_002b-member-fns_002c-automatically-_0040code_007binline_007d-2523"></a><a name="index-fno_002ddefault_002dinline-2524"></a>As required by ISO C++, GCC considers member functions defined within |
| the body of a class to be marked inline even if they are |
| not explicitly declared with the <code>inline</code> keyword. You can |
| 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>. |
| |
| <p>GCC does not inline any functions when not optimizing unless you specify |
| the ‘<samp><span class="samp">always_inline</span></samp>’ attribute for the function, like this: |
| |
| <pre class="smallexample"> /* <span class="roman">Prototype.</span> */ |
| inline void foo (const char) __attribute__((always_inline)); |
| </pre> |
| <p>The remainder of this section is specific to GNU C90 inlining. |
| |
| <p><a name="index-non_002dstatic-inline-function-2525"></a>When an inline function is not <code>static</code>, then the compiler must assume |
| that there may be calls from other source files; since a global symbol can |
| be defined only once in any program, the function must not be defined in |
| the other source files, so the calls therein cannot be integrated. |
| Therefore, a non-<code>static</code> inline function is always compiled on its |
| own in the usual fashion. |
| |
| <p>If you specify both <code>inline</code> and <code>extern</code> in the function |
| definition, then the definition is used only for inlining. In no case |
| is the function compiled on its own, not even if you refer to its |
| address explicitly. Such an address becomes an external reference, as |
| if you had only declared the function, and had not defined it. |
| |
| <p>This combination of <code>inline</code> and <code>extern</code> has almost the |
| effect of a macro. The way to use it is to put a function definition in |
| a header file with these keywords, and put another copy of the |
| definition (lacking <code>inline</code> and <code>extern</code>) in a library file. |
| The definition in the header file will cause most calls to the function |
| to be inlined. If any uses of the function remain, they will refer to |
| the single copy in the library. |
| |
| </body></html> |
| |
