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### 6.39 An Inline Function is As Fast As a Macro

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 `-finline-functions`.

GCC implements three different semantics of declaring a function inline. One is available with `-std=gnu89` or `-fgnu89-inline` or when `gnu_inline` attribute is present on all inline declarations, another when `-std=c99`, `-std=c11`, `-std=gnu99` or `-std=gnu11` (without `-fgnu89-inline`), and the third is used when compiling C++.

To declare a function inline, use the `inline` keyword in its declaration, like this:

         static inline int
         inc (int *a)
         {
           return (*a)++;
         }

If you are writing a header file to be included in ISO C90 programs, write `__inline__` instead of `inline`. See Alternate Keywords.

The three types of inlining behave similarly in two important cases: when the `inline` keyword is used on a `static` function, like the example above, and when a function is first declared without using the `inline` keyword and then is defined with `inline`, like this:

         extern int inc (int *a);
         inline int
         inc (int *a)
         {
           return (*a)++;
         }

In both of these common cases, the program behaves the same as if you had not used the `inline` keyword, except for its speed.

When a function is both inline and `static`, 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 `-fkeep-inline-functions`. 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.

Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: variadic functions, use of `alloca`, use of variable-length data types (see Variable Length), use of computed goto (see Labels as Values), use of nonlocal goto, and nested functions (see Nested Functions). Using `-Winline` warns when a function marked `inline` could not be substituted, and gives the reason for the failure.

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 `inline` keyword. You can override this with `-fno-default-inline`; see Options Controlling C++ Dialect.

GCC does not inline any functions when not optimizing unless you specify the ‘`always_inline`’ attribute for the function, like this:

         /* Prototype.  */
         inline void foo (const char) __attribute__((always_inline));

The remainder of this section is specific to GNU C90 inlining.

When an inline function is not `static`, 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-`static` inline function is always compiled on its own in the usual fashion.

If you specify both `inline` and `extern` 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.

This combination of `inline` and `extern` 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 `inline` and `extern`) in a library file. The definition in the header file causes most calls to the function to be inlined. If any uses of the function remain, they refer to the single copy in the library.