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.