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=gnu99
or an option for a later C version is used
(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
.
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 cannot 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 computed goto (see Labels as Values),
use of nonlocal goto, use of nested functions, use of setjmp
, use
of __builtin_longjmp
and use of __builtin_return
or
__builtin_apply_args
. 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.