5.33 An Inline Function is As Fast As a Macro
By declaring a function
inline, you can direct GCC 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. Inlining of functions is an optimization and it really “works” only in optimizing
compilation. If you don't use -O, no function is really inline.
Inline functions are included in the ISO C99 standard, but there are currently
substantial differences between what GCC implements and what the ISO C99
standard requires.
To declare a function inline, use the inline keyword in its declaration, like this:
קוד PHP:
inline int
inc (int *a)
{
(*a)++;
}
(If you are writing a header file to be included in ISO C programs, write
__inline__
instead of inline. See
Alternate Keywords.) You can also make all “simple enough”
functions inline with the option
-finline-functions.
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
Variable Length), use of computed goto (see
Labels as Values),
use of nonlocal goto, and nested functions (see
Nested Functions). Using
-Winline
will warn when a function marked
inline could not be substituted, and will give the
reason for the failure.
Note that in C and Objective-C, unlike C++, the
inline keyword does not affect the
linkage of the function.
GCC automatically inlines member functions defined within the class body of C++
programs even if they are not explicitly declared
inline. (You can override this with
-fno-default-inline; see
Options Controlling C++ Dialect.)
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.
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
inlineand
externin 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 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.
Since GCC eventually will implement ISO C99 semantics for inline functions, it is best
to use
static inline only to guarantee compatibility. (The existing semantics will
remain available when
-std=gnu89 is specified, but eventually the default will be
-std=gnu99 and that will implement the C99 semantics, though it does not do so
yet.)
GCC does not inline any functions when not optimizing unless you specify the
`always_inline' attribute for the function, like this:
קוד PHP:
/* Prototype. */
inline void foo (const char) __attribute__((always_inline));