An Inline Function is As Fast As a Macro(《Using the GNU Compiler Collection (GCC)》)

By declaring a function inline, you can direct GCC tointegrate that function's code into the code for its callers. Thismakes execution faster by eliminating the function-call overhead; inaddition, if any of the actual argument values are constant, their knownvalues may permit simplifications at compile time so that not all of theinline function's code needs to be included. The effect on code size isless predictable; object code may be larger or smaller with functioninlining, depending on the particular case. Inlining of functions is anoptimization and it really “works” only in optimizing compilation. Ifyou don't use -O, no function is really inline.

Inline functions are included in the ISO C99 standard, but there arecurrently substantial differences between what GCC implements and whatthe ISO C99 standard requires.

To declare a function inline, use the inline keyword in itsdeclaration, like this:

     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 unsuitablefor inline substitution. Among these usages are: use of varargs, use ofalloca, 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 -Winlinewill 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 keyworddoes not affect the linkage of the function.

GCC automatically inlines member functions defined within the classbody of C++ programs even if they are not explicitly declaredinline. (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 thefunction are integrated into the caller, and the function's address isnever used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for thefunction, 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, andneither can recursive calls within the definition). If there is anonintegrated call, then the function is compiled to assembler code asusual. The function must also be compiled as usual if the programrefers to its address, because that can't be inlined.

When an inline function is not static, then the compiler must assumethat there may be calls from other source files; since a global symbol canbe defined only once in any program, the function must not be defined inthe other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on itsown in the usual fashion.

If you specify both inline and extern in the functiondefinition, then the definition is used only for inlining. In no caseis the function compiled on its own, not even if you refer to itsaddress explicitly. Such an address becomes an external reference, asif you had only declared the function, and had not defined it.

This combination of inline and extern has almost theeffect of a macro. The way to use it is to put a function definition ina header file with these keywords, and put another copy of thedefinition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the functionto be inlined. If any uses of the function remain, they will refer tothe single copy in the library.

Since GCC eventually will implement ISO C99 semantics forinline functions, it is best to use static inline onlyto guarantee compatibility. (Theexisting semantics will remain available when -std=gnu89 isspecified, but eventually the default will be -std=gnu99 andthat will implement the C99 semantics, though it does not do so yet.)

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

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