6.34. 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:

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. Section 6.39 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 (Section 6.14 Arrays of Variable Length), use of computed goto (Section 6.3 Labels as Values), use of nonlocal goto, and nested functions (Section 6.4 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; Section 4.5 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 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 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:

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