15.9 Inline Functions

By declaring a function inline, you can direct the compiler to integrate that function’s code into the code for its callers. This usually 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. Machine code may be larger or smaller with inline functions, depending on the particular case.

Note: Function inlining will only take place when the function’s definition is visible (not just the prototype). In order to have a function inlined into more than one source file, the function definition may be placed into a header file that is included by each of the source files.
To declare a function inline, use the inline keyword in its declaration, like this: 
inline int inc (int *a) {
  (*a)++;
}

If you are using the -ansi option, write __inline__ instead of inline. You can also make all “simple enough” functions inline with the command-line option -finline-functions. The compiler heuristically decides which functions are simple enough to be worth integrating in this way, based on an -estimate of the function’s size.

Note: The inline keyword will only be recognized with -finline or optimizations enabled.

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, use of computed goto and use of nonlocal goto. Using the command-line option -Winline will warn when a function marked inline could not be substituted and will give the reason for the failure.

In compiler syntax, the inline keyword does not affect the linkage of the function.

When a function is both inline and static, and all direct calls to the function are integrated into the callers' code, and the function’s address is never used, then the function’s own assembler code is never referenced. In this case, the compiler does not actually output assembler code for the function itself, unless you specify the command-line 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 non-integrated 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. The compiler will only eliminate inline functions if they are declared to be static and if the function definition precedes all uses of the function.

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 a similar effect to a macro. 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.