At -O2 and -O3 levels of optimization, or when --autoline is specified, the compiler can automatically inline functions if it is practical and possible to do so, even if the functions are not declared as __inline or inline. This works best for static functions, because if all use of a static function can be inlined, no out-of-line copy is required. Unless a function is explicitly declared as static (or __inline), the compiler has to retain the out-of-line version of it in the object file in case it is called from some other module.

It is best to mark all non-inline functions as static if they are not used outside the translation unit where they are defined (a translation unit being the preprocessed output of a source file together with all of the headers and source files included as a result of the #include directive). Typically, you do not want to place definitions of non-inline functions in header files.

If you fail to declare functions that are never called from outside a module as static, code can be adversely affected. In particular, you might have:

Because of these problems, declare non-inline functions as static when you are sure that they can never be called from another module.