small functions defined in header files: inline or static?

Question

I have a number of small functions which are defined in a .h file. It is a small project (now) and I want to avoid the pain of having declarations and definitions separate, because they change all the time. To avoid multiply-defined symbols, I can either have them static or inline. What should be preferred and why?

I know it is in general bad practice to define functions in headers. You don't have to mention that in answers, this question is meant technically.

Answer 1

I'd use static inline, but static would work just as well.

extern and extern inline are out because you'd get multiple external definitions if the header is included in more than one translation unit, so you need to consider static, static inline and inline specification.

Heptic correctly states in his answer that most compilers consider functions for inlining regardless of whether inline is specified or not, ie the main impact of inline is its effect on linkage.

However, static definitions have internal linkage, so there's not much difference between static and static inline; I prefer static inline for function definitions in header files for purely stylistic reasons (rule of thumb: header files should only contain extern declarations, static const variable definitions and static inline function definitions).

inline without static or extern results in an inline definition, which the standard states (C99 6.7.4, §6)

provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition.

ie inline definitions should always be accompanied by external definitions, which is not what you're looking for.

Some more information about the subtleties of C99 inline semantics can be found in this answer, on the Clang homepage and the C99 Rationale (PDF).

Keep in mind that GCC will only use C99 semantics if -std=c99 or -std=gnu99 is present...

Answer 2

Since the question is about C (not C++), inline means that

1. You wish "that calls to the function be as fast as possible" (ISO9899-1999, 6.7.4(5)). The same paragraph also states that it is implementation-defined to which extent this suggestion is effective. In other words, it has little bearing and does not imply any inlining at all (in fact, non-inlining may quite possibly be faster due to instruction cache effects).
2. there are some restrictions and special cases in combination with extern (ISO9899-1999, 6.7.4(6), for example an inline funciton with external linkage must be defined in the same compilation unit, and an inline definition allows an extern definition elsewhere without an error (which is not necessarily a good thing, because the two functions need not be functionally equivalent, and it is unspecified which one the compiler uses at any time!).

The linker implications given by Heptic are required for C++, but not required by C (as far as I can tell). They are necessarily required by the "shall have the same address in all translation units" clause in ISO14882, 7.1.2(4). I am not aware of any similar clause in C99.
However, since the entirely different languages C and C++ usually go through the same C/C++ compiler and linker, it likely works identically for C, anyway.

So... how to answer your question? Use inline when you feel it's adequate. Be aware of the possible pitfalls of extern. Otherwise, leave it away and trust the compiler to do it right.