What's the difference between a static inline, extern inline and a normal inline function?
I've seen some vague explanations about this. As far as I've understood, static inline is not just an inline function that is meant to only be referred to within a certain file as the static keyword usually means. The same goes for extern inline too I guess, it's not the same explanation as with extern variables. Any answers would be greatly appreciated!
A function definition with static inline defines an inline function with internal linkage. Such function works "as expected" from the "usual" properties of these qualifiers: static gives it internal linkage and inline makes it inline. So, this function is "local" to a translation unit and inline in it.
A function definition with just inline defines an inline function with external linkage. However, such definition is referred to as inline definition and it does not work as external definition for that function. That means that even though this function has external linkage, it will be seen as undefined from other translation units, unless you provide a separate external definition for it somewhere.
A function definition with extern inline defines an inline function with external linkage and at the same time this definition serves as external definition for this function. It is possible to call such function from other translation units.
The last two paragraphs mean that you have a choice of providing a single extern inline definition for an inline function with external linkage, or providing two separate definitions for it: one inline and other extern. In the latter case, when you call the function the compiler is allowed to chose either of the two definitions.
来源：https://stackoverflow.com/questions/25000497/whats-the-difference-between-static-inline-extern-inline-and-a-normal-inline-f