问题
Is it possible to link with an instantiated non-static inline function by declaring it extern in another file?:
inline.c:
inline int foo(void) { return 42; }
extern inline int foo(void);
main.c:
extern int foo(void);
int main(){ return foo(); }
Empirically $CC main.c inline.c (where CC is gcc, clang or tcc) works. Is it a conforming C example?
回答1:
The first question here is about the behaviour of extern inline int foo(void); in the same translation unit as the apparent inline definition.
The text from C17 6.7.4/7 is:
If all of the file scope declarations for a function in a translation unit include the
inlinefunction specifier withoutextern, then the definition in that translation unit is an inline definition.
The "If all..." clause does not hold for inline.c, since the second line declares the identifier with extern. So the first line is actually an external definition, not an inline definition.
Then, using extern int foo(void); in the other translation unit is fine. foo is a function with external linkage which has an external definition. There is no rule precluding this.
It would also be fine if inline.c only contained the first line, which would be an inline definition, and extern inline int foo(void); appeared in a third translation unit. foo is still a function with external linkage and having an external definition.
Any inline definition is only relevant in the same translation unit as the inline definition.
来源：https://stackoverflow.com/questions/53042003/using-extern-to-refer-to-the-instantiation-of-a-non-static-inline-function