C inline functions

Question

As far as i know, inline functions in C work pretty the same way as in C++ when used in single translation unit and there's no need to dive into extern inline difficulties in such a case. However, the following program including three files seems not to compile in C and I am struggling to figure out why.

f.h

int f();
inline int g();

f.c

#include "f.h"

inline int g() {
    return 5;
}

int f() {
    return 3 + g();
}

main.c

#include "f.h"
#include <stdio.h>

int main() {
    printf("%d", f());
    return 0;
}

The linker

tells that there is an undefined reference to g. However, as g is used only in f.c file, I can not define where the problem lies exactly.

You need `extern inline int g();` in one translation unit, in ISO C — M.M, Dec 25 '19 at 22:17
It would help to show your build command, e.g. as GNU C behaves differently to ISO C — M.M, Dec 25 '19 at 22:18

score 2 · Accepted Answer · answered Dec 25 '19 at 22:19

From the C Standard (6.7.4 Function speciﬁers)

7,,,For a function with external linkage, the following restrictions apply: If a function is declared with an inline function speciﬁer, then it shall also be deﬁned in the same translation unit

In your project the function g is declared in the translation unit with main.

C inline functions

f.h

f.c

main.c

The linker

1 Answers1