Multiple definition of inline functions when linking static libs

First you have to understand the C99 inline model – perhaps there is something wrong with your headers. There are two kind of definitions for inline functions with external (non-static) linkage

• External definition
  This definition of a function can only appear once in the whole program, in a designated TU. It provides an exported function that can be used from other TUs.

• Inline definition
  These appear in every TU where declared as a separate definition. The definitions do not need to be identical to one another or to the external definition. If used internal in a library, they can do omit checking on function arguments that would otherwise be done in the external definition.

Each definition of the function has its own local static variables, because their local declarations have no linkage (they are not shared like in C++). A definition of a non-static inline function will be an inline definition if

• Every function declaration in a TU includes the specifier inline, and
• No function declaration in a TU includes the specifier extern.

Otherwise, the definition that must appear in that TU (because inline functions must be defined in the same TU where declared) is an external definition. In a call to an inline function it’s unspecified whether the external or the inline definition is used. However, because the function defined in all cases is still the same (because it has external linkage), the address of it compares equal in all cases, no matter how many inline definitions appear. So if you take the function’s address, it’s likely the compiler resolves to the external definition (especially if optimizations are disabled).

An example that demonstrates a wrong use of inline, because it includes an external definition of a function twice in two TUs, causing a multiple definition error

// included into two TUs
void f(void); // no inline specifier
inline void f(void) { }

The following program is dangerous, because the compiler is free to use the external definition, but the program does not provide one

// main.c, only TU of the program
inline void g(void) {
  printf("inline definition\n");
}

int main(void) {
  g(); // could use external definition!
}

I have made some test cases using GCC that demonstrate the mechanism further:

main.c

#include <stdio.h>

inline void f(void);

// inline definition of 'f'
inline void f(void) {
  printf("inline def main.c\n");
}

// defined in TU of second inline definition
void g(void);

// defined in TU of external definition
void h(void);

int main(void) {
  // unspecified whether external definition is used!
  f();
  g();
  h();

  // will probably use external definition. But since we won't compare
  // the address taken, the compiler can still use the inline definition.
  // To prevent it, i tried and succeeded using "volatile". 
  void (*volatile fp)() = &f;
  fp();
  return 0;
}

main1.c

#include <stdio.h>

inline void f(void);

// inline definition of 'f'
inline void f(void) {
  printf("inline def main1.c\n");
}

void g(void) {
  f();
}

main2.c

#include <stdio.h>

// external definition!
extern inline void f(void);

inline void f(void) {
  printf("external def\n");
}


void h(void) {
  f(); // calls external def
}

Now, the program outputs what we expected!

$ gcc -std=c99 -O2 main.c main1.c main2.c
inline def main.c
inline def main1.c
external def
external def

Looking at the symbol table, we will see that the symbol of an inline definition is not exported (from main1.o), while an external definition is exported (from main2.o).

---

Now, if your static libraries each have an external definition of their inline functions (as they should), they will naturally conflict with each other. The solution is to make the inline functions static or just to rename them. These will always provide external definitions (so they are full fledged definitions), but they are not exported because they have internal linkage, thus not conflicting

static inline void f(void) {
  printf("i'm unique in every TU\n");
}