c - How to declare an inline function in C99 multi-file project?

Question

I want to define an inline function in a project, compiled with c99. How can I do it? When I declare the function in a header file and give the detail in a .c file, the definition isn't recognized by other files. When I put the explicit function in a header file, I have a problem because all .o files who use it have a copy of the definition, so the linker gives me a "multiple definition" error.

What I am trying to do is something like:

header.h
inline void func()
{
    do things...
}


lib1.c
#include "header.h"
...

lib2.c
#include "header.h"

with a utility which uses both lib1.o and lib2.o

Answer 1

Unfortunately not all compilers are completely complying to C99 in that point even if they claim that they'd be.

An conforming way to do this is

// header file. an inline definition alone is
// not supposed to generate an external symbol
inline void toto(void) {
  // do something
}

// in one .c file, force the creation of an
// external symbol
extern inline void toto(void);

Newer versions of gcc, e.g, will work fine with that.

You may get away with it for other compilers (pretenders) by defining something like

#ifdef PRETENDER
# define inlDec static
# define inlIns static
#else
# define inlDec 
# define inlIns extern
#endif
// header file. an inline declaration alone is
// not supposed to generate an external symbol
inlDec inline void toto(void) {
  // do something
}

// in one .c file, force the creation of an
// external symbol
inlIns inline void toto(void);

Edit:

compilers with C99 support (usually option -std=c99) that I know of

• gcc (versions >= 4.3 IIRC) implements the correct inline model
• pcc is also correct
• ggc < 4.3 needs a special option to implement the correct model, otherwise they use their own model that results in multiple defined symbols if you are not careful
• icc just emits symbols in every unit if you don't take special care. But these symbols are "weak" symbols, so they don't generate a conflict. They just blow up your code.
• opencc, AFAIR, follows the old gcc specific model
• clang doesn't emit symbols for inline functions at all, unless you have an extern declaration and you use the function pointer in one compilation unit.
• tcc just ignores the inline keyword