static function declarations in header files

Question

I have a static function in a source file that is used by other functions in that same source file. Is it fine to put the declaration for that static function in the header file, even though that header file will be included into other source files? Example:

/* foo.c */
#include "foo.h"
 
/* exported function; calls g */ 
void f(void) {
    g(); 
}

static 
void g(void) {
    /* do something... */
}

/* foo.h */ 
void f(void); 
void g(void);


/* main.c */ 
#include "foo.h"

int main(void) {
   f();
}

Answer 1

Is it fine to put the declaration for that static function in the header file, even though that header file will be included into other source files?

No. Consider the conflict the other source files would have if they had a function/object/macro of the same name.

Even without conflict, an "unused function" warning may occur. @Adrian Mole

By putting static void g(void) in the .h file, it adds an unnecessary potential name conflict.

---

Simply declare/define that static function at the top of the .c in which it is used.

Answer 2

Is it fine to put the declaration for that static function in the header file, even though that header file will be included into other source files?

No. becuse it does not make any sense. and why? In C, functions are global by default. Unlike global functions in C, access to static functions is restricted to the file where they are declared! Therefore, when we want to restrict access to functions, we make them static.

Another reason for making functions static can be reuse of the same function name in other files. so declaring them static in the header file can cause to name collisons in other source files.

In header files we declare our API functions those we want to expose only. in c files we declare usually auxilary functions as static functions for restricting the scope of those aux functions to the c file only