Is it good practice to define a function on a code file, but not declare it on the header?

Question

What I want to do is have an auxiliary function inside my .c file that will only be used within another function in the file, and has no use outside that specific case. Therefore, it's not of great use for the interface to contain it, but I'm just wondering whether that is possible and/or a good practice, as well as if there's an alternative that is more elegant.

Answer 1

By not declaring the function in the header file, you are simply ommiting information about the function, not the visibility of the function.

Declare it static (and omit from the header file).

// barfoo.h
int bar(void);

---

//barfoo.c
#include "barfoo.h"
static int foo(void) { return 42; }
int bar(void) { return foo(); }

---

//main.c
#include "barfoo.h"
int main(void) {
    bar();
}

In this example, if foo() wasn't defined with static it could still be called from main(). With static the linker will report an error if you try to use it.