How to make a function access to only one file?

Question

What I want

I have two files: file.h and file.c. I want to define a function in file.h that is going to have its definition in file.c but I don't want the function to be included when I include file.h in other file.

Example:

file.h

#ifndef HEADER_INCLUDED
#define HEADER_INCLUDED

// This function should be declared here and defined in "file.c"
// but only used there (not in any other source file).
int private_func();

// This function can be used every where (if included of course)
// but it needs to be able to access the "private_func()".
int public_func();

#endif // HEADER_INCLUDED

file.c

#include "file.h"

int private_func()
{
  int a = 2;
  int b = 3;

  return a + b;
}

int public_func()
{
  // this functons uses the "private_func()" result to give its result
  int c = private_func();
  int d = private_func();

  return c + d; 
}

#endif // HEADER_INCLUDED

other.c
This file should not import private_func() when including its header

#include "file.h"

int main()
{
  // can call the "public_func()"
  int result1 = public_func();
  
  // but cannot call "private_func()"
  int result2 = private_func();

  return 0;
}

In short

I don't want private_func() to be imported by a file other than "file.c". (if possible)

Answer 1

You could use the pre-processor. In header:

#ifndef HEADER_INCLUDED
#define HEADER_INCLUDED

#ifdef _USE_PRIVATE_
// This function should be declared here and defined in "file.c"
// but only used there (not in any other source file).
int private_func();
#endif

// This function can be used every where (if included of course)
// but it needs to be able to access the "private_func()".
int public_func();

#endif // HEADER_INCLUDED

In the implementation file:

#define _USE_PRIVATE_
#include "file.h"

int private_func()
{
  int a = 2;
  int b = 3;

  return a + b;
}

int public_func()
{
  // this functons uses the "private_func()" result to give its result
  int c = private_func();
  int d = private_func();

  return c + d; 
}

#endif // HEADER_INCLUDED

other.c would not change. Including the header without the special define would omit the private definition.