Code Management Again

How can I write the header file, when I include header file, I can use the function of inside lib.c?
Can this lib.c compile to *.lib wihout any code added?
How to avoid double call of header file?

For example, the library program lib.c

Code:

---

unsigned int factorial(unsigned int n)
{
      if(n==0||n==1)
        return 1;
      else
        return factorial(n-1);
}

---

And the header file lib.h

Code:

---

unsigned int factorial(unsigned int);

---

The Main program

Code:

---

#include <stdio.h>
#include "lib.h"

int main(void)
{
  printf("%d",factorial(3));
  return 0;
}

---