undefined reference to `llist_init(LinkedList*)'

Question

I got a short question. I'm getting this error

undefined reference to `llist_init(LinkedList*)'

my Stack.cpp includes

#include "LinkedList.h"

LinkedList.h has a init function

 void llist_init(LinkedList * list);

I can't understand why it's telling me "undefined" since I include LinkedList.h and LinkedList.h has a llist_init function. help!