Doubly linked list

Hello, I recently started learning C and I encountered some problems with writing a good doubly linked list. I have done this in Java before, but pointers and memory management causes me some pain.

So I'm trying to write a simple dictionary like thing using linked list. When I declare my dict_entry struct in one main.c file w/o header files and a separate file for functions' implementation everything works pretty, but when I'm trying to do virtually the same thing with a header file, function implementation file, etc. It breaks down and I don't understand why.

The code below should only work in case when the list is empty.
Header file:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

typedef struct
{
   char *key;
   char *value;
   void *next_entry;
   void *previous_entry;	
}dict_entry;

// ADD ENTRY TO THE DICTIONARY	
int add_entry( dict_entry*, char*, char* );

Imprementation file:

Code:

#include "dict_lib.h"

// Return: SUCCES at successful entry addition
int add_entry( dict_entry *dictionary, char *new_key, char *new_value )
{
   if( dictionary == NULL )
   {
      dict_entry* new_entry = malloc(sizeof(dict_entry));
      new_entry->key = new_key;
      new_entry->value = new_value;
      new_entry->next_entry = NULL;
      new_entry->previous_entry = NULL;
      dictionary = new_entry;
      new_entry = NULL;
   } 
   return 0;
}

Main file:

Code:

#include "dict_lib.h"

void main()
{
   dict_entry *dictionary = NULL;
   char *a = (char*)malloc(10);
   char *b = (char*)malloc(10);
   a = "some";
   b = "text";
   add_entry( dictionary, a, b );
}