Copying one Linked LIst into another through Recursion in C

#include <stdio.h>
#include <conio.h>
#include <malloc.h>
#include <windows.h>
/* structure containing a data part and link part */
struct node
{
 int data ;
 struct node *link ;
} ;
void append ( struct node **, int ) ;
void copy ( struct node *, struct node ** ) ;
void display ( struct node * ) ;
int main( )
{
 struct node *first, *second ;
 first = second = NULL ;  /* empty linked lists */
 append ( &first, 1 ) ;
 append ( &first, 2 ) ;
 append ( &first, 3 ) ;
 append ( &first, 4 ) ;
 append ( &first, 5 ) ;
 append ( &first, 6 ) ;
 append ( &first, 7 ) ;
 system ( "cls" ) ;
 display ( first ) ;
 copy ( first, &second ) ;
 display ( second ) ;
 return 0 ;
}
/* adds a node at the end of the linked list */
void append ( struct node **q, int num )
{
 struct node *temp ;
 temp = *q ;
 if ( *q == NULL )  /* if the list is empty, create first node */
 {
  *q = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
  temp = *q ;
 }
 else
 {
  /* go to last node */
  while ( temp -> link != NULL )
   temp = temp -> link ;
  /* add node at the end */
  temp -> link = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
  temp = temp -> link ;
 }
 /* assign data to the last node */
 temp -> data = num ;
 temp -> link = NULL ;
}
/* copies a linked list into another */
void copy ( struct node *q, struct node **s )
{
 if ( q != NULL )
 {
  *s = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
  ( *s ) -> data = q -> data ;
  ( *s ) -> link = NULL ;
  copy ( q -> link, &( ( *s ) -> link ) ) ;
 }
}
/* displays the contents of the linked list */
void display ( struct node *q )
{
 /* traverse the entire linked list */
 while ( q != NULL )
 {
  printf ( "%d ", q -> data ) ;
  q = q -> link ;
 }
 printf ( "\n" ) ;
}