Thread: linked tree

I am going through a tutorial on linked lists and I am having trouble in telling what some piece of code achieves.
This are the structures used for constructing the list

Code:

 struct jsw_node {
       void*data;
       struct jsw_node *next;
};
struct jsw_list {
        struct jsw_node *head;
        int has_dummy_head; 
        size_t size;
};

The comes this function that serves to create a new list with an optional dummy head

Code:

struct jsw_list *new_list ( int has_dummy_head ){
      struct jsw_list *rv = malloc ( sizeof*rv );
      if ( rv != NULL )
            {   rv->head = has_dummy_head ? new_node ( NULL, NULL ) : NULL; 
                rv->has_dummy_head = has_dummy_head; 
                   rv->size =0; 
              if ( has_dummy_head && rv->head == NULL ) {
                 /* Release the list if a dummy couldn't be allocated */  
                  free ( rv );   
                    rv = NULL;   
                    } 
               }
            return rv;
}

There is a new_node function that create a new node given the data and the links

Code:

struct jsw_node * new_node(void * data,struct jsw_node * next/*this is the node to link to*/){
  struct jsw_node * rv=malloc(sizeof *rv);
  if(rv!=NULL){
    rv->data=data;
    rv->next=next;
  }
  return rv;
}

what does this part of the function new_list achieve?

Code:

 rv->head = has_dummy_head ? new_node ( NULL, NULL ) : NULL;