Why does the doubly linked list in sys/queue.h maintain the address of previous next element?

Question

I\'m studying sys/queue.h from FreeBSD and I have one question:

In sys/queue.h, LIST_ENTRY is defined as follows:

#define LIST_ENTRY(typ         


        

Answer 1

Let me try to explain. Actually the **le_prev* affords ablity to list defined by sys/queue.h to insert_before that forward-list can not. Compared with insert_before, the insert_after can both be implemented well in forward-list or list. So list is more functional.

insert_before(entry* list_elem, entry* elem, type val)
{
    elem->next = list_elem;
    *(list->prev) = elem;
    elem->prev = *(list->prev);
    list_elem->prev = elem->next;
}
insert_after(entry* list_elem, entry* elem, type val)
{
    if( ((elem)->next= (list_elem)->next) != NULL ) {
        (elem_list)->next->prev = &(elem)->next;
    }
    (list_elem)->next =  elem;
    elem->prev =  &(list_elem)->next;
}