What real use does a double pointer have? [closed]

Answer 1

One case where I've used it is a function manipulating a linked list, in C.

There is

struct node { struct node *next; ... };

for the list nodes, and

struct node *first;

to point to the first element. All the manipulation functions take a struct node **, because I can guarantee that this pointer is non-NULL even if the list is empty, and I don't need any special cases for insertion and deletion:

void link(struct node *new_node, struct node **list)
{
    new_node->next = *list;
    *list = new_node;
}

void unlink(struct node **prev_ptr)
{
    *prev_ptr = (*prev_ptr)->next;
}

To insert at the beginning of the list, just pass a pointer to the first pointer, and it will do the right thing even if the value of first is NULL.

struct node *new_node = (struct node *)malloc(sizeof *new_node);
link(new_node, &first);