What is the difference between struct node *head and struct node ** head?

Question

I am trying to sort a linked list. I'm confused about when to use struct node*head and when to use struct node **head, the implementation can be done using both of them.

When should I use:

void sortedinsert(struct node **head)

and when should I use:

void sortedinsert(struct node *head)

Answer 1

With this function signature:

void changeNode(struct node *head)

You have a pointer to the node, and so you can change that structure. You can't change what the variable head points to. Let's assume the following definition of struct node:

struct node
{
    int field1;
    struct node *next;
}

With the given function signature and struct node, consider the following operations can change the structure in the function:

void changeNode(struct node *head)
{
    head->field1 = 7;
    head->next = malloc(sizeof(struct node));
}

C is pass-by-value: when we pass a variable to a function, the function gets a copy. This is why we pass a pointer to a struct node, so that we can change it, and have the effects of those changes outside the function. But we still get only a copy of the pointer itself. So the following operation isn't useful:

void changeNode(struct node *head)
{
    // we're only changing the copy here
    head = malloc(sizeof(struct node));
}

The changes to head won't be reflected outside the function. In order to change what head points to, we must use an additional level of indirection:

void changeNode(struct node **head)
{
    // now we're changing head
    *head = malloc(sizeof(struct node));

    // alternately, we could also do this:
    *head = NULL;
}

Now the changes to head are reflected outside the function.

Answer 2

struct node **head you are passing the address of the pointer of head there by making it possible to make it refer/point to a different memory area. With struct node *head you can't modify head to point anywhere else