What is the role of the Node ** list double pointer in the enqueue function for queues using linked list?

Question

I was asked to implement a queue using a linked list without a header node. My teacher gave us this snippet for referring to the enqueue function. I'm familiar with how to use queue operations. But I keep getting confused when double-pointers are used along with functions. I'm not sure which variable the double-pointer points to in most cases. I would like to know what Node ** list means.

Any help or explanation is appreciated

Here is the snippet that I referred

void insertNode(Node * prev,int x) {
    Node * new = (Node *) malloc(sizeof(Node));
    new->val = x;
    new->next = prev->next;
    prev->next = new;
}

void enqueue(Node ** list, int x) {
    Node * new = (Node *) malloc(sizeof(Node));
    if (isEmpty(*list)) {
        *list = new;
        (*list)->val = x;
        (*list)->next = NULL;
    }
    else {
        new = *list;
        while (new->next != NULL) 
            new = new->next;
        insertNode(new,x);
    }
}

Answer 1

Usually double pointers as parameters of functions is used to modify the pointer itself.

Example:

int a = 5;
int b = 10;

void foo(int *x)
{
    x = &b;
}

void bar(int **x)
{
    *x = &b;
}


int main(void)
{
    int *ptr = &a;

    foo(ptr);
    printf("%d\n", *ptr);

    bar(&ptr);
    printf("%d\n", *ptr);
}

Output:

5
10

Function bar modifies the pointer ptr. Function foo does not as x is a local variable having the function scope.