Double pointers

[Anator]

Hi,

I have had a nagging doubt about the double pointers & I would appreciate some clarification. The following code is an e.g. of double pointers. It works on linked list & creates a copy of a linked list (q).

copy_linked_lists(struct node *q, struct node **s)
{
    if(q!=NULL)
    {
        *s=malloc(sizeof(struct node));
        (*s)->data=q->data;
        (*s)->link=NULL;
        copy_linked_list(q->link, &((*s)->link));
    }
}

I would like to know why this code has a double pointer passed to it. Suppose I "twist" the code to make it single pointered what could potentially go wrong with it.

copy_linked_lists(struct node *q, struct node *s)
{
    if(q!=NULL)
    {
        s=malloc(sizeof(struct node));
        s->data=q->data;
        s->link=NULL;
        copy_linked_list(q->link, s->link);
    }
}

My understanding is double pointers are used when you don't want the address you are working on being destroyed in stack (as in function exit). Am I right? If so why I would really appreciate if someone can explain to me why the double pointer is used in the above code.

[quzah]

To make a variable outside a function, keep a value assigned in a function, you need a pointer to it. Therefore, if you want a pointer outside a function, to keep a value you assign it in a function, you need its address (a pointer to it). Otherwise, you can't keep the value (unless you're assigning it via return).


Quzah.