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Question

Sorry for the silly title.

For part of an (very basic) assignment we're implementing a stack with pointers. I'm having a lot of trouble with one small part, so I've isolated it into this small problem.

I will try to explain my problem, but reading the code will probably be easier to understand.

There is a structure (named node) which has 2 members, a char (named data), and a pointer to another node (named next).

Inside the main function I have a pointer named head which points to node1, I want to pass this pointer to another function, and make it point to a new node (and make this new node point to yet another new node). I think I might be okay with setting the pointer to a new node, but I can't correctly get that new node to point to another new node correctly.

#include <stdio.h>

struct node {
    char data;
    struct node *next;
};

void modifyPtr(struct node **p);

int main(void)
{
    /* create first 2 nodes */
    struct node n1;
    n1.data = '1';

    struct node n2;
    n2.data = '2';

    /* set 1st node's next node to the 2nd node */
    n1.next = &n2;

    /* create a pointer to a node and make it point to the first node */
    struct node *head = &n1;

    /* this works as expected */
    printf("1. %c\n", head->data);
    printf("2. %c\n", head->next->data);

    /* this should set head to a new node (which in turn points to another new node) */
    modifyPtr(&head);

    /* this prints as expected. Am I just lucky here? */
    printf("3. %c\n", head->data);
    /* but this doesn't. I want it to print 4. */
    printf("4. %c\n", head->next->data);
}

void modifyPtr(struct node **p)
{
    /* create node 3 and 4 */
    struct node n3;
    n3.data = '3';

    struct node n4;
    n4.data = '4';

    /* set node3's next node to node4 */
    n3.next = &n4;

    /* make p point to node 3 */
    *p = &n3;
}

I expect to see the output as

1. 1
2. 2
3. 3
4. 4

but instead I get

1. 1
2. 2
3. 3
4. |

I've been trying to get this to work for ages. I'm thinking that maybe it's to do with creating the nodes in the local scope of modifyPtr and trying to use them in main. But then I don't see why #3 would work.

Can someone tell me what I'm doing wrong? Thanks.

Answer 1

void modifyPtr(struct node **p)
{
    struct node n3;
    n3.data = '3';
    ...
    *p = &n3;
}

n3 and n4 are local variables*, so they cease to exists once modifyPtr returns. You need to allocate them on the heap.

void modifyPtr(struct node **p)
{
    struct node *pn3 = malloc(sizeof(struct node));
    pn3->data = '3';
    ...
    *p = pn3;
}

You just got lucky that n3.data didn't get clobbered.

* — Laymen speak.

Answer 2

You're bang-on about the scope. The way to explain #3 is that just because it works doesn't mean it always will, and doesn't mean it's right. Time to learn dynamic memory allocation: new/delete or malloc/free