C Programming Listnode insert node resulting in infinite loop

Question

I was having some problem when trying to do a double deferencing when inserting node into ListNode. Here is the code:

#include "stdafx.h"
#include <stdlib.h>

typedef struct _listnode {
    int num;
    struct _listnode *next;
}ListNode;

void insertNode(ListNode **ptrHead, int index, int value);
ListNode *findNode(ListNode *head, int index);

int main()
{
    int index, value, i;
    ListNode **ptrHead, *head = NULL;

    ptrHead = &head;

    for (i = 0; i < 5; i++){
        printf("Enter value: ");
        scanf("%d", &value);
        printf("Enter index: ");
        scanf("%d", &index);

        insertNode(ptrHead, index, value);
    } 

    ptrHead = head;
    while (ptrHead != NULL) {
        printf("%d", head->num);
        ptrHead = head->next;
    }

    return 0;
}

void insertNode(ListNode **ptrHead, int index, int value) {
    ListNode *cur, *newNode;
    if (*ptrHead == NULL || index == 0) {
        newNode = malloc(sizeof(ListNode));
        newNode->num = value;
        newNode->next = *ptrHead;
        *ptrHead = newNode;
    }
    else if ((cur = findNode(*ptrHead, index - 1)) != NULL) {
        newNode = malloc(sizeof(ListNode));
        newNode->num = value;
        newNode->next = cur->next;
        cur->next = newNode;
    }
    else printf("Cannot insert the new item at index %d!\n", index);
}

ListNode *findNode(ListNode *head, int index) {
    ListNode *cur = head;
    if (head == NULL || index < 0)
        return NULL;
    while (index > 0) {
        cur = cur->next;
        if (cur == NULL) return NULL;
        index--;
    }
    return cur;
}

So basically I am taking 5 value and index inputs from the user. Then from there, I am inserting them into the ListNode. Inside insertNode(), there is a function called findNode which trying to find the cur so that I can point my cur to the next newNode.

However, with these code, when I try to print out the ListNode, it printed out the first value input infinitely. So I was thinking which part was my mistakes?

Thanks in advance.

Answer 1

In your main function, the following lines of code:

ptrHead = head;
while (ptrHead != NULL) {
    printf("%d", head->num);
    ptrHead = head->next;
}

shall be:

ListNode *cur = head;
while (cur != NULL) {
    printf("%d", cur->num);
    cur = cur->next;
}

EDITED:

But can I know why it does not work when I assign head to ptrHead. Is it because I am double deferencing the ptrHead?

They are of different types. ptrHead is ListNode** while head is ListNode*. Thus the assignment ptrHead = head; shall not do what you really want. Also a modern compiler shall have raised some warning on this line.