Debugging C Linked List Program caught in infinite loop

Question

So this is a very simple program to create and display a linked list. Here I'm getting caught in the display loop, and I'm seeing infinite '2->2->2->...' on screen.

After debugging, I can see that my program ALWAYS goes into the if statement of insertNode() whereas it should only go there ONCE i.e. when the linked list is initialized.

    #include <stdio.h>
#include <stdlib.h>

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

struct node * head = NULL;
struct node * curr = NULL;

void insertNode(struct node * temp2) {
    if (head == NULL) {
        head = temp2;
        head->next = NULL;
    }
    else {
        temp2->next = head;
        head = temp2;
    }
}

void display() {
    curr = head;
    while (curr->next != NULL)
    {   
        printf("%d->",curr->data);
        curr = curr->next;
    }
}

void main() {
    struct node * temp = (struct node *) malloc (sizeof(struct node));
    temp->data = 5;
    insertNode(temp);
    temp->data = 6;
    insertNode(temp);
    temp->data = 1;
    insertNode(temp);
    temp->data = 2;
    insertNode(temp);
    display();

}

it is better if you move curr inside of display instead of polluting the global area if you only use it in there. try to avoid globals as much as possible. — AndersK
– AndersK, Commented Aug 22, 2013 at 13:10
also check in display so that head really points to something before dereferencing it. — AndersK
– AndersK, Commented Aug 22, 2013 at 13:11

Rohan · Accepted Answer · 2013-08-22 12:48:13Z

2

You should allocate and create new node for each data when inserting it in the linked list.

void main() {
    struct node * temp = (struct node *) malloc (sizeof(struct node));
    temp->data = 5;
    insertNode(temp);

    temp = malloc (sizeof(struct node));
    temp->data = 6;
    insertNode(temp);

    temp = malloc (sizeof(struct node));
    temp->data = 1;
    insertNode(temp);

    temp = malloc (sizeof(struct node));
    temp->data = 2;
    insertNode(temp);
    display();

}

As you are using same node to add as multiple nodes, its making the circular list.

Your insertNode() looks ok.

answered Aug 22, 2013 at 12:48

Rohan

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1 Comment

AntonioCS Over a year ago

Is the cast in the first malloc really necessary?

Peter · Accepted Answer · 2013-08-22 12:50:37Z

1

You are setting next of temp2 to head, and then head to temp2. So in fact next node of temp2 is temp2, that's why you have infinite loop.

answered Aug 22, 2013 at 12:50

Peter

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Comments

sheu · Accepted Answer · 2013-08-22 12:52:11Z

1

In you main you have one pointer to the temp and you continue inserting the same node and it ends up pointing to itself. You now have a circular list that's why you have an infinite loop.

answered Aug 22, 2013 at 12:52

sheu

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Comments

AlexJ136 · Accepted Answer · 2013-08-22 12:50:09Z

0

You are getting an infinite loop because you only have one node. Doing temp->data = 5; and then temp->data = 6; does not create a new node. So when you add the same node into the list again, the next pointer in the node points to itself. So your while loop in display never terminates, because the next node is always the current node.

answered Aug 22, 2013 at 12:50

AlexJ136

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