Infinite while-loop while printing nodes in linkedlist

Question

If I call createnode(x) in main and then follow that with printNodes(); I will get an infinite while-loop that seems to be printing some memory address. I am guessing the issue lies in the fact that i set head = temp?

SinglyLinkedList *head = NULL;

void createNode(int data){
    SinglyLinkedList *temp = malloc(sizeof(SinglyLinkedList));
    temp-> data = data;

    if(head == NULL){
        head = temp;
        return;
    }

    temp->next = head;
    head = temp;
}

void printNodes(){

    SinglyLinkedList *temp = head;

    while ( temp != NULL ){
        printf("%d\r\n",temp->data);
        temp = temp->next;
    }

}

You only give a value to `temp->next` when `head` is not NULL. To be correct, the code must always set `temp->next`. The easy way to do that is to remove the whole `if` statement. — user3386109, Jan 12 '22 at 06:16
@user3386109 I totally forgot that I didn't set `temp->next`, it fixed the problem instantly.. thanks! :) — Randy, Jan 12 '22 at 06:23
next time please make sure you add the *complete* code (including the definition of `SingleLinkedList` and a minimal `main`). — OrenIshShalom, Jan 12 '22 at 06:28
Does this answer your question? [Difference between malloc and calloc?](https://stackoverflow.com/questions/1538420/difference-between-malloc-and-calloc) — OrenIshShalom, Jan 12 '22 at 06:38

score 3 · Accepted Answer · answered Jan 12 '22 at 06:56

SinglyLinkedList *temp = malloc(sizeof(SinglyLinkedList));
temp->data = data;

if(head == NULL){
    head = temp;
    return;
}

temp->next = head;
head = temp;

should be

SinglyLinkedList *temp = malloc(sizeof(SinglyLinkedList));
temp->data = data;
temp->next = head;    // Moved

if(head == NULL){
    head = temp;
    return;
}

head = temp;

which is a very complicate way of writing

SinglyLinkedList *temp = malloc(sizeof(SinglyLinkedList));
temp->data = data;
temp->next = head;
head = temp;

Without this fix, printNodes results in undefined behaviour as a result of the lack of initialization of temp->next.

Infinite while-loop while printing nodes in linkedlist

1 Answers1