Question

Just started with creating a Linked List in C. Getting runtime error in the code for returnLastNode. How do I know? Runs fine when it’s commented out.

Question#1 What am I doing wrong?

Code for my returnLastNode function

struct Node* returnLastNode(struct LinkedList *ll) {
    struct Node *n = ll->first;
    struct Node *result;
    while(1) {
        if(n->next == NULL) {
            result = n;
            break;
        } else {
            n = n->next;
        }
    }
    return result;
}

Definition of structs used.

struct Node {
    int val;
    struct Node *next;
};

struct LinkedList {
    struct Node *first;
};

LinkedList.h here, if needed/interested.

https://github.com/MoonStruckHorrors/LinkedListC/blob/master/LinkedList.h

Question #2 How a newbie should debug runtime errors?

Also, any other suggestions are appreciated. 🙂

Answer 1

You never check if n is NULL before dereferencing it. This means your code will crash when used on an empty list. Also you can get rid of the variable result and just return n; in the if (n->next == NULL) part. So a better version of your code might look like this:

struct Node* returnLastNode(struct LinkedList *ll) {
    struct Node *n = ll->first;

    // checking if n is NULL here
    while(n) {
        if(n->next == NULL)
             return n;

        n = n->next;
    }

    return NULL; // n was NULL immediately so there is no end node
}

As for debugging runtime errors, you can use printf to check what data is for simple things, and for more complex things you can use a debugger like gdb (or sometimes IDEs (such as Visual Studio) come with integrated debuggers).