C++ : segmentation Fault

Question

My C++ code

#include <iostream>
#include <cstdlib>
using namespace std ;

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

Node* Insert(Node *head,int data);

int main(){
    struct Node* head = NULL;
    struct Node* second = NULL;
    struct Node* third = NULL;
    head = (struct Node*)malloc(sizeof(struct Node));
    second = (struct Node*)malloc(sizeof(struct Node));
    third = (struct Node*)malloc(sizeof(struct Node));

    head->data = 7;
    head->next = second;
    second->data = 17;
    second->next = third;
    third->data = 37;
    third->next = NULL;

    head = Insert(head,3);
}
Node* Insert(Node *head,int data)
{
  while (true)
    {   cout << "hey" ;
        if (head == NULL)
        {
            cout << "hey";
            head->data = data ;
            head->next = NULL ;
            break ;
        }
        head = head->next ;
    }

    return head;
}

I am tring to learning Linked List in C++. I am passing a head and data to insert in the node using Insert() function that i have defined.

My output to terminal

Segmentation fault (core dumped)

I think i am invocating Insert() function in a wrong way. Please Help ! Thanks !

Answer 1

The problem lies in this part of your code:

if (head == NULL)
{
    cout << "hey";
    head->data = data ;
    head->next = NULL ;
    break ;
}

First you check if head is NULL, and if head is NULL you try to dereference ( head->data ) the head pointer, which is undefined behaviour and leads to a segfault in your case.

I propose the following algorithm:

if(head == NULL) {
    Node* newHead = new Node();
    newHead->data = data;
    newHead->next = NULL;
    return newHead;
}
while(head->next != NULL) head = head->next;
head->next = new Node();
head->next->data = data;
head->next->next = NULL;
return head;

Answer 2

You are missing out that head is NULL when it enters you if statement.

This should fix it:

Node* Insert(Node *head,int data) {
  while (head != NULL) {
        if (head->next == NULL) {
            struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
            newNode->data = data ;
            newNode->next = NULL ;
            head->next = newNode;
            head = newNode;
            break ;
        }
        head = head->next ;
    }

    return head;
}

Answer 3

Something is wrong here:

if (head == NULL)
{
    cout << "hey";
    head->data = data ;
    head->next = NULL ;
    break ;
}

If head is NULL , so you can't assign data to its head->data . This causes SIGSEGV.