Question

#include <iostream>
#include <string>
using namespace std;
struct Node{
    Node *next;
    int data;

 };

 int main(){
    Node* head = NULL;
    int data;
    cin >> data;
    Node*m = head;
    while(data >0){
    cout <<"enter a data";
    cin >> data;
    m -> data = data;
    m -> next = m;
    }

    while(m -> next != NULL){
            cout << m -> data << endl;

    }

  return 0;
}

Here is simple code that takes values when they are greater than 0 and make a linked list. after you enter a negative value, the while loop is terminated and prints the values.

However, the code gives me segmentation fault when it asks enter a data and after it takes the data. I could not solve it, what is the reason?

Answer 1

You get uninitialized pointer here. If you define link to next node as Node *next; you should initialise pointer with address to valid Node object before use.

And

Node* head = NULL;
...
Node*m = head;
...
m -> data = data; // NPE

as you can see, you trying to call member data of NULL object.

How to fix it:
Anways initialize pointer with valid address. For example:

int main(){
    Node head;
    head.next = NULL;
    int data;
    cout << "enter a data ";
    cin >> data;
    Node *prev = &head;
    prev->data = data;
    while(data >0){
      Node *next = new Node();
      cout <<"enter a data ";
      cin >> data;
      next->data = data;
      next->next = NULL;
      prev->next = next;
      prev = next;
    }
    Node* m = &head;
    while(m -> next ){
        cout << m -> data << endl;
        m = m->next;
    }

  return 0;
}