Segmentation fault (core dumped) in C++ while trying to implement a BST

Question

I am attempting to create a BST using linked lists. I am trying to traverse my tree going left or right when appropriate until I find a null then I try to create a node and give a value at that position.

Now I get the error

Segmentation fault (core dumped)

from this code(the logic may not be correct as this is a work in progress)

#include <iostream>
using namespace std;

struct Node
{
    int data;
    Node *left, *right;
    Node(int data)
    {
        this->data = data;
        left = right = NULL;
    }
};
void traverseIn(Node *node, int val);

int main()
{
    int numOfNodes;
    cout << "number of nodes ";
    cin >> numOfNodes;

    for(int i = 0;i<numOfNodes;i++){
        struct Node *root;
        int data;
        cout << "data ";
        cin >> data;
        root->data = data;
        traverseIn(root,data);
    }
}

void traverseIn(Node *node , int val){
    if (node == NULL){
        node->data = val;
        return;
    }

     //go leftdat
     if (val <= node->data) {
        cout << "\nleft " << val<<" "<< node->data;
        traverseIn(node->left,val);
     }
     else if(val > node->data){
         //go right
        cout << "\nright " << val<<" "<< node->data;
        traverseIn(node->right,val);
     }

     cout << node->data << " ";

}

sample output

number of nodes 5

data 12

left 12 12

Segmentation fault (core dumped)

What I want to know is

1) how can I debug this error as I often come across it. I am using VS Code in Ubuntu using the default compiler it came with and the C/C++ extensions from microsoft but when I attach a break point, I get just the call stack. How can I set it up so that I can step through like I would a java program.

2) How can I make C++ print out meaningful error messages rather than just seg fault message. For example I would like to know what line in the program is at fault(although it is obvious in this example).

3) How can I fix this error.

I have read the xkcd meme and the What is a segmentation fault?

`struct Node *root; ... root->data = data;` Where do you think `root` points to when you assign to `root->data`? (same for `node` in `if (node == NULL){node->data = val; ...}`) I suggest you to read a good C++ book or at least some tutorial on pointers. — HolyBlackCat, Mar 20 '19 at 12:21
Could you please help me understand what is wrong @HolyBlackCat rather than the typical "read a book" response. — uhexos, Mar 20 '19 at 12:27
A good book will explain it better than me. In short, a pointer has to point to an actual object for reading and writing to `*pointer` and `pointer->something` to be allowed. — HolyBlackCat, Mar 20 '19 at 12:35

stensal · Accepted Answer · 2020-05-24T08:31:26.107

root is not defined. You code causes a dereference of an undefined pointer. You can try your code here a live test

=========== Start of #0 stensal runtime message ===========

Runtime error: [dereferencing an undefined pointer]
Continuing execution can cause undefined behavior, abort!

-
- Writing 4 bytes to an undefined address (0x0).
- 
- Stack trace (most recent call first) of the write.
- [1]  file:/prog.cc::27, 9
- [2]  [libc-start-main]
-

============ End of #0 stensal runtime message ============

A simple fix (just for fixing the segfault, but not the logic of your code)

root = new Node(data);

Segmentation fault (core dumped) in C++ while trying to implement a BST

1 Answers1