Unknown NULL value accessed - c

Question

this a partial program of AVL tree, but right now its just more of a BST
#include <stdio.h>
#include <stdlib.h>
#include<conio.h>
typedef struct node node;
struct node
{
int data;
int height;
node *left;
node *right;
} *root = NULL, *temp = NULL;
int max(int n1, int n2)
{
return n1 > n2 ? n1 : n2;
}
node *new_node(int value)
{
temp = (node *)malloc(sizeof(node));
temp->data = value;
temp->left = NULL;
temp->right = NULL;
temp->height = 0;
return temp;
}
int get_height(node *x)
{
if (x == NULL)
{
return -1;
}
return x->height;
}
node *insert(node *r, int value)
{
if (r == NULL)
{
return new_node(value);
}
else
{
if (value < r->data)
r->left = insert(r->left, value);
else
r->right = insert(r->right, value);
int height = max(get_height(r->left), get_height(r->right)) + 1;
}
void inorder(node *r)
{
if (r == NULL)
return;
inorder(r->left);
printf("node = %d, height = %d\n", r->data, r->height);
inorder(r->right);
}
void main()
{
root = insert(root, 10);
root = insert(root, 20);
inorder(root);
}
The output of this program is following, as I'm not accessing any NULL values -
node = 10, height = 1
node = 20, height = 0
after just adding one line, I'm accessing a NULL value
#include <stdio.h>
#include <stdlib.h>
#include<conio.h>
typedef struct node node;
struct node
{
int data;
int height;
node *left;
node *right;
} *root = NULL, *temp = NULL;
int max(int n1, int n2)
{
return n1 > n2 ? n1 : n2;
}
node *new_node(int value)
{
temp = (node *)malloc(sizeof(node));
temp->data = value;
temp->left = NULL;
temp->right = NULL;
temp->height = 0;
return temp;
}
int get_height(node *x)
{
if (x == NULL)
{
return -1;
}
return x->height;
}
node *insert(node *r, int value)
{
if (r == NULL)
{
return new_node(value);
}
else
{
if (value < r->data)
r->left = insert(r->left, value);
else
r->right = insert(r->right, value);
int height = max(get_height(r->left), get_height(r->right)) + 1;
int balance_factor = get_height(r->left)- get_height(r->right); // extra line added
}
}
void inorder(node *r)
{
if (r == NULL)
return;
inorder(r->left);
printf("node = %d, height = %d\n", r->data, r->height);
inorder(r->right);
}
void main()
{
root = insert(root, 10);
root = insert(root, 20);
inorder(root);
}
Now my program is just hanging, its not an infinite call/ loop. It is surely a NULL value access. But how is this even possible?

Answer 1

Debugging by forum/Q&A is never efficient. Use a debugger:
For example at https://onlinegdb.com/S1ZansZNU
Reading symbols from a.out...done.
/usr/share/gdb/gdbinit: No such file or directory.
(gdb) run
Starting program: /home/a.out
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400703 in inorder (r=0xffffffff) at main.c:62
62 inorder(r->left);
(gdb)
r is not NULL, but neither is it a valid address.
The problem is clearly in:
root = insert(root, 10);
when insert does not explicitly return a value

Answer 2

Function insert is not returning any pointer in case of r != NULL collecting a return value when function is not returning anything is undefined behavior.
i.e
root = insert(root, 10);//it is returning valid pointer
root = insert(root, 20);//unknown value copied to root
please update your code as follows,
node *insert(node *r, int value)
{
if (r == NULL)
{
return new_node(value);
}
//rest of the logic
return r;//missing from both of your snippet
}
Refer for more details on UB