local verses global pointer variables in this particular code

Question

//binary tree
#include<stdio.h>
#include<stdlib.h>
struct node {
    int data;
    struct node* lc;
    struct node* rc;
    
}*ptr,*root;
void preorder(struct node* root){
    
    if(root!=NULL){
        printf("%d ",root->data);
        preorder(root->lc);
        preorder(root->rc);
    }
}

struct node* create(){
    int x;
    
    ptr = (struct node*)malloc(sizeof( struct node ));
    
    printf("type data value(enter -1 if not needed):\n");
    scanf("%d",&x);
    if(x!=-1){
    ptr->data = x;
    printf("Left child of %d:\n",x);
    ptr->lc = create();
    printf("Right child of %d:\n",x);
    ptr->rc = create();
    return ptr;
    }
    else{
        
        return NULL;
    }

}

void main(){
    root = NULL;
    root = create();
    preorder(root);
}

This code above runs into an endless loop when executed. But when i declared the structure pointer variables locally instead of declaring it globally. It worked. like this...

struct node* create(){
    int x;
    
   struct node* ptr = (struct node*)malloc(sizeof( struct node ));
.......
void main(){
    struct node* root = NULL;

Can someone explain to me why does the above code run into endless loop and how did declaring the variables locally fixed it?

i was trying to implement a binary tree data structure using recursive function create()... and the create function worked fine(i think) when i executed the code but the preorder() ran into an endless loop.

Answer 1

create() is a recursive functions. And when you call ptr->lc = create(); and ptr is a global variable, its value is overwritten by the recursive call and the assignment to ptr->lc is really made to that node.

Because of these traps you should really avoid using global variables in recursive functions (best: always avoid using global variables)