Pointer Doubt and help with code

#include<stdio.h>

typedef struct node{
 int data;
 struct node *link;
 }node;
node *headA = NULL;
node *headB = NULL;


void push(node *,int);
int pop(node *);
void display(node *);
void enQ();
void dQ();

main(){
       int choice;
             system("cls");
             printf("\n\tWelcome to Queue implementation using two stacks.\n");         
             do{
            /*system("clear"); */  
             /* Menu */      
             printf("\n\tWhat would you like with your queue?\n\n");
             printf("1. Insert an element at the front (EnQ)\n2. Delete the last element (DQ)\n3. Exit program\n\nEnter your desired choice with the corresponding serial number :\n\n");
             scanf("%d",&choice);
             system("cls");
             switch(choice){
                            case 1: enQ();
                                    break;
                            case 2: dQ();
                                    break;
                            case 3: printf("\n\tSorry to see you go, hope to see you back soon!\n");
                                    getch();
                                    exit(0);
                            default: printf("\n\tSorry, That option is unavailable. Try again!\n\n"); 
                                     getch(); 
             }
             }while (choice >= 0);
}


void enQ(){
     int add;
     int x,y;
     printf("\n\tEnter the item\n");
     scanf("%d",&add);
     if(headB == NULL){ 
          push(headA, add);
     }
     else{
          while(headB != NULL){
                x = pop(headB);
                push(headA, x);
          }
          push(headA, add);
     }
     while(headA != NULL){
                  y = pop(headA);
                  push(headB, y);
     }
     display(headB); 
}

void dQ(){
     int del;
     int x,y;
     if(headB == NULL){
               printf("Queue is empty.");
     }
     else{
          while(headB != NULL){
                      x = pop(headB);
                      push(headA, x);
          }
          del = pop(headA);
          printf("\n\tThe element deleted is : %d ", del);
          while(headA != NULL){
                       y = pop(headA);
                       push(headB, y);
          }
          display(headB);
     }
}


void push(node *head, int add){
     node *last_node;
     last_node = (node*)malloc(sizeof(node));
     last_node->data = add;
     last_node->link = head;
     head = last_node;
 }

int pop(node *head){
     node *last_node;
     int flag = 0;
     if(head==NULL)
          return flag;
     else{
          last_node = head;
          flag = last_node->data;
          head = head->link;
          free(last_node);
          return flag;
     }
}

void display(node *head){
   node *my_stack;
   if(head == NULL)
         printf("\n\tStack is empty\n");
   else{
        my_stack = head;
        printf("\n\tThe elements of the queue are : \n\n\t");
        while(my_stack != NULL){
             printf("%d\t", my_stack->data);   
             my_stack = my_stack->link;
        }
   }
   getch();
}