LINKED STACK IN PUSH, POP, PEEP & UPDATE OPERATIONS

CODING:
#include<stdio.h>
#include<conio.h>
#include<malloc.h>
struct stack
{
int no;
struct stack *next;
};
struct stack *top=NULL;
struct stack *push(struct stack *,int);
struct stack *pop(struct stack *);
struct stack *update(struct stack *);
struct stack *display(struct stack *);
int peep(struct stack *);
void main()
{
int val,ch;
clrscr();
do
{
 printf("\n\nS E L E C T  O P E R A T I O N");
 printf("\n1.PUSH");
 printf("\n2.POP");
 printf("\n3.PEEP");
 printf("\n4.UPDATE");
 printf("\n5.DISPLAY");
 printf("\n6.EXIT");
 printf("\nEnter The option:");
 scanf("%d",&ch);
 switch(ch)
 {
 case 1:
 printf("\nEnter the number to be pushed on to the stack:");
 scanf("%d",&val);
 top=push(top,val);
 break;
 case 2:
 top=pop(top);
 break;
 case 3:
 val=peep(top);
 printf("\nThe value stored at the top of the stack:%d",val);
 break; 
 case 4:
 top=update(top);
 break;
 case 5:
 top=display(top);
 break;
 case 6:
 printf("PRESS ENTER..");
 break;
 default:
 printf("INVALID ENTER,TRY AGAIN.");
 }
}while(ch!=6);
getch();
}
struct stack *push(struct stack *top,int val)
{
struct stack *new_stack;
new_stack=(struct stack *)malloc(sizeof(struct stack *));
new_stack->no=val;
if(top==NULL)
 {
 top=new_stack;
 top->next=NULL;
 }
else
 {
 new_stack->next=top;
 top=new_stack;
 }
return top;
}
struct stack *pop(struct stack *top)
{
struct stack *new_stack;
new_stack=top;
if(top==NULL)
 printf("STACK UNDERFLOW");
else
{
 top=top->next;
 printf("\nThe value being deleted is:%d",new_stack->no);
 free(new_stack);
}
return top;
}
int peep(struct stack *top)
{
if(top->no==0)
{
printf("Stack is Empty.");
return 0;
}
else
return top->no;
}
struct stack *display(struct stack *top)
{
struct stack *new_stack;
new_stack=top;
if(top==NULL)
 printf("STACK EMPTY");
else
{
 printf("STACK");
 while(new_stack!=NULL)
 {
 printf("\n%d",new_stack->no);
 new_stack=new_stack->next;
 }
}
return top;
}
struct stack *update(struct stack *top)
{
int temp,flag=0;
struct stack *up;
up=top;
if(top==NULL)
{
printf("STACK IS EMPTY");
flag=1;
}
else
{
printf("Enter the number you want to update:");
scanf("%d",&temp);
do
{
 if(up->no==temp)
 {
 temp=up->no;
 printf("\nEnter the number:");
 scanf("%d",&up->no);
 printf("%d is update.",temp);
 flag=1;
 }
 else
 up=up->next;
}
while(up!=NULL);
}
if(flag==0)
printf("%d is not in Stack",temp);
return top;
}