gistfile1.c

#include<stdio.h>
#include<iostream>
#include<malloc.h>
struct stack      
{
	int info;
	struct stack *next;
};

//typedef struct stack *s;
//dont use type def!, typedef makes the function parameter local to that function.
//since the scope of that function is local, the value of sptr outside the function was NULL

void empty(struct stack * sptr)
{
	sptr->info = NULL;
	sptr->next = NULL;
}

struct stack * newNode()
{
	return (struct stack *)malloc(sizeof(struct stack));
}

void push(struct stack * sptr ,int p)
{
	struct stack * s2;
	s2=newNode();
	if(s2==NULL)
	{
		printf("no memory");
//		exit(1);	//dont use exit
		return; //use return insted
	}

	else
	{
		s2->info=p;
		s2->next=sptr;
		sptr=s2;
	}
}

int pop(struct stack * sptr) //pop should return the popped value
{
	struct stack * s2=sptr;
	int px;
	if(sptr==NULL)
	{
//		exit(1);	//dont use exit
		return NULL; //use return insted
	}
	else
	{
		px=sptr->info;
        sptr=sptr->next;
        free(sptr);
		return px; //you were not returning
	}
}

void disp(struct stack * sptr)
{
	printf("\n address: %p\n",sptr);
    struct stack * temp=sptr;	
	printf("the elements in stack are:\n");
    while(sptr->next!=NULL)
	{
	    printf("%d\n",sptr->info);
	    sptr=sptr->next;
	}
	sptr=temp;
	
}

void main()
{
	int c,i;
    struct stack s1;
   empty(&s1);
   printf("\n address: %p\n",&s1);
   do{
	   printf("enter operation (0-exit, 1-push 2-pop, 3-display):-");
	   scanf("%d",&c);
	   switch(c)
	   {
	   case 1:  printf("Pushing: enter data:-");
	             scanf("%d",&i);
				 printf("\n address: %p\n",&s1);
	             push(&s1,i);
				 printf("\n address: %p\n",&s1);
				 break;
			   
	   case 2:printf("Popping: %d", pop(&s1));
		   printf("\n address: %p\n",&s1);
		   break;
       case 3:disp(&s1);
		   printf("\n address: %p\n",&s1);
		   break;
	   }
   }while(c!=NULL); //you had put s1 == NULL and s1 was always NULL, so it was executing only once!
}