#include <stdio.h>

typedef struct _node {
   char element;
   struct _node *next;
} node;

typedef node *stack;

stack init ()
{
   stack S;

   /* Create the dummy node */
   S = (node *)malloc(sizeof(node));
   S -> element = '\0';
   S -> next = NULL;
   return S;
}

int isEmpty ( stack S )
{
   return (S -> next == NULL);
}

int isFull ( stack S )
{
   return 0;
}

char top ( stack S )
{
   if (isEmpty(S)) {
      fprintf(stderr, "top: Empty stack\n");
      return '\0';
   }
   return S -> next -> element;
}

stack push ( stack S , char ch )
{
   node *T;

   if (isFull(S)) {
      fprintf(stderr, "push: Full stack\n");
      return S;
   }

   /* Copy the new element in the dummy node */
   S -> element = ch;

   /* Create a new dummy node */
   T = (node *)malloc(sizeof(node));
   T -> element = '\0';
   T -> next = S;
   return T;
}

stack pop ( stack S )
{
   if (isEmpty(S)) {
      fprintf(stderr, "pop: Empty stack\n");
      return S;
   }

   /* Treat the stack top as the new dummy node */
   S -> next -> element = '\0';
   return S -> next;
}

void print ( stack S )
{
   node *T;

   T = S -> next;
   while (T != NULL) {
      printf("%c", T -> element);
      T = T -> next;
   }
}

int main ()
{
   stack S;

   S = init(); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = push(S,'d'); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = push(S,'f'); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = push(S,'a'); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = pop(S); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = push(S,'x'); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = pop(S); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = pop(S); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = pop(S); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
   S = pop(S); printf("Current stack : "); print(S); printf(" with top = %c.\n", top(S));
}