stack.h

#define ERROR_CHAR '?'

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

typedef struct stack
{
	node *top;
} stack;

stack *create_stack()
{
	stack *new_stack;

	new_stack = malloc(sizeof(stack));
	new_stack->top = NULL;

	return new_stack;
}

node *create_node(char element)
{
	node *new_node = malloc(sizeof(node));
	new_node->next = NULL;
	new_node->element = element;

	return new_node;
}

void push(stack *st, char element)
{
	node *new_node;

	if (st == NULL)
		return;

	new_node = create_node(element);

	new_node->next = st->top;
	new_node->element = element;

	st->top = new_node;
}

// Returns ERROR_CHAR if the stack is empty
char pop(stack *st)
{
	char element;
	node *new_top;

	if (st == NULL)
		return ERROR_CHAR;

	if (st->top == NULL)
		return ERROR_CHAR;

	new_top = st->top->next;
	element = st->top->element;
	free(st->top);
	st->top = new_top;

	return element;
}

// Returns 1 if the stack is empty, returns 0 otherwise.
int is_empty(stack *st)
{
	if (st == NULL || st->top == NULL)
		return 1;

	return 0;
}

char top(stack *st)
{
	if (st == NULL || st->top == NULL)
		return ERROR_CHAR;

	return st->top->element;
}

void free_stack(stack *st)
{
	node *cur_node, *last_node;

	if (st == NULL)
		return;

	cur_node = st->top;

	while (cur_node != NULL)
	{
		last_node = cur_node;
		cur_node = cur_node->next;
		free(last_node);
	}

	free(st);
}