Stack by pointers in C working except at stack underflow - c

Question

I implemented stack by using pointers. It is compiling and working but it doesn't underflow when the stack is empty. It gives me some garbage value. I think the problem is something in the create_stack function. I am not getting segfaults no matter how much data is popped from the stack which is odd.
Can anyone help?
Here is my complete implementation of stack by pointers.
#include<assert.h>
#include<stdio.h>
#include<stdlib.h>
enum action {PUSH = 1, POP, TOP, QUIT};
typedef struct node
{
int data;
struct node *lower;
}stack_node;
void clear_screen(void)
{
system("cls");
}
static enum action get_user_action(void)
{
int choice = 0;
do
{
clear_screen();
printf("%d Push data\n"
"%d Pop Data\n"
"%d See the top of the stack\n"
"%d Exit\n\n"
"Enter your choice -> ", PUSH, POP, TOP, QUIT);
scanf("%d", &choice);
} while (choice != PUSH && choice != POP && choice != TOP && choice != QUIT);
return (enum action) choice;
}
void create_stack(stack_node **top, int *status)
{
*top = malloc(sizeof(stack_node));
*status = PUSH - 1;
if (*top == NULL){
*status = PUSH;
}
}
void push(stack_node **top_stack, int *status, int data)
{
*status = PUSH - 1;
stack_node *node = malloc(sizeof(node));
if (node == NULL)
{
*status = PUSH;
return;
}
node -> data = data;
if (*top_stack == NULL){
node -> lower = NULL;
}
else{
node -> lower = *top_stack;
}
*top_stack = node;
}
int pop(stack_node **top_stack, int *status)
{
*status = PUSH - 1;
if (*top_stack == NULL){
*status = POP;
return -1;
}
stack_node *node = *top_stack;
int data = node -> data;
*top_stack = node -> lower;
free(node);
return data;
}
int see_top(stack_node **top_stack, int *status)
{
*status = PUSH - 1;
if (*top_stack == NULL){
*status = POP;
return -1;
}
return (*top_stack) -> data;
}
int main(void)
{
enum action choice;
int status;
stack_node *top = NULL;
create_stack(&top, &status);
if (status == PUSH)
{
printf("Not enough memory\n");
return 1;
}
while ((choice = get_user_action()) != QUIT)
{
clear_screen();
int data;
switch (choice)
{
case PUSH:
printf("Enter data to be pushed -> ");
scanf("%d", &data);
push(&top, &status, data);
if (status == PUSH){
printf("Not enough memory\n");
}
break;
case POP:
data = pop(&top, &status);
if (status == POP){
printf("Stack underflow\n");
}
else{
printf("The data is %d\n", data);
}
break;
case TOP:
data = see_top(&top, &status);
switch (status)
{
case POP:
printf("Nothing in the stack\n");
break;
default:
printf("The data at top is %d\n", data);
}
break;
default:
assert(!"You should not have reached this.");
}
getchar();
getchar();
}
}

Answer 1

When you create the stack you allocate space for a node -- and don't fill it with anything. So after the call to create_stack() you have a blank node on your stack already. I guess you don't want that, doing just
void create_stack(stack_node **top, int *status)
{
*top = NULL;
*status = PUSH -1;
}
will work just fine. You allocate the memory during the push() call anyway any you check for top_stack == NULL during the functions. Alternatively you could have a flag in your stack node to indicate it's not used (then during push you'd not create a new one) but that's too complex for what you want here.

Answer 2

In create_stack() function you allocate memory and doesn't initialize it to any thing. Its data and lower part remain garbage.
When you pop elements if (*top_stack == NULL) condition never becomes true (becomes garbage value was not null) and so after remove all node it returns garbage value.