Pass a pointer to struct and a int to a function in C to implement a stack

Question

I'm trying to implement a stack in C. I have only implemented the struct that will contain an array and that currently only contains the size of the array and the position of the last item added to the stack

This is a partial implementation that is giving me some trouble.

in stack.h

#include <stdlib.h>
#include <stdbool.h>

typedef struct Stack
{
    int max_size;
    int top;
    // int *contents;
} Stack;

Stack *stack_create(int n);
bool stack_is_empty(Stack *stack);
bool stack_is_full(Stack *stack);
void stack_push(Stack *stack, int value);

in stack.c:

#include <stdio.h>
#ifndef STACK_H
#include "stack.h"
#endif

Stack *stack_create(int n)
{
    Stack stack;
    Stack *s = &stack;
    s->max_size = n;
    s->top = 0;
    // s->contents = (int *)malloc(sizeof(int) * n);
    return s;
}


bool stack_is_empty(Stack *stack)
{
    if (stack->top == 0)
    {
        return true;
    }
    return false;
}

bool stack_is_full(Stack *stack)
{
    if (stack->top == stack->max_size)
    {
         return true;
    }
    return false;
} 

void stack_push(Stack *stack, int value)
{

     if (!stack_is_full(stack))
     {
          printf("max_size: %d\n", stack->max_size);
          printf("top: %d (%p)\n", stack->top++, &stack->top);
          printf("value: %d (%p)\n", value, &value);
     }
     else
     {
          printf("Can't push. max_size==%d reached.\n", stack- >max_size);
          exit(EXIT_FAILURE);
     }
}

and in main.c:

 #include <stdio.h>
 #include <stdlib.h>
 #include "stack.h"

 #define SIZE 3

 int main()
 {
     Stack *s = stack_create(SIZE);
     printf("stack_is_empty: %d\n", stack_is_empty(s));
     stack_push(s, 100);
     printf("stack_is_empty: %d\n", stack_is_empty(s));
     stack_push(s, 30);
     printf("stack_is_empty: %d\n", stack_is_empty(s));
     stack_push(s, 20);
     printf("stack_is_empty: %d\n", stack_is_empty(s));

     return 0;
 }

main produces the following output:

stack_is_empty: 1
max_size: 3
top: 100 (0x7ffd5430dfb4)
value: 101 (0x7ffd5430dfb4)
stack_is_empty: 0
max_size: 3
top: 30 (0x7ffd5430dfb4)
value: 31 (0x7ffd5430dfb4)
stack_is_empty: 0
max_size: 3
top: 20 (0x7ffd5430dfb4)
value: 21 (0x7ffd5430dfb4)
stack_is_empty: 0

Why is value 's address the same of stack->top ?

Answer 1

Problem 1 : You are allocating memory for the stack locally in stack_create function. As soon as the function goes out of scope memory will be freed. Thus you will have a dangling pointer.

Problem 2 : You are allocating memory only for one instance regardless of value of 'n'

typedef struct Stack
{
    int max_size;
    int *contents;
    int top;
    // int *contents;
} Stack;

Stack *stack_create(int n) {
    Stack *s;
    s = (Stack *)malloc(sizeof(Stack));
    s->contents = (int *)malloc(sizeof(int) * n);
    s->max_size = n;
    s->top = 0;
    return s;
}