Manipulation of stack with full usage of pointers and arrays - arrays

Question

I was given this code from Uni and I am trying to do operations with it , the push function ask for two pointers and I could not figure out how can I pass the data to the item part.I have tried using another structure containing the data but I failed. I am stuck at the end of the code and I would like to learn how to push data using this code. How can I proceed ?
It would be preferable if the data itself was stored in a structur.
Thanks in advance.
typedef struct stk
{
struct stk* elems[MAX]; int top;
} stack, *stackptr;
void Init(stack* s)
{
s->top = 0;
}
int IsEmpty(stack s)
{
return (s.top == 0);
}
void Push(struct stk* item, stack* s)
{
if (s->top == MAX)
printf("Stack voll!");
s->elems[s->top] = item;
s->top++;
}
struct stk* Pop(stack* s)
{
if (IsEmpty(*s)) return NULL;
s->top--;
return s->elems[s->top];
}
int main()
{
stack* ptr = (stackptr)malloc(sizeof(stack));
Init(ptr);
printf("%d\n", ptr->top); // Ist 0 , OK
}

Answer 1

Here is a working stack implementation that store ints. This will afford you the opportunity to test that the operations work as expected. If you really want to store stack * replace the type. It seems unnecessarily confusing for a entry level class to have an assignment of storing pointers to the same thing you are building.
When you deal with pointers you want to make sure the object they point to outline the pointer. You may also want to think of shallow and deep copies with pointers. If you Pop followed by a Push the pointer that was returned from Pop now will point to the new value which would be surprising. Consider a different designs:
Pass in a reference to a variable (aka out parameter) so Pop(stack *s, *v) (and use an enum or define constants for error values).
return a value instead of a pointer; error would not be an out parameter.
return a pointer to a copy of the value and require client to free it.
#include <stdio.h>
#include <stdlib.h>
#define MAX 10
typedef struct stack {
int elems[MAX];
int top;
} stack;
void Init(stack *s) {
if(!s)
return;
s->top = 0;
}
int IsEmpty(stack *s) {
return (s->top == 0);
}
void Push(stack *s, int elem) {
if (s->top == MAX) {
printf("Stack voll!");
return;
}
s->elems[s->top++] = elem;
}
int *Pop(stack *s) {
if (IsEmpty(s))
return NULL;
return &s->elems[--(s->top)];
}
int main() {
stack *s = malloc(sizeof *s);
Init(s);
printf("%d\n", s->top); // Ist 0 , OK
Push(s, 42);
int *v = Pop(s);
printf("%d\n", *v);
}
and example run:
0
42
Consider using a name prefix like "Stack" for all your symbols to avoid name conflicts.
In c we don't cast void * (from malloc()).