Data saved in Stack content keep changing, can't increment - c

Question

So here is my issue, I have been trying to figure this out for the last 5 hours, I have a header file, a tester file, and a c source file. I would really like to understand what is happening and why so I can avoid the issue in the future. The header file declares the struct but does not define it:
typedef struct Stack *StackP;
and in my source file, Stack.c I have defined the stack:
struct Stack
{
int top;
int capacity;
int count;
ItemT items;
};
where ItemT is defined as char *
in the tester file, the call goes:
StackP stackPtr = newStack();
and what I have for my newStack function located in the c source file is:
StackP newStack(void) {
struct Stack stack1;
StackP stackPtr = &stack1;
(stackPtr->items) = (ItemT)malloc(DEFAULT_CAPACITY*sizeof(ItemT));
(stackPtr->top) = -1;
(stackPtr->capacity) = DEFAULT_CAPACITY;
(stackPtr->count) = 0;
fprintf(stderr, "\nSuccesfully allocated memory to items...\n");
return stackPtr;
}
now, my push function is:
void pushStack(StackP stackPtr, ItemT item) {
if ((stackPtr->count) == (stackPtr->capacity)) {
fprintf(stderr, "\nERROR: Full stack.\n");
}
else {
stackPtr->items = item;
fprintf(stderr, "\nSuccessfully pushed %s on to the stack...\n", stackPtr->items);
(stackPtr->items)++;
(stackPtr->top)++;
(stackPtr->count)++;
}
}
My question is this: Have I don't something wrong in any of these blocks of code.
If I call a function that says:
return (stackPtr->count);
it will return a random set of numbers instead of 0, or 1. For instance, if I push 2 strings to the stack, instead of count being 2, count is 479622 or some other random long number. Why is this happening?
Again, I would like to know what I'm doing wrong and not just correct syntax because I really HAVE to understand this.

Answer 1

The program has undefined behaviour as it is returning the address of a local variable from a function:
StackP newStack(void) {
struct Stack stack1;
StackP stackPtr = &stack1;
return stackPtr;
}
stack1 no longer exists when newStack exits. stackPtr must point to dynamically allocated memory if it is to exist beyond the scope of the function:
StackP newStack(void) {
struct Stack stack1;
StackP stackPtr = malloc(sizeof(*stackPtr));
if (stackPtr)
{
}
return stackPtr;
}
See Do I cast the result of malloc?