Pointers and structures and pointers again... when creating a stack - c

Question

So, I wanted to create a stack as follows:
#include <stdio.h>
#include <stdlib.h>
typedef struct Node{
int data;
struct node *link;
}node;
typedef struct Stack{
struct Node *topnode;
int count;
}stack;
void push(int data, stack *ourstack){
node newnode;
newnode.data = data;
ourstack.topnode = &newnode;
}
int main()
{
stack mystack;
push(1,mystack);
printf("%d",(mystack.(*topnode).data));
}
but I do get errors. I am a bit confused here. Inside the push() function, in the last line, I tried various ways of implementing it right but I failed each time. Now, my thinking is, ourstack is a pointer pointing to a struct Stack. And the topnode is also a pointer inside a structure of a stack which points to another node structure. So, should not (*ourstack).(*topnode) = newnode or ourstack.topnode = &newnode work? Why?

Answer 1

Neither work because newnode is on the stack and once the code exits push, it will no longer exist. You need to allocate it dynamically.
void push(int data, stack *ourstack){
node *newnode;
newnode = malloc(sizeof(*newnode));
newnode->next = ourstack->topnode; // Point the next of the new node to the top node of the stack
newnode->data = data;
ourstack->topnode = newnode;
}
And also you need to initialise mystack properly in main or else you risk undefined behaviour as topnode could be NULL or it could have a random value.
stack mystack;
mystack.topnode = NULL;
mystack.count = 0;

Answer 2

variable in a method, only exists in stack, will be unavailable after exit method. since you wish keep it after call of push, you need alloc a node with function like malloc, which will save data in heap. and you have to free it after you do not need it any more.

Answer 3

There are number of bugs in your program.
ourstack is pointer variable of structure type, while accessing data using ourstack use -> operator instead of dot operator
Replace
ourstack.topnode = &newnode;
with
ourstack->topnode = &newnode;
there is a conflict in push function definition and function call, you are passing just mystack and catching with struct pointer variable. pass the address of mystack, your function call should be
push(1,&mystack);
newnode is the local variable declared in push function and whatever you are trying with newnode it will reflects in this function only, not outside of this. So Allocate memory dynamically for newnode and do the operation.
void push(int data, stack *ourstack){
node *newnode;
newnode = malloc(sizeof(stack));
newnode->data = data;
ourstack->topnode = newnode;
}
Once compiled successfully debug the code using gdb and analyze.