Problem using two stacks with following code

Im getting crashes if I initialize more than one stack using this code.

Code:

struct stackNode{
	
	int data;
	struct stackNode *nextptr;
};

int pop(struct stackNode **topptr)
	{
		
		struct stackNode *tmp;
		
		if( *topptr != NULL)
		{
			tmp = *topptr;
			*topptr = (struct stackNode *) tmp->nextptr;
			int temp2;
			temp2 = tmp->data;
			free(tmp);
			return temp2;
		}
		else{
			printf("Pop function error.");
			exit(0);
		
		}
		
	
	}
	
	void push(struct stackNode **topptr, int info)
	{
		
		struct stackNode *temp;
		
		temp = (struct stackNode *)malloc(sizeof(struct stackNode));
		
			temp->data = info;
			temp->nextptr = *topptr;
			*topptr = temp;
			
	}

I am initializing in Main using

Code:

struct stackNode **xstack;
struct stackNode **ystack;

if I only initialize one I don't crash until I try to use the second initialized stack.

Example:

Code:

struct stackNode **xstack;
push(xstack, 5);
push(xstack, 5);


printf("%d %d", pop(xstack), pop(xstack));

works fine.

If I try

Code:

struct stackNode **xstack;
struct stackNode **ystack

push(xstack, 5);
push(ystack, 5);

printf("%d %d", pop(xstack), pop(ystack));

I will crash.

Code:

struct stackNode **xstack;
push(xstack, 5);
push(xstack, 5);


printf("%d %d", pop(xstack), pop(xstack));

u have just defined a stacknode pointer,that pointer needs to point somewhere, so use something like this:

Code:

struct stackNode xstack;
struct stackNode* xstack_p;
xstack_p = &xstack;


push(&xstack_p, 5);
push(&xstack_p, 5);


printf("%d %d", pop(&xstack_p), pop(&xstack_p));
}

use this.




for two stacks

Code:

struct stackNode xstack;
struct stackNode* xstack_p;
xstack_p = &xstack;

struct stackNode ystack;
struct stackNode* ystack_p;
ystack_p = &ystack;

thanks a bunch