Can't Solve Infinite Loop - c

Question

The program is supposed to take a user entered integer and convert it into binary through a stack of singly linked lists. I think it's either my toBin() function or my printStack() function that's causing the infinite loop.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
typedef struct node_def node;
struct node_def
{
int val;
node *next;
};
node *head;
void push(int val);
void pop(node *head);
int top();
void printStack();
int toBin(int val);
int main()
{
int num = 0;
printf("Enter an integer: ");
scanf("%d", &num);
push(num);
toBin(num);
printStack();
return 0;
}
void push(int val)
{
node *new;
new = malloc(sizeof(node));
if (head == NULL)
{
head = malloc(sizeof(node));
head->next = NULL;
head->val = val;
}
else
{
new->next = head;
new->val = val;
head = new;
}
return;
}
void pop(node *head)
{
node *tmp;
if(head == NULL)
{
printf("Stack is Empty\n");
return;
}
else
{
tmp = head;
head = head->next;
free(tmp);
}
return;
}
int top()
{
return(head->val);
}
void printStack()
{
node *tmp;
tmp = head;
if(head == NULL)
{
return;
}
while(head != NULL)
{
printf("%d ", head->val);
head = head->next;
}
printf("\n");
return;
}
int toBin(int val)
{
pop(head);
int i = 1, remainder, binary;
while(val != 0)
{
remainder = val % 2;
binary = binary + remainder * i;
val = val / 2;
i = i * 10;
push(binary);
}
return val;
}

Answer 1

You run into an infinite loop due to not properly initialising your variables. In particular, you have no guarantees that your node* head will be initialised to NULL, or that your int variables in toBin() will be initialised to zero.
Always, always, always initialise your variables when programming in C/C++.
Fixing these bugs and removing unused code leaves us with:
#include <stdio.h>
#include <stdlib.h>
typedef struct node_def node;
struct node_def
{
int val;
node *next;
};
/* Note that we are initialising the global variable to NULL. */
node *head = NULL;
void push(int val);
void printStack();
int toBin(int val);
int main()
{
int num = 0;
printf("Enter an integer: ");
scanf("%d", &num);
/* Removed push(num), as you're using parameters in the following call: */
toBin(num);
printStack();
return 0;
}
/* Changed printStack to use a tmp pointer to
traverse the stack without mutating it */
void printStack()
{
node* tmp = head;
while(tmp != NULL)
{
printf("%d ", tmp->val);
tmp = tmp->next;
}
printf("\n");
return;
}
int toBin(int val)
{
/* Removed pop() as you're getting val from parameters */
/* Also initialising remainder and binary variables */
int i = 1, remainder = 0, binary = 0;
while(val != 0)
{
remainder = val % 2;
binary = binary + remainder * i;
val = val / 2;
i = i * 10;
push(binary);
}
return val;
}
/* It's a stack so no if's are necessary for pushing */
void push(int val)
{
node *new = malloc(sizeof(node));
new->val = val;
new->next = head;
head = new;
return;
}