Error in initializing the instance of a structure to NULL - c

Question

I was writing a program to implement BST and I declareed two structures for that.
struct node
{
int data;
struct node *link[2];
};
struct tree
{
struct node *root;
};
typedef struct node node;
typedef struct tree tree;
And then i wrote a function to insert the elements in it.
void insert_iter(tree *tree, int data);
But in my main() function, I got confused that what thing shoud I initialise as NULL. So, I did this in my main() function.
tree *tree=NULL;
And it gave an error of segmentation fault.
But then I realised that the first condition in my insert_iter() function calls for tree->root, so this error was bound to happen.
But then I am confused about how to initialize this. How to go on for this?
EDIT: I have updated with the required code!!
this is the main() function
int main()
{
tree *tree=NULL; // all the confusion is regarding this
printf("hello"); // for debugging
//*tree->root=NULL; // tried this one but it was wrong
int value,choice;
while(1)
{
printf("enter the element : ");
scanf("%d",&value);
insert_iter(tree,value);
printf("do you want to enter more : 0 or 1 : ");
scanf("%d",&choice);
if(choice==0)
break;
}
return 0;
}
and this is the function that i used to insert the elements
void insert_iter(tree *tree, int data)
{
if(tree->root==NULL)
tree->root=newNode(data); // function to make a new node
else{
node *it=tree->root;
int dir;
while(1)
{
dir=data>it->data;
if(it->link[dir]!=NULL)
it=it->link[dir];
else
{
it->link[dir]=newNode(data);
break;
}
}}}

Answer 1

tree *tree=NULL; // all the confusion is regarding this
// the above is setting a pointer to NULL, which is ok
*tree->root=NULL; // tried this one but it was wrong
// the above is setting a
// (not initialized to point to any specific memory)
// offset in the tree struct variable
// (that offset is not pointing any specific memory)
// to set the first 4 bytes of that memory to NULL
// which should/will cause a seg fault event due to undefined behaviour
// a workable solution might be:
tree *pTree = malloc( sizeof(tree) );
// and then this would work:
tree->root = NULL; // notice no '*' dereference needed
//I would also strongly suggest that the variable names be different
//than the struct names, and not just in the capitalization of the name.
//The use of different names will greatly enhance
//the readability and understandability and clarity of the code
//(and will avoid any misunderstandings by the compiler and future maintainer)