Threaded Binary Tree's inorder successor

I'm building a C programme to generate a threaded binary tree and then discover the INORDER SUCCESSOR of a specific node. For this, I'm presenting the TBT's inorder sequence and then asking the user to select which node's successor should be displayed. I've built a function to accomplish this. However, I am not receiving a replacement for the FIRST NODE. Here is the whole schedule:

Code:

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#include<stdio.h>#include <stdlib.h>

struct tbtnode {
  int data;
  struct tbtnode *left, *right;
  int lbit, rbit, flag;
  int child;
} *root = NULL;

typedef struct tbtnode TBT;

TBT *insuc(TBT * t);

void inorder(TBT *);

void create(TBT *);

void create(TBT * root)
{
  int x, op, flag, y;
  flag = 0;
  char ch;
  TBT *curr = root;
  TBT *q, *p;
  do {

    printf("\nCurrent node %d \n\n 1.Left Direction.\n\n2.Right Direction",
          curr->data);
    printf("\nEnter ur choice :");
    scanf("%d", &op);
    switch (op) {
    case 1:
      if (curr->lbit == 1) {
        printf("Enter left child of %d : ", curr->data);
        scanf("%d", &x);
        q = (TBT *) malloc(sizeof(TBT));
        q->data = x;
        q->lbit = q->rbit = 1;
        q->left = curr->left;
        q->right = curr;
        curr->left = q;
        curr->lbit = 0;
        q->child = 0;
        flag = 1;
      } else
        curr = curr->left;
      break;
    case 2:
      if (curr->rbit == 1) {

        printf("Enter right child of %d :", curr->data);
        scanf("%d", &x);
        q = (TBT *) malloc(sizeof(TBT));
        q->data = x;
        q->lbit = q->rbit = 1;
        q->left = curr;
        q->right = curr->right;
        curr->right = q;
        curr->rbit = 0;
        q->child = 1;
        flag = 1;
      } else
        curr = curr->right;
      break;
    }
  } while (flag == 0);
}

void inorder(TBT * head)
{
  TBT *t;
  t = head->left;
  printf("\n");
  while (t->lbit == 0)
    t = t->left;
  while (t != head) {
    printf("  %d", t->data);
    t = insuc(t);
  }
}

TBT *insuc(TBT * t)
{
  if (t->rbit == 0) {
    t = t->right;
    while (t->lbit == 0)
      t = t->left;
    return (t);
  } else
    return (t->right);
}

void inorder_successor(TBT * head, int x)
{
  TBT *t;
  t = head->left;
  printf("\n");
  while (t->lbit == 0)
    t = t->left;
  while (t != head) {
    t = insuc(t);
    if (t->data == x) {
      t = insuc(t);
      printf("  %d", t->data);
    }
  }
}

int main()
{

  int op, x, n, i = 0, item;
  char ch;
  TBT *head, *root, *succ;      //here head indicates dummy variable

  head = (TBT *) malloc(sizeof(TBT));
  head->left = head;
  head->right = head;
  head->lbit = 1;
  head->rbit = 1;

  do {
    printf("\n****Threaded binary tree operations****");
    printf("\n1)create\n2)inorder\n3)Successor\n4)exit");
    printf("\nEnter ur choice: ");
    scanf("%d", &op);
    switch (op) {
    case 1:

      printf("\nEnter Number Of Nodes :");
      scanf("%d", &n);
      printf("\nEnter root data: ");
      scanf("%d", &x);

      root = (TBT *) malloc(sizeof(TBT));
      root->data = x;
      root->lbit = root->rbit = 1;
      root->child = 0;
      root->left = head->left;

      head->left = root;
      head->lbit = 0;
      root->right = head->right;

      for (i = 0; i < n - 1; i++)
        create(root);

      break;

    case 2:
      printf("\nInorder Traversal Is:\n");
      inorder(head);
      break;

    case 3:
      printf("Enter the node to which successor is to be found\n");
      scanf("%d", &item);
      inorder_successor(head, item);
      break;
    case 4:
      return (0);
      break;
    }
  } while (op <= 4);
  return 0;
}

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The successor of the last node is 0, yet everything is good. I read this <<snipped>>, they are trying to explain it but I didn't understand. Can somebody assist me in resolving this?