c语言关于vs19无缘报错问题

在中午

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

typedef char ElemType;

BiThrTree pre;

enum PointTag
{
    link, thread//link左右孩子指针，thread前驱后继线索
};

//二叉树结构
typedef struct BiThrNode
{
    ElemType data;
    PointTag ltag;
    PointTag rtag;
    BiThrNode* lchild;
    BiThrNode* rchild;
}BiThrNode, * BiThrTree;

//二叉树的建立
void CreatBiThrTree(BiThrTree& T)
{
    ElemType c;
    scanf_s("%c", &c);
    if (c == ' ')
    {
        T = nullptr;
    }
    else
    {
        T = (BiThrTree)malloc(sizeof(BiThrNode));
        T->data = c;
        T->ltag = link;
        T->rtag = link;

        CreatBiThrTree(T->lchild);
        CreatBiThrTree(T->rchild);
    }
}

//二叉树的线索化1，中序遍历
void InThreading(BiThrTree T)
{
    if (T)
    {
        InThreading(T->lchild);
        if (T->lchild == nullptr)
        {
            T->ltag = thread;
            T->lchild = pre;
        }
        if (pre->rchild == nullptr)
        {
            pre->rtag = thread;
            pre->rchild = T;
        }
        pre = T;
        InThreading(T->rchild);
    }
}

//二叉树的线索化2
void InOrderThreading(BiThrTree& T, BiThrTree p)
{
    p = (BiThrTree)malloc(sizeof(BiThrNode));
    p->ltag = link;
    p->rtag = thread;
    p->rchild = p;
    if (T != nullptr)
    {
        p->lchild = T;
        pre = p;
        InThreading(T);
        pre->rchild = p;
        pre->rtag = thread;
        p->rchild = pre;
    }
}

int main()
{
    BiThrTree T = nullptr, p;
    CreatBiThrTree(T);
    InOrderThreading(T, p);

    return 0;
}

运行结果及报错内容

我的解答思路和尝试过的方法

报了什么错？

报的是这种奇奇怪怪的错

结题了，我把全局变量写在结构前了