我是C编程的新手，需要帮助才能以“ sum”方法访问此值“ data”。

BigInteger sum(BigInteger A,BigInteger B) {
    long data = A -> list ->headnode->data
}

List.h

typedef struct ListObj* List;

List.c

    typedef struct NodeObj {
       long data;
       struct NodeObj* next;
       struct NodeObj* last;

    } NodeObj;

    typedef NodeObj* Node;
    typedef struct ListObj {
        Node cursor;
        Node headnode;
        Node lastnode;
    } ListObj;

BigInteger.h

typedef struct BigIntegerObj* BigInteger;

BigInteger.c

typedef struct BigIntegerObj {
     int sign;
     struct List list;

} BigIntegerObj;