How to Know the number of attrs?

typedef struct TupleDescData
{
    int            natts;            /* number of attributes in the tuple */
    Oid            tdtypeid;        /* composite type ID for tuple type */
    int32        tdtypmod;        /* typmod for tuple type */
    int            tdrefcount;        /* reference count, or -1 if not counting */
    TupleConstr *constr;        /* constraints, or NULL if none */
    /* attrs[N] is the description of Attribute Number N+1 */
    FormData_pg_attribute attrs[FLEXIBLE_ARRAY_MEMBER];
}            TupleDescData;

Hi, the attrs use FLEXIBLE_ARRAY_MEMBER, so which API should I use to get the real length of this array?