sinp = (struct sockaddr_in *)aip->ai_addr 会将struct sockaddr 变量强制转化为struct sockaddr_in 类型

inet_ntop函数用于在二进制格式与点分十进制格式表示（a.b.c.d）之间进行转换

-sinp = (struct sockaddr_in*) aip->　ai_addr　the struct sockaddr variables forced into the struct sockaddr_in the type inet_ntop function. used in binary format and dotted decimal format (abcd) to convert between