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