Question

我一直在关注Beej的网络编程指南，我很难理解为什么在调用getaddrinfo（）之前我们必须填写struct addrinfo的一些字段？另外，为什么它返回一个指向多个 addrinfo结构列表的指针？由于只有一个主机，为什么有多个地址？

Answer 1

为什么我们必须先填写struct addrinfo的一些字段调用getaddrinfo（）？

我假设你在询问hints参数。此参数可用于指定要从getaddrinfo()获取的响应类型。例如，您是否想要仅支持IPv4的DNS查询（AF_INET）或IPv6（AF_INET6）来自man page：

hints参数指向指定的addrinfo结构选择返回的套接字地址结构的标准 res指出的列表。如果提示不是NULL，则指向a addrinfo结构，其ai_family，ai_socktype和ai_protocol 指定限制返回的套接字地址集的条件 getaddrinfo（），如下：

   ai_family   This field specifies the desired address family for the
               returned addresses.  Valid values for this field include
               AF_INET and AF_INET6.  The value AF_UNSPEC indicates that
               getaddrinfo() should return socket addresses for any
               address family (either IPv4 or IPv6, for example) that
               can be used with node and service.

   ai_socktype This field specifies the preferred socket type, for
               example SOCK_STREAM or SOCK_DGRAM.  Specifying 0 in this
               field indicates that socket addresses of any type can be
               returned by getaddrinfo().

   ai_protocol This field specifies the protocol for the returned socket
               addresses.  Specifying 0 in this field indicates that
               socket addresses with any protocol can be returned by
               getaddrinfo().

   ai_flags    This field specifies additional options, described below.
               Multiple flags are specified by bitwise OR-ing them
               together.

   All the other fields in the structure pointed to by hints must
   contain either 0 or a null pointer, as appropriate.

为什么它返回一个指向多个addrinfo结构列表的指针？由于只有一个主机，为什么有多个地址？

域名通常被解析为多个IP地址。例如，对我来说，stackoverflow.com目前已解析为以下地址：

$ nslookup stackoverflow.com
Non-authoritative answer:
Server:  UnKnown
Address:  10.0.0.138

Name:    stackoverflow.com
Addresses:  151.101.65.69
          151.101.129.69
          151.101.1.69
          151.101.193.69

所有这些地址都是stackoverflow.com

为什么必须在使用getaddrinfo（）之前部分填充struct addrinfo？

1 个答案: