套接字编程中struct addrinfo {}中链接列表的用途

Question

我正在阅读Beejs的网络编程指南

我很难理解链表的目的,即这个结构中的最终参数:

struct addrinfo {
    int              ai_flags;     // AI_PASSIVE, AI_CANONNAME, etc.
    int              ai_family;    // AF_INET, AF_INET6, AF_UNSPEC
    int              ai_socktype;  // SOCK_STREAM, SOCK_DGRAM
    int              ai_protocol;  // use 0 for "any"
    size_t           ai_addrlen;   // size of ai_addr in bytes
    struct sockaddr *ai_addr;      // struct sockaddr_in or _in6
    char            *ai_canonname; // full canonical hostname

    struct addrinfo *ai_next;      // linked list, next node
};

这有什么需要？下一个节点是指下一个客户端还是什么？

Answer 1

主机可以拥有多个IP地址.例如,IPv4和IPv6地址,或多个IPv4地址.因此,getaddrinfo()为您提供指向一个或多个addrinfo结构的链接列表ai_next的指针,并且是指向下一个元素或NULL列表中最后一个元素的指针.

示例(打印主机的所有IP地址):

struct addrinfo hints;
memset(&hints, 0, sizeof(hints));
hints.ai_family = PF_UNSPEC;
hints.ai_protocol = IPPROTO_TCP;
struct addrinfo *addrs, *addr;

getaddrinfo("www.google.com", NULL, &hints, &addrs);
// addrs points to first addrinfo structure.

// Traverse the linked list:
for (addr = addrs; addr != NULL; addr = addr->ai_next) {

    char host[NI_MAXHOST];
    getnameinfo(addr->ai_addr, addr->ai_addrlen, host, sizeof(host), NULL, 0, NI_NUMERICHOST);
    printf("%s\n", host);

}
freeaddrinfo(addrs);

(为简洁起见,省略了错误检查.)