getifaddrs

getifaddrs

获取本地网络接口的信息。在路由器上可以用这个接口来获取wan/lan等接口当前的ip地址，广播地址等信息。

       #include <sys/types.h>
       #include <ifaddrs.h>

       int getifaddrs(struct ifaddrs **ifap);

       void freeifaddrs(struct ifaddrs *ifa);

getifaddrs创建一个链表，链表上的每个节点都是一个struct ifaddrs结构，getifaddrs()返回链表第一个元素的指针。成功返回0, 失败返回-1,同时errno会被赋允相应错误码。
struct ifaddrs结构描述如下：

           struct ifaddrs {
               struct ifaddrs  *ifa_next;    /* Next item in list */
               char            *ifa_name;    /* Name of interface */
               unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
               struct sockaddr *ifa_addr;    /* Address of interface */
               struct sockaddr *ifa_netmask; /* Netmask of interface */
               union {
                   struct sockaddr *ifu_broadaddr;
                                    /* Broadcast address of interface */
                   struct sockaddr *ifu_dstaddr;
                                    /* Point-to-point destination address */
               } ifa_ifu;
           #define              ifa_broadaddr ifa_ifu.ifu_broadaddr
           #define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
               void            *ifa_data;    /* Address-specific data */
           };

ifa_next 指向链表中下一个struct ifaddr结构
ifa_name 网络接口名
ifa_flags 网络接口标志，这些标志见下面描述。
ifa_addr 指向一个包含网络地址的sockaddr结构
ifa_netmask 指向一个包含网络掩码的结构
ifu_broadaddr 如果(ifa_flags&IFF_BROADCAST)有效，ifu_broadaddr指向一个包含广播地址的结构。
ifu_dstaddr 如果(ifa_flags&IFF_POINTOPOINT)有效，ifu_dstaddr指向一个包含p2p目的地址的结构。
ifa_addr 指向一个缓冲区，其中包含地址族私有数据。没有私有数据则为NULL。

ifa_flags 标志位：

                                      Device flags
              IFF_UP            Interface is running.
              IFF_BROADCAST     Valid broadcast address set.

              IFF_DEBUG         Internal debugging flag.
              IFF_LOOPBACK      Interface is a loopback interface.
              IFF_POINTOPOINT   Interface is a point-to-point link.
              IFF_RUNNING       Resources allocated.
              IFF_NOARP         No arp protocol, L2 destination address not
                                set.
              IFF_PROMISC       Interface is in promiscuous mode.
              IFF_NOTRAILERS    Avoid use of trailers.
              IFF_ALLMULTI      Receive all multicast packets.
              IFF_MASTER        Master of a load balancing bundle.
              IFF_SLAVE         Slave of a load balancing bundle.
              IFF_MULTICAST     Supports multicast
              IFF_PORTSEL       Is able to select media type via ifmap.
              IFF_AUTOMEDIA     Auto media selection active.
              IFF_DYNAMIC       The addresses are lost when the interface
                                goes down.
              IFF_LOWER_UP      Driver signals L1 up (since Linux 2.6.17)
              IFF_DORMANT       Driver signals dormant (since Linux 2.6.17)
              IFF_ECHO          Echo sent packets (since Linux 2.6.25)

使用实例，在man 3 getifaddrs的例子基础上加了一个broadcast address:

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <net/if.h>

int
main(int argc, char *argv[])
{
	struct ifaddrs *ifaddr, *ifa;
	int family, s;
	char host[NI_MAXHOST];

	if (getifaddrs(&ifaddr) == -1) {
		perror("getifaddrs");
		exit(EXIT_FAILURE);
	}

	/* Walk through linked list, maintaining head pointer so we
	   can free list later */

	for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
		if (ifa->ifa_addr == NULL)
			continue;
		family = ifa->ifa_addr->sa_family;

		/* Display interface name and family (including symbolic
		   form of the latter for the common families) */

		printf("%s  address family: %d%s
",
		       ifa->ifa_name, family,
		       (family == AF_PACKET) ? " (AF_PACKET)" :
		       (family == AF_INET) ?   " (AF_INET)" :
		       (family == AF_INET6) ?  " (AF_INET6)" : "");

		if (ifa->ifa_flags & IFF_BROADCAST)

		/* For an AF_INET* interface address, display the address */

		if (family == AF_INET || family == AF_INET6) {
			s = getnameinfo(ifa->ifa_addr,
					(family == AF_INET) ? sizeof(struct sockaddr_in) :
					sizeof(struct sockaddr_in6),
					host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
			if (s != 0) {
				printf("getnameinfo() failed: %s
", gai_strerror(s));
				exit(EXIT_FAILURE);
			}
			printf("	address: <%s>
", host);

			if (ifa->ifa_flags & IFF_BROADCAST) {
				s = getnameinfo(ifa->ifa_broadaddr,
						(family == AF_INET) ? sizeof(struct sockaddr_in) :
						sizeof(struct sockaddr_in6),
						host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
				printf("	broadcast address: <%s>
", host);
			}
			if (ifa->ifa_flags & IFF_POINTOPOINT) {
				s = getnameinfo(ifa->ifa_dstaddr,
						(family == AF_INET) ? sizeof(struct sockaddr_in) :
						sizeof(struct sockaddr_in6),
						host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
				printf("	broadcast address: <%s>
", host);
			}
		}
	}

	freeifaddrs(ifaddr);
	exit(EXIT_SUCCESS);
}