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_dstaddrvoid            *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 flagsIFF_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 notset.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 multicastIFF_PORTSEL       Is able to select media type via ifmap.IFF_AUTOMEDIA     Auto media selection active.IFF_DYNAMIC       The addresses are lost when the interfacegoes 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 wecan 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 symbolicform of the latter for the common families) */printf("%s  address family: %d%s\n",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\n", gai_strerror(s));exit(EXIT_FAILURE);}printf("\taddress: <%s>\n", 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("\tbroadcast address: <%s>\n", 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("\tbroadcast address: <%s>\n", host);}}}freeifaddrs(ifaddr);exit(EXIT_SUCCESS);
}