Linxu 의 struct ifaddrs 와 getifaddrs () 함수

man 함수 매 뉴 얼 에서 소개:
1. struct ifaddrs 에 대한 설명:

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 */
}; 

2, getifaddrs () 에 대하 여
The getifaddrs() function creates a linked list of structures describing the network interfaces of the local system, and stores the address of the first item of the list in *ifap.  The list consists of ifaddrs structures, defined as follows:        The ifa_next field contains a pointer to the next structure on the list, or        NULL if this is the last item of the list.        The ifa_name points to the null-terminated interface name.        The ifa_flags field contains the interface flags        The ifa_addr field points to a structure containing the interface address.        The ifa_netmask field points to a structure containing the netmask associated with ifa_addr, if applicable for the address family.        Depending on whether the bit IFF_BROADCAST or IFF_POINTOPOINT is set in ifa_flags (only one can be set at a time), either ifa_broadaddr will contain the broadcast address associated with ifa_addr (if applicable for the address family) or ifa_dstaddr will contain the destination address of the point-to-point interface.        The ifa_data field points to a buffer containing address-family-specific data;이 인터페이스 에 대한 데이터 가 없 는 경우 이 필드 는 NULL 일 수 있 습 니 다. 반환 값: 성공 시 getifaddrs () 는 0 을 반환 합 니 다.on error, - 1 is returned, and errno is set appropriately. 3, 주의:       The data returned by getifaddrs() is dynamically allocated and should be freed using freeifaddrs() when no longer needed.
4,  man 의 인 스 턴 스 코드:

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.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)" : "");

        /* 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("\taddress: <%s>
", host);
        }
    }

    freeifaddrs(ifaddr);
    exit(EXIT_SUCCESS);
}