关于getifaddrs()和struct ifaddrs的说明

1，关于struct ifaddrs的说明：

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，关于getifaddrs()

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;this field may be NULL if there is no such data for this interface.

返回值：
On success, getifaddrs() returns zero; on error, -1 is returned, and errno is set appropriately.

3，注意：
       The data returned by getifaddrs() is dynamically allocated and should be freed using freeifaddrs() when no longer needed.

4，一个小例子：

#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\n",
                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\n", gai_strerror(s));
                exit(EXIT_FAILURE);
            }
            printf("\taddress: <%s>\n", host);
        }
    }

    freeifaddrs(ifaddr); //一定要释放
    exit(EXIT_SUCCESS);
}

输出：

引用

$ ./a.out
lo      address family: 17 (AF_PACKET)
eth0    address family: 17 (AF_PACKET)
lo      address family: 2 (AF_INET)
        address: <127.0.0.1>
eth0    address family: 2 (AF_INET)
        address: <10.1.1.4>
lo      address family: 10 (AF_INET6)
        address: <::1>
eth0    address family: 10 (AF_INET6)
                   address: <fe80::2d0:59ff:feda:eb51%eth0