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>intmain(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