Linux下获取本机IP地址（不需要知道interface name的方式）

之前有写过一篇文章，介绍如何在linux下编程实现获取本机的IP地址，详见下文地址
http://blog.csdn.net/timsley/article/details/51062342

但是这种方法有一种限制，就是需要知道你的interface name，在阅读libupnp代码时，发现如果不知道interface name，也是有方法可以拿到本机的IP地址。

实现思路

通过struct ifconf拿到interface的configure信息，然后遍历struct ifreq，拿到合法的IP地址

struct ifconf是用来保存所有接口信息的一个结构体，具体定义可以在/usr/include/net/if.h文件中找到

struct ifconf
  {
    int ifc_len;            /* Size of buffer. */
    union
      {
    __caddr_t ifcu_buf;
    struct ifreq *ifcu_req;
      } ifc_ifcu;
  };
# define ifc_buf ifc_ifcu.ifcu_buf /* Buffer address. */
# define ifc_req ifc_ifcu.ifcu_req /* Array of structures. */
# define _IOT_ifconf _IOT(_IOTS(struct ifconf),1,0,0,0,0) /* not right */

struct ifreq是用来保存某个接口信息的，它的定义也是在/usr/include/net/if.h文件中找到，具体定义如下

struct ifreq
  {
# define IFHWADDRLEN 6
# define IFNAMSIZ IF_NAMESIZE
    union
      {
    char ifrn_name[IFNAMSIZ];   /* Interface name, e.g. "en0". */
      } ifr_ifrn;

    union
      {
    struct sockaddr ifru_addr;
    struct sockaddr ifru_dstaddr;
    struct sockaddr ifru_broadaddr;
    struct sockaddr ifru_netmask;
    struct sockaddr ifru_hwaddr;
    short int ifru_flags;
    int ifru_ivalue;
    int ifru_mtu;
    struct ifmap ifru_map;
    char ifru_slave[IFNAMSIZ];  /* Just fits the size */
    char ifru_newname[IFNAMSIZ];
    __caddr_t ifru_data;
      } ifr_ifru;
  };
# define ifr_name ifr_ifrn.ifrn_name /* interface name */
# define ifr_hwaddr ifr_ifru.ifru_hwaddr /* MAC address */
# define ifr_addr ifr_ifru.ifru_addr /* address */
# define ifr_dstaddr ifr_ifru.ifru_dstaddr /* other end of p-p lnk */
# define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */
# define ifr_netmask ifr_ifru.ifru_netmask /* interface net mask */
# define ifr_flags ifr_ifru.ifru_flags /* flags */
# define ifr_metric ifr_ifru.ifru_ivalue /* metric */
# define ifr_mtu ifr_ifru.ifru_mtu /* mtu */
# define ifr_map ifr_ifru.ifru_map /* device map */
# define ifr_slave ifr_ifru.ifru_slave /* slave device */
# define ifr_data ifr_ifru.ifru_data /* for use by interface */
# define ifr_ifindex ifr_ifru.ifru_ivalue /* interface index */
# define ifr_bandwidth ifr_ifru.ifru_ivalue /* link bandwidth */
# define ifr_qlen ifr_ifru.ifru_ivalue /* queue length */
# define ifr_newname ifr_ifru.ifru_newname /* New name */
# define _IOT_ifreq _IOT(_IOTS(char),IFNAMSIZ,_IOTS(char),16,0,0)
# define _IOT_ifreq_short _IOT(_IOTS(char),IFNAMSIZ,_IOTS(short),1,0,0)
# define _IOT_ifreq_int _IOT(_IOTS(char),IFNAMSIZ,_IOTS(int),1,0,0)

你可以通过struct ifreq里的ifr_name去获取Interface的名字，也可以通过ifr_addr去拿到对应interface的IP地址，还可以拿到netmask、broadaddr等，通过ifr_flags可以判断当前interface是否up或者是否是回环等等

具体代码实现如下，详细可以参考libupnp里的getlocalhostname函数

int get_local_ip(char *out, size_t out_len)
{
    const char *p = NULL;
    char szBuffer[256 * sizeof (struct ifreq)];
    struct ifconf ifConf;
    struct ifreq ifReq;
    int nResult;
    long unsigned int i;
    int LocalSock;
    struct sockaddr_in LocalAddr;
    int j = 0;

    /* purify */
    memset(&ifConf,  0, sizeof(ifConf));
    memset(&ifReq,   0, sizeof(ifReq));
    memset(szBuffer, 0, sizeof(szBuffer));
    memset(&LocalAddr, 0, sizeof(LocalAddr));

    /* Create an unbound datagram socket to do the SIOCGIFADDR ioctl on. */
    LocalSock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (LocalSock == -1) 
    {
        printf("Can't create addrlist socket\n");
        return -1;
    }
    /* Get the interface configuration information... */
    ifConf.ifc_len = (int)sizeof szBuffer;
    ifConf.ifc_ifcu.ifcu_buf = (caddr_t) szBuffer;
    nResult = ioctl(LocalSock, SIOCGIFCONF, &ifConf);
    if (nResult < 0) 
    {
        printf("DiscoverInterfaces: SIOCGIFCONF returned error\n");
        close(LocalSock);
        return -1;
    }

    /* Cycle through the list of interfaces looking for IP addresses. */
    for (i = 0lu; i < (long unsigned int)ifConf.ifc_len && j < 1; ) 
    {
        struct ifreq *pifReq = (struct ifreq *)((caddr_t)ifConf.ifc_req + i);
        i += sizeof *pifReq;
        /* See if this is the sort of interface we want to deal with. */

        memset(ifReq.ifr_name, 0, sizeof(ifReq.ifr_name));
        strncpy(ifReq.ifr_name, pifReq->ifr_name, sizeof(ifReq.ifr_name) - 1);
        if (ioctl(LocalSock, SIOCGIFFLAGS, &ifReq) < 0)
        {
            printf("Can't get interface flags for %s:\n", ifReq.ifr_name);
        }

        /* Skip loopback, point-to-point and down interfaces, * except don't skip down interfaces * if we're trying to get a list of configurable interfaces. */        
        if ((ifReq.ifr_flags & IFF_LOOPBACK) || (!(ifReq.ifr_flags & IFF_UP)))
        {
            continue;
        }

        if (pifReq->ifr_addr.sa_family == AF_INET) 
        {
            /* Get a pointer to the address...*/
            memcpy(&LocalAddr, &pifReq->ifr_addr, sizeof pifReq->ifr_addr);
            /* We don't want the loopback interface. */
            if (LocalAddr.sin_addr.s_addr == htonl(INADDR_LOOPBACK)) 
            {
                continue;
            }
        }
        /* increment j if we found an address which is not loopback * and is up */
        j++;
    }
    close(LocalSock);

    p = inet_ntoa(LocalAddr.sin_addr);  
    if (p) 
    {
        strncpy(out, p, out_len);
    } 
    else
    {
        printf("getlocalhostname: inet_ntop returned error\n" );
        return -1;
    }

    printf("Inside getlocalhostname: after strncpy %s\n", out);

    return 0;
}

关于struct ifreq里的ifr_flags变量设定，可以参考以下链接
SIOCGIFFLAGS, SIOCSIFFLAGS