使用ioctl获取本机ip地址列表

参见UPN VOLUM1 3rd 第17章

/*! \fn ILibGetLocalIPAddressList(int** pp_int)
    \brief Gets a list of IP Addresses
    \para
    \b NOTE: \a pp_int must be freed
    \param[out] pp_int Array of IP Addresses
    \returns Number of IP Addresses returned
*/
int ILibGetLocalIPAddressList(int** pp_int)
{
    // Posix will also use an Ioctl call to get the IPAddress List

    //

    char szBuffer[16*sizeof(struct ifreq)]; /*stevens: 一个host最多有16个网卡接口*/
    struct ifconf ifConf;
    struct ifreq ifReq;
    int nResult;
    int LocalSock;
    struct sockaddr_in LocalAddr;
    int tempresults[16];
    int ctr=0;
    int i;
    /* Create an unbound datagram socket to do the SIOCGIFADDR ioctl on. */
    if ((LocalSock = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
    {
        DEBUGSTATEMENT(printf("Can't do that\r\n"));
        exit(1);
    }
    /* Get the interface configuration information... */
    ifConf.ifc_len = sizeof szBuffer;
    ifConf.ifc_ifcu.ifcu_buf = (caddr_t)szBuffer; /*stevens:caddr_t就是void* */
    nResult = ioctl(LocalSock, SIOCGIFCONF, &ifConf);
    if (nResult < 0)
    {
        DEBUGSTATEMENT(printf("ioctl error\r\n"));
        exit(1);
    }
    /* Cycle through the list of interfaces looking for IP addresses. */
    for (i = 0;(i < ifConf.ifc_len);) /*stevens: 遍历取得的ip接口信息*/
    {
        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. */
        strcpy (ifReq.ifr_name, pifReq -> ifr_name);
        if (ioctl (LocalSock, SIOCGIFFLAGS, &ifReq) < 0) /* stevens: 在kernel中，#define ifr_flags     ifr_ifru.ifru_flags*/
        {
            DEBUGSTATEMENT(printf("Can't get flags\r\n"));
            exit(1);
        }
        /* 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);
            if (LocalAddr.sin_addr.s_addr != htonl (INADDR_LOOPBACK)) /*stevens: 前面已经判断了*/
            {                /*if ((ifReq.ifr_flags & IFF_LOOPBACK) || (!(ifReq.ifr_flags & IFF_UP))), 为什么这里还要判断呢?*/
                tempresults[ctr] = LocalAddr.sin_addr.s_addr;
                ++ctr;
            }
        }
    }
    close(LocalSock);
    *pp_int = (int*)malloc(sizeof(int)*(ctr));
    memcpy(*pp_int,tempresults,sizeof(int)*ctr); /*stevens:返回值放最多16个IP*/
    return(ctr);
}