在Linux下开发网络程序时，经常会遇到需要取本地网络接口名、IP、广播地址、子网掩码或者MAC地址等信息的需求，最常见的办法是配合宏SIOCGIFHWADDR、SIOCGIFADDR、SIOCGIFBRDADDR与SIOCGIFNETMASK作为参数调用函数ioctl分别获得MAC地址、IP地址、广播地址与子网掩码来实现。一次性获取此类信息的C语言代码实现如下。     

#include <stdio.h>
#include <string.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <errno.h>

int getLocalInfo(void)
{
   int fd;
   int interfaceNum = 0;
   struct ifreq buf[16];
   struct ifconf ifc;
   struct ifreq ifrcopy;
   char mac[16] = {0};
   char ip[32] = {0};
   char broadAddr[32] = {0};
   char subnetMask[32] = {0};

   if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
   {
      perror("socket");
      close(fd);
      return -1;
   }

   ifc.ifc_len = sizeof(buf);
   ifc.ifc_buf = (caddr_t)buf;
   if (!ioctl(fd, SIOCGIFCONF, (char *)&ifc))
   {
      interfaceNum = ifc.ifc_len / sizeof(struct ifreq);
      printf("interface num = %d\n", interfaceNum);
      while (interfaceNum-- > 0)
      {
             printf("\ndevice name: %s\n", buf[interfaceNum].ifr_name);

             //ignore the interface that not up or not runing
             ifrcopy = buf[interfaceNum];
             if (ioctl(fd, SIOCGIFFLAGS, &ifrcopy))
             {
                printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
                close(fd);
                return -1;
             }

             //get the mac of this interface
             if (!ioctl(fd, SIOCGIFHWADDR, (char *)(&buf[interfaceNum])))
             {
                memset(mac, 0, sizeof(mac));
                snprintf(mac, sizeof(mac), "%02x%02x%02x%02x%02x%02x",
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[0],
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[1],
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[2],
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[3],
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[4],
                            (unsigned char)buf[interfaceNum].ifr_hwaddr.sa_data[5]);
                printf("device mac: %s\n", mac);
             }
            else
            {
                printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
                close(fd);
                return -1;
            }
    
            //get the IP of this interface
            if (!ioctl(fd, SIOCGIFADDR, (char *)&buf[interfaceNum]))
            {
                snprintf(ip, sizeof(ip), "%s",
                            (char *)inet_ntoa(((struct sockaddr_in *)&(buf[interfaceNum].ifr_addr))->sin_addr));
                printf("device ip: %s\n", ip);
            }
            else
            {
                printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
                close(fd);
                return -1;
            }

            //get the broad address of this interface
            if (!ioctl(fd, SIOCGIFBRDADDR, &buf[interfaceNum]))
            {
                snprintf(broadAddr, sizeof(broadAddr), "%s",
                            (char *)inet_ntoa(((struct sockaddr_in *)&(buf[interfaceNum].ifr_broadaddr))->sin_addr));
                printf("device broadAddr: %s\n", broadAddr);
            }
            else
            {
                printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
                close(fd);
                return -1;
            }

            //get the subnet mask of this interface
            if (!ioctl(fd, SIOCGIFNETMASK, &buf[interfaceNum]))
            {
                snprintf(subnetMask, sizeof(subnetMask), "%s",
                            (char *)inet_ntoa(((struct sockaddr_in *)&(buf[interfaceNum].ifr_netmask))->sin_addr));
                printf("device subnetMask: %s\n", subnetMask);
            }
            else
            {
                printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
                close(fd);
                return -1;
            }
        }
    }
    else
    {
        printf("ioctl: %s [%s:%d]\n", strerror(errno), __FILE__, __LINE__);
        close(fd);
        return -1;
    }

    close(fd);

    return 0;
}

int main(void)
{
    getLocalInfo();

    return 0;
}

使用ioctl函数虽然可以获取所有的信息，但是使用起来比较麻烦，如果不需要获取MAC地址，那么使用getifaddrs函数来获取更加方便与简洁。值得一提的是，在MacOS或iOS系统上（如iPhone程序开发），上述iotcl函数没法获得mac地址跟子网掩码，这个使用，使用getifaddrs函数便更有优势了。下面是使用getiaddrs函数获取网卡信息的C语言代码实现。

#include <stdio.h>
#include <ifaddrs.h>
#include <arpa/inet.h>

int getSubnetMask()
{
    struct sockaddr_in *sin = NULL;
    struct ifaddrs *ifa = NULL, *ifList;

    if (getifaddrs(&ifList) < 0) return -1;

    for (ifa = ifList; ifa != NULL; ifa = ifa->ifa_next)
    {
        if(ifa->ifa_addr->sa_family == AF_INET)
        {
            printf("\n>>> interfaceName: %s\n", ifa->ifa_name);

            sin = (struct sockaddr_in *)ifa->ifa_addr;
            printf(">>> ipAddress: %s\n", inet_ntoa(sin->sin_addr));

            sin = (struct sockaddr_in *)ifa->ifa_dstaddr;
            printf(">>> broadcast: %s\n", inet_ntoa(sin->sin_addr));

            sin = (struct sockaddr_in *)ifa->ifa_netmask;
            printf(">>> subnetMask: %s\n", inet_ntoa(sin->sin_addr));
        }
    }

    freeifaddrs(ifList);

    return 0;
}

int main(void)
{
    getSubnetMask();

    return 0;
}

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