C/C++：与POSIX兼容的查找默认网络接口up/down的方法

#include <stdio.h>
#include <ifaddrs.h>
#include <net/if.h>

int main() {
    struct ifaddrs *ifaddr, *ifa;
    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        return 1;
    }

    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
        if (ifa->ifa_addr == NULL)
            continue;

        if (ifa->ifa_addr->sa_family == AF_PACKET) {
            struct ifreq ifr;
            int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
            strncpy(ifr.ifr_name, ifa->ifa_name, IFNAMSIZ - 1);
            ioctl(sockfd, SIOCGIFFLAGS, &ifr);
            close(sockfd);

            if (ifr.ifr_flags & IFF_UP)
                printf("%s is up\n", ifa->ifa_name);
            else
                printf("%s is down\n", ifa->ifa_name);
        }
    }

    freeifaddrs(ifaddr);
    return 0;
}

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

int main() {
    int sockfd;
    struct ifreq ifr;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    strncpy(ifr.ifr_name, "eth0", IFNAMSIZ - 1);

    if (ioctl(sockfd, SIOCGIFFLAGS, &ifr) == -1) {
        perror("ioctl");
        return 1;
    }

    if (ifr.ifr_flags & IFF_UP)
        printf("Interface is up\n");
    else
        printf("Interface is down\n");

    close(sockfd);
    return 0;
}