获取ANSI C中给定ip的网关

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

int getGatewayForIP(const char* ip, char* gateway, size_t size) {
    struct ifaddrs *ifaddr, *ifa;
    int family, s;
    char host[NI_MAXHOST];

    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        return -1;
    }

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

        family = ifa->ifa_addr->sa_family;

        if (family == AF_INET) {
            s = getnameinfo(ifa->ifa_addr, sizeof(struct sockaddr_in),
                            host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
            if (s != 0) {
                printf("getnameinfo() failed: %s\n", gai_strerror(s));
                return -1;
            }

            if (strcmp(host, ip) == 0) {
                struct sockaddr_in* gateway_addr = (struct sockaddr_in*)ifa->ifa_dstaddr;
                if (gateway_addr != NULL) {
                    strncpy(gateway, inet_ntoa(gateway_addr->sin_addr), size);
                    gateway[size - 1] = '\0';
                    freeifaddrs(ifaddr);
                    return 0;
                }
            }
        }
    }

    freeifaddrs(ifaddr);
    return -1;
}

int main() {
    char ip[] = "192.168.1.100";
    char gateway[INET_ADDRSTRLEN];

    if (getGatewayForIP(ip, gateway, sizeof(gateway)) == 0) {
        printf("Gateway for IP %s is %s\n", ip, gateway);
    } else {
        printf("Failed to get gateway for IP %s\n", ip);
    }

    return 0;
}