Получение шлюза использовать для данного IP в ANSI C

Это зависит от ОС, для этого не существует унифицированного (или ANSI C) API.

Предполагая, что Linux, лучший способ - просто проанализировать / proc / net / route, найти запись, где Destination - 00000000, шлюз по умолчанию находится в столбце Gateway, где вы можете прочитать шестнадцатеричное представление IP-адреса шлюза. (я полагаю, с прямым порядком байтов)

Если вы хотите сделать это с помощью более конкретных вызовов API, вам придется пройти через несколько обручей, вот пример программы:

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


#define BUFSIZE 8192
char gateway[255];

struct route_info {
    struct in_addr dstAddr;
    struct in_addr srcAddr;
    struct in_addr gateWay;
    char ifName[IF_NAMESIZE];
};

int readNlSock(int sockFd, char *bufPtr, int seqNum, int pId)
{
    struct nlmsghdr *nlHdr;
    int readLen = 0, msgLen = 0;

 do {
    /* Recieve response from the kernel */
        if ((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0) {
            perror("SOCK READ: ");
            return -1;
        }

        nlHdr = (struct nlmsghdr *) bufPtr;

    /* Check if the header is valid */
        if ((NLMSG_OK(nlHdr, readLen) == 0)
            || (nlHdr->nlmsg_type == NLMSG_ERROR)) {
            perror("Error in recieved packet");
            return -1;
        }

    /* Check if the its the last message */
        if (nlHdr->nlmsg_type == NLMSG_DONE) {
            break;
        } else {
    /* Else move the pointer to buffer appropriately */
            bufPtr += readLen;
            msgLen += readLen;
        }

    /* Check if its a multi part message */
        if ((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0) {
           /* return if its not */
            break;
        }
    } while ((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != pId));
    return msgLen;
}
/* For printing the routes. */
void printRoute(struct route_info *rtInfo)
{
    char tempBuf[512];

/* Print Destination address */
    if (rtInfo->dstAddr.s_addr != 0)
        strcpy(tempBuf,  inet_ntoa(rtInfo->dstAddr));
    else
        sprintf(tempBuf, "*.*.*.*\t");
    fprintf(stdout, "%s\t", tempBuf);

/* Print Gateway address */
    if (rtInfo->gateWay.s_addr != 0)
        strcpy(tempBuf, (char *) inet_ntoa(rtInfo->gateWay));
    else
        sprintf(tempBuf, "*.*.*.*\t");
    fprintf(stdout, "%s\t", tempBuf);

    /* Print Interface Name*/
    fprintf(stdout, "%s\t", rtInfo->ifName);

    /* Print Source address */
    if (rtInfo->srcAddr.s_addr != 0)
        strcpy(tempBuf, inet_ntoa(rtInfo->srcAddr));
    else
        sprintf(tempBuf, "*.*.*.*\t");
    fprintf(stdout, "%s\n", tempBuf);
}

void printGateway()
{
    printf("%s\n", gateway);
}
/* For parsing the route info returned */
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo)
{
    struct rtmsg *rtMsg;
    struct rtattr *rtAttr;
    int rtLen;

    rtMsg = (struct rtmsg *) NLMSG_DATA(nlHdr);

/* If the route is not for AF_INET or does not belong to main routing table
then return. */
    if ((rtMsg->rtm_family != AF_INET) || (rtMsg->rtm_table != RT_TABLE_MAIN))
        return;

/* get the rtattr field */
    rtAttr = (struct rtattr *) RTM_RTA(rtMsg);
    rtLen = RTM_PAYLOAD(nlHdr);
    for (; RTA_OK(rtAttr, rtLen); rtAttr = RTA_NEXT(rtAttr, rtLen)) {
        switch (rtAttr->rta_type) {
        case RTA_OIF:
            if_indextoname(*(int *) RTA_DATA(rtAttr), rtInfo->ifName);
            break;
        case RTA_GATEWAY:
            rtInfo->gateWay.s_addr= *(u_int *) RTA_DATA(rtAttr);
            break;
        case RTA_PREFSRC:
            rtInfo->srcAddr.s_addr= *(u_int *) RTA_DATA(rtAttr);
            break;
        case RTA_DST:
            rtInfo->dstAddr .s_addr= *(u_int *) RTA_DATA(rtAttr);
            break;
        }
    }
    //printf("%s\n", inet_ntoa(rtInfo->dstAddr));

    if (rtInfo->dstAddr.s_addr == 0)
        sprintf(gateway, (char *) inet_ntoa(rtInfo->gateWay));
    //printRoute(rtInfo);

    return;
}


int main()
{
    struct nlmsghdr *nlMsg;
    struct rtmsg *rtMsg;
    struct route_info *rtInfo;
    char msgBuf[BUFSIZE];

    int sock, len, msgSeq = 0;

/* Create Socket */
    if ((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
        perror("Socket Creation: ");

    memset(msgBuf, 0, BUFSIZE);

/* point the header and the msg structure pointers into the buffer */
    nlMsg = (struct nlmsghdr *) msgBuf;
    rtMsg = (struct rtmsg *) NLMSG_DATA(nlMsg);

/* Fill in the nlmsg header*/
    nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));  // Length of message.
    nlMsg->nlmsg_type = RTM_GETROUTE;   // Get the routes from kernel routing table .

    nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;    // The message is a request for dump.
    nlMsg->nlmsg_seq = msgSeq++;    // Sequence of the message packet.
    nlMsg->nlmsg_pid = getpid();    // PID of process sending the request.

/* Send the request */
    if (send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0) {
        printf("Write To Socket Failed...\n");
        return -1;
    }

/* Read the response */
    if ((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) {
        printf("Read From Socket Failed...\n");
    return -1;
    }
/* Parse and print the response */
    rtInfo = (struct route_info *) malloc(sizeof(struct route_info));
//fprintf(stdout, "Destination\tGateway\tInterface\tSource\n");
    for (; NLMSG_OK(nlMsg, len); nlMsg = NLMSG_NEXT(nlMsg, len)) {
        memset(rtInfo, 0, sizeof(struct route_info));
        parseRoutes(nlMsg, rtInfo);
    }
    free(rtInfo);
    close(sock);

    printGateway();
    return 0;
}