getifindex.c

#include <stdio.h>
#include <sys/socket.h>
#include <net/if.h>
#include <linux/ioctl.h>
#include <errno.h>
#include <string.h>
#include <sys/ioctl.h>

int main(int argc, char **argv)
{
    int fd;

    if (argc != 2) {
        fprintf(stderr, "<%s> interface name\n", argv[0]);
        return -1;
    }

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

    struct ifreq ifr;

    memset(&ifr, 0, sizeof(ifr));

    strcpy(ifr.ifr_name, argv[1]);

    int ret;

    ret = ioctl(fd, SIOCGIFINDEX, &ifr);
    if (ret < 0) {
        fprintf(stderr, "failed to get ifindex %s\n", strerror(errno));
        return -1;
    }

    printf("interface index for [%s] is %d\n", argv[1], ifr.ifr_ifindex);

    return 0;
}