How to check if interface is up

Question

Title pretty much says it all. If I run ifconfig, I get this:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet -snip-  netmask 255.255.255.0  broadcast -snip-
    ...

Using this, I can know if it's up or not (<UP,...), but I want to be able to do this in C (or C++, if there is a simpler solution there) without relying on parsing external processes.

---

Here is what I've got so far (doesn't work):

bool is_interface_online(std::string interface) {
    struct ifreq ifr;
    int sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP);
    memset(&ifr, 0, sizeof(ifr));
    strcpy(ifr.ifr_name, interface.c_str());
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) < 0) {
            perror("SIOCGIFFLAGS");
    }
    close(sock);
    return !!(ifr.ifr_flags | IFF_UP);
}

Can anyone point me in the correct direction for this?

Answer 1

Have you considered using the strace command to see how ifconfig works? you can even see what parameters are passed to functions and other interesting details of how ifconfig works ..

Answer 2

Answer was simple: I used the bitwise OR (|) operator instead of the AND (&) operator. Fixed code is:

bool is_interface_online(std::string interface) {
    struct ifreq ifr;
    int sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP);
    memset(&ifr, 0, sizeof(ifr));
    strcpy(ifr.ifr_name, interface.c_str());
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) < 0) {
            perror("SIOCGIFFLAGS");
    }
    close(sock);
    return !!(ifr.ifr_flags & IFF_UP);
}

Answer 3

If you care about the up/down state of the interface you might want to use the "IFF_RUNNING" flag instead of the "IFF_UP" flag provided by the current answer.