How can I get the IP address of a (Linux) machine?

I found the ioctl solution problematic on os x (which is POSIX compliant so should be similiar to linux). However getifaddress() will let you do the same thing easily, it works fine for me on os x 10.5 and should be the same below.

I've done a quick example below which will print all of the machine's IPv4 address, (you should also check the getifaddrs was successful ie returns 0).

I've updated it show IPv6 addresses too.

#include <stdio.h>      
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
            char addressBuffer[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } else if (ifa->ifa_addr->sa_family == AF_INET6) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
            char addressBuffer[INET6_ADDRSTRLEN];
            inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } 
    }
    if (ifAddrStruct!=NULL) freeifaddrs(ifAddrStruct);
    return 0;
}

1. Create a socket.
2. Perform ioctl(<socketfd>, SIOCGIFCONF, (struct ifconf)&buffer);

Read /usr/include/linux/if.h for information on the ifconf and ifreq structures. This should give you the IP address of each interface on the system. Also read /usr/include/linux/sockios.h for additional ioctls.

As you have found out there is no such thing as a single "local IP address". Here's how to find out the local address that can be sent out to a specific host.

1. Create a UDP socket
2. Connect the socket to an outside address (the host that will eventually receive the local address)
3. Use getsockname to get the local address

Further to what Steve Baker has said, you can find a description of the SIOCGIFCONF ioctl in the netdevice(7) man page.

Once you have the list of all the IP addresses on the host, you will have to use application specific logic to filter out the addresses you do not want and hope you have one IP address left.

How about you play with the stdio.h header only? (no network headers involved)

Here is the full C++ code for that:

(this is for the local IP; if you're over a home modem for example)

#include <stdio.h>
int main()
{
    static char ip[32];
    FILE *f = popen("ip a | grep 'scope global' | grep -v ':' | awk '{print $2}' | cut -d '/' -f1", "r");
    int c, i = 0;
    while ((c = getc(f)) != EOF) i += sprintf(ip+i, "%c", c);
    pclose(f);
    printf(ip);
}

• BONUS: you can cross-compile this for Android too over it's NDK C++ compiler

• JUST SAYING: I have a VPS where I ran this and it gets the external IP

You can do some integration with curl as something as easy as: curl www.whatismyip.org from the shell will get you your global ip. You're kind of reliant on some external server, but you will always be if you're behind a NAT.