Get the IP address of the 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.

I like jjvainio's answer. As Zan Lnyx says, it uses the local routing table to find the IP address of the ethernet interface that would be used for a connection to a specific external host. By using a connected UDP socket, you can get the information without actually sending any packets. The approach requires that you choose a specific external host. Most of the time, any well-known public IP should do the trick. I like Google's public DNS server address 8.8.8.8 for this purpose, but there may be times you'd want to choose a different external host IP. Here is some code that illustrates the full approach.

void GetPrimaryIp(char* buffer, size_t buflen) {    assert(buflen >= 16);    int sock = socket(AF_INET, SOCK_DGRAM, 0);    assert(sock != -1);    const char* kGoogleDnsIp = "8.8.8.8";    uint16_t kDnsPort = 53;    struct sockaddr_in serv;    memset(&serv, 0, sizeof(serv));    serv.sin_family = AF_INET;    serv.sin_addr.s_addr = inet_addr(kGoogleDnsIp);    serv.sin_port = htons(kDnsPort);    int err = connect(sock, (const sockaddr*) &serv, sizeof(serv));    assert(err != -1);    sockaddr_in name;    socklen_t namelen = sizeof(name);    err = getsockname(sock, (sockaddr*) &name, &namelen);    assert(err != -1);    const char* p = inet_ntop(AF_INET, &name.sin_addr, buffer, buflen);    assert(p);    close(sock);}