int main(int argc, char* argv[], char* enpv[])
{
    pid_t num;
    int status;
    num = fork();
    if(num == 0)
    {
        execl("ifconfig", "eth0", "inet", "10.0.2.16");
    }
    else
    {
        wait(&status);
        printf("ending");
    }
    return 0;
}

struct sockaddr_in sin;
int s;

s = socket(AF_INET,SOCK_DGRAM,0); //temporary socket

memset(&ifr, 0, sizeof(struct ifreq));
memset(&sin, 0, sizeof(struct sockaddr));
strncpy(ifr.ifr_name, iface, IF_NAMESIZE-1);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = inet_addr("192.168.0.123");

memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr));

if(ioctl(s, SIOCSIFADDR, &ifr)< 0){ //set IP-Adress and check for errors
    close(s);
    perror("ioctl()");
    exit(1);
}

s = socket(AF_INET,SOCK_DGRAM,0); //temporary socket

memset(&ifr, 0, sizeof(struct ifreq));
memset(&sin, 0, sizeof(struct sockaddr));
strncpy(ifr.ifr_name, "eth0", IF_NAMESIZE-1);
sin.sin_family = AF_INET;

sin.sin_addr.s_addr = inet_addr("192.168.0.123");

memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr));

if(ioctl(s, SIOCSIFADDR, &ifr)< 0) //set IP-Adress and check for errors
{ /*close(s);*/ }
close(s);

eth0      Link encap:Ethernet  HWaddr BC:6A:29:57:11:80
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth1      Link encap:Ethernet  HWaddr BC:6A:29:57:11:81
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

system("ifconfig eth0 10.0.5.20");

s = socket(...);
if(s < 0) {
  perror("socket");
  exit(1);
}
...
if(ioctl(...) < 0)
  perror("ioctl");

strace ifconfig eth0

        ioctl(s, SIOCGIFFLAGS, &ifr);
        // флаги интерфейса
        ifr.ifr_flags |= IFF_UP | IFF_RUNNING;

        ioctl(s, SIOCSIFFLAGS, &ifr);
        close(s);