Dumb Python implementation to get the IP address assigned to an interface

get_addr.py

import sys
import fcntl
import socket
import struct

'''
#include <stdio.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sys/ioctl.h>

int main(int argc, char **argv) {
        printf("IFNAMSIZ = %d\n", IFNAMSIZ);
        printf("sizeof(struct ifreq) = %d\n", sizeof(struct ifreq));
        printf("SIOCGIFADDR = %d\n", SIOCGIFADDR);
        printf("__builtin_offsetof(struct ifreq, ifr_addr) = %d\n", __builtin_offsetof(struct ifreq, ifr_addr));
        printf("__builtin_offsetof(struct sockaddr_in, sin_addr) = %d\n", __builtin_offsetof(struct sockaddr_in, sin_addr));
        printf("sizeof(struct in_addr) = %d\n", sizeof(struct in_addr));
        return 0;
}
'''

IFNAMSIZ = 16
SIZEOF_IFREQ = 40
SIOCGIFADDR = 35093
OFFSETOF_IFR_ADDR = 16
OFFSETOF_SIN_ADDR = 4
SIZEOF_IN_ADDR = 4

def get_address(name):
    assert len(name) < IFNAMSIZ
    req = struct.pack('{}s'.format(SIZEOF_IFREQ), name[:IFNAMSIZ - 1])

    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    try:
        res = fcntl.ioctl(sock.fileno(), SIOCGIFADDR, req)

        begin = OFFSETOF_IFR_ADDR + OFFSETOF_SIN_ADDR
        end = begin + SIZEOF_IN_ADDR
        ifr_addr = res[begin:end]

        return socket.inet_ntoa(ifr_addr)
    finally:
        sock.close()


if __name__ == '__main__':
    print(get_address(sys.argv[1]))