Question

To avoid relying on the wireless tools I want to get the essid directly from the device with ioctl, in C this wouldn’t be a problem, but in Ruby it’s quite different.

The problem is following struct from wireless.h that is used as input/reply of ioctl:

struct  iw_point
{
  void __user *pointer; /* Pointer to the data  (in user space) */
  __u16   length;   /* number of fields or size in bytes */
  __u16   flags;    /* Optional params */
};

The pointer part must be a valid address of a memory area, followed by the length in bytes, followed by a flag field. I tried with Array#pack and the bit-struct gem, but haven’t found a solution yet.

Is there a way to bypass this memory pointer problem?

Answer 1

I finally got it working, the solution was to use the ‘p’ argument of pack in this way:

require "socket"

# Copied from wireless.h
SIOCGIWESSID      = 0x8B1B
IW_ESSID_MAX_SIZE = 32

iwreq = [ "wlan0", " " * IW_ESSID_MAX_SIZE, IW_ESSID_MAX_SIZE, 0 ].pack("a16pII")

sock = Socket.new(Socket::AF_INET, Socket::SOCK_DGRAM, 0)

sock.ioctl(SIOCGIWESSID, iwreq)

interface, essid, len, flags = iwreq.unpack("a16pII")

puts essid