Getting essid via ioctl

Ruby
1

Hello.

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?

Wbr

Christoph

2

Christoph K. wrote:

__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?

Christoph,

How are you calling ioctl() ? If you’re using FFI, then googling for
FFI::Pointer should help. If you’re calling it in some other way and are
trying to access the binary data in ruby, does the P specifier in
pack/unpack work?

3

Hey Joel,

thanks for both hints - FFI looks interesting but I don’t want to wrap a
library.
Funny that I missed the ‘p’ of pack - it’s working now:

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

Now I can check if I can fetch the max. link quality via ioctl too.

Wbr, Christoph

---- On Tue, 22 Sep 2009 16:58:46 -0700 Joel VanderWerf
[email protected] wrote ----

Christoph K. wrote:

Hello.

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?

Christoph,

How are you calling ioctl() ? If you’re using FFI, then googling for
FFI::Pointer should help. If you’re calling it in some other way and
are
trying to access the binary data in ruby, does the P specifier in
pack/unpack work?

–
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407