[Openmcl-devel] %get-ip-interfaces returning nil

Gary Byers gb at clozure.com
Mon Jul 19 08:07:43 UTC 2010

We used to use an address returned by %GET-IP-INTERFACES as part of random-state
initialization, and my recollection is that that would sometimes fail on Windows
because %GET-IP-INTERFACES sometimes returned NIL.  Unless something's really
badly misconfigured on a Unix box, at least the loopback interface (
would always be there, and you'd generally see other interfaces known to the OS
regardless of whether they were up or down and regardless of whether or not they
had had IP(V4) addresses assigned to them or not; you'd pretty much see what
"ifconfig -a" would show you (and it was likely that the ifconfig program and
%GET-IP-INTERFACES were using the same mechanism to enumerate network interfaces.)

On a Win7 box running the CCL trunk - where the ethernet interface is active and
a WiFi interface is disabled - %GET-IP-INTERFACES returns a list describing the
loopback and ethernet interfaces.  I'm ssh'ed into that box and can't conveniently
try, but I'm fairly sure that if I disabled the ethernet interface it'd return
an empty list (the loopback interface might only be visible if some non-loopback
interface is active in some sense.)  Many laptop WiFi cards will go to sleep (to
save power) if they haven't seen traffic for a while, and that might also cause

If this is true ... well, what %G-I-I does on Windows isn't quite what it does
on Unix systems (it's more like "%GET-ACTIVE-INTERFACES", for a possibly strict
definition of "active.")  There's likely some way of enumerating all interfaces
on Windows (things like network control panels need some way of doing that), but
there may or may not be a public, supported way.  I don't know.

I also don't know how much I trust my theory about WiFi interfaces
turning themselves on and off - the SIO_GET_INTERFACE_LIST ioctl is supposed to
return a list of "configured" interfaces, and I don't know whether a power-saving
mode would involve loss of configuration.

(All of this is a long way of saying "I don't know"; I'm pretty confident in
saying that the CCL code for %GET-IP-INTERFACES hasn't changed in at least a year
or two and that what it returns on Windows depends on what interfaces are "active",
though I don't know precisely how "active" is defined.)

On Sun, 18 Jul 2010, Elliott Slaughter wrote:

> Hi,
> I have kind of a bizarre problem. I noticed recently that ccl::%get-ip-interfaces returns nil for me, but just 3 or 4 months
> ago I used to get a list of my network connections. I am running Windows 7 Pro 64-bit, and have not upgraded or changed my CCL
> installation in any way during that time frame. My CCL version is 1.4-r13122. The only thing I can think of that might changed
> on my system are security updates from Microsoft. I have tried changing network settings, and disabling the firewall, and
> nothing seems to help.
> The reason I ask is because several months ago I ported ccl::%get-ip-interfaces to CFFI. At the time, that implementation
> worked perfectly on 5 different CL implementations. Now it returns nil in all of them in Windows. I have not made any changes
> to the ip-interfaces code during that time. Everything still works fine on other operating systems.
> Any ideas would be appreciated. Thanks.
> --
> Elliott Slaughter
> "Don't worry about what anybody else is going to do. The best way to predict the future is to invent it." - Alan Kay

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