mp:process-run-function and a blocking foreign call

I am running the Allegro CL 7 Trial Edition on Windows XP, and am
having trouble using the multiprocessing features (via
mp:process-run-function) in conjunction with a blocking foreign
function call.

Background summary: I have defined foreign function calls out to much
of the Windows Smart Card subsystem and can call on these functions
from CL with ease. All these calls work as expected. The function I'm
working with today is SCardGetStatusChange¹, called on in the manner
sketched here.²

As discussed in the final paragraph on the second referenced page,
SCardGetStatusChange is a blocking function that is designed to be run
from a dedicated thread -- one which will presumably then communicate
with or start other threads in reaction to certain events.

I am able to use my wrapped SCardGetStatusChange function just fine
from CL. Running it from the listener, the Allegro CL IDE hangs while
SCardGetStatus change is blocked. That's fine for testing this
specific function, but of course I'd like to actually run the function
in a separate thread.

Today I attempted to run my card monitoring function in a separate
thread using mp:process-run-function, like so:

  (mp:process-run-function "monitor" #'monitor-changes 4000)

The function gets run, but the IDE still hangs while it's running. I
tried starting other threads in parallel as well, doing things like
printing random numbers, and while these other threads do get started
they obviously hang while the card monitoring function is running.

My suspicion is that the foreign function call is to blame for the
thread interference, but I don't know what to investigate next. To
recap, the blocking card monitoring function works properly, but I
can't figure out how to run it in such a way that it doesn't cause
other threads (including those running the IDE) to hang.

Any advice would be appreciated.


Footnotes: 
¹ http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secauthn/security/scardgetstatuschange.asp
² http://www.softpedal.net/CodeHelp/SCardGetStatusChange.htm

-- 
Steven E. Harris