Non-blocking read from sub-process?

[Dan Christian <danchristian65@...>]

I want to run a sub-process that may or may-not generate output.  How
can I make sure I don't block trying to read it.  I'm happy to use
luaposix, but I'm trying not to use a big event framework like lua-ev
(because I'm memory limited and also don't want to cross compile it).

Something like:

f = io.popen(unknown_command, "r")

while im_doing_something_else:
  data = f:read()
  if data:
    print ("Subproc says: ", data)

kill(sub_proc)

Thanks,
Dan