How do I programmatically detect if the Ethernet cable has been unplugged?  I tried the ioctl SIOCGIFFLAGS command to 
look at the IFF_UP and IFF_RUNNING flags, but these are always set.  ifconfig seems to know because when the cable is 
plugged in, it displays:

  status: active

Since the source code is nearly impossible to access, I can't see how it knows.  Does anyone know the magic function to 
determine this?