module Netsys_pollset_posix:Pollsets for POSIX operating systemssig..end
val poll_based_pollset : unit -> Netsys_pollset.pollsetpoll system
      call. 
      Win32: On Win32 this implementation works, but only for sockets,
      and is not cancellable in multi-threaded programs. (This is a 
      restriction because we have to map it to the select call of the
      WinSock layer.)
val reset : unit -> unitfork to free these descriptors.