I haven't used boost's locking mechanisms yet, but POSIX condvars
don't protect against spurious wakes.  Thus,

[snip]

>   //wait for an answer..
>   bool ready = _cond.timed_wait(lock, xt, boost::lambda::var(_data_ready));

you probably want this in a loop,

   while(!_data_ready) {
       _cond.timed_wait(... _data_ready);
   }