Interrupt handlers let tasks wait for external events (system
  signals), and allow the declaration of handler functions for these
  events.

  You can use classes derived from the Interrupt_handler class to
  overload the interrupt() function. When the signal is raised, the task
  system immediately calls the interrupt() function. The task system
  then schedules its own internal interrupt alerter for execution.
  Control returns to the task (if any) that was running when the signal
  was raised. When control returns to the scheduler, the interrupt
  alerter runs and schedules for execution those tasks that were waiting
  for the interrupt handler.

  If the run chain is empty, the scheduler does not cause the program
  to exit if there are any interrupt handlers that have been created but
  not yet destroyed.