Usage of UNIX signals

Several UNIX signals are currently used as defined in $DD_INC/dd_signal_def.h.

• BROADCAST_SIGNAL (default: SIGUSR2)
  This signal is used to broadcast events to the processes (that have their broadcast flag set to DD_BRC_SIGNAL).
• DD_USER_INTERRUPT (default SIGINT):
  This signal can be used if the user wants to interrupt a consumer process sleeping on a semaphore by hand. If DD_USER_INTERRUPT is set to NULL, then no signal will be linked.
• DD_SHUTDOWN_INTERRUPT (default: SIGHUP)
  When the garbage demon notices that the DD system has no more masters attached, it removes it. Before doing so, he sends the DD_USER_INTERRUPT to the remaining process (given he has the permission to do so) before removing the shared memory segments and semaphores.
  If the garbage demon receives a SIGHUP, he will shut down the DD system (regardless whether there are still master processes attached to it or not).