Signals

Processes can be sent special signals to indicate various error conditions.

Example signals:
STOP - pauses execution of the process
KILL - makes the process immediately stop executing, without a chance to cleanup
XCPU - the system sends this if a limit on CPU time has been set and exceeded
PIPE - the process was writing to a pipe and the receiving process has terminated.
USR1 - a "spare" signal that a program may use to perform some action (eg re-load a config file)
WINCH - the terminal window was resized