"Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children)."

"Signal(3) - SIGQUIT - quit program"

https://www.freebsd.org/cgi/man.cgi?query=signal&sektion=3&apropos=0&manpath=FreeBSD+11.0-RELEASE+and+Ports

See also https://sites.google.com/ste/sparks31commo/.