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Signals are messages passed by system to running processes.
Signal management functions
| sigaction | detailed signal management | 
| sigaltstack | set/get signal alternate stack context | 
| sigignore sigpause | simplified signal management | 
| sighold sigrelse | simplified signal management | 
| sigset signal | simplified signal management | 
| sigpending | examine blocked and pending signals | 
| sigprocmask | change or examine signal mask | 
| sigsuspend | install a signal mask and suspend process | 
| sigsend sigsendset | send a signal to a process or group of processes |