Signaling processes.


Signals

Signaling is the ability to communicate with a running process.
Generated by user from command line.
Using [ctrl] characters such as [ctrl]c, [ctrl]z, etc.
Using the kill command.
Generated by other processes.

man -s 7 signal

General actions by receiving process. 
Term - terminate process.
  [ctrl]c - interrupt - generates a Term type signal.

Ign - ignore signal.
  Child process has stopped or terminated.
  Socket needs attention.   
 
Core - terminate with a core dump. (debugging)
  See : man core

Stop - Stop - suspend process. 
  [ctr]z in the shell generates a Stop type signal.

Cont - continue a stopped process.
  fg and bg generate a Cont type signal.

Not all signals are available to user at the command prompt.

Many, but not all, signals can be trapped.

Signal value between 1 and 64, about 31 defined.

Common signals

SIGHUP 	1	kill -HUP	Hang up, trappable.
  If trapped, can signal program to re-read its configuration files.

SIGINT 	2	[ctrl]c	Interrupt, trappable.
  If trapped, can allow program to try to shutdown gracefully.

SIGQUIT	3	[ctrl]\	Interrupt (core dump), trappable.
  More useful for compiled programs.

SIGKILL	9	kill -KILL	Untrappable, kill (terminate).
  For when things have gone very wrong.
  
SIGTERM	15	kill	Standard termination, trappable.
  kill with no signal option, generates a 15.   

SIGCONT	18	bg	Restart a paused process, trappable.
  fg generates a CONT.

SIGSTOP	19	kill -STOP  Untrappable, suspend execution

SIGTSTP	20	[ctrl]z	Suspend execution, trappable. 
  Use care. If you trap [ctrl]z, 
    you will have to log into another term to kill/suspend process.


Because the numeric value can vary for different versions of Linux/Unix,
It is considered better form to use the symbolic aliases when handling the
signals.

The include.h that has actual definitions for our 64-bit systems :

  /usr/include/x86_64-linux-gnu/bits/signum.h