Discovering System Processes Part II

11/22/2000

In last week's article, we took a look at what processes are and how to view the running processes on your FreeBSD system. In this week's article, I'd like to look at how processes talk to each other, how you can talk to your processes, and why you might want to do so.

When processes talk to each other, it is called interprocess communication. Processes aren't allowed to say just anything to each other; instead, your FreeBSD system comes with 31 possible predefined messages. These messages are called signals; you can view these signals with this command:

kill -l

HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM URG STOP TSTP CONT CHLD TTIN TTOU IO XCPU XFSZ VTALRM PROF WINCH INFO USR1 USR2

Each signal has a numerical equivalent, and the signals are listed in numerical order; therefore, HUP=1, INT=2, etc. Processes are allowed to send these signals to each other; users are also allowed to send these signals to processes.

To find out what each of these signals does, read sigaction(2) by typing man 2 sigaction or signal(3) by typing man 3 signal. The following chart summarizes each of the 31 signals, their resultant actions, and a brief description of each. Although users can send any of these signals, I've included an asterisk next to the signals most commonly used by users.

Signal Name	#	Default Action	Description
*HUP	1	terminate process	terminal line hangup
*INT	2	terminate process	interrupt program
*QUIT	3	create core image	quit program
ILL	4	create core image	illegal instruction
TRAP	5	create core image	trace trap
ABRT	6	create core image	abort(3) call
EMT	7	create core image	emulate instruction executed
FPE	8	create core image	floating-point exception
*KILL	9	terminate process	kill program
BUS	10	create core image	bus error
SEGV	11	create core image	segmentation violation
SYS	12	create core image	non-existent system call
PIPE	13	terminate process	write on a pipe with no reader
ALRM	14	terminate process	real-time timer expired
*TERM	15	terminate process	software termination signal
URG	16	discard signal	urgent condition
*STOP	17	stop process	stop (cannot be ignored)
*TSTP	18	stop process	stop keyboard signal
CONT	19	discard signal	continue after stop
CHLD	20	discard signal	child status has changed
TTIN	21	stop process	background read attempted
TTOU	22	stop process	background write attempted
IO	23	discard signal	I/O is possible
XCPU	24	terminate process	CPU time limit exceeded
XFSZ	25	terminate process	file size limit exceeded
VTALRM	26	terminate process	virtual time alarm
PROF	27	terminate process	profiling timer alarm
WINCH	28	discard signal	Window size change
INFO	29	discard signal	status request from keyboard
USR1	30	terminate process	User defined signal 1
USR2	31	terminate process	User defined signal 2

Some signals are used so often by users that they have been mapped to control characters. To view your control character mappings, look at the last four lines of the output of:

stty -e

discard dsusp   eof     eol     eol2    erase   intr    kill    lnext   
^O      ^Y      ^D      <undef> <undef> ^H      ^C      ^U      ^V      
min     quit    reprint start   status  stop    susp    time    werase  
1       ^\      ^R      ^Q      ^T      ^S      ^Z      0       ^W

The ^ symbol means to press your CTRL key at the same time as the letter that follows it. Note that three signals have been mapped as control characters:

• ^C has been mapped to the INT signal, or signal 2
• ^\ has been mapped to the QUIT signal, or signal 3
• ^Z has been mapped to the TSTP signal, or signal 18 (even though it's called susp here)

Don't confuse the word kill in this output with the KILL signal (signal 9). ^U will erase an entire line, not send a signal 9. To prove this to yourself, type a long string of words at your prompt, then do a ^U.

So, how do you send signals that haven't been mapped to control characters? Use the kill command.

whatis kill
kill(1)   - terminate or signal a process
kill(2)   - send signal to a process

There's a couple of different ways to use kill; if you simply type:

kill PID

the default TERM signal will be sent to the process with the PID that you indicated. If you wish to change the type of signal to send, you can specify the signal either by its name or by its number like so:

kill -signal_name PID

or

kill -signal_number PID

So:

kill PID

kill -TERM PID

and

kill -15 PID

are equivalent commands. Remember that Unix is case-sensitive; if you type:

kill -term PID

you'll receive the following error message:

term: Unknown signal; kill -l lists all signals.

So, now that we've seen the 31 possible messages and how to send them, let's look at examples of reasons you would want to send a signal to a process. Often when you're working your way through the FreeBSD handbook or a tutorial, you'll learn how to make changes to a configuration file and will then be prompted to send a HUP signal. Most processes only read their configuration file when they first start up; the HUP signal tells a process to stop running; when that process restarts, it will reread its configuration file and your changes to that file will take effect. Also, every time you log out of a terminal, a HUP signal is sent to all processes running on that terminal; this means that all processes that were running on that terminal will be stopped.

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