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This chapter is from the book

Using Signals

Objective:

Clearing frozen processes.

Solaris supports the concept of sending software signals to a process. These signals are ways for other processes to interact with a running process outside the context of the hardware. The kill command is used to send a signal to a process. System administrators most often use the signals SIGHUP, SIGKILL, SIGSTOP, and SIGTERM. The SIGHUP signal is used by some utilities as a way to notify the process to do something, such as re-read its configuration file. The SIGHUP signal is also sent to a process if the remote connection is lost or hangs up. The SIGKILL signal is used to abort a process, and the SIGSTOP signal is used to pause a process. The SIGTERM signal is the default signal sent to processes by commands such as kill and pkill when no signal is specified. Table 5.12 describes the most common signals an administrator is likely to use.

Table 5.12. Signals Available Under Solaris

Signal

Number

Description

SIGHUP

1

Hangup. Usually means that the controlling terminal has been disconnected.

SIGINT

2

Interrupt. The user can generate this signal by pressing Ctrl+C or Delete.

SIGQUIT

3

Quits the process and produces a core dump.

SIGILL

4

Illegal instruction.

SIGTRAP

5

Trace or breakpoint trap.

SIGABRT

6

Abort.

SIGEMT

7

Emulation trap.

SIGFPE

8

Arithmetic exception. Informs a process of a floating-point error.

SIGKILL

9

Killed. Forces the process to terminate. This is a sure kill.

SIGBUS

10

Bus error.

SIGSEGV

11

Segmentation fault.

SIGSYS

12

Bad system call.

SIGPIPE

13

Broken pipe.

SIGALRM

14

Alarm clock.

SIGTERM

15

Terminated. A gentle kill that gives processes a chance to clean up.

SIGUSR1

16

User signal 1.

SIGUSR2

17

User signal 2.

SIGCHLD

18

Child status changed.

SIGPWR

19

Power fail or restart.

SIGWINCH

20

Window size change.

SIGURG

21

Urgent socket condition.

SIGPOLL

22

Pollable event.

SIGSTOP

23

Stopped (signal). Pauses a process.

SIGTSTP

24

Stopped (user).

SIGCONT

25

Continued.

SIGTTIN

26

Stopped (tty input).

SIGTTOU

27

Stopped (tty output).

SIGVTALRM

28

Virtual timer expired.

SIGPROF

29

Profiling timer expired.

SIGXCPU

30

CPU time limit exceeded.

SIGXFSZ

31

File size limit exceeded.

SIGWAITING

32

Concurrency signal reserved by threads library.

SIGLWP

33

Inter-LWP signal reserved by threads library.

SIGFREEZE

34

Checkpoint freeze.

SIGTHAW

35

Checkpoint thaw.

SIGCANCEL

36

Cancellation signal reserved by the threads library.

In addition, you can write a signal handler, or trap, in a program to respond to a signal being sent. For example, many system programs, such as the name server daemon, respond to the SIGHUP signal by re-reading their configuration files. This signal can then be used to update the process while running, without having to terminate and restart the process. Signal handlers cannot be installed for SIGSTOP (23) or SIGKILL (9). Because the process cannot install a signal handler for signal 9, an otherwise well-behaved process may leave temporary files around or not be able to finish out critical operations that it is in the middle of. Thus, kill -9 invites corruption of application data files and should only be used as a last resort.

Here's an example of how to trap a signal in a script:

# trap '/bin/rm tmp$$;exit 1' 1 2 3 15

As the name suggests, trap traps system interrupt until some command can be executed. The previous example traps the signals 1, 2, 3, and 15, and executes the /bin/rm tmp$$ command before exiting the program. The example deletes all tmp files even if the program terminates abnormally.

The kill command sends a terminate signal (signal 15) to the process, and the process is terminated. Signal 15, which is the default when no options are used with the kill command, is a gentle kill that allows a process to perform cleanup work before terminating. Signal 9, on the other hand, is called a sure, unconditional kill because it cannot be caught or ignored by a process. If the process is still around after a kill -9, either it is hung up in the Unix kernel, waiting for an event such as disk I/O to complete, or you are not the owner of the process.

The kill command is routinely used to send signals to a process. You can kill any process you own, and the superuser can kill all processes in the system except those that have process IDs 0, 1, 2, 3, and 4. The kill command is poorly named because not every signal sent by it is used to kill a process. This command gets its name from its most common use—terminating a process with the kill -15 signal.

You can send a signal to a process you own with the kill command. Many signals are available, as listed in Table 5.12. To send a signal to a process, first use the ps command to find the process ID (PID) number. For example, type ps -ef to list all processes and find the PID of the process you want to terminate:

# ps -ef

UID   PID  PPID  C    STIME  TTY  TIME  CMD
root    0     0  0   Nov 27  ?    0:01  sched
root    1     0  0   Nov 27  ?    0:01  /etc/init -
root    2     0  0   Nov 27  ?    0:00  pageout
root    3     0  0   Nov 27  ?   12:52  fsflush
root  101     1  0   Nov 27  ?    0:00  /usr/sbin/in.routed -q
root  298     1  0   Nov 27  ?    0:00  /usr/lib/saf/sac -t 300
root  111     1  0   Nov 27  ?    0:02  /usr/sbin/rpcbind
root  164     1  0   Nov 27  ?    0:01  /usr/sbin/syslogd -n -z 12
root  160     1  0   Nov 27  ?    0:01  /usr/lib/autofs/automountd
.
.
.
root 5497   433  1 09:58:02  pts/4  0:00  script psef

To kill the process with a PID number of 5497, type this:

# kill 5497

Another way to kill a process is to use the pkill command. pkill functions identically to pgrep, which was described earlier, except that instead of displaying information about each process, the process is terminated. A signal name or number may be specified as the first command-line option to pkill. The value for the signal can be any value described in Table 5.12. For example, to kill the process named psef with a SIGKILL signal, issue the following command:

# pkill -9 psef

In addition, the Desktop Process Manager, which was described earlier, can be used to kill processes. In the Process Manager window, highlight the process that you want to terminate, click Process from the toolbar at the top of the window, and then select Kill from the pull-down menu, as shown in Figure 5.9.

Figure 5.9

Figure 5.9 Killing processes.

The equivalent Unix command used by the Process Manager to terminate a process is shown here:

# kill -9 <PID>

<PID> is the process ID of the selected process.

The preap command forces the killing of a defunct process, known as a zombie. In previous Solaris releases, zombie processes that could not be killed off remained until the next system reboot. Defunct processes do not normally impact system operation; however, they do consume a small amount of system memory. See the preap manual page for further details of this command.

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