Meaning and typical use of -3 signal in kill Post: 302446802

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Top Forums UNIX for Dummies Questions & Answers Meaning and typical use of -3 signal in kill Post 302446802 by methyl on Thursday 19th of August 2010 07:02:13 PM

08-19-2010

Registered User

Okay here we go. The long explanation. Other correspondants may wish to add more.

When you issue "kill -n" the value of "n" is a numeric representation of the "signal". The list of signal numbers and their meaning for your Operating System is always available ... but we don't know which O/S you have.

The "quit" control code (in you case ctrl \) can be typed at a keyboard to deliberately crash a foreground process which is running on your terminal at that time and then produce a core dump. This is why many administrators disable this control code in user sessions. Unless you are in a development environment the core dump is of no value.
When typed at a keyboard the "quit" control code has no effect whatsoever on background processes.

The "kill -3 <pid>" unix command sends the signal to the background process with Process Identity <pid> (or for that matter to a process running in foreground on a terminal other than your own) asking that process to stop immediately and dump core. Unless you are in a development environment the core dump has no value.
Be aware that a process can be written to ignore any signal (except signal 9).

Though stating the obvious you cannot issue a "kill" command on your own terminal to your own foreground process because you do not have a command prompt at the time.

Most administrators would actually issue "kill -15" (hangup) to a broken background process unless of course the process is a database application which requires a specific different signal. The hangup signal simulates pulling out the cable on a serial terminal and most well-written applications will close tidily.

Most administrators (and users) would issue ctrl/C (interrupt) to a foreground process to ask it to stop. Administrators often disable this key at unix level where it needed by the application or when they don't want users crashing applications and being presented with a unix command prompt.
The ctrl/C keystroke has a similar function in Microsoft MSDOS and just happened to like unix and CP/M when MSDOS first came out.

The unix shell "trap" command can be used to intercept signals (excluding signal 9) and this can be useful for invoking a cleanup procedure.

Beware: There is rarely ever a reason to issue "kill -9" to a process and it should never ever be issued to a database server or client unless perhaps you are already in dire straits and cannot shut a server down just prior to restoring your backup.
You will find in you research that some critical unix processes (like "inetd") respond to specific signals and restart themselves cleanly after re-reading parameter files. This convenience is rarely found in commercial applications.

Footnote: Some unix kernels have a very limited default maximum process number and can recycle Process IDs rather too quickly. It may be that this is what is happening to you? A commercial unix will need a kernel build to match the expected load and this will usually cause a substantial increase in the maximum value of a Process ID and therefore less frequent recycling of Process IDs.

Last edited by methyl; 08-19-2010 at 08:13 PM.. Reason: typos,spellin

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ansi_ctrlu

term::ansi::ctrl::unix(n)					 Terminal control					 term::ansi::ctrl::unix(n)

__________________________________________________________________________________________________________________________________________________

NAME

       term::ansi::ctrl::unix - Control operations and queries

SYNOPSIS

       package require Tcl  8.4

       package require term::ansi::ctrl::unix  ?0.1?

       ::term::ansi::ctrl::unix::import ?ns? ?arg...?

       ::term::ansi::ctrl::unix::raw

       ::term::ansi::ctrl::unix::raw

       ::term::ansi::ctrl::unix::columns

       ::term::ansi::ctrl::unix::rows

_________________________________________________________________

DESCRIPTION

       WARNING:  This  package	is  unix-specific  and depends on the availability of two unix system commands for terminal control, i.e. stty and
       tput, both of which have to be found in the $PATH. If any of these two commands is missing the loading of the package will fail.

       The package provides commands to switch the standard input of the current process between raw and cooked input modes, and to query the size
       of terminals, i.e. the available number of columns and lines.

API

   INTROSPECTION
       ::term::ansi::ctrl::unix::import ?ns? ?arg...?
	      This  command imports some or all attribute commands into the namespace ns. This is by default the namespace ctrl. Note that this is
	      relative namespace name, placing the imported command into a child of the current namespace. By default all commands  are  imported,
	      this can howver be restricted by listing the names of the wanted commands after the namespace argument.

   OPERATIONS
       ::term::ansi::ctrl::unix::raw
	      This command switches the standard input of the current process to raw input mode. This means that from then on all characters typed
	      by the user are immediately reported to the application instead of waiting in the OS buffer until the Enter/Return key is received.

       ::term::ansi::ctrl::unix::raw
	      This command switches the standard input of the current process to cooked input mode. This means that from then  on  all	characters
	      typed by the user are kept in OS buffers for editing until the Enter/Return key is received.

       ::term::ansi::ctrl::unix::columns
	      This command queries the terminal connected to the standard input for the number of columns available for display.

       ::term::ansi::ctrl::unix::rows
	      This command queries the terminal connected to the standard input for the number of rows (aka lines) available for display.

BUGS, IDEAS, FEEDBACK
       This  document, and the package it describes, will undoubtedly contain bugs and other problems.	Please report such in the category term of
       the Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883].  Please also report any ideas for enhancements you  may  have  for
       either package and/or documentation.

KEYWORDS

       ansi, columns, control, cooked, input mode, lines, raw, rows, terminal

CATEGORY

       Terminal control

COPYRIGHT

       Copyright (c) 2006 Andreas Kupries <andreas_kupries@users.sourceforge.net>

term									0.1						 term::ansi::ctrl::unix(n)