1. A discrepancy between a computed, observed, or measured
	value or condition and the true, specified, or theoretically
	correct value or condition.

	2. <programming> A mental mistake made by a programmer that
	may result in a program {fault}.

	3. (verb) What a program does when it stops as result of a
	programming error.

	(2000-03-28)

1. {application program}.

	2. {function application}.

<operating system> /deb'ee`n/, *not* /deeb'ee`n/ The
	non-profit volunteer organisation responsible for Debian
	{GNU}/{Linux} and Debian {GNU}/{Hurd}.  Debian's {Linux}
	distribution is dedicated to free and {open source} software;
	the main goal of the distribution is to ensure that one can
	download and install a fully-functional {operating system}
	that is completely adherent to the Debian Free Software
	Guidelines (DFSG).

	Debian was begun in August 1993 by Ian Murdock, and was
	sponsored by the {Free Software Foundation} from November 1994
	to November 1995.  The name Debian is a contraction of DEB(ra)
	and IAN Murdock.

	Debian's packaging system (dpkg) is similar to other popular
	packaging systems like {RPM}.  There are over 2200 packages of
	precompiled software available in the main (free) section of
	the Debian 2.1 distribution alone -- this is what sets Debian
	apart from many other Linux distributions.  The high quality
	and huge number of official packages (most Debian systems'
	/usr/local/ remains empty -- almost everything most Linux
	users want is officially packaged) are what draw many people
	to use Debian.

	Another unique aspect to the Debian project is the open
	development; pre-releases are made available from Day 1 and if
	anyone wishes to become a Debian developer, all that is needed
	is proof of identification and a signed {PGP} or {GPG} key.
	There are over 400 Debian developers all around the world --
	many developers have never met face-to-face, and most
	development talks take place on the many {mailing lists} and
	the {IRC} network.

	{Home (http://www.debian.org/)}.

	{Debian Linux archives (ftp://ftp.debian.org/debian)}.

	(1999-02-23)

<operating system> ("Linus Unix") /li'nuks/ (but see below)
	An implementation of the {Unix} {kernel} originally written
	from scratch with no proprietary code.

	The kernel runs on {Intel} and {Alpha} hardware in the general
	release, with {SPARC}, {PowerPC}, {MIPS}, {ARM}, {Amiga},
	{Atari}, and {SGI} in active development.  The SPARC, PowerPC,
	ARM, {PowerMAC} - {OSF}, and 68k ports all support {shells},
	{X} and {networking}.  The Intel and SPARC versions have
	reliable {symmetric multiprocessing}.

	Work on the kernel is coordinated by Linus Torvalds, who holds
	the copyright on a large part of it.  The rest of the
	copyright is held by a large number of other contributors (or
	their employers).  Regardless of the copyright ownerships, the
	kernel as a whole is available under the {GNU} {General Public
	License}.  The GNU project supports Linux as its kernel until
	the research {Hurd} kernel is completed.

	This kernel would be no use without {application programs}.
	The GNU project has provided large numbers of quality tools,
	and together with other {public domain} software it is a rich
	Unix environment.  A compilation of the Linux kernel and these
	tools is known as a Linux distribution.  Compatibility modules
	and/or {emulators} exist for dozens of other computing
	environments.

	The kernel version numbers are significant: the odd numbered
	series (e.g. 1.3.xx) is the development (or beta) kernel which
	evolves very quickly.  Stable (or release) kernels have even
	major version numbers (e.g. 1.2.xx).

	There is a lot of commercial support for and use of Linux,
	both by hardware companies such as {Digital}, {IBM}, and
	{Apple} and numerous smaller network and integration
	specialists.  There are many commercially supported
	distributions which are generally entirely under the GPL.  At
	least one distribution vendor guarantees {Posix} compliance.
	Linux is particularly popular for {Internet Service
	Providers}, and there are ports to both parallel
	supercomputers and {embedded} {microcontrollers}.  {Debian} is
	one popular {open source} distribution.

	The pronunciation of "Linux" has been a matter of much debate.
	Many, including Torvalds, insist on the short I pronunciation
	/li'nuks/ because "Linus" has an /ee/ sound in Swedish
	(Linus's family is part of Finland's 6% ethnic-Swedish
	minority) and Linus considers English short /i/ to be closer
	to /ee/ than English long /i:/ dipthong.  This is consistent
	with the short I in words like "linen".  This doesn't stop
	others demanding a long I /li:'nuks/ following the english
	pronunciation of "Linus" and "minus".  Others say /li'niks/
	following {Minix}, which Torvalds was working on before Linux.

	{More on pronunciation
	(http://www.foldoc.org/pub/linux-pronunciation)}.

	{LinuxHQ (http://www.linuxhq.com/)}.  {slashdot
	(http://slashdot.org/)}.  {freshmeat (http://freshmeat.net/)}.
	{Woven Goods (http://www.fokus.gmd.de/linux/)}.  {Linux
	Gazette (http://www.ssc.com/lg)}.

	{funet Linux Archive (ftp://ftp.funet.fi/pub/Linux)}, {US
	mirror (ftp://sunsite.unc.edu/pub/Linux/)}, {UK Mirror
	(ftp://sunsite.doc.ic.ac.uk/packages/Linux/)}.

	(2000-06-09)

1. See {multithreading}.

	2. See {threaded code}.

	3. {topic thread}.

	[{Jargon File}]

<operating system> To move a program from fast-access memory
	to a slow-access memory ("swap out"), or vice versa ("swap
	in").  The term often refers specifically to the use of a
	{hard disk} (or a {swap file}) as {virtual memory} or "swap
	space".

	When a program is to be executed, possibly as determined by a
	{scheduler}, it is swapped into {core} for processing; when it
	can no longer continue executing for some reason, or the
	scheduler decides its {time slice} has expired, it is swapped
	out again.

	This contrasts with "paging" systems in which only parts of a
	program's memory is transfered.

	[{Jargon File}]

	(1996-11-22)

<character> The space character, {ASCII} 32.

	See {octal forty}.

<language, architecture> (JVM) A specification for software
	which interprets {Java} programs that have been compiled into
	{byte-codes}, and usually stored in a ".class" file.  The JVM
	{instruction set} is {stack}-oriented, with variable
	instruction length.  Unlike some other instruction sets, the
	JVM's supports {object-oriented} programming directly by
	including instructions for object {method} invocation (similar
	to {subroutine} call in other instruction sets).

	The JVM itself is written in {C} and so can be {ported} to run
	on most {platforms}.  It needs {thread} support and {I/O} (for
	{dynamic class loading}).  The Java byte-code is independent
	of the platform.

	There are also some hardware implementations of the JVM.

	{Specification
	(http://www.javasoft.com/docs/books/vmspec/html/VMSpecTOC.doc.html)}.

	{Sun's Java chip
	(http://news.com/News/Item/0,4,9328,00.html)}.

	[Documentation?  Versions?]

	(2000-01-03)

1. <programming> An area of memory used for {dynamic memory
	allocation} where blocks of memory are allocated and freed in
	an arbitrary order and the pattern of allocation and size of
	blocks is not known until {run time}.  Typically, a program
	has one heap which it may use for several different purposes.

	Heap is required by languages in which functions can return
	arbitrary data structures or functions with {free variables}
	(see {closure}).  In {C} functions {malloc} and {free} provide
	access to the heap.

	Contrast {stack}.  See also {dangling pointer}.

	2. <programming> A data structure with its elements partially
	ordered (sorted) such that finding either the minimum or the
	maximum (but not both) of the elements is computationally
	inexpensive (independent of the number of elements), while
	both adding a new item and finding each subsequent
	smallest/largest element can be done in O(log n) time, where n
	is the number of elements.

	Formally, a heap is a {binary tree} with a key in each {node},
	such that all the {leaves} of the tree are on two adjacent
	levels; all leaves on the lowest level occur to the left and
	all levels, except possibly the lowest, are filled; and the
	key in the {root} is at least as large as the keys in its
	children (if any), and the left and right subtrees (if they
	exist) are again heaps.

	Note that the last condition assumes that the goal is finding
	the minimum quickly.

	Heaps are often implemented as one-dimensional {arrays}.
	Still assuming that the goal is finding the minimum quickly
	the {invariant} is

	   heap[i] <= heap[2*i] and heap[i] <= heap[2*i+1] for all i,

	where heap[i] denotes the i-th element, heap[1] being the
	first.  Heaps can be used to implement {priority queues} or in
	{sort} algorithms.

	(1996-02-26)

1. The {supervisor} program or {operating system} on a
	computer.

	2. The entire computer system, including input/output devices,
	the {supervisor} program or {operating system} and possibly
	other {software}.

	3. Any large program.

	4. Any method or {algorithm}.

	[{Jargon File}]

<jargon> The opposite of {logical} in its jargon sense.
	Compare {real}, {virtual}, and {transparent}.

	It is said that what you can touch and see is real; what you
	can see but not touch is virtual; what you can touch but not
	see is transparent; and what you can neither touch nor see is
	probably imaginary.

	(2001-10-26)

<storage> These days, usually used synonymously with {Random
	Access Memory} or {Read-Only Memory}, but in the general sense
	it can be any device that can hold {data} in
	{machine-readable} format.

	(1996-05-25)

<programming> (See below for synonyms) A data structure for
	storing items which are to be accessed in last-in first-out
	order.

	The operations on a stack are to create a new stack, to "push"
	a new item onto the top of a stack and to "pop" the top item
	off.  Error conditions are raised by attempts to pop an empty
	stack or to push an item onto a stack which has no room for
	further items (because of its implementation).

	Most processors include support for stacks in their
	{instruction set architectures}.  Perhaps the most common use
	of stacks is to store {subroutine} arguments and return
	addresses.  This is usually supported at the {machine code}
	level either directly by "jump to subroutine" and "return from
	subroutine" instructions or by {auto-increment} and
	auto-decrement {addressing modes}, or both.  These allow a
	contiguous area of memory to be set aside for use as a stack
	and use either a special-purpose {register} or a general
	purpose register, chosen by the user, as a {stack pointer}.

	The use of a stack allows subroutines to be {recursive} since
	each call can have its own calling context, represented by a
	stack frame or {activation record}.  There are many other
	uses.  The programming language {Forth} uses a data stack in
	place of variables when possible.

	Although a stack may be considered an {object} by users,
	implementations of the object and its access details differ.
	For example, a stack may be either ascending (top of stack is
	at highest address) or descending.  It may also be "full" (the
	stack pointer points at the top of stack) or "empty" (the
	stack pointer points just past the top of stack, where the
	next element would be pushed).  The full/empty terminology is
	used in the {Acorn Risc Machine} and possibly elsewhere.

	In a list-based or {functional language}, a stack might be
	implemented as a {linked list} where a new stack is an empty
	list, push adds a new element to the head of the list and pop
	splits the list into its head (the popped element) and tail
	(the stack in its modified form).

	At {MIT}, {pdl} used to be a more common synonym for stack,
	and this may still be true.  {Knuth} ("The Art of Computer
	Programming", second edition, vol. 1, p. 236) says:

	  Many people who realised the importance of stacks and queues
	  independently have given other names to these structures:
	  stacks have been called push-down lists, reversion storages,
	  cellars, dumps, nesting stores, piles, last-in first-out
	  ("LIFO") lists, and even yo-yo lists!

	[{Jargon File}]

	(1995-04-10)

<programming> An unwanted and unintended property of a program
	or piece of hardware, especially one that causes it to
	malfunction.  Antonym of {feature}.  E.g. "There's a bug in
	the editor: it writes things out backward."  The
	identification and removal of bugs in a program is called
	"{debugging}".

	Admiral {Grace Hopper} (an early computing pioneer better
	known for inventing {COBOL}) liked to tell a story in which a
	technician solved a {glitch} in the {Harvard Mark II machine}
	by pulling an actual insect out from between the contacts of
	one of its relays, and she subsequently promulgated {bug} in
	its hackish sense as a joke about the incident (though, as she
	was careful to admit, she was not there when it happened).
	For many years the logbook associated with the incident and
	the actual bug in question (a moth) sat in a display case at
	the Naval Surface Warfare Center (NSWC).  The entire story,
	with a picture of the logbook and the moth taped into it, is
	recorded in the "Annals of the History of Computing", Vol. 3,
	No. 3 (July 1981), pp. 285--286.

	The text of the log entry (from September 9, 1947), reads
	"1545 Relay #70 Panel F (moth) in relay.  First actual case of
	bug being found".  This wording establishes that the term was
	already in use at the time in its current specific sense - and
	Hopper herself reports that the term "bug" was regularly
	applied to problems in radar electronics during WWII.

	Indeed, the use of "bug" to mean an industrial defect was
	already established in Thomas Edison's time, and a more
	specific and rather modern use can be found in an electrical
	handbook from 1896 ("Hawkin's New Catechism of Electricity",
	Theo. Audel & Co.)  which says: "The term "bug" is used to a
	limited extent to designate any fault or trouble in the
	connections or working of electric apparatus."  It further
	notes that the term is "said to have originated in
	{quadruplex} telegraphy and have been transferred to all
	electric apparatus."

	The latter observation may explain a common folk etymology of
	the term; that it came from telephone company usage, in which
	"bugs in a telephone cable" were blamed for noisy lines.
	Though this derivation seems to be mistaken, it may well be a
	distorted memory of a joke first current among *telegraph*
	operators more than a century ago!

	Actually, use of "bug" in the general sense of a disruptive
	event goes back to Shakespeare!  In the first edition of
	Samuel Johnson's dictionary one meaning of "bug" is "A
	frightful object; a walking spectre"; this is traced to
	"bugbear", a Welsh term for a variety of mythological monster
	which (to complete the circle) has recently been reintroduced
	into the popular lexicon through fantasy {role-playing games}.

	In any case, in jargon the word almost never refers to
	insects.  Here is a plausible conversation that never actually
	happened:

	"There is a bug in this ant farm!"

	"What do you mean?  I don't see any ants in it."

	"That's the bug."

	[There has been a widespread myth that the original bug was
	moved to the Smithsonian, and an earlier version of this entry
	so asserted.  A correspondent who thought to check discovered
	that the bug was not there.  While investigating this in late
	1990, your editor discovered that the NSWC still had the bug,
	but had unsuccessfully tried to get the Smithsonian to accept
	it - and that the present curator of their History of
	American Technology Museum didn't know this and agreed that it
	would make a worthwhile exhibit.  It was moved to the
	Smithsonian in mid-1991, but due to space and money
	constraints has not yet been exhibited.  Thus, the process of
	investigating the original-computer-bug bug fixed it in an
	entirely unexpected way, by making the myth true!  - ESR]

	[{Jargon File}]

	(1999-06-29)

<programming> A computer system or process that requests a
	service of another computer system or process (a "{server}")
	using some kind of {protocol} and accepts the server's
	responses.  A client is part of a {client-server} software
	architecture.

	For example, a {workstation} requesting the contents of a file
	from a {file server} is a client of the file server.

	(1997-10-27)

1. A program which provides some service to other ({client})
	programs.  The connection between client and server is
	normally by means of {message passing}, often over a
	{network}, and uses some {protocol} to encode the client's
	requests and the server's responses.  The server may run
	continuously (as a {daemon}), waiting for requests to arrive
	or it may be invoked by some higher level daemon which
	controls a number of specific servers ({inetd} on {Unix}).

	There are many servers associated with the {Internet}, such as
	those for {HTTP}, {Network File System}, {Network Information
	Service} (NIS), {Domain Name System} (DNS), {FTP}, {news},
	{finger}, {Network Time Protocol}.  On Unix, a long list can
	be found in /etc/services or in the {NIS} database "services".
	See {client-server}.

	2. A computer which provides some service for other computers
	connected to it via a network.  The most common example is a
	{file server} which has a local disk and services requests
	from remote clients to read and write files on that disk,
	often using {Sun}'s {Network File System} (NFS) {protocol} or
	{Novell Netware} on {PCs}.  Another common example is a {web
	server}.

	[{Jargon File}]

	(2003-12-29)