<theory, programming> (Or "data type") A set of values from
	which a {variable}, {constant}, {function}, or other
	{expression} may take its value.  A type is a classification
	of data that tells the {compiler} or {interpreter} how the
	programmer intends to use it.  For example, the process and
	result of adding two variables differs greatly according to
	whether they are integers, floating point numbers, or strings.

	Types supported by most programming languages include
	{integers} (usually limited to some range so they will fit in
	one {word} of storage), {Booleans}, {floating point numbers},
	and characters.  {Strings} are also common, and are
	represented as {lists} of characters in some languages.

	If s and t are types, then so is s -> t, the type of
	{functions} from s to t; that is, give them a term of type s,
	functions of type s -> t will return a term of type t.

	Some types are {primitive} - built-in to the language, with no
	visible internal structure - e.g. Boolean; others are
	composite - constructed from one or more other types (of
	either kind) - e.g. {lists}, {arrays}, {structures}, {unions}.
	{Object-oriented programming} extends this with {classes}
	which encapsulate both the structure of a type and the
	operations that can be performed on it.

	Some languages provide {strong typing}, others allow {implicit
	type conversion} and/or {explicit type conversion}.

	(2003-12-22)

<file system> An element of data storage in a {file system}.

	The history of computing is rich in varied kinds of files and
	{file systems}, whether ornate (e.g., {Macintosh file system}
	for a well-known case) or deficient (e.g., many simple
	pre-1980s file systems don't allow {directories}).

	However, the prototypical file has these characteristics:

	* It is a single sequence of bytes (but consider {Macintosh}
	{resource forks}).

	* It has a finite length, unlike, e.g. a {Unix} {device}.

	* It is stored in a {non-volatile storage} medium (but see
	{ramdrive}).

	* It exists (nominally) in a {directory}.

	* It has a name that it can be referred to by in file
	operations, possibly in combination with its {path}.

	Additionally, a file system may associate other information
	with a file, such as {permission} bits or other {file
	attributes}; timestamps for file creation, last revision, and
	last access; revision numbers (a` la VMS), and other kinds of
	{magic}.

	Compare: {document}.

	(1997-04-08)

1. <mathematics> (Or "map", "mapping") If D and C are sets
	(the domain and codomain) then a function f from D to C,
	normally written "f : D -> C" is a subset of D x C such that:

	1. For each d in D there exists some c in C such that (d,c) is
	an element of f.  I.e. the function is defined for every
	element of D.

	2. For each d in D, c1 and c2 in C, if both (d,c1) and (d,c2)
	are elements of f then c1 = c2.  I.e. the function is uniquely
	defined for every element of D.

	See also {image}, {inverse}, {partial function}.

	2. <programming> Computing usage derives from the mathematical
	term but is much less strict.  In programming (except in
	{functional programming}), a function may return different
	values each time it is called with the same argument values
	and may have {side effects}.

	A {procedure} is a function which returns no value but has
	only {side-effects}.  The {C} language, for example, has no
	procedures, only functions.  {ANSI C} even defines a {type},
	{void}, for the result of a function that has no result.

	(1996-09-01)

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

<programming> (CVS) A {cross-platform} {code management
	system} originally based on {RCS}.

	CVS tracks all revisions to a file in an associated file with
	the same name as the original file but with the string ",v"
	(for version) appended to the filename.  These files are
	stored in a (possibly centralised) repository.

	Changes are checked in or "committed" along with a comment
	(which appears in the the "commit log").  CVS has the notions
	of projects, branches, file locking and many others needed to
	provide a full-functioned repository.

	It is commonly accessed over over its own "anonCVS" {protocol}
	for read-only access (many {open source} projects are
	available by anonymous CVS) and over the {SSH} protocol by
	those with commit privileges ("committers").

	CVS has been rewritten several times and does not depend on
	RCS.  However, files are still largely compatible; one can
	easily migrate a project from RCS to CVS by copying the
	history files into a CVS repository.  A sub-project of the
	{OpenBSD} project is building a complete new implementation of
	CVS, to be called OpenCVS.

	{CVS Home (http://www.cvshome.org/)}.  {OpenCVS
	(http://www.opencvs.org/)}.

	(2005-01-17)

<operating system, programming> The process of carrying out
	the {instructions} in a computer program by a computer.

	See also {dry run}.

	(1996-05-13)

1. <unit, text> (Sometimes abbreviated "pt") The unit of
	length used in {typography} to specify text character height,
	{rule} width, and other small measurements.

	There are six slightly different definitions: {Truchet point},
	{Didot point}, {ATA point}, {TeX point}, {Postscript point},
	and {IN point}.

	In Europe, the most commonly used is Didot and in the US, the
	formerly standard ATA point has essentially been replaced by
	the PostScript point due to the demise of traditional
	typesetting systems and rise of desktop computer based systems
	running software such as {QuarkXPress}, {Adobe InDesign} and
	{Adobe Pagemaker}.

	There are 20 {twips} in a point and 12 points in a {pica}
	(known as a "Cicero" in the Didot system).

	{Different point systems
	(http://www.vakcer.com/oberon/dtp/fonts/point.htm)}.

	(2004-12-23)

	2. <hardware> To move a {pointing device} so that the
	on-screen pointer is positioned over a certain object on the
	screen such as a {button} in a {graphical user interface}.  In
	most {window systems} it is then necessary to {click} a
	(physical) button on the pointing device to activate or select
	the object.  In some systems, just pointing to an object is
	known as "mouse-over" {event} which may cause some help text
	(called a "tool tip" in {Windows}) to be displayed.

	(2001-05-21)

<abuse> Someone who's clueless.  From the {World-Wide Web}
	message "404, URL Not Found" meaning that the document you've
	tried to access can't be located.

	"Don't bother asking him...he's 404, man".

	404 is one of the standard response codes of the {telnet}
	{protocol} on which the web's {HTTP} is based.

	The first 4 indicates a client error such as a mistyped URL.
	The middle 0 refers to a general syntax error.  The last 4
	just indicates the specific error in the group of 40x, which
	also includes 400: Bad Request, 401: Unauthorized, etc.

	(2000-03-18)

1. To cause to be {broken} (in any sense).  "Your latest patch
	to the editor broke the paragraph commands."

	2. (Of a program) To stop temporarily, so that it may
	debugged.  The place where it stops is a "{breakpoint}".

	3. To send an {EIA-232} break (two character widths of line
	high) over a {serial line}.

	4. [Unix] To strike whatever key currently causes the tty
	driver to send SIGINT to the current process.  Normally,
	break, delete or {control-C} does this.

	5. "break break" may be said to interrupt a conversation (this
	is an example of verb doubling).  This usage comes from radio
	communications, which in turn probably came from landline
	telegraph/teleprinter usage, as badly abused in the Citizen's
	Band craze a few years ago.

	6. {pipeline break}.

	7. {break statement}.

	[{Jargon File}]

	(2004-03-24)

1. <hardware> An electrical conductor.  For distances larger
	than a breadbox, a single line may consist of two electrical
	conductors in twisted, parallel, or concentric arrangement
	used to transport one logical signal.

	By extension, a (usually physical) medium such as an {optical
	fibre} which carries a signal.

	(1995-09-29)

1. (Originally {Unix}, now also {MS-DOS}) A program that
	processes an input data stream into an output data stream in
	some well-defined way, and does no I/O to anywhere else except
	possibly on error conditions; one designed to be used as a
	stage in a {pipeline} (see {plumbing}).  Compare {sponge}.

	2. ({functional programming}) A {higher-order function} which
	takes a {predicate} and a list and returns those elements of
	the list for which the predicate is true.  In {Haskell}:

		filter p []     = []
		filter p (x:xs) = if p x then x : rest else rest
				  where
				  rest = filter p xs

	See also {filter promotion}.

	[{Jargon File}]

<operating system, programming> The process of carrying out
	the {instructions} in a computer program by a computer.

	See also {dry run}.

	(1996-05-13)

See also {complete graph}, {complete inference system},
	{complete lattice}, {complete metric space}, {complete partial
	ordering}, {complete theory}.

	[1. or 2. or both?]

	(1996-04-24)

<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)

<database> An action causing the automatic invocation of a
	procedure, for instance to preserve {referential integrity}.
	A triggers goes into effect when a user attempts to modify
	data with an insert, delete, or update command.  A trigger can
	instruct the system to take any number of actions when a
	specified change is attempted.  By preventing incorrect,
	unauthorized, or inconsistent changes to data, triggers help
	maintain the integrity of the database.

	[Sybase SQL Server Release 10.0 Transact-SQL User's Guide].

	(1995-02-22)

<World-Wide Web> (By analogy with "{applet}") A {Java} program
	that runs as part of a {network} service, typically an {HTTP
	server} and responds to requests from {clients}.

	The most common use for a servlet is to extend a {web server}
	by generating {web content} dynamically.  For example, a
	{client} may need {information} from a {database}; a servlet
	can be written that receives the request, gets and processes
	the data as needed by the {client} and then returns the result
	to the {client}.

	{Applets} are also written in {Java} but run inside the {JVM}
	of a {HTML} {browser} on the {client}.  Servlets and applets
	allow the server and client to be extended in a modular way by
	dynamically loading code which communicates with the main
	program via a standard programming interface.

	Servlets are more flexible than {CGI scripts} and, being
	written in {Java}, more portable.

	The spelling "servelet" is occasionally seen but {JavaSoft}
	spell it "servlet".  There is no such thing as a "serverlet".

	{(http://java.sun.com/products/servlet/)}.

	(2002-10-06)

In {object-oriented} systems, a set of {class}es that embodies
	an abstract design for solutions to a number of related
	problems.

	(1995-01-30)

After-sale handholding; something many software vendors
	promise but few deliver.  To hackers, most support people are
	useless - because by the time a hacker calls support he or she
	will usually know the software and the relevant manuals better
	than the support people (sadly, this is *not* a joke or
	exaggeration).  A hacker's idea of "support" is a tte--tte
	or exchange of {electronic mail} with the software's designer.

	[{Jargon File}]

	(1995-02-15)

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)

<language, tool> A {Unix} tool written in {Tcl} and a {script
	language} for automating the operation of {interactive}
	applications such as {telnet}, {FTP}, {passwd}, {fsck},
	{rlogin}, {tip}, etc..  Expect can feed input to other
	programs and perform {pattern matching} on their output.  It
	is also useful for testing these applications.  By adding
	{Tk}, you can also wrap interactive applications in {X11}
	{GUIs}.

	{Home (http://expect.nist.gov/)}.

	["expect: Scripts for Controlling Interactive Tasks", Don
	Libes, Comp Sys 4(2), U Cal Press Journals, Nov 1991].

	(1997-06-09)

1. A new or better version of some {hardware} or {software}.

	Often used in {marketroid}-speak to mean "{bug fix}".

	2. The act of developing or installing a new version.

	(1995-03-14)

<text> The spacing between {characters} in a line of {text}.
	This is defined when a {font} is designed but can often be
	altered in order to change the appearance of the text or for
	special effects.

	Tracking should not be confused with {kerning} which deals
	with the spacing between certain pairs of characters.

	See also {leading}.

	(1996-06-07)

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}]