<programming> Any piece of program code in a {high-level
	language} which, when (if) its execution terminates, returns a
	value.  In most programming languages, expressions consist of
	constants, variables, operators, functions, and {parentheses}.
	The operators and functions may be built-in or user defined.
	Languages differ on how expressions of different {types} may
	be combined - with some combination of explicit {casts} and
	implicit {coercions}.

	The {syntax} of expressions generally follows conventional
	mathematical notation, though some languages such as {Lisp} or
	{Forth} have their own idiosyncratic syntax.

	(2001-05-14)

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)

In {object-oriented programming} sending a message to an
	{object} (to invoke a {method}) is equivalent to calling a
	{procedure} in traditional programming languages, except that
	the actual code executed may only be selected at run time
	depending on the {class} of the object.  Thus, in response to
	the message "drawSelf", the method code invoked would be
	different if the target object were a circle or a square.

	(1995-02-16)

1. Not simulated.  Often used as a specific antonym to
	{virtual} in any of its jargon senses.

	2. <mathematics> {real number}.

	[{Jargon File}]

	(1997-03-12)

<jargon> A {marketroid} term for something he wants to sell
	you without bothering you with the often dizzying distinctions
	between {hardware}, {software}, {services}, {applications},
	{file formats}, companies, brand names and {operating
	systems}.

	"{Flash} is a perfect image-streaming solution."  "What is
	it?"  "Um...  about a thousand dollars."

	See also: {technology}.

	(1998-07-07)

<programming> A sequence of {data} values, usually {bytes},
	which usually stand for {characters} (a "character string").
	The {mapping} between values and characters is determined by
	the {character set} which is itself specified implcitly or
	explicitly by the environment in which the string is being
	interpreted.

	The most common character set is {ASCII} but, since the late
	1990s, there has been increased interest in larger character
	sets such as {Unicode} where each character is represented by
	more than eight {bits}.

	Most programming languages consider strings (e.g.
	"124:shabooya:\n", "hello world") basically distinct from
	numbers which are typically stored in fixed-length {binary} or
	{floating-point} representation.

	A {bit string} is a sequence of {bits}.

	(1999-12-21)

The {GNU} {regular expression} matching library.  See also
	{Rx}.

<character> An {atom} in a {character repertoire}.

	Compare with {glyph}.

	(1998-10-18)

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)

<operating system, standard> (POSIX) A set of {IEEE} standards
	designed to provide {application} {portability} between {Unix}
	variants.  IEEE 1003.1 defines a Unix-like operating system
	interface, IEEE 1003.2 defines the {shell} and utilities and
	IEEE 1003.4 defines {real-time} extensions.

	["More UNIX For Dummies", ISBN: 1-56884-361-5] says it stands
	for "Portable Operating System Interface with an X thrown in
	to make it sound cooler."

	(1997-12-04)

<storage> 1. {magnetic disk}.

	2. {compact disc}.

	3. {optical disk}.

	Note: the american spelling, "disk", is normal for most
	computer disks whereas "compact disc", having come to
	computers via the audio world, is correctly spelled with a
	"c", indeed, this spelling is part of the CD standard.

	(1995-07-30)

(FTP) A {client-server} protocol which allows a user on one
	computer to transfer files to and from another computer over a
	{TCP/IP} network.  Also the client program the user executes
	to transfer files.  It is defined in {STD 9}, {RFC 959}.

	See also {anonymous FTP}, {FSP}, {TFTP}.

	{Unix manual page}: ftp(1).

	(1994-12-01)

1. <tool> A program used primarily to create, manipulate,
	modify, or analyse other programs, such as a compiler or an
	editor or a cross-referencing program.  Opposite: {app},
	{operating system}.

	2. A {Unix} {application program} with a simple, "transparent"
	(typically text-stream) interface designed specifically to be
	used in programmed combination with other tools (see {filter},
	{plumbing}).

	3. <jargon> ({MIT}: general to students there) To work; to
	study (connotes tedium).  The {TMRC} Dictionary defined this
	as "to set one's brain to the grindstone".  See {hack}.

	4. <jargon, person> ({MIT}) A student who studies too much and
	hacks too little.  MIT's student humour magazine rejoices in
	the name "Tool and Die".

	[{Jargon File}]

	(1996-12-12)

<unit> (b) {binary} digit.

	The unit of information; the amount of information obtained by
	asking a yes-or-no question; a computational quantity that can
	take on one of two values, such as false and true or 0 and 1;
	the smallest unit of storage - sufficient to hold one bit.

	A bit is said to be "set" if its value is true or 1, and
	"reset" or "clear" if its value is false or 0.  One speaks of
	setting and clearing bits.  To {toggle} or "invert" a bit is
	to change it, either from 0 to 1 or from 1 to 0.

	The term "bit" first appeared in print in the computer-science
	sense in 1949, and seems to have been coined by the eminent
	statistician, {John Tukey}.  Tukey records that it evolved
	over a lunch table as a handier alternative to "bigit" or
	"binit".

	See also {flag}, {trit}, {mode bit}, {byte}, {word}.

	[{Jargon File}]

	(2002-01-22)

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)

1. See {multithreading}.

	2. See {threaded code}.

	3. {topic thread}.

	[{Jargon File}]

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)

<programming> (Or "remark") Explanatory text embedded in
	program {source} (or less often data) intended to help human
	readers understand it.

	Code completely without comments is often hard to read, but
	too heavily commented code isn't much better, especially if
	the comments are not kept up-to-date with changes to the code.
	Too much commenting may mean that the code is
	over-complicated.  A good rule is to comment everything that
	needs it but write code that doesn't need much of it.

	A particularly irksome form of over-commenting explains
	exactly what each statement does, even when it is obvious to
	any reasonably competant programmer, e.g.

		/* Open the input file */
		infd = open(input_file, O_RDONLY);

	(1998-04-28)

<chat, tool, networking, messaging> A {Unix} program and
	{protocol} supporting conversation between two or more users
	who may be logged into the same computer or different
	computers on a network.  Variants include {ntalk}, {ytalk},
	and {ports} or {emulators} of these programs for other
	{platforms}.

	{Unix} has the {talk} program and {protocol} and its variants
	{xtalk} and {ytalk} for the {X Window System}; {VMS} has
	{phone}; {Windows for Workgroups} has {chat}.  {ITS} also has
	a talk system.  These split the screen into separate areas for
	each user.

	{Unix}'s {write} command can also be used, though it does not
	attempt to separate input and output on the screen.

	Users of such systems are said to be in {talk mode} which has
	many conventional abbreviations and idioms.  Most of these
	survived into {chat} jargon, but many fell out of common use
	with the migration of {user} prattle from talk-like systems to
	{chat} systems in the early 1990s.  These disused
	talk-specific forms include:

	"BYE?" - are you ready to close the conversation?  This is the
	standard way to end a talk-mode conversation; the other person
	types "BYE" to confirm, or else continues the conversation.

	"JAM"/"MIN" - just a minute

	"O" - "over" (I have stopped talking).  Also "/" as in x/y - x
	over y, or two newlines (the latter being the most common).

	"OO" - "over and out" - end of conversation.

	"\" - Greek {lambda}.

	"R U THERE?" - are you there?

	"SEC" - wait a second.

	"/\/\/" - laughter.  But on a {MUD}, this usually means
	"earthquake fault".

	See also {talk bomb}.

	(1998-01-25)

<programming> A program which executes other programs.  This
	is in contrast to a {compiler} which does not execute its
	input program (the "{source code}") but translates it into
	executable "{machine code}" (also called "{object code}")
	which is output to a file for later execution.  It may be
	possible to execute the same source code either directly by an
	interpreter or by compiling it and then executing the {machine
	code} produced.

	It takes longer to run a program under an interpreter than to
	run the compiled code but it can take less time to interpret
	it than the total required to compile and run it.  This is
	especially important when prototyping and testing code when an
	edit-interpret-debug cycle can often be much shorter than an
	edit-compile-run-debug cycle.

	Interpreting code is slower than running the compiled code
	because the interpreter must analyse each statement in the
	program each time it is executed and then perform the desired
	action whereas the compiled code just performs the action.
	This run-time analysis is known as "interpretive overhead".
	Access to variables is also slower in an interpreter because
	the mapping of identifiers to storage locations must be done
	repeatedly at run time rather than at compile time.

	There are various compromises between the development speed
	when using an interpreter and the execution speed when using a
	compiler.  Some systems (e.g. some {Lisps}) allow interpreted
	and compiled code to call each other and to share variables.
	This means that once a routine has been tested and debugged
	under the interpreter it can be compiled and thus benefit from
	faster execution while other routines are being developed.
	Many interpreters do not execute the source code as it stands
	but convert it into some more compact internal form.  For
	example, some {BASIC} interpreters replace {keywords} with
	single byte tokens which can be used to {index} into a {jump
	table}.  An interpreter might well use the same {lexical
	analyser} and {parser} as the compiler and then interpret the
	resulting {abstract syntax tree}.

	There is thus a spectrum of possibilities between interpreting
	and compiling, depending on the amount of analysis performed
	before the program is executed.  For example {Emacs Lisp} is
	compiled to "{byte-code}" which is a highly compressed and
	optimised representation of the Lisp source but is not machine
	code (and therefore not tied to any particular hardware).
	This "compiled" code is then executed (interpreted) by a {byte
	code interpreter} (itself written in {C}).  The compiled code
	in this case is {machine code} for a {virtual machine} which
	is implemented not in hardware but in the byte-code
	interpreter.

	See also {partial evaluation}.

	(1995-01-30)

<jargon> 1. A piece of {hardware} that encapsulates some
	function but can't be used without some kind of {front end}.
	Today we have, especially, "{print engine}": the guts of a
	{laser printer}.

	2. An analogous piece of software; notionally, one that does a
	lot of noisy {crunching}, such as a "database engine", or
	"{search engine}".

	The hackish senses of "engine" are actually close to its
	original, pre-Industrial-Revolution sense of a skill, clever
	device, or instrument (the word is cognate to "ingenuity").
	This sense had not been completely eclipsed by the modern
	connotation of power-transducing machinery in {Charles
	Babbage}'s time, which explains why he named the
	stored-program computer that he designed in 1844 the
	"{Analytical Engine}".

	[{Jargon File}]

	(1996-05-31)

1. Not operating.  "The up escalator is down" is considered a
	humorous thing to say, and "The elevator is down" always
	means "The elevator isn't working" and never refers to what
	floor the elevator is on.  With respect to computers, this
	term has passed into the mainstream; the extension to other
	kinds of machine is still hackish.

	2. "go down" To stop functioning; usually said of the
	{system}.  The message from the {console} that every hacker
	hates to hear from the operator is "System going down in 5
	minutes".

	3. "take down", "bring down" To deactivate purposely, usually
	for repair work or {PM}.  "I'm taking the system down to work
	on that bug in the tape drive."  Occasionally one hears the
	word "down" by itself used as a verb in this sense.

	See {crash}; opposite: {up}.

	[{Jargon File}]

	(1994-12-07)

1. <programming, convention> /bar/ The second {metasyntactic
	variable}, after {foo} and before {baz}.  E.g. "Suppose
	function FOO calls functions BAR..."

	2. Often appended to {foo} to produce {foobar}.

	[{Jargon File}]

	(1995-03-07)

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

<programming> One of a sequence of copies of a program, each
	incorporating new modifications.  Each version is usually
	identified by a number, commonly of the form X.Y where X is
	the major version number and Y is the release number.
	Typically an increment in X (with Y reset to zero) signifies a
	substantial increase in the function of the program or a
	partial or total re-implementation, whereas Y increases each
	time the progam is changed in any way and re-released.

	Version numbers are useful so that the user can know if the
	program has changed ({bugs} have been fixed or new functions
	added) since he obtained his copy and the programmer can tell
	if a bug report relates to the current version.  It is thus
	always important to state the version when reporting bugs.
	Statements about compatibility between different software
	components should always say which versions they apply to.

	See {change management}.

	(1997-12-07)