<messaging> (Plural "fora" or "forums") Any discussion group
	accessible through a dial-in {BBS} (e.g. {GEnie}, {CI$}), a
	{mailing list}, or a {Usenet} {newsgroup} (see {network,
	the}).  A forum functions much like a {bulletin board}; users
	submit {postings} for all to read and discussion ensues.

	Contrast real-time {chat} or point-to-point personal {e-mail}.

	[{Jargon File}]

	(1998-01-18)

1. <data, graphics> Data representing a two-dimensional scene.
	A digital image is composed of {pixels} arranged in a
	rectangular array with a certain height and width.  Each pixel
	may consist of one or more {bits} of information, representing
	the brightness of the image at that point and possibly
	including colour information encoded as {RGB} triples.

	{Images} are usually taken from the real world via a {digital
	camera}, {frame grabber}, or {scanner}; or they may be
	generated by computer, e.g. by {ray tracing} software.

	See also {image formats}, {image processing}.

	(1994-10-21)

	2. <mathematics> The image (or range) of a {function} is the
	set of values obtained by applying the function to all
	elements of its {domain}.  So, if f : D -> C then the set f(D)
	= \{ f(d) | d in D \} is the image of D under f.  The image is
	a subset of C, the {codomain}.

	(2000-01-19)

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)

<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. <filename extension> {filename extension}.

	2. <programming> A {feature} or piece of {code} which
	{extends} a program's {functionality}, e.g. a {plug-in}.

	(1997-06-21)

<jargon> To transfer data from one computer to another.
	Downloading usually refers to transfer from a larger "host"
	system (especially a {server} or {mainframe}) to a smaller
	"client" system, especially a {microcomputer} or specialised
	peripheral, and "{upload}" usually means from small to large.

	Others hold that, technically, download means "receive" and
	upload means "send", irrespective of the size of the systems
	involved.

	Note that in communications between ground and space,
	space-to-earth transmission is always "down" and the reverse
	"up", regardless of size.  So far the in-space machines have
	invariably been smaller; thus the upload/download distinction
	has been reversed from its usual sense.

	[{Jargon File}]

	(2003-11-04)

See {followup}.

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. The portion of a {packet}, preceding the actual data,
	containing source and destination addresses, error checking
	and other fields.

	2. The part of an {electronic mail} message or {news} article
	that precedes the body of a message and contains, among other
	things, the sender's name and e-mail address and the date and
	time the message was sent.

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)

<jargon, architecture> (Via the technical term {virtual
	memory}, probably from the term "virtual image" in optics)
	1. Common alternative to {logical}; often used to refer to the
	artificial objects (like addressable {virtual memory} larger
	than physical memory) created by a computer system to help the
	system control access to shared resources.

	2. Simulated; performing the functions of something that isn't
	really there.  An imaginative child's doll may be a virtual
	playmate.

	Opposite of {real} or physical.

	[{Jargon File}]

	(1994-11-30)

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)

1. <application> Any specific type of {file} produced or
	edited by a specific {application}; usually capable of being
	printed.  E.g. "Word document", "Photoshop document", etc.

	2. <hypertext> A term used on some systems (e.g. {Intermedia})
	for a {hypertext} {node}.  It is sometimes used for a
	collection of nodes on related topics, possibly stored or
	distributed as one.

	3. <programming> To write {documentation} on a certain piece
	of code.

	(2003-10-25)

<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)

<networking, company> (IAP) A company or other origanisation
	which provides access to the {Internet} to businesses and/or
	consumers.  An IAP purchases an Internet link from another
	company that has a direct link to the Internet and resells
	portions of that {bandwidth} to the general public.

	For example, an IAP may purchase a {T1} link (1.544Mb/s) and
	resell that bandwidth in chunks consisting of {ISDN} (64Kb/s,
	128Kb/s) and analog {modems} (14.4Kb/s, 28.8Kb/s).  The IAP's
	customer base is likely to include both businesses and
	individuals.  Individual customers usually connect to the IAP
	via a modem and telephone line to a (preferably local) {point
	of presence}.

	An IAP may also be an {Internet Service Provider}.

	(1996-06-25)

<World-Wide Web, project> A {open source} {HTTP} server for
	{Unix}, {Windows NT}, and other {platforms}.  Apache was
	developed in early 1995, based on code and ideas found in the
	most popular HTTP server of the time, {NCSA httpd} 1.3.  It
	has since evolved to rival (and probably surpass) almost any
	other {Unix} based HTTP server in terms of functionality, and
	speed.  Since April 1996 Apache has been the most popular HTTP
	server on the {Internet}, in May 1999 it was running on 57% of
	all web servers.

	It features highly configurable error messages, {DBM}-based
	{authentication} {databases}, and {content negotiation}.

	Latest version: 1.3.9, as of 1999-10-27.

	{Home (http://www.apache.org/httpd.html)}.

	{FAQ (http://www.apache.org/docs/misc/FAQ.html)}.

	(1999-10-27)

1. <programming> An independent piece of {software} which
	forms part of one or more larger {programs}.  Different
	languages have different concepts of a module but there are
	several common ideas.

	Modules are usually compiled seperately (in compiled
	languages) and provide an {abstraction} or information hiding
	mechanism so that a module's implementation can be changed
	without requiring any change to other modules.  In this
	respect they are similar to {objects} in an {object-oriented
	language}, though a module may contain many {procedures}
	and/or {functions} which would correspond to many objects.

	A module often has its own {name space} for {identifiers} so
	the same identifier may be used to mean different things in
	different modules.

	[Difference from {package}?].

	2. <hardware> An independent assembly of electronic components
	with some distinct function, e.g. a RAM module consisting of
	several RAM chips mounted on a small circuit board.

	(1997-10-27)

<file format> /ping/ (PNG) An extensible {file format} for the
	{lossless}, {portable}, well-compressed storage of {raster
	images}.  PNG provides a patent-free replacement for {GIF} and
	can also replace many common uses of {TIFF}.
	{Indexed-colour}, {greyscale} and {truecolour} images are
	supported, plus an optional {alpha channel}.  Sample depths
	range from 1 to 16 bits.

	PNG is designed for on-line viewing applications, such as the
	{World Wide Web}, so it is fully {streamable} with a
	{progressive display} option.  PNG is robust, providing both
	full file integrity checking and simple detection of common
	transmission errors.  Also, PNG can store {gamma correction}
	and {chromaticity} data for improved colour matching on
	heterogeneous {platforms}.

	{Filename extension}: .png.

	{RFC 2083}.  {W3C PNG pages
	(http://www.w3.org/Graphics/PNG/)}.  {PNG home page
	(http://www.wco.com/~png/)}.

	(1997-08-07)

<hypertext> A program which allows a person to read
	{hypertext}.  The browser gives some means of viewing the
	contents of {nodes} (or "pages") and of {navigating} from one
	node to another.

	{Netscape Navigator}, {NCSA} {Mosaic}, {Lynx}, and {W3} are
	examples for browsers for the {World-Wide Web}.  They act as
	{clients} to remote {web servers}.

	(1996-05-31)

<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)

<tool, cryptography> (PGP) A high security {RSA} {public-key
	encryption} application for {MS-DOS}, {Unix}, {VAX/VMS}, and
	other computers.  It was written by {Philip R. Zimmermann}
	<pkz@acm.org> of Phil's Pretty Good(tm) Software and later
	augmented by a cast of thousands, especially including Hal
	Finney, Branko Lankester, and Peter Gutmann.

	PGP was distributed as "{guerrilla freeware}".  The authors
	don't mind if it is distributed widely, just don't ask Philip
	Zimmermann to send you a copy.  PGP uses a {public-key
	encryption} {algorithm} claimed by US patent #4,405,829.  The
	exclusive rights to this patent are held by a California
	company called {Public Key Partners}, and you may be
	infringing this patent if you use PGP in the USA.  This is
	explained in the PGP User's Guide, Volume II.

	PGP allows people to exchange files or messages with {privacy}
	and {authentication}.  Privacy and authentication are provided
	without managing the keys associated with conventional
	cryptographic software.  No secure channels are needed to
	exchange keys between users, which makes PGP much easier to
	use.  This is because PGP is based on {public-key
	cryptography}.

	PGP encrypts data using the {International Data Encryption
	Algorithm} with a random {session key}, and uses the {RSA}
	algorithm to encrypt the session key.

	In December 1994 Philip Zimmermann faced prosecution for
	"exporting" PGP out of the United States but in January 1996
	the US Goverment dropped the case.  A US law prohibits the
	export of {encryption} software out of the country.
	Zimmermann did not do this, but the US government hoped to
	establish the proposition that posting an encryption program
	on a {BBS} or on the {Internet} constitutes exporting it - in
	effect, stretching export control into domestic censorship.
	If the government had won it would have had a chilling effect
	on the free flow of information on the global network, as well
	as on everyone's privacy from government snooping.

	{FAQ (ftp://ftp.netcom.com/pub/mp/mpj/getpgp.asc)}.  {UK FTP
	(ftp://src.doc.ic.ac.uk/packages/pgp/)}.  {USA FTP
	(http://web.mit.edu/network/pgp-form.html)}.
	{(http://www.pegasus.esprit.ec.org/people/arne/pgp.html)}.

	{Justice Dept. announcement
	(http://www.eff.org/pub/Alerts/usatty_pgp_011196.announce)}.

	["Protect Your Privacy: A Guide for PGP Users", William
	Stallings, Prentice-Hall, ISBN 0-13-185596-4].

	(1996-04-07)

1. A set of function symbols with {arities}.

	2. <messaging> (Or sig) A few lines of information about the
	sender of an {electronic mail} message or {news} {posting}.
	Most {Unix} mail and news software will {automagically} append
	a signature from a file called .signature in the user's {home
	directory} to outgoing mail and news.

	A signature should give your real name and your {e-mail
	address} since, though these appear in the {headers} of your
	messages, they may be {munged} by intervening software.  It is
	currently (1994) hip to include the {URL} of your {home page}
	on the {World-Wide Web} in your sig.

	The composition of one's sig can be quite an art form,
	including an {ASCII} logo or one's choice of witty sayings
	(see {sig quote}, {fool file}).  However, large sigs are a
	waste of {bandwidth}, and it has been observed that the size
	of one's sig block is usually inversely proportional to one's
	prestige on the net.

	See also {doubled sig}, {sig virus}.

	2. <programming> A concept very similar to {abstract base
	classes} except that they have their own {hierarchy} and can
	be applied to compiled {classes}.  Signatures provide a means
	of separating {subtyping} and {inheritance}.  They are
	implemented in {C++} as patches to {GCC} 2.5.2 by Gerald
	Baumgartner <gb@cs.purdue.edu>.

	{(ftp://ftp.cs.purdue.edu/pub/gb/)}.

	(2001-01-05)

1. Executable code, especially a "pure code" portion shared
	between multiple instances of a program running in a
	{multitasking} {operating system}.

	Compare {English}.

	2. Textual material in the mainstream sense; data in ordinary
	{ASCII} or {EBCDIC} representation (see {flat ASCII}).  "Those
	are text files; you can review them using the editor."

	These two contradictory senses confuse hackers too.

	[{Jargon File}]

	(1995-03-16)

<jargon> ({MIT}) 1. To fail.  A program loses when it
	encounters an exceptional condition or fails to work in the
	expected manner.

	2. To be exceptionally unesthetic or crocky.

	3. Of people, to be obnoxious or unusually stupid (as opposed
	to ignorant).

	4. Refers to something that is {losing}, especially in the
	phrases "That's a lose!" and "What a lose!"

	[{Jargon File}]

	(1995-04-19)

<computer> A machine that can be programmed to manipulate
	symbols.  Computers can perform complex and repetitive
	procedures quickly, precisely and reliably and can quickly
	store and retrieve large amounts of data.

	The physical components from which a computer is constructed
	(electronic circuits and input/output devices) are known as
	"{hardware}".  Most computers have four types of hardware
	component: CPU, input, output and memory.  The CPU ({central
	processing unit}) executes programs ("{software}") which tell
	the computer what to do.  Input and output (I/O) devices allow
	the computer to communicate with the user and the outside
	world.  There are several kinds of memory - fast, expensive,
	short term memory (e.g. {RAM}) to hold intermediate results,
	and slower, cheaper, long-term memory (e.g. {magnetic disk} and
	{magnetic tape}) to hold programs and data between jobs.

	See also {analogue computer}.

	(1995-03-10)

1. <hardware> {monitor}.

	2. <language> A vector of pointers to {activation records}.
	The Nth element points to the activation record containing
	variables declared at {lexical depth} N.  This allows faster
	access to variables from outer {scopes} than the alternative
	of linked activation records (but most variable accesses are
	either local or global or occasionally to the immediately
	enclosing scope).  Displays were used in some {ALGOL}
	implementations.

	(1996-02-22)