public directory

public directory


The top-level "pub" (public) directory on a server that allows remote access, once upon a time via FTP, now probably by HTTP. The pub directory typically contains a collection of freely available files.

This server's pub directory.

Last updated: 2023-10-21

public domain

(PD) The total absence of copyright protection. If something is "in the public domain" then anyone can copy it or use it in any way they wish. The author has none of the exclusive rights which apply to a copyright work.

The phrase "public domain" is often used incorrectly to refer to freeware or shareware (software which is copyrighted but is distributed without (advance) payment). Public domain means no copyright -- no exclusive rights. In fact the phrase "public domain" has no legal status at all in the UK.

See also archive site, careware, charityware, copyleft, crippleware, guiltware, postcardware and -ware. Compare payware.

public domain software

public domain

public-key cryptography

public-key encryption

Public-Key Cryptography Standards

<cryptography, standard>

(PKCS) A set of standards for public-key cryptography, developed by RSA Data Security, Inc. in cooperation with an informal consortium, originally including Apple, Microsoft, DEC, Lotus, Sun and MIT. The PKCS have been cited by the OSI Implementers' Workshop (OIW) as a method for implementation of OSI standards.

PKCS includes both algorithm-specific and algorithm-independent implementation standards. Many algorithms are supported, including RSA and Diffie-Hellman key exchange, however, only the latter two are specifically detailed. PKCS also defines an algorithm-independent syntax for digital signatures, digital envelopes, and extended digital certificates; this enables someone implementing any cryptographic algorithm whatsoever to conform to a standard syntax, and thus achieve interoperability.

E-mail: [email protected].

Last updated: 1999-02-16

public-key encryption


(PKE, Or "public-key cryptography") An encryption scheme, introduced by Diffie and Hellman in 1976, where each person gets a pair of keys, called the public key and the private key. Each person's public key is published while the private key is kept secret. Messages are encrypted using the intended recipient's public key and can only be decrypted using his private key. This is often used in conjunction with a digital signature.

The need for sender and receiver to share secret information (keys) via some secure channel is eliminated: all communications involve only public keys, and no private key is ever transmitted or shared.

Public-key encryption can be used for authentication, confidentiality, integrity and non-repudiation.

RSA encryption is an example of a public-key cryptosystem. FAQ.

See also knapsack problem.

Last updated: 1995-03-27

Public Key Infrastructure

<cryptography, communications>

(PKI) A system of public key encryption using digital certificates from Certificate Authorities and other registration authorities that verify and authenticate the validity of each party involved in an electronic transaction.

PKIs are currently evolving and there is no single PKI nor even a single agreed-upon standard for setting up a PKI. However, nearly everyone agrees that reliable PKIs are necessary before electronic commerce can become widespread.



IETF PKIX Working Group.

Last updated: 1999-11-30

Public Switched Telephone Network


(PSTN, T.70) The collection of interconnected systems operated by the various telephone companies and administrations (telcos and PTTs) around the world. Also known as the Plain Old Telephone System (POTS) in contrast to xDSL and ISDN (not to mention other forms of PANS).

The PSTN started as human-operated analogue circuit switching systems (plugboards), progressed through electromechanical switches. By now this has almost completely been made digital, except for the final connection to the subscriber (the "last mile"): The signal coming out of the phone set is analogue. It is usually transmitted over a twisted pair cable still as an analogue signal. At the telco office this analogue signal is usually digitised, using 8000 samples per second and 8 bits per sample, yielding a 64 kb/s data stream (DS0). Several such data streams are usually combined into a fatter stream: in the US 24 channels are combined into a T1, in Europe 31 DS0 channels are combined into an E1 line. This can later be further combined into larger chunks for transmission over high-bandwidth core trunks. At the receiving end the channels are separated, the digital signals are converted back to analogue and delivered to the received phone.

While all these conversions are inaudible when voice is transmitted over the phone lines it can make digital communication difficult. Items of interest include A-law to mu-law conversion (and vice versa) on international calls; robbed bit signalling in North America (56 kbps <--> 64 kbps); data compression to save bandwidth on long-haul trunks; signal processing such as echo suppression and voice signal enhancement such as AT&T TrueVoice.

Last updated: 2000-07-09



(PUB) A 1972 text-formatting language for TOPS-10, with syntax based on SAIL. PUB influenced TeX and Scribe.

["PUB: The Document Compiler", Larry Tesler, Stanford AI Proj Op Note, Sept 1972].

Last updated: 2020-02-09



(The opposite of huff) To decompress data that has been compressed by Huffman coding. At least one widely distributed Huffman decoder program was actually *named* "PUFF", but these days it is usually packaged with the encoder.

[Jargon File]

Last updated: 1996-10-16


["The Purdue University Fast Fortran Translator", Saul Rosen et al, CACM 8(11):661-666 (Nov 1965)].

Last updated: 1995-01-05


pull media

pull-down list

<operating system>

(Or "drop-down list") A graphical user interface component that allows the user to choose one (or sometimes more than one) item from a list. The current choice is visible in a small rectangle and when the user clicks on it, a list of items is revealed below it. The user can then click on one of these to make it the current choice and the list disappears. In some cases, by holding down a modifier key such as Ctrl when clicking, the selection is added to (or removed from) the set of current choices rather than replacing it.

Last updated: 1999-09-25

pull-down menu

<operating system>

(Or "drop-down menu", "pop-down menu") A menu in a graphical user interface, whose title is normally visible but whose contents are revealed only when the user activates it, normally by pressing the mouse button while the pointer is over the title, whereupon the menu items appear below the title. The user may then select an item from the menu or click elsewhere, in either case the menu contents are hidden again. A menu item is selected either by dragging the mouse from the menu title to the item and releasing or by clicking the title and then the item.

When a pull-down menu appears in the main area of a window, as opposed to the menu bar, it may have a small, downward-pointing triangle to the right.

Compare: scrollable list.

Last updated: 1999-09-22

pull media


A model of media distribution were the bits of content have to be requested by the user, e.g. normal use of HTTP on the web.

Opposite: "push media".

Last updated: 1997-04-10

Pulse Code Modulation


(PCM) A method by which an audio signal is represented as digital data.

Virtually all digital audio systems use PCM, including, CD, DAT, F1 format, 1630 format, DASH, DCC, and MD. Many people get confused because "PCM" is also slang for Sony's F1 format which stores PCM digital audio on videotape.

Last updated: 1995-02-09



A humourous term for the token - the object (notional or real) that gives its possessor (the "pumpking" or the "pumpkineer") exclusive access to something, e.g. applying patches to a master copy of source (for which the pumpkin is called a "patch pumpkin").

Chip Salzenberg <[email protected]> wrote:

David Croy once told me once that at a previous job, there was one tape drive and multiple systems that used it for backups. But instead of some high-tech exclusion software, they used a low-tech method to prevent multiple simultaneous backups: a stuffed pumpkin. No one was allowed to make backups unless they had the "backup pumpkin".

Last updated: 1999-02-23





punch card

punched card

punched card

<storage, history>

(Or "punch card") The signature medium of computing's Stone Age, now long obsolete outside of a few legacy systems. The punched card actually predates computers considerably, originating in 1801 as a control device for Jacquard looms. Charles Babbage used them as a data and program storage medium for his Analytical Engine:

"To those who are acquainted with the principles of the Jacquard loom, and who are also familiar with analytical formulæ, a general idea of the means by which the Engine executes its operations may be obtained without much difficulty. In the Exhibition of 1862 there were many splendid examples of such looms. [...] These patterns are then sent to a peculiar artist, who, by means of a certain machine, punches holes in a set of pasteboard cards in such a manner that when those cards are placed in a Jacquard loom, it will then weave upon its produce the exact pattern designed by the artist. [...] The analogy of the Analytical Engine with this well-known process is nearly perfect. There are therefore two sets of cards, the first to direct the nature of the operations to be performed -- these are called operation cards: the other to direct the particular variables on which those cards are required to operate -- these latter are called variable cards. Now the symbol of each variable or constant, is placed at the top of a column capable of containing any required number of digits."

-- from Chapter 8 of Charles Babbage's "Passages from the Life of a Philosopher", 1864.

The version patented by Herman Hollerith and used with mechanical tabulating machines in the 1890 US Census was a piece of cardboard about 90 mm by 215 mm. There is a widespread myth that it was designed to fit in the currency trays used for that era's larger dollar bills, but recent investigations have falsified this.

IBM (which originated as a tabulating-machine manufacturer) married the punched card to computers, encoding binary information as patterns of small rectangular holes; one character per column, 80 columns per card. Other coding schemes, sizes of card, and hole shapes were tried at various times.

The 80-column width of most character terminals is a legacy of the IBM punched card; so is the size of the quick-reference cards distributed with many varieties of computers even today.

See chad, chad box, eighty-column mind, green card, dusty deck, lace card, card walloper.

[Jargon File]

Last updated: 1998-10-19


(From the punch line of an old joke referring to American football: "Drop back 15 yards and punt!") 1. To give up, typically without any intention of retrying. "Let's punt the movie tonight." "I was going to hack all night to get this feature in, but I decided to punt" may mean that you've decided not to stay up all night, and may also mean you're not ever even going to put in the feature.

2. More specifically, to give up on figuring out what the Right Thing is and resort to an inefficient hack.

3. A design decision to defer solving a problem, typically because one cannot define what is desirable sufficiently well to frame an algorithmic solution. "No way to know what the right form to dump the graph in is - we'll punt that for now."

4. To hand a tricky implementation problem off to some other section of the design. "It's too hard to get the compiler to do that; let's punt to the run-time system."

[Jargon File]

Purdue Compiler-Construction Tool Set


(PCCTS) A highly integrated lexical analser generator and parser generator by Terence J. Parr <[email protected]>, Will E. Cohen and Henry G. Dietz <[email protected]>, both of Purdue University.

ANTLR (ANother Tool for Language Recognition) corresponds to YACC and DLG (DFA-based Lexical analyser Generator) functions like LEX. PCCTS has many additional features which make it easier to use for a wide range of translation problems. PCCTS grammars contain specifications for lexical and syntactic analysis with selective backtracking ("infinite lookahead"), semantic predicates, intermediate-form construction and error reporting. Rules may employ Extended BNF (EBNF) grammar constructs and may define parameters, return values, and have local variables.

Languages described in PCCTS are recognised via LLk parsers constructed in pure, human-readable, C code. Selective backtracking is available to handle non-LL(k) constructs. PCCTS parsers may be compiled with a C++ compiler. PCCTS also includes the SORCERER tree parser generator.

UK FTP. Macintosh FTP.

Mailing list: [email protected] ("subscribe pccts-users your_name" in the message body).

E-mail: Terence J. Parr <[email protected]>, Roberto Avanzi <[email protected]> (Mac port).

Last updated: 2000-10-30

Purdue University

Last updated: 1995-01-05

pure functional language

purely functional language

pure lambda-calculus

Lambda-calculus with no constants, only functions expressed as lambda abstractions.

Last updated: 1994-10-27


An incremental linker from Pure Software.

Pure Lisp

A purely functional language derived from Lisp by excluding any feature which causes side-effects.

purely functional language


A language that supports only functional programming and does not allow functions to have side-effects. Program execution consists of evaluation of an expression and all subexpressions are referentially transparent.

Last updated: 2003-03-25


A debugging tool from Pure Software.

Purple Book


1. The "System V Interface Definition". The covers of the first editions were an amazingly nauseating shade of off-lavender.


2. The Wizard Book.

See also book titles.

[Jargon File]

purple wire

<jargon, hardware>

Wire installed by IBM Field Engineers to work around problems discovered during testing or debugging. These are called "purple wires" even when (as is frequently the case) they are yellow.

Compare blue wire, yellow wire, and red wire.

Last updated: 1995-04-11



A web server for Windows NT and Windows 95 (when available).

E-mail: <[email protected]>.

Last updated: 1995-04-11



1. To put something onto a stack or pdl.

Opposite: "pop".


2. push media.

[Jargon File]

Last updated: 1997-04-10



A roughly fingertip-sized plastic cover attached to a spring-loaded, normally-open switch, which, when pressed, closes the switch. Typical examples are the keys on a computer or calculator keyboard and mouse buttons.

Last updated: 1997-07-07

Push Down List


(PDL) In ITS days, the preferred MITism for stack.

See overflow pdl.

Last updated: 1995-12-21

push media


A model of media distribution where items of content are sent to the user (viewer, listener, etc.) in a sequence, and at a rate, determined by a server to which the user has connected. This contrasts with pull media where the user requests each item individually. Push media usually entail some notion of a "channel" which the user selects and which delivers a particular kind of content.

Broadcast television is (for the most part) the prototypical example of push media: you turn on the TV set, select a channel and shows and commercials stream out until you turn the set off.

By contrast, the web is (mostly) the prototypical example of pull media: each "page", each bit of content, comes to the user only if he requests it; put down the keyboard and the mouse, and everything stops.

At the time of writing (April 1997), much effort is being put into blurring the line between push media and pull media. Most of this is aimed at bringing more push media to the Internet, mainly as a way to disseminate advertising, since telling people about products they didn't know they wanted is very difficult in a strict pull media model.

These emergent forms of push media are generally variations on targeted advertising mixed in with bits of useful content. "At home on your computer, the same system will run soothing screensavers underneath regular news flashes, all while keeping track, in one corner, of press releases from companies whose stocks you own. With frequent commercial messages, of course." (Wired, March 1997, page 12).

Pointcast is probably the best known push system on the Internet at the time of writing.

As part of the eternal desire to apply a fun new words to boring old things, "push" is occasionally used to mean nothing more than email spam.

Last updated: 1997-04-10

Nearby terms:

PTNPtolemyPTTPUBpubpublic directorypublic domainpublic domain software

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