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PERLGLOSSARY(1) Perl Programmers Reference Guide PERLGLOSSARY(1)
NAME
perlglossary - Perl Glossary
DESCRIPTION
A glossary of terms (technical and otherwise) used in the Perl documen-
tation. Other useful sources include the Free On-Line Dictionary of
Computing , the Jargon
File , and Wikipedia
.
A
accessor methods
A "method" used to indirectly inspect or update an "object"'s state
(its instance variables).
actual arguments
The scalar values that you supply to a "function" or "subroutine"
when you call it. For instance, when you call "power("puff")", the
string "puff" is the actual argument. See also "argument" and
"formal arguments".
address operator
Some languages work directly with the memory addresses of values,
but this can be like playing with fire. Perl provides a set of
asbestos gloves for handling all memory management. The closest to
an address operator in Perl is the backslash operator, but it gives
you a "hard reference", which is much safer than a memory address.
algorithm
A well-defined sequence of steps, clearly enough explained that
even a computer could do them.
alias
A nickname for something, which behaves in all ways as though you'd
used the original name instead of the nickname. Temporary aliases
are implicitly created in the loop variable for "foreach" loops, in
the $_ variable for map or grep operators, in $a and $b during
sort's comparison function, and in each element of @_ for the
"actual arguments" of a subroutine call. Permanent aliases are
explicitly created in packages by importing symbols or by assign-
ment to typeglobs. Lexically scoped aliases for package variables
are explicitly created by the our declaration.
alternatives
A list of possible choices from which you may select only one, as
in "Would you like door A, B, or C?" Alternatives in regular
expressions are separated with a single vertical bar: "|". Alter-
natives in normal Perl expressions are separated with a double ver-
tical bar: "||". Logical alternatives in "Boolean" expressions are
separated with either "||" or "or".
anonymous
Used to describe a "referent" that is not directly accessible
through a named "variable". Such a referent must be indirectly
accessible through at least one "hard reference". When the last
hard reference goes away, the anonymous referent is destroyed with-
out pity.
architecture
The kind of computer you're working on, where one "kind" of com-
puter means all those computers sharing a compatible machine lan-
guage. Since Perl programs are (typically) simple text files, not
executable images, a Perl program is much less sensitive to the
architecture it's running on than programs in other languages, such
as C, that are compiled into machine code. See also "platform" and
"operating system".
argument
A piece of data supplied to a program, "subroutine", "function", or
"method" to tell it what it's supposed to do. Also called a
"parameter".
ARGV
The name of the array containing the "argument" "vector" from the
command line. If you use the empty "<>" operator, "ARGV" is the
name of both the "filehandle" used to traverse the arguments and
the "scalar" containing the name of the current input file.
arithmetical operator
A "symbol" such as "+" or "/" that tells Perl to do the arithmetic
you were supposed to learn in grade school.
array
An ordered sequence of values, stored such that you can easily
access any of the values using an integer "subscript" that speci-
fies the value's "offset" in the sequence.
array context
An archaic expression for what is more correctly referred to as
"list context".
ASCII
The American Standard Code for Information Interchange (a 7-bit
character set adequate only for poorly representing English text).
Often used loosely to describe the lowest 128 values of the various
ISO-8859-X character sets, a bunch of mutually incompatible 8-bit
codes best described as half ASCII. See also "Unicode".
assertion
A component of a "regular expression" that must be true for the
pattern to match but does not necessarily match any characters
itself. Often used specifically to mean a "zero width" assertion.
assignment
An "operator" whose assigned mission in life is to change the value
of a "variable".
assignment operator
Either a regular "assignment", or a compound "operator" composed of
an ordinary assignment and some other operator, that changes the
value of a variable in place, that is, relative to its old value.
For example, "$a += 2" adds 2 to $a.
associative array
See "hash". Please.
associativity
Determines whether you do the left "operator" first or the right
"operator" first when you have "A "operator" B "operator" C" and
the two operators are of the same precedence. Operators like "+"
are left associative, while operators like "**" are right associa-
tive. See perlop for a list of operators and their associativity.
asynchronous
Said of events or activities whose relative temporal ordering is
indeterminate because too many things are going on at once. Hence,
an asynchronous event is one you didn't know when to expect.
atom
A "regular expression" component potentially matching a "substring"
containing one or more characters and treated as an indivisible
syntactic unit by any following "quantifier". (Contrast with an
"assertion" that matches something of "zero width" and may not be
quantified.)
atomic operation
When Democritus gave the word "atom" to the indivisible bits of
matter, he meant literally something that could not be cut: a-
(not) + tomos (cuttable). An atomic operation is an action that
can't be interrupted, not one forbidden in a nuclear-free zone.
attribute
A new feature that allows the declaration of variables and subrou-
tines with modifiers as in "sub foo : locked method". Also,
another name for an "instance variable" of an "object".
autogeneration
A feature of "operator overloading" of objects, whereby the behav-
ior of certain operators can be reasonably deduced using more fun-
damental operators. This assumes that the overloaded operators
will often have the same relationships as the regular operators.
See perlop.
autoincrement
To add one to something automatically, hence the name of the "++"
operator. To instead subtract one from something automatically is
known as an "autodecrement".
autoload
To load on demand. (Also called "lazy" loading.) Specifically, to
call an AUTOLOAD subroutine on behalf of an undefined subroutine.
autosplit
To split a string automatically, as the -a "switch" does when run-
ning under -p or -n in order to emulate "awk". (See also the
AutoSplit module, which has nothing to do with the -a switch, but a
lot to do with autoloading.)
autovivification
A Greco-Roman word meaning "to bring oneself to life". In Perl,
storage locations (lvalues) spontaneously generate themselves as
needed, including the creation of any "hard reference" values to
point to the next level of storage. The assignment
"$a[5][5][5][5][5] = "quintet"" potentially creates five scalar
storage locations, plus four references (in the first four scalar
locations) pointing to four new anonymous arrays (to hold the last
four scalar locations). But the point of autovivification is that
you don't have to worry about it.
AV Short for "array value", which refers to one of Perl's internal
data types that holds an "array". The "AV" type is a subclass of
"SV".
awk Descriptive editing term--short for "awkward". Also coincidentally
refers to a venerable text-processing language from which Perl
derived some of its high-level ideas.
B
backreference
A substring captured by a subpattern within unadorned parentheses
in a "regex". Backslashed decimal numbers ("\1", "\2", etc.)
later in the same pattern refer back to the corresponding subpat-
tern in the current match. Outside the pattern, the numbered vari-
ables ($1, $2, etc.) continue to refer to these same values, as
long as the pattern was the last successful match of the current
dynamic scope.
backtracking
The practice of saying, "If I had to do it all over, I'd do it dif-
ferently," and then actually going back and doing it all over dif-
ferently. Mathematically speaking, it's returning from an unsuc-
cessful recursion on a tree of possibilities. Perl backtracks when
it attempts to match patterns with a "regular expression", and its
earlier attempts don't pan out. See "Backtracking" in perlre.
backward compatibility
Means you can still run your old program because we didn't break
any of the features or bugs it was relying on.
bareword
A word sufficiently ambiguous to be deemed illegal under use strict
'subs'. In the absence of that stricture, a bareword is treated as
if quotes were around it.
base class
A generic "object" type; that is, a "class" from which other, more
specific classes are derived genetically by "inheritance". Also
called a "superclass" by people who respect their ancestors.
big-endian
From Swift: someone who eats eggs big end first. Also used of com-
puters that store the most significant "byte" of a word at a lower
byte address than the least significant byte. Often considered
superior to little-endian machines. See also "little-endian".
binary
Having to do with numbers represented in base 2. That means
there's basically two numbers, 0 and 1. Also used to describe a
"non-text file", presumably because such a file makes full use of
all the binary bits in its bytes. With the advent of "Unicode",
this distinction, already suspect, loses even more of its meaning.
binary operator
An "operator" that takes two operands.
bind
To assign a specific "network address" to a "socket".
bit An integer in the range from 0 to 1, inclusive. The smallest pos-
sible unit of information storage. An eighth of a "byte" or of a
dollar. (The term "Pieces of Eight" comes from being able to split
the old Spanish dollar into 8 bits, each of which still counted for
money. That's why a 25-cent piece today is still "two bits".)
bit shift
The movement of bits left or right in a computer word, which has
the effect of multiplying or dividing by a power of 2.
bit string
A sequence of bits that is actually being thought of as a sequence
of bits, for once.
bless
In corporate life, to grant official approval to a thing, as in,
"The VP of Engineering has blessed our WebCruncher project." Simi-
larly in Perl, to grant official approval to a "referent" so that
it can function as an "object", such as a WebCruncher object. See
"bless" in perlfunc.
block
What a "process" does when it has to wait for something: "My
process blocked waiting for the disk." As an unrelated noun, it
refers to a large chunk of data, of a size that the "operating sys-
tem" likes to deal with (normally a power of two such as 512 or
8192). Typically refers to a chunk of data that's coming from or
going to a disk file.
BLOCK
A syntactic construct consisting of a sequence of Perl statements
that is delimited by braces. The "if" and "while" statements are
defined in terms of BLOCKs, for instance. Sometimes we also say
"block" to mean a lexical scope; that is, a sequence of statements
that act like a "BLOCK", such as within an eval or a file, even
though the statements aren't delimited by braces.
block buffering
A method of making input and output efficient by passing one
"block" at a time. By default, Perl does block buffering to disk
files. See "buffer" and "command buffering".
Boolean
A value that is either "true" or "false".
Boolean context
A special kind of "scalar context" used in conditionals to decide
whether the "scalar value" returned by an expression is "true" or
"false". Does not evaluate as either a string or a number. See
"context".
breakpoint
A spot in your program where you've told the debugger to stop exe-
cution so you can poke around and see whether anything is wrong
yet.
broadcast
To send a "datagram" to multiple destinations simultaneously.
BSD A psychoactive drug, popular in the 80s, probably developed at U.
C. Berkeley or thereabouts. Similar in many ways to the prescrip-
tion-only medication called "System V", but infinitely more useful.
(Or, at least, more fun.) The full chemical name is "Berkeley
Standard Distribution".
bucket
A location in a "hash table" containing (potentially) multiple
entries whose keys "hash" to the same hash value according to its
hash function. (As internal policy, you don't have to worry about
it, unless you're into internals, or policy.)
buffer
A temporary holding location for data. Block buffering means that
the data is passed on to its destination whenever the buffer is
full. Line buffering means that it's passed on whenever a complete
line is received. Command buffering means that it's passed every
time you do a print command (or equivalent). If your output is
unbuffered, the system processes it one byte at a time without the
use of a holding area. This can be rather inefficient.
built-in
A "function" that is predefined in the language. Even when hidden
by "overriding", you can always get at a built-in function by qual-
ifying its name with the "CORE::" pseudo-package.
bundle
A group of related modules on "CPAN". (Also, sometimes refers to a
group of command-line switches grouped into one "switch cluster".)
byte
A piece of data worth eight bits in most places.
bytecode
A pidgin-like language spoken among 'droids when they don't wish to
reveal their orientation (see "endian"). Named after some similar
languages spoken (for similar reasons) between compilers and inter-
preters in the late 20th century. These languages are character-
ized by representing everything as a non-architecture-dependent
sequence of bytes.
C
C A language beloved by many for its inside-out "type" definitions,
inscrutable "precedence" rules, and heavy "overloading" of the
function-call mechanism. (Well, actually, people first switched to
C because they found lowercase identifiers easier to read than
upper.) Perl is written in C, so it's not surprising that Perl
borrowed a few ideas from it.
C preprocessor
The typical C compiler's first pass, which processes lines begin-
ning with "#" for conditional compilation and macro definition and
does various manipulations of the program text based on the current
definitions. Also known as cpp(1).
call by reference
An "argument"-passing mechanism in which the "formal arguments"
refer directly to the "actual arguments", and the "subroutine" can
change the actual arguments by changing the formal arguments. That
is, the formal argument is an "alias" for the actual argument. See
also "call by value".
call by value
An "argument"-passing mechanism in which the "formal arguments"
refer to a copy of the "actual arguments", and the "subroutine"
cannot change the actual arguments by changing the formal argu-
ments. See also "call by reference".
callback
A "handler" that you register with some other part of your program
in the hope that the other part of your program will "trigger" your
handler when some event of interest transpires.
canonical
Reduced to a standard form to facilitate comparison.
capturing
The use of parentheses around a "subpattern" in a "regular expres-
sion" to store the matched "substring" as a "backreference". (Cap-
tured strings are also returned as a list in "list context".)
character
A small integer representative of a unit of orthography. Histori-
cally, characters were usually stored as fixed-width integers (typ-
ically in a byte, or maybe two, depending on the character set),
but with the advent of UTF-8, characters are often stored in a
variable number of bytes depending on the size of the integer that
represents the character. Perl manages this transparently for you,
for the most part.
character class
A square-bracketed list of characters used in a "regular expres-
sion" to indicate that any character of the set may occur at a
given point. Loosely, any predefined set of characters so used.
character property
A predefined "character class" matchable by the "\p" "metasymbol".
Many standard properties are defined for "Unicode".
circumfix operator
An "operator" that surrounds its "operand", like the angle opera-
tor, or parentheses, or a hug.
class
A user-defined "type", implemented in Perl via a "package" that
provides (either directly or by inheritance) methods (that is, sub-
routines) to handle instances of the class (its objects). See also
"inheritance".
class method
A "method" whose "invocant" is a "package" name, not an "object"
reference. A method associated with the class as a whole.
client
In networking, a "process" that initiates contact with a "server"
process in order to exchange data and perhaps receive a service.
cloister
A "cluster" used to restrict the scope of a "regular expression
modifier".
closure
An "anonymous" subroutine that, when a reference to it is generated
at run time, keeps track of the identities of externally visible
lexical variables even after those lexical variables have suppos-
edly gone out of "scope". They're called "closures" because this
sort of behavior gives mathematicians a sense of closure.
cluster
A parenthesized "subpattern" used to group parts of a "regular
expression" into a single "atom".
CODE
The word returned by the ref function when you apply it to a refer-
ence to a subroutine. See also "CV".
code generator
A system that writes code for you in a low-level language, such as
code to implement the backend of a compiler. See "program genera-
tor".
code subpattern
A "regular expression" subpattern whose real purpose is to execute
some Perl code, for example, the "(?{...})" and "(??{...})" subpat-
terns.
collating sequence
The order into which characters sort. This is used by "string"
comparison routines to decide, for example, where in this glossary
to put "collating sequence".
command
In "shell" programming, the syntactic combination of a program name
and its arguments. More loosely, anything you type to a shell (a
command interpreter) that starts it doing something. Even more
loosely, a Perl "statement", which might start with a "label" and
typically ends with a semicolon.
command buffering
A mechanism in Perl that lets you store up the output of each Perl
"command" and then flush it out as a single request to the "operat-
ing system". It's enabled by setting the $| ($AUTOFLUSH) variable
to a true value. It's used when you don't want data sitting around
not going where it's supposed to, which may happen because the
default on a "file" or "pipe" is to use "block buffering".
command name
The name of the program currently executing, as typed on the com-
mand line. In C, the "command" name is passed to the program as
the first command-line argument. In Perl, it comes in separately
as $0.
command-line arguments
The values you supply along with a program name when you tell a
"shell" to execute a "command". These values are passed to a Perl
program through @ARGV.
comment
A remark that doesn't affect the meaning of the program. In Perl,
a comment is introduced by a "#" character and continues to the end
of the line.
compilation unit
The "file" (or "string", in the case of eval) that is currently
being compiled.
compile phase
Any time before Perl starts running your main program. See also
"run phase". Compile phase is mostly spent in "compile time", but
may also be spent in "run time" when "BEGIN" blocks, use declara-
tions, or constant subexpressions are being evaluated. The startup
and import code of any use declaration is also run during compile
phase.
compile time
The time when Perl is trying to make sense of your code, as opposed
to when it thinks it knows what your code means and is merely try-
ing to do what it thinks your code says to do, which is "run time".
compiler
Strictly speaking, a program that munches up another program and
spits out yet another file containing the program in a "more exe-
cutable" form, typically containing native machine instructions.
The perl program is not a compiler by this definition, but it does
contain a kind of compiler that takes a program and turns it into a
more executable form (syntax trees) within the perl process itself,
which the "interpreter" then interprets. There are, however,
extension modules to get Perl to act more like a "real" compiler.
See O.
composer
A "constructor" for a "referent" that isn't really an "object",
like an anonymous array or a hash (or a sonata, for that matter).
For example, a pair of braces acts as a composer for a hash, and a
pair of brackets acts as a composer for an array. See "Making Ref-
erences" in perlref.
concatenation
The process of gluing one cat's nose to another cat's tail. Also,
a similar operation on two strings.
conditional
Something "iffy". See "Boolean context".
connection
In telephony, the temporary electrical circuit between the caller's
and the callee's phone. In networking, the same kind of temporary
circuit between a "client" and a "server".
construct
As a noun, a piece of syntax made up of smaller pieces. As a tran-
sitive verb, to create an "object" using a "constructor".
constructor
Any "class method", instance "method", or "subroutine" that com-
poses, initializes, blesses, and returns an "object". Sometimes we
use the term loosely to mean a "composer".
context
The surroundings, or environment. The context given by the sur-
rounding code determines what kind of data a particular "expres-
sion" is expected to return. The three primary contexts are "list
context", "scalar context", and "void context". Scalar context is
sometimes subdivided into "Boolean context", "numeric context",
"string context", and "void context". There's also a "don't care"
scalar context (which is dealt with in Programming Perl, Third Edi-
tion, Chapter 2, "Bits and Pieces" if you care).
continuation
The treatment of more than one physical "line" as a single logical
line. "Makefile" lines are continued by putting a backslash before
the "newline". Mail headers as defined by RFC 822 are continued by
putting a space or tab after the newline. In general, lines in
Perl do not need any form of continuation mark, because "white-
space" (including newlines) is gleefully ignored. Usually.
core dump
The corpse of a "process", in the form of a file left in the "work-
ing directory" of the process, usually as a result of certain kinds
of fatal error.
CPAN
The Comprehensive Perl Archive Network. (See "What modules and
extensions are available for Perl? What is CPAN? What does
CPAN/src/... mean?" in perlfaq2).
cracker
Someone who breaks security on computer systems. A cracker may be
a true "hacker" or only a "script kiddie".
current package
The "package" in which the current statement is compiled. Scan
backwards in the text of your program through the current lexical
scope or any enclosing lexical scopes till you find a package dec-
laration. That's your current package name.
current working directory
See "working directory".
currently selected output channel
The last "filehandle" that was designated with select("FILEHAN-
DLE"); "STDOUT", if no filehandle has been selected.
CV An internal "code value" typedef, holding a "subroutine". The "CV"
type is a subclass of "SV".
D
dangling statement
A bare, single "statement", without any braces, hanging off an "if"
or "while" conditional. C allows them. Perl doesn't.
data structure
How your various pieces of data relate to each other and what shape
they make when you put them all together, as in a rectangular table
or a triangular-shaped tree.
data type
A set of possible values, together with all the operations that
know how to deal with those values. For example, a numeric data
type has a certain set of numbers that you can work with and vari-
ous mathematical operations that you can do on the numbers but
would make little sense on, say, a string such as "Kilroy".
Strings have their own operations, such as "concatenation". Com-
pound types made of a number of smaller pieces generally have oper-
ations to compose and decompose them, and perhaps to rearrange
them. Objects that model things in the real world often have oper-
ations that correspond to real activities. For instance, if you
model an elevator, your elevator object might have an "open_door()"
"method".
datagram
A packet of data, such as a "UDP" message, that (from the viewpoint
of the programs involved) can be sent independently over the net-
work. (In fact, all packets are sent independently at the "IP"
level, but "stream" protocols such as "TCP" hide this from your
program.)
DBM Stands for "Data Base Management" routines, a set of routines that
emulate an "associative array" using disk files. The routines use
a dynamic hashing scheme to locate any entry with only two disk
accesses. DBM files allow a Perl program to keep a persistent
"hash" across multiple invocations. You can tie your hash vari-
ables to various DBM implementations--see AnyDBM_File and DB_File.
declaration
An "assertion" that states something exists and perhaps describes
what it's like, without giving any commitment as to how or where
you'll use it. A declaration is like the part of your recipe that
says, "two cups flour, one large egg, four or five tadpoles..."
See "statement" for its opposite. Note that some declarations also
function as statements. Subroutine declarations also act as defi-
nitions if a body is supplied.
decrement
To subtract a value from a variable, as in "decrement $x" (meaning
to remove 1 from its value) or "decrement $x by 3".
default
A "value" chosen for you if you don't supply a value of your own.
defined
Having a meaning. Perl thinks that some of the things people try
to do are devoid of meaning, in particular, making use of variables
that have never been given a "value" and performing certain opera-
tions on data that isn't there. For example, if you try to read
data past the end of a file, Perl will hand you back an undefined
value. See also "false" and "defined" in perlfunc.
delimiter
A "character" or "string" that sets bounds to an arbitrarily-sized
textual object, not to be confused with a "separator" or "termina-
tor". "To delimit" really just means "to surround" or "to enclose"
(like these parentheses are doing).
dereference
A fancy computer science term meaning "to follow a "reference" to
what it points to". The "de" part of it refers to the fact that
you're taking away one level of "indirection".
derived class
A "class" that defines some of its methods in terms of a more
generic class, called a "base class". Note that classes aren't
classified exclusively into base classes or derived classes: a
class can function as both a derived class and a base class simul-
taneously, which is kind of classy.
descriptor
See "file descriptor".
destroy
To deallocate the memory of a "referent" (first triggering its
"DESTROY" method, if it has one).
destructor
A special "method" that is called when an "object" is thinking
about destroying itself. A Perl program's "DESTROY" method doesn't
do the actual destruction; Perl just triggers the method in case
the "class" wants to do any associated cleanup.
device
A whiz-bang hardware gizmo (like a disk or tape drive or a modem or
a joystick or a mouse) attached to your computer, that the "operat-
ing system" tries to make look like a "file" (or a bunch of files).
Under Unix, these fake files tend to live in the /dev directory.
directive
A "pod" directive. See perlpod.
directory
A special file that contains other files. Some operating systems
call these "folders", "drawers", or "catalogs".
directory handle
A name that represents a particular instance of opening a directory
to read it, until you close it. See the opendir function.
dispatch
To send something to its correct destination. Often used metaphor-
ically to indicate a transfer of programmatic control to a destina-
tion selected algorithmically, often by lookup in a table of func-
tion references or, in the case of object methods, by traversing
the inheritance tree looking for the most specific definition for
the method.
distribution
A standard, bundled release of a system of software. The default
usage implies source code is included. If that is not the case, it
will be called a "binary-only" distribution.
dweomer
An enchantment, illusion, phantasm, or jugglery. Said when Perl's
magical "dwimmer" effects don't do what you expect, but rather seem
to be the product of arcane dweomercraft, sorcery, or wonder work-
ing. [From Old English]
dwimmer
DWIM is an acronym for "Do What I Mean", the principle that some-
thing should just do what you want it to do without an undue amount
of fuss. A bit of code that does "dwimming" is a "dwimmer". Dwim-
ming can require a great deal of behind-the-scenes magic, which (if
it doesn't stay properly behind the scenes) is called a "dweomer"
instead.
dynamic scoping
Dynamic scoping works over a dynamic scope, making variables visi-
ble throughout the rest of the "block" in which they are first used
and in any subroutines that are called by the rest of the block.
Dynamically scoped variables can have their values temporarily
changed (and implicitly restored later) by a local operator. (Com-
pare "lexical scoping".) Used more loosely to mean how a subrou-
tine that is in the middle of calling another subroutine "contains"
that subroutine at "run time".
E
eclectic
Derived from many sources. Some would say too many.
element
A basic building block. When you're talking about an "array", it's
one of the items that make up the array.
embedding
When something is contained in something else, particularly when
that might be considered surprising: "I've embedded a complete Perl
interpreter in my editor!"
empty subclass test
The notion that an empty "derived class" should behave exactly like
its "base class".
en passant
When you change a "value" as it is being copied. [From French, "in
passing", as in the exotic pawn-capturing maneuver in chess.]
encapsulation
The veil of abstraction separating the "interface" from the "imple-
mentation" (whether enforced or not), which mandates that all
access to an "object"'s state be through methods alone.
endian
See "little-endian" and "big-endian".
environment
The collective set of environment variables your "process" inherits
from its parent. Accessed via %ENV.
environment variable
A mechanism by which some high-level agent such as a user can pass
its preferences down to its future offspring (child processes,
grandchild processes, great-grandchild processes, and so on). Each
environment variable is a "key"/"value" pair, like one entry in a
"hash".
EOF End of File. Sometimes used metaphorically as the terminating
string of a "here document".
errno
The error number returned by a "syscall" when it fails. Perl
refers to the error by the name $! (or $OS_ERROR if you use the
English module).
error
See "exception" or "fatal error".
escape sequence
See "metasymbol".
exception
A fancy term for an error. See "fatal error".
exception handling
The way a program responds to an error. The exception handling
mechanism in Perl is the eval operator.
exec
To throw away the current "process"'s program and replace it with
another without exiting the process or relinquishing any resources
held (apart from the old memory image).
executable file
A "file" that is specially marked to tell the "operating system"
that it's okay to run this file as a program. Usually shortened to
"executable".
execute
To run a program or "subroutine". (Has nothing to do with the kill
built-in, unless you're trying to run a "signal handler".)
execute bit
The special mark that tells the operating system it can run this
program. There are actually three execute bits under Unix, and
which bit gets used depends on whether you own the file singularly,
collectively, or not at all.
exit status
See "status".
export
To make symbols from a "module" available for "import" by other
modules.
expression
Anything you can legally say in a spot where a "value" is required.
Typically composed of literals, variables, operators, functions,
and "subroutine" calls, not necessarily in that order.
extension
A Perl module that also pulls in compiled C or C++ code. More gen-
erally, any experimental option that can be compiled into Perl,
such as multithreading.
F
false
In Perl, any value that would look like "" or "0" if evaluated in a
string context. Since undefined values evaluate to "", all unde-
fined values are false, but not all false values are undefined.
FAQ Frequently Asked Question (although not necessarily frequently
answered, especially if the answer appears in the Perl FAQ shipped
standard with Perl).
fatal error
An uncaught "exception", which causes termination of the "process"
after printing a message on your "standard error" stream. Errors
that happen inside an eval are not fatal. Instead, the eval termi-
nates after placing the exception message in the $@ ($EVAL_ERROR)
variable. You can try to provoke a fatal error with the die opera-
tor (known as throwing or raising an exception), but this may be
caught by a dynamically enclosing eval. If not caught, the die
becomes a fatal error.
field
A single piece of numeric or string data that is part of a longer
"string", "record", or "line". Variable-width fields are usually
split up by separators (so use split to extract the fields), while
fixed-width fields are usually at fixed positions (so use unpack).
Instance variables are also known as fields.
FIFO
First In, First Out. See also "LIFO". Also, a nickname for a
"named pipe".
file
A named collection of data, usually stored on disk in a "directory"
in a "filesystem". Roughly like a document, if you're into office
metaphors. In modern filesystems, you can actually give a file
more than one name. Some files have special properties, like
directories and devices.
file descriptor
The little number the "operating system" uses to keep track of
which opened "file" you're talking about. Perl hides the file
descriptor inside a "standard I/O" stream and then attaches the
stream to a "filehandle".
file test operator
A built-in unary operator that you use to determine whether some-
thing is "true" about a file, such as "-o $filename" to test
whether you're the owner of the file.
fileglob
A "wildcard" match on filenames. See the glob function.
filehandle
An identifier (not necessarily related to the real name of a file)
that represents a particular instance of opening a file until you
close it. If you're going to open and close several different
files in succession, it's fine to open each of them with the same
filehandle, so you don't have to write out separate code to process
each file.
filename
One name for a file. This name is listed in a "directory", and you
can use it in an open to tell the "operating system" exactly which
file you want to open, and associate the file with a "filehandle"
which will carry the subsequent identity of that file in your pro-
gram, until you close it.
filesystem
A set of directories and files residing on a partition of the disk.
Sometimes known as a "partition". You can change the file's name
or even move a file around from directory to directory within a
filesystem without actually moving the file itself, at least under
Unix.
filter
A program designed to take a "stream" of input and transform it
into a stream of output.
flag
We tend to avoid this term because it means so many things. It may
mean a command-line "switch" that takes no argument itself (such as
Perl's -n and -p flags) or, less frequently, a single-bit indicator
(such as the "O_CREAT" and "O_EXCL" flags used in sysopen).
floating point
A method of storing numbers in "scientific notation", such that the
precision of the number is independent of its magnitude (the deci-
mal point "floats"). Perl does its numeric work with floating-
point numbers (sometimes called "floats"), when it can't get away
with using integers. Floating-point numbers are mere approxima-
tions of real numbers.
flush
The act of emptying a "buffer", often before it's full.
FMTEYEWTK
Far More Than Everything You Ever Wanted To Know. An exhaustive
treatise on one narrow topic, something of a super-"FAQ". See Tom
for far more.
fork
To create a child "process" identical to the parent process at its
moment of conception, at least until it gets ideas of its own. A
thread with protected memory.
formal arguments
The generic names by which a "subroutine" knows its arguments. In
many languages, formal arguments are always given individual names,
but in Perl, the formal arguments are just the elements of an
array. The formal arguments to a Perl program are $ARGV[0],
$ARGV[1], and so on. Similarly, the formal arguments to a Perl
subroutine are $_[0], $_[1], and so on. You may give the arguments
individual names by assigning the values to a my list. See also
"actual arguments".
format
A specification of how many spaces and digits and things to put
somewhere so that whatever you're printing comes out nice and
pretty.
freely available
Means you don't have to pay money to get it, but the copyright on
it may still belong to someone else (like Larry).
freely redistributable
Means you're not in legal trouble if you give a bootleg copy of it
to your friends and we find out about it. In fact, we'd rather you
gave a copy to all your friends.
freeware
Historically, any software that you give away, particularly if you
make the source code available as well. Now often called "open
source software". Recently there has been a trend to use the term
in contradistinction to "open source software", to refer only to
free software released under the Free Software Foundation's GPL
(General Public License), but this is difficult to justify etymo-
logically.
function
Mathematically, a mapping of each of a set of input values to a
particular output value. In computers, refers to a "subroutine" or
"operator" that returns a "value". It may or may not have input
values (called arguments).
funny character
Someone like Larry, or one of his peculiar friends. Also refers to
the strange prefixes that Perl requires as noun markers on its
variables.
garbage collection
A misnamed feature--it should be called, "expecting your mother to
pick up after you". Strictly speaking, Perl doesn't do this, but
it relies on a reference-counting mechanism to keep things tidy.
However, we rarely speak strictly and will often refer to the ref-
erence-counting scheme as a form of garbage collection. (If it's
any comfort, when your interpreter exits, a "real" garbage collec-
tor runs to make sure everything is cleaned up if you've been messy
with circular references and such.)
G
GID Group ID--in Unix, the numeric group ID that the "operating system"
uses to identify you and members of your "group".
glob
Strictly, the shell's "*" character, which will match a "glob" of
characters when you're trying to generate a list of filenames.
Loosely, the act of using globs and similar symbols to do pattern
matching. See also "fileglob" and "typeglob".
global
Something you can see from anywhere, usually used of variables and
subroutines that are visible everywhere in your program. In Perl,
only certain special variables are truly global--most variables
(and all subroutines) exist only in the current "package". Global
variables can be declared with our. See "our" in perlfunc.
global destruction
The "garbage collection" of globals (and the running of any associ-
ated object destructors) that takes place when a Perl "interpreter"
is being shut down. Global destruction should not be confused with
the Apocalypse, except perhaps when it should.
glue language
A language such as Perl that is good at hooking things together
that weren't intended to be hooked together.
granularity
The size of the pieces you're dealing with, mentally speaking.
greedy
A "subpattern" whose "quantifier" wants to match as many things as
possible.
grep
Originally from the old Unix editor command for "Globally search
for a Regular Expression and Print it", now used in the general
sense of any kind of search, especially text searches. Perl has a
built-in grep function that searches a list for elements matching
any given criterion, whereas the grep(1) program searches for lines
matching a "regular expression" in one or more files.
group
A set of users of which you are a member. In some operating sys-
tems (like Unix), you can give certain file access permissions to
other members of your group.
GV An internal "glob value" typedef, holding a "typeglob". The "GV"
type is a subclass of "SV".
H
hacker
Someone who is brilliantly persistent in solving technical prob-
lems, whether these involve golfing, fighting orcs, or programming.
Hacker is a neutral term, morally speaking. Good hackers are not
to be confused with evil crackers or clueless script kiddies. If
you confuse them, we will presume that you are either evil or clue-
less.
handler
A "subroutine" or "method" that is called by Perl when your program
needs to respond to some internal event, such as a "signal", or an
encounter with an operator subject to "operator overloading". See
also "callback".
hard reference
A "scalar" "value" containing the actual address of a "referent",
such that the referent's "reference" count accounts for it. (Some
hard references are held internally, such as the implicit reference
from one of a "typeglob"'s variable slots to its corresponding ref-
erent.) A hard reference is different from a "symbolic reference".
hash
An unordered association of "key"/"value" pairs, stored such that
you can easily use a string "key" to look up its associated data
"value". This glossary is like a hash, where the word to be
defined is the key, and the definition is the value. A hash is
also sometimes septisyllabically called an "associative array",
which is a pretty good reason for simply calling it a "hash"
instead.
hash table
A data structure used internally by Perl for implementing associa-
tive arrays (hashes) efficiently. See also "bucket".
header file
A file containing certain required definitions that you must
include "ahead" of the rest of your program to do certain obscure
operations. A C header file has a .h extension. Perl doesn't
really have header files, though historically Perl has sometimes
used translated .h files with a .ph extension. See "require" in
perlfunc. (Header files have been superseded by the "module" mech-
anism.)
here document
So called because of a similar construct in shells that pretends
that the lines following the "command" are a separate "file" to be
fed to the command, up to some terminating string. In Perl, how-
ever, it's just a fancy form of quoting.
hexadecimal
A number in base 16, "hex" for short. The digits for 10 through 16
are customarily represented by the letters "a" through "f". Hexa-
decimal constants in Perl start with "0x". See also "hex" in perl-
func.
home directory
The directory you are put into when you log in. On a Unix system,
the name is often placed into $ENV{HOME} or $ENV{LOGDIR} by login,
but you can also find it with "(getpwuid($<))[7]". (Some platforms
do not have a concept of a home directory.)
host
The computer on which a program or other data resides.
hubris
Excessive pride, the sort of thing Zeus zaps you for. Also the
quality that makes you write (and maintain) programs that other
people won't want to say bad things about. Hence, the third great
virtue of a programmer. See also "laziness" and "impatience".
HV Short for a "hash value" typedef, which holds Perl's internal rep-
resentation of a hash. The "HV" type is a subclass of "SV".
I
identifier
A legally formed name for most anything in which a computer program
might be interested. Many languages (including Perl) allow identi-
fiers that start with a letter and contain letters and digits.
Perl also counts the underscore character as a valid letter. (Perl
also has more complicated names, such as "qualified" names.)
impatience
The anger you feel when the computer is being lazy. This makes you
write programs that don't just react to your needs, but actually
anticipate them. Or at least that pretend to. Hence, the second
great virtue of a programmer. See also "laziness" and "hubris".
implementation
How a piece of code actually goes about doing its job. Users of
the code should not count on implementation details staying the
same unless they are part of the published "interface".
import
To gain access to symbols that are exported from another module.
See "use" in perlfunc.
increment
To increase the value of something by 1 (or by some other number,
if so specified).
indexing
In olden days, the act of looking up a "key" in an actual index
(such as a phone book), but now merely the act of using any kind of
key or position to find the corresponding "value", even if no index
is involved. Things have degenerated to the point that Perl's
index function merely locates the position (index) of one string in
another.
indirect filehandle
An "expression" that evaluates to something that can be used as a
"filehandle": a "string" (filehandle name), a "typeglob", a type-
glob "reference", or a low-level "IO" object.
indirect object
In English grammar, a short noun phrase between a verb and its
direct object indicating the beneficiary or recipient of the
action. In Perl, "print STDOUT "$foo\n";" can be understood as
"verb indirect-object object" where "STDOUT" is the recipient of
the print action, and "$foo" is the object being printed. Simi-
larly, when invoking a "method", you might place the invocant
between the method and its arguments:
$gollum = new Pathetic::Creature "Smeagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
indirect object slot
The syntactic position falling between a method call and its argu-
ments when using the indirect object invocation syntax. (The slot
is distinguished by the absence of a comma between it and the next
argument.) "STDERR" is in the indirect object slot here:
print STDERR "Awake! Awake! Fear, Fire,
Foes! Awake!\n";
indirection
If something in a program isn't the value you're looking for but
indicates where the value is, that's indirection. This can be done
with either symbolic references or hard references.
infix
An "operator" that comes in between its operands, such as multipli-
cation in "24 * 7".
inheritance
What you get from your ancestors, genetically or otherwise. If you
happen to be a "class", your ancestors are called base classes and
your descendants are called derived classes. See "single inheri-
tance" and "multiple inheritance".
instance
Short for "an instance of a class", meaning an "object" of that
"class".
instance variable
An "attribute" of an "object"; data stored with the particular
object rather than with the class as a whole.
integer
A number with no fractional (decimal) part. A counting number,
like 1, 2, 3, and so on, but including 0 and the negatives.
interface
The services a piece of code promises to provide forever, in con-
trast to its "implementation", which it should feel free to change
whenever it likes.
interpolation
The insertion of a scalar or list value somewhere in the middle of
another value, such that it appears to have been there all along.
In Perl, variable interpolation happens in double-quoted strings
and patterns, and list interpolation occurs when constructing the
list of values to pass to a list operator or other such construct
that takes a "LIST".
interpreter
Strictly speaking, a program that reads a second program and does
what the second program says directly without turning the program
into a different form first, which is what compilers do. Perl is
not an interpreter by this definition, because it contains a kind
of compiler that takes a program and turns it into a more exe-
cutable form (syntax trees) within the perl process itself, which
the Perl "run time" system then interprets.
invocant
The agent on whose behalf a "method" is invoked. In a "class"
method, the invocant is a package name. In an "instance" method,
the invocant is an object reference.
invocation
The act of calling up a deity, daemon, program, method, subroutine,
or function to get it do what you think it's supposed to do. We
usually "call" subroutines but "invoke" methods, since it sounds
cooler.
I/O Input from, or output to, a "file" or "device".
IO An internal I/O object. Can also mean "indirect object".
IP Internet Protocol, or Intellectual Property.
IPC Interprocess Communication.
is-a
A relationship between two objects in which one object is consid-
ered to be a more specific version of the other, generic object: "A
camel is a mammal." Since the generic object really only exists in
a Platonic sense, we usually add a little abstraction to the notion
of objects and think of the relationship as being between a generic
"base class" and a specific "derived class". Oddly enough, Pla-
tonic classes don't always have Platonic relationships--see "inher-
itance".
iteration
Doing something repeatedly.
iterator
A special programming gizmo that keeps track of where you are in
something that you're trying to iterate over. The "foreach" loop
in Perl contains an iterator; so does a hash, allowing you to each
through it.
IV The integer four, not to be confused with six, Tom's favorite edi-
tor. IV also means an internal Integer Value of the type a
"scalar" can hold, not to be confused with an "NV".
J
JAPH
"Just Another Perl Hacker," a clever but cryptic bit of Perl code
that when executed, evaluates to that string. Often used to illus-
trate a particular Perl feature, and something of an ungoing Obfus-
cated Perl Contest seen in Usenix signatures.
K
key The string index to a "hash", used to look up the "value" associ-
ated with that key.
keyword
See "reserved words".
L
label
A name you give to a "statement" so that you can talk about that
statement elsewhere in the program.
laziness
The quality that makes you go to great effort to reduce overall
energy expenditure. It makes you write labor-saving programs that
other people will find useful, and document what you wrote so you
don't have to answer so many questions about it. Hence, the first
great virtue of a programmer. Also hence, this book. See also
"impatience" and "hubris".
left shift
A "bit shift" that multiplies the number by some power of 2.
leftmost longest
The preference of the "regular expression" engine to match the
leftmost occurrence of a "pattern", then given a position at which
a match will occur, the preference for the longest match (presuming
the use of a "greedy" quantifier). See perlre for much more on
this subject.
lexeme
Fancy term for a "token".
lexer
Fancy term for a "tokener".
lexical analysis
Fancy term for "tokenizing".
lexical scoping
Looking at your Oxford English Dictionary through a microscope.
(Also known as "static scoping", because dictionaries don't change
very fast.) Similarly, looking at variables stored in a private
dictionary (namespace) for each scope, which are visible only from
their point of declaration down to the end of the lexical scope in
which they are declared. --Syn. "static scoping". --Ant. "dynamic
scoping".
lexical variable
A "variable" subject to "lexical scoping", declared by my. Often
just called a "lexical". (The our declaration declares a lexically
scoped name for a global variable, which is not itself a lexical
variable.)
library
Generally, a collection of procedures. In ancient days, referred
to a collection of subroutines in a .pl file. In modern times,
refers more often to the entire collection of Perl modules on your
system.
LIFO
Last In, First Out. See also "FIFO". A LIFO is usually called a
"stack".
line
In Unix, a sequence of zero or more non-newline characters termi-
nated with a "newline" character. On non-Unix machines, this is
emulated by the C library even if the underlying "operating system"
has different ideas.
line buffering
Used by a "standard I/O" output stream that flushes its "buffer"
after every "newline". Many standard I/O libraries automatically
set up line buffering on output that is going to the terminal.
line number
The number of lines read previous to this one, plus 1. Perl keeps
a separate line number for each source or input file it opens. The
current source file's line number is represented by "__LINE__".
The current input line number (for the file that was most recently
read via "") is represented by the $. ($INPUT_LINE_NUMBER)
variable. Many error messages report both values, if available.
link
Used as a noun, a name in a "directory", representing a "file". A
given file can have multiple links to it. It's like having the
same phone number listed in the phone directory under different
names. As a verb, to resolve a partially compiled file's unre-
solved symbols into a (nearly) executable image. Linking can gen-
erally be static or dynamic, which has nothing to do with static or
dynamic scoping.
LIST
A syntactic construct representing a comma-separated list of
expressions, evaluated to produce a "list value". Each "expres-
sion" in a "LIST" is evaluated in "list context" and interpolated
into the list value.
list
An ordered set of scalar values.
list context
The situation in which an "expression" is expected by its surround-
ings (the code calling it) to return a list of values rather than a
single value. Functions that want a "LIST" of arguments tell those
arguments that they should produce a list value. See also "con-
text".
list operator
An "operator" that does something with a list of values, such as
join or grep. Usually used for named built-in operators (such as
print, unlink, and system) that do not require parentheses around
their "argument" list.
list value
An unnamed list of temporary scalar values that may be passed
around within a program from any list-generating function to any
function or construct that provides a "list context".
literal
A token in a programming language such as a number or "string" that
gives you an actual "value" instead of merely representing possible
values as a "variable" does.
little-endian
From Swift: someone who eats eggs little end first. Also used of
computers that store the least significant "byte" of a word at a
lower byte address than the most significant byte. Often consid-
ered superior to big-endian machines. See also "big-endian".
local
Not meaning the same thing everywhere. A global variable in Perl
can be localized inside a dynamic scope via the local operator.
logical operator
Symbols representing the concepts "and", "or", "xor", and "not".
lookahead
An "assertion" that peeks at the string to the right of the current
match location.
lookbehind
An "assertion" that peeks at the string to the left of the current
match location.
loop
A construct that performs something repeatedly, like a roller
coaster.
loop control statement
Any statement within the body of a loop that can make a loop prema-
turely stop looping or skip an "iteration". Generally you
shouldn't try this on roller coasters.
loop label
A kind of key or name attached to a loop (or roller coaster) so
that loop control statements can talk about which loop they want to
control.
lvaluable
Able to serve as an "lvalue".
lvalue
Term used by language lawyers for a storage location you can assign
a new "value" to, such as a "variable" or an element of an "array".
The "l" is short for "left", as in the left side of an assignment,
a typical place for lvalues. An "lvaluable" function or expression
is one to which a value may be assigned, as in "pos($x) = 10".
lvalue modifier
An adjectival pseudofunction that warps the meaning of an "lvalue"
in some declarative fashion. Currently there are three lvalue mod-
ifiers: my, our, and local.
M
magic
Technically speaking, any extra semantics attached to a variable
such as $!, $0, %ENV, or %SIG, or to any tied variable. Magical
things happen when you diddle those variables.
magical increment
An "increment" operator that knows how to bump up alphabetics as
well as numbers.
magical variables
Special variables that have side effects when you access them or
assign to them. For example, in Perl, changing elements of the
%ENV array also changes the corresponding environment variables
that subprocesses will use. Reading the $! variable gives you the
current system error number or message.
Makefile
A file that controls the compilation of a program. Perl programs
don't usually need a "Makefile" because the Perl compiler has
plenty of self-control.
man The Unix program that displays online documentation (manual pages)
for you.
manpage
A "page" from the manuals, typically accessed via the man(1) com-
mand. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of
BUGS, and so on, and is typically longer than a page. There are
manpages documenting commands, syscalls, "library" functions,
devices, protocols, files, and such. In this book, we call any
piece of standard Perl documentation (like perlop or perldelta) a
manpage, no matter what format it's installed in on your system.
matching
See "pattern matching".
member data
See "instance variable".
memory
This always means your main memory, not your disk. Clouding the
issue is the fact that your machine may implement "virtual" memory;
that is, it will pretend that it has more memory than it really
does, and it'll use disk space to hold inactive bits. This can
make it seem like you have a little more memory than you really do,
but it's not a substitute for real memory. The best thing that can
be said about virtual memory is that it lets your performance
degrade gradually rather than suddenly when you run out of real
memory. But your program can die when you run out of virtual mem-
ory too, if you haven't thrashed your disk to death first.
metacharacter
A "character" that is not supposed to be treated normally. Which
characters are to be treated specially as metacharacters varies
greatly from context to context. Your "shell" will have certain
metacharacters, double-quoted Perl strings have other metacharac-
ters, and "regular expression" patterns have all the double-quote
metacharacters plus some extra ones of their own.
metasymbol
Something we'd call a "metacharacter" except that it's a sequence
of more than one character. Generally, the first character in the
sequence must be a true metacharacter to get the other characters
in the metasymbol to misbehave along with it.
method
A kind of action that an "object" can take if you tell it to. See
perlobj.
minimalism
The belief that "small is beautiful." Paradoxically, if you say
something in a small language, it turns out big, and if you say it
in a big language, it turns out small. Go figure.
mode
In the context of the stat syscall, refers to the field holding the
"permission bits" and the type of the "file".
modifier
See "statement modifier", "regular expression modifier", and
"lvalue modifier", not necessarily in that order.
module
A "file" that defines a "package" of (almost) the same name, which
can either "export" symbols or function as an "object" class. (A
module's main .pm file may also load in other files in support of
the module.) See the use built-in.
modulus
An integer divisor when you're interested in the remainder instead
of the quotient.
monger
Short for Perl Monger, a purveyor of Perl.
mortal
A temporary value scheduled to die when the current statement fin-
ishes.
multidimensional array
An array with multiple subscripts for finding a single element.
Perl implements these using references--see perllol and perldsc.
multiple inheritance
The features you got from your mother and father, mixed together
unpredictably. (See also "inheritance", and "single inheritance".)
In computer languages (including Perl), the notion that a given
class may have multiple direct ancestors or base classes.
N
named pipe
A "pipe" with a name embedded in the "filesystem" so that it can be
accessed by two unrelated processes.
namespace
A domain of names. You needn't worry about whether the names in
one such domain have been used in another. See "package".
network address
The most important attribute of a socket, like your telephone's
telephone number. Typically an IP address. See also "port".
newline
A single character that represents the end of a line, with the
ASCII value of 012 octal under Unix (but 015 on a Mac), and repre-
sented by "\n" in Perl strings. For Windows machines writing text
files, and for certain physical devices like terminals, the single
newline gets automatically translated by your C library into a line
feed and a carriage return, but normally, no translation is done.
NFS Network File System, which allows you to mount a remote filesystem
as if it were local.
null character
A character with the ASCII value of zero. It's used by C to termi-
nate strings, but Perl allows strings to contain a null.
null list
A "list value" with zero elements, represented in Perl by "()".
null string
A "string" containing no characters, not to be confused with a
string containing a "null character", which has a positive length
and is "true".
numeric context
The situation in which an expression is expected by its surround-
ings (the code calling it) to return a number. See also "context"
and "string context".
NV Short for Nevada, no part of which will ever be confused with civi-
lization. NV also means an internal floating-point Numeric Value
of the type a "scalar" can hold, not to be confused with an "IV".
nybble
Half a "byte", equivalent to one "hexadecimal" digit, and worth
four bits.
O
object
An "instance" of a "class". Something that "knows" what user-
defined type (class) it is, and what it can do because of what
class it is. Your program can request an object to do things, but
the object gets to decide whether it wants to do them or not. Some
objects are more accommodating than others.
octal
A number in base 8. Only the digits 0 through 7 are allowed.
Octal constants in Perl start with 0, as in 013. See also the oct
function.
offset
How many things you have to skip over when moving from the begin-
ning of a string or array to a specific position within it. Thus,
the minimum offset is zero, not one, because you don't skip any-
thing to get to the first item.
one-liner
An entire computer program crammed into one line of text.
open source software
Programs for which the source code is freely available and freely
redistributable, with no commercial strings attached. For a more
detailed definition, see .
operand
An "expression" that yields a "value" that an "operator" operates
on. See also "precedence".
operating system
A special program that runs on the bare machine and hides the gory
details of managing processes and devices. Usually used in a
looser sense to indicate a particular culture of programming. The
loose sense can be used at varying levels of specificity. At one
extreme, you might say that all versions of Unix and Unix-looka-
likes are the same operating system (upsetting many people, espe-
cially lawyers and other advocates). At the other extreme, you
could say this particular version of this particular vendor's oper-
ating system is different from any other version of this or any
other vendor's operating system. Perl is much more portable across
operating systems than many other languages. See also "architec-
ture" and "platform".
operator
A gizmo that transforms some number of input values to some number
of output values, often built into a language with a special syntax
or symbol. A given operator may have specific expectations about
what types of data you give as its arguments (operands) and what
type of data you want back from it.
operator overloading
A kind of "overloading" that you can do on built-in operators to
make them work on objects as if the objects were ordinary scalar
values, but with the actual semantics supplied by the object class.
This is set up with the overload "pragma".
options
See either switches or "regular expression modifier".
overloading
Giving additional meanings to a symbol or construct. Actually, all
languages do overloading to one extent or another, since people are
good at figuring out things from "context".
overriding
Hiding or invalidating some other definition of the same name.
(Not to be confused with "overloading", which adds definitions that
must be disambiguated some other way.) To confuse the issue fur-
ther, we use the word with two overloaded definitions: to describe
how you can define your own "subroutine" to hide a built-in "func-
tion" of the same name (see "Overriding Built-in Functions" in
perlsub) and to describe how you can define a replacement "method"
in a "derived class" to hide a "base class"'s method of the same
name (see perlobj).
owner
The one user (apart from the superuser) who has absolute control
over a "file". A file may also have a "group" of users who may
exercise joint ownership if the real owner permits it. See "per-
mission bits".
P
package
A "namespace" for global variables, subroutines, and the like, such
that they can be kept separate from like-named symbols in other
namespaces. In a sense, only the package is global, since the sym-
bols in the package's symbol table are only accessible from code
compiled outside the package by naming the package. But in another
sense, all package symbols are also globals--they're just well-
organized globals.
pad Short for "scratchpad".
parameter
See "argument".
parent class
See "base class".
parse tree
See "syntax tree".
parsing
The subtle but sometimes brutal art of attempting to turn your pos-
sibly malformed program into a valid "syntax tree".
patch
To fix by applying one, as it were. In the realm of hackerdom, a
listing of the differences between two versions of a program as
might be applied by the patch(1) program when you want to fix a bug
or upgrade your old version.
PATH
The list of directories the system searches to find a program you
want to "execute". The list is stored as one of your environment
variables, accessible in Perl as $ENV{PATH}.
pathname
A fully qualified filename such as /usr/bin/perl. Sometimes con-
fused with "PATH".
pattern
A template used in "pattern matching".
pattern matching
Taking a pattern, usually a "regular expression", and trying the
pattern various ways on a string to see whether there's any way to
make it fit. Often used to pick interesting tidbits out of a file.
permission bits
Bits that the "owner" of a file sets or unsets to allow or disallow
access to other people. These flag bits are part of the "mode"
word returned by the stat built-in when you ask about a file. On
Unix systems, you can check the ls(1) manpage for more information.
Pern
What you get when you do "Perl++" twice. Doing it only once will
curl your hair. You have to increment it eight times to shampoo
your hair. Lather, rinse, iterate.
pipe
A direct "connection" that carries the output of one "process" to
the input of another without an intermediate temporary file. Once
the pipe is set up, the two processes in question can read and
write as if they were talking to a normal file, with some caveats.
pipeline
A series of processes all in a row, linked by pipes, where each
passes its output stream to the next.
platform
The entire hardware and software context in which a program runs.
A
program written in a platform-dependent language might break if
you change any of: machine, operating system, libraries, compiler,
or system configuration. The perl interpreter has to be compiled
differently for each platform because it is implemented in C, but
programs written in the Perl language are largely platform-indepen-
dent.
pod The markup used to embed documentation into your Perl code. See
perlpod.
pointer
A "variable" in a language like C that contains the exact memory
location of some other item. Perl handles pointers internally so
you don't have to worry about them. Instead, you just use symbolic
pointers in the form of keys and "variable" names, or hard refer-
ences, which aren't pointers (but act like pointers and do in fact
contain pointers).
polymorphism
The notion that you can tell an "object" to do something generic,
and the object will interpret the command in different ways depend-
ing on its type. [mobile home or London Bridge.
porter
Someone who "carries" software from one "platform" to another.
Porting programs written in platform-dependent languages such as C
can be difficult work, but porting programs like Perl is very much
worth the agony.
POSIX
The Portable Operating System Interface specification.
postfix
An "operator" that follows its "operand", as in "$x++".
pp An internal shorthand for a "push-pop" code, that is, C code imple-
menting Perl's stack machine.
pragma
A standard module whose practical hints and suggestions are
received (and possibly ignored) at compile time. Pragmas are named
in all lowercase.
precedence
The rules of conduct that, in the absence of other guidance, deter-
mine what should happen first. For example, in the absence of
parentheses, you always do multiplication before addition.
prefix
An "operator" that precedes its "operand", as in "++$x".
preprocessing
What some helper "process" did to transform the incoming data into
a form more suitable for the current process. Often done with an
incoming "pipe". See also "C preprocessor".
procedure
A "subroutine".
process
An instance of a running program. Under multitasking systems like
Unix, two or more separate processes could be running the same pro-
gram independently at the same time--in fact, the fork function is
designed to bring about this happy state of affairs. Under other
operating systems, processes are sometimes called "threads",
"tasks", or "jobs", often with slight nuances in meaning.
program generator
A system that algorithmically writes code for you in a high-level
language. See also "code generator".
progressive matching
Pattern matching that picks up where it left off before.
property
See either "instance variable" or "character property".
protocol
In networking, an agreed-upon way of sending messages back and
forth so that neither correspondent will get too confused.
prototype
An optional part of a "subroutine" declaration telling the Perl
compiler how many and what flavor of arguments may be passed as
"actual arguments", so that you can write subroutine calls that
parse much like built-in functions. (Or don't parse, as the case
may be.)
pseudofunction
A construct that sometimes looks like a function but really isn't.
Usually reserved for "lvalue" modifiers like my, for "context" mod-
ifiers like scalar, and for the pick-your-own-quotes constructs,
"q//", "qq//", "qx//", "qw//", "qr//", "m//", "s///", "y///", and
"tr///".
pseudohash
A reference to an array whose initial element happens to hold a
reference to a hash. You can treat a pseudohash reference as
either an array reference or a hash reference.
pseudoliteral
An "operator" that looks something like a "literal", such as the
output-grabbing operator, "`""command""`".
public domain
Something not owned by anybody. Perl is copyrighted and is thus
not in the public domain--it's just "freely available" and "freely
redistributable".
pumpkin
A notional "baton" handed around the Perl community indicating who
is the lead integrator in some arena of development.
pumpking
A "pumpkin" holder, the person in charge of pumping the pump, or at
least priming it. Must be willing to play the part of the Great
Pumpkin now and then.
PV A "pointer value", which is Perl Internals Talk for a "char*".
Q
qualified
Possessing a complete name. The symbol $Ent::moot is qualified;
$moot is unqualified. A fully qualified filename is specified from
the top-level directory.
quantifier
A component of a "regular expression" specifying how many times the
foregoing "atom" may occur.
R
readable
With respect to files, one that has the proper permission bit set
to let you access the file. With respect to computer programs, one
that's written well enough that someone has a chance of figuring
out what it's trying to do.
reaping
The last rites performed by a parent "process" on behalf of a
deceased child process so that it doesn't remain a "zombie". See
the wait and waitpid function calls.
record
A set of related data values in a "file" or "stream", often associ-
ated with a unique "key" field. In Unix, often commensurate with a
"line", or a blank-line-terminated set of lines (a "paragraph").
Each line of the /etc/passwd file is a record, keyed on login name,
containing information about that user.
recursion
The art of defining something (at least partly) in terms of itself,
which is a naughty no-no in dictionaries but often works out okay
in computer programs if you're careful not to recurse forever,
which is like an infinite loop with more spectacular failure modes.
reference
Where you look to find a pointer to information somewhere else.
(See "indirection".) References come in two flavors, symbolic ref-
erences and hard references.
referent
Whatever a reference refers to, which may or may not have a name.
Common types of referents include scalars, arrays, hashes, and sub-
routines.
regex
See "regular expression".
regular expression
A single entity with various interpretations, like an elephant. To
a computer scientist, it's a grammar for a little language in which
some strings are legal and others aren't. To normal people, it's a
pattern you can use to find what you're looking for when it varies
from case to case. Perl's regular expressions are far from regular
in the theoretical sense, but in regular use they work quite well.
Here's a regular expression: "/Oh s.*t./". This will match strings
like ""Oh say can you see by the dawn's early light"" and ""Oh
sit!"". See perlre.
regular expression modifier
An option on a pattern or substitution, such as "/i" to render the
pattern case insensitive. See also "cloister".
regular file
A "file" that's not a "directory", a "device", a named "pipe" or
"socket", or a "symbolic link". Perl uses the "-f" file test oper-
ator to identify regular files. Sometimes called a "plain" file.
relational operator
An "operator" that says whether a particular ordering relationship
is "true" about a pair of operands. Perl has both numeric and
string relational operators. See "collating sequence".
reserved words
A word with a specific, built-in meaning to a "compiler", such as
"if" or delete. In many languages (not Perl), it's illegal to use
reserved words to name anything else. (Which is why they're
reserved, after all.) In Perl, you just can't use them to name
labels or filehandles. Also called "keywords".
return value
The "value" produced by a "subroutine" or "expression" when evalu-
ated. In Perl, a return value may be either a "list" or a
"scalar".
RFC Request For Comment, which despite the timid connotations is the
name of a series of important standards documents.
right shift
A "bit shift" that divides a number by some power of 2.
root
The superuser (UID == 0). Also, the top-level directory of the
filesystem.
RTFM
What you are told when someone thinks you should Read The Fine Man-
ual.
run phase
Any time after Perl starts running your main program. See also
"compile phase". Run phase is mostly spent in "run time" but may
also be spent in "compile time" when require, do "FILE", or eval
"STRING" operators are executed or when a substitution uses the
"/ee" modifier.
run time
The time when Perl is actually doing what your code says to do, as
opposed to the earlier period of time when it was trying to figure
out whether what you said made any sense whatsoever, which is "com-
pile time".
run-time pattern
A pattern that contains one or more variables to be interpolated
before parsing the pattern as a "regular expression", and that
therefore cannot be analyzed at compile time, but must be re-ana-
lyzed each time the pattern match operator is evaluated. Run-time
patterns are useful but expensive.
RV A recreational vehicle, not to be confused with vehicular recre-
ation. RV also means an internal Reference Value of the type a
"scalar" can hold. See also "IV" and "NV" if you're not confused
yet.
rvalue
A "value" that you might find on the right side of an "assignment".
See also "lvalue".
S
scalar
A simple, singular value; a number, "string", or "reference".
scalar context
The situation in which an "expression" is expected by its surround-
ings (the code calling it) to return a single "value" rather than a
"list" of values. See also "context" and "list context". A scalar
context sometimes imposes additional constraints on the return
value--see "string context" and "numeric context". Sometimes we
talk about a "Boolean context" inside conditionals, but this
imposes no additional constraints, since any scalar value, whether
numeric or "string", is already true or false.
scalar literal
A number or quoted "string"--an actual "value" in the text of your
program, as opposed to a "variable".
scalar value
A value that happens to be a "scalar" as opposed to a "list".
scalar variable
A "variable" prefixed with "$" that holds a single value.
scope
How far away you can see a variable from, looking through one.
Perl has two visibility mechanisms: it does "dynamic scoping" of
local variables, meaning that the rest of the "block", and any sub-
routines that are called by the rest of the block, can see the
variables that are local to the block. Perl does "lexical scoping"
of my variables, meaning that the rest of the block can see the
variable, but other subroutines called by the block cannot see the
variable.
scratchpad
The area in which a particular invocation of a particular file or
subroutine keeps some of its temporary values, including any lexi-
cally scoped variables.
script
A text "file" that is a program intended to be executed directly
rather than compiled to another form of file before execution.
Also, in the context of "Unicode", a writing system for a particu-
lar language or group of languages, such as Greek, Bengali, or
Klingon.
script kiddie
A "cracker" who is not a "hacker", but knows just enough to run
canned scripts. A cargo-cult programmer.
sed A venerable Stream EDitor from which Perl derives some of its
ideas.
semaphore
A fancy kind of interlock that prevents multiple threads or pro-
cesses from using up the same resources simultaneously.
separator
A "character" or "string" that keeps two surrounding strings from
being confused with each other. The split function works on sepa-
rators. Not to be confused with delimiters or terminators. The
"or" in the previous sentence separated the two alternatives.
serialization
Putting a fancy "data structure" into linear order so that it can
be stored as a "string" in a disk file or database or sent through
a "pipe". Also called marshalling.
server
In networking, a "process" that either advertises a "service" or
just hangs around at a known location and waits for clients who
need service to get in touch with it.
service
Something you do for someone else to make them happy, like giving
them the time of day (or of their life). On some machines, well-
known services are listed by the getservent function.
setgid
Same as "setuid", only having to do with giving away "group" privi-
leges.
setuid
Said of a program that runs with the privileges of its "owner"
rather than (as is usually the case) the privileges of whoever is
running it. Also describes the bit in the mode word ("permission
bits") that controls the feature. This bit must be explicitly set
by the owner to enable this feature, and the program must be care-
fully written not to give away more privileges than it ought to.
shared memory
A piece of "memory" accessible by two different processes who oth-
erwise would not see each other's memory.
shebang
Irish for the whole McGillicuddy. In Perl culture, a portmanteau
of "sharp" and "bang", meaning the "#!" sequence that tells the
system where to find the interpreter.
shell
A "command"-line "interpreter". The program that interactively
gives you a prompt, accepts one or more lines of input, and exe-
cutes the programs you mentioned, feeding each of them their proper
arguments and input data. Shells can also execute scripts contain-
ing such commands. Under Unix, typical shells include the Bourne
shell (/bin/sh), the C shell (/bin/csh), and the Korn shell
(/bin/ksh). Perl is not strictly a shell because it's not interac-
tive (although Perl programs can be interactive).
side effects
Something extra that happens when you evaluate an "expression".
Nowadays it can refer to almost anything. For example, evaluating
a simple assignment statement typically has the "side effect" of
assigning a value to a variable. (And you thought assigning the
value was your primary intent in the first place!) Likewise,
assigning a value to the special variable $| ($AUTOFLUSH) has the
side effect of forcing a flush after every write or print on the
currently selected filehandle.
signal
A bolt out of the blue; that is, an event triggered by the "operat-
ing system", probably when you're least expecting it.
signal handler
A "subroutine" that, instead of being content to be called in the
normal fashion, sits around waiting for a bolt out of the blue
before it will deign to "execute". Under Perl, bolts out of the
blue are called signals, and you send them with the kill built-in.
See "%SIG" in perlvar and "Signals" in perlipc.
single inheritance
The features you got from your mother, if she told you that you
don't have a father. (See also "inheritance" and "multiple inheri-
tance".) In computer languages, the notion that classes reproduce
asexually so that a given class can only have one direct ancestor
or "base class". Perl supplies no such restriction, though you may
certainly program Perl that way if you like.
slice
A selection of any number of elements from a "list", "array", or
"hash".
slurp
To read an entire "file" into a "string" in one operation.
socket
An endpoint for network communication among multiple processes that
works much like a telephone or a post office box. The most impor-
tant thing about a socket is its "network address" (like a phone
number). Different kinds of sockets have different kinds of
addresses--some look like filenames, and some don't.
soft reference
See "symbolic reference".
source filter
A special kind of "module" that does "preprocessing" on your script
just before it gets to the "tokener".
stack
A device you can put things on the top of, and later take them back
off in the opposite order in which you put them on. See "LIFO".
standard
Included in the official Perl distribution, as in a standard mod-
ule, a standard tool, or a standard Perl "manpage".
standard error
The default output "stream" for nasty remarks that don't belong in
"standard output". Represented within a Perl program by the "file-
handle" "STDERR". You can use this stream explicitly, but the die
and warn built-ins write to your standard error stream automati-
cally.
standard I/O
A standard C library for doing buffered input and output to the
"operating system". (The "standard" of standard I/O is only
marginally related to the "standard" of standard input and output.)
In general, Perl relies on whatever implementation of standard I/O
a given operating system supplies, so the buffering characteristics
of a Perl program on one machine may not exactly match those on
another machine. Normally this only influences efficiency, not
semantics. If your standard I/O package is doing block buffering
and you want it to "flush" the buffer more often, just set the $|
variable to a true value.
standard input
The default input "stream" for your program, which if possible
shouldn't care where its data is coming from. Represented within a
Perl program by the "filehandle" "STDIN".
standard output
The default output "stream" for your program, which if possible
shouldn't care where its data is going. Represented within a Perl
program by the "filehandle" "STDOUT".
stat structure
A special internal spot in which Perl keeps the information about
the last "file" on which you requested information.
statement
A "command" to the computer about what to do next, like a step in a
recipe: "Add marmalade to batter and mix until mixed." A statement
is distinguished from a "declaration", which doesn't tell the com-
puter to do anything, but just to learn something.
statement modifier
A "conditional" or "loop" that you put after the "statement"
instead of before, if you know what we mean.
static
Varying slowly compared to something else. (Unfortunately, every-
thing is relatively stable compared to something else, except for
certain elementary particles, and we're not so sure about them.)
In computers, where things are supposed to vary rapidly, "static"
has a derogatory connotation, indicating a slightly dysfunctional
"variable", "subroutine", or "method". In Perl culture, the word
is politely avoided.
static method
No such thing. See "class method".
static scoping
No such thing. See "lexical scoping".
static variable
No such thing. Just use a "lexical variable" in a scope larger
than your "subroutine".
status
The "value" returned to the parent "process" when one of its child
processes dies. This value is placed in the special variable $?.
Its upper eight bits are the exit status of the defunct process,
and its lower eight bits identify the signal (if any) that the
process died from. On Unix systems, this status value is the same
as the status word returned by wait(2). See "system" in perlfunc.
STDERR
See "standard error".
STDIN
See "standard input".
STDIO
See "standard I/O".
STDOUT
See "standard output".
stream
A flow of data into or out of a process as a steady sequence of
bytes or characters, without the appearance of being broken up into
packets. This is a kind of "interface"--the underlying "implemen-
tation" may well break your data up into separate packets for
delivery, but this is hidden from you.
string
A sequence of characters such as "He said !@#*&%@#*?!". A string
does not have to be entirely printable.
string context
The situation in which an expression is expected by its surround-
ings (the code calling it) to return a "string". See also "con-
text" and "numeric context".
stringification
The process of producing a "string" representation of an abstract
object.
struct
C keyword introducing a structure definition or name.
structure
See "data structure".
subclass
See "derived class".
subpattern
A component of a "regular expression" pattern.
subroutine
A named or otherwise accessible piece of program that can be
invoked from elsewhere in the program in order to accomplish some
sub-goal of the program. A subroutine is often parameterized to
accomplish different but related things depending on its input
arguments. If the subroutine returns a meaningful "value", it is
also called a "function".
subscript
A "value" that indicates the position of a particular "array" "ele-
ment" in an array.
substitution
Changing parts of a string via the "s///" operator. (We avoid use
of this term to mean "variable interpolation".)
substring
A portion of a "string", starting at a certain "character" position
("offset") and proceeding for a certain number of characters.
superclass
See "base class".
superuser
The person whom the "operating system" will let do almost anything.
Typically your system administrator or someone pretending to be
your system administrator. On Unix systems, the "root" user. On
Windows systems, usually the Administrator user.
SV Short for "scalar value". But within the Perl interpreter every
"referent" is treated as a member of a class derived from SV, in an
object-oriented sort of way. Every "value" inside Perl is passed
around as a C language "SV*" pointer. The SV "struct" knows its
own "referent type", and the code is smart enough (we hope) not to
try to call a "hash" function on a "subroutine".
switch
An option you give on a command line to influence the way your pro-
gram works, usually introduced with a minus sign. The word is also
used as a nickname for a "switch statement".
switch cluster
The combination of multiple command-line switches (e.g., -a -b -c)
into one switch (e.g., -abc). Any switch with an additional "argu-
ment" must be the last switch in a cluster.
switch statement
A program technique that lets you evaluate an "expression" and
then, based on the value of the expression, do a multiway branch to
the appropriate piece of code for that value. Also called a "case
structure", named after the similar Pascal construct. Most switch
statements in Perl are spelled "for". See "Basic BLOCKs and Switch
Statements" in perlsyn.
symbol
Generally, any "token" or "metasymbol". Often used more specifi-
cally to mean the sort of name you might find in a "symbol table".
symbol table
Where a "compiler" remembers symbols. A program like Perl must
somehow remember all the names of all the variables, filehandles,
and subroutines you've used. It does this by placing the names in
a symbol table, which is implemented in Perl using a "hash table".
There is a separate symbol table for each "package" to give each
package its own "namespace".
symbolic debugger
A program that lets you step through the execution of your program,
stopping or printing things out here and there to see whether any-
thing has gone wrong, and if so, what. The "symbolic" part just
means that you can talk to the debugger using the same symbols with
which your program is written.
symbolic link
An alternate filename that points to the real "filename", which in
turn points to the real "file". Whenever the "operating system" is
trying to parse a "pathname" containing a symbolic link, it merely
substitutes the new name and continues parsing.
symbolic reference
A variable whose value is the name of another variable or subrou-
tine. By dereferencing the first variable, you can get at the sec-
ond one. Symbolic references are illegal under use strict 'refs'.
synchronous
Programming in which the orderly sequence of events can be deter-
mined; that is, when things happen one after the other, not at the
same time.
syntactic sugar
An alternative way of writing something more easily; a shortcut.
syntax
From Greek, "with-arrangement". How things (particularly symbols)
are put together with each other.
syntax tree
An internal representation of your program wherein lower-level con-
structs dangle off the higher-level constructs enclosing them.
syscall
A "function" call directly to the "operating system". Many of the
important subroutines and functions you use aren't direct system
calls, but are built up in one or more layers above the system call
level. In general, Perl programmers don't need to worry about the
distinction. However, if you do happen to know which Perl func-
tions are really syscalls, you can predict which of these will set
the $! ($ERRNO) variable on failure. Unfortunately, beginning
programmers often confusingly employ the term "system call" to mean
what happens when you call the Perl system function, which actually
involves many syscalls. To avoid any confusion, we nearly always
use say "syscall" for something you could call indirectly via
Perl's syscall function, and never for something you would call
with Perl's system function.
T
tainted
Said of data derived from the grubby hands of a user and thus
unsafe for a secure program to rely on. Perl does taint checks if
you run a "setuid" (or "setgid") program, or if you use the -T
switch.
TCP Short for Transmission Control Protocol. A protocol wrapped around
the Internet Protocol to make an unreliable packet transmission
mechanism appear to the application program to be a reliable
"stream" of bytes. (Usually.)
term
Short for a "terminal", that is, a leaf node of a "syntax tree". A
thing that functions grammatically as an "operand" for the opera-
tors in an expression.
terminator
A "character" or "string" that marks the end of another string.
The $/ variable contains the string that terminates a readline
operation, which chomp deletes from the end. Not to be confused
with delimiters or separators. The period at the end of this sen-
tence is a terminator.
ternary
An "operator" taking three operands. Sometimes pronounced "tri-
nary".
text
A "string" or "file" containing primarily printable characters.
thread
Like a forked process, but without "fork"'s inherent memory protec-
tion. A thread is lighter weight than a full process, in that a
process could have multiple threads running around in it, all
fighting over the same process's memory space unless steps are
taken to protect threads from each other. See threads.
tie The bond between a magical variable and its implementation class.
See "tie" in perlfunc and perltie.
TMTOWTDI
There's More Than One Way To Do It, the Perl Motto. The notion
that there can be more than one valid path to solving a programming
problem in context. (This doesn't mean that more ways are always
better or that all possible paths are equally desirable--just that
there need not be One True Way.) Pronounced TimToady.
token
A morpheme in a programming language, the smallest unit of text
with semantic significance.
tokener
A module that breaks a program text into a sequence of tokens for
later analysis by a parser.
tokenizing
Splitting up a program text into tokens. Also known as "lexing",
in which case you get "lexemes" instead of tokens.
toolbox approach
The notion that, with a complete set of simple tools that work well
together, you can build almost anything you want. Which is fine if
you're assembling a tricycle, but if you're building a defranishiz-
ing comboflux regurgalator, you really want your own machine shop
in which to build special tools. Perl is sort of a machine shop.
transliterate
To turn one string representation into another by mapping each
character of the source string to its corresponding character in
the result string. See "tr/SEARCHLIST/REPLACEMENTLIST/cds" in per-
lop.
trigger
An event that causes a "handler" to be run.
trinary
Not a stellar system with three stars, but an "operator" taking
three operands. Sometimes pronounced "ternary".
troff
A venerable typesetting language from which Perl derives the name
of its $% variable and which is secretly used in the production of
Camel books.
true
Any scalar value that doesn't evaluate to 0 or "".
truncating
Emptying a file of existing contents, either automatically when
opening a file for writing or explicitly via the truncate function.
type
See "data type" and "class".
type casting
Converting data from one type to another. C permits this. Perl
does not need it. Nor want it.
typed lexical
A "lexical variable" that is declared with a "class" type: "my Pony
$bill".
typedef
A type definition in the C language.
typeglob
Use of a single identifier, prefixed with "*". For example, *name
stands for any or all of $name, @name, %name, &name, or just
"name". How you use it determines whether it is interpreted as all
or only one of them. See "Typeglobs and Filehandles" in perldata.
typemap
A description of how C types may be transformed to and from Perl
types within an "extension" module written in "XS".
U
UDP User Datagram Protocol, the typical way to send datagrams over the
Internet.
UID A user ID. Often used in the context of "file" or "process" owner-
ship.
umask
A mask of those "permission bits" that should be forced off when
creating files or directories, in order to establish a policy of
whom you'll ordinarily deny access to. See the umask function.
unary operator
An operator with only one "operand", like "!" or chdir. Unary
operators are usually prefix operators; that is, they precede their
operand. The "++" and "--" operators can be either prefix or post-
fix. (Their position does change their meanings.)
Unicode
A character set comprising all the major character sets of the
world, more or less. See .
Unix
A very large and constantly evolving language with several alterna-
tive and largely incompatible syntaxes, in which anyone can define
anything any way they choose, and usually do. Speakers of this
language think it's easy to learn because it's so easily twisted to
one's own ends, but dialectical differences make tribal intercommu-
nication nearly impossible, and travelers are often reduced to a
pidgin-like subset of the language. To be universally understood,
a Unix shell programmer must spend years of study in the art. Many
have abandoned this discipline and now communicate via an
Esperanto-like language called Perl.
In ancient times, Unix was also used to refer to some code that a
couple of people at Bell Labs wrote to make use of a PDP-7 computer
that wasn't doing much of anything else at the time.
V
value
An actual piece of data, in contrast to all the variables, refer-
ences, keys, indexes, operators, and whatnot that you need to
access the value.
variable
A named storage location that can hold any of various kinds of
"value", as your program sees fit.
variable interpolation
The "interpolation" of a scalar or array variable into a string.
variadic
Said of a "function" that happily receives an indeterminate number
of "actual arguments".
vector
Mathematical jargon for a list of scalar values.
virtual
Providing the appearance of something without the reality, as in:
virtual memory is not real memory. (See also "memory".) The oppo-
site of "virtual" is "transparent", which means providing the real-
ity of something without the appearance, as in: Perl handles the
variable-length UTF-8 character encoding transparently.
void context
A form of "scalar context" in which an "expression" is not expected
to return any "value" at all and is evaluated for its "side
effects" alone.
v-string
A "version" or "vector" "string" specified with a "v" followed by a
series of decimal integers in dot notation, for instance,
"v1.20.300.4000". Each number turns into a "character" with the
specified ordinal value. (The "v" is optional when there are at
least three integers.)
W
warning
A message printed to the "STDERR" stream to the effect that some-
thing might be wrong but isn't worth blowing up over. See "warn"
in perlfunc and the warnings pragma.
watch expression
An expression which, when its value changes, causes a breakpoint in
the Perl debugger.
whitespace
A "character" that moves your cursor but doesn't otherwise put any-
thing on your screen. Typically refers to any of: space, tab, line
feed, carriage return, or form feed.
word
In normal "computerese", the piece of data of the size most effi-
ciently handled by your computer, typically 32 bits or so, give or
take a few powers of 2. In Perl culture, it more often refers to
an alphanumeric "identifier" (including underscores), or to a
string of nonwhitespace characters bounded by whitespace or string
boundaries.
working directory
Your current "directory", from which relative pathnames are inter-
preted by the "operating system". The operating system knows your
current directory because you told it with a chdir or because you
started out in the place where your parent "process" was when you
were born.
wrapper
A program or subroutine that runs some other program or subroutine
for you, modifying some of its input or output to better suit your
purposes.
WYSIWYG
What You See Is What You Get. Usually used when something that
appears on the screen matches how it will eventually look, like
Perl's format declarations. Also used to mean the opposite of
magic because everything works exactly as it appears, as in the
three-argument form of open.
X
XS An extraordinarily exported, expeditiously excellent, expressly
eXternal Subroutine, executed in existing C or C++ or in an excit-
ing new extension language called (exasperatingly) XS. Examine
perlxs for the exact explanation or perlxstut for an exemplary
unexacting one.
XSUB
An external "subroutine" defined in "XS".
Y
yacc
Yet Another Compiler Compiler. A parser generator without which
Perl probably would not have existed. See the file perly.y in the
Perl source distribution.
Z
zero width
A subpattern "assertion" matching the "null string" between charac-
ters.
zombie
A process that has died (exited) but whose parent has not yet
received proper notification of its demise by virtue of having
called wait or waitpid. If you fork, you must clean up after your
child processes when they exit, or else the process table will fill
up and your system administrator will Not Be Happy with you.
AUTHOR AND COPYRIGHT
Based on the Glossary of Programming Perl, Third Edition, by Larry
Wall, Tom Christiansen & Jon Orwant. Copyright (c) 2000, 1996, 1991
O'Reilly Media, Inc. This document may be distributed under the same
terms as Perl itself.
perl v5.8.8 2006-01-07 PERLGLOSSARY(1)
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