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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 documentation. Other useful sources
include the Free On-Line Dictionary of Computing <http://foldoc.org/>, the Jargon File
<http://catb.org/~esr/jargon/>, and Wikipedia <http://www.wikipedia.org/>.
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 assignment 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: "|".
Alternatives in normal Perl expressions are separated with a double vertical 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 without pity.
architecture
The kind of computer you're working on, where one "kind" of computer means all those computers
sharing a compatible machine language. 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 specifies 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 sometimes
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 associative. 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 subroutines 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 behavior of certain operators can be
reasonably deduced using more fundamental 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 running 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 textprocessing
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", also referred to
as a capture group. The sequences ("\g1", "\g2", etc.) later in the same pattern refer back to
the corresponding subpattern in the current match. Outside the pattern, the numbered variables
($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. "\g{-1}" can be used to refer to a group by
relative rather than absolute position; and groups can be also be named, and referred to later by
name rather than number. See "Capture groups" in perlre.
backtracking
The practice of saying, "If I had to do it all over, I'd do it differently," and then actually
going back and doing it all over differently. Mathematically speaking, it's returning from an
unsuccessful 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 computers 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 possible 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." Similarly 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
system" 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 execution 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 prescription-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 qualifying 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
interpreters in the late 20th century. These languages are characterized 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 beginning 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
arguments. 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.
capture buffer, capture group
These two terms are synonymous: a captured substring by a regex subpattern.
capturing
The use of parentheses around a "subpattern" in a "regular expression" to store the matched
"substring" as a "backreference" or capture group. (Captured strings are also returned as a list
in "list context".)
character
A small integer representative of a unit of orthography. Historically, characters were usually
stored as fixed-width integers (typically 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 expression" 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 operator, 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, subroutines) 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
supposedly 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 reference 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 generator".
code point
The position of a character in a character set encoding. The character "NULL" is almost
certainly at the zeroth position in all character sets, so its code point is 0. The code point
for the "SPACE" character in the ASCII character set is 0x20, or 32 decimal; in EBCDIC it is
0x40, or 64 decimal. The ord function returns the code point of a character.
"code position" and "ordinal" mean the same thing as "code point".
code subpattern
A "regular expression" subpattern whose real purpose is to execute some Perl code, for example,
the "(?{...})" and "(??{...})" subpatterns.
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 "operating 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 command 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
declarations, 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 trying 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 executable" 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 References" 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 transitive verb, to create an
"object" using a "constructor".
constructor
Any "class method", instance "method", or "subroutine" that composes, 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 surrounding code determines what kind
of data a particular "expression" 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 Edition, 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 "whitespace" (including newlines) is gleefully ignored.
Usually.
core dump
The corpse of a "process", in the form of a file left in the "working 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?" 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
declaration. That's your current package name.
current working directory
See "working directory".
currently selected output channel
The last "filehandle" that was designated with select("FILEHANDLE"); "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
various 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".
Compound types made of a number of smaller pieces generally have operations to compose and
decompose them, and perhaps to rearrange them. Objects that model things in the real world often
have operations 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 network. (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 variables 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 definitions 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
operations 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 "terminator". "To delimit" really just means "to surround" or "to
enclose" (like these parentheses are doing).
deprecated modules and features
Deprecated modules and features are those which were part of a stable release, but later found to
be subtly flawed, and which should be avoided. They are subject to removal and/or bug-incompatible bugincompatible
incompatible reimplementation in the next major release (but they will be preserved through
maintenance releases). Deprecation warnings are issued under -w or "use diagnostics", and
notices are found in perldeltas, as well as various other PODs. Coding practices that misuse
features, such as "my $foo if 0", can also be deprecated.
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 simultaneously, 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 "operating 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 metaphorically to indicate a transfer
of programmatic control to a destination selected algorithmically, often by lookup in a table of
function 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.
(to be) dropped modules
When Perl 5 was first released (see perlhist), several modules were included, which have now
fallen out of common use. It has been suggested that these modules should be removed, since the
distribution became rather large, and the common criterion for new module additions is now
limited to modules that help to build, test, and extend perl itself. Furthermore, the CPAN
(which didn't exist at the time of Perl 5.0) can become the new home of dropped modules. Dropping
modules is currently not an option, but further developments may clear the last barriers.
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 working. [From Old English]
dwimmer
DWIM is an acronym for "Do What I Mean", the principle that something 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". Dwimming 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 visible 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. (Compare "lexical scoping".) Used more loosely to mean how
a subroutine 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 list
See </null list>.
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 "implementation" (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 generally, 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 undefined values are false (including the "null list"), 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 terminates after placing the exception message in the $@ ($EVAL_ERROR) variable. You can
try to provoke a fatal error with the die operator (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 something 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 program, 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 decimal 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 approximations 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 etymologically.
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.
G
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 reference-counting scheme as
a form of garbage collection. (If it's any comfort, when your interpreter exits, a "real"
garbage collector runs to make sure everything is cleaned up if you've been messy with circular
references and such.)
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 associated 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 systems (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 problems, 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 clueless.
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 referent.) 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. A hash can optionally be restricted to a fixed set of keys.
hash table
A data structure used internally by Perl for implementing associative 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"
mechanism.)
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, however, 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". Hexadecimal constants in Perl start with "0x". See also "hex"
in perlfunc.
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 representation 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 identifiers 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 typeglob "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. Similarly, 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!";
In modern Perl, calling methods this way is often considered bad practice and to be avoided.
indirect object slot
The syntactic position falling between a method call and its arguments 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 multiplication 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
inheritance" 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 contrast 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 executable 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 considered 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, Platonic classes don't always have Platonic relationships--see "inheritance".
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 editor. 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 illustrate a particular Perl feature, and something of an ongoing
Obfuscated Perl Contest seen in Usenix signatures.
K
key The string index to a "hash", used to look up the "value" associated 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 terminated 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 "<FH>")
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 unresolved symbols into a
(nearly) executable image. Linking can generally 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 "expression" 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 surroundings (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 "context".
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
considered 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 prematurely 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 modifiers: 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) command. 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 memory 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 metacharacters, 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(2) 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 finishes.
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 represented 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 terminate strings, but Perl allows
strings to contain a null.
null list
A valueless value represented in Perl by "()". It is not really a "LIST", but an expression that
yields "undef" in "scalar context" and a "list value" with zero elements in "list context".
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 surroundings (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 civilization. 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 beginning of a string or array to a
specific position within it. Thus, the minimum offset is zero, not one, because you don't skip
anything 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
<http://www.opensource.org/osd.html>.
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-lookalikes are the same operating system (upsetting many people,
especially lawyers and other advocates). At the other extreme, you could say this particular
version of this particular vendor's operating 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 "architecture" 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".
ordinal
Another name for "code point"
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 further, we use the word with two overloaded definitions: to describe how you can define
your own "subroutine" to hide a built-in "function" 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
"permission 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 symbols 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 possibly 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 confused 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-independent.
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 references, 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 depending on its type. [<Gk many shapes]
port
The part of the address of a TCP or UDP socket that directs packets to the correct process after
finding the right machine, something like the phone extension you give when you reach the company
operator. Also, the result of converting code to run on a different platform than originally
intended, or the verb denoting this conversion.
portable
Once upon a time, C code compilable under both BSD and SysV. In general, code that can be easily
converted to run on another "platform", where "easily" can be defined however you like, and
usually is. Anything may be considered portable if you try hard enough. See 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 implementing 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, determine 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 program 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" modifiers 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 associated 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 references 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 subroutines.
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 operator 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".
restricted hash
A "hash" with a closed set of allowed keys. See Hash::Util.
return value
The "value" produced by a "subroutine" or "expression" when evaluated. 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 Manual.
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 "compile 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-
analyzed 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 recreation. 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 surroundings (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 subroutines 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 lexically 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
particular 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 processes 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 separators. 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" privileges.
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 carefully written not to give away more privileges
than it ought to.
shared memory
A piece of "memory" accessible by two different processes who otherwise 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 executes the programs you mentioned, feeding each of them their
proper arguments and input data. Shells can also execute scripts containing 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 interactive (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 "operating 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 inheritance".) 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 important 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 module, 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 "filehandle" "STDERR". You can use this stream
explicitly, but the die and warn built-ins write to your standard error stream automatically.
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 computer 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, everything 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
"implementation" 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 surroundings (the code calling it) to
return a "string". See also "context" 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" "element" 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 program 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 "argument" 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. See See "Basic BLOCKs" in perlsyn.
symbol
Generally, any "token" or "metasymbol". Often used more specifically 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 anything 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 subroutine. By dereferencing the first
variable, you can get at the second one. Symbolic references are illegal under use strict
'refs'.
synchronous
Programming in which the orderly sequence of events can be determined; 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 constructs dangle off the higher-level higherlevel
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 functions 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 operators 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 sentence is a terminator.
ternary
An "operator" taking three operands. Sometimes pronounced "trinary".
text
A "string" or "file" containing primarily printable characters.
thread
Like a forked process, but without "fork"'s inherent memory protection. 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 defranishizing 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/cdsr" in
perlop.
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" ownership.
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 postfix. (Their position does change their meanings.)
Unicode
A character set comprising all the major character sets of the world, more or less. See
perlunicode and <http://www.unicode.org>.
Unix
A very large and constantly evolving language with several alternative 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 intercommunication 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, references, 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 opposite of "virtual" is "transparent", which means providing
the reality 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 something 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 anything 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 efficiently 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 interpreted 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 A language to extend Perl with C and C++. XS is an interface description file format used to
create an extension interface between Perl and C code (or a C library) which one wishes to use
with Perl. See perlxs for the exact explanation or read the perlxstut tutorial.
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 characters.
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 v5.16.2 2012-10-25 PERLGLOSSARY(1)
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