PERLGLOSSARY(1) Perl Programmers Reference Guide PERLGLOSSARY(1)
NAME
perlglossary - Perl Glossary
VERSION
version 5.20240218
DESCRIPTION
A glossary of terms (technical and otherwise) used in the Perl
documentation, derived from the Glossary of Programming Perl, Fourth
Edition. Words or phrases in bold are defined elsewhere in this
glossary.
Other useful sources include the Unicode Glossary
<http://unicode.org/glossary/>, 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, explained clearly enough 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.
alphabetic
The sort of characters we put into words. In Unicode, this is all
letters including all ideographs and certain diacritics, letter
numbers like Roman numerals, and various combining marks.
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.
application
A bigger, fancier sort of program with a fancier name so people
don’t realize they are using a program.
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.
Artistic License
The open source license that Larry Wall created for Perl, maximizing
Perl’s usefulness, availability, and modifiability. The current
version is 2.
(<http://www.opensource.org/licenses/artistic-license.php>).
ASCII
The American Standard Code for Information Interchange (a 7-bit
character set adequate only for poorly representing English text).
Often used loosely to describe the lowest 128 values of the various
ISO-8859-X character sets, a bunch of mutually incompatible 8-bit
codes best described as half ASCII. See also Unicode.
assertion
A component of a regular expression that must be true for the
pattern to match but does not necessarily match any characters
itself. Often used specifically to mean a zero-width assertion.
assignment
An operator whose assigned mission in life is to change the value of
a variable.
assignment operator
Either a regular assignment or a compound operator composed of an
ordinary assignment and some other operator, that changes the value
of a variable in place; that is, relative to its old value. For
example, "$a += 2" adds 2 to $a.
associative array
See hash. Please. The term associative array is the old Perl 4 term
for a hash. Some languages call it a dictionary.
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
Camel chapter 3, “Unary and Binary Operators” 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: ἀ- (not)
+ -τομος (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
Camel chapter 13, “Overloading”.
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 Graeco-Roman word meaning “to bring oneself to life”. In Perl,
storage locations (lvalues) spontaneously generate themselves as
needed, including the creation of any hard reference values to point
to the next level of storage. The assignment "$a[5][5][5][5][5] =
"quintet"" potentially creates five scalar storage locations, plus
four references (in the first four scalar locations) pointing to
four new anonymous arrays (to hold the last four scalar locations).
But the point of autovivification is that you don’t have to worry
about it.
AV Short for “array value”, which refers to one of Perl’s internal data
types that holds an array. The "AV" type is a subclass of SV.
awk Descriptive editing term—short for “awkward”. Also coincidentally
refers to a venerable text-processing language from which Perl
derived some of its high-level ideas.
B
backreference
A substring captured by a subpattern within unadorned parentheses in
a regex. Backslashed decimal numbers ("\1", "\2", etc.) later in the
same pattern refer back to the corresponding 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.
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 the section “The Little
Engine That /Couldn(n’t)” in Camel chapter 5, “Pattern Matching”.
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 file of
“nontext”, 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 the
"bless" function in Camel chapter 27, “Functions”.
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 2 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 "BLOCK"s, for instance. Sometimes we also say
“block” to mean a lexical scope; that is, a sequence of statements
that acts 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 UC
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. Data that are 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::" pseudopackage.
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 lingo 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 nonarchitecture-
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.
cache
A data repository. Instead of computing expensive answers several
times, compute it once and save the result.
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.
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.
canonical
Reduced to a standard form to facilitate comparison.
capture variables
The variables—such as $1 and $2, and "%+" and "%– "—that hold the
text remembered in a pattern match. See Camel chapter 5, “Pattern
Matching”.
capturing
The use of parentheses around a subpattern in a regular expression
to store the matched substring as a backreference. (Captured strings
are also returned as a list in list context.) See Camel chapter 5,
“Pattern Matching”.
cargo cult
Copying and pasting code without understanding it, while
superstitiously believing in its value. This term originated from
preindustrial cultures dealing with the detritus of explorers and
colonizers of technologically advanced cultures. See The Gods Must
Be Crazy.
case
A property of certain characters. Originally, typesetter stored
capital letters in the upper of two cases and small letters in the
lower one. Unicode recognizes three cases: lowercase (character
property "\p{lower}"), titlecase ("\p{title}"), and uppercase
("\p{upper}"). A fourth casemapping called foldcase is not itself a
distinct case, but it is used internally to implement casefolding.
Not all letters have case, and some nonletters have case.
casefolding
Comparing or matching a string case-insensitively. In Perl, it is
implemented with the "/i" pattern modifier, the "fc" function, and
the "\F" double-quote translation escape.
casemapping
The process of converting a string to one of the four Unicode
casemaps; in Perl, it is implemented with the "fc", "lc", "ucfirst",
and "uc" functions.
character
The smallest individual element of a string. Computers store
characters as integers, but Perl lets you operate on them as text.
The integer used to represent a particular character is called that
character’s codepoint.
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" or "\P"
metasymbol. Unicode defines hundreds of standard properties for
every possible codepoint, and Perl defines a few of its own, too.
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. Also see instance
method.
client
In networking, a process that initiates contact with a server
process in order to exchange data and perhaps receive a service.
closure
An anonymous subroutine that, when a reference to it is generated at
runtime, 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.
codepoint
The integer a computer uses to represent a given character. ASCII
codepoints are in the range 0 to 127; Unicode codepoints are in the
range 0 to 0x10_FFFF; and Perl codepoints are in the range 0 to
2³²−1 or 0 to 2⁶⁴−1, depending on your native integer size. In Perl
Culture, sometimes called ordinals.
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”.
co-maintainer
A person with permissions to index a namespace in PAUSE. Anyone can
upload any namespace, but only primary and co-maintainers get their
contributions indexed.
combining character
Any character with the General Category of Combining Mark
("\p{GC=M}"), which may be spacing or nonspacing. Some are even
invisible. A sequence of combining characters following a grapheme
base character together make up a single user-visible character
called a grapheme. Most but not all diacritics are combining
characters, and vice versa.
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-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.
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.
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
The process of turning source code into a machine-usable form. See
compile phase.
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 runtime when "BEGIN" blocks, "use" or "no" declarations,
or constant subexpressions are being evaluated. The startup and
import code of any "use" declaration is also run during compile
phase.
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 Camel
chapter 16, “Compiling”.
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 runtime.
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 the section
“Creating References” in Camel chapter 8, “References”.
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, 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, 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” context (which is
dealt with in Camel 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 errors.
CPAN
The Comprehensive Perl Archive Network. (See the Camel Preface and
Camel chapter 19, “CPAN” for details.)
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).
cracker
Someone who breaks security on computer systems. A cracker may be a
true hacker or only a script kiddie.
currently selected output channel
The last filehandle that was designated with select(FILEHANDLE);
"STDOUT", if no filehandle has been selected.
current package
The package in which the current statement is compiled. Scan
backward in the text of your program through the current lexical
scope or any enclosing lexical scopes until you find a package
declaration. That’s your current package name.
current working directory
See working directory.
CV In academia, a curriculum vitæ, a fancy kind of résumé. In Perl, 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.
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.)
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 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, as well as 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.
DBM Stands for “Database 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.
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.
declarator
Something that tells your program what sort of variable you’d like.
Perl doesn’t require you to declare variables, but you can use "my",
"our", or "state" to denote that you want something other than the
default.
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 the "defined" entry in Camel chapter 27, “Functions”.
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).
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, which 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 Camel chapter 23, “Plain Old Documentation”.
directory
A special file that contains other files. Some operating systems
call these “folders”, “drawers”, “catalogues”, 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.
discipline
Some people need this and some people avoid it. For Perl, it’s an
old way to say I/O layer.
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.
dual-lived
Some modules live both in the Standard Library and on CPAN. These
modules might be developed on two tracks as people modify either
version. The trend currently is to untangle these situations.
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 Middle 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 runtime.
E
eclectic
Derived from many sources. Some would say too many.
element
A basic building block. When you’re talking about an array, it’s one
of the items that make up the array.
embedding
When something is contained in something else, particularly when
that might be considered surprising: “I’ve embedded a complete Perl
interpreter in my editor!”
empty subclass test
The notion that an empty derived class should behave exactly like
its base class.
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.
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.]
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.
exploit
Used as a noun in this case, this refers to a known way to
compromise a program to get it to do something the author didn’t
intend. Your task is to write unexploitable programs.
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, 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.
feeping creaturism
A spoonerism of “creeping featurism”, noting the biological urge to
add just one more feature to a program.
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.
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. 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.
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.
filter
A program designed to take a stream of input and transform it into a
stream of output.
first-come
The first PAUSE author to upload a namespace automatically becomes
the primary maintainer for that namespace. The “first come”
permissions distinguish a primary maintainer who was assigned that
role from one who received it automatically.
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"). Sometimes informally used to refer to certain regex
modifiers.
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.
foldcase
The casemap used in Unicode when comparing or matching without
regard to case. Comparing lower-, title-, or uppercase are all
unreliable due to Unicode’s complex, one-to-many case mappings.
Foldcase is a lowercase variant (using a partially decomposed
normalization form for certain codepoints) created specifically to
resolve this.
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; 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 “Global Declarations” in
Camel chapter 4, “Statements and Declarations”.
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.
grapheme
A graphene is an allotrope of carbon arranged in a hexagonal crystal
lattice one atom thick. A grapheme, or more fully, a grapheme
cluster string is a single user-visible character, which may in turn
be several characters (codepoints) long. For example, a carriage
return plus a line feed is a single grapheme but two characters,
while a “ȫ” is a single grapheme but one, two, or even three
characters, depending on normalization.
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 Perl calls 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.
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 Camel
chapter 27, “Functions”. (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 15
are customarily represented by the letters "a" through "f".
Hexadecimal constants in Perl start with "0x". See also the "hex"
function in Camel chapter 27, “Functions”.
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 $ENV{LOGDIR} by login,
but you can also find it with "(get""pwuid($<))[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 for which Zeus zaps you. 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 to start with an alphabetic character, and then contain
alphabetics and digits. Perl also allows connector punctuation like
the underscore character wherever it allows alphabetics. (Perl also
has more complicated names, like 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 Camel chapter 27, “Functions”.
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 it's 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.
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.
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 in the dative
slot between the method and its arguments:
$gollum = new Pathetic::Creature "Sméagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
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";
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 data
See instance variable.
instance method
A method of an object, as opposed to a class method.
A method whose invocant is an object, not a package name. Every
object of a class shares all the methods of that class, so an
instance method applies to all instances of the class, rather than
applying to a particular instance. Also see class method.
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
runtime 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 to 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.
I/O layer
One of the filters between the data and what you get as input or
what you end up with as output.
IPA India Pale Ale. Also the International Phonetic Alphabet, the
standard alphabet used for phonetic notation worldwide. Draws
heavily on Unicode, including many combining characters.
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 USENET 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 then 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.
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 Camel chapter 5, “Pattern
Matching” for much more on this subject.
left shift
A bit shift that multiplies the number by some power of 2.
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 nonnewline 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.
linebreak
A grapheme consisting of either a carriage return followed by a line
feed or any character with the Unicode Vertical Space character
property.
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 that represents 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.
lowercase
In Unicode, not just characters with the General Category of
Lowercase Letter, but any character with the Lowercase property,
including Modifier Letters, Letter Numbers, some Other Symbols, and
one Combining Mark.
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 ASCII 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
Camel chapter 12, “Objects”.
method resolution order
The path Perl takes through @INC. By default, this is a double depth
first search, once looking for defined methods and once for
"AUTOLOAD". However, Perl lets you configure this with "mro".
minicpan
A CPAN mirror that includes just the latest versions for each
distribution, probably created with "CPAN::Mini". See Camel chapter
19, “CPAN”.
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, and lvalue, 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.
mojibake
When you speak one language and the computer thinks you’re speaking
another. You’ll see odd translations when you send UTF‑8, for
instance, but the computer thinks you sent Latin-1, showing all
sorts of weird characters instead. The term is written 「文字化け」in
Japanese and means “character rot”, an apt description. Pronounced
["modʑibake"] in standard IPA phonetics, or approximately
“moh-jee-bah-keh”.
monger
Short for one member of Perl mongers, a purveyor of Perl.
mortal
A temporary value scheduled to die when the current statement
finishes.
mro See method resolution order.
multidimensional array
An array with multiple subscripts for finding a single element. Perl
implements these using references—see Camel chapter 9, “Data
Structures”.
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), it is 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.
NaN Not a number. The value Perl uses for certain invalid or
inexpressible floating-point operations.
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.
normalization
Converting a text string into an alternate but equivalent canonical
(or compatible) representation that can then be compared for
equivalence. Unicode recognizes four different normalization forms:
NFD, NFC, NFKD, and NFKC.
null character
A character with the numeric value of zero. It’s used by C to
terminate strings, but Perl allows strings to contain a null.
null list
A list value with zero elements, represented in Perl by "()".
null string
A string containing no characters, not to be confused with a string
containing a null character, which has a positive length and is
true.
numeric context
The situation in which an expression is expected by its surroundings
(the code calling it) to return a number. See also context and
string context.
numification
(Sometimes spelled nummification and nummify.) Perl lingo for
implicit conversion into a number; the related verb is numify.
Numification is intended to rhyme with mummification, and numify
with mummify. It is unrelated to English numen, numina, numinous. We
originally forgot the extra m a long time ago, and some people got
used to our funny spelling, and so just as with "HTTP_REFERER"’s own
missing letter, our weird spelling has stuck around.
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—see Camel chapter 13, “Overloading”.
options
See either switches or regular expression modifiers.
ordinal
An abstract character’s integer value. Same thing as codepoint.
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 the section “Overriding Built-in Functions” in Camel
chapter 11, “Modules”), 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 Camel chapter 12, “Objects”).
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.
PAUSE
The Perl Authors Upload SErver (<http://pause.perl.org>), the
gateway for modules on their way to CPAN.
Perl mongers
A Perl user group, taking the form of its name from the New York
Perl mongers, the first Perl user group. Find one near you at
<http://www.pm.org>.
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 the following: 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. Pod
stands for “Plain old documentation”. See Camel chapter 23, “Plain
Old Documentation”.
pod command
A sequence, such as "=head1", that denotes the start of a pod
section.
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. [< Greek πολυ- + μορϕή, many forms.]
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, such as a 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.
possessive
Said of quantifiers and groups in patterns that refuse to give up
anything once they’ve gotten their mitts on it. Catchier and easier
to say than the even more formal nonbacktrackable.
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.
primary maintainer
The author that PAUSE allows to assign co-maintainer permissions to
a namespace. A primary maintainer can give up this distinction by
assigning it to another PAUSE author. See Camel chapter 19, “CPAN”.
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
See script.
program generator
A system that algorithmically writes code for you in a high-level
language. See also code generator.
progressive matching
Pattern matching 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 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
Formerly, a reference to an array whose initial element happens to
hold a reference to a hash. You used to be able to treat a
pseudohash reference as either an array reference or a hash
reference. Pseudohashes are no longer supported.
pseudoliteral
An operator X"that looks something like a literal, such as the
output-grabbing operator, <literal moreinfo="none""`>"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
race condition
A race condition exists when the result of several interrelated
events depends on the ordering of those events, but that order
cannot be guaranteed due to nondeterministic timing effects. If two
or more programs, or parts of the same program, try to go through
the same series of events, one might interrupt the work of the
other. This is a good way to find an exploit.
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 Camel chapter 5, “Pattern Matching”.
regular expression modifier
An option on a pattern or substitution, such as "/i" to render the
pattern case- insensitive.
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”.
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.
role
A name for a concrete set of behaviors. A role is a way to add
behavior to a class without inheritance.
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 runtime 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.
runtime
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.
runtime 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 reanalyzed
each time the pattern match operator is evaluated. Runtime 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
sandbox
A walled off area that’s not supposed to affect beyond its walls.
You let kids play in the sandbox instead of running in the road.
See Camel chapter 20, “Security”.
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
From how far away you can see a variable, 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 Tengwar.
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.
sigil
A glyph used in magic. Or, for Perl, the symbol in front of a
variable name, such as "$", "@", and "%".
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
the %SIG hash in Camel chapter 25, “Special Names” and the section
“Signals” in Camel chapter 15, “Interprocess Communication”.
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 idea 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 output>
filehandle "STDERR". You can use this stream explicitly, but the
"die" and "warn" built-ins write to your standard error stream
automatically (unless trapped or otherwise intercepted).
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 I/O
A standard C library for doing buffered input and output to the
operating system. (The “standard” of standard I/O is at most
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 Library
Everything that comes with the official perl distribution. Some
vendor versions of perl change their distributions, leaving out some
parts or including extras. See also dual-lived.
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".
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.
If you’re a C or C++ programmer, you might be looking for Perl’s
"state" keyword.
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, or declare it with "state" instead of with "my".
stat structure
A special internal spot in which Perl keeps the information about
the last file on which you requested information.
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 Camel chapter 27,
“Functions”.
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 subgoal 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. Most switch
statements in Perl are spelled "given". See “The "given" statement”
in Camel chapter 4, “Statements and Declarations”.
The "given", "when" keywords and the smartmatch ("~~") operator will
be removed in Perl 5.42.
symbol
Generally, any token or metasymbol. Often used more specifically to
mean the sort of name you might find in a symbol table.
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"".
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.
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 σύνταξις, “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 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 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
taint checks
The special bookkeeping Perl does to track the flow of external data
through your program and disallow their use in system commands.
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.
taint mode
Running under the "–T" switch, marking all external data as suspect
and refusing to use it with system commands. See Camel chapter 20,
“Security”.
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 one another.
tie The bond between a magical variable and its implementation class.
See the "tie" function in Camel chapter 27, “Functions” and Camel
chapter 14, “Tied Variables”.
titlecase
The case used for capitals that are followed by lowercase characters
instead of by more capitals. Sometimes called sentence case or
headline case. English doesn’t use Unicode titlecase, but casing
rules for English titles are more complicated than simply
capitalizing each word’s first character.
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.)
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.
topic
The thing you’re working on. Structures like while(<>), "for",
"foreach", and "given" set the topic for you by assigning to $_, the
default (topic) variable.
transliterate
To turn one string representation into another by mapping each
character of the source string to its corresponding character in the
result string. Not to be confused with translation: for example,
Greek πολύχρωμος transliterates into polychromos but translates into
many-colored. See the "tr///" operator in Camel chapter 5, “Pattern
Matching”.
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.
typedef
A type definition in the C and C++ languages.
typed lexical
A lexical variable lexical>that is declared with a class type: "my
Pony $bill".
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 Camel chapter 2,
“Bits and Pieces”.
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 <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.
uppercase
In Unicode, not just characters with the General Category of
Uppercase Letter, but any character with the Uppercase property,
including some Letter Numbers and Symbols. Not to be confused with
titlecase.
UTF-8 string
A "string" whose ordinals represent a valid sequence of UTF-8 bytes.
Sometimes called a "UTF-8 encoded string".
(IMPORTANT: This is unrelated to Perl’s internal “UTF8 flag”, which
only Perl itself should usually care about. UTF-8 strings may have
that flag set or unset.)
V
value
An actual piece of data, in contrast to all the variables,
references, keys, indices, 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 Camel chapter 27, “Functions” and the "warnings" pragma in Camel
chapter 28, “Pragmantic Modules”.
watch expression
An expression which, when its value changes, causes a breakpoint in
the Perl debugger.
weak reference
A reference that doesn’t get counted normally. When all the normal
references to data disappear, the data disappears. These are useful
for circular references that would never disappear otherwise.
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. In Unicode, matches many
other characters that Unicode considers whitespace, including the
ɴ-ʙʀ .
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 An extraordinarily exported, expeditiously excellent, expressly
eXternal Subroutine, executed in existing C or C++ or in an exciting
extension language called (exasperatingly) XS.
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; otherwise, 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, Fourth Edition, by Tom
Christiansen, brian d foy, Larry Wall, & Jon Orwant. Copyright (c)
2000, 1996, 1991, 2012 O'Reilly Media, Inc. This document may be
distributed under the same terms as Perl itself.
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