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PERLGLOSSARY(1) Perl Programmers Reference Guide PERLGLOSSARY(1)
NAME
perlglossary - Perl Glossary
DESCRIPTION
A glossary of terms (technical and otherwise) used in the
Perl documentation. Other useful sources include the Free
On-Line Dictionary of Computing
, the Jargon
File , and Wikipedia
.
A
accessor methods
A "method" used to indirectly inspect or update an
"object"'s state (its instance variables).
actual arguments
The scalar values that you supply to a "function" or
"subroutine" when you call it. For instance, when you
call "power("puff")", the string "puff" is the actual
argument. See also "argument" and "formal arguments".
address operator
Some languages work directly with the memory addresses
of values, but this can be like playing with fire.
Perl provides a set of asbestos gloves for handling
all memory management. The closest to an address
operator in Perl is the backslash operator, but it
gives you a "hard reference", which is much safer than
a memory address.
algorithm
A well-defined sequence of steps, clearly enough
explained that even a computer could do them.
alias
A nickname for something, which behaves in all ways as
though you'd used the original name instead of the
nickname. Temporary aliases are implicitly created in
the loop variable for "foreach" loops, in the $_ vari-
able 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. Per-
manent aliases are explicitly created in packages by
importing symbols or by assignment to typeglobs. Lex-
ically scoped aliases for package variables are
explicitly created by the our declaration.
alternatives
A list of possible choices from which you may select
only one, as in "Would you like door A, B, or C?"
Alternatives in regular expressions are separated with
a single vertical bar: "|". Alternatives in normal
Perl expressions are separated with a double vertical
bar: "||". Logical alternatives in "Boolean" expres-
sions 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 refer-
ence goes away, the anonymous referent is destroyed
without pity.
architecture
The kind of computer you're working on, where one
"kind" of computer means all those computers sharing a
compatible machine language. Since Perl programs are
(typically) simple text files, not executable images,
a Perl program is much less sensitive to the architec-
ture 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" "vec-
tor" 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" con-
taining the name of the current input file.
arithmetical operator
A "symbol" such as "+" or "/" that tells Perl to do
the arithmetic you were supposed to learn in grade
school.
array
An ordered sequence of values, stored such that you
can easily access any of the values using an integer
"subscript" that specifies the value's "offset" in the
sequence.
array context
An archaic expression for what is more correctly
referred to as "list context".
ASCII
The American Standard Code for Information Interchange
(a 7-bit character set adequate only for poorly repre-
senting English text). Often used loosely to describe
the lowest 128 values of the various ISO-8859-X char-
acter sets, a bunch of mutually incompatible 8-bit
codes best described as half ASCII. See also "Uni-
code".
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 "opera-
tor" composed of an ordinary assignment and some other
operator, that changes the value of a variable in
place, that is, relative to its old value. For
example, "$a += 2" adds 2 to $a.
associative array
See "hash". Please.
associativity
Determines whether you do the left "operator" first or
the right "operator" first when you have "A "operator"
B "operator" C" and the two operators are of the same
precedence. Operators like "+" are left associative,
while operators like "**" are right associative. See
perlop for a list of operators and their associativ-
ity.
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 fol-
lowing "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 indivisi-
ble bits of matter, he meant literally something that
could not be cut: a- (not) + tomos (cuttable). An
atomic operation is an action that can't be inter-
rupted, 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 vari-
able" of an "object".
autogeneration
A feature of "operator overloading" of objects,
whereby the behavior of certain operators can be rea-
sonably deduced using more fundamental operators.
This assumes that the overloaded operators will often
have the same relationships as the regular operators.
See perlop.
autoincrement
To add one to something automatically, hence the name
of the "++" operator. To instead subtract one from
something automatically is known as an "autodecre-
ment".
autoload
To load on demand. (Also called "lazy" loading.)
Specifically, to call an AUTOLOAD subroutine on behalf
of an undefined subroutine.
autosplit
To split a string automatically, as the -a "switch"
does when running under -p or -n in order to emulate
"awk". (See also the AutoSplit module, which has
nothing to do with the -a switch, but a lot to do with
autoloading.)
autovivification
A Greco-Roman word meaning "to bring oneself to life".
In Perl, storage locations (lvalues) spontaneously
generate themselves as needed, including the creation
of any "hard reference" values to point to the next
level of storage. The assignment "$a[5][5][5][5][5] =
"quintet"" potentially creates five scalar storage
locations, plus four references (in the first four
scalar locations) pointing to four new anonymous
arrays (to hold the last four scalar locations). But
the point of autovivification is that you don't have
to worry about it.
AV Short for "array value", which refers to one of Perl's
internal data types that holds an "array". The "AV"
type is a subclass of "SV".
awk Descriptive editing term--short for "awkward". Also
coincidentally refers to a venerable text-processing
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 recur-
sion on a tree of possibilities. Perl backtracks when
it attempts to match patterns with a "regular expres-
sion", and its earlier attempts don't pan out. See
"Backtracking" in perlre.
backward compatibility
Means you can still run your old program because we
didn't break any of the features or bugs it was rely-
ing 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 peo-
ple who respect their ancestors.
big-endian
From Swift: someone who eats eggs big end first. Also
used of computers that store the most significant
"byte" of a word at a lower byte address than the
least significant byte. Often considered superior to
little-endian machines. See also "little-endian".
binary
Having to do with numbers represented in base 2. That
means there's basically two numbers, 0 and 1. Also
used to describe a "non-text file", presumably because
such a file makes full use of all the binary bits in
its bytes. With the advent of "Unicode", this dis-
tinction, 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 Span-
ish 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 func-
tion as an "object", such as a WebCruncher object.
See "bless" in perlfunc.
block
What a "process" does when it has to wait for some-
thing: "My process blocked waiting for the disk." As
an unrelated noun, it refers to a large chunk of data,
of a size that the "operating system" likes to deal
with (normally a power of two such as 512 or 8192).
Typically refers to a chunk of data that's coming from
or going to a disk file.
BLOCK
A syntactic construct consisting of a sequence of Perl
statements that is delimited by braces. The "if" and
"while" statements are defined in terms of BLOCKs, for
instance. Sometimes we also say "block" to mean a
lexical scope; that is, a sequence of statements that
act like a "BLOCK", such as within an eval or a file,
even though the statements aren't delimited by braces.
block buffering
A method of making input and output efficient by pass-
ing 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 condition-
als 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 simulta-
neously.
BSD A psychoactive drug, popular in the 80s, probably
developed at U. C. Berkeley or thereabouts. Similar
in many ways to the prescription-only medication
called "System V", but infinitely more useful. (Or,
at least, more fun.) The full chemical name is
"Berkeley Standard Distribution".
bucket
A location in a "hash table" containing (potentially)
multiple entries whose keys "hash" to the same hash
value according to its hash function. (As internal
policy, you don't have to worry about it, unless
you're into internals, or policy.)
buffer
A temporary holding location for data. Block buffer-
ing means that the data is passed on to its destina-
tion whenever the buffer is full. Line buffering
means that it's passed on whenever a complete line is
received. Command buffering means that it's passed
every time you do a print command (or equivalent). If
your output is unbuffered, the system processes it one
byte at a time without the use of a holding area.
This can be rather inefficient.
built-in
A "function" that is predefined in the language. Even
when hidden by "overriding", you can always get at a
built-in function by qualifying its name with the
"CORE::" pseudo-package.
bundle
A group of related modules on "CPAN". (Also, some-
times refers to a group of command-line switches
grouped into one "switch cluster".)
byte
A piece of data worth eight bits in most places.
bytecode
A pidgin-like language spoken among 'droids when they
don't wish to reveal their orientation (see "endian").
Named after some similar languages spoken (for similar
reasons) between compilers and interpreters in the
late 20th century. These languages are characterized
by representing everything as a non-architecture-
dependent sequence of bytes.
C
C A language beloved by many for its inside-out "type"
definitions, inscrutable "precedence" rules, and heavy
"overloading" of the function-call mechanism. (Well,
actually, people first switched to C because they
found lowercase identifiers easier to read than
upper.) Perl is written in C, so it's not surprising
that Perl borrowed a few ideas from it.
C preprocessor
The typical C compiler's first pass, which processes
lines beginning with "#" for conditional compilation
and macro definition and does various manipulations of
the program text based on the current definitions.
Also known as cpp(1).
call by reference
An "argument"-passing mechanism in which the "formal
arguments" refer directly to the "actual arguments",
and the "subroutine" can change the actual arguments
by changing the formal arguments. That is, the formal
argument is an "alias" for the actual argument. See
also "call by value".
call by value
An "argument"-passing mechanism in which the "formal
arguments" refer to a copy of the "actual arguments",
and the "subroutine" cannot change the actual argu-
ments by changing the formal arguments. See also
"call by reference".
callback
A "handler" that you register with some other part of
your program in the hope that the other part of your
program will "trigger" your handler when some event of
interest transpires.
canonical
Reduced to a standard form to facilitate comparison.
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".)
character
A small integer representative of a unit of orthogra-
phy. Historically, characters were usually stored as
fixed-width integers (typically in a byte, or maybe
two, depending on the character set), but with the
advent of UTF-8, characters are often stored in a
variable number of bytes depending on the size of the
integer that represents the character. Perl manages
this transparently for you, for the most part.
character class
A square-bracketed list of characters used in a "regu-
lar expression" to indicate that any character of the
set may occur at a given point. Loosely, any prede-
fined set of characters so used.
character property
A predefined "character class" matchable by the "\p"
"metasymbol". Many standard properties are defined
for "Unicode".
circumfix operator
An "operator" that surrounds its "operand", like the
angle operator, or parentheses, or a hug.
class
A user-defined "type", implemented in Perl via a
"package" that provides (either directly or by inheri-
tance) methods (that is, subroutines) to handle
instances of the class (its objects). See also
"inheritance".
class method
A "method" whose "invocant" is a "package" name, not
an "object" reference. A method associated with the
class as a whole.
client
In networking, a "process" that initiates contact with
a "server" process in order to exchange data and per-
haps receive a service.
cloister
A "cluster" used to restrict the scope of a "regular
expression modifier".
closure
An "anonymous" subroutine that, when a reference to it
is generated at run time, keeps track of the identi-
ties 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 clo-
sure.
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 lan-
guage, such as code to implement the backend of a com-
piler. See "program generator".
code subpattern
A "regular expression" subpattern whose real purpose
is to execute some Perl code, for example, the
"(?{...})" and "(??{...})" subpatterns.
collating sequence
The order into which characters sort. This is used by
"string" comparison routines to decide, for example,
where in this glossary to put "collating sequence".
command
In "shell" programming, the syntactic combination of a
program name and its arguments. More loosely, any-
thing 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 typ-
ically 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 sin-
gle request to the "operating system". It's enabled
by setting the $| ($AUTOFLUSH) variable to a true
value. It's used when you don't want data sitting
around not going where it's supposed to, which may
happen because the default on a "file" or "pipe" is to
use "block buffering".
command name
The name of the program currently executing, as typed
on the command line. In C, the "command" name is
passed to the program as the first command-line argu-
ment. In Perl, it comes in separately as $0.
command-line arguments
The values you supply along with a program name when
you tell a "shell" to execute a "command". These val-
ues are passed to a Perl program through @ARGV.
comment
A remark that doesn't affect the meaning of the pro-
gram. In Perl, a comment is introduced by a "#" char-
acter and continues to the end of the line.
compilation unit
The "file" (or "string", in the case of eval) that is
currently being compiled.
compile phase
Any time before Perl starts running your main program.
See also "run phase". Compile phase is mostly spent
in "compile time", but may also be spent in "run time"
when "BEGIN" blocks, use declarations, or constant
subexpressions are being evaluated. The startup and
import code of any use declaration is also run during
compile phase.
compile time
The time when Perl is trying to make sense of your
code, as opposed to when it thinks it knows what your
code means and is merely trying to do what it thinks
your code says to do, which is "run time".
compiler
Strictly speaking, a program that munches up another
program and spits out yet another file containing the
program in a "more executable" form, typically con-
taining native machine instructions. The perl program
is not a compiler by this definition, but it does con-
tain a kind of compiler that takes a program and turns
it into a more executable form (syntax trees) within
the perl process itself, which the "interpreter" then
interprets. There are, however, extension modules to
get Perl to act more like a "real" compiler. See O.
composer
A "constructor" for a "referent" that isn't really an
"object", like an anonymous array or a hash (or a
sonata, for that matter). For example, a pair of
braces acts as a composer for a hash, and a pair of
brackets acts as a composer for an array. See "Making
References" in perlref.
concatenation
The process of gluing one cat's nose to another cat's
tail. Also, a similar operation on two strings.
conditional
Something "iffy". See "Boolean context".
connection
In telephony, the temporary electrical circuit between
the caller's and the callee's phone. In networking,
the same kind of temporary circuit between a "client"
and a "server".
construct
As a noun, a piece of syntax made up of smaller
pieces. As a transitive verb, to create an "object"
using a "constructor".
constructor
Any "class method", instance "method", or "subroutine"
that composes, initializes, blesses, and returns an
"object". Sometimes we use the term loosely to mean a
"composer".
context
The surroundings, or environment. The context given
by the surrounding code determines what kind of data a
particular "expression" is expected to return. The
three primary contexts are "list context", "scalar
context", and "void context". Scalar context is some-
times subdivided into "Boolean context", "numeric con-
text", "string context", and "void context". There's
also a "don't care" scalar context (which is dealt
with in Programming Perl, Third Edition, Chapter 2,
"Bits and Pieces" if you care).
continuation
The treatment of more than one physical "line" as a
single logical line. "Makefile" lines are continued
by putting a backslash before the "newline". Mail
headers as defined by RFC 822 are continued by putting
a space or tab after the newline. In general, lines
in Perl do not need any form of continuation mark,
because "whitespace" (including newlines) is gleefully
ignored. Usually.
core dump
The corpse of a "process", in the form of a file left
in the "working directory" of the process, usually as
a result of certain kinds of fatal error.
CPAN
The Comprehensive Perl Archive Network. (See "What
modules and extensions are available for Perl? What is
CPAN? What does CPAN/src/... mean?" in perlfaq2).
cracker
Someone who breaks security on computer systems. A
cracker may be a true "hacker" or only a "script kid-
die".
current package
The "package" in which the current statement is
compiled. Scan backwards in the text of your program
through the current lexical scope or any enclosing
lexical scopes till you find a package declaration.
That's your current package name.
current working directory
See "working directory".
currently selected output channel
The last "filehandle" that was designated with
select("FILEHANDLE"); "STDOUT", if no filehandle has
been selected.
CV An internal "code value" typedef, holding a "subrou-
tine". The "CV" type is a subclass of "SV".
D
dangling statement
A bare, single "statement", without any braces, hang-
ing off an "if" or "while" conditional. C allows
them. Perl doesn't.
data structure
How your various pieces of data relate to each other
and what shape they make when you put them all
together, as in a rectangular table or a triangular-
shaped tree.
data type
A set of possible values, together with all the opera-
tions that know how to deal with those values. For
example, a numeric data type has a certain set of num-
bers that you can work with and various mathematical
operations that you can do on the numbers but would
make little sense on, say, a string such as "Kilroy".
Strings have their own operations, such as "concatena-
tion". Compound types made of a number of smaller
pieces generally have operations to compose and decom-
pose them, and perhaps to rearrange them. Objects
that model things in the real world often have opera-
tions that correspond to real activities. For
instance, if you model an elevator, your elevator
object might have an "open_door()" "method".
datagram
A packet of data, such as a "UDP" message, that (from
the viewpoint of the programs involved) can be sent
independently over the network. (In fact, all packets
are sent independently at the "IP" level, but "stream"
protocols such as "TCP" hide this from your program.)
DBM Stands for "Data Base Management" routines, a set of
routines that emulate an "associative array" using
disk files. The routines use a dynamic hashing scheme
to locate any entry with only two disk accesses. DBM
files allow a Perl program to keep a persistent "hash"
across multiple invocations. You can tie your hash
variables to various DBM implementations--see Any-
DBM_File and DB_File.
declaration
An "assertion" that states something exists and per-
haps describes what it's like, without giving any com-
mitment 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 tad-
poles..." See "statement" for its opposite. Note
that some declarations also function as statements.
Subroutine declarations also act as definitions if a
body is supplied.
decrement
To subtract a value from a variable, as in "decrement
$x" (meaning to remove 1 from its value) or "decrement
$x by 3".
default
A "value" chosen for you if you don't supply a value
of your own.
defined
Having a meaning. Perl thinks that some of the things
people try to do are devoid of meaning, in particular,
making use of variables that have never been given a
"value" and performing certain operations on data that
isn't there. For example, if you try to read data
past the end of a file, Perl will hand you back an
undefined value. See also "false" and "defined" in
perlfunc.
delimiter
A "character" or "string" that sets bounds to an arbi-
trarily-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 trig-
gering its "DESTROY" method, if it has one).
destructor
A special "method" that is called when an "object" is
thinking about destroying itself. A Perl program's
"DESTROY" method doesn't do the actual destruction;
Perl just triggers the method in case the "class"
wants to do any associated cleanup.
device
A whiz-bang hardware gizmo (like a disk or tape drive
or a modem or a joystick or a mouse) attached to your
computer, that the "operating system" tries to make
look like a "file" (or a bunch of files). Under Unix,
these fake files tend to live in the /dev directory.
directive
A "pod" directive. See perlpod.
directory
A special file that contains other files. Some oper-
ating systems call these "folders", "drawers", or
"catalogs".
directory handle
A name that represents a particular instance of open-
ing a directory to read it, until you close it. See
the opendir function.
dispatch
To send something to its correct destination. Often
used metaphorically to indicate a transfer of program-
matic control to a destination selected algorithmi-
cally, often by lookup in a table of function refer-
ences or, in the case of object methods, by traversing
the inheritance tree looking for the most specific
definition for the method.
distribution
A standard, bundled release of a system of software.
The default usage implies source code is included. If
that is not the case, it will be called a
"binary-only" distribution.
dweomer
An enchantment, illusion, phantasm, or jugglery. Said
when Perl's magical "dwimmer" effects don't do what
you expect, but rather seem to be the product of
arcane dweomercraft, sorcery, or wonder working.
[From Old English]
dwimmer
DWIM is an acronym for "Do What I Mean", the principle
that something should just do what you want it to do
without an undue amount of fuss. A bit of code that
does "dwimming" is a "dwimmer". Dwimming can require
a great deal of behind-the-scenes magic, which (if it
doesn't stay properly behind the scenes) is called a
"dweomer" instead.
dynamic scoping
Dynamic scoping works over a dynamic scope, making
variables visible throughout the rest of the "block"
in which they are first used and in any subroutines
that are called by the rest of the block. Dynamically
scoped variables can have their values temporarily
changed (and implicitly restored later) by a local
operator. (Compare "lexical scoping".) Used more
loosely to mean how a subroutine that is in the middle
of calling another subroutine "contains" that subrou-
tine at "run time".
E
eclectic
Derived from many sources. Some would say too many.
element
A basic building block. When you're talking about an
"array", it's one of the items that make up the array.
embedding
When something is contained in something else, partic-
ularly when that might be considered surprising: "I've
embedded a complete Perl interpreter in my editor!"
empty subclass test
The notion that an empty "derived class" should behave
exactly like its "base class".
en passant
When you change a "value" as it is being copied.
[From French, "in passing", as in the exotic pawn-cap-
turing maneuver in chess.]
encapsulation
The veil of abstraction separating the "interface"
from the "implementation" (whether enforced or not),
which mandates that all access to an "object"'s state
be through methods alone.
endian
See "little-endian" and "big-endian".
environment
The collective set of environment variables your "pro-
cess" 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 off-
spring (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 exe-
cute bits under Unix, and which bit gets used depends
on whether you own the file singularly, collectively,
or not at all.
exit status
See "status".
export
To make symbols from a "module" available for "import"
by other modules.
expression
Anything you can legally say in a spot where a "value"
is required. Typically composed of literals, vari-
ables, 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 "stan-
dard error" stream. Errors that happen inside an eval
are not fatal. Instead, the eval terminates after
placing the exception message in the $@ ($EVAL_ERROR)
variable. You can try to provoke a fatal error with
the die operator (known as throwing or raising an
exception), but this may be caught by a dynamically
enclosing eval. If not caught, the die becomes a
fatal error.
field
A single piece of numeric or string data that is part
of a longer "string", "record", or "line". Variable-
width fields are usually split up by separators (so
use split to extract the fields), while fixed-width
fields are usually at fixed positions (so use unpack).
Instance variables are also known as fields.
FIFO
First In, First Out. See also "LIFO". Also, a nick-
name for a "named pipe".
file
A named collection of data, usually stored on disk in
a "directory" in a "filesystem". Roughly like a docu-
ment, if you're into office metaphors. In modern
filesystems, you can actually give a file more than
one name. Some files have special properties, like
directories and devices.
file descriptor
The little number the "operating system" uses to keep
track of which opened "file" you're talking about.
Perl hides the file descriptor inside a "standard I/O"
stream and then attaches the stream to a "filehandle".
file test operator
A built-in unary operator that you use to determine
whether something is "true" about a file, such as "-o
$filename" to test whether you're the owner of the
file.
fileglob
A "wildcard" match on filenames. See the glob func-
tion.
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 succes-
sion, 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 "direc-
tory", and you can use it in an open to tell the
"operating system" exactly which file you want to
open, and associate the file with a "filehandle" which
will carry the subsequent identity of that file in
your program, until you close it.
filesystem
A set of directories and files residing on a partition
of the disk. Sometimes known as a "partition". You
can change the file's name or even move a file around
from directory to directory within a filesystem with-
out actually moving the file itself, at least under
Unix.
filter
A program designed to take a "stream" of input and
transform it into a stream of output.
flag
We tend to avoid this term because it means so many
things. It may mean a command-line "switch" that
takes no argument itself (such as Perl's -n and -p
flags) or, less frequently, a single-bit indicator
(such as the "O_CREAT" and "O_EXCL" flags used in
sysopen).
floating point
A method of storing numbers in "scientific notation",
such that the precision of the number is independent
of its magnitude (the decimal point "floats"). Perl
does its numeric work with floating-point numbers
(sometimes called "floats"), when it can't get away
with using integers. Floating-point numbers are mere
approximations of real numbers.
flush
The act of emptying a "buffer", often before it's
full.
FMTEYEWTK
Far More Than Everything You Ever Wanted To Know. An
exhaustive treatise on one narrow topic, something of
a super-"FAQ". See Tom for far more.
fork
To create a child "process" identical to the parent
process at its moment of conception, at least until it
gets ideas of its own. A thread with protected mem-
ory.
formal arguments
The generic names by which a "subroutine" knows its
arguments. In many languages, formal arguments are
always given individual names, but in Perl, the formal
arguments are just the elements of an array. The for-
mal 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 print-
ing 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 boot-
leg 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, partic-
ularly 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 contradis-
tinction to "open source software", to refer only to
free software released under the Free Software Founda-
tion's GPL (General Public License), but this is dif-
ficult 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.
garbage collection
A misnamed feature--it should be called, "expecting
your mother to pick up after you". Strictly speaking,
Perl doesn't do this, but it relies on a reference-
counting mechanism to keep things tidy. However, we
rarely speak strictly and will often refer to the ref-
erence-counting scheme as a form of garbage collec-
tion. (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.)
G
GID Group ID--in Unix, the numeric group ID that the
"operating system" uses to identify you and members of
your "group".
glob
Strictly, the shell's "*" character, which will match
a "glob" of characters when you're trying to generate
a list of filenames. Loosely, the act of using globs
and similar symbols to do pattern matching. See also
"fileglob" and "typeglob".
global
Something you can see from anywhere, usually used of
variables and subroutines that are visible everywhere
in your program. In Perl, only certain special vari-
ables are truly global--most variables (and all sub-
routines) exist only in the current "package". Global
variables can be declared with our. See "our" in
perlfunc.
global destruction
The "garbage collection" of globals (and the running
of any associated object destructors) that takes place
when a Perl "interpreter" is being shut down. Global
destruction should not be confused with the Apoca-
lypse, except perhaps when it should.
glue language
A language such as Perl that is good at hooking things
together that weren't intended to be hooked together.
granularity
The size of the pieces you're dealing with, mentally
speaking.
greedy
A "subpattern" whose "quantifier" wants to match as
many things as possible.
grep
Originally from the old Unix editor command for "Glob-
ally 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 "type-
glob". The "GV" type is a subclass of "SV".
H
hacker
Someone who is brilliantly persistent in solving tech-
nical problems, whether these involve golfing, fight-
ing orcs, or programming. Hacker is a neutral term,
morally speaking. Good hackers are not to be confused
with evil crackers or clueless script kiddies. If you
confuse them, we will presume that you are either evil
or clueless.
handler
A "subroutine" or "method" that is called by Perl when
your program needs to respond to some internal event,
such as a "signal", or an encounter with an operator
subject to "operator overloading". See also "call-
back".
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 "sym-
bolic 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 some-
times 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 imple-
menting associative arrays (hashes) efficiently. See
also "bucket".
header file
A file containing certain required definitions that
you must include "ahead" of the rest of your program
to do certain obscure operations. A C header file has
a .h extension. Perl doesn't really have header
files, though historically Perl has sometimes used
translated .h files with a .ph extension. See
"require" in perlfunc. (Header files have been super-
seded by the "module" mechanism.)
here document
So called because of a similar construct in shells
that pretends that the lines following the "command"
are a separate "file" to be fed to the command, up to
some terminating string. In Perl, however, it's just
a fancy form of quoting.
hexadecimal
A number in base 16, "hex" for short. The digits for
10 through 16 are customarily represented by the let-
ters "a" through "f". Hexadecimal constants in Perl
start with "0x". See also "hex" in perlfunc.
home directory
The directory you are put into when you log in. On a
Unix system, the name is often placed into $ENV{HOME}
or $ENV{LOGDIR} by login, but you can also find it
with "(getpwuid($<))[7]". (Some platforms do not have
a concept of a home directory.)
host
The computer on which a program or other data resides.
hubris
Excessive pride, the sort of thing Zeus zaps you for.
Also the quality that makes you write (and maintain)
programs that other people won't want to say bad
things about. Hence, the third great virtue of a pro-
grammer. See also "laziness" and "impatience".
HV Short for a "hash value" typedef, which holds Perl's
internal representation of a hash. The "HV" type is a
subclass of "SV".
I
identifier
A legally formed name for most anything in which a
computer program might be interested. Many languages
(including Perl) allow identifiers that start with a
letter and contain letters and digits. Perl also
counts the underscore character as a valid letter.
(Perl also has more complicated names, such as "quali-
fied" names.)
impatience
The anger you feel when the computer is being lazy.
This makes you write programs that don't just react to
your needs, but actually anticipate them. Or at least
that pretend to. Hence, the second great virtue of a
programmer. See also "laziness" and "hubris".
implementation
How a piece of code actually goes about doing its job.
Users of the code should not count on implementation
details staying the same unless they are part of the
published "interface".
import
To gain access to symbols that are exported from
another module. See "use" in perlfunc.
increment
To increase the value of something by 1 (or by some
other number, if so specified).
indexing
In olden days, the act of looking up a "key" in an
actual index (such as a phone book), but now merely
the act of using any kind of key or position to find
the corresponding "value", even if no index is
involved. Things have degenerated to the point that
Perl's index function merely locates the position
(index) of one string in another.
indirect filehandle
An "expression" that evaluates to something that can
be used as a "filehandle": a "string" (filehandle
name), a "typeglob", a typeglob "reference", or a low-
level "IO" object.
indirect object
In English grammar, a short noun phrase between a verb
and its direct object indicating the beneficiary or
recipient of the action. In Perl, "print STDOUT
"$foo\n";" can be understood as "verb indirect-object
object" where "STDOUT" is the recipient of the print
action, and "$foo" is the object being printed. Simi-
larly, when invoking a "method", you might place the
invocant between the method and its arguments:
$gollum = new Pathetic::Creature "Smeagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
indirect object slot
The syntactic position falling between a method call
and its arguments when using the indirect object invo-
cation syntax. (The slot is distinguished by the
absence of a comma between it and the next argument.)
"STDERR" is in the indirect object slot here:
print STDERR "Awake! Awake! Fear, Fire,
Foes! Awake!\n";
indirection
If something in a program isn't the value you're look-
ing for but indicates where the value is, that's indi-
rection. This can be done with either symbolic refer-
ences or hard references.
infix
An "operator" that comes in between its operands, such
as multiplication in "24 * 7".
inheritance
What you get from your ancestors, genetically or oth-
erwise. If you happen to be a "class", your ancestors
are called base classes and your descendants are
called derived classes. See "single inheritance" and
"multiple inheritance".
instance
Short for "an instance of a class", meaning an
"object" of that "class".
instance variable
An "attribute" of an "object"; data stored with the
particular object rather than with the class as a
whole.
integer
A number with no fractional (decimal) part. A count-
ing number, like 1, 2, 3, and so on, but including 0
and the negatives.
interface
The services a piece of code promises to provide for-
ever, 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 interpo-
lation 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 pro-
gram and does what the second program says directly
without turning the program into a different form
first, which is what compilers do. Perl is not an
interpreter by this definition, because it contains a
kind of compiler that takes a program and turns it
into a more executable form (syntax trees) within the
perl process itself, which the Perl "run time" system
then interprets.
invocant
The agent on whose behalf a "method" is invoked. In a
"class" method, the invocant is a package name. In an
"instance" method, the invocant is an object refer-
ence.
invocation
The act of calling up a deity, daemon, program,
method, subroutine, or function to get it do what you
think it's supposed to do. We usually "call" subrou-
tines but "invoke" methods, since it sounds cooler.
I/O Input from, or output to, a "file" or "device".
IO An internal I/O object. Can also mean "indirect
object".
IP Internet Protocol, or Intellectual Property.
IPC Interprocess Communication.
is-a
A relationship between two objects in which one object
is considered to be a more specific version of the
other, generic object: "A camel is a mammal." Since
the generic object really only exists in a Platonic
sense, we usually add a little abstraction to the
notion of objects and think of the relationship as
being between a generic "base class" and a specific
"derived class". Oddly enough, Platonic classes don't
always have Platonic relationships--see "inheritance".
iteration
Doing something repeatedly.
iterator
A special programming gizmo that keeps track of where
you are in something that you're trying to iterate
over. The "foreach" loop in Perl contains an itera-
tor; 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 con-
fused 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 ungoing Obfuscated Perl
Contest seen in Usenix signatures.
K
key The string index to a "hash", used to look up the
"value" associated with that key.
keyword
See "reserved words".
L
label
A name you give to a "statement" so that you can talk
about that statement elsewhere in the program.
laziness
The quality that makes you go to great effort to
reduce overall energy expenditure. It makes you write
labor-saving programs that other people will find use-
ful, and document what you wrote so you don't have to
answer so many questions about it. Hence, the first
great virtue of a programmer. Also hence, this book.
See also "impatience" and "hubris".
left shift
A "bit shift" that multiplies the number by some power
of 2.
leftmost longest
The preference of the "regular expression" engine to
match the leftmost occurrence of a "pattern", then
given a position at which a match will occur, the
preference for the longest match (presuming the use of
a "greedy" quantifier). See perlre for much more on
this subject.
lexeme
Fancy term for a "token".
lexer
Fancy term for a "tokener".
lexical analysis
Fancy term for "tokenizing".
lexical scoping
Looking at your Oxford English Dictionary through a
microscope. (Also known as "static scoping", because
dictionaries don't change very fast.) Similarly,
looking at variables stored in a private dictionary
(namespace) for each scope, which are visible only
from their point of declaration down to the end of the
lexical scope in which they are declared. --Syn.
"static scoping". --Ant. "dynamic scoping".
lexical variable
A "variable" subject to "lexical scoping", declared by
my. Often just called a "lexical". (The our declara-
tion 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 usu-
ally called a "stack".
line
In Unix, a sequence of zero or more non-newline char-
acters terminated with a "newline" character. On non-
Unix machines, this is emulated by the C library even
if the underlying "operating system" has different
ideas.
line buffering
Used by a "standard I/O" output stream that flushes
its "buffer" after every "newline". Many standard I/O
libraries automatically set up line buffering on out-
put that is going to the terminal.
line number
The number of lines read previous to this one, plus 1.
Perl keeps a separate line number for each source or
input file it opens. The current source file's line
number is represented by "__LINE__". The current
input line number (for the file that was most recently
read via "") is represented by the $.
($INPUT_LINE_NUMBER) variable. Many error messages
report both values, if available.
link
Used as a noun, a name in a "directory", representing
a "file". A given file can have multiple links to it.
It's like having the same phone number listed in the
phone directory under different names. As a verb, to
resolve a partially compiled file's unresolved symbols
into a (nearly) executable image. Linking can gener-
ally 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 "con-
text".
list operator
An "operator" that does something with a list of val-
ues, 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 "argu-
ment" list.
list value
An unnamed list of temporary scalar values that may be
passed around within a program from any list-generat-
ing function to any function or construct that pro-
vides 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 signifi-
cant "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 vari-
able 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 "itera-
tion". Generally you shouldn't try this on roller
coasters.
loop label
A kind of key or name attached to a loop (or roller
coaster) so that loop control statements can talk
about which loop they want to control.
lvaluable
Able to serve as an "lvalue".
lvalue
Term used by language lawyers for a storage location
you can assign a new "value" to, such as a "variable"
or an element of an "array". The "l" is short for
"left", as in the left side of an assignment, a typi-
cal place for lvalues. An "lvaluable" function or
expression is one to which a value may be assigned, as
in "pos($x) = 10".
lvalue modifier
An adjectival pseudofunction that warps the meaning of
an "lvalue" in some declarative fashion. Currently
there are three lvalue modifiers: my, our, and local.
M
magic
Technically speaking, any extra semantics attached to
a variable such as $!, $0, %ENV, or %SIG, or to any
tied variable. Magical things happen when you diddle
those variables.
magical increment
An "increment" operator that knows how to bump up
alphabetics as well as numbers.
magical variables
Special variables that have side effects when you
access them or assign to them. For example, in Perl,
changing elements of the %ENV array also changes the
corresponding environment variables that subprocesses
will use. Reading the $! variable gives you the cur-
rent 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 typi-
cally longer than a page. There are manpages docu-
menting 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 nor-
mally. Which characters are to be treated specially
as metacharacters varies greatly from context to con-
text. 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 meta-
symbol to misbehave along with it.
method
A kind of action that an "object" can take if you tell
it to. See perlobj.
minimalism
The belief that "small is beautiful." Paradoxically,
if you say something in a small language, it turns out
big, and if you say it in a big language, it turns out
small. Go figure.
mode
In the context of the stat syscall, refers to the
field holding the "permission bits" and the type of
the "file".
modifier
See "statement modifier", "regular expression modi-
fier", and "lvalue modifier", not necessarily in that
order.
module
A "file" that defines a "package" of (almost) the same
name, which can either "export" symbols or function as
an "object" class. (A module's main .pm file may also
load in other files in support of the module.) See
the use built-in.
modulus
An integer divisor when you're interested in the
remainder instead of the quotient.
monger
Short for Perl Monger, a purveyor of Perl.
mortal
A temporary value scheduled to die when the current
statement finishes.
multidimensional array
An array with multiple subscripts for finding a single
element. Perl implements these using references--see
perllol and perldsc.
multiple inheritance
The features you got from your mother and father,
mixed together unpredictably. (See also "inheri-
tance", and "single inheritance".) In computer lan-
guages (including Perl), the notion that a given class
may have multiple direct ancestors or base classes.
N
named pipe
A "pipe" with a name embedded in the "filesystem" so
that it can be accessed by two unrelated processes.
namespace
A domain of names. You needn't worry about whether
the names in one such domain have been used in
another. See "package".
network address
The most important attribute of a socket, like your
telephone's telephone number. Typically an IP
address. See also "port".
newline
A single character that represents the end of a line,
with the ASCII value of 012 octal under Unix (but 015
on a Mac), and represented by "\n" in Perl strings.
For Windows machines writing text files, and for cer-
tain physical devices like terminals, the single new-
line gets automatically translated by your C library
into a line feed and a carriage return, but normally,
no translation is done.
NFS Network File System, which allows you to mount a
remote filesystem as if it were local.
null character
A character with the ASCII value of zero. It's used
by C to terminate strings, but Perl allows strings to
contain a null.
null list
A "list value" with zero elements, represented in Perl
by "()".
null string
A "string" containing no characters, not to be con-
fused with a string containing a "null character",
which has a positive length and is "true".
numeric context
The situation in which an expression is expected by
its surroundings (the code calling it) to return a
number. See also "context" and "string context".
NV Short for Nevada, no part of which will ever be con-
fused 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 posi-
tion 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
.
operand
An "expression" that yields a "value" that an "opera-
tor" 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 (upset-
ting many people, especially lawyers and other advo-
cates). At the other extreme, you could say this par-
ticular 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 lan-
guage with a special syntax or symbol. A given opera-
tor may have specific expectations about what types of
data you give as its arguments (operands) and what
type of data you want back from it.
operator overloading
A kind of "overloading" that you can do on built-in
operators to make them work on objects as if the
objects were ordinary scalar values, but with the
actual semantics supplied by the object class. This
is set up with the overload "pragma".
options
See either switches or "regular expression modifier".
overloading
Giving additional meanings to a symbol or construct.
Actually, all languages do overloading to one extent
or another, since people are good at figuring out
things from "context".
overriding
Hiding or invalidating some other definition of the
same name. (Not to be confused with "overloading",
which adds definitions that must be disambiguated some
other way.) To confuse the issue further, we use the
word with two overloaded definitions: to describe how
you can define your own "subroutine" to hide a built-
in "function" of the same name (see "Overriding Built-
in Functions" in perlsub) and to describe how you can
define a replacement "method" in a "derived class" to
hide a "base class"'s method of the same name (see
perlobj).
owner
The one user (apart from the superuser) who has abso-
lute control over a "file". A file may also have a
"group" of users who may exercise joint ownership if
the real owner permits it. See "permission bits".
P
package
A "namespace" for global variables, subroutines, and
the like, such that they can be kept separate from
like-named symbols in other namespaces. In a sense,
only the package is global, since the symbols in the
package's symbol table are only accessible from code
compiled outside the package by naming the package.
But in another sense, all package symbols are also
globals--they're just well-organized globals.
pad Short for "scratchpad".
parameter
See "argument".
parent class
See "base class".
parse tree
See "syntax tree".
parsing
The subtle but sometimes brutal art of attempting to
turn your possibly malformed program into a valid
"syntax tree".
patch
To fix by applying one, as it were. In the realm of
hackerdom, a listing of the differences between two
versions of a program as might be applied by the
patch(1) program when you want to fix a bug or upgrade
your old version.
PATH
The list of directories the system searches to find a
program you want to "execute". The list is stored as
one of your environment variables, accessible in Perl
as $ENV{PATH}.
pathname
A fully qualified filename such as /usr/bin/perl.
Sometimes confused with "PATH".
pattern
A template used in "pattern matching".
pattern matching
Taking a pattern, usually a "regular expression", and
trying the pattern various ways on a string to see
whether there's any way to make it fit. Often used to
pick interesting tidbits out of a file.
permission bits
Bits that the "owner" of a file sets or unsets to
allow or disallow access to other people. These flag
bits are part of the "mode" word returned by the stat
built-in when you ask about a file. On Unix systems,
you can check the ls(1) manpage for more information.
Pern
What you get when you do "Perl++" twice. Doing it
only once will curl your hair. You have to increment
it eight times to shampoo your hair. Lather, rinse,
iterate.
pipe
A direct "connection" that carries the output of one
"process" to the input of another without an interme-
diate temporary file. Once the pipe is set up, the
two processes in question can read and write as if
they were talking to a normal file, with some caveats.
pipeline
A series of processes all in a row, linked by pipes,
where each passes its output stream to the next.
platform
The entire hardware and software context in which a
program runs. A
program written in a platform-dependent language
might break if you change any of: machine, operating
system, libraries, compiler, or system configuration.
The perl interpreter has to be compiled differently
for each platform because it is implemented in C, but
programs written in the Perl language are largely
platform-independent.
pod The markup used to embed documentation into your Perl
code. See perlpod.
pointer
A "variable" in a language like C that contains the
exact memory location of some other item. Perl han-
dles 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 ref-
erences, 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 some-
thing generic, and the object will interpret the com-
mand in different ways depending on its type. [wait(2). See
"system" in perlfunc.
STDERR
See "standard error".
STDIN
See "standard input".
STDIO
See "standard I/O".
STDOUT
See "standard output".
stream
A flow of data into or out of a process as a steady
sequence of bytes or characters, without the appear-
ance of being broken up into packets. This is a kind
of "interface"--the underlying "implementation" may
well break your data up into separate packets for
delivery, but this is hidden from you.
string
A sequence of characters such as "He said
!@#*&%@#*?!". A string does not have to be entirely
printable.
string context
The situation in which an expression is expected by
its surroundings (the code calling it) to return a
"string". See also "context" and "numeric context".
stringification
The process of producing a "string" representation of
an abstract object.
struct
C keyword introducing a structure definition or name.
structure
See "data structure".
subclass
See "derived class".
subpattern
A component of a "regular expression" pattern.
subroutine
A named or otherwise accessible piece of program that
can be invoked from elsewhere in the program in order
to accomplish some sub-goal of the program. A subrou-
tine 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 interpo-
lation".)
substring
A portion of a "string", starting at a certain "char-
acter" position ("offset") and proceeding for a cer-
tain 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 inter-
preter 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 "sub-
routine".
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 "expres-
sion" and then, based on the value of the expression,
do a multiway branch to the appropriate piece of code
for that value. Also called a "case structure", named
after the similar Pascal construct. Most switch
statements in Perl are spelled "for". See "Basic
BLOCKs and Switch Statements" in perlsyn.
symbol
Generally, any "token" or "metasymbol". Often used
more specifically to mean the sort of name you might
find in a "symbol table".
symbol table
Where a "compiler" remembers symbols. A program like
Perl must somehow remember all the names of all the
variables, filehandles, and subroutines you've used.
It does this by placing the names in a symbol table,
which is implemented in Perl using a "hash table".
There is a separate symbol table for each "package" to
give each package its own "namespace".
symbolic debugger
A program that lets you step through the execution of
your program, stopping or printing things out here and
there to see whether anything has gone wrong, and if
so, what. The "symbolic" part just means that you can
talk to the debugger using the same symbols with which
your program is written.
symbolic link
An alternate filename that points to the real "file-
name", which in turn points to the real "file". When-
ever the "operating system" is trying to parse a
"pathname" containing a symbolic link, it merely sub-
stitutes the new name and continues parsing.
symbolic reference
A variable whose value is the name of another variable
or subroutine. By dereferencing the first variable,
you can get at the second one. Symbolic references
are illegal under use strict 'refs'.
synchronous
Programming in which the orderly sequence of events
can be determined; that is, when things happen one
after the other, not at the same time.
syntactic sugar
An alternative way of writing something more easily; a
shortcut.
syntax
From Greek, "with-arrangement". How things (particu-
larly 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 pre-
dict 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 func-
tion, which actually involves many syscalls. To avoid
any confusion, we nearly always use say "syscall" for
something you could call indirectly via Perl's syscall
function, and never for something you would call with
Perl's system function.
T
tainted
Said of data derived from the grubby hands of a user
and thus unsafe for a secure program to rely on. Perl
does taint checks if you run a "setuid" (or "setgid")
program, or if you use the -T switch.
TCP Short for Transmission Control Protocol. A protocol
wrapped around the Internet Protocol to make an unre-
liable 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 delim-
iters or separators. The period at the end of this
sentence is a terminator.
ternary
An "operator" taking three operands. Sometimes pro-
nounced "trinary".
text
A "string" or "file" containing primarily printable
characters.
thread
Like a forked process, but without "fork"'s inherent
memory protection. A thread is lighter weight than a
full process, in that a process could have multiple
threads running around in it, all fighting over the
same process's memory space unless steps are taken to
protect threads from each other. See threads.
tie The bond between a magical variable and its
implementation class. See "tie" in perlfunc and
perltie.
TMTOWTDI
There's More Than One Way To Do It, the Perl Motto.
The notion that there can be more than one valid path
to solving a programming problem in context. (This
doesn't mean that more ways are always better or that
all possible paths are equally desirable--just that
there need not be One True Way.) Pronounced TimToady.
token
A morpheme in a programming language, the smallest
unit of text with semantic significance.
tokener
A module that breaks a program text into a sequence of
tokens for later analysis by a parser.
tokenizing
Splitting up a program text into tokens. Also known
as "lexing", in which case you get "lexemes" instead
of tokens.
toolbox approach
The notion that, with a complete set of simple tools
that work well together, you can build almost anything
you want. Which is fine if you're assembling a tricy-
cle, but if you're building a defranishizing comboflux
regurgalator, you really want your own machine shop in
which to build special tools. Perl is sort of a
machine shop.
transliterate
To turn one string representation into another by map-
ping each character of the source string to its corre-
sponding character in the result string. See
"tr/SEARCHLIST/REPLACEMENTLIST/cds" in perlop.
trigger
An event that causes a "handler" to be run.
trinary
Not a stellar system with three stars, but an "opera-
tor" 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 automati-
cally when opening a file for writing or explicitly
via the truncate function.
type
See "data type" and "class".
type casting
Converting data from one type to another. C permits
this. Perl does not need it. Nor want it.
typed lexical
A "lexical variable" that is declared with a "class"
type: "my Pony $bill".
typedef
A type definition in the C language.
typeglob
Use of a single identifier, prefixed with "*". For
example, *name stands for any or all of $name, @name,
%name, &name, or just "name". How you use it deter-
mines whether it is interpreted as all or only one of
them. See "Typeglobs and Filehandles" in perldata.
typemap
A description of how C types may be transformed to and
from Perl types within an "extension" module written
in "XS".
U
UDP User Datagram Protocol, the typical way to send data-
grams 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 .
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 communi-
cate via an Esperanto-like language called Perl.
In ancient times, Unix was also used to refer to some
code that a couple of people at Bell Labs wrote to
make use of a PDP-7 computer that wasn't doing much of
anything else at the time.
V
value
An actual piece of data, in contrast to all the vari-
ables, references, keys, indexes, operators, and what-
not 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 indeter-
minate 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 transpar-
ently.
void context
A form of "scalar context" in which an "expression" is
not expected to return any "value" at all and is eval-
uated 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 nota-
tion, for instance, "v1.20.300.4000". Each number
turns into a "character" with the specified ordinal
value. (The "v" is optional when there are at least
three integers.)
W
warning
A message printed to the "STDERR" stream to the effect
that something might be wrong but isn't worth blowing
up over. See "warn" in perlfunc and the warnings
pragma.
watch expression
An expression which, when its value changes, causes a
breakpoint in the Perl debugger.
whitespace
A "character" that moves your cursor but doesn't oth-
erwise put anything on your screen. Typically refers
to any of: space, tab, line feed, carriage return, or
form feed.
word
In normal "computerese", the piece of data of the size
most efficiently handled by your computer, typically
32 bits or so, give or take a few powers of 2. In
Perl culture, it more often refers to an alphanumeric
"identifier" (including underscores), or to a string
of nonwhitespace characters bounded by whitespace or
string boundaries.
working directory
Your current "directory", from which relative path-
names 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 some-
thing 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 every-
thing 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 new extension language called
(exasperatingly) XS. Examine perlxs for the exact
explanation or perlxstut for an exemplary unexacting
one.
XSUB
An external "subroutine" defined in "XS".
Y
yacc
Yet Another Compiler Compiler. A parser generator
without which Perl probably would not have existed.
See the file perly.y in the Perl source distribution.
Z
zero width
A subpattern "assertion" matching the "null string"
between characters.
zombie
A process that has died (exited) but whose parent has
not yet received proper notification of its demise by
virtue of having called wait or waitpid. If you fork,
you must clean up after your child processes when they
exit, or else the process table will fill up and your
system administrator will Not Be Happy with you.
AUTHOR AND COPYRIGHT
Based on the Glossary of Programming Perl, Third Edition,
by Larry Wall, Tom Christiansen & Jon Orwant. Copyright
(c) 2000, 1996, 1991 O'Reilly Media, Inc. This document
may be distributed under the same terms as Perl itself.
perl v5.8.8 2006-01-07 PERLGLOSSARY(1)