perlglossary - Perl Glossary
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/>.
- accessor methods
- A method used to indirectly inspect or update an objectXs
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 youXd 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"Xs 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,
XWould you like door A, B, or C?X 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
donXt realize they are using a program.
- architecture
- The kind of computer youXre working on, where one Xkind of computerX 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 itXs 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 itXs supposed to do.
Also called a XparameterX.
- 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 valueXs 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
PerlXs 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 XA operator B operator
CX, and the two operators are of the same precedence. Operators like
"+" are left associative, while
operators like "**" are right
associative. See Camel chapter 3, XUnary and Binary OperatorsX 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 didnXt 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 XatomX to the indivisible bits of matter, he
meant literally something that could not be cut: X- (not) +
-XXXXX (cuttable). An atomic operation is an action that canXt 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, XOverloadingX.
- autoincrement
- To add one to something automatically, hence the name of the
"++" operator. To instead subtract one
from something automatically is known as an XautodecrementX.
- autoload
- To load on demand. (Also called XlazyX loading.) Specifically, to call an
"AUTOLOAD" subroutine on behalf of an
undefined subroutine.
- autosplit
- To split a string automatically, as the Xa switch does when
running under Xp or Xn in order to emulate awk. (See
also the "AutoSplit" module, which has
nothing to do with the "Xa" switch but a
lot to do with autoloading.)
- autovivification
- A Graeco-Roman word meaning Xto bring oneself to lifeX. 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 donXt have to
worry about it.
- AV
- Short for Xarray valueX, which refers to one of PerlXs internal data types
that holds an array. The "AV"
type is a subclass of SV.
- awk
- Descriptive editing termXshort for XawkwardX. Also coincidentally refers
to a venerable text-processing language from which Perl derived some of
its high-level ideas.
- 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, XIf I had to do it all over, IXd do it
differently,X and then actually going back and doing it all over
differently. Mathematically speaking, itXs 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
donXt pan out. See the section XThe Little Engine That /Couldn(nXt)X in
Camel chapter 5, XPattern MatchingX.
- backward compatibility
- Means you can still run your old program because we didnXt 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 XsuperclassX 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 thereXs
basically two numbers: 0 and 1. Also used to describe a file of XnontextX,
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 XPieces of EightX comes from being able to split the old Spanish
dollar into 8 bits, each of which still counted for money. ThatXs why a
25- cent piece today is still Xtwo bitsX.)
- 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, XThe VP
of Engineering has blessed our WebCruncher project.X 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,
XFunctionsX.
- block
- What a process does when it has to wait for something: XMy process
blocked waiting for the disk.X 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 thatXs 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 XblockX 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 arenXt 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 youXve 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 X80s, probably developed at UC
Berkeley or thereabouts. Similar in many ways to the prescription-only
medication called XSystem VX, but infinitely more useful. (Or, at least,
more fun.) The full chemical name is XBerkeley Standard
DistributionX.
- bucket
- A location in a hash table containing (potentially) multiple
entries whose keys XhashX to the same hash value according to its hash
function. (As internal policy, you donXt have to worry about it unless
youXre 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 itXs passed on
whenever a complete line is received. Command buffering means that
itXs 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 Xdroids when they donXt wish to reveal
their orientation (see endian). Named after some similar languages
spoken (for similar reasons) between compilers and interpreters in the
late 20XX century. These languages are characterized by representing
everything as a nonarchitecture-dependent sequence of bytes.
- 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 itXs 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 variablesXsuch as $1 and
$2, and "%+" and
%X Xthat hold the text remembered in a pattern
match. See Camel chapter 5, XPattern MatchingX.
- 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, XPattern MatchingX.
- 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 characterXs
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. TheyXre called XclosuresX 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 0x1F_FFFF; and Perl codepoints are in the range 0 to 2XXX1 or 0 to
2XXX1, 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 codeXfor 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
Xcollating sequenceX.
- 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. ItXs enabled by setting the
$| ($AUTOFLUSH) variable
to a true value. ItXs used when you donXt want data sitting around, not
going where itXs 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 doesnXt 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 Xmore executableX 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 XrealX compiler. See Camel
chapter 16, XCompilingX.
- 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 XconstructorX for a referent that isnXt 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 XCreating
ReferencesX in Camel chapter 8, XReferencesX.
- concatenation
- The process of gluing one catXs nose to another catXs tail. Also a similar
operation on two strings.
- conditional
- Something XiffyX. See Boolean context.
- connection
- In telephony, the temporary electrical circuit between the callerXs and
the calleeXs 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. ThereXs also a XdonXt careX context (which is
dealt with in Camel chapter 2, XBits and PiecesX, 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, XCPANX for details.)
- C preprocessor
- The typical C compilerXs 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. ThatXs your current package name.
- current working directory
- See working directory.
- CV
- In academia, a curriculum vitae, a fancy kind of resume. In Perl, an
internal Xcode valueX typedef holding a subroutine. The
"CV" type is a subclass of
SV.
- dangling statement
- A bare, single statement, without any braces, hanging off an
"if" or
"while" conditional. C allows them. Perl
doesnXt.
- 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 XDatabase ManagementX 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 itXs like, without giving any commitment as to how or where youXll
use it. A declaration is like the part of your recipe that says, Xtwo cups
flour, one large egg, four or five tadpolesXX 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 youXd like. Perl
doesnXt 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 Xdecrement
$xX (meaning to remove 1 from its value) or
Xdecrement $x by 3X.
- default
- A value chosen for you if you donXt 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
isnXt 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,
XFunctionsX.
- delimiter
- A character or string that sets bounds to an arbitrarily
sized textual object, not to be confused with a separator or
terminator. XTo delimitX really just means Xto surroundX or Xto
encloseX (like these parentheses are doing).
- dereference
- A fancy computer science term meaning Xto follow a reference to
what it points toX. The XdeX part of it refers to the fact that youXre
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 arenXt
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 programXs
"DESTROY" method doesnXt 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, XPlain Old
DocumentationX.
- directory
- A special file that contains other files. Some operating systems
call these XfoldersX, XdrawersX, XcataloguesX, or XcatalogsX.
- 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, itXs 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 Xbinary-onlyX 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 PerlXs magical
dwimmer effects donXt 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 XDo What I MeanX, 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 XdwimmingX is a XdwimmerX. Dwimming can require a
great deal of behind-the-scenes magic, which (if it doesnXt 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 XcontainsX that subroutine at
runtime.
- eclectic
- Derived from many sources. Some would say too many.
- element
- A basic building block. When youXre talking about an array, itXs
one of the items that make up the array.
- embedding
- When something is contained in something else, particularly when that
might be considered surprising: XIXve embedded a complete Perl interpreter
in my editor!X
- 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 objectXs 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 Xin
passingX, 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 processXs 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 itXs okay to run this file as a program. Usually shortened to
XexecutableX.
- execute
- To run a program or subroutine. (Has nothing to do with the
"kill" built-in, unless youXre 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 didnXt 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.
- 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 Xcreeping featurismX, 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 XfieldsX.
- 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 youXre 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 youXre talking about. Perl hides the file
descriptor inside a standard I/O stream and then attaches the
stream to a filehandle.
- fileglob
- A XwildcardX 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
youXre going to open and close several different files in succession, itXs
fine to open each of them with the same filehandle, so you donXt 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 XpartitionX. You can change the fileXs 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 "Xo
$filename" to test whether youXre 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
Xfirst comeX 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 PerlXs
"Xn" and
"Xp" 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 Xscientific notationX, such that the
precision of the number is independent of its magnitude (the decimal point
XfloatsX). Perl does its numeric work with floating-point numbers
(sometimes called XfloatsX) when it canXt get away with using
integers. Floating-point numbers are mere approximations of real
numbers.
- flush
- The act of emptying a buffer, often before itXs 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
UnicodeXs 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 youXre printing comes out nice and pretty.
- freely available
- Means you donXt have to pay money to get it, but the copyright on it may
still belong to someone else (like Larry).
- freely redistributable
- Means youXre not in legal trouble if you give a bootleg copy of it to your
friends and we find out about it. In fact, weXd 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 FoundationXs 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.
- garbage collection
- A misnamed featureXit should be called, Xexpecting your mother to pick up
after youX. Strictly speaking, Perl doesnXt 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 itXs any comfort, when your interpreter exits,
a XrealX garbage collector runs to make sure everything is cleaned up if
youXve been messy with circular references and such.)
- GID
- Group IDXin Unix, the numeric group ID that the operating system
uses to identify you and members of your group.
- glob
- Strictly, the shellXs "*" character,
which will match a XglobX of characters when youXre 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 globalXmost variables (and all
subroutines) exist only in the current package. Global variables
can be declared with "our". See XGlobal
DeclarationsX in Camel chapter 4, XStatements and DeclarationsX.
- 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
werenXt intended to be hooked together.
- granularity
- The size of the pieces youXre 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 XXX 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 XGlobally search for a
Regular Expression and Print itX, 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 Xglob valueX typedef, holding a typeglob. The
"GV" type is a subclass of
SV.
- 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 referentXs reference count accounts
for it. (Some hard references are held internally, such as the implicit
reference from one of a typeglobXs 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 Xassociative arrayX, which is a pretty good
reason for simply calling it a XhashX 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
XaheadX of the rest of your program to do certain obscure operations. A C
header file has a .h extension. Perl doesnXt really have header
files, though historically Perl has sometimes used translated .h
files with a .ph extension. See
"require" in Camel chapter 27,
XFunctionsX. (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, itXs just a fancy form of quoting.
- hexadecimal
- A number in base 16, XhexX 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,
XFunctionsX.
- 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
wonXt want to say bad things about. Hence, the third great virtue of a
programmer. See also laziness and impatience.
- HV
- Short for a Xhash valueX typedef, which holds PerlXs internal
representation of a hash. The "HV" type
is a subclass of SV.
- 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 donXt 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,
XFunctionsX.
- 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 PerlXs
"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 isnXt the value youXre looking for but indicates
where the value is, thatXs 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 "$foo\n";" can
be understood as Xverb indirect-object objectX, where
"STDOUT" is the recipient of the
"print" action, and
"$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 "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 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.  See single
inheritance and multiple inheritance.
- instance
- Short for Xan instance of a classX, 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 itXs supposed to do. We usually
XcallX subroutines but XinvokeX 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: XA
camel is a mammal.X 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 donXt always have Platonic relationshipsXsee
inheritance.
- iteration
- Doing something repeatedly.
- iterator
- A special programming gizmo that keeps track of where you are in something
that youXre 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, TomXs favorite editor. IV
also means an internal Integer Value of the type a scalar can hold,
not to be confused with an NV.
- JAPH
- XJust Another Perl HackerX, 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.
- key
- The string index to a hash, used to look up the value
associated with that key.
- keyword
- See reserved words.
- 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 donXt 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, XPattern MatchingX
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
donXt 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. XSyn. static scoping. XAnt. dynamic
scoping.
- lexical variable
- A variable subject to lexical scoping, declared by
"my". Often just called a XlexicalX.
(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 fileXs 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. ItXs like having
the same phone number listed in the phone directory under different names.
As a verb, to resolve a partially compiled fileXs 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 XandX, XorX, XxorX, and XnotX.
- 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 shouldnXt 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 XlX is short for XleftX, 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".
- 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 donXt
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 XpageX 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 itXs 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 itXll use
disk space to hold inactive bits. This can make it seem like you have a
little more memory than you really do, but itXs 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.
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 weXd call a metacharacter except that itXs 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, XObjectsX.
- 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,
XCPANX.
- minimalism
- The belief that Xsmall is beautifulX. 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 moduleXs main .pm file may also load in other files in
support of the module.) See the "use"
built-in.
- modulus
- An integer divisor when youXre interested in the remainder instead of the
quotient.
- mojibake
- When you speak one language and the computer thinks youXre speaking
another. YouXll see odd translations when you send UTFX8, for instance,
but the computer thinks you sent Latin-1, showing all sorts of weird
characters instead. The term is written XXXXXXin Japanese and means
Xcharacter rotX, an apt description. Pronounced
["modXibake"] in standard IPA
phonetics, or approximately Xmoh-jee-bah-kehX.
- 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 referencesXsee Camel chapter 9, XData
StructuresX.
- 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.
- 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 neednXt 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 telephoneXs 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. ItXs 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"Xs 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.
- object
- An instance of a class. Something that XknowsX 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 donXt 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 vendorXs operating system is different from any other version
of this or any other vendorXs 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 pragmaXsee Camel chapter 13,
XOverloadingX.
- options
- See either switches or regular expression modifiers.
- ordinal
- An abstract characterXs 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 XOverriding Built-in FunctionsX in Camel chapter 11,
XModulesX), and to describe how you can define a replacement method
in a derived class to hide a base classXs method of the same
name (see Camel chapter 12, XObjectsX).
- 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.
- 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 packageXs symbol table are only
accessible from code compiled outside the package by naming the
package. But in another sense, all package symbols are also
globalsXtheyXre 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 thereXs 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
XPlain old documentationX. See Camel chapter 23, XPlain Old
DocumentationX.
- 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 donXt
have to worry about them. Instead, you just use symbolic pointers in the
form of keys and variable names, or hard references,
which arenXt 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 XXXX- + XXXXX, 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
XeasilyX 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 XcarriesX 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 theyXve 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 Xpush- popX code; that is, C code implementing
PerlXs 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, XCPANX.
- 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 timeXin fact, the
"fork" function is designed to bring
about this happy state of affairs. Under other operating systems,
processes are sometimes called XthreadsX, XtasksX, or XjobsX, 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 donXt parse, as the case may be.)
- pseudofunction
- A construct that sometimes looks like a function but really isnXt. 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. Pseduohashes 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 domainXitXs just freely available and freely
redistributable.
- pumpkin
- A notional XbatonX 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 Xpointer valueX, which is Perl Internals Talk for a
"char*".
- 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.
- 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 thatXs written
well enough that someone has a chance of figuring out what itXs trying to
do.
- reaping
- The last rites performed by a parent process on behalf of a
deceased child process so that it doesnXt 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-lineXterminated set of lines (a
XparagraphX). 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 youXre 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, itXs a grammar for a little language in which some
strings are legal and others arenXt. To normal people, itXs a pattern you
can use to find what youXre looking for when it varies from case to case.
PerlXs regular expressions are far from regular in the theoretical sense,
but in regular use they work quite well. HereXs a regular expression:
"/Oh s.*t./". This will match strings
like X"Oh say can you see by the dawn's early
light"X and X"Oh sit!"X. See
Camel chapter 5, XPattern MatchingX.
- regular expression modifier
- An option on a pattern or substitution, such as
"/i" to render the pattern case-
insensitive.
- regular file
- A file thatXs not a directory, a device, a named
pipe or socket, or a symbolic link. Perl uses the
"Xf" file test operator to identify
regular files. Sometimes called a XplainX 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),
itXs illegal to use reserved words to name anything else. (Which is why
theyXre reserved, after all.) In Perl, you just canXt use them to name
labels or filehandles. Also called XkeywordsX.
- 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 youXre not confused yet.
- rvalue
- A value that you might find on the right side of an
assignment. See also lvalue.
- sandbox
- A walled off area thatXs not supposed to affect beyond its walls. You let
kids play in the sandbox instead of running in the road. See Camel chapter
20, XSecurityX.
- 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 valueXsee 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 stringXan 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 XorX
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 otherXs memory.
- shebang
- Irish for the whole McGillicuddy. In Perl culture, a portmanteau of
XsharpX and XbangX, 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 itXs 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 Xside effectX 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 youXre 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, XSpecial NamesX and
the section XSignalsX in Camel chapter 15, XInterprocess
CommunicationX.
- single inheritance
- The features you got from your mother, if she told you that you donXt 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 addressesXsome look
like filenames, and some donXt.
- 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 donXt 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
shouldnXt 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 XstandardX of standard I/O is at most
marginally related to the XstandardX 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
shouldnXt 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: XAdd marmalade to batter and mix until mixed.X A statement is
distinguished from a declaration, which doesnXt 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 weXre not so sure about them.) In computers,
where things are supposed to vary rapidly, XstaticX has a derogatory
connotation, indicating a slightly dysfunctional variable,
subroutine, or method. In Perl culture, the word is politely
avoided.
If youXre a C or C++ programmer, you might be looking for
PerlXs "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,
XFunctionsX.
- 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 interfaceXthe 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 XHe said !@#*&%@#*?!X. 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 Xscalar valueX. 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 Xreferent typeX, 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.,
"Xa Xb Xc") into one switch
(e.g., "Xabc"). 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 Xcase structureX,
named after the similar Pascal construct. Most switch statements in Perl
are spelled "given". See XThe
"given" statementX in Camel chapter 4,
XStatements and DeclarationsX.
- 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 XsymbolicX 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 youXve 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 XXXXXXXX, Xwith-arrangementX. 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 arenXt direct
system calls, but are built up in one or more layers above the system call
level. In general, Perl programmers donXt 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 Xsystem callX to mean what happens when you call the Perl
"system" function, which actually
involves many syscalls. To avoid any confusion, we nearly always say
XsyscallX for something you could call indirectly via PerlXs
"syscall" function, and never for
something you would call with PerlXs
"system" function.
- 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
"XT" switch.
- taint mode
- Running under the "XT" switch, marking
all external data as suspect and refusing to use it with system commands.
See Camel chapter 20, XSecurityX.
- 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 XterminalXXthat 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 forkXs 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 processXs 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,
XFunctionsX and Camel chapter 14, XTied VariablesX.
- 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 doesnXt use Unicode titlecase, but casing rules for English
titles are more complicated than simply capitalizing each wordXs first
character.
- TMTOWTDI
- ThereXs 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 doesnXt mean that more ways are always better or that all
possible paths are equally desirableXjust 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 XlexingX, in
which case you get XlexemesX 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 youXre
assembling a tricycle, but if youXre 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 youXre 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 XXXXXXXXXX
transliterates into polychromos but translates into
many-colored. See the "tr///"
operator in Camel chapter 5, XPattern MatchingX.
- 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 doesnXt 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 XTypeglobs and
FilehandlesX in Camel chapter 2, XBits and PiecesX.
- typemap
- A description of how C types may be transformed to and from Perl types
within an extension module written in XS.
- 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
youXll 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
"XX" 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 itXs easy to
learn because itXs so easily twisted to oneXs 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 wasnXt 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.
- 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
XvirtualX is XtransparentX, which means providing the reality of something
without the appearance, as in: Perl handles the variable-length UTFX8
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 XversionX or XvectorX 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.)
- warning
- A message printed to the "STDERR" stream
to the effect that something might be wrong but isnXt worth blowing up
over. See "warn" in Camel chapter 27,
XFunctionsX and the "warnings" pragma in
Camel chapter 28, XPragmantic ModulesX.
- watch expression
- An expression which, when its value changes, causes a breakpoint in the
Perl debugger.
- weak reference
- A reference that doesnXt 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 doesnXt 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 X-XX .
- word
- In normal XcomputereseX, 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 PerlXs
"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".
- 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.
- 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.
- 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.
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.