NAME¶
perlglossary - Perl Glossary
DESCRIPTION¶
A glossary of terms (technical and otherwise) used in the Perl documentation.
Other useful sources include the Free On-Line Dictionary of Computing
<
http://foldoc.doc.ic.ac.uk/foldoc/index.html>, 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 "object"'s state (its instance variables).
- actual arguments
- The scalar values that you supply to a "function"
or "subroutine" when you call it. For instance, when you call
"power("puff")", the string "puff" is the
actual argument. See also "argument" and "formal
arguments".
- address operator
- Some languages work directly with the memory addresses of
values, but this can be like playing with fire. Perl provides a set of
asbestos gloves for handling all memory management. The closest to an
address operator in Perl is the backslash operator, but it gives you a
"hard reference", which is much safer than a memory
address.
- algorithm
- A well-defined sequence of steps, clearly enough explained
that even a computer could do them.
- alias
- A nickname for something, which behaves in all ways as
though you'd used the original name instead of the nickname. Temporary
aliases are implicitly created in the loop variable for
"foreach" loops, in the $_ variable for map or grep operators,
in $a and $b during sort's comparison function, and in each element of @_
for the "actual arguments" of a subroutine call. Permanent
aliases are explicitly created in packages by importing symbols or by
assignment to typeglobs. Lexically scoped aliases for package variables
are explicitly created by the our declaration.
- alternatives
- A list of possible choices from which you may select only
one, as in "Would you like door A, B, or C?" Alternatives in
regular expressions are separated with a single vertical bar:
"|". Alternatives in normal Perl expressions are separated with
a double vertical bar: "||". Logical alternatives in
"Boolean" expressions are separated with either "||"
or "or".
- anonymous
- Used to describe a "referent" that is not
directly accessible through a named "variable". Such a referent
must be indirectly accessible through at least one "hard
reference". When the last hard reference goes away, the anonymous
referent is destroyed without pity.
- architecture
- The kind of computer you're working on, where one
"kind" of computer means all those computers sharing a
compatible machine language. Since Perl programs are (typically) simple
text files, not executable images, a Perl program is much less sensitive
to the architecture it's running on than programs in other languages, such
as C, that are compiled into machine code. See also "platform"
and "operating system".
- argument
- A piece of data supplied to a program,
"subroutine", "function", or "method" to
tell it what it's supposed to do. Also called a
"parameter".
- ARGV
- The name of the array containing the "argument"
"vector" from the command line. If you use the empty
"<>" operator, "ARGV" is the name of both the
"filehandle" used to traverse the arguments and the
"scalar" containing the name of the current input file.
- arithmetical operator
- A "symbol" such as "+" or "/"
that tells Perl to do the arithmetic you were supposed to learn in grade
school.
- array
- An ordered sequence of values, stored such that you can
easily access any of the values using an integer "subscript"
that specifies the value's "offset" in the sequence.
- array context
- An archaic expression for what is more correctly referred
to as "list context".
- ASCII
- The American Standard Code for Information Interchange (a
7-bit character set adequate only for poorly representing English text).
Often used loosely to describe the lowest 128 values of the various
ISO-8859-X character sets, a bunch of mutually incompatible 8-bit codes
sometimes described as half ASCII. See also "Unicode".
- assertion
- A component of a "regular expression" that must
be true for the pattern to match but does not necessarily match any
characters itself. Often used specifically to mean a "zero
width" assertion.
- assignment
- An "operator" whose assigned mission in life is
to change the value of a "variable".
- assignment operator
- Either a regular "assignment", or a compound
"operator" composed of an ordinary assignment and some other
operator, that changes the value of a variable in place, that is, relative
to its old value. For example, "$a += 2" adds 2 to $a.
- associative array
- See "hash". Please.
- associativity
- Determines whether you do the left "operator"
first or the right "operator" first when you have "A
"operator" B "operator" C" and the two operators
are of the same precedence. Operators like "+" are left
associative, while operators like "**" are right associative.
See perlop for a list of operators and their associativity.
- asynchronous
- Said of events or activities whose relative temporal
ordering is indeterminate because too many things are going on at once.
Hence, an asynchronous event is one you didn't know when to expect.
- atom
- A "regular expression" component potentially
matching a "substring" containing one or more characters and
treated as an indivisible syntactic unit by any following
"quantifier". (Contrast with an "assertion" that
matches something of "zero width" and may not be
quantified.)
- atomic operation
- When Democritus gave the word "atom" to the
indivisible bits of matter, he meant literally something that could not be
cut: a- (not) + tomos (cuttable). An atomic operation is an
action that can't be interrupted, not one forbidden in a nuclear-free
zone.
- attribute
- A new feature that allows the declaration of variables and
subroutines with modifiers as in "sub foo : locked method".
Also, another name for an "instance variable" of an
"object".
- autogeneration
- A feature of "operator overloading" of objects,
whereby the behavior of certain operators can be reasonably deduced using
more fundamental operators. This assumes that the overloaded operators
will often have the same relationships as the regular operators. See
perlop.
- autoincrement
- To add one to something automatically, hence the name of
the "++" operator. To instead subtract one from something
automatically is known as an "autodecrement".
- autoload
- To load on demand. (Also called "lazy" loading.)
Specifically, to call an AUTOLOAD subroutine on behalf of an undefined
subroutine.
- autosplit
- To split a string automatically, as the -a
"switch" does when running under -p or -n in order
to emulate "awk". (See also the AutoSplit module, which has
nothing to do with the -a switch, but a lot to do with
autoloading.)
- autovivification
- A Greco-Roman word meaning "to bring oneself to
life". In Perl, storage locations (lvalues) spontaneously generate
themselves as needed, including the creation of any "hard
reference" values to point to the next level of storage. The
assignment "$a[5][5][5][5][5] = "quintet"" potentially
creates five scalar storage locations, plus four references (in the first
four scalar locations) pointing to four new anonymous arrays (to hold the
last four scalar locations). But the point of autovivification is that you
don't have to worry about it.
- AV
- Short for "array value", which refers to one of
Perl's internal data types that holds an "array". The
"AV" type is a subclass of "SV".
- awk
- Descriptive editing term--short for "awkward".
Also coincidentally refers to a venerable text-processing language from
which Perl derived some of its high-level ideas.
- backreference
- A substring captured by a subpattern within unadorned
parentheses in a "regex", also referred to as a capture group.
The sequences ("\g1", "\g2", etc.) later in the same
pattern refer back to the corresponding subpattern in the current match.
Outside the pattern, the numbered variables ($1, $2, etc.) continue to
refer to these same values, as long as the pattern was the last successful
match of the current dynamic scope. "\g{-1}" can be used to
refer to a group by relative rather than absolute position; and groups can
be also be named, and referred to later by name rather than number. See
"Capture groups" in perlre.
- backtracking
- The practice of saying, "If I had to do it all over,
I'd do it differently," and then actually going back and doing it all
over differently. Mathematically speaking, it's returning from an
unsuccessful recursion on a tree of possibilities. Perl backtracks when it
attempts to match patterns with a "regular expression", and its
earlier attempts don't pan out. See "Backtracking" in
perlre.
- backward compatibility
- Means you can still run your old program because we didn't
break any of the features or bugs it was relying on.
- bareword
- A word sufficiently ambiguous to be deemed illegal under
use strict 'subs'. In the absence of that stricture, a bareword is treated
as if quotes were around it.
- base class
- A generic "object" type; that is, a
"class" from which other, more specific classes are derived
genetically by "inheritance". Also called a
"superclass" by people who respect their ancestors.
- big-endian
- From Swift: someone who eats eggs big end first. Also used
of computers that store the most significant "byte" of a word at
a lower byte address than the least significant byte. Often considered
superior to little-endian machines. See also
"little-endian".
- binary
- Having to do with numbers represented in base 2. That means
there's basically two numbers, 0 and 1. Also used to describe a
"non-text file", presumably because such a file makes full use
of all the binary bits in its bytes. With the advent of
"Unicode", this distinction, already suspect, loses even more of
its meaning.
- binary operator
- An "operator" that takes two operands.
- bind
- To assign a specific "network address" to a
"socket".
- bit
- An integer in the range from 0 to 1, inclusive. The
smallest possible unit of information storage. An eighth of a
"byte" or of a dollar. (The term "Pieces of Eight"
comes from being able to split the old Spanish dollar into 8 bits, each of
which still counted for money. That's why a 25-cent piece today is still
"two bits".)
- bit shift
- The movement of bits left or right in a computer word,
which has the effect of multiplying or dividing by a power of 2.
- bit string
- A sequence of bits that is actually being thought of as a
sequence of bits, for once.
- bless
- In corporate life, to grant official approval to a thing,
as in, "The VP of Engineering has blessed our WebCruncher
project." Similarly in Perl, to grant official approval to a
"referent" so that it can function as an "object",
such as a WebCruncher object. See "bless" in perlfunc.
- block
- What a "process" does when it has to wait for
something: "My process blocked waiting for the disk." As an
unrelated noun, it refers to a large chunk of data, of a size that the
"operating system" likes to deal with (normally a power of two
such as 512 or 8192). Typically refers to a chunk of data that's coming
from or going to a disk file.
- BLOCK
- A syntactic construct consisting of a sequence of Perl
statements that is delimited by braces. The "if" and
"while" statements are defined in terms of BLOCKs, for instance.
Sometimes we also say "block" to mean a lexical scope; that is,
a sequence of statements that act like a "BLOCK", such as within
an eval or a file, even though the statements aren't delimited by
braces.
- block buffering
- A method of making input and output efficient by passing
one "block" at a time. By default, Perl does block buffering to
disk files. See "buffer" and "command buffering".
- Boolean
- A value that is either "true" or
"false".
- Boolean context
- A special kind of "scalar context" used in
conditionals to decide whether the "scalar value" returned by an
expression is "true" or "false". Does not evaluate as
either a string or a number. See "context".
- breakpoint
- A spot in your program where you've told the debugger to
stop execution so you can poke around and see whether anything is wrong
yet.
- broadcast
- To send a "datagram" to multiple destinations
simultaneously.
- BSD
- A psychoactive drug, popular in the 80s, probably developed
at U. C. Berkeley or thereabouts. Similar in many ways to the
prescription-only medication called "System V", but infinitely
more useful. (Or, at least, more fun.) The full chemical name is
"Berkeley Standard Distribution".
- bucket
- A location in a "hash table" containing
(potentially) multiple entries whose keys "hash" to the same
hash value according to its hash function. (As internal policy, you don't
have to worry about it, unless you're into internals, or policy.)
- buffer
- A temporary holding location for data. Block buffering
means that the data is passed on to its destination whenever the buffer is
full. Line buffering means that it's passed on whenever a complete line is
received. Command buffering means that it's passed every time you do a
print command (or equivalent). If your output is unbuffered, the system
processes it one byte at a time without the use of a holding area. This
can be rather inefficient.
- built-in
- A "function" that is predefined in the language.
Even when hidden by "overriding", you can always get at a
built-in function by qualifying its name with the "CORE::"
pseudo-package.
- bundle
- A group of related modules on "CPAN". (Also,
sometimes refers to a group of command-line switches grouped into one
"switch cluster".)
- byte
- A piece of data worth eight bits in most places.
- bytecode
- A pidgin-like language spoken among 'droids when they don't
wish to reveal their orientation (see "endian"). Named after
some similar languages spoken (for similar reasons) between compilers and
interpreters in the late 20th century. These languages are characterized
by representing everything as a non-architecture-dependent sequence of
bytes.
- C
- A language beloved by many for its inside-out
"type" definitions, inscrutable "precedence" rules,
and heavy "overloading" of the function-call mechanism. (Well,
actually, people first switched to C because they found lowercase
identifiers easier to read than upper.) Perl is written in C, so it's not
surprising that Perl borrowed a few ideas from it.
- C preprocessor
- The typical C compiler's first pass, which processes lines
beginning with "#" for conditional compilation and macro
definition and does various manipulations of the program text based on the
current definitions. Also known as cpp(1).
- call by reference
- An "argument"-passing mechanism in which the
"formal arguments" refer directly to the "actual
arguments", and the "subroutine" can change the actual
arguments by changing the formal arguments. That is, the formal argument
is an "alias" for the actual argument. See also "call by
value".
- call by value
- An "argument"-passing mechanism in which the
"formal arguments" refer to a copy of the "actual
arguments", and the "subroutine" cannot change the actual
arguments by changing the formal arguments. See also "call by
reference".
- callback
- A "handler" that you register with some other
part of your program in the hope that the other part of your program will
"trigger" your handler when some event of interest
transpires.
- canonical
- Reduced to a standard form to facilitate comparison.
- capture buffer, capture group
- These two terms are synonymous: a captured substring by a
regex subpattern.
- capturing
- The use of parentheses around a "subpattern" in a
"regular expression" to store the matched "substring"
as a "backreference" or capture group. (Captured strings are
also returned as a list in "list context".)
- character
- A small integer representative of a unit of orthography.
Historically, characters were usually stored as fixed-width integers
(typically in a byte, or maybe two, depending on the character set), but
with the advent of UTF-8, characters are often stored in a variable number
of bytes depending on the size of the integer that represents the
character. Perl manages this transparently for you, for the most
part.
- character class
- A square-bracketed list of characters used in a
"regular expression" to indicate that any character of the set
may occur at a given point. Loosely, any predefined set of characters so
used.
- character property
- A predefined "character class" matchable by the
"\p" "metasymbol". Many standard properties are
defined for "Unicode".
- circumfix operator
- An "operator" that surrounds its
"operand", like the angle operator, or parentheses, or a
hug.
- class
- A user-defined "type", implemented in Perl via a
"package" that provides (either directly or by inheritance)
methods (that is, subroutines) to handle instances of the class (its
objects). See also "inheritance".
- class method
- A "method" whose "invocant" is a
"package" name, not an "object" reference. A method
associated with the class as a whole.
- client
- In networking, a "process" that initiates contact
with a "server" process in order to exchange data and perhaps
receive a service.
- cloister
- A "cluster" used to restrict the scope of a
"regular expression modifier".
- closure
- An "anonymous" subroutine that, when a reference
to it is generated at run time, keeps track of the identities of
externally visible lexical variables even after those lexical variables
have supposedly gone out of "scope". They're called
"closures" because this sort of behavior gives mathematicians a
sense of closure.
- cluster
- A parenthesized "subpattern" used to group parts
of a "regular expression" into a single "atom".
- CODE
- The word returned by the ref function when you apply it to
a reference to a subroutine. See also "CV".
- code generator
- A system that writes code for you in a low-level language,
such as code to implement the backend of a compiler. See "program
generator".
- code point
- The position of a character in a character set encoding.
The character "NULL" is almost certainly at the zeroth position
in all character sets, so its code point is 0. The code point for the
"SPACE" character in the ASCII character set is 0x20, or 32
decimal; in EBCDIC it is 0x40, or 64 decimal. The ord function returns the
code point of a character.
"code position" and "ordinal" mean the same thing as
"code point".
- code subpattern
- A "regular expression" subpattern whose real
purpose is to execute some Perl code, for example, the
"(?{...})" and "(??{...})" subpatterns.
- collating sequence
- The order into which characters sort. This is used by
"string" comparison routines to decide, for example, where in
this glossary to put "collating sequence".
- command
- In "shell" programming, the syntactic combination
of a program name and its arguments. More loosely, anything you type to a
shell (a command interpreter) that starts it doing something. Even more
loosely, a Perl "statement", which might start with a
"label" and typically ends with a semicolon.
- command buffering
- A mechanism in Perl that lets you store up the output of
each Perl "command" and then flush it out as a single request to
the "operating system". It's enabled by setting the $|
($AUTOFLUSH) variable to a true value. It's used when you don't want data
sitting around not going where it's supposed to, which may happen because
the default on a "file" or "pipe" is to use
"block buffering".
- command name
- The name of the program currently executing, as typed on
the command line. In C, the "command" name is passed to the
program as the first command-line argument. In Perl, it comes in
separately as $0.
- command-line arguments
- The values you supply along with a program name when you
tell a "shell" to execute a "command". These values
are passed to a Perl program through @ARGV.
- comment
- A remark that doesn't affect the meaning of the program. In
Perl, a comment is introduced by a "#" character and continues
to the end of the line.
- compilation unit
- The "file" (or "string", in the case of
eval) that is currently being compiled.
- compile phase
- Any time before Perl starts running your main program. See
also "run phase". Compile phase is mostly spent in "compile
time", but may also be spent in "run time" when
"BEGIN" blocks, use declarations, or constant subexpressions are
being evaluated. The startup and import code of any use declaration is
also run during compile phase.
- compile time
- The time when Perl is trying to make sense of your code, as
opposed to when it thinks it knows what your code means and is merely
trying to do what it thinks your code says to do, which is "run
time".
- compiler
- Strictly speaking, a program that munches up another
program and spits out yet another file containing the program in a
"more executable" form, typically containing native machine
instructions. The perl program is not a compiler by this
definition, but it does contain a kind of compiler that takes a program
and turns it into a more executable form (syntax trees) within the
perl process itself, which the "interpreter" then
interprets. There are, however, extension modules to get Perl to act more
like a "real" compiler. See O.
- composer
- A "constructor" for a "referent" that
isn't really an "object", like an anonymous array or a hash (or
a sonata, for that matter). For example, a pair of braces acts as a
composer for a hash, and a pair of brackets acts as a composer for an
array. See "Making References" in perlref.
- concatenation
- The process of gluing one cat's nose to another cat's tail.
Also, a similar operation on two strings.
- conditional
- Something "iffy". See "Boolean
context".
- connection
- In telephony, the temporary electrical circuit between the
caller's and the callee's phone. In networking, the same kind of temporary
circuit between a "client" and a "server".
- construct
- As a noun, a piece of syntax made up of smaller pieces. As
a transitive verb, to create an "object" using a
"constructor".
- constructor
- Any "class method", instance "method",
or "subroutine" that composes, initializes, blesses, and returns
an "object". Sometimes we use the term loosely to mean a
"composer".
- context
- The surroundings, or environment. The context given by the
surrounding code determines what kind of data a particular
"expression" is expected to return. The three primary contexts
are "list context", "scalar context", and "void
context". Scalar context is sometimes subdivided into "Boolean
context", "numeric context", "string context",
and "void context". There's also a "don't care" scalar
context (which is dealt with in Programming Perl, Third Edition, Chapter
2, "Bits and Pieces" if you care).
- continuation
- The treatment of more than one physical "line" as
a single logical line. "Makefile" lines are continued by putting
a backslash before the "newline". Mail headers as defined by RFC
822 are continued by putting a space or tab after the newline. In
general, lines in Perl do not need any form of continuation mark, because
"whitespace" (including newlines) is gleefully ignored.
Usually.
- core dump
- The corpse of a "process", in the form of a file
left in the "working directory" of the process, usually as a
result of certain kinds of fatal error.
- CPAN
- The Comprehensive Perl Archive Network. (See "What
modules and extensions are available for Perl? What is CPAN? What does
CPAN/src/... mean?" in perlfaq2).
- cracker
- Someone who breaks security on computer systems. A cracker
may be a true "hacker" or only a "script kiddie".
- current package
- The "package" in which the current statement is
compiled. Scan backwards in the text of your program through the current
lexical scope or any enclosing lexical scopes till you find a package
declaration. That's your current package name.
- current working directory
- See "working directory".
- currently selected output channel
- The last "filehandle" that was designated with
select("FILEHANDLE"); "STDOUT", if no filehandle has
been selected.
- CV
- An internal "code value" typedef, holding a
"subroutine". The "CV" type is a subclass of
"SV".
- dangling statement
- A bare, single "statement", without any braces,
hanging off an "if" or "while" conditional. C allows
them. Perl doesn't.
- data structure
- How your various pieces of data relate to each other and
what shape they make when you put them all together, as in a rectangular
table or a triangular-shaped tree.
- data type
- A set of possible values, together with all the operations
that know how to deal with those values. For example, a numeric data type
has a certain set of numbers that you can work with and various
mathematical operations that you can do on the numbers but would make
little sense on, say, a string such as "Kilroy". Strings have
their own operations, such as "concatenation". Compound types
made of a number of smaller pieces generally have operations to compose
and decompose them, and perhaps to rearrange them. Objects that model
things in the real world often have operations that correspond to real
activities. For instance, if you model an elevator, your elevator object
might have an "open_door()" "method".
- datagram
- A packet of data, such as a "UDP" message, that
(from the viewpoint of the programs involved) can be sent independently
over the network. (In fact, all packets are sent independently at the
"IP" level, but "stream" protocols such as
"TCP" hide this from your program.)
- DBM
- Stands for "Data Base Management" routines, a set
of routines that emulate an "associative array" using disk
files. The routines use a dynamic hashing scheme to locate any entry with
only two disk accesses. DBM files allow a Perl program to keep a
persistent "hash" across multiple invocations. You can tie your
hash variables to various DBM implementations--see AnyDBM_File and
DB_File.
- declaration
- An "assertion" that states something exists and
perhaps describes what it's like, without giving any commitment as to how
or where you'll use it. A declaration is like the part of your recipe that
says, "two cups flour, one large egg, four or five tadpoles..."
See "statement" for its opposite. Note that some declarations
also function as statements. Subroutine declarations also act as
definitions if a body is supplied.
- decrement
- To subtract a value from a variable, as in "decrement
$x" (meaning to remove 1 from its value) or "decrement $x by
3".
- default
- A "value" chosen for you if you don't supply a
value of your own.
- defined
- Having a meaning. Perl thinks that some of the things
people try to do are devoid of meaning, in particular, making use of
variables that have never been given a "value" and performing
certain operations on data that isn't there. For example, if you try to
read data past the end of a file, Perl will hand you back an undefined
value. See also "false" and "defined" in
perlfunc.
- delimiter
- A "character" or "string" that sets
bounds to an arbitrarily-sized textual object, not to be confused with a
"separator" or "terminator". "To delimit"
really just means "to surround" or "to enclose" (like
these parentheses are doing).
- deprecated modules and features
- Deprecated modules and features are those which were part
of a stable release, but later found to be subtly flawed, and which should
be avoided. They are subject to removal and/or bug-incompatible
reimplementation in the next major release (but they will be preserved
through maintenance releases). Deprecation warnings are issued under
-w or "use diagnostics", and notices are found in
perldeltas, as well as various other PODs. Coding practices that misuse
features, such as "my $foo if 0", can also be deprecated.
- dereference
- A fancy computer science term meaning "to follow a
"reference" to what it points to". The "de" part
of it refers to the fact that you're taking away one level of
"indirection".
- derived class
- A "class" that defines some of its methods in
terms of a more generic class, called a "base class". Note that
classes aren't classified exclusively into base classes or derived
classes: a class can function as both a derived class and a base class
simultaneously, which is kind of classy.
- descriptor
- See "file descriptor".
- destroy
- To deallocate the memory of a "referent" (first
triggering its "DESTROY" method, if it has one).
- destructor
- A special "method" that is called when an
"object" is thinking about destroying itself. A Perl program's
"DESTROY" method doesn't do the actual destruction; Perl just
triggers the method in case the "class" wants to do any
associated cleanup.
- device
- A whiz-bang hardware gizmo (like a disk or tape drive or a
modem or a joystick or a mouse) attached to your computer, that the
"operating system" tries to make look like a "file"
(or a bunch of files). Under Unix, these fake files tend to live in the
/dev directory.
- directive
- A "pod" directive. See perlpod.
- directory
- A special file that contains other files. Some operating
systems call these "folders", "drawers", or
"catalogs".
- directory handle
- A name that represents a particular instance of opening a
directory to read it, until you close it. See the opendir function.
- dispatch
- To send something to its correct destination. Often used
metaphorically to indicate a transfer of programmatic control to a
destination selected algorithmically, often by lookup in a table of
function references or, in the case of object methods, by traversing the
inheritance tree looking for the most specific definition for the
method.
- distribution
- A standard, bundled release of a system of software. The
default usage implies source code is included. If that is not the case, it
will be called a "binary-only" distribution.
- (to be) dropped modules
- When Perl 5 was first released (see perlhist), several
modules were included, which have now fallen out of common use. It has
been suggested that these modules should be removed, since the
distribution became rather large, and the common criterion for new module
additions is now limited to modules that help to build, test, and extend
perl itself. Furthermore, the CPAN (which didn't exist at the time of Perl
5.0) can become the new home of dropped modules. Dropping modules is
currently not an option, but further developments may clear the last
barriers.
- dweomer
- An enchantment, illusion, phantasm, or jugglery. Said when
Perl's magical "dwimmer" effects don't do what you expect, but
rather seem to be the product of arcane dweomercraft, sorcery, or wonder
working. [From Old English]
- dwimmer
- DWIM is an acronym for "Do What I Mean", the
principle that something should just do what you want it to do without an
undue amount of fuss. A bit of code that does "dwimming" is a
"dwimmer". Dwimming can require a great deal of
behind-the-scenes magic, which (if it doesn't stay properly behind the
scenes) is called a "dweomer" instead.
- dynamic scoping
- Dynamic scoping works over a dynamic scope, making
variables visible throughout the rest of the "block" in which
they are first used and in any subroutines that are called by the rest of
the block. Dynamically scoped variables can have their values temporarily
changed (and implicitly restored later) by a local operator. (Compare
"lexical scoping".) Used more loosely to mean how a subroutine
that is in the middle of calling another subroutine "contains"
that subroutine at "run time".
- eclectic
- Derived from many sources. Some would say too
many.
- element
- A basic building block. When you're talking about an
"array", it's one of the items that make up the array.
- embedding
- When something is contained in something else, particularly
when that might be considered surprising: "I've embedded a complete
Perl interpreter in my editor!"
- empty subclass test
- The notion that an empty "derived class" should
behave exactly like its "base class".
- en passant
- When you change a "value" as it is being copied.
[From French, "in passing", as in the exotic pawn-capturing
maneuver in chess.]
- encapsulation
- The veil of abstraction separating the
"interface" from the "implementation" (whether
enforced or not), which mandates that all access to an
"object"'s state be through methods alone.
- endian
- See "little-endian" and
"big-endian".
- environment
- The collective set of environment variables your
"process" inherits from its parent. Accessed via %ENV.
- environment variable
- A mechanism by which some high-level agent such as a user
can pass its preferences down to its future offspring (child processes,
grandchild processes, great-grandchild processes, and so on). Each
environment variable is a "key"/"value" pair, like one
entry in a "hash".
- EOF
- End of File. Sometimes used metaphorically as the
terminating string of a "here document".
- errno
- The error number returned by a "syscall" when it
fails. Perl refers to the error by the name $! (or $OS_ERROR if you use
the English module).
- error
- See "exception" or "fatal error".
- escape sequence
- See "metasymbol".
- exception
- A fancy term for an error. See "fatal
error".
- exception handling
- The way a program responds to an error. The exception
handling mechanism in Perl is the eval operator.
- exec
- To throw away the current "process"'s program and
replace it with another without exiting the process or relinquishing any
resources held (apart from the old memory image).
- executable file
- A "file" that is specially marked to tell the
"operating system" that it's okay to run this file as a program.
Usually shortened to "executable".
- execute
- To run a program or "subroutine". (Has nothing to
do with the kill built-in, unless you're trying to run a "signal
handler".)
- execute bit
- The special mark that tells the operating system it can run
this program. There are actually three execute bits under Unix, and which
bit gets used depends on whether you own the file singularly,
collectively, or not at all.
- exit status
- See "status".
- export
- To make symbols from a "module" available for
"import" by other modules.
- expression
- Anything you can legally say in a spot where a
"value" is required. Typically composed of literals, variables,
operators, functions, and "subroutine" calls, not necessarily in
that order.
- extension
- A Perl module that also pulls in compiled C or C++ code.
More generally, any experimental option that can be compiled into Perl,
such as multithreading.
- 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.
- field
- A single piece of numeric or string data that is part of a
longer "string", "record", or "line".
Variable-width fields are usually split up by separators (so use split to
extract the fields), while fixed-width fields are usually at fixed
positions (so use unpack). Instance variables are also known as
fields.
- FIFO
- First In, First Out. See also "LIFO". Also, a
nickname for a "named pipe".
- file
- A named collection of data, usually stored on disk in a
"directory" in a "filesystem". Roughly like a
document, if you're into office metaphors. In modern filesystems, you can
actually give a file more than one name. Some files have special
properties, like directories and devices.
- file descriptor
- The little number the "operating system" uses to
keep track of which opened "file" you're talking about. Perl
hides the file descriptor inside a "standard I/O" stream and
then attaches the stream to a "filehandle".
- file test operator
- A built-in unary operator that you use to determine whether
something is "true" about a file, such as "-o
$filename" to test whether you're the owner of the file.
- fileglob
- A "wildcard" match on filenames. See the glob
function.
- filehandle
- An identifier (not necessarily related to the real name of
a file) that represents a particular instance of opening a file until you
close it. If you're going to open and close several different files in
succession, it's fine to open each of them with the same filehandle, so
you don't have to write out separate code to process each file.
- filename
- One name for a file. This name is listed in a
"directory", and you can use it in an open to tell the
"operating system" exactly which file you want to open, and
associate the file with a "filehandle" which will carry the
subsequent identity of that file in your program, until you close it.
- filesystem
- A set of directories and files residing on a partition of
the disk. Sometimes known as a "partition". You can change the
file's name or even move a file around from directory to directory within
a filesystem without actually moving the file itself, at least under
Unix.
- filter
- A program designed to take a "stream" of input
and transform it into a stream of output.
- flag
- We tend to avoid this term because it means so many things.
It may mean a command-line "switch" that takes no argument
itself (such as Perl's -n and -p flags) or, less frequently,
a single-bit indicator (such as the "O_CREAT" and
"O_EXCL" flags used in sysopen).
- floating point
- A method of storing numbers in "scientific
notation", such that the precision of the number is independent of
its magnitude (the decimal point "floats"). Perl does its
numeric work with floating-point numbers (sometimes called
"floats"), when it can't get away with using integers.
Floating-point numbers are mere approximations of real numbers.
- flush
- The act of emptying a "buffer", often before it's
full.
- FMTEYEWTK
- Far More Than Everything You Ever Wanted To Know. An
exhaustive treatise on one narrow topic, something of a
super-"FAQ". See Tom for far more.
- fork
- To create a child "process" identical to the
parent process at its moment of conception, at least until it gets ideas
of its own. A thread with protected memory.
- formal arguments
- The generic names by which a "subroutine" knows
its arguments. In many languages, formal arguments are always given
individual names, but in Perl, the formal arguments are just the elements
of an array. The formal arguments to a Perl program are $ARGV[0],
$ARGV[1], and so on. Similarly, the formal arguments to a Perl subroutine
are $_[0], $_[1], and so on. You may give the arguments individual names
by assigning the values to a my list. See also "actual
arguments".
- format
- A specification of how many spaces and digits and things to
put somewhere so that whatever you're printing comes out nice and
pretty.
- freely available
- Means you don't have to pay money to get it, but the
copyright on it may still belong to someone else (like Larry).
- freely redistributable
- Means you're not in legal trouble if you give a bootleg
copy of it to your friends and we find out about it. In fact, we'd rather
you gave a copy to all your friends.
- freeware
- Historically, any software that you give away, particularly
if you make the source code available as well. Now often called "open
source software". Recently there has been a trend to use the term in
contradistinction to "open source software", to refer only to
free software released under the Free Software Foundation's GPL (General
Public License), but this is difficult to justify etymologically.
- function
- Mathematically, a mapping of each of a set of input values
to a particular output value. In computers, refers to a
"subroutine" or "operator" that returns a
"value". It may or may not have input values (called
arguments).
- funny character
- Someone like Larry, or one of his peculiar friends. Also
refers to the strange prefixes that Perl requires as noun markers on its
variables.
- garbage collection
- A misnamed feature--it should be called, "expecting
your mother to pick up after you". Strictly speaking, Perl doesn't do
this, but it relies on a reference-counting mechanism to keep things tidy.
However, we rarely speak strictly and will often refer to the
reference-counting scheme as a form of garbage collection. (If it's any
comfort, when your interpreter exits, a "real" garbage collector
runs to make sure everything is cleaned up if you've been messy with
circular references and such.)
- GID
- Group ID--in Unix, the numeric group ID that the
"operating system" uses to identify you and members of your
"group".
- glob
- Strictly, the shell's "*" character, which will
match a "glob" of characters when you're trying to generate a
list of filenames. Loosely, the act of using globs and similar symbols to
do pattern matching. See also "fileglob" and
"typeglob".
- global
- Something you can see from anywhere, usually used of
variables and subroutines that are visible everywhere in your program. In
Perl, only certain special variables are truly global--most variables (and
all subroutines) exist only in the current "package". Global
variables can be declared with our. See "our" in perlfunc.
- global destruction
- The "garbage collection" of globals (and the
running of any associated object destructors) that takes place when a Perl
"interpreter" is being shut down. Global destruction should not
be confused with the Apocalypse, except perhaps when it should.
- glue language
- A language such as Perl that is good at hooking things
together that weren't intended to be hooked together.
- granularity
- The size of the pieces you're dealing with, mentally
speaking.
- greedy
- A "subpattern" whose "quantifier" wants
to match as many things as possible.
- grep
- Originally from the old Unix editor command for
"Globally search for a Regular Expression and Print it", now
used in the general sense of any kind of search, especially text searches.
Perl has a built-in grep function that searches a list for elements
matching any given criterion, whereas the grep(1) program searches
for lines matching a "regular expression" in one or more
files.
- group
- A set of users of which you are a member. In some operating
systems (like Unix), you can give certain file access permissions to other
members of your group.
- GV
- An internal "glob value" typedef, holding a
"typeglob". The "GV" type is a subclass of
"SV".
- hacker
- Someone who is brilliantly persistent in solving technical
problems, whether these involve golfing, fighting orcs, or programming.
Hacker is a neutral term, morally speaking. Good hackers are not to be
confused with evil crackers or clueless script kiddies. If you confuse
them, we will presume that you are either evil or clueless.
- handler
- A "subroutine" or "method" that is
called by Perl when your program needs to respond to some internal event,
such as a "signal", or an encounter with an operator subject to
"operator overloading". See also "callback".
- hard reference
- A "scalar" "value" containing the
actual address of a "referent", such that the referent's
"reference" count accounts for it. (Some hard references are
held internally, such as the implicit reference from one of a
"typeglob"'s variable slots to its corresponding referent.) A
hard reference is different from a "symbolic reference".
- hash
- An unordered association of
"key"/"value" pairs, stored such that you can easily
use a string "key" to look up its associated data
"value". This glossary is like a hash, where the word to be
defined is the key, and the definition is the value. A hash is also
sometimes septisyllabically called an "associative array", which
is a pretty good reason for simply calling it a "hash"
instead.
- hash table
- A data structure used internally by Perl for implementing
associative arrays (hashes) efficiently. See also "bucket".
- header file
- A file containing certain required definitions that you
must include "ahead" of the rest of your program to do certain
obscure operations. A C header file has a .h extension. Perl
doesn't really have header files, though historically Perl has sometimes
used translated .h files with a .ph extension. See
"require" in perlfunc. (Header files have been superseded by the
"module" mechanism.)
- here document
- So called because of a similar construct in shells that
pretends that the lines following the "command" are a separate
"file" to be fed to the command, up to some terminating string.
In Perl, however, it's just a fancy form of quoting.
- hexadecimal
- A number in base 16, "hex" for short. The digits
for 10 through 16 are customarily represented by the letters "a"
through "f". Hexadecimal constants in Perl start with
"0x". See also "hex" in perlfunc.
- home directory
- The directory you are put into when you log in. On a Unix
system, the name is often placed into $ENV{HOME} or $ENV{LOGDIR} by
login, but you can also find it with
"(getpwuid($<))[7]". (Some platforms do not have a concept of
a home directory.)
- host
- The computer on which a program or other data resides.
- hubris
- Excessive pride, the sort of thing Zeus zaps you for. Also
the quality that makes you write (and maintain) programs that other people
won't want to say bad things about. Hence, the third great virtue of a
programmer. See also "laziness" and "impatience".
- HV
- Short for a "hash value" typedef, which holds
Perl's internal representation of a hash. The "HV" type is a
subclass of "SV".
- identifier
- A legally formed name for most anything in which a computer
program might be interested. Many languages (including Perl) allow
identifiers that start with a letter and contain letters and digits. Perl
also counts the underscore character as a valid letter. (Perl also has
more complicated names, such as "qualified" names.)
- impatience
- The anger you feel when the computer is being lazy. This
makes you write programs that don't just react to your needs, but actually
anticipate them. Or at least that pretend to. Hence, the second great
virtue of a programmer. See also "laziness" and
"hubris".
- implementation
- How a piece of code actually goes about doing its job.
Users of the code should not count on implementation details staying the
same unless they are part of the published "interface".
- import
- To gain access to symbols that are exported from another
module. See "use" in perlfunc.
- increment
- To increase the value of something by 1 (or by some other
number, if so specified).
- indexing
- In olden days, the act of looking up a "key" in
an actual index (such as a phone book), but now merely the act of using
any kind of key or position to find the corresponding "value",
even if no index is involved. Things have degenerated to the point that
Perl's index function merely locates the position (index) of one string in
another.
- indirect filehandle
- An "expression" that evaluates to something that
can be used as a "filehandle": a "string" (filehandle
name), a "typeglob", a typeglob "reference", or a
low-level "IO" object.
- indirect object
- In English grammar, a short noun phrase between a verb and
its direct object indicating the beneficiary or recipient of the action.
In Perl, "print STDOUT "$foo\n";" can be understood as
"verb indirect-object object" where "STDOUT" is the
recipient of the print action, and "$foo" is the object being
printed. Similarly, when invoking a "method", you might place
the invocant between the method and its arguments:
$gollum = new Pathetic::Creature "Smeagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
In modern Perl, calling methods this way is often considered bad practice
and to be avoided.
- indirect object slot
- The syntactic position falling between a method call and
its arguments when using the indirect object invocation syntax. (The slot
is distinguished by the absence of a comma between it and the next
argument.) "STDERR" is in the indirect object slot here:
print STDERR "Awake! Awake! Fear, Fire,
Foes! Awake!\n";
- indirection
- If something in a program isn't the value you're looking
for but indicates where the value is, that's indirection. This can be done
with either symbolic references or hard references.
- infix
- An "operator" that comes in between its operands,
such as multiplication in "24 * 7".
- inheritance
- What you get from your ancestors, genetically or otherwise.
If you happen to be a "class", your ancestors are called base
classes and your descendants are called derived classes. See "single
inheritance" and "multiple inheritance".
- instance
- Short for "an instance of a class", meaning an
"object" of that "class".
- instance variable
- An "attribute" of an "object"; data
stored with the particular object rather than with the class as a
whole.
- integer
- A number with no fractional (decimal) part. A counting
number, like 1, 2, 3, and so on, but including 0 and the negatives.
- interface
- The services a piece of code promises to provide forever,
in contrast to its "implementation", which it should feel free
to change whenever it likes.
- interpolation
- The insertion of a scalar or list value somewhere in the
middle of another value, such that it appears to have been there all
along. In Perl, variable interpolation happens in double-quoted strings
and patterns, and list interpolation occurs when constructing the list of
values to pass to a list operator or other such construct that takes a
"LIST".
- interpreter
- Strictly speaking, a program that reads a second program
and does what the second program says directly without turning the program
into a different form first, which is what compilers do. Perl is not an
interpreter by this definition, because it contains a kind of compiler
that takes a program and turns it into a more executable form (syntax
trees) within the perl process itself, which the Perl "run
time" system then interprets.
- invocant
- The agent on whose behalf a "method" is invoked.
In a "class" method, the invocant is a package name. In an
"instance" method, the invocant is an object reference.
- invocation
- The act of calling up a deity, daemon, program, method,
subroutine, or function to get it do what you think it's supposed to do.
We usually "call" subroutines but "invoke" methods,
since it sounds cooler.
- I/O
- Input from, or output to, a "file" or
"device".
- IO
- An internal I/O object. Can also mean "indirect
object".
- IP
- Internet Protocol, or Intellectual Property.
- IPC
- Interprocess Communication.
- is-a
- A relationship between two objects in which one object is
considered to be a more specific version of the other, generic object:
"A camel is a mammal." Since the generic object really only
exists in a Platonic sense, we usually add a little abstraction to the
notion of objects and think of the relationship as being between a generic
"base class" and a specific "derived class". Oddly
enough, Platonic classes don't always have Platonic relationships--see
"inheritance".
- iteration
- Doing something repeatedly.
- iterator
- A special programming gizmo that keeps track of where you
are in something that you're trying to iterate over. The
"foreach" loop in Perl contains an iterator; so does a hash,
allowing you to each through it.
- IV
- The integer four, not to be confused with six, Tom's
favorite editor. IV also means an internal Integer Value of the type a
"scalar" can hold, not to be confused with an
"NV".
- JAPH
- "Just Another Perl Hacker," a clever but cryptic
bit of Perl code that when executed, evaluates to that string. Often used
to illustrate a particular Perl feature, and something of an ongoing
Obfuscated Perl Contest seen in Usenix signatures.
- 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 document what you wrote so you don't
have to answer so many questions about it. Hence, the first great virtue
of a programmer. Also hence, this book. See also "impatience"
and "hubris".
- left shift
- A "bit shift" that multiplies the number by some
power of 2.
- leftmost longest
- The preference of the "regular expression" engine
to match the leftmost occurrence of a "pattern", then given a
position at which a match will occur, the preference for the longest match
(presuming the use of a "greedy" quantifier). See perlre for
much more on this subject.
- lexeme
- Fancy term for a "token".
- lexer
- Fancy term for a "tokener".
- lexical analysis
- Fancy term for "tokenizing".
- lexical scoping
- Looking at your Oxford English Dictionary through a
microscope. (Also known as "static scoping", because
dictionaries don't change very fast.) Similarly, looking at variables
stored in a private dictionary (namespace) for each scope, which are
visible only from their point of declaration down to the end of the
lexical scope in which they are declared. --Syn. "static
scoping". --Ant. "dynamic scoping".
- lexical variable
- A "variable" subject to "lexical
scoping", declared by my. Often just called a "lexical".
(The our declaration declares a lexically scoped name for a global
variable, which is not itself a lexical variable.)
- library
- Generally, a collection of procedures. In ancient days,
referred to a collection of subroutines in a .pl file. In modern
times, refers more often to the entire collection of Perl modules on your
system.
- LIFO
- Last In, First Out. See also "FIFO". A LIFO is
usually called a "stack".
- line
- In Unix, a sequence of zero or more non-newline characters
terminated with a "newline" character. On non-Unix machines,
this is emulated by the C library even if the underlying "operating
system" has different ideas.
- line buffering
- Used by a "standard I/O" output stream that
flushes its "buffer" after every "newline". Many
standard I/O libraries automatically set up line buffering on output that
is going to the terminal.
- line number
- The number of lines read previous to this one, plus 1. Perl
keeps a separate line number for each source or input file it opens. The
current source file's line number is represented by "__LINE__".
The current input line number (for the file that was most recently read
via "<FH>") is represented by the $. ($INPUT_LINE_NUMBER)
variable. Many error messages report both values, if available.
- link
- Used as a noun, a name in a "directory",
representing a "file". A given file can have multiple links to
it. It's like having the same phone number listed in the phone directory
under different names. As a verb, to resolve a partially compiled file's
unresolved symbols into a (nearly) executable image. Linking can generally
be static or dynamic, which has nothing to do with static or dynamic
scoping.
- LIST
- A syntactic construct representing a comma-separated list
of expressions, evaluated to produce a "list value". Each
"expression" in a "LIST" is evaluated in "list
context" and interpolated into the list value.
- list
- An ordered set of scalar values.
- list context
- The situation in which an "expression" is
expected by its surroundings (the code calling it) to return a list of
values rather than a single value. Functions that want a "LIST"
of arguments tell those arguments that they should produce a list value.
See also "context".
- list operator
- An "operator" that does something with a list of
values, such as join or grep. Usually used for named built-in operators
(such as print, unlink, and system) that do not require parentheses around
their "argument" list.
- list value
- An unnamed list of temporary scalar values that may be
passed around within a program from any list-generating function to any
function or construct that provides a "list context".
- literal
- A token in a programming language such as a number or
"string" that gives you an actual "value" instead of
merely representing possible values as a "variable" does.
- little-endian
- From Swift: someone who eats eggs little end first. Also
used of computers that store the least significant "byte" of a
word at a lower byte address than the most significant byte. Often
considered superior to big-endian machines. See also
"big-endian".
- local
- Not meaning the same thing everywhere. A global variable in
Perl can be localized inside a dynamic scope via the local operator.
- logical operator
- Symbols representing the concepts "and",
"or", "xor", and "not".
- lookahead
- An "assertion" that peeks at the string to the
right of the current match location.
- lookbehind
- An "assertion" that peeks at the string to the
left of the current match location.
- loop
- A construct that performs something repeatedly, like a
roller coaster.
- loop control statement
- Any statement within the body of a loop that can make a
loop prematurely stop looping or skip an "iteration". Generally
you shouldn't try this on roller coasters.
- loop label
- A kind of key or name attached to a loop (or roller
coaster) so that loop control statements can talk about which loop they
want to control.
- lvaluable
- Able to serve as an "lvalue".
- lvalue
- Term used by language lawyers for a storage location you
can assign a new "value" to, such as a "variable" or
an element of an "array". The "l" is short for
"left", as in the left side of an assignment, a typical place
for lvalues. An "lvaluable" function or expression is one to
which a value may be assigned, as in "pos($x) = 10".
- lvalue modifier
- An adjectival pseudofunction that warps the meaning of an
"lvalue" in some declarative fashion. Currently there are three
lvalue modifiers: my, our, and local.
- magic
- Technically speaking, any extra semantics attached to a
variable such as $!, $0, %ENV, or %SIG, or to any tied variable. Magical
things happen when you diddle those variables.
- magical increment
- An "increment" operator that knows how to bump up
alphabetics as well as numbers.
- magical variables
- Special variables that have side effects when you access
them or assign to them. For example, in Perl, changing elements of the
%ENV array also changes the corresponding environment variables that
subprocesses will use. Reading the $! variable gives you the current
system error number or message.
- Makefile
- A file that controls the compilation of a program. Perl
programs don't usually need a "Makefile" because the Perl
compiler has plenty of self-control.
- man
- The Unix program that displays online documentation (manual
pages) for you.
- manpage
- A "page" from the manuals, typically accessed via
the man(1) command. A manpage contains a SYNOPSIS, a DESCRIPTION, a
list of BUGS, and so on, and is typically longer than a page. There are
manpages documenting commands, syscalls, "library" functions,
devices, protocols, files, and such. In this book, we call any piece of
standard Perl documentation (like perlop or perldelta) a
manpage, no matter what format it's installed in on your system.
- matching
- See "pattern matching".
- member data
- See "instance variable".
- memory
- This always means your main memory, not your disk. Clouding
the issue is the fact that your machine may implement "virtual"
memory; that is, it will pretend that it has more memory than it really
does, and it'll use disk space to hold inactive bits. This can make it
seem like you have a little more memory than you really do, but it's not a
substitute for real memory. The best thing that can be said about virtual
memory is that it lets your performance degrade gradually rather than
suddenly when you run out of real memory. But your program can die when
you run out of virtual memory too, if you haven't thrashed your disk to
death first.
- metacharacter
- A "character" that is not supposed to be
treated normally. Which characters are to be treated specially as
metacharacters varies greatly from context to context. Your
"shell" will have certain metacharacters, double-quoted Perl
strings have other metacharacters, and "regular expression"
patterns have all the double-quote metacharacters plus some extra ones of
their own.
- metasymbol
- Something we'd call a "metacharacter" except that
it's a sequence of more than one character. Generally, the first character
in the sequence must be a true metacharacter to get the other characters
in the metasymbol to misbehave along with it.
- method
- A kind of action that an "object" can take if you
tell it to. See perlobj.
- minimalism
- The belief that "small is beautiful."
Paradoxically, if you say something in a small language, it turns out big,
and if you say it in a big language, it turns out small. Go figure.
- mode
- In the context of the stat syscall, refers to the field
holding the "permission bits" and the type of the
"file".
- modifier
- See "statement modifier", "regular
expression modifier", and "lvalue modifier", not
necessarily in that order.
- module
- A "file" that defines a "package" of
(almost) the same name, which can either "export" symbols or
function as an "object" class. (A module's main .pm file
may also load in other files in support of the module.) See the use
built-in.
- modulus
- An integer divisor when you're interested in the remainder
instead of the quotient.
- monger
- Short for Perl Monger, a purveyor of Perl.
- mortal
- A temporary value scheduled to die when the current
statement finishes.
- multidimensional array
- An array with multiple subscripts for finding a single
element. Perl implements these using references--see perllol and
perldsc.
- multiple inheritance
- The features you got from your mother and father, mixed
together unpredictably. (See also "inheritance", and
"single inheritance".) In computer languages (including Perl),
the notion that a given class may have multiple direct ancestors or base
classes.
- named pipe
- A "pipe" with a name embedded in the
"filesystem" so that it can be accessed by two unrelated
processes.
- namespace
- A domain of names. You needn't worry about whether the
names in one such domain have been used in another. See
"package".
- network address
- The most important attribute of a socket, like your
telephone's telephone number. Typically an IP address. See also
"port".
- newline
- A single character that represents the end of a line, with
the ASCII value of 012 octal under Unix (but 015 on a Mac), and
represented by "\n" in Perl strings. For Windows machines
writing text files, and for certain physical devices like terminals, the
single newline gets automatically translated by your C library into a line
feed and a carriage return, but normally, no translation is done.
- NFS
- Network File System, which allows you to mount a remote
filesystem as if it were local.
- null character
- A character with the ASCII value of zero. It's used by C to
terminate strings, but Perl allows strings to contain a null.
- null list
- A "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".
- 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 "knows" what user-defined type (class) it is, and what it
can do because of what class it is. Your program can request an object to
do things, but the object gets to decide whether it wants to do them or
not. Some objects are more accommodating than others.
- octal
- A number in base 8. Only the digits 0 through 7 are
allowed. Octal constants in Perl start with 0, as in 013. See also the oct
function.
- offset
- How many things you have to skip over when moving from the
beginning of a string or array to a specific position within it. Thus, the
minimum offset is zero, not one, because you don't skip anything to get to
the first item.
- one-liner
- An entire computer program crammed into one line of
text.
- open source software
- Programs for which the source code is freely available and
freely redistributable, with no commercial strings attached. For a more
detailed definition, see <http://www.opensource.org/osd.html>.
- operand
- An "expression" that yields a "value"
that an "operator" operates on. See also
"precedence".
- operating system
- A special program that runs on the bare machine and hides
the gory details of managing processes and devices. Usually used in a
looser sense to indicate a particular culture of programming. The loose
sense can be used at varying levels of specificity. At one extreme, you
might say that all versions of Unix and Unix-lookalikes are the same
operating system (upsetting many people, especially lawyers and other
advocates). At the other extreme, you could say this particular version of
this particular vendor's operating system is different from any other
version of this or any other vendor's operating system. Perl is much more
portable across operating systems than many other languages. See also
"architecture" and "platform".
- operator
- A gizmo that transforms some number of input values to some
number of output values, often built into a language with a special syntax
or symbol. A given operator may have specific expectations about what
types of data you give as its arguments (operands) and what type of data
you want back from it.
- operator overloading
- A kind of "overloading" that you can do on
built-in operators to make them work on objects as if the objects were
ordinary scalar values, but with the actual semantics supplied by the
object class. This is set up with the overload "pragma".
- options
- See either switches or "regular expression
modifier".
- ordinal
- Another name for "code point"
- overloading
- Giving additional meanings to a symbol or construct.
Actually, all languages do overloading to one extent or another, since
people are good at figuring out things from "context".
- overriding
- Hiding or invalidating some other definition of the same
name. (Not to be confused with "overloading", which adds
definitions that must be disambiguated some other way.) To confuse the
issue further, we use the word with two overloaded definitions: to
describe how you can define your own "subroutine" to hide a
built-in "function" of the same name (see "Overriding
Built-in Functions" in perlsub) and to describe how you can define a
replacement "method" in a "derived class" to hide a
"base class"'s method of the same name (see perlobj).
- owner
- The one user (apart from the superuser) who has absolute
control over a "file". A file may also have a "group"
of users who may exercise joint ownership if the real owner permits it.
See "permission bits".
- package
- A "namespace" for global variables, subroutines,
and the like, such that they can be kept separate from like-named symbols
in other namespaces. In a sense, only the package is global, since the
symbols in the package's symbol table are only accessible from code
compiled outside the package by naming the package. But in another sense,
all package symbols are also globals--they're just well-organized
globals.
- pad
- Short for "scratchpad".
- parameter
- See "argument".
- parent class
- See "base class".
- parse tree
- See "syntax tree".
- parsing
- The subtle but sometimes brutal art of attempting to turn
your possibly malformed program into a valid "syntax tree".
- patch
- To fix by applying one, as it were. In the realm of
hackerdom, a listing of the differences between two versions of a program
as might be applied by the patch(1) program when you want to fix a
bug or upgrade your old version.
- PATH
- The list of directories the system searches to find a
program you want to "execute". The list is stored as one of your
environment variables, accessible in Perl as $ENV{PATH}.
- pathname
- A fully qualified filename such as /usr/bin/perl.
Sometimes confused with "PATH".
- pattern
- A template used in "pattern matching".
- pattern matching
- Taking a pattern, usually a "regular expression",
and trying the pattern various ways on a string to see whether there's any
way to make it fit. Often used to pick interesting tidbits out of a
file.
- permission bits
- Bits that the "owner" of a file sets or unsets to
allow or disallow access to other people. These flag bits are part of the
"mode" word returned by the stat built-in when you ask about a
file. On Unix systems, you can check the ls(1) manpage for more
information.
- Pern
- What you get when you do "Perl++" twice. Doing it
only once will curl your hair. You have to increment it eight times to
shampoo your hair. Lather, rinse, iterate.
- pipe
- A direct "connection" that carries the output of
one "process" to the input of another without an intermediate
temporary file. Once the pipe is set up, the two processes in question can
read and write as if they were talking to a normal file, with some
caveats.
- pipeline
- A series of processes all in a row, linked by pipes, where
each passes its output stream to the next.
- platform
- The entire hardware and software context in which a program
runs. A
program written in a platform-dependent language might break if you change
any of: machine, operating system, libraries, compiler, or system
configuration. The perl interpreter has to be compiled differently
for each platform because it is implemented in C, but programs written in
the Perl language are largely platform-independent.
- pod
- The markup used to embed documentation into your Perl code.
See perlpod.
- pointer
- A "variable" in a language like C that contains
the exact memory location of some other item. Perl handles pointers
internally so you don't have to worry about them. Instead, you just use
symbolic pointers in the form of keys and "variable" names, or
hard references, which aren't pointers (but act like pointers and do in
fact contain pointers).
- polymorphism
- The notion that you can tell an "object" to do
something generic, and the object will interpret the command in different
ways depending on its type. [<Gk many shapes]
- port
- The part of the address of a TCP or UDP socket that directs
packets to the correct process after finding the right machine, something
like the phone extension you give when you reach the company operator.
Also, the result of converting code to run on a different platform than
originally intended, or the verb denoting this conversion.
- portable
- Once upon a time, C code compilable under both BSD and
SysV. In general, code that can be easily converted to run on another
"platform", where "easily" can be defined however you
like, and usually is. Anything may be considered portable if you try hard
enough. See mobile home or London Bridge.
- porter
- Someone who "carries" software from one
"platform" to another. Porting programs written in
platform-dependent languages such as C can be difficult work, but porting
programs like Perl is very much worth the agony.
- POSIX
- The Portable Operating System Interface specification.
- postfix
- An "operator" that follows its
"operand", as in "$x++".
- pp
- An internal shorthand for a "push-pop" code, that
is, C code implementing Perl's stack machine.
- pragma
- A standard module whose practical hints and suggestions are
received (and possibly ignored) at compile time. Pragmas are named in all
lowercase.
- precedence
- The rules of conduct that, in the absence of other
guidance, determine what should happen first. For example, in the absence
of parentheses, you always do multiplication before addition.
- prefix
- An "operator" that precedes its
"operand", as in "++$x".
- preprocessing
- What some helper "process" did to transform the
incoming data into a form more suitable for the current process. Often
done with an incoming "pipe". See also "C
preprocessor".
- procedure
- A "subroutine".
- process
- An instance of a running program. Under multitasking
systems like Unix, two or more separate processes could be running the
same program independently at the same time--in fact, the fork function is
designed to bring about this happy state of affairs. Under other operating
systems, processes are sometimes called "threads",
"tasks", or "jobs", often with slight nuances in
meaning.
- program generator
- A system that algorithmically writes code for you in a
high-level language. See also "code generator".
- progressive matching
- Pattern matching that picks up where it left off
before.
- property
- See either "instance variable" or "character
property".
- protocol
- In networking, an agreed-upon way of sending messages back
and forth so that neither correspondent will get too confused.
- prototype
- An optional part of a "subroutine" declaration
telling the Perl compiler how many and what flavor of arguments may be
passed as "actual arguments", so that you can write subroutine
calls that parse much like built-in functions. (Or don't parse, as the
case may be.)
- pseudofunction
- A construct that sometimes looks like a function but really
isn't. Usually reserved for "lvalue" modifiers like my, for
"context" modifiers like scalar, and for the
pick-your-own-quotes constructs, "q//", "qq//",
"qx//", "qw//", "qr//", "m//",
"s///", "y///", and "tr///".
- pseudohash
- A reference to an array whose initial element happens to
hold a reference to a hash. You can treat a pseudohash reference as either
an array reference or a hash reference.
- pseudoliteral
- An "operator" that looks something like a
"literal", such as the output-grabbing operator, "`"
"command""`".
- public domain
- Something not owned by anybody. Perl is copyrighted and is
thus not in the public domain--it's just "freely
available" and "freely redistributable".
- pumpkin
- A notional "baton" handed around the Perl
community indicating who is the lead integrator in some arena of
development.
- pumpking
- A "pumpkin" holder, the person in charge of
pumping the pump, or at least priming it. Must be willing to play the part
of the Great Pumpkin now and then.
- PV
- A "pointer value", which is Perl Internals Talk
for a "char*".
- 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.
- readable
- With respect to files, one that has the proper permission
bit set to let you access the file. With respect to computer programs, one
that's written well enough that someone has a chance of figuring out what
it's trying to do.
- reaping
- The last rites performed by a parent "process" on
behalf of a deceased child process so that it doesn't remain a
"zombie". See the wait and waitpid function calls.
- record
- A set of related data values in a "file" or
"stream", often associated with a unique "key" field.
In Unix, often commensurate with a "line", or a
blank-line-terminated set of lines (a "paragraph"). Each line of
the /etc/passwd file is a record, keyed on login name, containing
information about that user.
- recursion
- The art of defining something (at least partly) in terms of
itself, which is a naughty no-no in dictionaries but often works out okay
in computer programs if you're careful not to recurse forever, which is
like an infinite loop with more spectacular failure modes.
- reference
- Where you look to find a pointer to information somewhere
else. (See "indirection".) References come in two flavors,
symbolic references and hard references.
- referent
- Whatever a reference refers to, which may or may not have a
name. Common types of referents include scalars, arrays, hashes, and
subroutines.
- regex
- See "regular expression".
- regular expression
- A single entity with various interpretations, like an
elephant. To a computer scientist, it's a grammar for a little language in
which some strings are legal and others aren't. To normal people, it's a
pattern you can use to find what you're looking for when it varies from
case to case. Perl's regular expressions are far from regular in the
theoretical sense, but in regular use they work quite well. Here's a
regular expression: "/Oh s.*t./". This will match strings like
""Oh say can you see by the dawn's early light"" and
""Oh sit!"". See perlre.
- regular expression modifier
- An option on a pattern or substitution, such as
"/i" to render the pattern case insensitive. See also
"cloister".
- regular file
- A "file" that's not a "directory", a
"device", a named "pipe" or "socket", or a
"symbolic link". Perl uses the "-f" file test operator
to identify regular files. Sometimes called a "plain" file.
- relational operator
- An "operator" that says whether a particular
ordering relationship is "true" about a pair of operands. Perl
has both numeric and string relational operators. See "collating
sequence".
- reserved words
- A word with a specific, built-in meaning to a
"compiler", such as "if" or delete. In many languages
(not Perl), it's illegal to use reserved words to name anything else.
(Which is why they're reserved, after all.) In Perl, you just can't use
them to name labels or filehandles. Also called "keywords".
- return value
- The "value" produced by a "subroutine"
or "expression" when evaluated. In Perl, a return value may be
either a "list" or a "scalar".
- RFC
- Request For Comment, which despite the timid connotations
is the name of a series of important standards documents.
- right shift
- A "bit shift" that divides a number by some power
of 2.
- root
- The superuser (UID == 0). Also, the top-level directory of
the filesystem.
- RTFM
- What you are told when someone thinks you should Read The
Fine Manual.
- run phase
- Any time after Perl starts running your main program. See
also "compile phase". Run phase is mostly spent in "run
time" but may also be spent in "compile time" when require,
do "FILE", or eval "STRING" operators are executed or
when a substitution uses the "/ee" modifier.
- run time
- The time when Perl is actually doing what your code says to
do, as opposed to the earlier period of time when it was trying to figure
out whether what you said made any sense whatsoever, which is
"compile time".
- run-time pattern
- A pattern that contains one or more variables to be
interpolated before parsing the pattern as a "regular
expression", and that therefore cannot be analyzed at compile time,
but must be re-analyzed each time the pattern match operator is evaluated.
Run-time patterns are useful but expensive.
- RV
- A recreational vehicle, not to be confused with vehicular
recreation. RV also means an internal Reference Value of the type a
"scalar" can hold. See also "IV" and "NV" if
you're not confused yet.
- rvalue
- A "value" that you might find on the right side
of an "assignment". See also "lvalue".
- scalar
- A simple, singular value; a number, "string", or
"reference".
- scalar context
- The situation in which an "expression" is
expected by its surroundings (the code calling it) to return a single
"value" rather than a "list" of values. See also
"context" and "list context". A scalar context
sometimes imposes additional constraints on the return value--see
"string context" and "numeric context". Sometimes we
talk about a "Boolean context" inside conditionals, but this
imposes no additional constraints, since any scalar value, whether numeric
or "string", is already true or false.
- scalar literal
- A number or quoted "string"--an actual
"value" in the text of your program, as opposed to a
"variable".
- scalar value
- A value that happens to be a "scalar" as opposed
to a "list".
- scalar variable
- A "variable" prefixed with "$" that
holds a single value.
- scope
- How far away you can see a variable from, looking through
one. Perl has two visibility mechanisms: it does "dynamic
scoping" of local variables, meaning that the rest of the
"block", and any subroutines that are called by the rest of the
block, can see the variables that are local to the block. Perl does
"lexical scoping" of my variables, meaning that the rest of the
block can see the variable, but other subroutines called by the block
cannot see the variable.
- scratchpad
- The area in which a particular invocation of a particular
file or subroutine keeps some of its temporary values, including any
lexically scoped variables.
- script
- A text "file" that is a program intended to be
executed directly rather than compiled to another form of file before
execution. Also, in the context of "Unicode", a writing system
for a particular language or group of languages, such as Greek, Bengali,
or Klingon.
- script kiddie
- A "cracker" who is not a "hacker", but
knows just enough to run canned scripts. A cargo-cult programmer.
- sed
- A venerable Stream EDitor from which Perl derives some of
its ideas.
- semaphore
- A fancy kind of interlock that prevents multiple threads or
processes from using up the same resources simultaneously.
- separator
- A "character" or "string" that keeps
two surrounding strings from being confused with each other. The split
function works on separators. Not to be confused with delimiters or
terminators. The "or" in the previous sentence separated the two
alternatives.
- serialization
- Putting a fancy "data structure" into linear
order so that it can be stored as a "string" in a disk file or
database or sent through a "pipe". Also called marshalling.
- server
- In networking, a "process" that either advertises
a "service" or just hangs around at a known location and waits
for clients who need service to get in touch with it.
- service
- Something you do for someone else to make them happy, like
giving them the time of day (or of their life). On some machines,
well-known services are listed by the getservent function.
- setgid
- Same as "setuid", only having to do with giving
away "group" privileges.
- setuid
- Said of a program that runs with the privileges of its
"owner" rather than (as is usually the case) the privileges of
whoever is running it. Also describes the bit in the mode word
("permission bits") that controls the feature. This bit must be
explicitly set by the owner to enable this feature, and the program must
be carefully written not to give away more privileges than it ought
to.
- shared memory
- A piece of "memory" accessible by two different
processes who otherwise would not see each other's memory.
- shebang
- Irish for the whole McGillicuddy. In Perl culture, a
portmanteau of "sharp" and "bang", meaning the
"#!" sequence that tells the system where to find the
interpreter.
- shell
- A "command"-line "interpreter". The
program that interactively gives you a prompt, accepts one or more lines
of input, and executes the programs you mentioned, feeding each of them
their proper arguments and input data. Shells can also execute scripts
containing such commands. Under Unix, typical shells include the Bourne
shell ( /bin/sh), the C shell (/bin/csh), and the Korn shell
( /bin/ksh). Perl is not strictly a shell because it's not
interactive (although Perl programs can be interactive).
- side effects
- Something extra that happens when you evaluate an
"expression". Nowadays it can refer to almost anything. For
example, evaluating a simple assignment statement typically has the
"side effect" of assigning a value to a variable. (And you
thought assigning the value was your primary intent in the first place!)
Likewise, assigning a value to the special variable $| ($AUTOFLUSH) has
the side effect of forcing a flush after every write or print on the
currently selected filehandle.
- signal
- A bolt out of the blue; that is, an event triggered by the
"operating system", probably when you're least expecting
it.
- signal handler
- A "subroutine" that, instead of being content to
be called in the normal fashion, sits around waiting for a bolt out of the
blue before it will deign to "execute". Under Perl, bolts out of
the blue are called signals, and you send them with the kill built-in. See
"%SIG" in perlvar and "Signals" in perlipc.
- single inheritance
- The features you got from your mother, if she told you that
you don't have a father. (See also "inheritance" and
"multiple inheritance".) In computer languages, the notion that
classes reproduce asexually so that a given class can only have one direct
ancestor or "base class". Perl supplies no such restriction,
though you may certainly program Perl that way if you like.
- slice
- A selection of any number of elements from a
"list", "array", or "hash".
- slurp
- To read an entire "file" into a
"string" in one operation.
- socket
- An endpoint for network communication among multiple
processes that works much like a telephone or a post office box. The most
important thing about a socket is its "network address" (like a
phone number). Different kinds of sockets have different kinds of
addresses--some look like filenames, and some don't.
- soft reference
- See "symbolic reference".
- source filter
- A special kind of "module" that does
"preprocessing" on your script just before it gets to the
"tokener".
- stack
- A device you can put things on the top of, and later take
them back off in the opposite order in which you put them on. See
"LIFO".
- standard
- Included in the official Perl distribution, as in a
standard module, a standard tool, or a standard Perl
"manpage".
- standard error
- The default output "stream" for nasty remarks
that don't belong in "standard output". Represented within a
Perl program by the "filehandle" "STDERR". You can use
this stream explicitly, but the die and warn built-ins write to your
standard error stream automatically.
- standard I/O
- A standard C library for doing buffered input and output to
the "operating system". (The "standard" of standard
I/O is only marginally related to the "standard" of standard
input and output.) In general, Perl relies on whatever implementation of
standard I/O a given operating system supplies, so the buffering
characteristics of a Perl program on one machine may not exactly match
those on another machine. Normally this only influences efficiency, not
semantics. If your standard I/O package is doing block buffering and you
want it to "flush" the buffer more often, just set the $|
variable to a true value.
- standard input
- The default input "stream" for your program,
which if possible shouldn't care where its data is coming from.
Represented within a Perl program by the "filehandle"
"STDIN".
- standard output
- The default output "stream" for your program,
which if possible shouldn't care where its data is going. Represented
within a Perl program by the "filehandle"
"STDOUT".
- stat structure
- A special internal spot in which Perl keeps the information
about the last "file" on which you requested information.
- statement
- A "command" to the computer about what to do
next, like a step in a recipe: "Add marmalade to batter and mix until
mixed." A statement is distinguished from a "declaration",
which doesn't tell the computer to do anything, but just to learn
something.
- statement modifier
- A "conditional" or "loop" that you put
after the "statement" instead of before, if you know what we
mean.
- static
- Varying slowly compared to something else. (Unfortunately,
everything is relatively stable compared to something else, except for
certain elementary particles, and we're not so sure about them.) In
computers, where things are supposed to vary rapidly, "static"
has a derogatory connotation, indicating a slightly dysfunctional
"variable", "subroutine", or "method". In
Perl culture, the word is politely avoided.
- static method
- No such thing. See "class method".
- static scoping
- No such thing. See "lexical scoping".
- static variable
- No such thing. Just use a "lexical variable" in a
scope larger than your "subroutine".
- status
- The "value" returned to the parent
"process" when one of its child processes dies. This value is
placed in the special variable $?. Its upper eight bits are the exit
status of the defunct process, and its lower eight bits identify the
signal (if any) that the process died from. On Unix systems, this status
value is the same as the status word returned by wait(2). See
"system" in perlfunc.
- STDERR
- See "standard error".
- STDIN
- See "standard input".
- STDIO
- See "standard I/O".
- STDOUT
- See "standard output".
- stream
- A flow of data into or out of a process as a steady
sequence of bytes or characters, without the appearance of being broken up
into packets. This is a kind of "interface"--the underlying
"implementation" may well break your data up into separate
packets for delivery, but this is hidden from you.
- string
- A sequence of characters such as "He said
!@#*&%@#*?!". A string does not have to be entirely
printable.
- string context
- The situation in which an expression is expected by its
surroundings (the code calling it) to return a "string". See
also "context" and "numeric context".
- stringification
- The process of producing a "string"
representation of an abstract object.
- struct
- C keyword introducing a structure definition or name.
- structure
- See "data structure".
- subclass
- See "derived class".
- subpattern
- A component of a "regular expression"
pattern.
- subroutine
- A named or otherwise accessible piece of program that can
be invoked from elsewhere in the program in order to accomplish some
sub-goal of the program. A subroutine is often parameterized to accomplish
different but related things depending on its input arguments. If the
subroutine returns a meaningful "value", it is also called a
"function".
- subscript
- A "value" that indicates the position of a
particular "array" "element" in an array.
- substitution
- Changing parts of a string via the "s///"
operator. (We avoid use of this term to mean "variable
interpolation".)
- substring
- A portion of a "string", starting at a certain
"character" position ("offset") and proceeding for a
certain number of characters.
- superclass
- See "base class".
- superuser
- The person whom the "operating system" will let
do almost anything. Typically your system administrator or someone
pretending to be your system administrator. On Unix systems, the
"root" user. On Windows systems, usually the Administrator
user.
- SV
- Short for "scalar value". But within the Perl
interpreter every "referent" is treated as a member of a class
derived from SV, in an object-oriented sort of way. Every
"value" inside Perl is passed around as a C language
"SV*" pointer. The SV "struct" knows its own
"referent type", and the code is smart enough (we hope) not to
try to call a "hash" function on a "subroutine".
- switch
- An option you give on a command line to influence the way
your program works, usually introduced with a minus sign. The word is also
used as a nickname for a "switch statement".
- switch cluster
- The combination of multiple command-line switches (e.g.,
-a -b -c) into one switch (e.g., -abc). Any switch with an
additional "argument" must be the last switch in a cluster.
- switch statement
- A program technique that lets you evaluate an
"expression" and then, based on the value of the expression, do
a multiway branch to the appropriate piece of code for that value. Also
called a "case structure", named after the similar Pascal
construct. See "Switch statements" in perlsyn.
- symbol
- Generally, any "token" or "metasymbol".
Often used more specifically to mean the sort of name you might find in a
"symbol table".
- symbol table
- Where a "compiler" remembers symbols. A program
like Perl must somehow remember all the names of all the variables,
filehandles, and subroutines you've used. It does this by placing the
names in a symbol table, which is implemented in Perl using a "hash
table". There is a separate symbol table for each "package"
to give each package its own "namespace".
- symbolic debugger
- A program that lets you step through the execution of your
program, stopping or printing things out here and there to see whether
anything has gone wrong, and if so, what. The "symbolic" part
just means that you can talk to the debugger using the same symbols with
which your program is written.
- symbolic link
- An alternate filename that points to the real
"filename", which in turn points to the real "file".
Whenever the "operating system" is trying to parse a
"pathname" containing a symbolic link, it merely substitutes the
new name and continues parsing.
- symbolic reference
- A variable whose value is the name of another variable or
subroutine. By dereferencing the first variable, you can get at the second
one. Symbolic references are illegal under use strict 'refs'.
- synchronous
- Programming in which the orderly sequence of events can be
determined; that is, when things happen one after the other, not at the
same time.
- syntactic sugar
- An alternative way of writing something more easily; a
shortcut.
- syntax
- From Greek, "with-arrangement". How things
(particularly symbols) are put together with each other.
- syntax tree
- An internal representation of your program wherein
lower-level constructs dangle off the higher-level constructs enclosing
them.
- syscall
- A "function" call directly to the "operating
system". Many of the important subroutines and functions you use
aren't direct system calls, but are built up in one or more layers above
the system call level. In general, Perl programmers don't need to worry
about the distinction. However, if you do happen to know which Perl
functions are really syscalls, you can predict which of these will set the
$! ($ERRNO) variable on failure. Unfortunately, beginning programmers
often confusingly employ the term "system call" to mean what
happens when you call the Perl system function, which actually involves
many syscalls. To avoid any confusion, we nearly always use say
"syscall" for something you could call indirectly via Perl's
syscall function, and never for something you would call with Perl's
system function.
- tainted
- Said of data derived from the grubby hands of a user and
thus unsafe for a secure program to rely on. Perl does taint checks if you
run a "setuid" (or "setgid") program, or if you use
the -T switch.
- TCP
- Short for Transmission Control Protocol. A protocol wrapped
around the Internet Protocol to make an unreliable packet transmission
mechanism appear to the application program to be a reliable
"stream" of bytes. (Usually.)
- term
- Short for a "terminal", that is, a leaf node of a
"syntax tree". A thing that functions grammatically as an
"operand" for the operators in an expression.
- terminator
- A "character" or "string" that marks
the end of another string. The $/ variable contains the string that
terminates a readline operation, which chomp deletes from the end. Not to
be confused with delimiters or separators. The period at the end of this
sentence is a terminator.
- ternary
- An "operator" taking three operands. Sometimes
pronounced "trinary".
- text
- A "string" or "file" containing
primarily printable characters.
- thread
- Like a forked process, but without "fork"'s
inherent memory protection. A thread is lighter weight than a full
process, in that a process could have multiple threads running around in
it, all fighting over the same process's memory space unless steps are
taken to protect threads from each other. See threads.
- tie
- The bond between a magical variable and its implementation
class. See "tie" in perlfunc and perltie.
- TMTOWTDI
- There's More Than One Way To Do It, the Perl Motto. The
notion that there can be more than one valid path to solving a programming
problem in context. (This doesn't mean that more ways are always better or
that all possible paths are equally desirable--just that there need not be
One True Way.) Pronounced TimToady.
- token
- A morpheme in a programming language, the smallest unit of
text with semantic significance.
- tokener
- A module that breaks a program text into a sequence of
tokens for later analysis by a parser.
- tokenizing
- Splitting up a program text into tokens. Also known as
"lexing", in which case you get "lexemes" instead of
tokens.
- toolbox approach
- The notion that, with a complete set of simple tools that
work well together, you can build almost anything you want. Which is fine
if you're assembling a tricycle, but if you're building a defranishizing
comboflux regurgalator, you really want your own machine shop in which to
build special tools. Perl is sort of a machine shop.
- transliterate
- To turn one string representation into another by mapping
each character of the source string to its corresponding character in the
result string. See "tr/SEARCHLIST/REPLACEMENTLIST/cdsr" in
perlop.
- trigger
- An event that causes a "handler" to be run.
- trinary
- Not a stellar system with three stars, but an
"operator" taking three operands. Sometimes pronounced
"ternary".
- troff
- A venerable typesetting language from which Perl derives
the name of its $% variable and which is secretly used in the production
of Camel books.
- true
- Any scalar value that doesn't evaluate to 0 or
"".
- truncating
- Emptying a file of existing contents, either automatically
when opening a file for writing or explicitly via the truncate
function.
- type
- See "data type" and "class".
- type casting
- Converting data from one type to another. C permits this.
Perl does not need it. Nor want it.
- typed lexical
- A "lexical variable" that is declared with a
"class" type: "my Pony $bill".
- typedef
- A type definition in the C language.
- typeglob
- Use of a single identifier, prefixed with "*".
For example, *name stands for any or all of $name, @name, %name,
&name, or just "name". How you use it determines whether it
is interpreted as all or only one of them. See "Typeglobs and
Filehandles" in perldata.
- typemap
- A description of how C types may be transformed to and from
Perl types within an "extension" module written in
"XS".
- UDP
- User Datagram Protocol, the typical way to send datagrams
over the Internet.
- UID
- A user ID. Often used in the context of "file" or
"process" ownership.
- umask
- A mask of those "permission bits" that should be
forced off when creating files or directories, in order to establish a
policy of whom you'll ordinarily deny access to. See the umask
function.
- unary operator
- An operator with only one "operand", like
"!" or chdir. Unary operators are usually prefix operators; that
is, they precede their operand. The "++" and "--"
operators can be either prefix or postfix. (Their position does
change their meanings.)
- Unicode
- A character set comprising all the major character sets of
the world, more or less. See perlunicode and
<http://www.unicode.org>.
- Unix
- A very large and constantly evolving language with several
alternative and largely incompatible syntaxes, in which anyone can define
anything any way they choose, and usually do. Speakers of this language
think it's easy to learn because it's so easily twisted to one's own ends,
but dialectical differences make tribal intercommunication nearly
impossible, and travelers are often reduced to a pidgin-like subset of the
language. To be universally understood, a Unix shell programmer must spend
years of study in the art. Many have abandoned this discipline and now
communicate via an Esperanto-like language called Perl.
In ancient times, Unix was also used to refer to some code that a couple of
people at Bell Labs wrote to make use of a PDP-7 computer that wasn't
doing much of anything else at the time.
- value
- An actual piece of data, in contrast to all the variables,
references, keys, indexes, operators, and whatnot that you need to access
the value.
- variable
- A named storage location that can hold any of various kinds
of "value", as your program sees fit.
- variable interpolation
- The "interpolation" of a scalar or array variable
into a string.
- variadic
- Said of a "function" that happily receives an
indeterminate number of "actual arguments".
- vector
- Mathematical jargon for a list of scalar values.
- virtual
- Providing the appearance of something without the reality,
as in: virtual memory is not real memory. (See also "memory".)
The opposite of "virtual" is "transparent", which
means providing the reality of something without the appearance, as in:
Perl handles the variable-length UTF-8 character encoding
transparently.
- void context
- A form of "scalar context" in which an
"expression" is not expected to return any "value" at
all and is evaluated for its "side effects" alone.
- v-string
- A "version" or "vector"
"string" specified with a "v" followed by a series of
decimal integers in dot notation, for instance,
"v1.20.300.4000". Each number turns into a "character"
with the specified ordinal value. (The "v" is optional when
there are at least three integers.)
- warning
- A message printed to the "STDERR" stream to the
effect that something might be wrong but isn't worth blowing up over. See
"warn" in perlfunc and the warnings pragma.
- watch expression
- An expression which, when its value changes, causes a
breakpoint in the Perl debugger.
- whitespace
- A "character" that moves your cursor but doesn't
otherwise put anything on your screen. Typically refers to any of: space,
tab, line feed, carriage return, or form feed.
- word
- In normal "computerese", the piece of data of the
size most efficiently handled by your computer, typically 32 bits or so,
give or take a few powers of 2. In Perl culture, it more often refers to
an alphanumeric "identifier" (including underscores), or to a
string of nonwhitespace characters bounded by whitespace or string
boundaries.
- working directory
- Your current "directory", from which relative
pathnames are interpreted by the "operating system". The
operating system knows your current directory because you told it with a
chdir or because you started out in the place where your parent
"process" was when you were born.
- wrapper
- A program or subroutine that runs some other program or
subroutine for you, modifying some of its input or output to better suit
your purposes.
- WYSIWYG
- What You See Is What You Get. Usually used when something
that appears on the screen matches how it will eventually look, like
Perl's format declarations. Also used to mean the opposite of magic
because everything works exactly as it appears, as in the three-argument
form of open.
- XS
- An extraordinarily exported, expeditiously excellent,
expressly eXternal Subroutine, executed in existing C or C++ or in an
exciting new extension language called (exasperatingly) XS. Examine perlxs
for the exact explanation or perlxstut for an exemplary unexacting
one.
- XSUB
- An external "subroutine" defined in
"XS".
- 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, or else the process table will fill up and your system
administrator will Not Be Happy with you.
AUTHOR AND COPYRIGHT¶
Based on the Glossary of Programming Perl, Third Edition, by Larry Wall, Tom
Christiansen & Jon Orwant. Copyright (c) 2000, 1996, 1991 O'Reilly Media,
Inc. This document may be distributed under the same terms as Perl
itself.