3 perlglossary - Perl Glossary
7 A glossary of terms (technical and otherwise) used in the Perl documentation.
8 Other useful sources include the Free On-Line Dictionary of Computing
9 L<http://foldoc.doc.ic.ac.uk/foldoc/index.html>, the Jargon File
10 L<http://catb.org/~esr/jargon/>, and Wikipedia L<http://www.wikipedia.org/>.
16 =item accessor methods
18 A L</method> used to indirectly inspect or update an L</object>'s
19 state (its L<instance variables|/instance variable>).
21 =item actual arguments
23 The L<scalar values|/scalar value> that you supply to a L</function>
24 or L</subroutine> when you call it. For instance, when you call
25 C<power("puff")>, the string C<"puff"> is the actual argument. See
26 also L</argument> and L</formal arguments>.
28 =item address operator
30 Some languages work directly with the memory addresses of values, but
31 this can be like playing with fire. Perl provides a set of asbestos
32 gloves for handling all memory management. The closest to an address
33 operator in Perl is the backslash operator, but it gives you a L</hard
34 reference>, which is much safer than a memory address.
38 A well-defined sequence of steps, clearly enough explained that even a
39 computer could do them.
43 A nickname for something, which behaves in all ways as though you'd
44 used the original name instead of the nickname. Temporary aliases are
45 implicitly created in the loop variable for C<foreach> loops, in the
46 C<$_> variable for L<map|perlfunc/map> or L<grep|perlfunc/grep>
47 operators, in C<$a> and C<$b> during L<sort|perlfunc/sort>'s
48 comparison function, and in each element of C<@_> for the L</actual
49 arguments> of a subroutine call. Permanent aliases are explicitly
50 created in L<packages|/package> by L<importing|/import> symbols or by
51 assignment to L<typeglobs|/typeglob>. Lexically scoped aliases for
52 package variables are explicitly created by the L<our|perlfunc/our>
57 A list of possible choices from which you may select only one, as in
58 "Would you like door A, B, or C?" Alternatives in regular expressions
59 are separated with a single vertical bar: C<|>. Alternatives in
60 normal Perl expressions are separated with a double vertical bar:
61 C<||>. Logical alternatives in L</Boolean> expressions are separated
62 with either C<||> or C<or>.
66 Used to describe a L</referent> that is not directly accessible
67 through a named L</variable>. Such a referent must be indirectly
68 accessible through at least one L</hard reference>. When the last
69 hard reference goes away, the anonymous referent is destroyed without
74 The kind of computer you're working on, where one "kind" of computer
75 means all those computers sharing a compatible machine language.
76 Since Perl programs are (typically) simple text files, not executable
77 images, a Perl program is much less sensitive to the architecture it's
78 running on than programs in other languages, such as C, that are
79 compiled into machine code. See also L</platform> and L</operating
84 A piece of data supplied to a L<program|/executable file>,
85 L</subroutine>, L</function>, or L</method> to tell it what it's
86 supposed to do. Also called a "parameter".
90 The name of the array containing the L</argument> L</vector> from the
91 command line. If you use the empty C<< E<lt>E<gt> >> operator, L</ARGV> is
92 the name of both the L</filehandle> used to traverse the arguments and
93 the L</scalar> containing the name of the current input file.
95 =item arithmetical operator
97 A L</symbol> such as C<+> or C</> that tells Perl to do the arithmetic
98 you were supposed to learn in grade school.
102 An ordered sequence of L<values|/value>, stored such that you can
103 easily access any of the values using an integer L</subscript>
104 that specifies the value's L</offset> in the sequence.
108 An archaic expression for what is more correctly referred to as
113 The American Standard Code for Information Interchange (a 7-bit
114 character set adequate only for poorly representing English text).
115 Often used loosely to describe the lowest 128 values of the various
116 ISO-8859-X character sets, a bunch of mutually incompatible 8-bit
117 codes best described as half ASCII. See also L</Unicode>.
121 A component of a L</regular expression> that must be true for the
122 pattern to match but does not necessarily match any characters itself.
123 Often used specifically to mean a L</zero width> assertion.
127 An L</operator> whose assigned mission in life is to change the value
130 =item assignment operator
132 Either a regular L</assignment>, or a compound L</operator> composed
133 of an ordinary assignment and some other operator, that changes the
134 value of a variable in place, that is, relative to its old value. For
135 example, C<$a += 2> adds C<2> to C<$a>.
137 =item associative array
139 See L</hash>. Please.
143 Determines whether you do the left L</operator> first or the right
144 L</operator> first when you have "A L</operator> B L</operator> C" and
145 the two operators are of the same precedence. Operators like C<+> are
146 left associative, while operators like C<**> are right associative.
147 See L<perlop> for a list of operators and their associativity.
151 Said of events or activities whose relative temporal ordering is
152 indeterminate because too many things are going on at once. Hence, an
153 asynchronous event is one you didn't know when to expect.
157 A L</regular expression> component potentially matching a
158 L</substring> containing one or more characters and treated as an
159 indivisible syntactic unit by any following L</quantifier>. (Contrast
160 with an L</assertion> that matches something of L</zero width> and may
163 =item atomic operation
165 When Democritus gave the word "atom" to the indivisible bits of
166 matter, he meant literally something that could not be cut: I<a->
167 (not) + I<tomos> (cuttable). An atomic operation is an action that
168 can't be interrupted, not one forbidden in a nuclear-free zone.
172 A new feature that allows the declaration of L<variables|/variable>
173 and L<subroutines|/subroutine> with modifiers as in C<sub foo : locked
174 method>. Also, another name for an L</instance variable> of an
179 A feature of L</operator overloading> of L<objects|/object>, whereby
180 the behavior of certain L<operators|/operator> can be reasonably
181 deduced using more fundamental operators. This assumes that the
182 overloaded operators will often have the same relationships as the
183 regular operators. See L<perlop>.
187 To add one to something automatically, hence the name of the C<++>
188 operator. To instead subtract one from something automatically is
189 known as an "autodecrement".
193 To load on demand. (Also called "lazy" loading.) Specifically, to
194 call an L<AUTOLOAD|perlsub/Autoloading> subroutine on behalf of an
195 undefined subroutine.
199 To split a string automatically, as the B<-a> L</switch> does when
200 running under B<-p> or B<-n> in order to emulate L</awk>. (See also
201 the L<AutoSplit> module, which has nothing to do with the B<-a>
202 switch, but a lot to do with autoloading.)
204 =item autovivification
206 A Greco-Roman word meaning "to bring oneself to life". In Perl,
207 storage locations (L<lvalues|/lvalue>) spontaneously generate
208 themselves as needed, including the creation of any L</hard reference>
209 values to point to the next level of storage. The assignment
210 C<$a[5][5][5][5][5] = "quintet"> potentially creates five scalar
211 storage locations, plus four references (in the first four scalar
212 locations) pointing to four new anonymous arrays (to hold the last
213 four scalar locations). But the point of autovivification is that you
214 don't have to worry about it.
218 Short for "array value", which refers to one of Perl's internal data
219 types that holds an L</array>. The L</AV> type is a subclass of
224 Descriptive editing term--short for "awkward". Also coincidentally
225 refers to a venerable text-processing language from which Perl derived
226 some of its high-level ideas.
236 A substring L<captured|/capturing> by a subpattern within
237 unadorned parentheses in a L</regex>. Backslashed decimal numbers
238 (C<\1>, C<\2>, etc.) later in the same pattern refer back to the
239 corresponding subpattern in the current match. Outside the pattern,
240 the numbered variables (C<$1>, C<$2>, etc.) continue to refer to these
241 same values, as long as the pattern was the last successful match of
242 the current dynamic scope.
246 The practice of saying, "If I had to do it all over, I'd do it
247 differently," and then actually going back and doing it all over
248 differently. Mathematically speaking, it's returning from an
249 unsuccessful recursion on a tree of possibilities. Perl backtracks
250 when it attempts to match patterns with a L</regular expression>, and
251 its earlier attempts don't pan out. See L<perlre/Backtracking>.
253 =item backward compatibility
255 Means you can still run your old program because we didn't break any
256 of the features or bugs it was relying on.
260 A word sufficiently ambiguous to be deemed illegal under L<use strict
261 'subs'|strict/strict subs>. In the absence of that stricture, a
262 bareword is treated as if quotes were around it.
266 A generic L</object> type; that is, a L</class> from which other, more
267 specific classes are derived genetically by L</inheritance>. Also
268 called a "superclass" by people who respect their ancestors.
272 From Swift: someone who eats eggs big end first. Also used of
273 computers that store the most significant L</byte> of a word at a
274 lower byte address than the least significant byte. Often considered
275 superior to little-endian machines. See also L</little-endian>.
279 Having to do with numbers represented in base 2. That means there's
280 basically two numbers, 0 and 1. Also used to describe a "non-text
281 file", presumably because such a file makes full use of all the binary
282 bits in its bytes. With the advent of L</Unicode>, this distinction,
283 already suspect, loses even more of its meaning.
285 =item binary operator
287 An L</operator> that takes two L<operands|/operand>.
291 To assign a specific L</network address> to a L</socket>.
295 An integer in the range from 0 to 1, inclusive. The smallest possible
296 unit of information storage. An eighth of a L</byte> or of a dollar.
297 (The term "Pieces of Eight" comes from being able to split the old
298 Spanish dollar into 8 bits, each of which still counted for money.
299 That's why a 25-cent piece today is still "two bits".)
303 The movement of bits left or right in a computer word, which has the
304 effect of multiplying or dividing by a power of 2.
308 A sequence of L<bits|/bit> that is actually being thought of as a
309 sequence of bits, for once.
313 In corporate life, to grant official approval to a thing, as in, "The
314 VP of Engineering has blessed our WebCruncher project." Similarly in
315 Perl, to grant official approval to a L</referent> so that it can
316 function as an L</object>, such as a WebCruncher object. See
321 What a L</process> does when it has to wait for something: "My process
322 blocked waiting for the disk." As an unrelated noun, it refers to a
323 large chunk of data, of a size that the L</operating system> likes to
324 deal with (normally a power of two such as 512 or 8192). Typically
325 refers to a chunk of data that's coming from or going to a disk file.
329 A syntactic construct consisting of a sequence of Perl
330 L<statements|/statement> that is delimited by braces. The C<if> and
331 C<while> statements are defined in terms of L<BLOCKs|/BLOCK>, for instance.
332 Sometimes we also say "block" to mean a lexical scope; that is, a
333 sequence of statements that act like a L</BLOCK>, such as within an
334 L<eval|perlfunc/eval> or a file, even though the statements aren't
337 =item block buffering
339 A method of making input and output efficient by passing one L</block>
340 at a time. By default, Perl does block buffering to disk files. See
341 L</buffer> and L</command buffering>.
345 A value that is either L</true> or L</false>.
347 =item Boolean context
349 A special kind of L</scalar context> used in conditionals to decide
350 whether the L</scalar value> returned by an expression is L</true> or
351 L</false>. Does not evaluate as either a string or a number. See
356 A spot in your program where you've told the debugger to stop
357 L<execution|/execute> so you can poke around and see whether anything
362 To send a L</datagram> to multiple destinations simultaneously.
366 A psychoactive drug, popular in the 80s, probably developed at
367 U. C. Berkeley or thereabouts. Similar in many ways to the
368 prescription-only medication called "System V", but infinitely more
369 useful. (Or, at least, more fun.) The full chemical name is
370 "Berkeley Standard Distribution".
374 A location in a L</hash table> containing (potentially) multiple
375 entries whose keys "hash" to the same hash value according to its hash
376 function. (As internal policy, you don't have to worry about it,
377 unless you're into internals, or policy.)
381 A temporary holding location for data. L<Block buffering|/block
382 buffering> means that the data is passed on to its destination
383 whenever the buffer is full. L<Line buffering|/line buffering> means
384 that it's passed on whenever a complete line is received. L<Command
385 buffering|/command buffering> means that it's passed every time you do
386 a L<print|perlfunc/print> command (or equivalent). If your output is
387 unbuffered, the system processes it one byte at a time without the use
388 of a holding area. This can be rather inefficient.
392 A L</function> that is predefined in the language. Even when hidden
393 by L</overriding>, you can always get at a built-in function by
394 L<qualifying|/qualified> its name with the C<CORE::> pseudo-package.
398 A group of related modules on L</CPAN>. (Also, sometimes refers to a
399 group of command-line switches grouped into one L</switch cluster>.)
403 A piece of data worth eight L<bits|/bit> in most places.
407 A pidgin-like language spoken among 'droids when they don't wish to
408 reveal their orientation (see L</endian>). Named after some similar
409 languages spoken (for similar reasons) between compilers and
410 interpreters in the late 20th century. These languages are
411 characterized by representing everything as a
412 non-architecture-dependent sequence of bytes.
422 A language beloved by many for its inside-out L</type> definitions,
423 inscrutable L</precedence> rules, and heavy L</overloading> of the
424 function-call mechanism. (Well, actually, people first switched to C
425 because they found lowercase identifiers easier to read than upper.)
426 Perl is written in C, so it's not surprising that Perl borrowed a few
431 The typical C compiler's first pass, which processes lines beginning
432 with C<#> for conditional compilation and macro definition and does
433 various manipulations of the program text based on the current
434 definitions. Also known as I<cpp>(1).
436 =item call by reference
438 An L</argument>-passing mechanism in which the L</formal arguments>
439 refer directly to the L</actual arguments>, and the L</subroutine> can
440 change the actual arguments by changing the formal arguments. That
441 is, the formal argument is an L</alias> for the actual argument. See
442 also L</call by value>.
446 An L</argument>-passing mechanism in which the L</formal arguments>
447 refer to a copy of the L</actual arguments>, and the L</subroutine>
448 cannot change the actual arguments by changing the formal arguments.
449 See also L</call by reference>.
453 A L</handler> that you register with some other part of your program
454 in the hope that the other part of your program will L</trigger> your
455 handler when some event of interest transpires.
459 Reduced to a standard form to facilitate comparison.
463 The use of parentheses around a L</subpattern> in a L</regular
464 expression> to store the matched L</substring> as a L</backreference>.
465 (Captured strings are also returned as a list in L</list context>.)
469 A small integer representative of a unit of orthography.
470 Historically, characters were usually stored as fixed-width integers
471 (typically in a byte, or maybe two, depending on the character set),
472 but with the advent of UTF-8, characters are often stored in a
473 variable number of bytes depending on the size of the integer that
474 represents the character. Perl manages this transparently for you,
477 =item character class
479 A square-bracketed list of characters used in a L</regular expression>
480 to indicate that any character of the set may occur at a given point.
481 Loosely, any predefined set of characters so used.
483 =item character property
485 A predefined L</character class> matchable by the C<\p>
486 L</metasymbol>. Many standard properties are defined for L</Unicode>.
488 =item circumfix operator
490 An L</operator> that surrounds its L</operand>, like the angle
491 operator, or parentheses, or a hug.
495 A user-defined L</type>, implemented in Perl via a L</package> that
496 provides (either directly or by inheritance) L<methods|/method> (that
497 is, L<subroutines|/subroutine>) to handle L<instances|/instance> of
498 the class (its L<objects|/object>). See also L</inheritance>.
502 A L</method> whose L</invocant> is a L</package> name, not an
503 L</object> reference. A method associated with the class as a whole.
507 In networking, a L</process> that initiates contact with a L</server>
508 process in order to exchange data and perhaps receive a service.
512 A L</cluster> used to restrict the scope of a L</regular expression
517 An L</anonymous> subroutine that, when a reference to it is generated
518 at run time, keeps track of the identities of externally visible
519 L<lexical variables|/lexical variable> even after those lexical
520 variables have supposedly gone out of L</scope>. They're called
521 "closures" because this sort of behavior gives mathematicians a sense
526 A parenthesized L</subpattern> used to group parts of a L</regular
527 expression> into a single L</atom>.
531 The word returned by the L<ref|perlfunc/ref> function when you apply
532 it to a reference to a subroutine. See also L</CV>.
536 A system that writes code for you in a low-level language, such as
537 code to implement the backend of a compiler. See L</program
540 =item code subpattern
542 A L</regular expression> subpattern whose real purpose is to execute
543 some Perl code, for example, the C<(?{...})> and C<(??{...})>
546 =item collating sequence
548 The order into which L<characters|/character> sort. This is used by
549 L</string> comparison routines to decide, for example, where in this
550 glossary to put "collating sequence".
554 In L</shell> programming, the syntactic combination of a program name
555 and its arguments. More loosely, anything you type to a shell (a
556 command interpreter) that starts it doing something. Even more
557 loosely, a Perl L</statement>, which might start with a L</label> and
558 typically ends with a semicolon.
560 =item command buffering
562 A mechanism in Perl that lets you store up the output of each Perl
563 L</command> and then flush it out as a single request to the
564 L</operating system>. It's enabled by setting the C<$|>
565 (C<$AUTOFLUSH>) variable to a true value. It's used when you don't
566 want data sitting around not going where it's supposed to, which may
567 happen because the default on a L</file> or L</pipe> is to use
572 The name of the program currently executing, as typed on the command
573 line. In C, the L</command> name is passed to the program as the
574 first command-line argument. In Perl, it comes in separately as
577 =item command-line arguments
579 The L<values|/value> you supply along with a program name when you
580 tell a L</shell> to execute a L</command>. These values are passed to
581 a Perl program through C<@ARGV>.
585 A remark that doesn't affect the meaning of the program. In Perl, a
586 comment is introduced by a C<#> character and continues to the end of
589 =item compilation unit
591 The L</file> (or L</string>, in the case of L<eval|perlfunc/eval>)
592 that is currently being compiled.
596 Any time before Perl starts running your main program. See also
597 L</run phase>. Compile phase is mostly spent in L</compile time>, but
598 may also be spent in L</run time> when C<BEGIN> blocks,
599 L<use|perlfunc/use> declarations, or constant subexpressions are being
600 evaluated. The startup and import code of any L<use|perlfunc/use>
601 declaration is also run during compile phase.
605 The time when Perl is trying to make sense of your code, as opposed to
606 when it thinks it knows what your code means and is merely trying to
607 do what it thinks your code says to do, which is L</run time>.
611 Strictly speaking, a program that munches up another program and spits
612 out yet another file containing the program in a "more executable"
613 form, typically containing native machine instructions. The I<perl>
614 program is not a compiler by this definition, but it does contain a
615 kind of compiler that takes a program and turns it into a more
616 executable form (L<syntax trees|/syntax tree>) within the I<perl>
617 process itself, which the L</interpreter> then interprets. There are,
618 however, extension L<modules|/module> to get Perl to act more like a
619 "real" compiler. See L<O>.
623 A "constructor" for a L</referent> that isn't really an L</object>,
624 like an anonymous array or a hash (or a sonata, for that matter). For
625 example, a pair of braces acts as a composer for a hash, and a pair of
626 brackets acts as a composer for an array. See L<perlref/Making
631 The process of gluing one cat's nose to another cat's tail. Also, a
632 similar operation on two L<strings|/string>.
636 Something "iffy". See L</Boolean context>.
640 In telephony, the temporary electrical circuit between the caller's
641 and the callee's phone. In networking, the same kind of temporary
642 circuit between a L</client> and a L</server>.
646 As a noun, a piece of syntax made up of smaller pieces. As a
647 transitive verb, to create an L</object> using a L</constructor>.
651 Any L</class method>, instance L</method>, or L</subroutine>
652 that composes, initializes, blesses, and returns an L</object>.
653 Sometimes we use the term loosely to mean a L</composer>.
657 The surroundings, or environment. The context given by the
658 surrounding code determines what kind of data a particular
659 L</expression> is expected to return. The three primary contexts are
660 L</list context>, L</scalar context>, and L</void context>. Scalar
661 context is sometimes subdivided into L</Boolean context>, L</numeric
662 context>, L</string context>, and L</void context>. There's also a
663 "don't care" scalar context (which is dealt with in Programming Perl,
664 Third Edition, Chapter 2, "Bits and Pieces" if you care).
668 The treatment of more than one physical L</line> as a single logical
669 line. L</Makefile> lines are continued by putting a backslash before
670 the L</newline>. Mail headers as defined by RFC 822 are continued by
671 putting a space or tab I<after> the newline. In general, lines in
672 Perl do not need any form of continuation mark, because L</whitespace>
673 (including newlines) is gleefully ignored. Usually.
677 The corpse of a L</process>, in the form of a file left in the
678 L</working directory> of the process, usually as a result of certain
679 kinds of fatal error.
683 The Comprehensive Perl Archive Network. (See L<perlfaq2/What modules and extensions are available for Perl? What is CPAN? What does CPANE<sol>srcE<sol>... mean?>).
687 Someone who breaks security on computer systems. A cracker may be a
688 true L</hacker> or only a L</script kiddie>.
690 =item current package
692 The L</package> in which the current statement is compiled. Scan
693 backwards in the text of your program through the current L<lexical
694 scope|/lexical scoping> or any enclosing lexical scopes till you find
695 a package declaration. That's your current package name.
697 =item current working directory
699 See L</working directory>.
701 =item currently selected output channel
703 The last L</filehandle> that was designated with
704 L<select|perlfunc/select>(C<FILEHANDLE>); L</STDOUT>, if no filehandle
709 An internal "code value" typedef, holding a L</subroutine>. The L</CV>
710 type is a subclass of L</SV>.
718 =item dangling statement
720 A bare, single L</statement>, without any braces, hanging off an C<if>
721 or C<while> conditional. C allows them. Perl doesn't.
725 How your various pieces of data relate to each other and what shape
726 they make when you put them all together, as in a rectangular table or
727 a triangular-shaped tree.
731 A set of possible values, together with all the operations that know
732 how to deal with those values. For example, a numeric data type has a
733 certain set of numbers that you can work with and various mathematical
734 operations that you can do on the numbers but would make little sense
735 on, say, a string such as C<"Kilroy">. Strings have their own
736 operations, such as L</concatenation>. Compound types made of a
737 number of smaller pieces generally have operations to compose and
738 decompose them, and perhaps to rearrange them. L<Objects|/object>
739 that model things in the real world often have operations that
740 correspond to real activities. For instance, if you model an
741 elevator, your elevator object might have an C<open_door()>
746 A packet of data, such as a L</UDP> message, that (from the viewpoint
747 of the programs involved) can be sent independently over the network.
748 (In fact, all packets are sent independently at the L</IP> level, but
749 L</stream> protocols such as L</TCP> hide this from your program.)
753 Stands for "Data Base Management" routines, a set of routines that
754 emulate an L</associative array> using disk files. The routines use a
755 dynamic hashing scheme to locate any entry with only two disk
756 accesses. DBM files allow a Perl program to keep a persistent
757 L</hash> across multiple invocations. You can L<tie|perlfunc/tie>
758 your hash variables to various DBM implementations--see L<AnyDBM_File>
763 An L</assertion> that states something exists and perhaps describes
764 what it's like, without giving any commitment as to how or where
765 you'll use it. A declaration is like the part of your recipe that
766 says, "two cups flour, one large egg, four or five tadpoles..." See
767 L</statement> for its opposite. Note that some declarations also
768 function as statements. Subroutine declarations also act as
769 definitions if a body is supplied.
773 To subtract a value from a variable, as in "decrement C<$x>" (meaning
774 to remove 1 from its value) or "decrement C<$x> by 3".
778 A L</value> chosen for you if you don't supply a value of your own.
782 Having a meaning. Perl thinks that some of the things people try to
783 do are devoid of meaning, in particular, making use of variables that
784 have never been given a L</value> and performing certain operations on
785 data that isn't there. For example, if you try to read data past the
786 end of a file, Perl will hand you back an undefined value. See also
787 L</false> and L<perlfunc/defined>.
791 A L</character> or L</string> that sets bounds to an arbitrarily-sized
792 textual object, not to be confused with a L</separator> or
793 L</terminator>. "To delimit" really just means "to surround" or "to
794 enclose" (like these parentheses are doing).
798 A fancy computer science term meaning "to follow a L</reference> to
799 what it points to". The "de" part of it refers to the fact that
800 you're taking away one level of L</indirection>.
804 A L</class> that defines some of its L<methods|/method> in terms of a
805 more generic class, called a L</base class>. Note that classes aren't
806 classified exclusively into base classes or derived classes: a class
807 can function as both a derived class and a base class simultaneously,
808 which is kind of classy.
812 See L</file descriptor>.
816 To deallocate the memory of a L</referent> (first triggering its
817 C<DESTROY> method, if it has one).
821 A special L</method> that is called when an L</object> is thinking
822 about L<destroying|/destroy> itself. A Perl program's C<DESTROY>
823 method doesn't do the actual destruction; Perl just
824 L<triggers|/trigger> the method in case the L</class> wants to do any
829 A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a
830 joystick or a mouse) attached to your computer, that the L</operating
831 system> tries to make look like a L</file> (or a bunch of files).
832 Under Unix, these fake files tend to live in the I</dev> directory.
836 A L</pod> directive. See L<perlpod>.
840 A special file that contains other files. Some L<operating
841 systems|/operating system> call these "folders", "drawers", or
844 =item directory handle
846 A name that represents a particular instance of opening a directory to
847 read it, until you close it. See the L<opendir|perlfunc/opendir>
852 To send something to its correct destination. Often used
853 metaphorically to indicate a transfer of programmatic control to a
854 destination selected algorithmically, often by lookup in a table of
855 function L<references|/reference> or, in the case of object
856 L<methods|/method>, by traversing the inheritance tree looking for the
857 most specific definition for the method.
861 A standard, bundled release of a system of software. The default
862 usage implies source code is included. If that is not the case, it
863 will be called a "binary-only" distribution.
867 An enchantment, illusion, phantasm, or jugglery. Said when Perl's
868 magical L</dwimmer> effects don't do what you expect, but rather seem
869 to be the product of arcane dweomercraft, sorcery, or wonder working.
874 DWIM is an acronym for "Do What I Mean", the principle that something
875 should just do what you want it to do without an undue amount of fuss.
876 A bit of code that does "dwimming" is a "dwimmer". Dwimming can
877 require a great deal of behind-the-scenes magic, which (if it doesn't
878 stay properly behind the scenes) is called a L</dweomer> instead.
880 =item dynamic scoping
882 Dynamic scoping works over a dynamic scope, making variables visible
883 throughout the rest of the L</block> in which they are first used and
884 in any L<subroutines|/subroutine> that are called by the rest of the
885 block. Dynamically scoped variables can have their values temporarily
886 changed (and implicitly restored later) by a L<local|perlfunc/local>
887 operator. (Compare L</lexical scoping>.) Used more loosely to mean
888 how a subroutine that is in the middle of calling another subroutine
889 "contains" that subroutine at L</run time>.
899 Derived from many sources. Some would say I<too> many.
903 A basic building block. When you're talking about an L</array>, it's
904 one of the items that make up the array.
908 When something is contained in something else, particularly when that
909 might be considered surprising: "I've embedded a complete Perl
910 interpreter in my editor!"
912 =item empty subclass test
914 The notion that an empty L</derived class> should behave exactly like
919 When you change a L</value> as it is being copied. [From French, "in
920 passing", as in the exotic pawn-capturing maneuver in chess.]
924 The veil of abstraction separating the L</interface> from the
925 L</implementation> (whether enforced or not), which mandates that all
926 access to an L</object>'s state be through L<methods|/method> alone.
930 See L</little-endian> and L</big-endian>.
934 The collective set of L<environment variables|/environment variable>
935 your L</process> inherits from its parent. Accessed via C<%ENV>.
937 =item environment variable
939 A mechanism by which some high-level agent such as a user can pass its
940 preferences down to its future offspring (child L<processes|/process>,
941 grandchild processes, great-grandchild processes, and so on). Each
942 environment variable is a L</key>/L</value> pair, like one entry in a
947 End of File. Sometimes used metaphorically as the terminating string
948 of a L</here document>.
952 The error number returned by a L</syscall> when it fails. Perl refers
953 to the error by the name C<$!> (or C<$OS_ERROR> if you use the English
958 See L</exception> or L</fatal error>.
960 =item escape sequence
966 A fancy term for an error. See L</fatal error>.
968 =item exception handling
970 The way a program responds to an error. The exception handling
971 mechanism in Perl is the L<eval|perlfunc/eval> operator.
975 To throw away the current L</process>'s program and replace it with
976 another without exiting the process or relinquishing any resources
977 held (apart from the old memory image).
979 =item executable file
981 A L</file> that is specially marked to tell the L</operating system>
982 that it's okay to run this file as a program. Usually shortened to
987 To run a L<program|/executable file> or L</subroutine>. (Has nothing
988 to do with the L<kill|perlfunc/kill> built-in, unless you're trying to
989 run a L</signal handler>.)
993 The special mark that tells the operating system it can run this
994 program. There are actually three execute bits under Unix, and which
995 bit gets used depends on whether you own the file singularly,
996 collectively, or not at all.
1004 To make symbols from a L</module> available for L</import> by other modules.
1008 Anything you can legally say in a spot where a L</value> is required.
1009 Typically composed of L<literals|/literal>, L<variables|/variable>,
1010 L<operators|/operator>, L<functions|/function>, and L</subroutine>
1011 calls, not necessarily in that order.
1015 A Perl module that also pulls in compiled C or C++ code. More
1016 generally, any experimental option that can be compiled into Perl,
1017 such as multithreading.
1027 In Perl, any value that would look like C<""> or C<"0"> if evaluated
1028 in a string context. Since undefined values evaluate to C<"">, all
1029 undefined values are false, but not all false values are undefined.
1033 Frequently Asked Question (although not necessarily frequently
1034 answered, especially if the answer appears in the Perl FAQ shipped
1035 standard with Perl).
1039 An uncaught L</exception>, which causes termination of the L</process>
1040 after printing a message on your L</standard error> stream. Errors
1041 that happen inside an L<eval|perlfunc/eval> are not fatal. Instead,
1042 the L<eval|perlfunc/eval> terminates after placing the exception
1043 message in the C<$@> (C<$EVAL_ERROR>) variable. You can try to
1044 provoke a fatal error with the L<die|perlfunc/die> operator (known as
1045 throwing or raising an exception), but this may be caught by a
1046 dynamically enclosing L<eval|perlfunc/eval>. If not caught, the
1047 L<die|perlfunc/die> becomes a fatal error.
1051 A single piece of numeric or string data that is part of a longer
1052 L</string>, L</record>, or L</line>. Variable-width fields are usually
1053 split up by L<separators|/separator> (so use L<split|perlfunc/split> to
1054 extract the fields), while fixed-width fields are usually at fixed
1055 positions (so use L<unpack|perlfunc/unpack>). L<Instance
1056 variables|/instance variable> are also known as fields.
1060 First In, First Out. See also L</LIFO>. Also, a nickname for a
1065 A named collection of data, usually stored on disk in a L</directory>
1066 in a L</filesystem>. Roughly like a document, if you're into office
1067 metaphors. In modern filesystems, you can actually give a file more
1068 than one name. Some files have special properties, like directories
1071 =item file descriptor
1073 The little number the L</operating system> uses to keep track of which
1074 opened L</file> you're talking about. Perl hides the file descriptor
1075 inside a L</standard IE<sol>O> stream and then attaches the stream to
1078 =item file test operator
1080 A built-in unary operator that you use to determine whether something
1081 is L</true> about a file, such as C<-o $filename> to test whether
1082 you're the owner of the file.
1086 A "wildcard" match on L<filenames|/filename>. See the
1087 L<glob|perlfunc/glob> function.
1091 An identifier (not necessarily related to the real name of a file)
1092 that represents a particular instance of opening a file until you
1093 close it. If you're going to open and close several different files
1094 in succession, it's fine to open each of them with the same
1095 filehandle, so you don't have to write out separate code to process
1100 One name for a file. This name is listed in a L</directory>, and you
1101 can use it in an L<open|perlfunc/open> to tell the L</operating
1102 system> exactly which file you want to open, and associate the file
1103 with a L</filehandle> which will carry the subsequent identity of that
1104 file in your program, until you close it.
1108 A set of L<directories|/directory> and L<files|/file> residing on a
1109 partition of the disk. Sometimes known as a "partition". You can
1110 change the file's name or even move a file around from directory to
1111 directory within a filesystem without actually moving the file itself,
1112 at least under Unix.
1116 A program designed to take a L</stream> of input and transform it into
1121 We tend to avoid this term because it means so many things. It may
1122 mean a command-line L</switch> that takes no argument
1123 itself (such as Perl's B<-n> and B<-p>
1124 flags) or, less frequently, a single-bit indicator (such as the
1125 C<O_CREAT> and C<O_EXCL> flags used in
1126 L<sysopen|perlfunc/sysopen>).
1128 =item floating point
1130 A method of storing numbers in "scientific notation", such that the
1131 precision of the number is independent of its magnitude (the decimal
1132 point "floats"). Perl does its numeric work with floating-point
1133 numbers (sometimes called "floats"), when it can't get away with
1134 using L<integers|/integer>. Floating-point numbers are mere
1135 approximations of real numbers.
1139 The act of emptying a L</buffer>, often before it's full.
1143 Far More Than Everything You Ever Wanted To Know. An exhaustive
1144 treatise on one narrow topic, something of a super-L</FAQ>. See Tom
1149 To create a child L</process> identical to the parent process at its
1150 moment of conception, at least until it gets ideas of its own. A
1151 thread with protected memory.
1153 =item formal arguments
1155 The generic names by which a L</subroutine> knows its
1156 L<arguments|/argument>. In many languages, formal arguments are
1157 always given individual names, but in Perl, the formal arguments are
1158 just the elements of an array. The formal arguments to a Perl program
1159 are C<$ARGV[0]>, C<$ARGV[1]>, and so on. Similarly, the formal
1160 arguments to a Perl subroutine are C<$_[0]>, C<$_[1]>, and so on. You
1161 may give the arguments individual names by assigning the values to a
1162 L<my|perlfunc/my> list. See also L</actual arguments>.
1166 A specification of how many spaces and digits and things to put
1167 somewhere so that whatever you're printing comes out nice and pretty.
1169 =item freely available
1171 Means you don't have to pay money to get it, but the copyright on it
1172 may still belong to someone else (like Larry).
1174 =item freely redistributable
1176 Means you're not in legal trouble if you give a bootleg copy of it to
1177 your friends and we find out about it. In fact, we'd rather you gave
1178 a copy to all your friends.
1182 Historically, any software that you give away, particularly if you
1183 make the source code available as well. Now often called C<open
1184 source software>. Recently there has been a trend to use the term in
1185 contradistinction to L</open source software>, to refer only to free
1186 software released under the Free Software Foundation's GPL (General
1187 Public License), but this is difficult to justify etymologically.
1191 Mathematically, a mapping of each of a set of input values to a
1192 particular output value. In computers, refers to a L</subroutine> or
1193 L</operator> that returns a L</value>. It may or may not have input
1194 values (called L<arguments|/argument>).
1196 =item funny character
1198 Someone like Larry, or one of his peculiar friends. Also refers to
1199 the strange prefixes that Perl requires as noun markers on its
1202 =item garbage collection
1204 A misnamed feature--it should be called, "expecting your mother to
1205 pick up after you". Strictly speaking, Perl doesn't do this, but it
1206 relies on a reference-counting mechanism to keep things tidy.
1207 However, we rarely speak strictly and will often refer to the
1208 reference-counting scheme as a form of garbage collection. (If it's
1209 any comfort, when your interpreter exits, a "real" garbage collector
1210 runs to make sure everything is cleaned up if you've been messy with
1211 circular references and such.)
1221 Group ID--in Unix, the numeric group ID that the L</operating system>
1222 uses to identify you and members of your L</group>.
1226 Strictly, the shell's C<*> character, which will match a "glob" of
1227 characters when you're trying to generate a list of filenames.
1228 Loosely, the act of using globs and similar symbols to do pattern
1229 matching. See also L</fileglob> and L</typeglob>.
1233 Something you can see from anywhere, usually used of
1234 L<variables|/variable> and L<subroutines|/subroutine> that are visible
1235 everywhere in your program. In Perl, only certain special variables
1236 are truly global--most variables (and all subroutines) exist only in
1237 the current L</package>. Global variables can be declared with
1238 L<our|perlfunc/our>. See L<perlfunc/our>.
1240 =item global destruction
1242 The L</garbage collection> of globals (and the running of any
1243 associated object destructors) that takes place when a Perl
1244 L</interpreter> is being shut down. Global destruction should not be
1245 confused with the Apocalypse, except perhaps when it should.
1249 A language such as Perl that is good at hooking things together that
1250 weren't intended to be hooked together.
1254 The size of the pieces you're dealing with, mentally speaking.
1258 A L</subpattern> whose L</quantifier> wants to match as many things as
1263 Originally from the old Unix editor command for "Globally search for a
1264 Regular Expression and Print it", now used in the general sense of any
1265 kind of search, especially text searches. Perl has a built-in
1266 L<grep|perlfunc/grep> function that searches a list for elements
1267 matching any given criterion, whereas the I<grep>(1) program searches
1268 for lines matching a L</regular expression> in one or more files.
1272 A set of users of which you are a member. In some operating systems
1273 (like Unix), you can give certain file access permissions to other
1274 members of your group.
1278 An internal "glob value" typedef, holding a L</typeglob>. The L</GV>
1279 type is a subclass of L</SV>.
1289 Someone who is brilliantly persistent in solving technical problems,
1290 whether these involve golfing, fighting orcs, or programming. Hacker
1291 is a neutral term, morally speaking. Good hackers are not to be
1292 confused with evil L<crackers|/cracker> or clueless L<script
1293 kiddies|/script kiddie>. If you confuse them, we will presume that
1294 you are either evil or clueless.
1298 A L</subroutine> or L</method> that is called by Perl when your
1299 program needs to respond to some internal event, such as a L</signal>,
1300 or an encounter with an operator subject to L</operator overloading>.
1301 See also L</callback>.
1303 =item hard reference
1305 A L</scalar> L</value> containing the actual address of a
1306 L</referent>, such that the referent's L</reference> count accounts
1307 for it. (Some hard references are held internally, such as the
1308 implicit reference from one of a L</typeglob>'s variable slots to its
1309 corresponding referent.) A hard reference is different from a
1310 L</symbolic reference>.
1314 An unordered association of L</key>/L</value> pairs, stored such that
1315 you can easily use a string L</key> to look up its associated data
1316 L</value>. This glossary is like a hash, where the word to be defined
1317 is the key, and the definition is the value. A hash is also sometimes
1318 septisyllabically called an "associative array", which is a pretty
1319 good reason for simply calling it a "hash" instead.
1323 A data structure used internally by Perl for implementing associative
1324 arrays (hashes) efficiently. See also L</bucket>.
1328 A file containing certain required definitions that you must include
1329 "ahead" of the rest of your program to do certain obscure operations.
1330 A C header file has a I<.h> extension. Perl doesn't really have
1331 header files, though historically Perl has sometimes used translated
1332 I<.h> files with a I<.ph> extension. See L<perlfunc/require>.
1333 (Header files have been superseded by the L</module> mechanism.)
1337 So called because of a similar construct in L<shells|/shell> that
1338 pretends that the L<lines|/line> following the L</command> are a
1339 separate L</file> to be fed to the command, up to some terminating
1340 string. In Perl, however, it's just a fancy form of quoting.
1344 A number in base 16, "hex" for short. The digits for 10 through 16
1345 are customarily represented by the letters C<a> through C<f>.
1346 Hexadecimal constants in Perl start with C<0x>. See also
1349 =item home directory
1351 The directory you are put into when you log in. On a Unix system, the
1352 name is often placed into C<$ENV{HOME}> or C<$ENV{LOGDIR}> by
1353 I<login>, but you can also find it with C<(getpwuid($E<lt>))[7]>.
1354 (Some platforms do not have a concept of a home directory.)
1358 The computer on which a program or other data resides.
1362 Excessive pride, the sort of thing Zeus zaps you for. Also the
1363 quality that makes you write (and maintain) programs that other people
1364 won't want to say bad things about. Hence, the third great virtue of
1365 a programmer. See also L</laziness> and L</impatience>.
1369 Short for a "hash value" typedef, which holds Perl's internal
1370 representation of a hash. The L</HV> type is a subclass of L</SV>.
1380 A legally formed name for most anything in which a computer program
1381 might be interested. Many languages (including Perl) allow
1382 identifiers that start with a letter and contain letters and digits.
1383 Perl also counts the underscore character as a valid letter. (Perl
1384 also has more complicated names, such as L</qualified> names.)
1388 The anger you feel when the computer is being lazy. This makes you
1389 write programs that don't just react to your needs, but actually
1390 anticipate them. Or at least that pretend to. Hence, the second
1391 great virtue of a programmer. See also L</laziness> and L</hubris>.
1393 =item implementation
1395 How a piece of code actually goes about doing its job. Users of the
1396 code should not count on implementation details staying the same
1397 unless they are part of the published L</interface>.
1401 To gain access to symbols that are exported from another module. See
1406 To increase the value of something by 1 (or by some other number, if
1411 In olden days, the act of looking up a L</key> in an actual index
1412 (such as a phone book), but now merely the act of using any kind of
1413 key or position to find the corresponding L</value>, even if no index
1414 is involved. Things have degenerated to the point that Perl's
1415 L<index|perlfunc/index> function merely locates the position (index)
1416 of one string in another.
1418 =item indirect filehandle
1420 An L</expression> that evaluates to something that can be used as a
1421 L</filehandle>: a L</string> (filehandle name), a L</typeglob>, a
1422 typeglob L</reference>, or a low-level L</IO> object.
1424 =item indirect object
1426 In English grammar, a short noun phrase between a verb and its direct
1427 object indicating the beneficiary or recipient of the action. In
1428 Perl, C<print STDOUT "$foo\n";> can be understood as "verb
1429 indirect-object object" where L</STDOUT> is the recipient of the
1430 L<print|perlfunc/print> action, and C<"$foo"> is the object being
1431 printed. Similarly, when invoking a L</method>, you might place the
1432 invocant between the method and its arguments:
1434 $gollum = new Pathetic::Creature "Smeagol";
1435 give $gollum "Fisssssh!";
1436 give $gollum "Precious!";
1438 =item indirect object slot
1440 The syntactic position falling between a method call and its arguments
1441 when using the indirect object invocation syntax. (The slot is
1442 distinguished by the absence of a comma between it and the next
1443 argument.) L</STDERR> is in the indirect object slot here:
1445 print STDERR "Awake! Awake! Fear, Fire,
1450 If something in a program isn't the value you're looking for but
1451 indicates where the value is, that's indirection. This can be done
1452 with either L<symbolic references|/symbolic reference> or L<hard
1453 references|/hard reference>.
1457 An L</operator> that comes in between its L<operands|/operand>, such
1458 as multiplication in C<24 * 7>.
1462 What you get from your ancestors, genetically or otherwise. If you
1463 happen to be a L</class>, your ancestors are called L<base
1464 classes|/base class> and your descendants are called L<derived
1465 classes|/derived class>. See L</single inheritance> and L</multiple
1470 Short for "an instance of a class", meaning an L</object> of that L</class>.
1472 =item instance variable
1474 An L</attribute> of an L</object>; data stored with the particular
1475 object rather than with the class as a whole.
1479 A number with no fractional (decimal) part. A counting number, like
1480 1, 2, 3, and so on, but including 0 and the negatives.
1484 The services a piece of code promises to provide forever, in contrast to
1485 its L</implementation>, which it should feel free to change whenever it
1490 The insertion of a scalar or list value somewhere in the middle of
1491 another value, such that it appears to have been there all along. In
1492 Perl, variable interpolation happens in double-quoted strings and
1493 patterns, and list interpolation occurs when constructing the list of
1494 values to pass to a list operator or other such construct that takes a
1499 Strictly speaking, a program that reads a second program and does what
1500 the second program says directly without turning the program into a
1501 different form first, which is what L<compilers|/compiler> do. Perl
1502 is not an interpreter by this definition, because it contains a kind
1503 of compiler that takes a program and turns it into a more executable
1504 form (L<syntax trees|/syntax tree>) within the I<perl> process itself,
1505 which the Perl L</run time> system then interprets.
1509 The agent on whose behalf a L</method> is invoked. In a L</class>
1510 method, the invocant is a package name. In an L</instance> method,
1511 the invocant is an object reference.
1515 The act of calling up a deity, daemon, program, method, subroutine, or
1516 function to get it do what you think it's supposed to do. We usually
1517 "call" subroutines but "invoke" methods, since it sounds cooler.
1521 Input from, or output to, a L</file> or L</device>.
1525 An internal I/O object. Can also mean L</indirect object>.
1529 Internet Protocol, or Intellectual Property.
1533 Interprocess Communication.
1537 A relationship between two L<objects|/object> in which one object is
1538 considered to be a more specific version of the other, generic object:
1539 "A camel is a mammal." Since the generic object really only exists in
1540 a Platonic sense, we usually add a little abstraction to the notion of
1541 objects and think of the relationship as being between a generic
1542 L</base class> and a specific L</derived class>. Oddly enough,
1543 Platonic classes don't always have Platonic relationships--see
1548 Doing something repeatedly.
1552 A special programming gizmo that keeps track of where you are in
1553 something that you're trying to iterate over. The C<foreach> loop in
1554 Perl contains an iterator; so does a hash, allowing you to
1555 L<each|perlfunc/each> through it.
1559 The integer four, not to be confused with six, Tom's favorite editor.
1560 IV also means an internal Integer Value of the type a L</scalar> can
1561 hold, not to be confused with an L</NV>.
1571 "Just Another Perl Hacker," a clever but cryptic bit of Perl code that
1572 when executed, evaluates to that string. Often used to illustrate a
1573 particular Perl feature, and something of an ungoing Obfuscated Perl
1574 Contest seen in Usenix signatures.
1584 The string index to a L</hash>, used to look up the L</value>
1585 associated with that key.
1589 See L</reserved words>.
1599 A name you give to a L</statement> so that you can talk about that
1600 statement elsewhere in the program.
1604 The quality that makes you go to great effort to reduce overall energy
1605 expenditure. It makes you write labor-saving programs that other
1606 people will find useful, and document what you wrote so you don't have
1607 to answer so many questions about it. Hence, the first great virtue
1608 of a programmer. Also hence, this book. See also L</impatience> and
1613 A L</bit shift> that multiplies the number by some power of 2.
1615 =item leftmost longest
1617 The preference of the L</regular expression> engine to match the
1618 leftmost occurrence of a L</pattern>, then given a position at which a
1619 match will occur, the preference for the longest match (presuming the
1620 use of a L</greedy> quantifier). See L<perlre> for I<much> more on
1625 Fancy term for a L</token>.
1629 Fancy term for a L</tokener>.
1631 =item lexical analysis
1633 Fancy term for L</tokenizing>.
1635 =item lexical scoping
1637 Looking at your I<Oxford English Dictionary> through a microscope.
1638 (Also known as L</static scoping>, because dictionaries don't change
1639 very fast.) Similarly, looking at variables stored in a private
1640 dictionary (namespace) for each scope, which are visible only from
1641 their point of declaration down to the end of the lexical scope in
1642 which they are declared. --Syn. L</static scoping>.
1643 --Ant. L</dynamic scoping>.
1645 =item lexical variable
1647 A L</variable> subject to L</lexical scoping>, declared by
1648 L<my|perlfunc/my>. Often just called a "lexical". (The
1649 L<our|perlfunc/our> declaration declares a lexically scoped name for a
1650 global variable, which is not itself a lexical variable.)
1654 Generally, a collection of procedures. In ancient days, referred to a
1655 collection of subroutines in a I<.pl> file. In modern times, refers
1656 more often to the entire collection of Perl L<modules|/module> on your
1661 Last In, First Out. See also L</FIFO>. A LIFO is usually called a
1666 In Unix, a sequence of zero or more non-newline characters terminated
1667 with a L</newline> character. On non-Unix machines, this is emulated
1668 by the C library even if the underlying L</operating system> has
1671 =item line buffering
1673 Used by a L</standard IE<sol>O> output stream that flushes its
1674 L</buffer> after every L</newline>. Many standard I/O libraries
1675 automatically set up line buffering on output that is going to the
1680 The number of lines read previous to this one, plus 1. Perl keeps a
1681 separate line number for each source or input file it opens. The
1682 current source file's line number is represented by C<__LINE__>. The
1683 current input line number (for the file that was most recently read
1684 via C<< E<lt>FHE<gt> >>) is represented by the C<$.>
1685 (C<$INPUT_LINE_NUMBER>) variable. Many error messages report both
1686 values, if available.
1690 Used as a noun, a name in a L</directory>, representing a L</file>. A
1691 given file can have multiple links to it. It's like having the same
1692 phone number listed in the phone directory under different names. As
1693 a verb, to resolve a partially compiled file's unresolved symbols into
1694 a (nearly) executable image. Linking can generally be static or
1695 dynamic, which has nothing to do with static or dynamic scoping.
1699 A syntactic construct representing a comma-separated list of
1700 expressions, evaluated to produce a L</list value>. Each
1701 L</expression> in a L</LIST> is evaluated in L</list context> and
1702 interpolated into the list value.
1706 An ordered set of scalar values.
1710 The situation in which an L</expression> is expected by its
1711 surroundings (the code calling it) to return a list of values rather
1712 than a single value. Functions that want a L</LIST> of arguments tell
1713 those arguments that they should produce a list value. See also
1718 An L</operator> that does something with a list of values, such as
1719 L<join|perlfunc/join> or L<grep|perlfunc/grep>. Usually used for
1720 named built-in operators (such as L<print|perlfunc/print>,
1721 L<unlink|perlfunc/unlink>, and L<system|perlfunc/system>) that do not
1722 require parentheses around their L</argument> list.
1726 An unnamed list of temporary scalar values that may be passed around
1727 within a program from any list-generating function to any function or
1728 construct that provides a L</list context>.
1732 A token in a programming language such as a number or L</string> that
1733 gives you an actual L</value> instead of merely representing possible
1734 values as a L</variable> does.
1738 From Swift: someone who eats eggs little end first. Also used of
1739 computers that store the least significant L</byte> of a word at a
1740 lower byte address than the most significant byte. Often considered
1741 superior to big-endian machines. See also L</big-endian>.
1745 Not meaning the same thing everywhere. A global variable in Perl can
1746 be localized inside a L<dynamic scope|/dynamic scoping> via the
1747 L<local|perlfunc/local> operator.
1749 =item logical operator
1751 Symbols representing the concepts "and", "or", "xor", and "not".
1755 An L</assertion> that peeks at the string to the right of the current
1760 An L</assertion> that peeks at the string to the left of the current
1765 A construct that performs something repeatedly, like a roller coaster.
1767 =item loop control statement
1769 Any statement within the body of a loop that can make a loop
1770 prematurely stop looping or skip an L</iteration>. Generally you
1771 shouldn't try this on roller coasters.
1775 A kind of key or name attached to a loop (or roller coaster) so that
1776 loop control statements can talk about which loop they want to
1781 Able to serve as an L</lvalue>.
1785 Term used by language lawyers for a storage location you can assign a
1786 new L</value> to, such as a L</variable> or an element of an
1787 L</array>. The "l" is short for "left", as in the left side of an
1788 assignment, a typical place for lvalues. An L</lvaluable> function or
1789 expression is one to which a value may be assigned, as in C<pos($x) =
1792 =item lvalue modifier
1794 An adjectival pseudofunction that warps the meaning of an L</lvalue>
1795 in some declarative fashion. Currently there are three lvalue
1796 modifiers: L<my|perlfunc/my>, L<our|perlfunc/our>, and
1797 L<local|perlfunc/local>.
1807 Technically speaking, any extra semantics attached to a variable such
1808 as C<$!>, C<$0>, C<%ENV>, or C<%SIG>, or to any tied variable.
1809 Magical things happen when you diddle those variables.
1811 =item magical increment
1813 An L</increment> operator that knows how to bump up alphabetics as
1816 =item magical variables
1818 Special variables that have side effects when you access them or
1819 assign to them. For example, in Perl, changing elements of the
1820 C<%ENV> array also changes the corresponding environment variables
1821 that subprocesses will use. Reading the C<$!> variable gives you the
1822 current system error number or message.
1826 A file that controls the compilation of a program. Perl programs
1827 don't usually need a L</Makefile> because the Perl compiler has plenty
1832 The Unix program that displays online documentation (manual pages) for
1837 A "page" from the manuals, typically accessed via the I<man>(1)
1838 command. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of
1839 BUGS, and so on, and is typically longer than a page. There are
1840 manpages documenting L<commands|/command>, L<syscalls|/syscall>,
1841 L</library> L<functions|/function>, L<devices|/device>,
1842 L<protocols|/protocol>, L<files|/file>, and such. In this book, we
1843 call any piece of standard Perl documentation (like I<perlop> or
1844 I<perldelta>) a manpage, no matter what format it's installed in on
1849 See L</pattern matching>.
1853 See L</instance variable>.
1857 This always means your main memory, not your disk. Clouding the issue
1858 is the fact that your machine may implement L</virtual> memory; that
1859 is, it will pretend that it has more memory than it really does, and
1860 it'll use disk space to hold inactive bits. This can make it seem
1861 like you have a little more memory than you really do, but it's not a
1862 substitute for real memory. The best thing that can be said about
1863 virtual memory is that it lets your performance degrade gradually
1864 rather than suddenly when you run out of real memory. But your
1865 program can die when you run out of virtual memory too, if you haven't
1866 thrashed your disk to death first.
1870 A L</character> that is I<not> supposed to be treated normally. Which
1871 characters are to be treated specially as metacharacters varies
1872 greatly from context to context. Your L</shell> will have certain
1873 metacharacters, double-quoted Perl L<strings|/string> have other
1874 metacharacters, and L</regular expression> patterns have all the
1875 double-quote metacharacters plus some extra ones of their own.
1879 Something we'd call a L</metacharacter> except that it's a sequence of
1880 more than one character. Generally, the first character in the
1881 sequence must be a true metacharacter to get the other characters in
1882 the metasymbol to misbehave along with it.
1886 A kind of action that an L</object> can take if you tell it to. See
1891 The belief that "small is beautiful." Paradoxically, if you say
1892 something in a small language, it turns out big, and if you say it in
1893 a big language, it turns out small. Go figure.
1897 In the context of the L<stat> syscall, refers to the field holding
1898 the L</permission bits> and the type of the L</file>.
1902 See L</statement modifier>, L</regular expression modifier>, and
1903 L</lvalue modifier>, not necessarily in that order.
1907 A L</file> that defines a L</package> of (almost) the same name, which
1908 can either L</export> symbols or function as an L</object> class. (A
1909 module's main I<.pm> file may also load in other files in support of
1910 the module.) See the L<use|perlfunc/use> built-in.
1914 An integer divisor when you're interested in the remainder instead of
1919 Short for Perl Monger, a purveyor of Perl.
1923 A temporary value scheduled to die when the current statement
1926 =item multidimensional array
1928 An array with multiple subscripts for finding a single element. Perl
1929 implements these using L<references|/reference>--see L<perllol> and
1932 =item multiple inheritance
1934 The features you got from your mother and father, mixed together
1935 unpredictably. (See also L</inheritance>, and L</single
1936 inheritance>.) In computer languages (including Perl), the notion
1937 that a given class may have multiple direct ancestors or L<base
1938 classes|/base class>.
1948 A L</pipe> with a name embedded in the L</filesystem> so that it can
1949 be accessed by two unrelated L<processes|/process>.
1953 A domain of names. You needn't worry about whether the names in one
1954 such domain have been used in another. See L</package>.
1956 =item network address
1958 The most important attribute of a socket, like your telephone's
1959 telephone number. Typically an IP address. See also L</port>.
1963 A single character that represents the end of a line, with the ASCII
1964 value of 012 octal under Unix (but 015 on a Mac), and represented by
1965 C<\n> in Perl strings. For Windows machines writing text files, and
1966 for certain physical devices like terminals, the single newline gets
1967 automatically translated by your C library into a line feed and a
1968 carriage return, but normally, no translation is done.
1972 Network File System, which allows you to mount a remote filesystem as
1975 =item null character
1977 A character with the ASCII value of zero. It's used by C to terminate
1978 strings, but Perl allows strings to contain a null.
1982 A L</list value> with zero elements, represented in Perl by C<()>.
1986 A L</string> containing no characters, not to be confused with a
1987 string containing a L</null character>, which has a positive length
1990 =item numeric context
1992 The situation in which an expression is expected by its surroundings
1993 (the code calling it) to return a number. See also L</context> and
1998 Short for Nevada, no part of which will ever be confused with
1999 civilization. NV also means an internal floating-point Numeric Value
2000 of the type a L</scalar> can hold, not to be confused with an L</IV>.
2004 Half a L</byte>, equivalent to one L</hexadecimal> digit, and worth
2015 An L</instance> of a L</class>. Something that "knows" what
2016 user-defined type (class) it is, and what it can do because of what
2017 class it is. Your program can request an object to do things, but the
2018 object gets to decide whether it wants to do them or not. Some
2019 objects are more accommodating than others.
2023 A number in base 8. Only the digits 0 through 7 are allowed. Octal
2024 constants in Perl start with 0, as in 013. See also the
2025 L<oct|perlfunc/oct> function.
2029 How many things you have to skip over when moving from the beginning
2030 of a string or array to a specific position within it. Thus, the
2031 minimum offset is zero, not one, because you don't skip anything to
2032 get to the first item.
2036 An entire computer program crammed into one line of text.
2038 =item open source software
2040 Programs for which the source code is freely available and freely
2041 redistributable, with no commercial strings attached. For a more
2042 detailed definition, see L<http://www.opensource.org/osd.html>.
2046 An L</expression> that yields a L</value> that an L</operator>
2047 operates on. See also L</precedence>.
2049 =item operating system
2051 A special program that runs on the bare machine and hides the gory
2052 details of managing L<processes|/process> and L<devices|/device>.
2053 Usually used in a looser sense to indicate a particular culture of
2054 programming. The loose sense can be used at varying levels of
2055 specificity. At one extreme, you might say that all versions of Unix
2056 and Unix-lookalikes are the same operating system (upsetting many
2057 people, especially lawyers and other advocates). At the other
2058 extreme, you could say this particular version of this particular
2059 vendor's operating system is different from any other version of this
2060 or any other vendor's operating system. Perl is much more portable
2061 across operating systems than many other languages. See also
2062 L</architecture> and L</platform>.
2066 A gizmo that transforms some number of input values to some number of
2067 output values, often built into a language with a special syntax or
2068 symbol. A given operator may have specific expectations about what
2069 L<types|/type> of data you give as its arguments
2070 (L<operands|/operand>) and what type of data you want back from it.
2072 =item operator overloading
2074 A kind of L</overloading> that you can do on built-in
2075 L<operators|/operator> to make them work on L<objects|/object> as if
2076 the objects were ordinary scalar values, but with the actual semantics
2077 supplied by the object class. This is set up with the L<overload>
2082 See either L<switches|/switch> or L</regular expression modifier>.
2086 Giving additional meanings to a symbol or construct. Actually, all
2087 languages do overloading to one extent or another, since people are
2088 good at figuring out things from L</context>.
2092 Hiding or invalidating some other definition of the same name. (Not
2093 to be confused with L</overloading>, which adds definitions that must
2094 be disambiguated some other way.) To confuse the issue further, we use
2095 the word with two overloaded definitions: to describe how you can
2096 define your own L</subroutine> to hide a built-in L</function> of the
2097 same name (see L<perlsub/Overriding Built-in Functions>) and to
2098 describe how you can define a replacement L</method> in a L</derived
2099 class> to hide a L</base class>'s method of the same name (see
2104 The one user (apart from the superuser) who has absolute control over
2105 a L</file>. A file may also have a L</group> of users who may
2106 exercise joint ownership if the real owner permits it. See
2107 L</permission bits>.
2117 A L</namespace> for global L<variables|/variable>,
2118 L<subroutines|/subroutine>, and the like, such that they can be kept
2119 separate from like-named L<symbols|/symbol> in other namespaces. In a
2120 sense, only the package is global, since the symbols in the package's
2121 symbol table are only accessible from code compiled outside the
2122 package by naming the package. But in another sense, all package
2123 symbols are also globals--they're just well-organized globals.
2127 Short for L</scratchpad>.
2139 See L</syntax tree>.
2143 The subtle but sometimes brutal art of attempting to turn your
2144 possibly malformed program into a valid L</syntax tree>.
2148 To fix by applying one, as it were. In the realm of hackerdom, a
2149 listing of the differences between two versions of a program as might
2150 be applied by the I<patch>(1) program when you want to fix a bug or
2151 upgrade your old version.
2155 The list of L<directories|/directory> the system searches to find a
2156 program you want to L</execute>. The list is stored as one of your
2157 L<environment variables|/environment variable>, accessible in Perl as
2162 A fully qualified filename such as I</usr/bin/perl>. Sometimes
2163 confused with L</PATH>.
2167 A template used in L</pattern matching>.
2169 =item pattern matching
2171 Taking a pattern, usually a L</regular expression>, and trying the
2172 pattern various ways on a string to see whether there's any way to
2173 make it fit. Often used to pick interesting tidbits out of a file.
2175 =item permission bits
2177 Bits that the L</owner> of a file sets or unsets to allow or disallow
2178 access to other people. These flag bits are part of the L</mode> word
2179 returned by the L<stat|perlfunc/stat> built-in when you ask about a
2180 file. On Unix systems, you can check the I<ls>(1) manpage for more
2185 What you get when you do C<Perl++> twice. Doing it only once will
2186 curl your hair. You have to increment it eight times to shampoo your
2187 hair. Lather, rinse, iterate.
2191 A direct L</connection> that carries the output of one L</process> to
2192 the input of another without an intermediate temporary file. Once the
2193 pipe is set up, the two processes in question can read and write as if
2194 they were talking to a normal file, with some caveats.
2198 A series of L<processes|/process> all in a row, linked by
2199 L<pipes|/pipe>, where each passes its output stream to the next.
2203 The entire hardware and software context in which a program runs. A
2204 program written in a platform-dependent language might break if you
2205 change any of: machine, operating system, libraries, compiler, or
2206 system configuration. The I<perl> interpreter has to be compiled
2207 differently for each platform because it is implemented in C, but
2208 programs written in the Perl language are largely
2209 platform-independent.
2213 The markup used to embed documentation into your Perl code. See
2218 A L</variable> in a language like C that contains the exact memory
2219 location of some other item. Perl handles pointers internally so you
2220 don't have to worry about them. Instead, you just use symbolic
2221 pointers in the form of L<keys|/key> and L</variable> names, or L<hard
2222 references|/hard reference>, which aren't pointers (but act like
2223 pointers and do in fact contain pointers).
2227 The notion that you can tell an L</object> to do something generic,
2228 and the object will interpret the command in different ways depending
2229 on its type. [E<lt>Gk many shapes]
2233 The part of the address of a TCP or UDP socket that directs packets to
2234 the correct process after finding the right machine, something like
2235 the phone extension you give when you reach the company operator.
2236 Also, the result of converting code to run on a different platform
2237 than originally intended, or the verb denoting this conversion.
2241 Once upon a time, C code compilable under both BSD and SysV. In
2242 general, code that can be easily converted to run on another
2243 L</platform>, where "easily" can be defined however you like, and
2244 usually is. Anything may be considered portable if you try hard
2245 enough. See I<mobile home> or I<London Bridge>.
2249 Someone who "carries" software from one L</platform> to another.
2250 Porting programs written in platform-dependent languages such as C can
2251 be difficult work, but porting programs like Perl is very much worth
2256 The Portable Operating System Interface specification.
2260 An L</operator> that follows its L</operand>, as in C<$x++>.
2264 An internal shorthand for a "push-pop" code, that is, C code
2265 implementing Perl's stack machine.
2269 A standard module whose practical hints and suggestions are received
2270 (and possibly ignored) at compile time. Pragmas are named in all
2275 The rules of conduct that, in the absence of other guidance, determine
2276 what should happen first. For example, in the absence of parentheses,
2277 you always do multiplication before addition.
2281 An L</operator> that precedes its L</operand>, as in C<++$x>.
2285 What some helper L</process> did to transform the incoming data into a
2286 form more suitable for the current process. Often done with an
2287 incoming L</pipe>. See also L</C preprocessor>.
2295 An instance of a running program. Under multitasking systems like
2296 Unix, two or more separate processes could be running the same program
2297 independently at the same time--in fact, the L<fork|perlfunc/fork>
2298 function is designed to bring about this happy state of affairs.
2299 Under other operating systems, processes are sometimes called
2300 "threads", "tasks", or "jobs", often with slight nuances in meaning.
2302 =item program generator
2304 A system that algorithmically writes code for you in a high-level
2305 language. See also L</code generator>.
2307 =item progressive matching
2309 L<Pattern matching|/pattern matching> that picks up where it left off before.
2313 See either L</instance variable> or L</character property>.
2317 In networking, an agreed-upon way of sending messages back and forth
2318 so that neither correspondent will get too confused.
2322 An optional part of a L</subroutine> declaration telling the Perl
2323 compiler how many and what flavor of arguments may be passed as
2324 L</actual arguments>, so that you can write subroutine calls that
2325 parse much like built-in functions. (Or don't parse, as the case may
2328 =item pseudofunction
2330 A construct that sometimes looks like a function but really isn't.
2331 Usually reserved for L</lvalue> modifiers like L<my|perlfunc/my>, for
2332 L</context> modifiers like L<scalar|perlfunc/scalar>, and for the
2333 pick-your-own-quotes constructs, C<q//>, C<qq//>, C<qx//>, C<qw//>,
2334 C<qr//>, C<m//>, C<s///>, C<y///>, and C<tr///>.
2338 A reference to an array whose initial element happens to hold a
2339 reference to a hash. You can treat a pseudohash reference as either
2340 an array reference or a hash reference.
2344 An L</operator> that looks something like a L</literal>, such as the
2345 output-grabbing operator, C<`>I<C<command>>C<`>.
2349 Something not owned by anybody. Perl is copyrighted and is thus
2350 I<not> in the public domain--it's just L</freely available> and
2351 L</freely redistributable>.
2355 A notional "baton" handed around the Perl community indicating who is
2356 the lead integrator in some arena of development.
2360 A L</pumpkin> holder, the person in charge of pumping the pump, or at
2361 least priming it. Must be willing to play the part of the Great
2362 Pumpkin now and then.
2366 A "pointer value", which is Perl Internals Talk for a C<char*>.
2376 Possessing a complete name. The symbol C<$Ent::moot> is qualified;
2377 C<$moot> is unqualified. A fully qualified filename is specified from
2378 the top-level directory.
2382 A component of a L</regular expression> specifying how many times the
2383 foregoing L</atom> may occur.
2393 With respect to files, one that has the proper permission bit set to
2394 let you access the file. With respect to computer programs, one
2395 that's written well enough that someone has a chance of figuring out
2396 what it's trying to do.
2400 The last rites performed by a parent L</process> on behalf of a
2401 deceased child process so that it doesn't remain a L</zombie>. See
2402 the L<wait|perlfunc/wait> and L<waitpid|perlfunc/waitpid> function
2407 A set of related data values in a L</file> or L</stream>, often
2408 associated with a unique L</key> field. In Unix, often commensurate
2409 with a L</line>, or a blank-line-terminated set of lines (a
2410 "paragraph"). Each line of the I</etc/passwd> file is a record, keyed
2411 on login name, containing information about that user.
2415 The art of defining something (at least partly) in terms of itself,
2416 which is a naughty no-no in dictionaries but often works out okay in
2417 computer programs if you're careful not to recurse forever, which is
2418 like an infinite loop with more spectacular failure modes.
2422 Where you look to find a pointer to information somewhere else. (See
2423 L</indirection>.) References come in two flavors, L<symbolic
2424 references|/symbolic reference> and L<hard references|/hard
2429 Whatever a reference refers to, which may or may not have a name.
2430 Common types of referents include scalars, arrays, hashes, and
2435 See L</regular expression>.
2437 =item regular expression
2439 A single entity with various interpretations, like an elephant. To a
2440 computer scientist, it's a grammar for a little language in which some
2441 strings are legal and others aren't. To normal people, it's a pattern
2442 you can use to find what you're looking for when it varies from case
2443 to case. Perl's regular expressions are far from regular in the
2444 theoretical sense, but in regular use they work quite well. Here's a
2445 regular expression: C</Oh s.*t./>. This will match strings like "C<Oh
2446 say can you see by the dawn's early light>" and "C<Oh sit!>". See
2449 =item regular expression modifier
2451 An option on a pattern or substitution, such as C</i> to render the
2452 pattern case insensitive. See also L</cloister>.
2456 A L</file> that's not a L</directory>, a L</device>, a named L</pipe>
2457 or L</socket>, or a L</symbolic link>. Perl uses the C<-f> file test
2458 operator to identify regular files. Sometimes called a "plain" file.
2460 =item relational operator
2462 An L</operator> that says whether a particular ordering relationship
2463 is L</true> about a pair of L<operands|/operand>. Perl has both
2464 numeric and string relational operators. See L</collating sequence>.
2466 =item reserved words
2468 A word with a specific, built-in meaning to a L</compiler>, such as
2469 C<if> or L<delete|perlfunc/delete>. In many languages (not Perl),
2470 it's illegal to use reserved words to name anything else. (Which is
2471 why they're reserved, after all.) In Perl, you just can't use them to
2472 name L<labels|/label> or L<filehandles|/filehandle>. Also called
2477 The L</value> produced by a L</subroutine> or L</expression> when
2478 evaluated. In Perl, a return value may be either a L</list> or a
2483 Request For Comment, which despite the timid connotations is the name
2484 of a series of important standards documents.
2488 A L</bit shift> that divides a number by some power of 2.
2492 The superuser (UID == 0). Also, the top-level directory of the
2497 What you are told when someone thinks you should Read The Fine Manual.
2501 Any time after Perl starts running your main program. See also
2502 L</compile phase>. Run phase is mostly spent in L</run time> but may
2503 also be spent in L</compile time> when L<require|perlfunc/require>,
2504 L<do|perlfunc/do> C<FILE>, or L<eval|perlfunc/eval> C<STRING>
2505 operators are executed or when a substitution uses the C</ee>
2510 The time when Perl is actually doing what your code says to do, as
2511 opposed to the earlier period of time when it was trying to figure out
2512 whether what you said made any sense whatsoever, which is L</compile
2515 =item run-time pattern
2517 A pattern that contains one or more variables to be interpolated
2518 before parsing the pattern as a L</regular expression>, and that
2519 therefore cannot be analyzed at compile time, but must be re-analyzed
2520 each time the pattern match operator is evaluated. Run-time patterns
2521 are useful but expensive.
2525 A recreational vehicle, not to be confused with vehicular recreation.
2526 RV also means an internal Reference Value of the type a L</scalar> can
2527 hold. See also L</IV> and L</NV> if you're not confused yet.
2531 A L</value> that you might find on the right side of an
2532 L</assignment>. See also L</lvalue>.
2542 A simple, singular value; a number, L</string>, or L</reference>.
2544 =item scalar context
2546 The situation in which an L</expression> is expected by its
2547 surroundings (the code calling it) to return a single L</value> rather
2548 than a L</list> of values. See also L</context> and L</list context>.
2549 A scalar context sometimes imposes additional constraints on the
2550 return value--see L</string context> and L</numeric context>.
2551 Sometimes we talk about a L</Boolean context> inside conditionals, but
2552 this imposes no additional constraints, since any scalar value,
2553 whether numeric or L</string>, is already true or false.
2555 =item scalar literal
2557 A number or quoted L</string>--an actual L</value> in the text of your
2558 program, as opposed to a L</variable>.
2562 A value that happens to be a L</scalar> as opposed to a L</list>.
2564 =item scalar variable
2566 A L</variable> prefixed with C<$> that holds a single value.
2570 How far away you can see a variable from, looking through one. Perl
2571 has two visibility mechanisms: it does L</dynamic scoping> of
2572 L<local|perlfunc/local> L<variables|/variable>, meaning that the rest
2573 of the L</block>, and any L<subroutines|/subroutine> that are called
2574 by the rest of the block, can see the variables that are local to the
2575 block. Perl does L</lexical scoping> of L<my|perlfunc/my> variables,
2576 meaning that the rest of the block can see the variable, but other
2577 subroutines called by the block I<cannot> see the variable.
2581 The area in which a particular invocation of a particular file or
2582 subroutine keeps some of its temporary values, including any lexically
2587 A text L</file> that is a program intended to be L<executed|/execute>
2588 directly rather than L<compiled|/compiler> to another form of file
2589 before execution. Also, in the context of L</Unicode>, a writing
2590 system for a particular language or group of languages, such as Greek,
2591 Bengali, or Klingon.
2595 A L</cracker> who is not a L</hacker>, but knows just enough to run
2596 canned scripts. A cargo-cult programmer.
2600 A venerable Stream EDitor from which Perl derives some of its ideas.
2604 A fancy kind of interlock that prevents multiple L<threads|/thread> or
2605 L<processes|/process> from using up the same resources simultaneously.
2609 A L</character> or L</string> that keeps two surrounding strings from
2610 being confused with each other. The L<split|perlfunc/split> function
2611 works on separators. Not to be confused with L<delimiters|/delimiter>
2612 or L<terminators|/terminator>. The "or" in the previous sentence
2613 separated the two alternatives.
2617 Putting a fancy L</data structure> into linear order so that it can be
2618 stored as a L</string> in a disk file or database or sent through a
2619 L</pipe>. Also called marshalling.
2623 In networking, a L</process> that either advertises a L</service> or
2624 just hangs around at a known location and waits for L<clients|/client>
2625 who need service to get in touch with it.
2629 Something you do for someone else to make them happy, like giving them
2630 the time of day (or of their life). On some machines, well-known
2631 services are listed by the L<getservent|perlfunc/getservent> function.
2635 Same as L</setuid>, only having to do with giving away L</group>
2640 Said of a program that runs with the privileges of its L</owner>
2641 rather than (as is usually the case) the privileges of whoever is
2642 running it. Also describes the bit in the mode word (L</permission
2643 bits>) that controls the feature. This bit must be explicitly set by
2644 the owner to enable this feature, and the program must be carefully
2645 written not to give away more privileges than it ought to.
2649 A piece of L</memory> accessible by two different
2650 L<processes|/process> who otherwise would not see each other's memory.
2654 Irish for the whole McGillicuddy. In Perl culture, a portmanteau of
2655 "sharp" and "bang", meaning the C<#!> sequence that tells the system
2656 where to find the interpreter.
2660 A L</command>-line L</interpreter>. The program that interactively
2661 gives you a prompt, accepts one or more L<lines|/line> of input, and
2662 executes the programs you mentioned, feeding each of them their proper
2663 L<arguments|/argument> and input data. Shells can also execute
2664 scripts containing such commands. Under Unix, typical shells include
2665 the Bourne shell (I</bin/sh>), the C shell (I</bin/csh>), and the Korn
2666 shell (I</bin/ksh>). Perl is not strictly a shell because it's not
2667 interactive (although Perl programs can be interactive).
2671 Something extra that happens when you evaluate an L</expression>.
2672 Nowadays it can refer to almost anything. For example, evaluating a
2673 simple assignment statement typically has the "side effect" of
2674 assigning a value to a variable. (And you thought assigning the value
2675 was your primary intent in the first place!) Likewise, assigning a
2676 value to the special variable C<$|> (C<$AUTOFLUSH>) has the side
2677 effect of forcing a flush after every L<write|perlfunc/write> or
2678 L<print|perlfunc/print> on the currently selected filehandle.
2682 A bolt out of the blue; that is, an event triggered by the
2683 L</operating system>, probably when you're least expecting it.
2685 =item signal handler
2687 A L</subroutine> that, instead of being content to be called in the
2688 normal fashion, sits around waiting for a bolt out of the blue before
2689 it will deign to L</execute>. Under Perl, bolts out of the blue are
2690 called signals, and you send them with the L<kill|perlfunc/kill>
2691 built-in. See L<perlvar/%SIG> and L<perlipc/Signals>.
2693 =item single inheritance
2695 The features you got from your mother, if she told you that you don't
2696 have a father. (See also L</inheritance> and L</multiple
2697 inheritance>.) In computer languages, the notion that
2698 L<classes|/class> reproduce asexually so that a given class can only
2699 have one direct ancestor or L</base class>. Perl supplies no such
2700 restriction, though you may certainly program Perl that way if you
2705 A selection of any number of L<elements|/element> from a L</list>,
2706 L</array>, or L</hash>.
2710 To read an entire L</file> into a L</string> in one operation.
2714 An endpoint for network communication among multiple
2715 L<processes|/process> that works much like a telephone or a post
2716 office box. The most important thing about a socket is its L</network
2717 address> (like a phone number). Different kinds of sockets have
2718 different kinds of addresses--some look like filenames, and some
2721 =item soft reference
2723 See L</symbolic reference>.
2727 A special kind of L</module> that does L</preprocessing> on your
2728 script just before it gets to the L</tokener>.
2732 A device you can put things on the top of, and later take them back
2733 off in the opposite order in which you put them on. See L</LIFO>.
2737 Included in the official Perl distribution, as in a standard module, a
2738 standard tool, or a standard Perl L</manpage>.
2740 =item standard error
2742 The default output L</stream> for nasty remarks that don't belong in
2743 L</standard output>. Represented within a Perl program by the
2744 L</filehandle> L</STDERR>. You can use this stream explicitly, but the
2745 L<die|perlfunc/die> and L<warn|perlfunc/warn> built-ins write to your
2746 standard error stream automatically.
2750 A standard C library for doing L<buffered|/buffer> input and output to
2751 the L</operating system>. (The "standard" of standard I/O is only
2752 marginally related to the "standard" of standard input and output.)
2753 In general, Perl relies on whatever implementation of standard I/O a
2754 given operating system supplies, so the buffering characteristics of a
2755 Perl program on one machine may not exactly match those on another
2756 machine. Normally this only influences efficiency, not semantics. If
2757 your standard I/O package is doing block buffering and you want it to
2758 L</flush> the buffer more often, just set the C<$|> variable to a true
2761 =item standard input
2763 The default input L</stream> for your program, which if possible
2764 shouldn't care where its data is coming from. Represented within a
2765 Perl program by the L</filehandle> L</STDIN>.
2767 =item standard output
2769 The default output L</stream> for your program, which if possible
2770 shouldn't care where its data is going. Represented within a Perl
2771 program by the L</filehandle> L</STDOUT>.
2773 =item stat structure
2775 A special internal spot in which Perl keeps the information about the
2776 last L</file> on which you requested information.
2780 A L</command> to the computer about what to do next, like a step in a
2781 recipe: "Add marmalade to batter and mix until mixed." A statement is
2782 distinguished from a L</declaration>, which doesn't tell the computer
2783 to do anything, but just to learn something.
2785 =item statement modifier
2787 A L</conditional> or L</loop> that you put after the L</statement>
2788 instead of before, if you know what we mean.
2792 Varying slowly compared to something else. (Unfortunately, everything
2793 is relatively stable compared to something else, except for certain
2794 elementary particles, and we're not so sure about them.) In
2795 computers, where things are supposed to vary rapidly, "static" has a
2796 derogatory connotation, indicating a slightly dysfunctional
2797 L</variable>, L</subroutine>, or L</method>. In Perl culture, the
2798 word is politely avoided.
2802 No such thing. See L</class method>.
2804 =item static scoping
2806 No such thing. See L</lexical scoping>.
2808 =item static variable
2810 No such thing. Just use a L</lexical variable> in a scope larger than
2811 your L</subroutine>.
2815 The L</value> returned to the parent L</process> when one of its child
2816 processes dies. This value is placed in the special variable C<$?>.
2817 Its upper eight L<bits|/bit> are the exit status of the defunct
2818 process, and its lower eight bits identify the signal (if any) that
2819 the process died from. On Unix systems, this status value is the same
2820 as the status word returned by I<wait>(2). See L<perlfunc/system>.
2824 See L</standard error>.
2828 See L</standard input>.
2832 See L</standard IE<sol>O>.
2836 See L</standard output>.
2840 A flow of data into or out of a process as a steady sequence of bytes
2841 or characters, without the appearance of being broken up into packets.
2842 This is a kind of L</interface>--the underlying L</implementation> may
2843 well break your data up into separate packets for delivery, but this
2848 A sequence of characters such as "He said !@#*&%@#*?!". A string does
2849 not have to be entirely printable.
2851 =item string context
2853 The situation in which an expression is expected by its surroundings
2854 (the code calling it) to return a L</string>. See also L</context>
2855 and L</numeric context>.
2857 =item stringification
2859 The process of producing a L</string> representation of an abstract
2864 C keyword introducing a structure definition or name.
2868 See L</data structure>.
2872 See L</derived class>.
2876 A component of a L</regular expression> pattern.
2880 A named or otherwise accessible piece of program that can be invoked
2881 from elsewhere in the program in order to accomplish some sub-goal of
2882 the program. A subroutine is often parameterized to accomplish
2883 different but related things depending on its input
2884 L<arguments|/argument>. If the subroutine returns a meaningful
2885 L</value>, it is also called a L</function>.
2889 A L</value> that indicates the position of a particular L</array>
2890 L</element> in an array.
2894 Changing parts of a string via the C<s///> operator. (We avoid use of
2895 this term to mean L</variable interpolation>.)
2899 A portion of a L</string>, starting at a certain L</character>
2900 position (L</offset>) and proceeding for a certain number of
2909 The person whom the L</operating system> will let do almost anything.
2910 Typically your system administrator or someone pretending to be your
2911 system administrator. On Unix systems, the L</root> user. On Windows
2912 systems, usually the Administrator user.
2916 Short for "scalar value". But within the Perl interpreter every
2917 L</referent> is treated as a member of a class derived from SV, in an
2918 object-oriented sort of way. Every L</value> inside Perl is passed
2919 around as a C language C<SV*> pointer. The SV L</struct> knows its
2920 own "referent type", and the code is smart enough (we hope) not to try
2921 to call a L</hash> function on a L</subroutine>.
2925 An option you give on a command line to influence the way your program
2926 works, usually introduced with a minus sign. The word is also used as
2927 a nickname for a L</switch statement>.
2929 =item switch cluster
2931 The combination of multiple command-line switches (e.g., B<-a -b -c>)
2932 into one switch (e.g., B<-abc>). Any switch with an additional
2933 L</argument> must be the last switch in a cluster.
2935 =item switch statement
2937 A program technique that lets you evaluate an L</expression> and then,
2938 based on the value of the expression, do a multiway branch to the
2939 appropriate piece of code for that value. Also called a "case
2940 structure", named after the similar Pascal construct. Most switch
2941 statements in Perl are spelled C<for>. See L<perlsyn/Basic BLOCKs and
2946 Generally, any L</token> or L</metasymbol>. Often used more
2947 specifically to mean the sort of name you might find in a L</symbol
2952 Where a L</compiler> remembers symbols. A program like Perl must
2953 somehow remember all the names of all the L<variables|/variable>,
2954 L<filehandles|/filehandle>, and L<subroutines|/subroutine> you've
2955 used. It does this by placing the names in a symbol table, which is
2956 implemented in Perl using a L</hash table>. There is a separate
2957 symbol table for each L</package> to give each package its own
2960 =item symbolic debugger
2962 A program that lets you step through the L<execution|/execute> of your
2963 program, stopping or printing things out here and there to see whether
2964 anything has gone wrong, and if so, what. The "symbolic" part just
2965 means that you can talk to the debugger using the same symbols with
2966 which your program is written.
2970 An alternate filename that points to the real L</filename>, which in
2971 turn points to the real L</file>. Whenever the L</operating system>
2972 is trying to parse a L</pathname> containing a symbolic link, it
2973 merely substitutes the new name and continues parsing.
2975 =item symbolic reference
2977 A variable whose value is the name of another variable or subroutine.
2978 By L<dereferencing|/dereference> the first variable, you can get at
2979 the second one. Symbolic references are illegal under L<use strict
2980 'refs'|strict/strict refs>.
2984 Programming in which the orderly sequence of events can be determined;
2985 that is, when things happen one after the other, not at the same time.
2987 =item syntactic sugar
2989 An alternative way of writing something more easily; a shortcut.
2993 From Greek, "with-arrangement". How things (particularly symbols) are
2994 put together with each other.
2998 An internal representation of your program wherein lower-level
2999 L<constructs|/construct> dangle off the higher-level constructs
3004 A L</function> call directly to the L</operating system>. Many of the
3005 important subroutines and functions you use aren't direct system
3006 calls, but are built up in one or more layers above the system call
3007 level. In general, Perl programmers don't need to worry about the
3008 distinction. However, if you do happen to know which Perl functions
3009 are really syscalls, you can predict which of these will set the C<$!>
3010 (C<$ERRNO>) variable on failure. Unfortunately, beginning programmers
3011 often confusingly employ the term "system call" to mean what happens
3012 when you call the Perl L<system|perlfunc/system> function, which
3013 actually involves many syscalls. To avoid any confusion, we nearly
3014 always use say "syscall" for something you could call indirectly via
3015 Perl's L<syscall|perlfunc/syscall> function, and never for something
3016 you would call with Perl's L<system|perlfunc/system> function.
3026 Said of data derived from the grubby hands of a user and thus unsafe
3027 for a secure program to rely on. Perl does taint checks if you run a
3028 L</setuid> (or L</setgid>) program, or if you use the B<-T> switch.
3032 Short for Transmission Control Protocol. A protocol wrapped around
3033 the Internet Protocol to make an unreliable packet transmission
3034 mechanism appear to the application program to be a reliable
3035 L</stream> of bytes. (Usually.)
3039 Short for a "terminal", that is, a leaf node of a L</syntax tree>. A
3040 thing that functions grammatically as an L</operand> for the operators
3045 A L</character> or L</string> that marks the end of another string.
3046 The C<$/> variable contains the string that terminates a
3047 L<readline|perlfunc/readline> operation, which L<chomp|perlfunc/chomp>
3048 deletes from the end. Not to be confused with
3049 L<delimiters|/delimiter> or L<separators|/separator>. The period at
3050 the end of this sentence is a terminator.
3054 An L</operator> taking three L<operands|/operand>. Sometimes
3055 pronounced L</trinary>.
3059 A L</string> or L</file> containing primarily printable characters.
3063 Like a forked process, but without L</fork>'s inherent memory
3064 protection. A thread is lighter weight than a full process, in that a
3065 process could have multiple threads running around in it, all fighting
3066 over the same process's memory space unless steps are taken to protect
3067 threads from each other. See L<threads>.
3071 The bond between a magical variable and its implementation class. See
3072 L<perlfunc/tie> and L<perltie>.
3076 There's More Than One Way To Do It, the Perl Motto. The notion that
3077 there can be more than one valid path to solving a programming problem
3078 in context. (This doesn't mean that more ways are always better or
3079 that all possible paths are equally desirable--just that there need
3080 not be One True Way.) Pronounced TimToady.
3084 A morpheme in a programming language, the smallest unit of text with
3085 semantic significance.
3089 A module that breaks a program text into a sequence of
3090 L<tokens|/token> for later analysis by a parser.
3094 Splitting up a program text into L<tokens|/token>. Also known as
3095 "lexing", in which case you get "lexemes" instead of tokens.
3097 =item toolbox approach
3099 The notion that, with a complete set of simple tools that work well
3100 together, you can build almost anything you want. Which is fine if
3101 you're assembling a tricycle, but if you're building a defranishizing
3102 comboflux regurgalator, you really want your own machine shop in which
3103 to build special tools. Perl is sort of a machine shop.
3107 To turn one string representation into another by mapping each
3108 character of the source string to its corresponding character in the
3110 L<perlop/trE<sol>SEARCHLISTE<sol>REPLACEMENTLISTE<sol>cds>.
3114 An event that causes a L</handler> to be run.
3118 Not a stellar system with three stars, but an L</operator> taking
3119 three L<operands|/operand>. Sometimes pronounced L</ternary>.
3123 A venerable typesetting language from which Perl derives the name of
3124 its C<$%> variable and which is secretly used in the production of
3129 Any scalar value that doesn't evaluate to 0 or C<"">.
3133 Emptying a file of existing contents, either automatically when
3134 opening a file for writing or explicitly via the
3135 L<truncate|perlfunc/truncate> function.
3139 See L</data type> and L</class>.
3143 Converting data from one type to another. C permits this. Perl does
3144 not need it. Nor want it.
3148 A L</lexical variable> that is declared with a L</class> type: C<my
3153 A type definition in the C language.
3157 Use of a single identifier, prefixed with C<*>. For example, C<*name>
3158 stands for any or all of C<$name>, C<@name>, C<%name>, C<&name>, or
3159 just C<name>. How you use it determines whether it is interpreted as
3160 all or only one of them. See L<perldata/Typeglobs and Filehandles>.
3164 A description of how C types may be transformed to and from Perl types
3165 within an L</extension> module written in L</XS>.
3175 User Datagram Protocol, the typical way to send L<datagrams|/datagram>
3180 A user ID. Often used in the context of L</file> or L</process>
3185 A mask of those L</permission bits> that should be forced off when
3186 creating files or directories, in order to establish a policy of whom
3187 you'll ordinarily deny access to. See the L<umask|perlfunc/umask>
3190 =item unary operator
3192 An operator with only one L</operand>, like C<!> or
3193 L<chdir|perlfunc/chdir>. Unary operators are usually prefix
3194 operators; that is, they precede their operand. The C<++> and C<-->
3195 operators can be either prefix or postfix. (Their position I<does>
3196 change their meanings.)
3200 A character set comprising all the major character sets of the world,
3201 more or less. See L<http://www.unicode.org>.
3205 A very large and constantly evolving language with several alternative
3206 and largely incompatible syntaxes, in which anyone can define anything
3207 any way they choose, and usually do. Speakers of this language think
3208 it's easy to learn because it's so easily twisted to one's own ends,
3209 but dialectical differences make tribal intercommunication nearly
3210 impossible, and travelers are often reduced to a pidgin-like subset of
3211 the language. To be universally understood, a Unix shell programmer
3212 must spend years of study in the art. Many have abandoned this
3213 discipline and now communicate via an Esperanto-like language called
3216 In ancient times, Unix was also used to refer to some code that a
3217 couple of people at Bell Labs wrote to make use of a PDP-7 computer
3218 that wasn't doing much of anything else at the time.
3228 An actual piece of data, in contrast to all the variables, references,
3229 keys, indexes, operators, and whatnot that you need to access the
3234 A named storage location that can hold any of various kinds of
3235 L</value>, as your program sees fit.
3237 =item variable interpolation
3239 The L</interpolation> of a scalar or array variable into a string.
3243 Said of a L</function> that happily receives an indeterminate number
3244 of L</actual arguments>.
3248 Mathematical jargon for a list of L<scalar values|/scalar value>.
3252 Providing the appearance of something without the reality, as in:
3253 virtual memory is not real memory. (See also L</memory>.) The
3254 opposite of "virtual" is "transparent", which means providing the
3255 reality of something without the appearance, as in: Perl handles the
3256 variable-length UTF-8 character encoding transparently.
3260 A form of L</scalar context> in which an L</expression> is not
3261 expected to return any L</value> at all and is evaluated for its
3262 L</side effects> alone.
3266 A "version" or "vector" L</string> specified with a C<v> followed by a
3267 series of decimal integers in dot notation, for instance,
3268 C<v1.20.300.4000>. Each number turns into a L</character> with the
3269 specified ordinal value. (The C<v> is optional when there are at
3270 least three integers.)
3280 A message printed to the L</STDERR> stream to the effect that something
3281 might be wrong but isn't worth blowing up over. See L<perlfunc/warn>
3282 and the L<warnings> pragma.
3284 =item watch expression
3286 An expression which, when its value changes, causes a breakpoint in
3291 A L</character> that moves your cursor but doesn't otherwise put
3292 anything on your screen. Typically refers to any of: space, tab, line
3293 feed, carriage return, or form feed.
3297 In normal "computerese", the piece of data of the size most
3298 efficiently handled by your computer, typically 32 bits or so, give or
3299 take a few powers of 2. In Perl culture, it more often refers to an
3300 alphanumeric L</identifier> (including underscores), or to a string of
3301 nonwhitespace L<characters|/character> bounded by whitespace or string
3304 =item working directory
3306 Your current L</directory>, from which relative pathnames are
3307 interpreted by the L</operating system>. The operating system knows
3308 your current directory because you told it with a
3309 L<chdir|perlfunc/chdir> or because you started out in the place where
3310 your parent L</process> was when you were born.
3314 A program or subroutine that runs some other program or subroutine for
3315 you, modifying some of its input or output to better suit your
3320 What You See Is What You Get. Usually used when something that
3321 appears on the screen matches how it will eventually look, like Perl's
3322 L<format|perlfunc/format> declarations. Also used to mean the
3323 opposite of magic because everything works exactly as it appears, as
3324 in the three-argument form of L<open|perlfunc/open>.
3334 An extraordinarily exported, expeditiously excellent, expressly
3335 eXternal Subroutine, executed in existing C or C++ or in an exciting
3336 new extension language called (exasperatingly) XS. Examine L<perlxs>
3337 for the exact explanation or L<perlxstut> for an exemplary unexacting
3342 An external L</subroutine> defined in L</XS>.
3352 Yet Another Compiler Compiler. A parser generator without which Perl
3353 probably would not have existed. See the file I<perly.y> in the Perl
3354 source distribution.
3364 A subpattern L</assertion> matching the L</null string> between
3365 L<characters|/character>.
3369 A process that has died (exited) but whose parent has not yet received
3370 proper notification of its demise by virtue of having called
3371 L<wait|perlfunc/wait> or L<waitpid|perlfunc/waitpid>. If you
3372 L<fork|perlfunc/fork>, you must clean up after your child processes
3373 when they exit, or else the process table will fill up and your system
3374 administrator will Not Be Happy with you.
3378 =head1 AUTHOR AND COPYRIGHT
3380 Based on the Glossary of Programming Perl, Third Edition,
3381 by Larry Wall, Tom Christiansen & Jon Orwant.
3382 Copyright (c) 2000, 1996, 1991 O'Reilly Media, Inc.
3383 This document may be distributed under the same terms as Perl itself.