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 sometimes 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>, also referred to as a capture group. The
238 sequences (C<\g1>, C<\g2>, etc.) later in the same pattern refer back to
239 the 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. C<\g{-1}> can be used to refer to a group by
243 relative rather than absolute position; and groups can be also be named, and
244 referred to later by name rather than number. See L<perlre/Capture Buffers>.
248 The practice of saying, "If I had to do it all over, I'd do it
249 differently," and then actually going back and doing it all over
250 differently. Mathematically speaking, it's returning from an
251 unsuccessful recursion on a tree of possibilities. Perl backtracks
252 when it attempts to match patterns with a L</regular expression>, and
253 its earlier attempts don't pan out. See L<perlre/Backtracking>.
255 =item backward compatibility
257 Means you can still run your old program because we didn't break any
258 of the features or bugs it was relying on.
262 A word sufficiently ambiguous to be deemed illegal under L<use strict
263 'subs'|strict/strict subs>. In the absence of that stricture, a
264 bareword is treated as if quotes were around it.
268 A generic L</object> type; that is, a L</class> from which other, more
269 specific classes are derived genetically by L</inheritance>. Also
270 called a "superclass" by people who respect their ancestors.
274 From Swift: someone who eats eggs big end first. Also used of
275 computers that store the most significant L</byte> of a word at a
276 lower byte address than the least significant byte. Often considered
277 superior to little-endian machines. See also L</little-endian>.
281 Having to do with numbers represented in base 2. That means there's
282 basically two numbers, 0 and 1. Also used to describe a "non-text
283 file", presumably because such a file makes full use of all the binary
284 bits in its bytes. With the advent of L</Unicode>, this distinction,
285 already suspect, loses even more of its meaning.
287 =item binary operator
289 An L</operator> that takes two L<operands|/operand>.
293 To assign a specific L</network address> to a L</socket>.
297 An integer in the range from 0 to 1, inclusive. The smallest possible
298 unit of information storage. An eighth of a L</byte> or of a dollar.
299 (The term "Pieces of Eight" comes from being able to split the old
300 Spanish dollar into 8 bits, each of which still counted for money.
301 That's why a 25-cent piece today is still "two bits".)
305 The movement of bits left or right in a computer word, which has the
306 effect of multiplying or dividing by a power of 2.
310 A sequence of L<bits|/bit> that is actually being thought of as a
311 sequence of bits, for once.
315 In corporate life, to grant official approval to a thing, as in, "The
316 VP of Engineering has blessed our WebCruncher project." Similarly in
317 Perl, to grant official approval to a L</referent> so that it can
318 function as an L</object>, such as a WebCruncher object. See
323 What a L</process> does when it has to wait for something: "My process
324 blocked waiting for the disk." As an unrelated noun, it refers to a
325 large chunk of data, of a size that the L</operating system> likes to
326 deal with (normally a power of two such as 512 or 8192). Typically
327 refers to a chunk of data that's coming from or going to a disk file.
331 A syntactic construct consisting of a sequence of Perl
332 L<statements|/statement> that is delimited by braces. The C<if> and
333 C<while> statements are defined in terms of L<BLOCKs|/BLOCK>, for instance.
334 Sometimes we also say "block" to mean a lexical scope; that is, a
335 sequence of statements that act like a L</BLOCK>, such as within an
336 L<eval|perlfunc/eval> or a file, even though the statements aren't
339 =item block buffering
341 A method of making input and output efficient by passing one L</block>
342 at a time. By default, Perl does block buffering to disk files. See
343 L</buffer> and L</command buffering>.
347 A value that is either L</true> or L</false>.
349 =item Boolean context
351 A special kind of L</scalar context> used in conditionals to decide
352 whether the L</scalar value> returned by an expression is L</true> or
353 L</false>. Does not evaluate as either a string or a number. See
358 A spot in your program where you've told the debugger to stop
359 L<execution|/execute> so you can poke around and see whether anything
364 To send a L</datagram> to multiple destinations simultaneously.
368 A psychoactive drug, popular in the 80s, probably developed at
369 U. C. Berkeley or thereabouts. Similar in many ways to the
370 prescription-only medication called "System V", but infinitely more
371 useful. (Or, at least, more fun.) The full chemical name is
372 "Berkeley Standard Distribution".
376 A location in a L</hash table> containing (potentially) multiple
377 entries whose keys "hash" to the same hash value according to its hash
378 function. (As internal policy, you don't have to worry about it,
379 unless you're into internals, or policy.)
383 A temporary holding location for data. L<Block buffering|/block
384 buffering> means that the data is passed on to its destination
385 whenever the buffer is full. L<Line buffering|/line buffering> means
386 that it's passed on whenever a complete line is received. L<Command
387 buffering|/command buffering> means that it's passed every time you do
388 a L<print|perlfunc/print> command (or equivalent). If your output is
389 unbuffered, the system processes it one byte at a time without the use
390 of a holding area. This can be rather inefficient.
394 A L</function> that is predefined in the language. Even when hidden
395 by L</overriding>, you can always get at a built-in function by
396 L<qualifying|/qualified> its name with the C<CORE::> pseudo-package.
400 A group of related modules on L</CPAN>. (Also, sometimes refers to a
401 group of command-line switches grouped into one L</switch cluster>.)
405 A piece of data worth eight L<bits|/bit> in most places.
409 A pidgin-like language spoken among 'droids when they don't wish to
410 reveal their orientation (see L</endian>). Named after some similar
411 languages spoken (for similar reasons) between compilers and
412 interpreters in the late 20th century. These languages are
413 characterized by representing everything as a
414 non-architecture-dependent sequence of bytes.
424 A language beloved by many for its inside-out L</type> definitions,
425 inscrutable L</precedence> rules, and heavy L</overloading> of the
426 function-call mechanism. (Well, actually, people first switched to C
427 because they found lowercase identifiers easier to read than upper.)
428 Perl is written in C, so it's not surprising that Perl borrowed a few
433 The typical C compiler's first pass, which processes lines beginning
434 with C<#> for conditional compilation and macro definition and does
435 various manipulations of the program text based on the current
436 definitions. Also known as I<cpp>(1).
438 =item call by reference
440 An L</argument>-passing mechanism in which the L</formal arguments>
441 refer directly to the L</actual arguments>, and the L</subroutine> can
442 change the actual arguments by changing the formal arguments. That
443 is, the formal argument is an L</alias> for the actual argument. See
444 also L</call by value>.
448 An L</argument>-passing mechanism in which the L</formal arguments>
449 refer to a copy of the L</actual arguments>, and the L</subroutine>
450 cannot change the actual arguments by changing the formal arguments.
451 See also L</call by reference>.
455 A L</handler> that you register with some other part of your program
456 in the hope that the other part of your program will L</trigger> your
457 handler when some event of interest transpires.
461 Reduced to a standard form to facilitate comparison.
463 =item capture buffer, capture group
465 These two terms are synonymous:
466 a L<captured substring|/capturing> by a regex subpattern.
470 The use of parentheses around a L</subpattern> in a L</regular
471 expression> to store the matched L</substring> as a L</backreference>
472 or L<capture group|/capture buffer, capture group>.
473 (Captured strings are also returned as a list in L</list context>.)
477 A small integer representative of a unit of orthography.
478 Historically, characters were usually stored as fixed-width integers
479 (typically in a byte, or maybe two, depending on the character set),
480 but with the advent of UTF-8, characters are often stored in a
481 variable number of bytes depending on the size of the integer that
482 represents the character. Perl manages this transparently for you,
485 =item character class
487 A square-bracketed list of characters used in a L</regular expression>
488 to indicate that any character of the set may occur at a given point.
489 Loosely, any predefined set of characters so used.
491 =item character property
493 A predefined L</character class> matchable by the C<\p>
494 L</metasymbol>. Many standard properties are defined for L</Unicode>.
496 =item circumfix operator
498 An L</operator> that surrounds its L</operand>, like the angle
499 operator, or parentheses, or a hug.
503 A user-defined L</type>, implemented in Perl via a L</package> that
504 provides (either directly or by inheritance) L<methods|/method> (that
505 is, L<subroutines|/subroutine>) to handle L<instances|/instance> of
506 the class (its L<objects|/object>). See also L</inheritance>.
510 A L</method> whose L</invocand> is a L</package> name, not an
511 L</object> reference. A method associated with the class as a whole.
515 In networking, a L</process> that initiates contact with a L</server>
516 process in order to exchange data and perhaps receive a service.
520 A L</cluster> used to restrict the scope of a L</regular expression
525 An L</anonymous> subroutine that, when a reference to it is generated
526 at run time, keeps track of the identities of externally visible
527 L<lexical variables|/lexical variable> even after those lexical
528 variables have supposedly gone out of L</scope>. They're called
529 "closures" because this sort of behavior gives mathematicians a sense
534 A parenthesized L</subpattern> used to group parts of a L</regular
535 expression> into a single L</atom>.
539 The word returned by the L<ref|perlfunc/ref> function when you apply
540 it to a reference to a subroutine. See also L</CV>.
544 A system that writes code for you in a low-level language, such as
545 code to implement the backend of a compiler. See L</program
550 The position of a character in a character set encoding. The character
551 C<NULL> is almost certainly at the zeroth position in all character
552 sets, so its code point is 0. The code point for the C<SPACE>
553 character in the ASCII character set is 0x20, or 32 decimal; in EBCDIC
554 it is 0x40, or 64 decimal. The L<ord|perlfunc/ord> function returns
555 the code point of a character.
557 "code position" and "ordinal" mean the same thing as "code point".
559 =item code subpattern
561 A L</regular expression> subpattern whose real purpose is to execute
562 some Perl code, for example, the C<(?{...})> and C<(??{...})>
565 =item collating sequence
567 The order into which L<characters|/character> sort. This is used by
568 L</string> comparison routines to decide, for example, where in this
569 glossary to put "collating sequence".
573 In L</shell> programming, the syntactic combination of a program name
574 and its arguments. More loosely, anything you type to a shell (a
575 command interpreter) that starts it doing something. Even more
576 loosely, a Perl L</statement>, which might start with a L</label> and
577 typically ends with a semicolon.
579 =item command buffering
581 A mechanism in Perl that lets you store up the output of each Perl
582 L</command> and then flush it out as a single request to the
583 L</operating system>. It's enabled by setting the C<$|>
584 (C<$AUTOFLUSH>) variable to a true value. It's used when you don't
585 want data sitting around not going where it's supposed to, which may
586 happen because the default on a L</file> or L</pipe> is to use
591 The name of the program currently executing, as typed on the command
592 line. In C, the L</command> name is passed to the program as the
593 first command-line argument. In Perl, it comes in separately as
596 =item command-line arguments
598 The L<values|/value> you supply along with a program name when you
599 tell a L</shell> to execute a L</command>. These values are passed to
600 a Perl program through C<@ARGV>.
604 A remark that doesn't affect the meaning of the program. In Perl, a
605 comment is introduced by a C<#> character and continues to the end of
608 =item compilation unit
610 The L</file> (or L</string>, in the case of L<eval|perlfunc/eval>)
611 that is currently being compiled.
615 Any time before Perl starts running your main program. See also
616 L</run phase>. Compile phase is mostly spent in L</compile time>, but
617 may also be spent in L</run time> when C<BEGIN> blocks,
618 L<use|perlfunc/use> declarations, or constant subexpressions are being
619 evaluated. The startup and import code of any L<use|perlfunc/use>
620 declaration is also run during compile phase.
624 The time when Perl is trying to make sense of your code, as opposed to
625 when it thinks it knows what your code means and is merely trying to
626 do what it thinks your code says to do, which is L</run time>.
630 Strictly speaking, a program that munches up another program and spits
631 out yet another file containing the program in a "more executable"
632 form, typically containing native machine instructions. The I<perl>
633 program is not a compiler by this definition, but it does contain a
634 kind of compiler that takes a program and turns it into a more
635 executable form (L<syntax trees|/syntax tree>) within the I<perl>
636 process itself, which the L</interpreter> then interprets. There are,
637 however, extension L<modules|/module> to get Perl to act more like a
638 "real" compiler. See L<O>.
642 A "constructor" for a L</referent> that isn't really an L</object>,
643 like an anonymous array or a hash (or a sonata, for that matter). For
644 example, a pair of braces acts as a composer for a hash, and a pair of
645 brackets acts as a composer for an array. See L<perlref/Making
650 The process of gluing one cat's nose to another cat's tail. Also, a
651 similar operation on two L<strings|/string>.
655 Something "iffy". See L</Boolean context>.
659 In telephony, the temporary electrical circuit between the caller's
660 and the callee's phone. In networking, the same kind of temporary
661 circuit between a L</client> and a L</server>.
665 As a noun, a piece of syntax made up of smaller pieces. As a
666 transitive verb, to create an L</object> using a L</constructor>.
670 Any L</class method>, instance L</method>, or L</subroutine>
671 that composes, initializes, blesses, and returns an L</object>.
672 Sometimes we use the term loosely to mean a L</composer>.
676 The surroundings, or environment. The context given by the
677 surrounding code determines what kind of data a particular
678 L</expression> is expected to return. The three primary contexts are
679 L</list context>, L</scalar context>, and L</void context>. Scalar
680 context is sometimes subdivided into L</Boolean context>, L</numeric
681 context>, L</string context>, and L</void context>. There's also a
682 "don't care" scalar context (which is dealt with in Programming Perl,
683 Third Edition, Chapter 2, "Bits and Pieces" if you care).
687 The treatment of more than one physical L</line> as a single logical
688 line. L</Makefile> lines are continued by putting a backslash before
689 the L</newline>. Mail headers as defined by RFC 822 are continued by
690 putting a space or tab I<after> the newline. In general, lines in
691 Perl do not need any form of continuation mark, because L</whitespace>
692 (including newlines) is gleefully ignored. Usually.
696 The corpse of a L</process>, in the form of a file left in the
697 L</working directory> of the process, usually as a result of certain
698 kinds of fatal error.
702 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?>).
706 Someone who breaks security on computer systems. A cracker may be a
707 true L</hacker> or only a L</script kiddie>.
709 =item current package
711 The L</package> in which the current statement is compiled. Scan
712 backwards in the text of your program through the current L<lexical
713 scope|/lexical scoping> or any enclosing lexical scopes till you find
714 a package declaration. That's your current package name.
716 =item current working directory
718 See L</working directory>.
720 =item currently selected output channel
722 The last L</filehandle> that was designated with
723 L<select|perlfunc/select>(C<FILEHANDLE>); L</STDOUT>, if no filehandle
728 An internal "code value" typedef, holding a L</subroutine>. The L</CV>
729 type is a subclass of L</SV>.
737 =item dangling statement
739 A bare, single L</statement>, without any braces, hanging off an C<if>
740 or C<while> conditional. C allows them. Perl doesn't.
744 How your various pieces of data relate to each other and what shape
745 they make when you put them all together, as in a rectangular table or
746 a triangular-shaped tree.
750 A set of possible values, together with all the operations that know
751 how to deal with those values. For example, a numeric data type has a
752 certain set of numbers that you can work with and various mathematical
753 operations that you can do on the numbers but would make little sense
754 on, say, a string such as C<"Kilroy">. Strings have their own
755 operations, such as L</concatenation>. Compound types made of a
756 number of smaller pieces generally have operations to compose and
757 decompose them, and perhaps to rearrange them. L<Objects|/object>
758 that model things in the real world often have operations that
759 correspond to real activities. For instance, if you model an
760 elevator, your elevator object might have an C<open_door()>
765 A packet of data, such as a L</UDP> message, that (from the viewpoint
766 of the programs involved) can be sent independently over the network.
767 (In fact, all packets are sent independently at the L</IP> level, but
768 L</stream> protocols such as L</TCP> hide this from your program.)
772 Stands for "Data Base Management" routines, a set of routines that
773 emulate an L</associative array> using disk files. The routines use a
774 dynamic hashing scheme to locate any entry with only two disk
775 accesses. DBM files allow a Perl program to keep a persistent
776 L</hash> across multiple invocations. You can L<tie|perlfunc/tie>
777 your hash variables to various DBM implementations--see L<AnyDBM_File>
782 An L</assertion> that states something exists and perhaps describes
783 what it's like, without giving any commitment as to how or where
784 you'll use it. A declaration is like the part of your recipe that
785 says, "two cups flour, one large egg, four or five tadpoles..." See
786 L</statement> for its opposite. Note that some declarations also
787 function as statements. Subroutine declarations also act as
788 definitions if a body is supplied.
792 To subtract a value from a variable, as in "decrement C<$x>" (meaning
793 to remove 1 from its value) or "decrement C<$x> by 3".
797 A L</value> chosen for you if you don't supply a value of your own.
801 Having a meaning. Perl thinks that some of the things people try to
802 do are devoid of meaning, in particular, making use of variables that
803 have never been given a L</value> and performing certain operations on
804 data that isn't there. For example, if you try to read data past the
805 end of a file, Perl will hand you back an undefined value. See also
806 L</false> and L<perlfunc/defined>.
810 A L</character> or L</string> that sets bounds to an arbitrarily-sized
811 textual object, not to be confused with a L</separator> or
812 L</terminator>. "To delimit" really just means "to surround" or "to
813 enclose" (like these parentheses are doing).
815 =item deprecated modules and features
817 Deprecated modules and features are those which were part of a stable
818 release, but later found to be subtly flawed, and which should be avoided.
819 They are subject to removal and/or bug-incompatible reimplementation in
820 the next major release (but they will be preserved through maintenance
821 releases). Deprecation warnings are issued under B<-w> or C<use
822 diagnostics>, and notices are found in L<perldelta>s, as well as various
823 other PODs. Coding practices that misuse features, such as C<my $foo if
824 0>, can also be deprecated.
828 A fancy computer science term meaning "to follow a L</reference> to
829 what it points to". The "de" part of it refers to the fact that
830 you're taking away one level of L</indirection>.
834 A L</class> that defines some of its L<methods|/method> in terms of a
835 more generic class, called a L</base class>. Note that classes aren't
836 classified exclusively into base classes or derived classes: a class
837 can function as both a derived class and a base class simultaneously,
838 which is kind of classy.
842 See L</file descriptor>.
846 To deallocate the memory of a L</referent> (first triggering its
847 C<DESTROY> method, if it has one).
851 A special L</method> that is called when an L</object> is thinking
852 about L<destroying|/destroy> itself. A Perl program's C<DESTROY>
853 method doesn't do the actual destruction; Perl just
854 L<triggers|/trigger> the method in case the L</class> wants to do any
859 A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a
860 joystick or a mouse) attached to your computer, that the L</operating
861 system> tries to make look like a L</file> (or a bunch of files).
862 Under Unix, these fake files tend to live in the I</dev> directory.
866 A L</pod> directive. See L<perlpod>.
870 A special file that contains other files. Some L<operating
871 systems|/operating system> call these "folders", "drawers", or
874 =item directory handle
876 A name that represents a particular instance of opening a directory to
877 read it, until you close it. See the L<opendir|perlfunc/opendir>
882 To send something to its correct destination. Often used
883 metaphorically to indicate a transfer of programmatic control to a
884 destination selected algorithmically, often by lookup in a table of
885 function L<references|/reference> or, in the case of object
886 L<methods|/method>, by traversing the inheritance tree looking for the
887 most specific definition for the method.
891 A standard, bundled release of a system of software. The default
892 usage implies source code is included. If that is not the case, it
893 will be called a "binary-only" distribution.
895 =item (to be) dropped modules
897 When Perl 5 was first released (see L<perlhistory>), several modules were
898 included, which have now fallen out of common use. It has been suggested
899 that these modules should be removed, since the distribution became rather
900 large, and the common criterion for new module additions is now limited to
901 modules that help to build, test, and extend perl itself. Furthermore,
902 the CPAN (which didn't exist at the time of Perl 5.0) can become the new
903 home of dropped modules. Dropping modules is currently not an option, but
904 further developments may clear the last barriers.
908 An enchantment, illusion, phantasm, or jugglery. Said when Perl's
909 magical L</dwimmer> effects don't do what you expect, but rather seem
910 to be the product of arcane dweomercraft, sorcery, or wonder working.
915 DWIM is an acronym for "Do What I Mean", the principle that something
916 should just do what you want it to do without an undue amount of fuss.
917 A bit of code that does "dwimming" is a "dwimmer". Dwimming can
918 require a great deal of behind-the-scenes magic, which (if it doesn't
919 stay properly behind the scenes) is called a L</dweomer> instead.
921 =item dynamic scoping
923 Dynamic scoping works over a dynamic scope, making variables visible
924 throughout the rest of the L</block> in which they are first used and
925 in any L<subroutines|/subroutine> that are called by the rest of the
926 block. Dynamically scoped variables can have their values temporarily
927 changed (and implicitly restored later) by a L<local|perlfunc/local>
928 operator. (Compare L</lexical scoping>.) Used more loosely to mean
929 how a subroutine that is in the middle of calling another subroutine
930 "contains" that subroutine at L</run time>.
940 Derived from many sources. Some would say I<too> many.
944 A basic building block. When you're talking about an L</array>, it's
945 one of the items that make up the array.
949 When something is contained in something else, particularly when that
950 might be considered surprising: "I've embedded a complete Perl
951 interpreter in my editor!"
953 =item empty subclass test
955 The notion that an empty L</derived class> should behave exactly like
960 When you change a L</value> as it is being copied. [From French, "in
961 passing", as in the exotic pawn-capturing maneuver in chess.]
965 The veil of abstraction separating the L</interface> from the
966 L</implementation> (whether enforced or not), which mandates that all
967 access to an L</object>'s state be through L<methods|/method> alone.
971 See L</little-endian> and L</big-endian>.
975 The collective set of L<environment variables|/environment variable>
976 your L</process> inherits from its parent. Accessed via C<%ENV>.
978 =item environment variable
980 A mechanism by which some high-level agent such as a user can pass its
981 preferences down to its future offspring (child L<processes|/process>,
982 grandchild processes, great-grandchild processes, and so on). Each
983 environment variable is a L</key>/L</value> pair, like one entry in a
988 End of File. Sometimes used metaphorically as the terminating string
989 of a L</here document>.
993 The error number returned by a L</syscall> when it fails. Perl refers
994 to the error by the name C<$!> (or C<$OS_ERROR> if you use the English
999 See L</exception> or L</fatal error>.
1001 =item escape sequence
1007 A fancy term for an error. See L</fatal error>.
1009 =item exception handling
1011 The way a program responds to an error. The exception handling
1012 mechanism in Perl is the L<eval|perlfunc/eval> operator.
1016 To throw away the current L</process>'s program and replace it with
1017 another without exiting the process or relinquishing any resources
1018 held (apart from the old memory image).
1020 =item executable file
1022 A L</file> that is specially marked to tell the L</operating system>
1023 that it's okay to run this file as a program. Usually shortened to
1028 To run a L<program|/executable file> or L</subroutine>. (Has nothing
1029 to do with the L<kill|perlfunc/kill> built-in, unless you're trying to
1030 run a L</signal handler>.)
1034 The special mark that tells the operating system it can run this
1035 program. There are actually three execute bits under Unix, and which
1036 bit gets used depends on whether you own the file singularly,
1037 collectively, or not at all.
1045 To make symbols from a L</module> available for L</import> by other modules.
1049 Anything you can legally say in a spot where a L</value> is required.
1050 Typically composed of L<literals|/literal>, L<variables|/variable>,
1051 L<operators|/operator>, L<functions|/function>, and L</subroutine>
1052 calls, not necessarily in that order.
1056 A Perl module that also pulls in compiled C or C++ code. More
1057 generally, any experimental option that can be compiled into Perl,
1058 such as multithreading.
1068 In Perl, any value that would look like C<""> or C<"0"> if evaluated
1069 in a string context. Since undefined values evaluate to C<"">, all
1070 undefined values are false, but not all false values are undefined.
1074 Frequently Asked Question (although not necessarily frequently
1075 answered, especially if the answer appears in the Perl FAQ shipped
1076 standard with Perl).
1080 An uncaught L</exception>, which causes termination of the L</process>
1081 after printing a message on your L</standard error> stream. Errors
1082 that happen inside an L<eval|perlfunc/eval> are not fatal. Instead,
1083 the L<eval|perlfunc/eval> terminates after placing the exception
1084 message in the C<$@> (C<$EVAL_ERROR>) variable. You can try to
1085 provoke a fatal error with the L<die|perlfunc/die> operator (known as
1086 throwing or raising an exception), but this may be caught by a
1087 dynamically enclosing L<eval|perlfunc/eval>. If not caught, the
1088 L<die|perlfunc/die> becomes a fatal error.
1092 A single piece of numeric or string data that is part of a longer
1093 L</string>, L</record>, or L</line>. Variable-width fields are usually
1094 split up by L<separators|/separator> (so use L<split|perlfunc/split> to
1095 extract the fields), while fixed-width fields are usually at fixed
1096 positions (so use L<unpack|perlfunc/unpack>). L<Instance
1097 variables|/instance variable> are also known as fields.
1101 First In, First Out. See also L</LIFO>. Also, a nickname for a
1106 A named collection of data, usually stored on disk in a L</directory>
1107 in a L</filesystem>. Roughly like a document, if you're into office
1108 metaphors. In modern filesystems, you can actually give a file more
1109 than one name. Some files have special properties, like directories
1112 =item file descriptor
1114 The little number the L</operating system> uses to keep track of which
1115 opened L</file> you're talking about. Perl hides the file descriptor
1116 inside a L</standard IE<sol>O> stream and then attaches the stream to
1119 =item file test operator
1121 A built-in unary operator that you use to determine whether something
1122 is L</true> about a file, such as C<-o $filename> to test whether
1123 you're the owner of the file.
1127 A "wildcard" match on L<filenames|/filename>. See the
1128 L<glob|perlfunc/glob> function.
1132 An identifier (not necessarily related to the real name of a file)
1133 that represents a particular instance of opening a file until you
1134 close it. If you're going to open and close several different files
1135 in succession, it's fine to open each of them with the same
1136 filehandle, so you don't have to write out separate code to process
1141 One name for a file. This name is listed in a L</directory>, and you
1142 can use it in an L<open|perlfunc/open> to tell the L</operating
1143 system> exactly which file you want to open, and associate the file
1144 with a L</filehandle> which will carry the subsequent identity of that
1145 file in your program, until you close it.
1149 A set of L<directories|/directory> and L<files|/file> residing on a
1150 partition of the disk. Sometimes known as a "partition". You can
1151 change the file's name or even move a file around from directory to
1152 directory within a filesystem without actually moving the file itself,
1153 at least under Unix.
1157 A program designed to take a L</stream> of input and transform it into
1162 We tend to avoid this term because it means so many things. It may
1163 mean a command-line L</switch> that takes no argument
1164 itself (such as Perl's B<-n> and B<-p>
1165 flags) or, less frequently, a single-bit indicator (such as the
1166 C<O_CREAT> and C<O_EXCL> flags used in
1167 L<sysopen|perlfunc/sysopen>).
1169 =item floating point
1171 A method of storing numbers in "scientific notation", such that the
1172 precision of the number is independent of its magnitude (the decimal
1173 point "floats"). Perl does its numeric work with floating-point
1174 numbers (sometimes called "floats"), when it can't get away with
1175 using L<integers|/integer>. Floating-point numbers are mere
1176 approximations of real numbers.
1180 The act of emptying a L</buffer>, often before it's full.
1184 Far More Than Everything You Ever Wanted To Know. An exhaustive
1185 treatise on one narrow topic, something of a super-L</FAQ>. See Tom
1190 To create a child L</process> identical to the parent process at its
1191 moment of conception, at least until it gets ideas of its own. A
1192 thread with protected memory.
1194 =item formal arguments
1196 The generic names by which a L</subroutine> knows its
1197 L<arguments|/argument>. In many languages, formal arguments are
1198 always given individual names, but in Perl, the formal arguments are
1199 just the elements of an array. The formal arguments to a Perl program
1200 are C<$ARGV[0]>, C<$ARGV[1]>, and so on. Similarly, the formal
1201 arguments to a Perl subroutine are C<$_[0]>, C<$_[1]>, and so on. You
1202 may give the arguments individual names by assigning the values to a
1203 L<my|perlfunc/my> list. See also L</actual arguments>.
1207 A specification of how many spaces and digits and things to put
1208 somewhere so that whatever you're printing comes out nice and pretty.
1210 =item freely available
1212 Means you don't have to pay money to get it, but the copyright on it
1213 may still belong to someone else (like Larry).
1215 =item freely redistributable
1217 Means you're not in legal trouble if you give a bootleg copy of it to
1218 your friends and we find out about it. In fact, we'd rather you gave
1219 a copy to all your friends.
1223 Historically, any software that you give away, particularly if you
1224 make the source code available as well. Now often called C<open
1225 source software>. Recently there has been a trend to use the term in
1226 contradistinction to L</open source software>, to refer only to free
1227 software released under the Free Software Foundation's GPL (General
1228 Public License), but this is difficult to justify etymologically.
1232 Mathematically, a mapping of each of a set of input values to a
1233 particular output value. In computers, refers to a L</subroutine> or
1234 L</operator> that returns a L</value>. It may or may not have input
1235 values (called L<arguments|/argument>).
1237 =item funny character
1239 Someone like Larry, or one of his peculiar friends. Also refers to
1240 the strange prefixes that Perl requires as noun markers on its
1243 =item garbage collection
1245 A misnamed feature--it should be called, "expecting your mother to
1246 pick up after you". Strictly speaking, Perl doesn't do this, but it
1247 relies on a reference-counting mechanism to keep things tidy.
1248 However, we rarely speak strictly and will often refer to the
1249 reference-counting scheme as a form of garbage collection. (If it's
1250 any comfort, when your interpreter exits, a "real" garbage collector
1251 runs to make sure everything is cleaned up if you've been messy with
1252 circular references and such.)
1262 Group ID--in Unix, the numeric group ID that the L</operating system>
1263 uses to identify you and members of your L</group>.
1267 Strictly, the shell's C<*> character, which will match a "glob" of
1268 characters when you're trying to generate a list of filenames.
1269 Loosely, the act of using globs and similar symbols to do pattern
1270 matching. See also L</fileglob> and L</typeglob>.
1274 Something you can see from anywhere, usually used of
1275 L<variables|/variable> and L<subroutines|/subroutine> that are visible
1276 everywhere in your program. In Perl, only certain special variables
1277 are truly global--most variables (and all subroutines) exist only in
1278 the current L</package>. Global variables can be declared with
1279 L<our|perlfunc/our>. See L<perlfunc/our>.
1281 =item global destruction
1283 The L</garbage collection> of globals (and the running of any
1284 associated object destructors) that takes place when a Perl
1285 L</interpreter> is being shut down. Global destruction should not be
1286 confused with the Apocalypse, except perhaps when it should.
1290 A language such as Perl that is good at hooking things together that
1291 weren't intended to be hooked together.
1295 The size of the pieces you're dealing with, mentally speaking.
1299 A L</subpattern> whose L</quantifier> wants to match as many things as
1304 Originally from the old Unix editor command for "Globally search for a
1305 Regular Expression and Print it", now used in the general sense of any
1306 kind of search, especially text searches. Perl has a built-in
1307 L<grep|perlfunc/grep> function that searches a list for elements
1308 matching any given criterion, whereas the I<grep>(1) program searches
1309 for lines matching a L</regular expression> in one or more files.
1313 A set of users of which you are a member. In some operating systems
1314 (like Unix), you can give certain file access permissions to other
1315 members of your group.
1319 An internal "glob value" typedef, holding a L</typeglob>. The L</GV>
1320 type is a subclass of L</SV>.
1330 Someone who is brilliantly persistent in solving technical problems,
1331 whether these involve golfing, fighting orcs, or programming. Hacker
1332 is a neutral term, morally speaking. Good hackers are not to be
1333 confused with evil L<crackers|/cracker> or clueless L<script
1334 kiddies|/script kiddie>. If you confuse them, we will presume that
1335 you are either evil or clueless.
1339 A L</subroutine> or L</method> that is called by Perl when your
1340 program needs to respond to some internal event, such as a L</signal>,
1341 or an encounter with an operator subject to L</operator overloading>.
1342 See also L</callback>.
1344 =item hard reference
1346 A L</scalar> L</value> containing the actual address of a
1347 L</referent>, such that the referent's L</reference> count accounts
1348 for it. (Some hard references are held internally, such as the
1349 implicit reference from one of a L</typeglob>'s variable slots to its
1350 corresponding referent.) A hard reference is different from a
1351 L</symbolic reference>.
1355 An unordered association of L</key>/L</value> pairs, stored such that
1356 you can easily use a string L</key> to look up its associated data
1357 L</value>. This glossary is like a hash, where the word to be defined
1358 is the key, and the definition is the value. A hash is also sometimes
1359 septisyllabically called an "associative array", which is a pretty
1360 good reason for simply calling it a "hash" instead.
1364 A data structure used internally by Perl for implementing associative
1365 arrays (hashes) efficiently. See also L</bucket>.
1369 A file containing certain required definitions that you must include
1370 "ahead" of the rest of your program to do certain obscure operations.
1371 A C header file has a I<.h> extension. Perl doesn't really have
1372 header files, though historically Perl has sometimes used translated
1373 I<.h> files with a I<.ph> extension. See L<perlfunc/require>.
1374 (Header files have been superseded by the L</module> mechanism.)
1378 So called because of a similar construct in L<shells|/shell> that
1379 pretends that the L<lines|/line> following the L</command> are a
1380 separate L</file> to be fed to the command, up to some terminating
1381 string. In Perl, however, it's just a fancy form of quoting.
1385 A number in base 16, "hex" for short. The digits for 10 through 16
1386 are customarily represented by the letters C<a> through C<f>.
1387 Hexadecimal constants in Perl start with C<0x>. See also
1390 =item home directory
1392 The directory you are put into when you log in. On a Unix system, the
1393 name is often placed into C<$ENV{HOME}> or C<$ENV{LOGDIR}> by
1394 I<login>, but you can also find it with C<(getpwuid($E<lt>))[7]>.
1395 (Some platforms do not have a concept of a home directory.)
1399 The computer on which a program or other data resides.
1403 Excessive pride, the sort of thing Zeus zaps you for. Also the
1404 quality that makes you write (and maintain) programs that other people
1405 won't want to say bad things about. Hence, the third great virtue of
1406 a programmer. See also L</laziness> and L</impatience>.
1410 Short for a "hash value" typedef, which holds Perl's internal
1411 representation of a hash. The L</HV> type is a subclass of L</SV>.
1421 A legally formed name for most anything in which a computer program
1422 might be interested. Many languages (including Perl) allow
1423 identifiers that start with a letter and contain letters and digits.
1424 Perl also counts the underscore character as a valid letter. (Perl
1425 also has more complicated names, such as L</qualified> names.)
1429 The anger you feel when the computer is being lazy. This makes you
1430 write programs that don't just react to your needs, but actually
1431 anticipate them. Or at least that pretend to. Hence, the second
1432 great virtue of a programmer. See also L</laziness> and L</hubris>.
1434 =item implementation
1436 How a piece of code actually goes about doing its job. Users of the
1437 code should not count on implementation details staying the same
1438 unless they are part of the published L</interface>.
1442 To gain access to symbols that are exported from another module. See
1447 To increase the value of something by 1 (or by some other number, if
1452 In olden days, the act of looking up a L</key> in an actual index
1453 (such as a phone book), but now merely the act of using any kind of
1454 key or position to find the corresponding L</value>, even if no index
1455 is involved. Things have degenerated to the point that Perl's
1456 L<index|perlfunc/index> function merely locates the position (index)
1457 of one string in another.
1459 =item indirect filehandle
1461 An L</expression> that evaluates to something that can be used as a
1462 L</filehandle>: a L</string> (filehandle name), a L</typeglob>, a
1463 typeglob L</reference>, or a low-level L</IO> object.
1465 =item indirect object
1467 In English grammar, a short noun phrase between a verb and its direct
1468 object indicating the beneficiary or recipient of the action. In
1469 Perl, C<print STDOUT "$foo\n";> can be understood as "verb
1470 indirect-object object" where L</STDOUT> is the recipient of the
1471 L<print|perlfunc/print> action, and C<"$foo"> is the object being
1472 printed. Similarly, when invoking a L</method>, you might place the
1473 invocand between the method and its arguments:
1475 $gollum = new Pathetic::Creature "Smeagol";
1476 give $gollum "Fisssssh!";
1477 give $gollum "Precious!";
1479 In modern Perl, calling methods this way is often considered bad practice and
1482 =item indirect object slot
1484 The syntactic position falling between a method call and its arguments
1485 when using the indirect object invocation syntax. (The slot is
1486 distinguished by the absence of a comma between it and the next
1487 argument.) L</STDERR> is in the indirect object slot here:
1489 print STDERR "Awake! Awake! Fear, Fire,
1494 If something in a program isn't the value you're looking for but
1495 indicates where the value is, that's indirection. This can be done
1496 with either L<symbolic references|/symbolic reference> or L<hard
1497 references|/hard reference>.
1501 An L</operator> that comes in between its L<operands|/operand>, such
1502 as multiplication in C<24 * 7>.
1506 What you get from your ancestors, genetically or otherwise. If you
1507 happen to be a L</class>, your ancestors are called L<base
1508 classes|/base class> and your descendants are called L<derived
1509 classes|/derived class>. See L</single inheritance> and L</multiple
1514 Short for "an instance of a class", meaning an L</object> of that L</class>.
1516 =item instance variable
1518 An L</attribute> of an L</object>; data stored with the particular
1519 object rather than with the class as a whole.
1523 A number with no fractional (decimal) part. A counting number, like
1524 1, 2, 3, and so on, but including 0 and the negatives.
1528 The services a piece of code promises to provide forever, in contrast to
1529 its L</implementation>, which it should feel free to change whenever it
1534 The insertion of a scalar or list value somewhere in the middle of
1535 another value, such that it appears to have been there all along. In
1536 Perl, variable interpolation happens in double-quoted strings and
1537 patterns, and list interpolation occurs when constructing the list of
1538 values to pass to a list operator or other such construct that takes a
1543 Strictly speaking, a program that reads a second program and does what
1544 the second program says directly without turning the program into a
1545 different form first, which is what L<compilers|/compiler> do. Perl
1546 is not an interpreter by this definition, because it contains a kind
1547 of compiler that takes a program and turns it into a more executable
1548 form (L<syntax trees|/syntax tree>) within the I<perl> process itself,
1549 which the Perl L</run time> system then interprets.
1553 The agent on whose behalf a L</method> is invoked. In a L</class>
1554 method, the invocand is a package name. In an L</instance> method,
1555 the invocand is an object reference.
1559 The act of calling up a deity, daemon, program, method, subroutine, or
1560 function to get it do what you think it's supposed to do. We usually
1561 "call" subroutines but "invoke" methods, since it sounds cooler.
1565 Input from, or output to, a L</file> or L</device>.
1569 An internal I/O object. Can also mean L</indirect object>.
1573 Internet Protocol, or Intellectual Property.
1577 Interprocess Communication.
1581 A relationship between two L<objects|/object> in which one object is
1582 considered to be a more specific version of the other, generic object:
1583 "A camel is a mammal." Since the generic object really only exists in
1584 a Platonic sense, we usually add a little abstraction to the notion of
1585 objects and think of the relationship as being between a generic
1586 L</base class> and a specific L</derived class>. Oddly enough,
1587 Platonic classes don't always have Platonic relationships--see
1592 Doing something repeatedly.
1596 A special programming gizmo that keeps track of where you are in
1597 something that you're trying to iterate over. The C<foreach> loop in
1598 Perl contains an iterator; so does a hash, allowing you to
1599 L<each|perlfunc/each> through it.
1603 The integer four, not to be confused with six, Tom's favorite editor.
1604 IV also means an internal Integer Value of the type a L</scalar> can
1605 hold, not to be confused with an L</NV>.
1615 "Just Another Perl Hacker," a clever but cryptic bit of Perl code that
1616 when executed, evaluates to that string. Often used to illustrate a
1617 particular Perl feature, and something of an ongoing Obfuscated Perl
1618 Contest seen in Usenix signatures.
1628 The string index to a L</hash>, used to look up the L</value>
1629 associated with that key.
1633 See L</reserved words>.
1643 A name you give to a L</statement> so that you can talk about that
1644 statement elsewhere in the program.
1648 The quality that makes you go to great effort to reduce overall energy
1649 expenditure. It makes you write labor-saving programs that other
1650 people will find useful, and document what you wrote so you don't have
1651 to answer so many questions about it. Hence, the first great virtue
1652 of a programmer. Also hence, this book. See also L</impatience> and
1657 A L</bit shift> that multiplies the number by some power of 2.
1659 =item leftmost longest
1661 The preference of the L</regular expression> engine to match the
1662 leftmost occurrence of a L</pattern>, then given a position at which a
1663 match will occur, the preference for the longest match (presuming the
1664 use of a L</greedy> quantifier). See L<perlre> for I<much> more on
1669 Fancy term for a L</token>.
1673 Fancy term for a L</tokener>.
1675 =item lexical analysis
1677 Fancy term for L</tokenizing>.
1679 =item lexical scoping
1681 Looking at your I<Oxford English Dictionary> through a microscope.
1682 (Also known as L</static scoping>, because dictionaries don't change
1683 very fast.) Similarly, looking at variables stored in a private
1684 dictionary (namespace) for each scope, which are visible only from
1685 their point of declaration down to the end of the lexical scope in
1686 which they are declared. --Syn. L</static scoping>.
1687 --Ant. L</dynamic scoping>.
1689 =item lexical variable
1691 A L</variable> subject to L</lexical scoping>, declared by
1692 L<my|perlfunc/my>. Often just called a "lexical". (The
1693 L<our|perlfunc/our> declaration declares a lexically scoped name for a
1694 global variable, which is not itself a lexical variable.)
1698 Generally, a collection of procedures. In ancient days, referred to a
1699 collection of subroutines in a I<.pl> file. In modern times, refers
1700 more often to the entire collection of Perl L<modules|/module> on your
1705 Last In, First Out. See also L</FIFO>. A LIFO is usually called a
1710 In Unix, a sequence of zero or more non-newline characters terminated
1711 with a L</newline> character. On non-Unix machines, this is emulated
1712 by the C library even if the underlying L</operating system> has
1715 =item line buffering
1717 Used by a L</standard IE<sol>O> output stream that flushes its
1718 L</buffer> after every L</newline>. Many standard I/O libraries
1719 automatically set up line buffering on output that is going to the
1724 The number of lines read previous to this one, plus 1. Perl keeps a
1725 separate line number for each source or input file it opens. The
1726 current source file's line number is represented by C<__LINE__>. The
1727 current input line number (for the file that was most recently read
1728 via C<< E<lt>FHE<gt> >>) is represented by the C<$.>
1729 (C<$INPUT_LINE_NUMBER>) variable. Many error messages report both
1730 values, if available.
1734 Used as a noun, a name in a L</directory>, representing a L</file>. A
1735 given file can have multiple links to it. It's like having the same
1736 phone number listed in the phone directory under different names. As
1737 a verb, to resolve a partially compiled file's unresolved symbols into
1738 a (nearly) executable image. Linking can generally be static or
1739 dynamic, which has nothing to do with static or dynamic scoping.
1743 A syntactic construct representing a comma-separated list of
1744 expressions, evaluated to produce a L</list value>. Each
1745 L</expression> in a L</LIST> is evaluated in L</list context> and
1746 interpolated into the list value.
1750 An ordered set of scalar values.
1754 The situation in which an L</expression> is expected by its
1755 surroundings (the code calling it) to return a list of values rather
1756 than a single value. Functions that want a L</LIST> of arguments tell
1757 those arguments that they should produce a list value. See also
1762 An L</operator> that does something with a list of values, such as
1763 L<join|perlfunc/join> or L<grep|perlfunc/grep>. Usually used for
1764 named built-in operators (such as L<print|perlfunc/print>,
1765 L<unlink|perlfunc/unlink>, and L<system|perlfunc/system>) that do not
1766 require parentheses around their L</argument> list.
1770 An unnamed list of temporary scalar values that may be passed around
1771 within a program from any list-generating function to any function or
1772 construct that provides a L</list context>.
1776 A token in a programming language such as a number or L</string> that
1777 gives you an actual L</value> instead of merely representing possible
1778 values as a L</variable> does.
1782 From Swift: someone who eats eggs little end first. Also used of
1783 computers that store the least significant L</byte> of a word at a
1784 lower byte address than the most significant byte. Often considered
1785 superior to big-endian machines. See also L</big-endian>.
1789 Not meaning the same thing everywhere. A global variable in Perl can
1790 be localized inside a L<dynamic scope|/dynamic scoping> via the
1791 L<local|perlfunc/local> operator.
1793 =item logical operator
1795 Symbols representing the concepts "and", "or", "xor", and "not".
1799 An L</assertion> that peeks at the string to the right of the current
1804 An L</assertion> that peeks at the string to the left of the current
1809 A construct that performs something repeatedly, like a roller coaster.
1811 =item loop control statement
1813 Any statement within the body of a loop that can make a loop
1814 prematurely stop looping or skip an L</iteration>. Generally you
1815 shouldn't try this on roller coasters.
1819 A kind of key or name attached to a loop (or roller coaster) so that
1820 loop control statements can talk about which loop they want to
1825 Able to serve as an L</lvalue>.
1829 Term used by language lawyers for a storage location you can assign a
1830 new L</value> to, such as a L</variable> or an element of an
1831 L</array>. The "l" is short for "left", as in the left side of an
1832 assignment, a typical place for lvalues. An L</lvaluable> function or
1833 expression is one to which a value may be assigned, as in C<pos($x) =
1836 =item lvalue modifier
1838 An adjectival pseudofunction that warps the meaning of an L</lvalue>
1839 in some declarative fashion. Currently there are three lvalue
1840 modifiers: L<my|perlfunc/my>, L<our|perlfunc/our>, and
1841 L<local|perlfunc/local>.
1851 Technically speaking, any extra semantics attached to a variable such
1852 as C<$!>, C<$0>, C<%ENV>, or C<%SIG>, or to any tied variable.
1853 Magical things happen when you diddle those variables.
1855 =item magical increment
1857 An L</increment> operator that knows how to bump up alphabetics as
1860 =item magical variables
1862 Special variables that have side effects when you access them or
1863 assign to them. For example, in Perl, changing elements of the
1864 C<%ENV> array also changes the corresponding environment variables
1865 that subprocesses will use. Reading the C<$!> variable gives you the
1866 current system error number or message.
1870 A file that controls the compilation of a program. Perl programs
1871 don't usually need a L</Makefile> because the Perl compiler has plenty
1876 The Unix program that displays online documentation (manual pages) for
1881 A "page" from the manuals, typically accessed via the I<man>(1)
1882 command. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of
1883 BUGS, and so on, and is typically longer than a page. There are
1884 manpages documenting L<commands|/command>, L<syscalls|/syscall>,
1885 L</library> L<functions|/function>, L<devices|/device>,
1886 L<protocols|/protocol>, L<files|/file>, and such. In this book, we
1887 call any piece of standard Perl documentation (like I<perlop> or
1888 I<perldelta>) a manpage, no matter what format it's installed in on
1893 See L</pattern matching>.
1897 See L</instance variable>.
1901 This always means your main memory, not your disk. Clouding the issue
1902 is the fact that your machine may implement L</virtual> memory; that
1903 is, it will pretend that it has more memory than it really does, and
1904 it'll use disk space to hold inactive bits. This can make it seem
1905 like you have a little more memory than you really do, but it's not a
1906 substitute for real memory. The best thing that can be said about
1907 virtual memory is that it lets your performance degrade gradually
1908 rather than suddenly when you run out of real memory. But your
1909 program can die when you run out of virtual memory too, if you haven't
1910 thrashed your disk to death first.
1914 A L</character> that is I<not> supposed to be treated normally. Which
1915 characters are to be treated specially as metacharacters varies
1916 greatly from context to context. Your L</shell> will have certain
1917 metacharacters, double-quoted Perl L<strings|/string> have other
1918 metacharacters, and L</regular expression> patterns have all the
1919 double-quote metacharacters plus some extra ones of their own.
1923 Something we'd call a L</metacharacter> except that it's a sequence of
1924 more than one character. Generally, the first character in the
1925 sequence must be a true metacharacter to get the other characters in
1926 the metasymbol to misbehave along with it.
1930 A kind of action that an L</object> can take if you tell it to. See
1935 The belief that "small is beautiful." Paradoxically, if you say
1936 something in a small language, it turns out big, and if you say it in
1937 a big language, it turns out small. Go figure.
1941 In the context of the L<stat> syscall, refers to the field holding
1942 the L</permission bits> and the type of the L</file>.
1946 See L</statement modifier>, L</regular expression modifier>, and
1947 L</lvalue modifier>, not necessarily in that order.
1951 A L</file> that defines a L</package> of (almost) the same name, which
1952 can either L</export> symbols or function as an L</object> class. (A
1953 module's main I<.pm> file may also load in other files in support of
1954 the module.) See the L<use|perlfunc/use> built-in.
1958 An integer divisor when you're interested in the remainder instead of
1963 Short for Perl Monger, a purveyor of Perl.
1967 A temporary value scheduled to die when the current statement
1970 =item multidimensional array
1972 An array with multiple subscripts for finding a single element. Perl
1973 implements these using L<references|/reference>--see L<perllol> and
1976 =item multiple inheritance
1978 The features you got from your mother and father, mixed together
1979 unpredictably. (See also L</inheritance>, and L</single
1980 inheritance>.) In computer languages (including Perl), the notion
1981 that a given class may have multiple direct ancestors or L<base
1982 classes|/base class>.
1992 A L</pipe> with a name embedded in the L</filesystem> so that it can
1993 be accessed by two unrelated L<processes|/process>.
1997 A domain of names. You needn't worry about whether the names in one
1998 such domain have been used in another. See L</package>.
2000 =item network address
2002 The most important attribute of a socket, like your telephone's
2003 telephone number. Typically an IP address. See also L</port>.
2007 A single character that represents the end of a line, with the ASCII
2008 value of 012 octal under Unix (but 015 on a Mac), and represented by
2009 C<\n> in Perl strings. For Windows machines writing text files, and
2010 for certain physical devices like terminals, the single newline gets
2011 automatically translated by your C library into a line feed and a
2012 carriage return, but normally, no translation is done.
2016 Network File System, which allows you to mount a remote filesystem as
2019 =item null character
2021 A character with the ASCII value of zero. It's used by C to terminate
2022 strings, but Perl allows strings to contain a null.
2026 A L</list value> with zero elements, represented in Perl by C<()>.
2030 A L</string> containing no characters, not to be confused with a
2031 string containing a L</null character>, which has a positive length
2034 =item numeric context
2036 The situation in which an expression is expected by its surroundings
2037 (the code calling it) to return a number. See also L</context> and
2042 Short for Nevada, no part of which will ever be confused with
2043 civilization. NV also means an internal floating-point Numeric Value
2044 of the type a L</scalar> can hold, not to be confused with an L</IV>.
2048 Half a L</byte>, equivalent to one L</hexadecimal> digit, and worth
2059 An L</instance> of a L</class>. Something that "knows" what
2060 user-defined type (class) it is, and what it can do because of what
2061 class it is. Your program can request an object to do things, but the
2062 object gets to decide whether it wants to do them or not. Some
2063 objects are more accommodating than others.
2067 A number in base 8. Only the digits 0 through 7 are allowed. Octal
2068 constants in Perl start with 0, as in 013. See also the
2069 L<oct|perlfunc/oct> function.
2073 How many things you have to skip over when moving from the beginning
2074 of a string or array to a specific position within it. Thus, the
2075 minimum offset is zero, not one, because you don't skip anything to
2076 get to the first item.
2080 An entire computer program crammed into one line of text.
2082 =item open source software
2084 Programs for which the source code is freely available and freely
2085 redistributable, with no commercial strings attached. For a more
2086 detailed definition, see L<http://www.opensource.org/osd.html>.
2090 An L</expression> that yields a L</value> that an L</operator>
2091 operates on. See also L</precedence>.
2093 =item operating system
2095 A special program that runs on the bare machine and hides the gory
2096 details of managing L<processes|/process> and L<devices|/device>.
2097 Usually used in a looser sense to indicate a particular culture of
2098 programming. The loose sense can be used at varying levels of
2099 specificity. At one extreme, you might say that all versions of Unix
2100 and Unix-lookalikes are the same operating system (upsetting many
2101 people, especially lawyers and other advocates). At the other
2102 extreme, you could say this particular version of this particular
2103 vendor's operating system is different from any other version of this
2104 or any other vendor's operating system. Perl is much more portable
2105 across operating systems than many other languages. See also
2106 L</architecture> and L</platform>.
2110 A gizmo that transforms some number of input values to some number of
2111 output values, often built into a language with a special syntax or
2112 symbol. A given operator may have specific expectations about what
2113 L<types|/type> of data you give as its arguments
2114 (L<operands|/operand>) and what type of data you want back from it.
2116 =item operator overloading
2118 A kind of L</overloading> that you can do on built-in
2119 L<operators|/operator> to make them work on L<objects|/object> as if
2120 the objects were ordinary scalar values, but with the actual semantics
2121 supplied by the object class. This is set up with the L<overload>
2126 See either L<switches|/switch> or L</regular expression modifier>.
2130 Another name for L</code point>
2134 Giving additional meanings to a symbol or construct. Actually, all
2135 languages do overloading to one extent or another, since people are
2136 good at figuring out things from L</context>.
2140 Hiding or invalidating some other definition of the same name. (Not
2141 to be confused with L</overloading>, which adds definitions that must
2142 be disambiguated some other way.) To confuse the issue further, we use
2143 the word with two overloaded definitions: to describe how you can
2144 define your own L</subroutine> to hide a built-in L</function> of the
2145 same name (see L<perlsub/Overriding Built-in Functions>) and to
2146 describe how you can define a replacement L</method> in a L</derived
2147 class> to hide a L</base class>'s method of the same name (see
2152 The one user (apart from the superuser) who has absolute control over
2153 a L</file>. A file may also have a L</group> of users who may
2154 exercise joint ownership if the real owner permits it. See
2155 L</permission bits>.
2165 A L</namespace> for global L<variables|/variable>,
2166 L<subroutines|/subroutine>, and the like, such that they can be kept
2167 separate from like-named L<symbols|/symbol> in other namespaces. In a
2168 sense, only the package is global, since the symbols in the package's
2169 symbol table are only accessible from code compiled outside the
2170 package by naming the package. But in another sense, all package
2171 symbols are also globals--they're just well-organized globals.
2175 Short for L</scratchpad>.
2187 See L</syntax tree>.
2191 The subtle but sometimes brutal art of attempting to turn your
2192 possibly malformed program into a valid L</syntax tree>.
2196 To fix by applying one, as it were. In the realm of hackerdom, a
2197 listing of the differences between two versions of a program as might
2198 be applied by the I<patch>(1) program when you want to fix a bug or
2199 upgrade your old version.
2203 The list of L<directories|/directory> the system searches to find a
2204 program you want to L</execute>. The list is stored as one of your
2205 L<environment variables|/environment variable>, accessible in Perl as
2210 A fully qualified filename such as I</usr/bin/perl>. Sometimes
2211 confused with L</PATH>.
2215 A template used in L</pattern matching>.
2217 =item pattern matching
2219 Taking a pattern, usually a L</regular expression>, and trying the
2220 pattern various ways on a string to see whether there's any way to
2221 make it fit. Often used to pick interesting tidbits out of a file.
2223 =item permission bits
2225 Bits that the L</owner> of a file sets or unsets to allow or disallow
2226 access to other people. These flag bits are part of the L</mode> word
2227 returned by the L<stat|perlfunc/stat> built-in when you ask about a
2228 file. On Unix systems, you can check the I<ls>(1) manpage for more
2233 What you get when you do C<Perl++> twice. Doing it only once will
2234 curl your hair. You have to increment it eight times to shampoo your
2235 hair. Lather, rinse, iterate.
2239 A direct L</connection> that carries the output of one L</process> to
2240 the input of another without an intermediate temporary file. Once the
2241 pipe is set up, the two processes in question can read and write as if
2242 they were talking to a normal file, with some caveats.
2246 A series of L<processes|/process> all in a row, linked by
2247 L<pipes|/pipe>, where each passes its output stream to the next.
2251 The entire hardware and software context in which a program runs. A
2252 program written in a platform-dependent language might break if you
2253 change any of: machine, operating system, libraries, compiler, or
2254 system configuration. The I<perl> interpreter has to be compiled
2255 differently for each platform because it is implemented in C, but
2256 programs written in the Perl language are largely
2257 platform-independent.
2261 The markup used to embed documentation into your Perl code. See
2266 A L</variable> in a language like C that contains the exact memory
2267 location of some other item. Perl handles pointers internally so you
2268 don't have to worry about them. Instead, you just use symbolic
2269 pointers in the form of L<keys|/key> and L</variable> names, or L<hard
2270 references|/hard reference>, which aren't pointers (but act like
2271 pointers and do in fact contain pointers).
2275 The notion that you can tell an L</object> to do something generic,
2276 and the object will interpret the command in different ways depending
2277 on its type. [E<lt>Gk many shapes]
2281 The part of the address of a TCP or UDP socket that directs packets to
2282 the correct process after finding the right machine, something like
2283 the phone extension you give when you reach the company operator.
2284 Also, the result of converting code to run on a different platform
2285 than originally intended, or the verb denoting this conversion.
2289 Once upon a time, C code compilable under both BSD and SysV. In
2290 general, code that can be easily converted to run on another
2291 L</platform>, where "easily" can be defined however you like, and
2292 usually is. Anything may be considered portable if you try hard
2293 enough. See I<mobile home> or I<London Bridge>.
2297 Someone who "carries" software from one L</platform> to another.
2298 Porting programs written in platform-dependent languages such as C can
2299 be difficult work, but porting programs like Perl is very much worth
2304 The Portable Operating System Interface specification.
2308 An L</operator> that follows its L</operand>, as in C<$x++>.
2312 An internal shorthand for a "push-pop" code, that is, C code
2313 implementing Perl's stack machine.
2317 A standard module whose practical hints and suggestions are received
2318 (and possibly ignored) at compile time. Pragmas are named in all
2323 The rules of conduct that, in the absence of other guidance, determine
2324 what should happen first. For example, in the absence of parentheses,
2325 you always do multiplication before addition.
2329 An L</operator> that precedes its L</operand>, as in C<++$x>.
2333 What some helper L</process> did to transform the incoming data into a
2334 form more suitable for the current process. Often done with an
2335 incoming L</pipe>. See also L</C preprocessor>.
2343 An instance of a running program. Under multitasking systems like
2344 Unix, two or more separate processes could be running the same program
2345 independently at the same time--in fact, the L<fork|perlfunc/fork>
2346 function is designed to bring about this happy state of affairs.
2347 Under other operating systems, processes are sometimes called
2348 "threads", "tasks", or "jobs", often with slight nuances in meaning.
2350 =item program generator
2352 A system that algorithmically writes code for you in a high-level
2353 language. See also L</code generator>.
2355 =item progressive matching
2357 L<Pattern matching|/pattern matching> that picks up where it left off before.
2361 See either L</instance variable> or L</character property>.
2365 In networking, an agreed-upon way of sending messages back and forth
2366 so that neither correspondent will get too confused.
2370 An optional part of a L</subroutine> declaration telling the Perl
2371 compiler how many and what flavor of arguments may be passed as
2372 L</actual arguments>, so that you can write subroutine calls that
2373 parse much like built-in functions. (Or don't parse, as the case may
2376 =item pseudofunction
2378 A construct that sometimes looks like a function but really isn't.
2379 Usually reserved for L</lvalue> modifiers like L<my|perlfunc/my>, for
2380 L</context> modifiers like L<scalar|perlfunc/scalar>, and for the
2381 pick-your-own-quotes constructs, C<q//>, C<qq//>, C<qx//>, C<qw//>,
2382 C<qr//>, C<m//>, C<s///>, C<y///>, and C<tr///>.
2386 A reference to an array whose initial element happens to hold a
2387 reference to a hash. You can treat a pseudohash reference as either
2388 an array reference or a hash reference.
2392 An L</operator> that looks something like a L</literal>, such as the
2393 output-grabbing operator, C<`>I<C<command>>C<`>.
2397 Something not owned by anybody. Perl is copyrighted and is thus
2398 I<not> in the public domain--it's just L</freely available> and
2399 L</freely redistributable>.
2403 A notional "baton" handed around the Perl community indicating who is
2404 the lead integrator in some arena of development.
2408 A L</pumpkin> holder, the person in charge of pumping the pump, or at
2409 least priming it. Must be willing to play the part of the Great
2410 Pumpkin now and then.
2414 A "pointer value", which is Perl Internals Talk for a C<char*>.
2424 Possessing a complete name. The symbol C<$Ent::moot> is qualified;
2425 C<$moot> is unqualified. A fully qualified filename is specified from
2426 the top-level directory.
2430 A component of a L</regular expression> specifying how many times the
2431 foregoing L</atom> may occur.
2441 With respect to files, one that has the proper permission bit set to
2442 let you access the file. With respect to computer programs, one
2443 that's written well enough that someone has a chance of figuring out
2444 what it's trying to do.
2448 The last rites performed by a parent L</process> on behalf of a
2449 deceased child process so that it doesn't remain a L</zombie>. See
2450 the L<wait|perlfunc/wait> and L<waitpid|perlfunc/waitpid> function
2455 A set of related data values in a L</file> or L</stream>, often
2456 associated with a unique L</key> field. In Unix, often commensurate
2457 with a L</line>, or a blank-line-terminated set of lines (a
2458 "paragraph"). Each line of the I</etc/passwd> file is a record, keyed
2459 on login name, containing information about that user.
2463 The art of defining something (at least partly) in terms of itself,
2464 which is a naughty no-no in dictionaries but often works out okay in
2465 computer programs if you're careful not to recurse forever, which is
2466 like an infinite loop with more spectacular failure modes.
2470 Where you look to find a pointer to information somewhere else. (See
2471 L</indirection>.) References come in two flavors, L<symbolic
2472 references|/symbolic reference> and L<hard references|/hard
2477 Whatever a reference refers to, which may or may not have a name.
2478 Common types of referents include scalars, arrays, hashes, and
2483 See L</regular expression>.
2485 =item regular expression
2487 A single entity with various interpretations, like an elephant. To a
2488 computer scientist, it's a grammar for a little language in which some
2489 strings are legal and others aren't. To normal people, it's a pattern
2490 you can use to find what you're looking for when it varies from case
2491 to case. Perl's regular expressions are far from regular in the
2492 theoretical sense, but in regular use they work quite well. Here's a
2493 regular expression: C</Oh s.*t./>. This will match strings like "C<Oh
2494 say can you see by the dawn's early light>" and "C<Oh sit!>". See
2497 =item regular expression modifier
2499 An option on a pattern or substitution, such as C</i> to render the
2500 pattern case insensitive. See also L</cloister>.
2504 A L</file> that's not a L</directory>, a L</device>, a named L</pipe>
2505 or L</socket>, or a L</symbolic link>. Perl uses the C<-f> file test
2506 operator to identify regular files. Sometimes called a "plain" file.
2508 =item relational operator
2510 An L</operator> that says whether a particular ordering relationship
2511 is L</true> about a pair of L<operands|/operand>. Perl has both
2512 numeric and string relational operators. See L</collating sequence>.
2514 =item reserved words
2516 A word with a specific, built-in meaning to a L</compiler>, such as
2517 C<if> or L<delete|perlfunc/delete>. In many languages (not Perl),
2518 it's illegal to use reserved words to name anything else. (Which is
2519 why they're reserved, after all.) In Perl, you just can't use them to
2520 name L<labels|/label> or L<filehandles|/filehandle>. Also called
2525 The L</value> produced by a L</subroutine> or L</expression> when
2526 evaluated. In Perl, a return value may be either a L</list> or a
2531 Request For Comment, which despite the timid connotations is the name
2532 of a series of important standards documents.
2536 A L</bit shift> that divides a number by some power of 2.
2540 The superuser (UID == 0). Also, the top-level directory of the
2545 What you are told when someone thinks you should Read The Fine Manual.
2549 Any time after Perl starts running your main program. See also
2550 L</compile phase>. Run phase is mostly spent in L</run time> but may
2551 also be spent in L</compile time> when L<require|perlfunc/require>,
2552 L<do|perlfunc/do> C<FILE>, or L<eval|perlfunc/eval> C<STRING>
2553 operators are executed or when a substitution uses the C</ee>
2558 The time when Perl is actually doing what your code says to do, as
2559 opposed to the earlier period of time when it was trying to figure out
2560 whether what you said made any sense whatsoever, which is L</compile
2563 =item run-time pattern
2565 A pattern that contains one or more variables to be interpolated
2566 before parsing the pattern as a L</regular expression>, and that
2567 therefore cannot be analyzed at compile time, but must be re-analyzed
2568 each time the pattern match operator is evaluated. Run-time patterns
2569 are useful but expensive.
2573 A recreational vehicle, not to be confused with vehicular recreation.
2574 RV also means an internal Reference Value of the type a L</scalar> can
2575 hold. See also L</IV> and L</NV> if you're not confused yet.
2579 A L</value> that you might find on the right side of an
2580 L</assignment>. See also L</lvalue>.
2590 A simple, singular value; a number, L</string>, or L</reference>.
2592 =item scalar context
2594 The situation in which an L</expression> is expected by its
2595 surroundings (the code calling it) to return a single L</value> rather
2596 than a L</list> of values. See also L</context> and L</list context>.
2597 A scalar context sometimes imposes additional constraints on the
2598 return value--see L</string context> and L</numeric context>.
2599 Sometimes we talk about a L</Boolean context> inside conditionals, but
2600 this imposes no additional constraints, since any scalar value,
2601 whether numeric or L</string>, is already true or false.
2603 =item scalar literal
2605 A number or quoted L</string>--an actual L</value> in the text of your
2606 program, as opposed to a L</variable>.
2610 A value that happens to be a L</scalar> as opposed to a L</list>.
2612 =item scalar variable
2614 A L</variable> prefixed with C<$> that holds a single value.
2618 How far away you can see a variable from, looking through one. Perl
2619 has two visibility mechanisms: it does L</dynamic scoping> of
2620 L<local|perlfunc/local> L<variables|/variable>, meaning that the rest
2621 of the L</block>, and any L<subroutines|/subroutine> that are called
2622 by the rest of the block, can see the variables that are local to the
2623 block. Perl does L</lexical scoping> of L<my|perlfunc/my> variables,
2624 meaning that the rest of the block can see the variable, but other
2625 subroutines called by the block I<cannot> see the variable.
2629 The area in which a particular invocation of a particular file or
2630 subroutine keeps some of its temporary values, including any lexically
2635 A text L</file> that is a program intended to be L<executed|/execute>
2636 directly rather than L<compiled|/compiler> to another form of file
2637 before execution. Also, in the context of L</Unicode>, a writing
2638 system for a particular language or group of languages, such as Greek,
2639 Bengali, or Klingon.
2643 A L</cracker> who is not a L</hacker>, but knows just enough to run
2644 canned scripts. A cargo-cult programmer.
2648 A venerable Stream EDitor from which Perl derives some of its ideas.
2652 A fancy kind of interlock that prevents multiple L<threads|/thread> or
2653 L<processes|/process> from using up the same resources simultaneously.
2657 A L</character> or L</string> that keeps two surrounding strings from
2658 being confused with each other. The L<split|perlfunc/split> function
2659 works on separators. Not to be confused with L<delimiters|/delimiter>
2660 or L<terminators|/terminator>. The "or" in the previous sentence
2661 separated the two alternatives.
2665 Putting a fancy L</data structure> into linear order so that it can be
2666 stored as a L</string> in a disk file or database or sent through a
2667 L</pipe>. Also called marshalling.
2671 In networking, a L</process> that either advertises a L</service> or
2672 just hangs around at a known location and waits for L<clients|/client>
2673 who need service to get in touch with it.
2677 Something you do for someone else to make them happy, like giving them
2678 the time of day (or of their life). On some machines, well-known
2679 services are listed by the L<getservent|perlfunc/getservent> function.
2683 Same as L</setuid>, only having to do with giving away L</group>
2688 Said of a program that runs with the privileges of its L</owner>
2689 rather than (as is usually the case) the privileges of whoever is
2690 running it. Also describes the bit in the mode word (L</permission
2691 bits>) that controls the feature. This bit must be explicitly set by
2692 the owner to enable this feature, and the program must be carefully
2693 written not to give away more privileges than it ought to.
2697 A piece of L</memory> accessible by two different
2698 L<processes|/process> who otherwise would not see each other's memory.
2702 Irish for the whole McGillicuddy. In Perl culture, a portmanteau of
2703 "sharp" and "bang", meaning the C<#!> sequence that tells the system
2704 where to find the interpreter.
2708 A L</command>-line L</interpreter>. The program that interactively
2709 gives you a prompt, accepts one or more L<lines|/line> of input, and
2710 executes the programs you mentioned, feeding each of them their proper
2711 L<arguments|/argument> and input data. Shells can also execute
2712 scripts containing such commands. Under Unix, typical shells include
2713 the Bourne shell (I</bin/sh>), the C shell (I</bin/csh>), and the Korn
2714 shell (I</bin/ksh>). Perl is not strictly a shell because it's not
2715 interactive (although Perl programs can be interactive).
2719 Something extra that happens when you evaluate an L</expression>.
2720 Nowadays it can refer to almost anything. For example, evaluating a
2721 simple assignment statement typically has the "side effect" of
2722 assigning a value to a variable. (And you thought assigning the value
2723 was your primary intent in the first place!) Likewise, assigning a
2724 value to the special variable C<$|> (C<$AUTOFLUSH>) has the side
2725 effect of forcing a flush after every L<write|perlfunc/write> or
2726 L<print|perlfunc/print> on the currently selected filehandle.
2730 A bolt out of the blue; that is, an event triggered by the
2731 L</operating system>, probably when you're least expecting it.
2733 =item signal handler
2735 A L</subroutine> that, instead of being content to be called in the
2736 normal fashion, sits around waiting for a bolt out of the blue before
2737 it will deign to L</execute>. Under Perl, bolts out of the blue are
2738 called signals, and you send them with the L<kill|perlfunc/kill>
2739 built-in. See L<perlvar/%SIG> and L<perlipc/Signals>.
2741 =item single inheritance
2743 The features you got from your mother, if she told you that you don't
2744 have a father. (See also L</inheritance> and L</multiple
2745 inheritance>.) In computer languages, the notion that
2746 L<classes|/class> reproduce asexually so that a given class can only
2747 have one direct ancestor or L</base class>. Perl supplies no such
2748 restriction, though you may certainly program Perl that way if you
2753 A selection of any number of L<elements|/element> from a L</list>,
2754 L</array>, or L</hash>.
2758 To read an entire L</file> into a L</string> in one operation.
2762 An endpoint for network communication among multiple
2763 L<processes|/process> that works much like a telephone or a post
2764 office box. The most important thing about a socket is its L</network
2765 address> (like a phone number). Different kinds of sockets have
2766 different kinds of addresses--some look like filenames, and some
2769 =item soft reference
2771 See L</symbolic reference>.
2775 A special kind of L</module> that does L</preprocessing> on your
2776 script just before it gets to the L</tokener>.
2780 A device you can put things on the top of, and later take them back
2781 off in the opposite order in which you put them on. See L</LIFO>.
2785 Included in the official Perl distribution, as in a standard module, a
2786 standard tool, or a standard Perl L</manpage>.
2788 =item standard error
2790 The default output L</stream> for nasty remarks that don't belong in
2791 L</standard output>. Represented within a Perl program by the
2792 L</filehandle> L</STDERR>. You can use this stream explicitly, but the
2793 L<die|perlfunc/die> and L<warn|perlfunc/warn> built-ins write to your
2794 standard error stream automatically.
2798 A standard C library for doing L<buffered|/buffer> input and output to
2799 the L</operating system>. (The "standard" of standard I/O is only
2800 marginally related to the "standard" of standard input and output.)
2801 In general, Perl relies on whatever implementation of standard I/O a
2802 given operating system supplies, so the buffering characteristics of a
2803 Perl program on one machine may not exactly match those on another
2804 machine. Normally this only influences efficiency, not semantics. If
2805 your standard I/O package is doing block buffering and you want it to
2806 L</flush> the buffer more often, just set the C<$|> variable to a true
2809 =item standard input
2811 The default input L</stream> for your program, which if possible
2812 shouldn't care where its data is coming from. Represented within a
2813 Perl program by the L</filehandle> L</STDIN>.
2815 =item standard output
2817 The default output L</stream> for your program, which if possible
2818 shouldn't care where its data is going. Represented within a Perl
2819 program by the L</filehandle> L</STDOUT>.
2821 =item stat structure
2823 A special internal spot in which Perl keeps the information about the
2824 last L</file> on which you requested information.
2828 A L</command> to the computer about what to do next, like a step in a
2829 recipe: "Add marmalade to batter and mix until mixed." A statement is
2830 distinguished from a L</declaration>, which doesn't tell the computer
2831 to do anything, but just to learn something.
2833 =item statement modifier
2835 A L</conditional> or L</loop> that you put after the L</statement>
2836 instead of before, if you know what we mean.
2840 Varying slowly compared to something else. (Unfortunately, everything
2841 is relatively stable compared to something else, except for certain
2842 elementary particles, and we're not so sure about them.) In
2843 computers, where things are supposed to vary rapidly, "static" has a
2844 derogatory connotation, indicating a slightly dysfunctional
2845 L</variable>, L</subroutine>, or L</method>. In Perl culture, the
2846 word is politely avoided.
2850 No such thing. See L</class method>.
2852 =item static scoping
2854 No such thing. See L</lexical scoping>.
2856 =item static variable
2858 No such thing. Just use a L</lexical variable> in a scope larger than
2859 your L</subroutine>.
2863 The L</value> returned to the parent L</process> when one of its child
2864 processes dies. This value is placed in the special variable C<$?>.
2865 Its upper eight L<bits|/bit> are the exit status of the defunct
2866 process, and its lower eight bits identify the signal (if any) that
2867 the process died from. On Unix systems, this status value is the same
2868 as the status word returned by I<wait>(2). See L<perlfunc/system>.
2872 See L</standard error>.
2876 See L</standard input>.
2880 See L</standard IE<sol>O>.
2884 See L</standard output>.
2888 A flow of data into or out of a process as a steady sequence of bytes
2889 or characters, without the appearance of being broken up into packets.
2890 This is a kind of L</interface>--the underlying L</implementation> may
2891 well break your data up into separate packets for delivery, but this
2896 A sequence of characters such as "He said !@#*&%@#*?!". A string does
2897 not have to be entirely printable.
2899 =item string context
2901 The situation in which an expression is expected by its surroundings
2902 (the code calling it) to return a L</string>. See also L</context>
2903 and L</numeric context>.
2905 =item stringification
2907 The process of producing a L</string> representation of an abstract
2912 C keyword introducing a structure definition or name.
2916 See L</data structure>.
2920 See L</derived class>.
2924 A component of a L</regular expression> pattern.
2928 A named or otherwise accessible piece of program that can be invoked
2929 from elsewhere in the program in order to accomplish some sub-goal of
2930 the program. A subroutine is often parameterized to accomplish
2931 different but related things depending on its input
2932 L<arguments|/argument>. If the subroutine returns a meaningful
2933 L</value>, it is also called a L</function>.
2937 A L</value> that indicates the position of a particular L</array>
2938 L</element> in an array.
2942 Changing parts of a string via the C<s///> operator. (We avoid use of
2943 this term to mean L</variable interpolation>.)
2947 A portion of a L</string>, starting at a certain L</character>
2948 position (L</offset>) and proceeding for a certain number of
2957 The person whom the L</operating system> will let do almost anything.
2958 Typically your system administrator or someone pretending to be your
2959 system administrator. On Unix systems, the L</root> user. On Windows
2960 systems, usually the Administrator user.
2964 Short for "scalar value". But within the Perl interpreter every
2965 L</referent> is treated as a member of a class derived from SV, in an
2966 object-oriented sort of way. Every L</value> inside Perl is passed
2967 around as a C language C<SV*> pointer. The SV L</struct> knows its
2968 own "referent type", and the code is smart enough (we hope) not to try
2969 to call a L</hash> function on a L</subroutine>.
2973 An option you give on a command line to influence the way your program
2974 works, usually introduced with a minus sign. The word is also used as
2975 a nickname for a L</switch statement>.
2977 =item switch cluster
2979 The combination of multiple command-line switches (e.g., B<-a -b -c>)
2980 into one switch (e.g., B<-abc>). Any switch with an additional
2981 L</argument> must be the last switch in a cluster.
2983 =item switch statement
2985 A program technique that lets you evaluate an L</expression> and then,
2986 based on the value of the expression, do a multiway branch to the
2987 appropriate piece of code for that value. Also called a "case
2988 structure", named after the similar Pascal construct. Most switch
2989 statements in Perl are spelled C<for>. See L<perlsyn/Basic BLOCKs and
2994 Generally, any L</token> or L</metasymbol>. Often used more
2995 specifically to mean the sort of name you might find in a L</symbol
3000 Where a L</compiler> remembers symbols. A program like Perl must
3001 somehow remember all the names of all the L<variables|/variable>,
3002 L<filehandles|/filehandle>, and L<subroutines|/subroutine> you've
3003 used. It does this by placing the names in a symbol table, which is
3004 implemented in Perl using a L</hash table>. There is a separate
3005 symbol table for each L</package> to give each package its own
3008 =item symbolic debugger
3010 A program that lets you step through the L<execution|/execute> of your
3011 program, stopping or printing things out here and there to see whether
3012 anything has gone wrong, and if so, what. The "symbolic" part just
3013 means that you can talk to the debugger using the same symbols with
3014 which your program is written.
3018 An alternate filename that points to the real L</filename>, which in
3019 turn points to the real L</file>. Whenever the L</operating system>
3020 is trying to parse a L</pathname> containing a symbolic link, it
3021 merely substitutes the new name and continues parsing.
3023 =item symbolic reference
3025 A variable whose value is the name of another variable or subroutine.
3026 By L<dereferencing|/dereference> the first variable, you can get at
3027 the second one. Symbolic references are illegal under L<use strict
3028 'refs'|strict/strict refs>.
3032 Programming in which the orderly sequence of events can be determined;
3033 that is, when things happen one after the other, not at the same time.
3035 =item syntactic sugar
3037 An alternative way of writing something more easily; a shortcut.
3041 From Greek, "with-arrangement". How things (particularly symbols) are
3042 put together with each other.
3046 An internal representation of your program wherein lower-level
3047 L<constructs|/construct> dangle off the higher-level constructs
3052 A L</function> call directly to the L</operating system>. Many of the
3053 important subroutines and functions you use aren't direct system
3054 calls, but are built up in one or more layers above the system call
3055 level. In general, Perl programmers don't need to worry about the
3056 distinction. However, if you do happen to know which Perl functions
3057 are really syscalls, you can predict which of these will set the C<$!>
3058 (C<$ERRNO>) variable on failure. Unfortunately, beginning programmers
3059 often confusingly employ the term "system call" to mean what happens
3060 when you call the Perl L<system|perlfunc/system> function, which
3061 actually involves many syscalls. To avoid any confusion, we nearly
3062 always use say "syscall" for something you could call indirectly via
3063 Perl's L<syscall|perlfunc/syscall> function, and never for something
3064 you would call with Perl's L<system|perlfunc/system> function.
3074 Said of data derived from the grubby hands of a user and thus unsafe
3075 for a secure program to rely on. Perl does taint checks if you run a
3076 L</setuid> (or L</setgid>) program, or if you use the B<-T> switch.
3080 Short for Transmission Control Protocol. A protocol wrapped around
3081 the Internet Protocol to make an unreliable packet transmission
3082 mechanism appear to the application program to be a reliable
3083 L</stream> of bytes. (Usually.)
3087 Short for a "terminal", that is, a leaf node of a L</syntax tree>. A
3088 thing that functions grammatically as an L</operand> for the operators
3093 A L</character> or L</string> that marks the end of another string.
3094 The C<$/> variable contains the string that terminates a
3095 L<readline|perlfunc/readline> operation, which L<chomp|perlfunc/chomp>
3096 deletes from the end. Not to be confused with
3097 L<delimiters|/delimiter> or L<separators|/separator>. The period at
3098 the end of this sentence is a terminator.
3102 An L</operator> taking three L<operands|/operand>. Sometimes
3103 pronounced L</trinary>.
3107 A L</string> or L</file> containing primarily printable characters.
3111 Like a forked process, but without L</fork>'s inherent memory
3112 protection. A thread is lighter weight than a full process, in that a
3113 process could have multiple threads running around in it, all fighting
3114 over the same process's memory space unless steps are taken to protect
3115 threads from each other. See L<threads>.
3119 The bond between a magical variable and its implementation class. See
3120 L<perlfunc/tie> and L<perltie>.
3124 There's More Than One Way To Do It, the Perl Motto. The notion that
3125 there can be more than one valid path to solving a programming problem
3126 in context. (This doesn't mean that more ways are always better or
3127 that all possible paths are equally desirable--just that there need
3128 not be One True Way.) Pronounced TimToady.
3132 A morpheme in a programming language, the smallest unit of text with
3133 semantic significance.
3137 A module that breaks a program text into a sequence of
3138 L<tokens|/token> for later analysis by a parser.
3142 Splitting up a program text into L<tokens|/token>. Also known as
3143 "lexing", in which case you get "lexemes" instead of tokens.
3145 =item toolbox approach
3147 The notion that, with a complete set of simple tools that work well
3148 together, you can build almost anything you want. Which is fine if
3149 you're assembling a tricycle, but if you're building a defranishizing
3150 comboflux regurgalator, you really want your own machine shop in which
3151 to build special tools. Perl is sort of a machine shop.
3155 To turn one string representation into another by mapping each
3156 character of the source string to its corresponding character in the
3158 L<perlop/trE<sol>SEARCHLISTE<sol>REPLACEMENTLISTE<sol>cds>.
3162 An event that causes a L</handler> to be run.
3166 Not a stellar system with three stars, but an L</operator> taking
3167 three L<operands|/operand>. Sometimes pronounced L</ternary>.
3171 A venerable typesetting language from which Perl derives the name of
3172 its C<$%> variable and which is secretly used in the production of
3177 Any scalar value that doesn't evaluate to 0 or C<"">.
3181 Emptying a file of existing contents, either automatically when
3182 opening a file for writing or explicitly via the
3183 L<truncate|perlfunc/truncate> function.
3187 See L</data type> and L</class>.
3191 Converting data from one type to another. C permits this. Perl does
3192 not need it. Nor want it.
3196 A L</lexical variable> that is declared with a L</class> type: C<my
3201 A type definition in the C language.
3205 Use of a single identifier, prefixed with C<*>. For example, C<*name>
3206 stands for any or all of C<$name>, C<@name>, C<%name>, C<&name>, or
3207 just C<name>. How you use it determines whether it is interpreted as
3208 all or only one of them. See L<perldata/Typeglobs and Filehandles>.
3212 A description of how C types may be transformed to and from Perl types
3213 within an L</extension> module written in L</XS>.
3223 User Datagram Protocol, the typical way to send L<datagrams|/datagram>
3228 A user ID. Often used in the context of L</file> or L</process>
3233 A mask of those L</permission bits> that should be forced off when
3234 creating files or directories, in order to establish a policy of whom
3235 you'll ordinarily deny access to. See the L<umask|perlfunc/umask>
3238 =item unary operator
3240 An operator with only one L</operand>, like C<!> or
3241 L<chdir|perlfunc/chdir>. Unary operators are usually prefix
3242 operators; that is, they precede their operand. The C<++> and C<-->
3243 operators can be either prefix or postfix. (Their position I<does>
3244 change their meanings.)
3248 A character set comprising all the major character sets of the world,
3249 more or less. See L<perlunicode> and L<http://www.unicode.org>.
3253 A very large and constantly evolving language with several alternative
3254 and largely incompatible syntaxes, in which anyone can define anything
3255 any way they choose, and usually do. Speakers of this language think
3256 it's easy to learn because it's so easily twisted to one's own ends,
3257 but dialectical differences make tribal intercommunication nearly
3258 impossible, and travelers are often reduced to a pidgin-like subset of
3259 the language. To be universally understood, a Unix shell programmer
3260 must spend years of study in the art. Many have abandoned this
3261 discipline and now communicate via an Esperanto-like language called
3264 In ancient times, Unix was also used to refer to some code that a
3265 couple of people at Bell Labs wrote to make use of a PDP-7 computer
3266 that wasn't doing much of anything else at the time.
3276 An actual piece of data, in contrast to all the variables, references,
3277 keys, indexes, operators, and whatnot that you need to access the
3282 A named storage location that can hold any of various kinds of
3283 L</value>, as your program sees fit.
3285 =item variable interpolation
3287 The L</interpolation> of a scalar or array variable into a string.
3291 Said of a L</function> that happily receives an indeterminate number
3292 of L</actual arguments>.
3296 Mathematical jargon for a list of L<scalar values|/scalar value>.
3300 Providing the appearance of something without the reality, as in:
3301 virtual memory is not real memory. (See also L</memory>.) The
3302 opposite of "virtual" is "transparent", which means providing the
3303 reality of something without the appearance, as in: Perl handles the
3304 variable-length UTF-8 character encoding transparently.
3308 A form of L</scalar context> in which an L</expression> is not
3309 expected to return any L</value> at all and is evaluated for its
3310 L</side effects> alone.
3314 A "version" or "vector" L</string> specified with a C<v> followed by a
3315 series of decimal integers in dot notation, for instance,
3316 C<v1.20.300.4000>. Each number turns into a L</character> with the
3317 specified ordinal value. (The C<v> is optional when there are at
3318 least three integers.)
3328 A message printed to the L</STDERR> stream to the effect that something
3329 might be wrong but isn't worth blowing up over. See L<perlfunc/warn>
3330 and the L<warnings> pragma.
3332 =item watch expression
3334 An expression which, when its value changes, causes a breakpoint in
3339 A L</character> that moves your cursor but doesn't otherwise put
3340 anything on your screen. Typically refers to any of: space, tab, line
3341 feed, carriage return, or form feed.
3345 In normal "computerese", the piece of data of the size most
3346 efficiently handled by your computer, typically 32 bits or so, give or
3347 take a few powers of 2. In Perl culture, it more often refers to an
3348 alphanumeric L</identifier> (including underscores), or to a string of
3349 nonwhitespace L<characters|/character> bounded by whitespace or string
3352 =item working directory
3354 Your current L</directory>, from which relative pathnames are
3355 interpreted by the L</operating system>. The operating system knows
3356 your current directory because you told it with a
3357 L<chdir|perlfunc/chdir> or because you started out in the place where
3358 your parent L</process> was when you were born.
3362 A program or subroutine that runs some other program or subroutine for
3363 you, modifying some of its input or output to better suit your
3368 What You See Is What You Get. Usually used when something that
3369 appears on the screen matches how it will eventually look, like Perl's
3370 L<format|perlfunc/format> declarations. Also used to mean the
3371 opposite of magic because everything works exactly as it appears, as
3372 in the three-argument form of L<open|perlfunc/open>.
3382 An extraordinarily exported, expeditiously excellent, expressly
3383 eXternal Subroutine, executed in existing C or C++ or in an exciting
3384 new extension language called (exasperatingly) XS. Examine L<perlxs>
3385 for the exact explanation or L<perlxstut> for an exemplary unexacting
3390 An external L</subroutine> defined in L</XS>.
3400 Yet Another Compiler Compiler. A parser generator without which Perl
3401 probably would not have existed. See the file I<perly.y> in the Perl
3402 source distribution.
3412 A subpattern L</assertion> matching the L</null string> between
3413 L<characters|/character>.
3417 A process that has died (exited) but whose parent has not yet received
3418 proper notification of its demise by virtue of having called
3419 L<wait|perlfunc/wait> or L<waitpid|perlfunc/waitpid>. If you
3420 L<fork|perlfunc/fork>, you must clean up after your child processes
3421 when they exit, or else the process table will fill up and your system
3422 administrator will Not Be Happy with you.
3426 =head1 AUTHOR AND COPYRIGHT
3428 Based on the Glossary of Programming Perl, Third Edition,
3429 by Larry Wall, Tom Christiansen & Jon Orwant.
3430 Copyright (c) 2000, 1996, 1991 O'Reilly Media, Inc.
3431 This document may be distributed under the same terms as Perl itself.