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1=head1 NAME
2
3perlglossary - Perl Glossary
4
5=head1 DESCRIPTION
6
7A glossary of terms (technical and otherwise) used in the Perl documentation.
8Other useful sources include the Free On-Line Dictionary of Computing
9L<http://foldoc.doc.ic.ac.uk/foldoc/index.html>, the Jargon File
10L<http://catb.org/~esr/jargon/>, and Wikipedia L<http://www.wikipedia.org/>.
11
5bbd0522 12=head2 A
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13
14=over 4
15
16=item accessor methods
17
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18A L</method> used to indirectly inspect or update an L</object>'s
19state (its L<instance variables|/instance variable>).
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20
21=item actual arguments
22
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23The L<scalar values|/scalar value> that you supply to a L</function>
24or L</subroutine> when you call it. For instance, when you call
97a1d740 25C<power("puff")>, the string C<"puff"> is the actual argument. See
27ed30b8 26also L</argument> and L</formal arguments>.
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27
28=item address operator
29
30Some languages work directly with the memory addresses of values, but
31this can be like playing with fire. Perl provides a set of asbestos
32gloves for handling all memory management. The closest to an address
27ed30b8 33operator in Perl is the backslash operator, but it gives you a L</hard
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34reference>, which is much safer than a memory address.
35
36=item algorithm
37
38A well-defined sequence of steps, clearly enough explained that even a
39computer could do them.
40
41=item alias
42
43A nickname for something, which behaves in all ways as though you'd
44used the original name instead of the nickname. Temporary aliases are
45implicitly created in the loop variable for C<foreach> loops, in the
46C<$_> variable for L<map|perlfunc/map> or L<grep|perlfunc/grep>
47operators, in C<$a> and C<$b> during L<sort|perlfunc/sort>'s
27ed30b8 48comparison function, and in each element of C<@_> for the L</actual
97a1d740 49arguments> of a subroutine call. Permanent aliases are explicitly
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50created in L<packages|/package> by L<importing|/import> symbols or by
51assignment to L<typeglobs|/typeglob>. Lexically scoped aliases for
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52package variables are explicitly created by the L<our|perlfunc/our>
53declaration.
54
55=item alternatives
56
57A 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
59are separated with a single vertical bar: C<|>. Alternatives in
60normal Perl expressions are separated with a double vertical bar:
27ed30b8 61C<||>. Logical alternatives in L</Boolean> expressions are separated
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62with either C<||> or C<or>.
63
64=item anonymous
65
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66Used to describe a L</referent> that is not directly accessible
67through a named L</variable>. Such a referent must be indirectly
68accessible through at least one L</hard reference>. When the last
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69hard reference goes away, the anonymous referent is destroyed without
70pity.
71
72=item architecture
73
d7f8936a 74The kind of computer you're working on, where one "kind" of computer
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75means all those computers sharing a compatible machine language.
76Since Perl programs are (typically) simple text files, not executable
77images, a Perl program is much less sensitive to the architecture it's
78running on than programs in other languages, such as C, that are
27ed30b8 79compiled into machine code. See also L</platform> and L</operating
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80system>.
81
82=item argument
83
84A piece of data supplied to a L<program|/executable file>,
27ed30b8 85L</subroutine>, L</function>, or L</method> to tell it what it's
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86supposed to do. Also called a "parameter".
87
88=item ARGV
89
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90The name of the array containing the L</argument> L</vector> from the
91command line. If you use the empty C<< E<lt>E<gt> >> operator, L</ARGV> is
92the name of both the L</filehandle> used to traverse the arguments and
93the L</scalar> containing the name of the current input file.
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94
95=item arithmetical operator
96
27ed30b8 97A L</symbol> such as C<+> or C</> that tells Perl to do the arithmetic
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98you were supposed to learn in grade school.
99
100=item array
101
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102An ordered sequence of L<values|/value>, stored such that you can
103easily access any of the values using an integer L</subscript>
104that specifies the value's L</offset> in the sequence.
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105
106=item array context
107
108An archaic expression for what is more correctly referred to as
27ed30b8 109L</list context>.
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110
111=item ASCII
112
113The American Standard Code for Information Interchange (a 7-bit
114character set adequate only for poorly representing English text).
115Often used loosely to describe the lowest 128 values of the various
116ISO-8859-X character sets, a bunch of mutually incompatible 8-bit
27ed30b8 117codes best described as half ASCII. See also L</Unicode>.
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118
119=item assertion
120
27ed30b8 121A component of a L</regular expression> that must be true for the
97a1d740 122pattern to match but does not necessarily match any characters itself.
27ed30b8 123Often used specifically to mean a L</zero width> assertion.
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124
125=item assignment
126
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127An L</operator> whose assigned mission in life is to change the value
128of a L</variable>.
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129
130=item assignment operator
131
27ed30b8 132Either a regular L</assignment>, or a compound L</operator> composed
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133of an ordinary assignment and some other operator, that changes the
134value of a variable in place, that is, relative to its old value. For
135example, C<$a += 2> adds C<2> to C<$a>.
136
137=item associative array
138
27ed30b8 139See L</hash>. Please.
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140
141=item associativity
142
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143Determines whether you do the left L</operator> first or the right
144L</operator> first when you have "A L</operator> B L</operator> C" and
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145the two operators are of the same precedence. Operators like C<+> are
146left associative, while operators like C<**> are right associative.
147See L<perlop> for a list of operators and their associativity.
148
149=item asynchronous
150
151Said of events or activities whose relative temporal ordering is
152indeterminate because too many things are going on at once. Hence, an
153asynchronous event is one you didn't know when to expect.
154
155=item atom
156
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157A L</regular expression> component potentially matching a
158L</substring> containing one or more characters and treated as an
159indivisible syntactic unit by any following L</quantifier>. (Contrast
160with an L</assertion> that matches something of L</zero width> and may
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161not be quantified.)
162
163=item atomic operation
164
165When Democritus gave the word "atom" to the indivisible bits of
166matter, he meant literally something that could not be cut: I<a->
167(not) + I<tomos> (cuttable). An atomic operation is an action that
168can't be interrupted, not one forbidden in a nuclear-free zone.
169
170=item attribute
171
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172A new feature that allows the declaration of L<variables|/variable>
173and L<subroutines|/subroutine> with modifiers as in C<sub foo : locked
174method>. Also, another name for an L</instance variable> of an
175L</object>.
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176
177=item autogeneration
178
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179A feature of L</operator overloading> of L<objects|/object>, whereby
180the behavior of certain L<operators|/operator> can be reasonably
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181deduced using more fundamental operators. This assumes that the
182overloaded operators will often have the same relationships as the
183regular operators. See L<perlop>.
184
185=item autoincrement
186
cf525c36 187To add one to something automatically, hence the name of the C<++>
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188operator. To instead subtract one from something automatically is
189known as an "autodecrement".
190
191=item autoload
192
193To load on demand. (Also called "lazy" loading.) Specifically, to
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194call an L<AUTOLOAD|perlsub/Autoloading> subroutine on behalf of an
195undefined subroutine.
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196
197=item autosplit
198
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199To split a string automatically, as the B<-a> L</switch> does when
200running under B<-p> or B<-n> in order to emulate L</awk>. (See also
201the L<AutoSplit> module, which has nothing to do with the B<-a>
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202switch, but a lot to do with autoloading.)
203
204=item autovivification
205
206A Greco-Roman word meaning "to bring oneself to life". In Perl,
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207storage locations (L<lvalues|/lvalue>) spontaneously generate
208themselves as needed, including the creation of any L</hard reference>
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209values to point to the next level of storage. The assignment
210C<$a[5][5][5][5][5] = "quintet"> potentially creates five scalar
211storage locations, plus four references (in the first four scalar
212locations) pointing to four new anonymous arrays (to hold the last
213four scalar locations). But the point of autovivification is that you
214don't have to worry about it.
215
216=item AV
217
218Short for "array value", which refers to one of Perl's internal data
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219types that holds an L</array>. The L</AV> type is a subclass of
220L</SV>.
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221
222=item awk
223
224Descriptive editing term--short for "awkward". Also coincidentally
225refers to a venerable text-processing language from which Perl derived
226some of its high-level ideas.
227
228=back
229
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230=head2 B
231
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232=over 4
233
234=item backreference
235
236A substring L<captured|/capturing> by a subpattern within
27ed30b8 237unadorned parentheses in a L</regex>. Backslashed decimal numbers
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238(C<\1>, C<\2>, etc.) later in the same pattern refer back to the
239corresponding subpattern in the current match. Outside the pattern,
240the numbered variables (C<$1>, C<$2>, etc.) continue to refer to these
241same values, as long as the pattern was the last successful match of
242the current dynamic scope.
243
244=item backtracking
245
246The practice of saying, "If I had to do it all over, I'd do it
247differently," and then actually going back and doing it all over
248differently. Mathematically speaking, it's returning from an
249unsuccessful recursion on a tree of possibilities. Perl backtracks
27ed30b8 250when it attempts to match patterns with a L</regular expression>, and
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251its earlier attempts don't pan out. See L<perlre/Backtracking>.
252
253=item backward compatibility
254
255Means you can still run your old program because we didn't break any
256of the features or bugs it was relying on.
257
258=item bareword
259
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260A word sufficiently ambiguous to be deemed illegal under L<use strict
261'subs'|strict/strict subs>. In the absence of that stricture, a
262bareword is treated as if quotes were around it.
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263
264=item base class
265
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266A generic L</object> type; that is, a L</class> from which other, more
267specific classes are derived genetically by L</inheritance>. Also
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268called a "superclass" by people who respect their ancestors.
269
270=item big-endian
271
272From Swift: someone who eats eggs big end first. Also used of
27ed30b8 273computers that store the most significant L</byte> of a word at a
97a1d740 274lower byte address than the least significant byte. Often considered
27ed30b8 275superior to little-endian machines. See also L</little-endian>.
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276
277=item binary
278
279Having to do with numbers represented in base 2. That means there's
280basically two numbers, 0 and 1. Also used to describe a "non-text
281file", presumably because such a file makes full use of all the binary
27ed30b8 282bits in its bytes. With the advent of L</Unicode>, this distinction,
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283already suspect, loses even more of its meaning.
284
285=item binary operator
286
27ed30b8 287An L</operator> that takes two L<operands|/operand>.
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288
289=item bind
290
27ed30b8 291To assign a specific L</network address> to a L</socket>.
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292
293=item bit
294
295An integer in the range from 0 to 1, inclusive. The smallest possible
27ed30b8 296unit of information storage. An eighth of a L</byte> or of a dollar.
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297(The term "Pieces of Eight" comes from being able to split the old
298Spanish dollar into 8 bits, each of which still counted for money.
299That's why a 25-cent piece today is still "two bits".)
300
301=item bit shift
302
303The movement of bits left or right in a computer word, which has the
304effect of multiplying or dividing by a power of 2.
305
306=item bit string
307
27ed30b8 308A sequence of L<bits|/bit> that is actually being thought of as a
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309sequence of bits, for once.
310
311=item bless
312
313In corporate life, to grant official approval to a thing, as in, "The
314VP of Engineering has blessed our WebCruncher project." Similarly in
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315Perl, to grant official approval to a L</referent> so that it can
316function as an L</object>, such as a WebCruncher object. See
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317L<perlfunc/"bless">.
318
319=item block
320
27ed30b8 321What a L</process> does when it has to wait for something: "My process
97a1d740 322blocked waiting for the disk." As an unrelated noun, it refers to a
27ed30b8 323large chunk of data, of a size that the L</operating system> likes to
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324deal with (normally a power of two such as 512 or 8192). Typically
325refers to a chunk of data that's coming from or going to a disk file.
326
327=item BLOCK
328
329A syntactic construct consisting of a sequence of Perl
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330L<statements|/statement> that is delimited by braces. The C<if> and
331C<while> statements are defined in terms of L<BLOCKs|/BLOCK>, for instance.
97a1d740 332Sometimes we also say "block" to mean a lexical scope; that is, a
27ed30b8 333sequence of statements that act like a L</BLOCK>, such as within an
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334L<eval|perlfunc/eval> or a file, even though the statements aren't
335delimited by braces.
336
337=item block buffering
338
27ed30b8 339A method of making input and output efficient by passing one L</block>
97a1d740 340at a time. By default, Perl does block buffering to disk files. See
27ed30b8 341L</buffer> and L</command buffering>.
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342
343=item Boolean
344
27ed30b8 345A value that is either L</true> or L</false>.
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346
347=item Boolean context
348
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349A special kind of L</scalar context> used in conditionals to decide
350whether the L</scalar value> returned by an expression is L</true> or
351L</false>. Does not evaluate as either a string or a number. See
352L</context>.
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353
354=item breakpoint
355
356A spot in your program where you've told the debugger to stop
357L<execution|/execute> so you can poke around and see whether anything
358is wrong yet.
359
360=item broadcast
361
27ed30b8 362To send a L</datagram> to multiple destinations simultaneously.
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363
364=item BSD
365
366A psychoactive drug, popular in the 80s, probably developed at
367U. C. Berkeley or thereabouts. Similar in many ways to the
368prescription-only medication called "System V", but infinitely more
369useful. (Or, at least, more fun.) The full chemical name is
370"Berkeley Standard Distribution".
371
372=item bucket
373
27ed30b8 374A location in a L</hash table> containing (potentially) multiple
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375entries whose keys "hash" to the same hash value according to its hash
376function. (As internal policy, you don't have to worry about it,
377unless you're into internals, or policy.)
378
379=item buffer
380
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381A temporary holding location for data. L<Block buffering|/block
382buffering> means that the data is passed on to its destination
383whenever the buffer is full. L<Line buffering|/line buffering> means
384that it's passed on whenever a complete line is received. L<Command
385buffering|/command buffering> means that it's passed every time you do
386a L<print|perlfunc/print> command (or equivalent). If your output is
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387unbuffered, the system processes it one byte at a time without the use
388of a holding area. This can be rather inefficient.
389
390=item built-in
391
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392A L</function> that is predefined in the language. Even when hidden
393by L</overriding>, you can always get at a built-in function by
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394L<qualifying|/qualified> its name with the C<CORE::> pseudo-package.
395
396=item bundle
397
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398A group of related modules on L</CPAN>. (Also, sometimes refers to a
399group of command-line switches grouped into one L</switch cluster>.)
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400
401=item byte
402
27ed30b8 403A piece of data worth eight L<bits|/bit> in most places.
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404
405=item bytecode
406
407A pidgin-like language spoken among 'droids when they don't wish to
27ed30b8 408reveal their orientation (see L</endian>). Named after some similar
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409languages spoken (for similar reasons) between compilers and
410interpreters in the late 20th century. These languages are
411characterized by representing everything as a
412non-architecture-dependent sequence of bytes.
413
414=back
415
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416=head2 C
417
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418=over 4
419
420=item C
421
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422A language beloved by many for its inside-out L</type> definitions,
423inscrutable L</precedence> rules, and heavy L</overloading> of the
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424function-call mechanism. (Well, actually, people first switched to C
425because they found lowercase identifiers easier to read than upper.)
426Perl is written in C, so it's not surprising that Perl borrowed a few
427ideas from it.
428
429=item C preprocessor
430
431The typical C compiler's first pass, which processes lines beginning
432with C<#> for conditional compilation and macro definition and does
433various manipulations of the program text based on the current
434definitions. Also known as I<cpp>(1).
435
436=item call by reference
437
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438An L</argument>-passing mechanism in which the L</formal arguments>
439refer directly to the L</actual arguments>, and the L</subroutine> can
97a1d740 440change the actual arguments by changing the formal arguments. That
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441is, the formal argument is an L</alias> for the actual argument. See
442also L</call by value>.
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443
444=item call by value
445
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446An L</argument>-passing mechanism in which the L</formal arguments>
447refer to a copy of the L</actual arguments>, and the L</subroutine>
97a1d740 448cannot change the actual arguments by changing the formal arguments.
27ed30b8 449See also L</call by reference>.
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450
451=item callback
452
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453A L</handler> that you register with some other part of your program
454in the hope that the other part of your program will L</trigger> your
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455handler when some event of interest transpires.
456
457=item canonical
458
459Reduced to a standard form to facilitate comparison.
460
461=item capturing
462
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463The use of parentheses around a L</subpattern> in a L</regular
464expression> to store the matched L</substring> as a L</backreference>.
465(Captured strings are also returned as a list in L</list context>.)
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466
467=item character
468
469A small integer representative of a unit of orthography.
470Historically, characters were usually stored as fixed-width integers
471(typically in a byte, or maybe two, depending on the character set),
472but with the advent of UTF-8, characters are often stored in a
473variable number of bytes depending on the size of the integer that
474represents the character. Perl manages this transparently for you,
475for the most part.
476
477=item character class
478
27ed30b8 479A square-bracketed list of characters used in a L</regular expression>
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480to indicate that any character of the set may occur at a given point.
481Loosely, any predefined set of characters so used.
482
483=item character property
484
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485A predefined L</character class> matchable by the C<\p>
486L</metasymbol>. Many standard properties are defined for L</Unicode>.
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487
488=item circumfix operator
489
27ed30b8 490An L</operator> that surrounds its L</operand>, like the angle
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491operator, or parentheses, or a hug.
492
493=item class
494
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495A user-defined L</type>, implemented in Perl via a L</package> that
496provides (either directly or by inheritance) L<methods|/method> (that
497is, L<subroutines|/subroutine>) to handle L<instances|/instance> of
498the class (its L<objects|/object>). See also L</inheritance>.
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499
500=item class method
501
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502A L</method> whose L</invocant> is a L</package> name, not an
503L</object> reference. A method associated with the class as a whole.
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504
505=item client
506
27ed30b8 507In networking, a L</process> that initiates contact with a L</server>
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508process in order to exchange data and perhaps receive a service.
509
510=item cloister
511
27ed30b8 512A L</cluster> used to restrict the scope of a L</regular expression
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513modifier>.
514
515=item closure
516
27ed30b8 517An L</anonymous> subroutine that, when a reference to it is generated
97a1d740 518at run time, keeps track of the identities of externally visible
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519L<lexical variables|/lexical variable> even after those lexical
520variables have supposedly gone out of L</scope>. They're called
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521"closures" because this sort of behavior gives mathematicians a sense
522of closure.
523
524=item cluster
525
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526A parenthesized L</subpattern> used to group parts of a L</regular
527expression> into a single L</atom>.
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528
529=item CODE
530
531The word returned by the L<ref|perlfunc/ref> function when you apply
27ed30b8 532it to a reference to a subroutine. See also L</CV>.
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533
534=item code generator
535
536A system that writes code for you in a low-level language, such as
27ed30b8 537code to implement the backend of a compiler. See L</program
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538generator>.
539
540=item code subpattern
541
27ed30b8 542A L</regular expression> subpattern whose real purpose is to execute
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543some Perl code, for example, the C<(?{...})> and C<(??{...})>
544subpatterns.
545
546=item collating sequence
547
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548The order into which L<characters|/character> sort. This is used by
549L</string> comparison routines to decide, for example, where in this
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550glossary to put "collating sequence".
551
552=item command
553
27ed30b8 554In L</shell> programming, the syntactic combination of a program name
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555and its arguments. More loosely, anything you type to a shell (a
556command interpreter) that starts it doing something. Even more
27ed30b8 557loosely, a Perl L</statement>, which might start with a L</label> and
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558typically ends with a semicolon.
559
560=item command buffering
561
562A mechanism in Perl that lets you store up the output of each Perl
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563L</command> and then flush it out as a single request to the
564L</operating system>. It's enabled by setting the C<$|>
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565(C<$AUTOFLUSH>) variable to a true value. It's used when you don't
566want data sitting around not going where it's supposed to, which may
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567happen because the default on a L</file> or L</pipe> is to use
568L</block buffering>.
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569
570=item command name
571
572The name of the program currently executing, as typed on the command
27ed30b8 573line. In C, the L</command> name is passed to the program as the
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574first command-line argument. In Perl, it comes in separately as
575C<$0>.
576
577=item command-line arguments
578
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YST
579The L<values|/value> you supply along with a program name when you
580tell a L</shell> to execute a L</command>. These values are passed to
97a1d740
YST
581a Perl program through C<@ARGV>.
582
583=item comment
584
585A remark that doesn't affect the meaning of the program. In Perl, a
586comment is introduced by a C<#> character and continues to the end of
587the line.
588
589=item compilation unit
590
27ed30b8 591The L</file> (or L</string>, in the case of L<eval|perlfunc/eval>)
97a1d740
YST
592that is currently being compiled.
593
594=item compile phase
595
596Any time before Perl starts running your main program. See also
27ed30b8
YST
597L</run phase>. Compile phase is mostly spent in L</compile time>, but
598may also be spent in L</run time> when C<BEGIN> blocks,
97a1d740
YST
599L<use|perlfunc/use> declarations, or constant subexpressions are being
600evaluated. The startup and import code of any L<use|perlfunc/use>
601declaration is also run during compile phase.
602
603=item compile time
604
605The time when Perl is trying to make sense of your code, as opposed to
606when it thinks it knows what your code means and is merely trying to
27ed30b8 607do what it thinks your code says to do, which is L</run time>.
97a1d740
YST
608
609=item compiler
610
611Strictly speaking, a program that munches up another program and spits
612out yet another file containing the program in a "more executable"
613form, typically containing native machine instructions. The I<perl>
614program is not a compiler by this definition, but it does contain a
615kind of compiler that takes a program and turns it into a more
27ed30b8
YST
616executable form (L<syntax trees|/syntax tree>) within the I<perl>
617process itself, which the L</interpreter> then interprets. There are,
618however, extension L<modules|/module> to get Perl to act more like a
97a1d740
YST
619"real" compiler. See L<O>.
620
621=item composer
622
27ed30b8 623A "constructor" for a L</referent> that isn't really an L</object>,
97a1d740
YST
624like an anonymous array or a hash (or a sonata, for that matter). For
625example, a pair of braces acts as a composer for a hash, and a pair of
626brackets acts as a composer for an array. See L<perlref/Making
627References>.
628
629=item concatenation
630
631The process of gluing one cat's nose to another cat's tail. Also, a
27ed30b8 632similar operation on two L<strings|/string>.
97a1d740
YST
633
634=item conditional
635
27ed30b8 636Something "iffy". See L</Boolean context>.
97a1d740
YST
637
638=item connection
639
640In telephony, the temporary electrical circuit between the caller's
641and the callee's phone. In networking, the same kind of temporary
27ed30b8 642circuit between a L</client> and a L</server>.
97a1d740
YST
643
644=item construct
645
646As a noun, a piece of syntax made up of smaller pieces. As a
27ed30b8 647transitive verb, to create an L</object> using a L</constructor>.
97a1d740
YST
648
649=item constructor
650
27ed30b8
YST
651Any L</class method>, instance L</method>, or L</subroutine>
652that composes, initializes, blesses, and returns an L</object>.
653Sometimes we use the term loosely to mean a L</composer>.
97a1d740
YST
654
655=item context
656
657The surroundings, or environment. The context given by the
658surrounding code determines what kind of data a particular
27ed30b8
YST
659L</expression> is expected to return. The three primary contexts are
660L</list context>, L</scalar context>, and L</void context>. Scalar
661context is sometimes subdivided into L</Boolean context>, L</numeric
662context>, L</string context>, and L</void context>. There's also a
97a1d740
YST
663"don't care" scalar context (which is dealt with in Programming Perl,
664Third Edition, Chapter 2, "Bits and Pieces" if you care).
665
666=item continuation
667
27ed30b8
YST
668The treatment of more than one physical L</line> as a single logical
669line. L</Makefile> lines are continued by putting a backslash before
670the L</newline>. Mail headers as defined by RFC 822 are continued by
97a1d740 671putting a space or tab I<after> the newline. In general, lines in
27ed30b8 672Perl do not need any form of continuation mark, because L</whitespace>
97a1d740
YST
673(including newlines) is gleefully ignored. Usually.
674
675=item core dump
676
27ed30b8
YST
677The corpse of a L</process>, in the form of a file left in the
678L</working directory> of the process, usually as a result of certain
97a1d740
YST
679kinds of fatal error.
680
681=item CPAN
682
683The 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?>).
684
685=item cracker
686
687Someone who breaks security on computer systems. A cracker may be a
27ed30b8 688true L</hacker> or only a L</script kiddie>.
97a1d740
YST
689
690=item current package
691
27ed30b8 692The L</package> in which the current statement is compiled. Scan
97a1d740
YST
693backwards in the text of your program through the current L<lexical
694scope|/lexical scoping> or any enclosing lexical scopes till you find
695a package declaration. That's your current package name.
696
697=item current working directory
698
27ed30b8 699See L</working directory>.
97a1d740
YST
700
701=item currently selected output channel
702
27ed30b8
YST
703The last L</filehandle> that was designated with
704L<select|perlfunc/select>(C<FILEHANDLE>); L</STDOUT>, if no filehandle
705has been selected.
97a1d740
YST
706
707=item CV
708
27ed30b8
YST
709An internal "code value" typedef, holding a L</subroutine>. The L</CV>
710type is a subclass of L</SV>.
97a1d740
YST
711
712=back
713
5bbd0522
YST
714=head2 D
715
97a1d740
YST
716=over 4
717
718=item dangling statement
719
27ed30b8 720A bare, single L</statement>, without any braces, hanging off an C<if>
97a1d740
YST
721or C<while> conditional. C allows them. Perl doesn't.
722
723=item data structure
724
725How your various pieces of data relate to each other and what shape
726they make when you put them all together, as in a rectangular table or
727a triangular-shaped tree.
728
729=item data type
730
731A set of possible values, together with all the operations that know
732how to deal with those values. For example, a numeric data type has a
733certain set of numbers that you can work with and various mathematical
734operations that you can do on the numbers but would make little sense
735on, say, a string such as C<"Kilroy">. Strings have their own
27ed30b8 736operations, such as L</concatenation>. Compound types made of a
97a1d740 737number of smaller pieces generally have operations to compose and
27ed30b8
YST
738decompose them, and perhaps to rearrange them. L<Objects|/object>
739that model things in the real world often have operations that
97a1d740
YST
740correspond to real activities. For instance, if you model an
741elevator, your elevator object might have an C<open_door()>
27ed30b8 742L</method>.
97a1d740
YST
743
744=item datagram
745
27ed30b8 746A packet of data, such as a L</UDP> message, that (from the viewpoint
97a1d740 747of the programs involved) can be sent independently over the network.
27ed30b8
YST
748(In fact, all packets are sent independently at the L</IP> level, but
749L</stream> protocols such as L</TCP> hide this from your program.)
97a1d740
YST
750
751=item DBM
752
753Stands for "Data Base Management" routines, a set of routines that
27ed30b8 754emulate an L</associative array> using disk files. The routines use a
97a1d740
YST
755dynamic hashing scheme to locate any entry with only two disk
756accesses. DBM files allow a Perl program to keep a persistent
27ed30b8 757L</hash> across multiple invocations. You can L<tie|perlfunc/tie>
97a1d740
YST
758your hash variables to various DBM implementations--see L<AnyDBM_File>
759and L<DB_File>.
760
761=item declaration
762
27ed30b8 763An L</assertion> that states something exists and perhaps describes
97a1d740
YST
764what it's like, without giving any commitment as to how or where
765you'll use it. A declaration is like the part of your recipe that
766says, "two cups flour, one large egg, four or five tadpoles..." See
27ed30b8 767L</statement> for its opposite. Note that some declarations also
97a1d740
YST
768function as statements. Subroutine declarations also act as
769definitions if a body is supplied.
770
771=item decrement
772
773To subtract a value from a variable, as in "decrement C<$x>" (meaning
774to remove 1 from its value) or "decrement C<$x> by 3".
775
776=item default
777
27ed30b8 778A L</value> chosen for you if you don't supply a value of your own.
97a1d740
YST
779
780=item defined
781
782Having a meaning. Perl thinks that some of the things people try to
783do are devoid of meaning, in particular, making use of variables that
27ed30b8 784have never been given a L</value> and performing certain operations on
97a1d740
YST
785data that isn't there. For example, if you try to read data past the
786end of a file, Perl will hand you back an undefined value. See also
27ed30b8 787L</false> and L<perlfunc/defined>.
97a1d740
YST
788
789=item delimiter
790
27ed30b8
YST
791A L</character> or L</string> that sets bounds to an arbitrarily-sized
792textual object, not to be confused with a L</separator> or
793L</terminator>. "To delimit" really just means "to surround" or "to
97a1d740
YST
794enclose" (like these parentheses are doing).
795
2dd6f5a3
JC
796=item deprecated modules and features
797
798Deprecated modules and features are those which were part of a stable
799release, but later found to be subtly flawed, and which should be avoided.
800They are subject to removal and/or bug-incompatible reimplementation in
801the next major release (but they will be preserved through maintainance
802releases). Deprecation warnings are issued under B<-w> or C<use
803diagnostics>, and notices are found in L<perldelta>s, as well as various
804other PODs. Coding practices that misuse features, such as C<my $foo if
8050>, can also be deprecated.
806
97a1d740
YST
807=item dereference
808
27ed30b8 809A fancy computer science term meaning "to follow a L</reference> to
97a1d740 810what it points to". The "de" part of it refers to the fact that
27ed30b8 811you're taking away one level of L</indirection>.
97a1d740
YST
812
813=item derived class
814
27ed30b8
YST
815A L</class> that defines some of its L<methods|/method> in terms of a
816more generic class, called a L</base class>. Note that classes aren't
97a1d740
YST
817classified exclusively into base classes or derived classes: a class
818can function as both a derived class and a base class simultaneously,
819which is kind of classy.
820
821=item descriptor
822
27ed30b8 823See L</file descriptor>.
97a1d740
YST
824
825=item destroy
826
27ed30b8 827To deallocate the memory of a L</referent> (first triggering its
97a1d740
YST
828C<DESTROY> method, if it has one).
829
830=item destructor
831
27ed30b8
YST
832A special L</method> that is called when an L</object> is thinking
833about L<destroying|/destroy> itself. A Perl program's C<DESTROY>
97a1d740 834method doesn't do the actual destruction; Perl just
27ed30b8 835L<triggers|/trigger> the method in case the L</class> wants to do any
97a1d740
YST
836associated cleanup.
837
838=item device
839
840A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a
27ed30b8
YST
841joystick or a mouse) attached to your computer, that the L</operating
842system> tries to make look like a L</file> (or a bunch of files).
97a1d740
YST
843Under Unix, these fake files tend to live in the I</dev> directory.
844
845=item directive
846
27ed30b8 847A L</pod> directive. See L<perlpod>.
97a1d740
YST
848
849=item directory
850
851A special file that contains other files. Some L<operating
852systems|/operating system> call these "folders", "drawers", or
853"catalogs".
854
855=item directory handle
856
857A name that represents a particular instance of opening a directory to
858read it, until you close it. See the L<opendir|perlfunc/opendir>
859function.
860
861=item dispatch
862
863To send something to its correct destination. Often used
864metaphorically to indicate a transfer of programmatic control to a
865destination selected algorithmically, often by lookup in a table of
27ed30b8
YST
866function L<references|/reference> or, in the case of object
867L<methods|/method>, by traversing the inheritance tree looking for the
97a1d740
YST
868most specific definition for the method.
869
870=item distribution
871
872A standard, bundled release of a system of software. The default
873usage implies source code is included. If that is not the case, it
874will be called a "binary-only" distribution.
875
2dd6f5a3
JC
876=item (to be) dropped modules
877
878When Perl 5 was first released (see L<perlhistory>), several modules were
879included, which have now fallen out of common use. It has been suggested
880that these modules should be removed, since the distribution became rather
881large, and the common criterion for new module additions is now limited to
882modules that help to build, test, and extend perl itself. Furthermore,
883the CPAN (which didn't exist by the time of Perl 5.0) can become a new
884home dropped modules. Dropping modules is currently not an option, but
885further developments may clear the last barriers.
886
97a1d740
YST
887=item dweomer
888
889An enchantment, illusion, phantasm, or jugglery. Said when Perl's
27ed30b8 890magical L</dwimmer> effects don't do what you expect, but rather seem
97a1d740
YST
891to be the product of arcane dweomercraft, sorcery, or wonder working.
892[From Old English]
893
894=item dwimmer
895
896DWIM is an acronym for "Do What I Mean", the principle that something
897should just do what you want it to do without an undue amount of fuss.
898A bit of code that does "dwimming" is a "dwimmer". Dwimming can
899require a great deal of behind-the-scenes magic, which (if it doesn't
27ed30b8 900stay properly behind the scenes) is called a L</dweomer> instead.
97a1d740
YST
901
902=item dynamic scoping
903
904Dynamic scoping works over a dynamic scope, making variables visible
27ed30b8
YST
905throughout the rest of the L</block> in which they are first used and
906in any L<subroutines|/subroutine> that are called by the rest of the
97a1d740
YST
907block. Dynamically scoped variables can have their values temporarily
908changed (and implicitly restored later) by a L<local|perlfunc/local>
27ed30b8 909operator. (Compare L</lexical scoping>.) Used more loosely to mean
97a1d740 910how a subroutine that is in the middle of calling another subroutine
27ed30b8 911"contains" that subroutine at L</run time>.
97a1d740
YST
912
913=back
914
5bbd0522
YST
915=head2 E
916
97a1d740
YST
917=over 4
918
919=item eclectic
920
921Derived from many sources. Some would say I<too> many.
922
923=item element
924
27ed30b8 925A basic building block. When you're talking about an L</array>, it's
97a1d740
YST
926one of the items that make up the array.
927
928=item embedding
929
930When something is contained in something else, particularly when that
931might be considered surprising: "I've embedded a complete Perl
932interpreter in my editor!"
933
934=item empty subclass test
935
27ed30b8
YST
936The notion that an empty L</derived class> should behave exactly like
937its L</base class>.
97a1d740
YST
938
939=item en passant
940
27ed30b8 941When you change a L</value> as it is being copied. [From French, "in
97a1d740
YST
942passing", as in the exotic pawn-capturing maneuver in chess.]
943
944=item encapsulation
945
27ed30b8
YST
946The veil of abstraction separating the L</interface> from the
947L</implementation> (whether enforced or not), which mandates that all
948access to an L</object>'s state be through L<methods|/method> alone.
97a1d740
YST
949
950=item endian
951
27ed30b8 952See L</little-endian> and L</big-endian>.
97a1d740
YST
953
954=item environment
955
27ed30b8
YST
956The collective set of L<environment variables|/environment variable>
957your L</process> inherits from its parent. Accessed via C<%ENV>.
97a1d740
YST
958
959=item environment variable
960
961A mechanism by which some high-level agent such as a user can pass its
27ed30b8 962preferences down to its future offspring (child L<processes|/process>,
97a1d740 963grandchild processes, great-grandchild processes, and so on). Each
27ed30b8
YST
964environment variable is a L</key>/L</value> pair, like one entry in a
965L</hash>.
97a1d740
YST
966
967=item EOF
968
969End of File. Sometimes used metaphorically as the terminating string
27ed30b8 970of a L</here document>.
97a1d740
YST
971
972=item errno
973
27ed30b8 974The error number returned by a L</syscall> when it fails. Perl refers
97a1d740
YST
975to the error by the name C<$!> (or C<$OS_ERROR> if you use the English
976module).
977
978=item error
979
27ed30b8 980See L</exception> or L</fatal error>.
97a1d740
YST
981
982=item escape sequence
983
27ed30b8 984See L</metasymbol>.
97a1d740
YST
985
986=item exception
987
27ed30b8 988A fancy term for an error. See L</fatal error>.
97a1d740
YST
989
990=item exception handling
991
992The way a program responds to an error. The exception handling
993mechanism in Perl is the L<eval|perlfunc/eval> operator.
994
995=item exec
996
27ed30b8 997To throw away the current L</process>'s program and replace it with
97a1d740
YST
998another without exiting the process or relinquishing any resources
999held (apart from the old memory image).
1000
1001=item executable file
1002
27ed30b8 1003A L</file> that is specially marked to tell the L</operating system>
97a1d740
YST
1004that it's okay to run this file as a program. Usually shortened to
1005"executable".
1006
1007=item execute
1008
27ed30b8 1009To run a L<program|/executable file> or L</subroutine>. (Has nothing
97a1d740 1010to do with the L<kill|perlfunc/kill> built-in, unless you're trying to
27ed30b8 1011run a L</signal handler>.)
97a1d740
YST
1012
1013=item execute bit
1014
1015The special mark that tells the operating system it can run this
1016program. There are actually three execute bits under Unix, and which
1017bit gets used depends on whether you own the file singularly,
1018collectively, or not at all.
1019
1020=item exit status
1021
27ed30b8 1022See L</status>.
97a1d740
YST
1023
1024=item export
1025
27ed30b8 1026To make symbols from a L</module> available for L</import> by other modules.
97a1d740
YST
1027
1028=item expression
1029
27ed30b8
YST
1030Anything you can legally say in a spot where a L</value> is required.
1031Typically composed of L<literals|/literal>, L<variables|/variable>,
1032L<operators|/operator>, L<functions|/function>, and L</subroutine>
97a1d740
YST
1033calls, not necessarily in that order.
1034
1035=item extension
1036
1037A Perl module that also pulls in compiled C or C++ code. More
1038generally, any experimental option that can be compiled into Perl,
1039such as multithreading.
1040
1041=back
1042
5bbd0522
YST
1043=head2 F
1044
97a1d740
YST
1045=over 4
1046
1047=item false
1048
1049In Perl, any value that would look like C<""> or C<"0"> if evaluated
1050in a string context. Since undefined values evaluate to C<"">, all
1051undefined values are false, but not all false values are undefined.
1052
1053=item FAQ
1054
1055Frequently Asked Question (although not necessarily frequently
1056answered, especially if the answer appears in the Perl FAQ shipped
1057standard with Perl).
1058
1059=item fatal error
1060
27ed30b8
YST
1061An uncaught L</exception>, which causes termination of the L</process>
1062after printing a message on your L</standard error> stream. Errors
97a1d740
YST
1063that happen inside an L<eval|perlfunc/eval> are not fatal. Instead,
1064the L<eval|perlfunc/eval> terminates after placing the exception
1065message in the C<$@> (C<$EVAL_ERROR>) variable. You can try to
1066provoke a fatal error with the L<die|perlfunc/die> operator (known as
1067throwing or raising an exception), but this may be caught by a
1068dynamically enclosing L<eval|perlfunc/eval>. If not caught, the
1069L<die|perlfunc/die> becomes a fatal error.
1070
1071=item field
1072
1073A single piece of numeric or string data that is part of a longer
27ed30b8
YST
1074L</string>, L</record>, or L</line>. Variable-width fields are usually
1075split up by L<separators|/separator> (so use L<split|perlfunc/split> to
1076extract the fields), while fixed-width fields are usually at fixed
1077positions (so use L<unpack|perlfunc/unpack>). L<Instance
1078variables|/instance variable> are also known as fields.
97a1d740
YST
1079
1080=item FIFO
1081
27ed30b8
YST
1082First In, First Out. See also L</LIFO>. Also, a nickname for a
1083L</named pipe>.
97a1d740
YST
1084
1085=item file
1086
27ed30b8
YST
1087A named collection of data, usually stored on disk in a L</directory>
1088in a L</filesystem>. Roughly like a document, if you're into office
97a1d740
YST
1089metaphors. In modern filesystems, you can actually give a file more
1090than one name. Some files have special properties, like directories
1091and devices.
1092
1093=item file descriptor
1094
27ed30b8
YST
1095The little number the L</operating system> uses to keep track of which
1096opened L</file> you're talking about. Perl hides the file descriptor
1097inside a L</standard IE<sol>O> stream and then attaches the stream to
1098a L</filehandle>.
97a1d740
YST
1099
1100=item file test operator
1101
1102A built-in unary operator that you use to determine whether something
27ed30b8 1103is L</true> about a file, such as C<-o $filename> to test whether
97a1d740
YST
1104you're the owner of the file.
1105
1106=item fileglob
1107
27ed30b8 1108A "wildcard" match on L<filenames|/filename>. See the
97a1d740
YST
1109L<glob|perlfunc/glob> function.
1110
1111=item filehandle
1112
1113An identifier (not necessarily related to the real name of a file)
1114that represents a particular instance of opening a file until you
1115close it. If you're going to open and close several different files
1116in succession, it's fine to open each of them with the same
1117filehandle, so you don't have to write out separate code to process
1118each file.
1119
1120=item filename
1121
27ed30b8
YST
1122One name for a file. This name is listed in a L</directory>, and you
1123can use it in an L<open|perlfunc/open> to tell the L</operating
97a1d740 1124system> exactly which file you want to open, and associate the file
27ed30b8 1125with a L</filehandle> which will carry the subsequent identity of that
97a1d740
YST
1126file in your program, until you close it.
1127
1128=item filesystem
1129
27ed30b8 1130A set of L<directories|/directory> and L<files|/file> residing on a
97a1d740
YST
1131partition of the disk. Sometimes known as a "partition". You can
1132change the file's name or even move a file around from directory to
1133directory within a filesystem without actually moving the file itself,
1134at least under Unix.
1135
1136=item filter
1137
27ed30b8 1138A program designed to take a L</stream> of input and transform it into
97a1d740
YST
1139a stream of output.
1140
1141=item flag
1142
1143We tend to avoid this term because it means so many things. It may
27ed30b8 1144mean a command-line L</switch> that takes no argument
97a1d740
YST
1145itself (such as Perl's B<-n> and B<-p>
1146flags) or, less frequently, a single-bit indicator (such as the
1147C<O_CREAT> and C<O_EXCL> flags used in
1148L<sysopen|perlfunc/sysopen>).
1149
1150=item floating point
1151
1152A method of storing numbers in "scientific notation", such that the
1153precision of the number is independent of its magnitude (the decimal
1154point "floats"). Perl does its numeric work with floating-point
1155numbers (sometimes called "floats"), when it can't get away with
27ed30b8 1156using L<integers|/integer>. Floating-point numbers are mere
97a1d740
YST
1157approximations of real numbers.
1158
1159=item flush
1160
27ed30b8 1161The act of emptying a L</buffer>, often before it's full.
97a1d740
YST
1162
1163=item FMTEYEWTK
1164
1165Far More Than Everything You Ever Wanted To Know. An exhaustive
27ed30b8 1166treatise on one narrow topic, something of a super-L</FAQ>. See Tom
97a1d740
YST
1167for far more.
1168
1169=item fork
1170
27ed30b8 1171To create a child L</process> identical to the parent process at its
97a1d740
YST
1172moment of conception, at least until it gets ideas of its own. A
1173thread with protected memory.
1174
1175=item formal arguments
1176
27ed30b8
YST
1177The generic names by which a L</subroutine> knows its
1178L<arguments|/argument>. In many languages, formal arguments are
97a1d740
YST
1179always given individual names, but in Perl, the formal arguments are
1180just the elements of an array. The formal arguments to a Perl program
1181are C<$ARGV[0]>, C<$ARGV[1]>, and so on. Similarly, the formal
1182arguments to a Perl subroutine are C<$_[0]>, C<$_[1]>, and so on. You
1183may give the arguments individual names by assigning the values to a
27ed30b8 1184L<my|perlfunc/my> list. See also L</actual arguments>.
97a1d740
YST
1185
1186=item format
1187
1188A specification of how many spaces and digits and things to put
1189somewhere so that whatever you're printing comes out nice and pretty.
1190
1191=item freely available
1192
1193Means you don't have to pay money to get it, but the copyright on it
1194may still belong to someone else (like Larry).
1195
1196=item freely redistributable
1197
1198Means you're not in legal trouble if you give a bootleg copy of it to
1199your friends and we find out about it. In fact, we'd rather you gave
1200a copy to all your friends.
1201
1202=item freeware
1203
1204Historically, any software that you give away, particularly if you
1205make the source code available as well. Now often called C<open
1206source software>. Recently there has been a trend to use the term in
27ed30b8 1207contradistinction to L</open source software>, to refer only to free
97a1d740
YST
1208software released under the Free Software Foundation's GPL (General
1209Public License), but this is difficult to justify etymologically.
1210
1211=item function
1212
1213Mathematically, a mapping of each of a set of input values to a
27ed30b8
YST
1214particular output value. In computers, refers to a L</subroutine> or
1215L</operator> that returns a L</value>. It may or may not have input
1216values (called L<arguments|/argument>).
97a1d740
YST
1217
1218=item funny character
1219
1220Someone like Larry, or one of his peculiar friends. Also refers to
1221the strange prefixes that Perl requires as noun markers on its
1222variables.
1223
1224=item garbage collection
1225
1226A misnamed feature--it should be called, "expecting your mother to
1227pick up after you". Strictly speaking, Perl doesn't do this, but it
1228relies on a reference-counting mechanism to keep things tidy.
1229However, we rarely speak strictly and will often refer to the
1230reference-counting scheme as a form of garbage collection. (If it's
1231any comfort, when your interpreter exits, a "real" garbage collector
1232runs to make sure everything is cleaned up if you've been messy with
1233circular references and such.)
1234
1235=back
1236
5bbd0522
YST
1237=head2 G
1238
97a1d740
YST
1239=over 4
1240
1241=item GID
1242
27ed30b8
YST
1243Group ID--in Unix, the numeric group ID that the L</operating system>
1244uses to identify you and members of your L</group>.
97a1d740
YST
1245
1246=item glob
1247
1248Strictly, the shell's C<*> character, which will match a "glob" of
1249characters when you're trying to generate a list of filenames.
1250Loosely, the act of using globs and similar symbols to do pattern
27ed30b8 1251matching. See also L</fileglob> and L</typeglob>.
97a1d740
YST
1252
1253=item global
1254
1255Something you can see from anywhere, usually used of
27ed30b8 1256L<variables|/variable> and L<subroutines|/subroutine> that are visible
97a1d740
YST
1257everywhere in your program. In Perl, only certain special variables
1258are truly global--most variables (and all subroutines) exist only in
27ed30b8 1259the current L</package>. Global variables can be declared with
97a1d740
YST
1260L<our|perlfunc/our>. See L<perlfunc/our>.
1261
1262=item global destruction
1263
27ed30b8 1264The L</garbage collection> of globals (and the running of any
97a1d740 1265associated object destructors) that takes place when a Perl
27ed30b8 1266L</interpreter> is being shut down. Global destruction should not be
97a1d740
YST
1267confused with the Apocalypse, except perhaps when it should.
1268
1269=item glue language
1270
1271A language such as Perl that is good at hooking things together that
1272weren't intended to be hooked together.
1273
1274=item granularity
1275
1276The size of the pieces you're dealing with, mentally speaking.
1277
1278=item greedy
1279
27ed30b8 1280A L</subpattern> whose L</quantifier> wants to match as many things as
97a1d740
YST
1281possible.
1282
1283=item grep
1284
1285Originally from the old Unix editor command for "Globally search for a
1286Regular Expression and Print it", now used in the general sense of any
1287kind of search, especially text searches. Perl has a built-in
1288L<grep|perlfunc/grep> function that searches a list for elements
27ed30b8
YST
1289matching any given criterion, whereas the I<grep>(1) program searches
1290for lines matching a L</regular expression> in one or more files.
97a1d740
YST
1291
1292=item group
1293
1294A set of users of which you are a member. In some operating systems
1295(like Unix), you can give certain file access permissions to other
1296members of your group.
1297
1298=item GV
1299
27ed30b8
YST
1300An internal "glob value" typedef, holding a L</typeglob>. The L</GV>
1301type is a subclass of L</SV>.
97a1d740
YST
1302
1303=back
1304
5bbd0522
YST
1305=head2 H
1306
97a1d740
YST
1307=over 4
1308
1309=item hacker
1310
1311Someone who is brilliantly persistent in solving technical problems,
1312whether these involve golfing, fighting orcs, or programming. Hacker
1313is a neutral term, morally speaking. Good hackers are not to be
27ed30b8 1314confused with evil L<crackers|/cracker> or clueless L<script
97a1d740
YST
1315kiddies|/script kiddie>. If you confuse them, we will presume that
1316you are either evil or clueless.
1317
1318=item handler
1319
27ed30b8
YST
1320A L</subroutine> or L</method> that is called by Perl when your
1321program needs to respond to some internal event, such as a L</signal>,
1322or an encounter with an operator subject to L</operator overloading>.
1323See also L</callback>.
97a1d740
YST
1324
1325=item hard reference
1326
27ed30b8
YST
1327A L</scalar> L</value> containing the actual address of a
1328L</referent>, such that the referent's L</reference> count accounts
97a1d740 1329for it. (Some hard references are held internally, such as the
27ed30b8 1330implicit reference from one of a L</typeglob>'s variable slots to its
97a1d740 1331corresponding referent.) A hard reference is different from a
27ed30b8 1332L</symbolic reference>.
97a1d740
YST
1333
1334=item hash
1335
27ed30b8
YST
1336An unordered association of L</key>/L</value> pairs, stored such that
1337you can easily use a string L</key> to look up its associated data
1338L</value>. This glossary is like a hash, where the word to be defined
97a1d740
YST
1339is the key, and the definition is the value. A hash is also sometimes
1340septisyllabically called an "associative array", which is a pretty
1341good reason for simply calling it a "hash" instead.
1342
1343=item hash table
1344
1345A data structure used internally by Perl for implementing associative
27ed30b8 1346arrays (hashes) efficiently. See also L</bucket>.
97a1d740
YST
1347
1348=item header file
1349
1350A file containing certain required definitions that you must include
1351"ahead" of the rest of your program to do certain obscure operations.
1352A C header file has a I<.h> extension. Perl doesn't really have
1353header files, though historically Perl has sometimes used translated
1354I<.h> files with a I<.ph> extension. See L<perlfunc/require>.
27ed30b8 1355(Header files have been superseded by the L</module> mechanism.)
97a1d740
YST
1356
1357=item here document
1358
27ed30b8
YST
1359So called because of a similar construct in L<shells|/shell> that
1360pretends that the L<lines|/line> following the L</command> are a
1361separate L</file> to be fed to the command, up to some terminating
97a1d740
YST
1362string. In Perl, however, it's just a fancy form of quoting.
1363
1364=item hexadecimal
1365
1366A number in base 16, "hex" for short. The digits for 10 through 16
1367are customarily represented by the letters C<a> through C<f>.
1368Hexadecimal constants in Perl start with C<0x>. See also
27ed30b8 1369L<perlfunc/hex>.
97a1d740
YST
1370
1371=item home directory
1372
1373The directory you are put into when you log in. On a Unix system, the
1374name is often placed into C<$ENV{HOME}> or C<$ENV{LOGDIR}> by
1375I<login>, but you can also find it with C<(getpwuid($E<lt>))[7]>.
1376(Some platforms do not have a concept of a home directory.)
1377
1378=item host
1379
1380The computer on which a program or other data resides.
1381
1382=item hubris
1383
1384Excessive pride, the sort of thing Zeus zaps you for. Also the
1385quality that makes you write (and maintain) programs that other people
1386won't want to say bad things about. Hence, the third great virtue of
27ed30b8 1387a programmer. See also L</laziness> and L</impatience>.
97a1d740
YST
1388
1389=item HV
1390
1391Short for a "hash value" typedef, which holds Perl's internal
27ed30b8 1392representation of a hash. The L</HV> type is a subclass of L</SV>.
97a1d740
YST
1393
1394=back
1395
5bbd0522
YST
1396=head2 I
1397
97a1d740
YST
1398=over 4
1399
1400=item identifier
1401
1402A legally formed name for most anything in which a computer program
1403might be interested. Many languages (including Perl) allow
1404identifiers that start with a letter and contain letters and digits.
1405Perl also counts the underscore character as a valid letter. (Perl
27ed30b8 1406also has more complicated names, such as L</qualified> names.)
97a1d740
YST
1407
1408=item impatience
1409
1410The anger you feel when the computer is being lazy. This makes you
1411write programs that don't just react to your needs, but actually
1412anticipate them. Or at least that pretend to. Hence, the second
27ed30b8 1413great virtue of a programmer. See also L</laziness> and L</hubris>.
97a1d740
YST
1414
1415=item implementation
1416
1417How a piece of code actually goes about doing its job. Users of the
1418code should not count on implementation details staying the same
27ed30b8 1419unless they are part of the published L</interface>.
97a1d740
YST
1420
1421=item import
1422
1423To gain access to symbols that are exported from another module. See
1424L<perlfunc/use>.
1425
1426=item increment
1427
1428To increase the value of something by 1 (or by some other number, if
1429so specified).
1430
1431=item indexing
1432
27ed30b8 1433In olden days, the act of looking up a L</key> in an actual index
97a1d740 1434(such as a phone book), but now merely the act of using any kind of
27ed30b8 1435key or position to find the corresponding L</value>, even if no index
97a1d740
YST
1436is involved. Things have degenerated to the point that Perl's
1437L<index|perlfunc/index> function merely locates the position (index)
1438of one string in another.
1439
1440=item indirect filehandle
1441
27ed30b8
YST
1442An L</expression> that evaluates to something that can be used as a
1443L</filehandle>: a L</string> (filehandle name), a L</typeglob>, a
1444typeglob L</reference>, or a low-level L</IO> object.
97a1d740
YST
1445
1446=item indirect object
1447
1448In English grammar, a short noun phrase between a verb and its direct
1449object indicating the beneficiary or recipient of the action. In
1450Perl, C<print STDOUT "$foo\n";> can be understood as "verb
27ed30b8 1451indirect-object object" where L</STDOUT> is the recipient of the
97a1d740 1452L<print|perlfunc/print> action, and C<"$foo"> is the object being
27ed30b8 1453printed. Similarly, when invoking a L</method>, you might place the
97a1d740
YST
1454invocant between the method and its arguments:
1455
1456 $gollum = new Pathetic::Creature "Smeagol";
1457 give $gollum "Fisssssh!";
1458 give $gollum "Precious!";
1459
1460=item indirect object slot
1461
1462The syntactic position falling between a method call and its arguments
1463when using the indirect object invocation syntax. (The slot is
1464distinguished by the absence of a comma between it and the next
27ed30b8 1465argument.) L</STDERR> is in the indirect object slot here:
97a1d740
YST
1466
1467 print STDERR "Awake! Awake! Fear, Fire,
1468 Foes! Awake!\n";
1469
1470=item indirection
1471
1472If something in a program isn't the value you're looking for but
1473indicates where the value is, that's indirection. This can be done
27ed30b8 1474with either L<symbolic references|/symbolic reference> or L<hard
97a1d740
YST
1475references|/hard reference>.
1476
1477=item infix
1478
27ed30b8 1479An L</operator> that comes in between its L<operands|/operand>, such
97a1d740
YST
1480as multiplication in C<24 * 7>.
1481
1482=item inheritance
1483
1484What you get from your ancestors, genetically or otherwise. If you
27ed30b8 1485happen to be a L</class>, your ancestors are called L<base
97a1d740 1486classes|/base class> and your descendants are called L<derived
27ed30b8 1487classes|/derived class>. See L</single inheritance> and L</multiple
97a1d740
YST
1488inheritance>.
1489
1490=item instance
1491
27ed30b8 1492Short for "an instance of a class", meaning an L</object> of that L</class>.
97a1d740
YST
1493
1494=item instance variable
1495
27ed30b8 1496An L</attribute> of an L</object>; data stored with the particular
97a1d740
YST
1497object rather than with the class as a whole.
1498
1499=item integer
1500
1501A number with no fractional (decimal) part. A counting number, like
15021, 2, 3, and so on, but including 0 and the negatives.
1503
1504=item interface
1505
1506The services a piece of code promises to provide forever, in contrast to
27ed30b8 1507its L</implementation>, which it should feel free to change whenever it
97a1d740
YST
1508likes.
1509
1510=item interpolation
1511
1512The insertion of a scalar or list value somewhere in the middle of
1513another value, such that it appears to have been there all along. In
1514Perl, variable interpolation happens in double-quoted strings and
1515patterns, and list interpolation occurs when constructing the list of
1516values to pass to a list operator or other such construct that takes a
27ed30b8 1517L</LIST>.
97a1d740
YST
1518
1519=item interpreter
1520
1521Strictly speaking, a program that reads a second program and does what
1522the second program says directly without turning the program into a
27ed30b8 1523different form first, which is what L<compilers|/compiler> do. Perl
97a1d740
YST
1524is not an interpreter by this definition, because it contains a kind
1525of compiler that takes a program and turns it into a more executable
27ed30b8
YST
1526form (L<syntax trees|/syntax tree>) within the I<perl> process itself,
1527which the Perl L</run time> system then interprets.
97a1d740
YST
1528
1529=item invocant
1530
27ed30b8
YST
1531The agent on whose behalf a L</method> is invoked. In a L</class>
1532method, the invocant is a package name. In an L</instance> method,
97a1d740
YST
1533the invocant is an object reference.
1534
1535=item invocation
1536
1537The act of calling up a deity, daemon, program, method, subroutine, or
1538function to get it do what you think it's supposed to do. We usually
1539"call" subroutines but "invoke" methods, since it sounds cooler.
1540
1541=item I/O
1542
27ed30b8 1543Input from, or output to, a L</file> or L</device>.
97a1d740
YST
1544
1545=item IO
1546
27ed30b8 1547An internal I/O object. Can also mean L</indirect object>.
97a1d740
YST
1548
1549=item IP
1550
1551Internet Protocol, or Intellectual Property.
1552
1553=item IPC
1554
1555Interprocess Communication.
1556
1557=item is-a
1558
27ed30b8 1559A relationship between two L<objects|/object> in which one object is
97a1d740
YST
1560considered to be a more specific version of the other, generic object:
1561"A camel is a mammal." Since the generic object really only exists in
1562a Platonic sense, we usually add a little abstraction to the notion of
1563objects and think of the relationship as being between a generic
27ed30b8 1564L</base class> and a specific L</derived class>. Oddly enough,
97a1d740 1565Platonic classes don't always have Platonic relationships--see
27ed30b8 1566L</inheritance>.
97a1d740
YST
1567
1568=item iteration
1569
1570Doing something repeatedly.
1571
1572=item iterator
1573
1574A special programming gizmo that keeps track of where you are in
1575something that you're trying to iterate over. The C<foreach> loop in
1576Perl contains an iterator; so does a hash, allowing you to
1577L<each|perlfunc/each> through it.
1578
1579=item IV
1580
1581The integer four, not to be confused with six, Tom's favorite editor.
27ed30b8
YST
1582IV also means an internal Integer Value of the type a L</scalar> can
1583hold, not to be confused with an L</NV>.
97a1d740
YST
1584
1585=back
1586
5bbd0522
YST
1587=head2 J
1588
97a1d740
YST
1589=over 4
1590
1591=item JAPH
1592
1593"Just Another Perl Hacker," a clever but cryptic bit of Perl code that
1594when executed, evaluates to that string. Often used to illustrate a
1595particular Perl feature, and something of an ungoing Obfuscated Perl
1596Contest seen in Usenix signatures.
1597
1598=back
1599
5bbd0522
YST
1600=head2 K
1601
97a1d740
YST
1602=over 4
1603
1604=item key
1605
27ed30b8 1606The string index to a L</hash>, used to look up the L</value>
97a1d740
YST
1607associated with that key.
1608
1609=item keyword
1610
27ed30b8 1611See L</reserved words>.
97a1d740
YST
1612
1613=back
1614
5bbd0522
YST
1615=head2 L
1616
97a1d740
YST
1617=over 4
1618
1619=item label
1620
27ed30b8 1621A name you give to a L</statement> so that you can talk about that
97a1d740
YST
1622statement elsewhere in the program.
1623
1624=item laziness
1625
1626The quality that makes you go to great effort to reduce overall energy
1627expenditure. It makes you write labor-saving programs that other
1628people will find useful, and document what you wrote so you don't have
1629to answer so many questions about it. Hence, the first great virtue
27ed30b8
YST
1630of a programmer. Also hence, this book. See also L</impatience> and
1631L</hubris>.
97a1d740
YST
1632
1633=item left shift
1634
27ed30b8 1635A L</bit shift> that multiplies the number by some power of 2.
97a1d740
YST
1636
1637=item leftmost longest
1638
27ed30b8
YST
1639The preference of the L</regular expression> engine to match the
1640leftmost occurrence of a L</pattern>, then given a position at which a
97a1d740 1641match will occur, the preference for the longest match (presuming the
27ed30b8 1642use of a L</greedy> quantifier). See L<perlre> for I<much> more on
97a1d740
YST
1643this subject.
1644
1645=item lexeme
1646
27ed30b8 1647Fancy term for a L</token>.
97a1d740
YST
1648
1649=item lexer
1650
27ed30b8 1651Fancy term for a L</tokener>.
97a1d740
YST
1652
1653=item lexical analysis
1654
27ed30b8 1655Fancy term for L</tokenizing>.
97a1d740
YST
1656
1657=item lexical scoping
1658
1659Looking at your I<Oxford English Dictionary> through a microscope.
27ed30b8 1660(Also known as L</static scoping>, because dictionaries don't change
97a1d740
YST
1661very fast.) Similarly, looking at variables stored in a private
1662dictionary (namespace) for each scope, which are visible only from
1663their point of declaration down to the end of the lexical scope in
27ed30b8
YST
1664which they are declared. --Syn. L</static scoping>.
1665--Ant. L</dynamic scoping>.
97a1d740
YST
1666
1667=item lexical variable
1668
27ed30b8 1669A L</variable> subject to L</lexical scoping>, declared by
97a1d740
YST
1670L<my|perlfunc/my>. Often just called a "lexical". (The
1671L<our|perlfunc/our> declaration declares a lexically scoped name for a
1672global variable, which is not itself a lexical variable.)
1673
1674=item library
1675
1676Generally, a collection of procedures. In ancient days, referred to a
1677collection of subroutines in a I<.pl> file. In modern times, refers
27ed30b8 1678more often to the entire collection of Perl L<modules|/module> on your
97a1d740
YST
1679system.
1680
1681=item LIFO
1682
27ed30b8
YST
1683Last In, First Out. See also L</FIFO>. A LIFO is usually called a
1684L</stack>.
97a1d740
YST
1685
1686=item line
1687
1688In Unix, a sequence of zero or more non-newline characters terminated
27ed30b8
YST
1689with a L</newline> character. On non-Unix machines, this is emulated
1690by the C library even if the underlying L</operating system> has
97a1d740
YST
1691different ideas.
1692
1693=item line buffering
1694
27ed30b8
YST
1695Used by a L</standard IE<sol>O> output stream that flushes its
1696L</buffer> after every L</newline>. Many standard I/O libraries
97a1d740
YST
1697automatically set up line buffering on output that is going to the
1698terminal.
1699
1700=item line number
1701
1702The number of lines read previous to this one, plus 1. Perl keeps a
1703separate line number for each source or input file it opens. The
1704current source file's line number is represented by C<__LINE__>. The
1705current input line number (for the file that was most recently read
27ed30b8 1706via C<< E<lt>FHE<gt> >>) is represented by the C<$.>
97a1d740
YST
1707(C<$INPUT_LINE_NUMBER>) variable. Many error messages report both
1708values, if available.
1709
1710=item link
1711
27ed30b8 1712Used as a noun, a name in a L</directory>, representing a L</file>. A
97a1d740
YST
1713given file can have multiple links to it. It's like having the same
1714phone number listed in the phone directory under different names. As
1715a verb, to resolve a partially compiled file's unresolved symbols into
1716a (nearly) executable image. Linking can generally be static or
1717dynamic, which has nothing to do with static or dynamic scoping.
1718
1719=item LIST
1720
1721A syntactic construct representing a comma-separated list of
27ed30b8
YST
1722expressions, evaluated to produce a L</list value>. Each
1723L</expression> in a L</LIST> is evaluated in L</list context> and
97a1d740
YST
1724interpolated into the list value.
1725
1726=item list
1727
1728An ordered set of scalar values.
1729
1730=item list context
1731
27ed30b8 1732The situation in which an L</expression> is expected by its
97a1d740 1733surroundings (the code calling it) to return a list of values rather
27ed30b8 1734than a single value. Functions that want a L</LIST> of arguments tell
97a1d740 1735those arguments that they should produce a list value. See also
27ed30b8 1736L</context>.
97a1d740
YST
1737
1738=item list operator
1739
27ed30b8 1740An L</operator> that does something with a list of values, such as
97a1d740
YST
1741L<join|perlfunc/join> or L<grep|perlfunc/grep>. Usually used for
1742named built-in operators (such as L<print|perlfunc/print>,
1743L<unlink|perlfunc/unlink>, and L<system|perlfunc/system>) that do not
27ed30b8 1744require parentheses around their L</argument> list.
97a1d740
YST
1745
1746=item list value
1747
1748An unnamed list of temporary scalar values that may be passed around
1749within a program from any list-generating function to any function or
27ed30b8 1750construct that provides a L</list context>.
97a1d740
YST
1751
1752=item literal
1753
27ed30b8
YST
1754A token in a programming language such as a number or L</string> that
1755gives you an actual L</value> instead of merely representing possible
1756values as a L</variable> does.
97a1d740
YST
1757
1758=item little-endian
1759
1760From Swift: someone who eats eggs little end first. Also used of
27ed30b8 1761computers that store the least significant L</byte> of a word at a
97a1d740 1762lower byte address than the most significant byte. Often considered
27ed30b8 1763superior to big-endian machines. See also L</big-endian>.
97a1d740
YST
1764
1765=item local
1766
1767Not meaning the same thing everywhere. A global variable in Perl can
1768be localized inside a L<dynamic scope|/dynamic scoping> via the
1769L<local|perlfunc/local> operator.
1770
1771=item logical operator
1772
1773Symbols representing the concepts "and", "or", "xor", and "not".
1774
1775=item lookahead
1776
27ed30b8 1777An L</assertion> that peeks at the string to the right of the current
97a1d740
YST
1778match location.
1779
1780=item lookbehind
1781
27ed30b8 1782An L</assertion> that peeks at the string to the left of the current
97a1d740
YST
1783match location.
1784
1785=item loop
1786
1787A construct that performs something repeatedly, like a roller coaster.
1788
1789=item loop control statement
1790
1791Any statement within the body of a loop that can make a loop
27ed30b8 1792prematurely stop looping or skip an L</iteration>. Generally you
97a1d740
YST
1793shouldn't try this on roller coasters.
1794
1795=item loop label
1796
1797A kind of key or name attached to a loop (or roller coaster) so that
1798loop control statements can talk about which loop they want to
1799control.
1800
1801=item lvaluable
1802
27ed30b8 1803Able to serve as an L</lvalue>.
97a1d740
YST
1804
1805=item lvalue
1806
1807Term used by language lawyers for a storage location you can assign a
27ed30b8
YST
1808new L</value> to, such as a L</variable> or an element of an
1809L</array>. The "l" is short for "left", as in the left side of an
1810assignment, a typical place for lvalues. An L</lvaluable> function or
97a1d740
YST
1811expression is one to which a value may be assigned, as in C<pos($x) =
181210>.
1813
1814=item lvalue modifier
1815
27ed30b8 1816An adjectival pseudofunction that warps the meaning of an L</lvalue>
97a1d740
YST
1817in some declarative fashion. Currently there are three lvalue
1818modifiers: L<my|perlfunc/my>, L<our|perlfunc/our>, and
1819L<local|perlfunc/local>.
1820
1821=back
1822
5bbd0522
YST
1823=head2 M
1824
97a1d740
YST
1825=over 4
1826
1827=item magic
1828
1829Technically speaking, any extra semantics attached to a variable such
1830as C<$!>, C<$0>, C<%ENV>, or C<%SIG>, or to any tied variable.
1831Magical things happen when you diddle those variables.
1832
1833=item magical increment
1834
27ed30b8 1835An L</increment> operator that knows how to bump up alphabetics as
97a1d740
YST
1836well as numbers.
1837
1838=item magical variables
1839
1840Special variables that have side effects when you access them or
1841assign to them. For example, in Perl, changing elements of the
1842C<%ENV> array also changes the corresponding environment variables
1843that subprocesses will use. Reading the C<$!> variable gives you the
1844current system error number or message.
1845
1846=item Makefile
1847
1848A file that controls the compilation of a program. Perl programs
27ed30b8 1849don't usually need a L</Makefile> because the Perl compiler has plenty
97a1d740
YST
1850of self-control.
1851
1852=item man
1853
1854The Unix program that displays online documentation (manual pages) for
1855you.
1856
1857=item manpage
1858
27ed30b8 1859A "page" from the manuals, typically accessed via the I<man>(1)
97a1d740
YST
1860command. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of
1861BUGS, and so on, and is typically longer than a page. There are
27ed30b8
YST
1862manpages documenting L<commands|/command>, L<syscalls|/syscall>,
1863L</library> L<functions|/function>, L<devices|/device>,
1864L<protocols|/protocol>, L<files|/file>, and such. In this book, we
97a1d740
YST
1865call any piece of standard Perl documentation (like I<perlop> or
1866I<perldelta>) a manpage, no matter what format it's installed in on
1867your system.
1868
1869=item matching
1870
27ed30b8 1871See L</pattern matching>.
97a1d740
YST
1872
1873=item member data
1874
27ed30b8 1875See L</instance variable>.
97a1d740
YST
1876
1877=item memory
1878
1879This always means your main memory, not your disk. Clouding the issue
27ed30b8 1880is the fact that your machine may implement L</virtual> memory; that
97a1d740
YST
1881is, it will pretend that it has more memory than it really does, and
1882it'll use disk space to hold inactive bits. This can make it seem
1883like you have a little more memory than you really do, but it's not a
1884substitute for real memory. The best thing that can be said about
1885virtual memory is that it lets your performance degrade gradually
1886rather than suddenly when you run out of real memory. But your
1887program can die when you run out of virtual memory too, if you haven't
1888thrashed your disk to death first.
1889
1890=item metacharacter
1891
27ed30b8 1892A L</character> that is I<not> supposed to be treated normally. Which
97a1d740 1893characters are to be treated specially as metacharacters varies
27ed30b8
YST
1894greatly from context to context. Your L</shell> will have certain
1895metacharacters, double-quoted Perl L<strings|/string> have other
1896metacharacters, and L</regular expression> patterns have all the
97a1d740
YST
1897double-quote metacharacters plus some extra ones of their own.
1898
1899=item metasymbol
1900
27ed30b8 1901Something we'd call a L</metacharacter> except that it's a sequence of
97a1d740
YST
1902more than one character. Generally, the first character in the
1903sequence must be a true metacharacter to get the other characters in
1904the metasymbol to misbehave along with it.
1905
1906=item method
1907
27ed30b8 1908A kind of action that an L</object> can take if you tell it to. See
97a1d740
YST
1909L<perlobj>.
1910
1911=item minimalism
1912
1913The belief that "small is beautiful." Paradoxically, if you say
1914something in a small language, it turns out big, and if you say it in
1915a big language, it turns out small. Go figure.
1916
1917=item mode
1918
27ed30b8
YST
1919In the context of the L<stat> syscall, refers to the field holding
1920the L</permission bits> and the type of the L</file>.
97a1d740
YST
1921
1922=item modifier
1923
27ed30b8
YST
1924See L</statement modifier>, L</regular expression modifier>, and
1925L</lvalue modifier>, not necessarily in that order.
97a1d740
YST
1926
1927=item module
1928
27ed30b8
YST
1929A L</file> that defines a L</package> of (almost) the same name, which
1930can either L</export> symbols or function as an L</object> class. (A
97a1d740
YST
1931module's main I<.pm> file may also load in other files in support of
1932the module.) See the L<use|perlfunc/use> built-in.
1933
1934=item modulus
1935
1936An integer divisor when you're interested in the remainder instead of
1937the quotient.
1938
1939=item monger
1940
1941Short for Perl Monger, a purveyor of Perl.
1942
1943=item mortal
1944
1945A temporary value scheduled to die when the current statement
1946finishes.
1947
1948=item multidimensional array
1949
1950An array with multiple subscripts for finding a single element. Perl
27ed30b8 1951implements these using L<references|/reference>--see L<perllol> and
97a1d740
YST
1952L<perldsc>.
1953
1954=item multiple inheritance
1955
1956The features you got from your mother and father, mixed together
27ed30b8 1957unpredictably. (See also L</inheritance>, and L</single
97a1d740
YST
1958inheritance>.) In computer languages (including Perl), the notion
1959that a given class may have multiple direct ancestors or L<base
1960classes|/base class>.
1961
1962=back
1963
5bbd0522
YST
1964=head2 N
1965
97a1d740
YST
1966=over 4
1967
1968=item named pipe
1969
27ed30b8
YST
1970A L</pipe> with a name embedded in the L</filesystem> so that it can
1971be accessed by two unrelated L<processes|/process>.
97a1d740
YST
1972
1973=item namespace
1974
1975A domain of names. You needn't worry about whether the names in one
27ed30b8 1976such domain have been used in another. See L</package>.
97a1d740
YST
1977
1978=item network address
1979
1980The most important attribute of a socket, like your telephone's
27ed30b8 1981telephone number. Typically an IP address. See also L</port>.
97a1d740
YST
1982
1983=item newline
1984
1985A single character that represents the end of a line, with the ASCII
1986value of 012 octal under Unix (but 015 on a Mac), and represented by
1987C<\n> in Perl strings. For Windows machines writing text files, and
1988for certain physical devices like terminals, the single newline gets
1989automatically translated by your C library into a line feed and a
1990carriage return, but normally, no translation is done.
1991
1992=item NFS
1993
1994Network File System, which allows you to mount a remote filesystem as
1995if it were local.
1996
1997=item null character
1998
1999A character with the ASCII value of zero. It's used by C to terminate
2000strings, but Perl allows strings to contain a null.
2001
2002=item null list
2003
27ed30b8 2004A L</list value> with zero elements, represented in Perl by C<()>.
97a1d740
YST
2005
2006=item null string
2007
27ed30b8
YST
2008A L</string> containing no characters, not to be confused with a
2009string containing a L</null character>, which has a positive length
2010and is L</true>.
97a1d740
YST
2011
2012=item numeric context
2013
2014The situation in which an expression is expected by its surroundings
27ed30b8
YST
2015(the code calling it) to return a number. See also L</context> and
2016L</string context>.
97a1d740
YST
2017
2018=item NV
2019
2020Short for Nevada, no part of which will ever be confused with
2021civilization. NV also means an internal floating-point Numeric Value
27ed30b8 2022of the type a L</scalar> can hold, not to be confused with an L</IV>.
97a1d740
YST
2023
2024=item nybble
2025
27ed30b8
YST
2026Half a L</byte>, equivalent to one L</hexadecimal> digit, and worth
2027four L<bits|/bit>.
97a1d740
YST
2028
2029=back
2030
5bbd0522
YST
2031=head2 O
2032
97a1d740
YST
2033=over 4
2034
2035=item object
2036
27ed30b8 2037An L</instance> of a L</class>. Something that "knows" what
97a1d740
YST
2038user-defined type (class) it is, and what it can do because of what
2039class it is. Your program can request an object to do things, but the
2040object gets to decide whether it wants to do them or not. Some
2041objects are more accommodating than others.
2042
2043=item octal
2044
2045A number in base 8. Only the digits 0 through 7 are allowed. Octal
2046constants in Perl start with 0, as in 013. See also the
2047L<oct|perlfunc/oct> function.
2048
2049=item offset
2050
2051How many things you have to skip over when moving from the beginning
2052of a string or array to a specific position within it. Thus, the
2053minimum offset is zero, not one, because you don't skip anything to
2054get to the first item.
2055
2056=item one-liner
2057
2058An entire computer program crammed into one line of text.
2059
2060=item open source software
2061
2062Programs for which the source code is freely available and freely
2063redistributable, with no commercial strings attached. For a more
2064detailed definition, see L<http://www.opensource.org/osd.html>.
2065
2066=item operand
2067
27ed30b8
YST
2068An L</expression> that yields a L</value> that an L</operator>
2069operates on. See also L</precedence>.
97a1d740
YST
2070
2071=item operating system
2072
2073A special program that runs on the bare machine and hides the gory
27ed30b8 2074details of managing L<processes|/process> and L<devices|/device>.
97a1d740
YST
2075Usually used in a looser sense to indicate a particular culture of
2076programming. The loose sense can be used at varying levels of
2077specificity. At one extreme, you might say that all versions of Unix
2078and Unix-lookalikes are the same operating system (upsetting many
2079people, especially lawyers and other advocates). At the other
2080extreme, you could say this particular version of this particular
2081vendor's operating system is different from any other version of this
2082or any other vendor's operating system. Perl is much more portable
2083across operating systems than many other languages. See also
27ed30b8 2084L</architecture> and L</platform>.
97a1d740
YST
2085
2086=item operator
2087
2088A gizmo that transforms some number of input values to some number of
2089output values, often built into a language with a special syntax or
2090symbol. A given operator may have specific expectations about what
27ed30b8
YST
2091L<types|/type> of data you give as its arguments
2092(L<operands|/operand>) and what type of data you want back from it.
97a1d740
YST
2093
2094=item operator overloading
2095
27ed30b8
YST
2096A kind of L</overloading> that you can do on built-in
2097L<operators|/operator> to make them work on L<objects|/object> as if
97a1d740
YST
2098the objects were ordinary scalar values, but with the actual semantics
2099supplied by the object class. This is set up with the L<overload>
27ed30b8 2100L</pragma>.
97a1d740
YST
2101
2102=item options
2103
27ed30b8 2104See either L<switches|/switch> or L</regular expression modifier>.
97a1d740
YST
2105
2106=item overloading
2107
2108Giving additional meanings to a symbol or construct. Actually, all
2109languages do overloading to one extent or another, since people are
27ed30b8 2110good at figuring out things from L</context>.
97a1d740
YST
2111
2112=item overriding
2113
2114Hiding or invalidating some other definition of the same name. (Not
27ed30b8 2115to be confused with L</overloading>, which adds definitions that must
97a1d740
YST
2116be disambiguated some other way.) To confuse the issue further, we use
2117the word with two overloaded definitions: to describe how you can
27ed30b8 2118define your own L</subroutine> to hide a built-in L</function> of the
97a1d740 2119same name (see L<perlsub/Overriding Built-in Functions>) and to
27ed30b8
YST
2120describe how you can define a replacement L</method> in a L</derived
2121class> to hide a L</base class>'s method of the same name (see
97a1d740
YST
2122L<perlobj>).
2123
2124=item owner
2125
2126The one user (apart from the superuser) who has absolute control over
27ed30b8 2127a L</file>. A file may also have a L</group> of users who may
97a1d740 2128exercise joint ownership if the real owner permits it. See
27ed30b8 2129L</permission bits>.
97a1d740
YST
2130
2131=back
2132
5bbd0522
YST
2133=head2 P
2134
97a1d740
YST
2135=over 4
2136
2137=item package
2138
27ed30b8
YST
2139A L</namespace> for global L<variables|/variable>,
2140L<subroutines|/subroutine>, and the like, such that they can be kept
2141separate from like-named L<symbols|/symbol> in other namespaces. In a
97a1d740
YST
2142sense, only the package is global, since the symbols in the package's
2143symbol table are only accessible from code compiled outside the
2144package by naming the package. But in another sense, all package
2145symbols are also globals--they're just well-organized globals.
2146
2147=item pad
2148
27ed30b8 2149Short for L</scratchpad>.
97a1d740
YST
2150
2151=item parameter
2152
27ed30b8 2153See L</argument>.
97a1d740
YST
2154
2155=item parent class
2156
27ed30b8 2157See L</base class>.
97a1d740
YST
2158
2159=item parse tree
2160
27ed30b8 2161See L</syntax tree>.
97a1d740
YST
2162
2163=item parsing
2164
2165The subtle but sometimes brutal art of attempting to turn your
27ed30b8 2166possibly malformed program into a valid L</syntax tree>.
97a1d740
YST
2167
2168=item patch
2169
2170To fix by applying one, as it were. In the realm of hackerdom, a
2171listing of the differences between two versions of a program as might
27ed30b8 2172be applied by the I<patch>(1) program when you want to fix a bug or
97a1d740
YST
2173upgrade your old version.
2174
2175=item PATH
2176
2177The list of L<directories|/directory> the system searches to find a
27ed30b8
YST
2178program you want to L</execute>. The list is stored as one of your
2179L<environment variables|/environment variable>, accessible in Perl as
97a1d740
YST
2180C<$ENV{PATH}>.
2181
2182=item pathname
2183
2184A fully qualified filename such as I</usr/bin/perl>. Sometimes
27ed30b8 2185confused with L</PATH>.
97a1d740
YST
2186
2187=item pattern
2188
27ed30b8 2189A template used in L</pattern matching>.
97a1d740
YST
2190
2191=item pattern matching
2192
27ed30b8 2193Taking a pattern, usually a L</regular expression>, and trying the
97a1d740
YST
2194pattern various ways on a string to see whether there's any way to
2195make it fit. Often used to pick interesting tidbits out of a file.
2196
2197=item permission bits
2198
27ed30b8
YST
2199Bits that the L</owner> of a file sets or unsets to allow or disallow
2200access to other people. These flag bits are part of the L</mode> word
97a1d740
YST
2201returned by the L<stat|perlfunc/stat> built-in when you ask about a
2202file. On Unix systems, you can check the I<ls>(1) manpage for more
2203information.
2204
2205=item Pern
2206
2207What you get when you do C<Perl++> twice. Doing it only once will
2208curl your hair. You have to increment it eight times to shampoo your
2209hair. Lather, rinse, iterate.
2210
2211=item pipe
2212
27ed30b8 2213A direct L</connection> that carries the output of one L</process> to
97a1d740
YST
2214the input of another without an intermediate temporary file. Once the
2215pipe is set up, the two processes in question can read and write as if
2216they were talking to a normal file, with some caveats.
2217
2218=item pipeline
2219
27ed30b8
YST
2220A series of L<processes|/process> all in a row, linked by
2221L<pipes|/pipe>, where each passes its output stream to the next.
97a1d740
YST
2222
2223=item platform
2224
2225The entire hardware and software context in which a program runs. A
2226 program written in a platform-dependent language might break if you
2227change any of: machine, operating system, libraries, compiler, or
2228system configuration. The I<perl> interpreter has to be compiled
2229differently for each platform because it is implemented in C, but
2230programs written in the Perl language are largely
2231platform-independent.
2232
2233=item pod
2234
2235The markup used to embed documentation into your Perl code. See
2236L<perlpod>.
2237
2238=item pointer
2239
27ed30b8 2240A L</variable> in a language like C that contains the exact memory
97a1d740
YST
2241location of some other item. Perl handles pointers internally so you
2242don't have to worry about them. Instead, you just use symbolic
27ed30b8 2243pointers in the form of L<keys|/key> and L</variable> names, or L<hard
97a1d740
YST
2244references|/hard reference>, which aren't pointers (but act like
2245pointers and do in fact contain pointers).
2246
2247=item polymorphism
2248
27ed30b8 2249The notion that you can tell an L</object> to do something generic,
97a1d740
YST
2250and the object will interpret the command in different ways depending
2251on its type. [E<lt>Gk many shapes]
2252
2253=item port
2254
2255The part of the address of a TCP or UDP socket that directs packets to
2256the correct process after finding the right machine, something like
2257the phone extension you give when you reach the company operator.
2258Also, the result of converting code to run on a different platform
2259than originally intended, or the verb denoting this conversion.
2260
2261=item portable
2262
2263Once upon a time, C code compilable under both BSD and SysV. In
2264general, code that can be easily converted to run on another
27ed30b8 2265L</platform>, where "easily" can be defined however you like, and
97a1d740
YST
2266usually is. Anything may be considered portable if you try hard
2267enough. See I<mobile home> or I<London Bridge>.
2268
2269=item porter
2270
27ed30b8 2271Someone who "carries" software from one L</platform> to another.
97a1d740
YST
2272Porting programs written in platform-dependent languages such as C can
2273be difficult work, but porting programs like Perl is very much worth
2274the agony.
2275
2276=item POSIX
2277
2278The Portable Operating System Interface specification.
2279
2280=item postfix
2281
27ed30b8 2282An L</operator> that follows its L</operand>, as in C<$x++>.
97a1d740
YST
2283
2284=item pp
2285
2286An internal shorthand for a "push-pop" code, that is, C code
2287implementing Perl's stack machine.
2288
2289=item pragma
2290
2291A standard module whose practical hints and suggestions are received
2292(and possibly ignored) at compile time. Pragmas are named in all
2293lowercase.
2294
2295=item precedence
2296
2297The rules of conduct that, in the absence of other guidance, determine
2298what should happen first. For example, in the absence of parentheses,
2299you always do multiplication before addition.
2300
2301=item prefix
2302
27ed30b8 2303An L</operator> that precedes its L</operand>, as in C<++$x>.
97a1d740
YST
2304
2305=item preprocessing
2306
27ed30b8 2307What some helper L</process> did to transform the incoming data into a
97a1d740 2308form more suitable for the current process. Often done with an
27ed30b8 2309incoming L</pipe>. See also L</C preprocessor>.
97a1d740
YST
2310
2311=item procedure
2312
27ed30b8 2313A L</subroutine>.
97a1d740
YST
2314
2315=item process
2316
2317An instance of a running program. Under multitasking systems like
2318Unix, two or more separate processes could be running the same program
2319independently at the same time--in fact, the L<fork|perlfunc/fork>
2320function is designed to bring about this happy state of affairs.
2321Under other operating systems, processes are sometimes called
2322"threads", "tasks", or "jobs", often with slight nuances in meaning.
2323
2324=item program generator
2325
2326A system that algorithmically writes code for you in a high-level
27ed30b8 2327language. See also L</code generator>.
97a1d740
YST
2328
2329=item progressive matching
2330
27ed30b8 2331L<Pattern matching|/pattern matching> that picks up where it left off before.
97a1d740
YST
2332
2333=item property
2334
27ed30b8 2335See either L</instance variable> or L</character property>.
97a1d740
YST
2336
2337=item protocol
2338
2339In networking, an agreed-upon way of sending messages back and forth
2340so that neither correspondent will get too confused.
2341
2342=item prototype
2343
27ed30b8 2344An optional part of a L</subroutine> declaration telling the Perl
97a1d740 2345compiler how many and what flavor of arguments may be passed as
27ed30b8 2346L</actual arguments>, so that you can write subroutine calls that
97a1d740
YST
2347parse much like built-in functions. (Or don't parse, as the case may
2348be.)
2349
2350=item pseudofunction
2351
2352A construct that sometimes looks like a function but really isn't.
27ed30b8
YST
2353Usually reserved for L</lvalue> modifiers like L<my|perlfunc/my>, for
2354L</context> modifiers like L<scalar|perlfunc/scalar>, and for the
97a1d740
YST
2355pick-your-own-quotes constructs, C<q//>, C<qq//>, C<qx//>, C<qw//>,
2356C<qr//>, C<m//>, C<s///>, C<y///>, and C<tr///>.
2357
2358=item pseudohash
2359
2360A reference to an array whose initial element happens to hold a
2361reference to a hash. You can treat a pseudohash reference as either
2362an array reference or a hash reference.
2363
2364=item pseudoliteral
2365
27ed30b8
YST
2366An L</operator> that looks something like a L</literal>, such as the
2367output-grabbing operator, C<`>I<C<command>>C<`>.
97a1d740
YST
2368
2369=item public domain
2370
2371Something not owned by anybody. Perl is copyrighted and is thus
27ed30b8
YST
2372I<not> in the public domain--it's just L</freely available> and
2373L</freely redistributable>.
97a1d740
YST
2374
2375=item pumpkin
2376
2377A notional "baton" handed around the Perl community indicating who is
2378the lead integrator in some arena of development.
2379
2380=item pumpking
2381
27ed30b8 2382A L</pumpkin> holder, the person in charge of pumping the pump, or at
97a1d740
YST
2383least priming it. Must be willing to play the part of the Great
2384Pumpkin now and then.
2385
2386=item PV
2387
2388A "pointer value", which is Perl Internals Talk for a C<char*>.
2389
2390=back
2391
5bbd0522
YST
2392=head2 Q
2393
97a1d740
YST
2394=over 4
2395
2396=item qualified
2397
2398Possessing a complete name. The symbol C<$Ent::moot> is qualified;
2399C<$moot> is unqualified. A fully qualified filename is specified from
2400the top-level directory.
2401
2402=item quantifier
2403
27ed30b8
YST
2404A component of a L</regular expression> specifying how many times the
2405foregoing L</atom> may occur.
97a1d740
YST
2406
2407=back
2408
5bbd0522
YST
2409=head2 R
2410
97a1d740
YST
2411=over 4
2412
2413=item readable
2414
2415With respect to files, one that has the proper permission bit set to
2416let you access the file. With respect to computer programs, one
2417that's written well enough that someone has a chance of figuring out
2418what it's trying to do.
2419
2420=item reaping
2421
27ed30b8
YST
2422The last rites performed by a parent L</process> on behalf of a
2423deceased child process so that it doesn't remain a L</zombie>. See
97a1d740
YST
2424the L<wait|perlfunc/wait> and L<waitpid|perlfunc/waitpid> function
2425calls.
2426
2427=item record
2428
27ed30b8
YST
2429A set of related data values in a L</file> or L</stream>, often
2430associated with a unique L</key> field. In Unix, often commensurate
2431with a L</line>, or a blank-line-terminated set of lines (a
97a1d740
YST
2432"paragraph"). Each line of the I</etc/passwd> file is a record, keyed
2433on login name, containing information about that user.
2434
2435=item recursion
2436
2437The art of defining something (at least partly) in terms of itself,
2438which is a naughty no-no in dictionaries but often works out okay in
2439computer programs if you're careful not to recurse forever, which is
2440like an infinite loop with more spectacular failure modes.
2441
2442=item reference
2443
2444Where you look to find a pointer to information somewhere else. (See
27ed30b8 2445L</indirection>.) References come in two flavors, L<symbolic
97a1d740
YST
2446references|/symbolic reference> and L<hard references|/hard
2447reference>.
2448
2449=item referent
2450
2451Whatever a reference refers to, which may or may not have a name.
2452Common types of referents include scalars, arrays, hashes, and
2453subroutines.
2454
2455=item regex
2456
27ed30b8 2457See L</regular expression>.
97a1d740
YST
2458
2459=item regular expression
2460
2461A single entity with various interpretations, like an elephant. To a
2462computer scientist, it's a grammar for a little language in which some
2463strings are legal and others aren't. To normal people, it's a pattern
2464you can use to find what you're looking for when it varies from case
2465to case. Perl's regular expressions are far from regular in the
2466theoretical sense, but in regular use they work quite well. Here's a
2467regular expression: C</Oh s.*t./>. This will match strings like "C<Oh
2468say can you see by the dawn's early light>" and "C<Oh sit!>". See
2469L<perlre>.
2470
2471=item regular expression modifier
2472
2473An option on a pattern or substitution, such as C</i> to render the
27ed30b8 2474pattern case insensitive. See also L</cloister>.
97a1d740
YST
2475
2476=item regular file
2477
27ed30b8
YST
2478A L</file> that's not a L</directory>, a L</device>, a named L</pipe>
2479or L</socket>, or a L</symbolic link>. Perl uses the C<-f> file test
97a1d740
YST
2480operator to identify regular files. Sometimes called a "plain" file.
2481
2482=item relational operator
2483
27ed30b8
YST
2484An L</operator> that says whether a particular ordering relationship
2485is L</true> about a pair of L<operands|/operand>. Perl has both
2486numeric and string relational operators. See L</collating sequence>.
97a1d740
YST
2487
2488=item reserved words
2489
27ed30b8 2490A word with a specific, built-in meaning to a L</compiler>, such as
97a1d740
YST
2491C<if> or L<delete|perlfunc/delete>. In many languages (not Perl),
2492it's illegal to use reserved words to name anything else. (Which is
2493why they're reserved, after all.) In Perl, you just can't use them to
27ed30b8 2494name L<labels|/label> or L<filehandles|/filehandle>. Also called
97a1d740
YST
2495"keywords".
2496
2497=item return value
2498
27ed30b8
YST
2499The L</value> produced by a L</subroutine> or L</expression> when
2500evaluated. In Perl, a return value may be either a L</list> or a
2501L</scalar>.
97a1d740
YST
2502
2503=item RFC
2504
2505Request For Comment, which despite the timid connotations is the name
2506of a series of important standards documents.
2507
2508=item right shift
2509
27ed30b8 2510A L</bit shift> that divides a number by some power of 2.
97a1d740
YST
2511
2512=item root
2513
2514The superuser (UID == 0). Also, the top-level directory of the
2515filesystem.
2516
2517=item RTFM
2518
2519What you are told when someone thinks you should Read The Fine Manual.
2520
2521=item run phase
2522
2523Any time after Perl starts running your main program. See also
27ed30b8
YST
2524L</compile phase>. Run phase is mostly spent in L</run time> but may
2525also be spent in L</compile time> when L<require|perlfunc/require>,
97a1d740
YST
2526L<do|perlfunc/do> C<FILE>, or L<eval|perlfunc/eval> C<STRING>
2527operators are executed or when a substitution uses the C</ee>
2528modifier.
2529
2530=item run time
2531
2532The time when Perl is actually doing what your code says to do, as
2533opposed to the earlier period of time when it was trying to figure out
27ed30b8 2534whether what you said made any sense whatsoever, which is L</compile
97a1d740
YST
2535time>.
2536
2537=item run-time pattern
2538
2539A pattern that contains one or more variables to be interpolated
27ed30b8 2540before parsing the pattern as a L</regular expression>, and that
97a1d740
YST
2541therefore cannot be analyzed at compile time, but must be re-analyzed
2542each time the pattern match operator is evaluated. Run-time patterns
2543are useful but expensive.
2544
2545=item RV
2546
2547A recreational vehicle, not to be confused with vehicular recreation.
27ed30b8
YST
2548RV also means an internal Reference Value of the type a L</scalar> can
2549hold. See also L</IV> and L</NV> if you're not confused yet.
97a1d740
YST
2550
2551=item rvalue
2552
27ed30b8
YST
2553A L</value> that you might find on the right side of an
2554L</assignment>. See also L</lvalue>.
97a1d740
YST
2555
2556=back
2557
5bbd0522
YST
2558=head2 S
2559
97a1d740
YST
2560=over 4
2561
2562=item scalar
2563
27ed30b8 2564A simple, singular value; a number, L</string>, or L</reference>.
97a1d740
YST
2565
2566=item scalar context
2567
27ed30b8
YST
2568The situation in which an L</expression> is expected by its
2569surroundings (the code calling it) to return a single L</value> rather
2570than a L</list> of values. See also L</context> and L</list context>.
97a1d740 2571A scalar context sometimes imposes additional constraints on the
27ed30b8
YST
2572return value--see L</string context> and L</numeric context>.
2573Sometimes we talk about a L</Boolean context> inside conditionals, but
97a1d740 2574this imposes no additional constraints, since any scalar value,
27ed30b8 2575whether numeric or L</string>, is already true or false.
97a1d740
YST
2576
2577=item scalar literal
2578
27ed30b8
YST
2579A number or quoted L</string>--an actual L</value> in the text of your
2580program, as opposed to a L</variable>.
97a1d740
YST
2581
2582=item scalar value
2583
27ed30b8 2584A value that happens to be a L</scalar> as opposed to a L</list>.
97a1d740
YST
2585
2586=item scalar variable
2587
27ed30b8 2588A L</variable> prefixed with C<$> that holds a single value.
97a1d740
YST
2589
2590=item scope
2591
2592How far away you can see a variable from, looking through one. Perl
27ed30b8
YST
2593has two visibility mechanisms: it does L</dynamic scoping> of
2594L<local|perlfunc/local> L<variables|/variable>, meaning that the rest
2595of the L</block>, and any L<subroutines|/subroutine> that are called
97a1d740 2596by the rest of the block, can see the variables that are local to the
27ed30b8 2597block. Perl does L</lexical scoping> of L<my|perlfunc/my> variables,
97a1d740
YST
2598meaning that the rest of the block can see the variable, but other
2599subroutines called by the block I<cannot> see the variable.
2600
2601=item scratchpad
2602
2603The area in which a particular invocation of a particular file or
2604subroutine keeps some of its temporary values, including any lexically
2605scoped variables.
2606
2607=item script
2608
27ed30b8 2609A text L</file> that is a program intended to be L<executed|/execute>
97a1d740 2610directly rather than L<compiled|/compiler> to another form of file
27ed30b8 2611before execution. Also, in the context of L</Unicode>, a writing
97a1d740
YST
2612system for a particular language or group of languages, such as Greek,
2613Bengali, or Klingon.
2614
2615=item script kiddie
2616
27ed30b8 2617A L</cracker> who is not a L</hacker>, but knows just enough to run
97a1d740
YST
2618canned scripts. A cargo-cult programmer.
2619
2620=item sed
2621
2622A venerable Stream EDitor from which Perl derives some of its ideas.
2623
2624=item semaphore
2625
27ed30b8
YST
2626A fancy kind of interlock that prevents multiple L<threads|/thread> or
2627L<processes|/process> from using up the same resources simultaneously.
97a1d740
YST
2628
2629=item separator
2630
27ed30b8 2631A L</character> or L</string> that keeps two surrounding strings from
97a1d740 2632being confused with each other. The L<split|perlfunc/split> function
27ed30b8
YST
2633works on separators. Not to be confused with L<delimiters|/delimiter>
2634or L<terminators|/terminator>. The "or" in the previous sentence
97a1d740
YST
2635separated the two alternatives.
2636
2637=item serialization
2638
27ed30b8
YST
2639Putting a fancy L</data structure> into linear order so that it can be
2640stored as a L</string> in a disk file or database or sent through a
2641L</pipe>. Also called marshalling.
97a1d740
YST
2642
2643=item server
2644
27ed30b8
YST
2645In networking, a L</process> that either advertises a L</service> or
2646just hangs around at a known location and waits for L<clients|/client>
97a1d740
YST
2647who need service to get in touch with it.
2648
2649=item service
2650
2651Something you do for someone else to make them happy, like giving them
2652the time of day (or of their life). On some machines, well-known
2653services are listed by the L<getservent|perlfunc/getservent> function.
2654
2655=item setgid
2656
27ed30b8 2657Same as L</setuid>, only having to do with giving away L</group>
97a1d740
YST
2658privileges.
2659
2660=item setuid
2661
27ed30b8 2662Said of a program that runs with the privileges of its L</owner>
97a1d740 2663rather than (as is usually the case) the privileges of whoever is
27ed30b8 2664running it. Also describes the bit in the mode word (L</permission
97a1d740
YST
2665bits>) that controls the feature. This bit must be explicitly set by
2666the owner to enable this feature, and the program must be carefully
2667written not to give away more privileges than it ought to.
2668
2669=item shared memory
2670
27ed30b8
YST
2671A piece of L</memory> accessible by two different
2672L<processes|/process> who otherwise would not see each other's memory.
97a1d740
YST
2673
2674=item shebang
2675
2676Irish for the whole McGillicuddy. In Perl culture, a portmanteau of
2677"sharp" and "bang", meaning the C<#!> sequence that tells the system
2678where to find the interpreter.
2679
2680=item shell
2681
27ed30b8
YST
2682A L</command>-line L</interpreter>. The program that interactively
2683gives you a prompt, accepts one or more L<lines|/line> of input, and
97a1d740 2684executes the programs you mentioned, feeding each of them their proper
27ed30b8 2685L<arguments|/argument> and input data. Shells can also execute
97a1d740
YST
2686scripts containing such commands. Under Unix, typical shells include
2687the Bourne shell (I</bin/sh>), the C shell (I</bin/csh>), and the Korn
2688shell (I</bin/ksh>). Perl is not strictly a shell because it's not
2689interactive (although Perl programs can be interactive).
2690
2691=item side effects
2692
27ed30b8 2693Something extra that happens when you evaluate an L</expression>.
97a1d740
YST
2694Nowadays it can refer to almost anything. For example, evaluating a
2695simple assignment statement typically has the "side effect" of
2696assigning a value to a variable. (And you thought assigning the value
2697was your primary intent in the first place!) Likewise, assigning a
27ed30b8
YST
2698value to the special variable C<$|> (C<$AUTOFLUSH>) has the side
2699effect of forcing a flush after every L<write|perlfunc/write> or
2700L<print|perlfunc/print> on the currently selected filehandle.
97a1d740
YST
2701
2702=item signal
2703
2704A bolt out of the blue; that is, an event triggered by the
27ed30b8 2705L</operating system>, probably when you're least expecting it.
97a1d740
YST
2706
2707=item signal handler
2708
27ed30b8 2709A L</subroutine> that, instead of being content to be called in the
97a1d740 2710normal fashion, sits around waiting for a bolt out of the blue before
27ed30b8 2711it will deign to L</execute>. Under Perl, bolts out of the blue are
97a1d740
YST
2712called signals, and you send them with the L<kill|perlfunc/kill>
2713built-in. See L<perlvar/%SIG> and L<perlipc/Signals>.
2714
2715=item single inheritance
2716
2717The features you got from your mother, if she told you that you don't
27ed30b8 2718have a father. (See also L</inheritance> and L</multiple
97a1d740 2719inheritance>.) In computer languages, the notion that
27ed30b8
YST
2720L<classes|/class> reproduce asexually so that a given class can only
2721have one direct ancestor or L</base class>. Perl supplies no such
97a1d740
YST
2722restriction, though you may certainly program Perl that way if you
2723like.
2724
2725=item slice
2726
27ed30b8
YST
2727A selection of any number of L<elements|/element> from a L</list>,
2728L</array>, or L</hash>.
97a1d740
YST
2729
2730=item slurp
2731
27ed30b8 2732To read an entire L</file> into a L</string> in one operation.
97a1d740
YST
2733
2734=item socket
2735
2736An endpoint for network communication among multiple
27ed30b8
YST
2737L<processes|/process> that works much like a telephone or a post
2738office box. The most important thing about a socket is its L</network
97a1d740
YST
2739address> (like a phone number). Different kinds of sockets have
2740different kinds of addresses--some look like filenames, and some
2741don't.
2742
2743=item soft reference
2744
27ed30b8 2745See L</symbolic reference>.
97a1d740
YST
2746
2747=item source filter
2748
27ed30b8
YST
2749A special kind of L</module> that does L</preprocessing> on your
2750script just before it gets to the L</tokener>.
97a1d740
YST
2751
2752=item stack
2753
2754A device you can put things on the top of, and later take them back
27ed30b8 2755off in the opposite order in which you put them on. See L</LIFO>.
97a1d740
YST
2756
2757=item standard
2758
2759Included in the official Perl distribution, as in a standard module, a
27ed30b8 2760standard tool, or a standard Perl L</manpage>.
97a1d740
YST
2761
2762=item standard error
2763
27ed30b8
YST
2764The default output L</stream> for nasty remarks that don't belong in
2765L</standard output>. Represented within a Perl program by the
2766L</filehandle> L</STDERR>. You can use this stream explicitly, but the
97a1d740
YST
2767L<die|perlfunc/die> and L<warn|perlfunc/warn> built-ins write to your
2768standard error stream automatically.
2769
2770=item standard I/O
2771
27ed30b8
YST
2772A standard C library for doing L<buffered|/buffer> input and output to
2773the L</operating system>. (The "standard" of standard I/O is only
97a1d740
YST
2774marginally related to the "standard" of standard input and output.)
2775In general, Perl relies on whatever implementation of standard I/O a
2776given operating system supplies, so the buffering characteristics of a
2777Perl program on one machine may not exactly match those on another
2778machine. Normally this only influences efficiency, not semantics. If
2779your standard I/O package is doing block buffering and you want it to
27ed30b8 2780L</flush> the buffer more often, just set the C<$|> variable to a true
97a1d740
YST
2781value.
2782
2783=item standard input
2784
27ed30b8 2785The default input L</stream> for your program, which if possible
97a1d740 2786shouldn't care where its data is coming from. Represented within a
27ed30b8 2787Perl program by the L</filehandle> L</STDIN>.
97a1d740
YST
2788
2789=item standard output
2790
27ed30b8 2791The default output L</stream> for your program, which if possible
97a1d740 2792shouldn't care where its data is going. Represented within a Perl
27ed30b8 2793program by the L</filehandle> L</STDOUT>.
97a1d740
YST
2794
2795=item stat structure
2796
2797A special internal spot in which Perl keeps the information about the
27ed30b8 2798last L</file> on which you requested information.
97a1d740
YST
2799
2800=item statement
2801
27ed30b8 2802A L</command> to the computer about what to do next, like a step in a
97a1d740 2803recipe: "Add marmalade to batter and mix until mixed." A statement is
27ed30b8 2804distinguished from a L</declaration>, which doesn't tell the computer
97a1d740
YST
2805to do anything, but just to learn something.
2806
2807=item statement modifier
2808
27ed30b8 2809A L</conditional> or L</loop> that you put after the L</statement>
97a1d740
YST
2810instead of before, if you know what we mean.
2811
2812=item static
2813
2814Varying slowly compared to something else. (Unfortunately, everything
2815is relatively stable compared to something else, except for certain
2816elementary particles, and we're not so sure about them.) In
2817computers, where things are supposed to vary rapidly, "static" has a
2818derogatory connotation, indicating a slightly dysfunctional
27ed30b8 2819L</variable>, L</subroutine>, or L</method>. In Perl culture, the
97a1d740
YST
2820word is politely avoided.
2821
2822=item static method
2823
27ed30b8 2824No such thing. See L</class method>.
97a1d740
YST
2825
2826=item static scoping
2827
27ed30b8 2828No such thing. See L</lexical scoping>.
97a1d740
YST
2829
2830=item static variable
2831
27ed30b8
YST
2832No such thing. Just use a L</lexical variable> in a scope larger than
2833your L</subroutine>.
97a1d740
YST
2834
2835=item status
2836
27ed30b8 2837The L</value> returned to the parent L</process> when one of its child
97a1d740 2838processes dies. This value is placed in the special variable C<$?>.
27ed30b8 2839Its upper eight L<bits|/bit> are the exit status of the defunct
97a1d740
YST
2840process, and its lower eight bits identify the signal (if any) that
2841the process died from. On Unix systems, this status value is the same
2842as the status word returned by I<wait>(2). See L<perlfunc/system>.
2843
2844=item STDERR
2845
27ed30b8 2846See L</standard error>.
97a1d740
YST
2847
2848=item STDIN
2849
27ed30b8 2850See L</standard input>.
97a1d740
YST
2851
2852=item STDIO
2853
27ed30b8 2854See L</standard IE<sol>O>.
97a1d740
YST
2855
2856=item STDOUT
2857
27ed30b8 2858See L</standard output>.
97a1d740
YST
2859
2860=item stream
2861
2862A flow of data into or out of a process as a steady sequence of bytes
2863or characters, without the appearance of being broken up into packets.
27ed30b8 2864This is a kind of L</interface>--the underlying L</implementation> may
97a1d740
YST
2865well break your data up into separate packets for delivery, but this
2866is hidden from you.
2867
2868=item string
2869
2870A sequence of characters such as "He said !@#*&%@#*?!". A string does
2871not have to be entirely printable.
2872
2873=item string context
2874
2875The situation in which an expression is expected by its surroundings
27ed30b8
YST
2876(the code calling it) to return a L</string>. See also L</context>
2877and L</numeric context>.
97a1d740
YST
2878
2879=item stringification
2880
27ed30b8 2881The process of producing a L</string> representation of an abstract
97a1d740
YST
2882object.
2883
2884=item struct
2885
2886C keyword introducing a structure definition or name.
2887
2888=item structure
2889
27ed30b8 2890See L</data structure>.
97a1d740
YST
2891
2892=item subclass
2893
27ed30b8 2894See L</derived class>.
97a1d740
YST
2895
2896=item subpattern
2897
27ed30b8 2898A component of a L</regular expression> pattern.
97a1d740
YST
2899
2900=item subroutine
2901
2902A named or otherwise accessible piece of program that can be invoked
2903from elsewhere in the program in order to accomplish some sub-goal of
2904the program. A subroutine is often parameterized to accomplish
2905different but related things depending on its input
27ed30b8
YST
2906L<arguments|/argument>. If the subroutine returns a meaningful
2907L</value>, it is also called a L</function>.
97a1d740
YST
2908
2909=item subscript
2910
27ed30b8
YST
2911A L</value> that indicates the position of a particular L</array>
2912L</element> in an array.
97a1d740
YST
2913
2914=item substitution
2915
2916Changing parts of a string via the C<s///> operator. (We avoid use of
27ed30b8 2917this term to mean L</variable interpolation>.)
97a1d740
YST
2918
2919=item substring
2920
27ed30b8
YST
2921A portion of a L</string>, starting at a certain L</character>
2922position (L</offset>) and proceeding for a certain number of
97a1d740
YST
2923characters.
2924
2925=item superclass
2926
27ed30b8 2927See L</base class>.
97a1d740
YST
2928
2929=item superuser
2930
27ed30b8 2931The person whom the L</operating system> will let do almost anything.
97a1d740 2932Typically your system administrator or someone pretending to be your
27ed30b8 2933system administrator. On Unix systems, the L</root> user. On Windows
97a1d740
YST
2934systems, usually the Administrator user.
2935
2936=item SV
2937
2938Short for "scalar value". But within the Perl interpreter every
27ed30b8
YST
2939L</referent> is treated as a member of a class derived from SV, in an
2940object-oriented sort of way. Every L</value> inside Perl is passed
2941around as a C language C<SV*> pointer. The SV L</struct> knows its
97a1d740 2942own "referent type", and the code is smart enough (we hope) not to try
27ed30b8 2943to call a L</hash> function on a L</subroutine>.
97a1d740
YST
2944
2945=item switch
2946
2947An option you give on a command line to influence the way your program
2948works, usually introduced with a minus sign. The word is also used as
27ed30b8 2949a nickname for a L</switch statement>.
97a1d740
YST
2950
2951=item switch cluster
2952
2953The combination of multiple command-line switches (e.g., B<-a -b -c>)
2954into one switch (e.g., B<-abc>). Any switch with an additional
27ed30b8 2955L</argument> must be the last switch in a cluster.
97a1d740
YST
2956
2957=item switch statement
2958
27ed30b8 2959A program technique that lets you evaluate an L</expression> and then,
97a1d740
YST
2960based on the value of the expression, do a multiway branch to the
2961appropriate piece of code for that value. Also called a "case
2962structure", named after the similar Pascal construct. Most switch
2963statements in Perl are spelled C<for>. See L<perlsyn/Basic BLOCKs and
2964Switch Statements>.
2965
2966=item symbol
2967
27ed30b8
YST
2968Generally, any L</token> or L</metasymbol>. Often used more
2969specifically to mean the sort of name you might find in a L</symbol
97a1d740
YST
2970table>.
2971
2972=item symbol table
2973
27ed30b8
YST
2974Where a L</compiler> remembers symbols. A program like Perl must
2975somehow remember all the names of all the L<variables|/variable>,
2976L<filehandles|/filehandle>, and L<subroutines|/subroutine> you've
97a1d740 2977used. It does this by placing the names in a symbol table, which is
27ed30b8
YST
2978implemented in Perl using a L</hash table>. There is a separate
2979symbol table for each L</package> to give each package its own
2980L</namespace>.
97a1d740
YST
2981
2982=item symbolic debugger
2983
2984A program that lets you step through the L<execution|/execute> of your
2985program, stopping or printing things out here and there to see whether
2986anything has gone wrong, and if so, what. The "symbolic" part just
2987means that you can talk to the debugger using the same symbols with
2988which your program is written.
2989
2990=item symbolic link
2991
27ed30b8
YST
2992An alternate filename that points to the real L</filename>, which in
2993turn points to the real L</file>. Whenever the L</operating system>
2994is trying to parse a L</pathname> containing a symbolic link, it
97a1d740
YST
2995merely substitutes the new name and continues parsing.
2996
2997=item symbolic reference
2998
2999A variable whose value is the name of another variable or subroutine.
3000By L<dereferencing|/dereference> the first variable, you can get at
27ed30b8
YST
3001the second one. Symbolic references are illegal under L<use strict
3002'refs'|strict/strict refs>.
97a1d740
YST
3003
3004=item synchronous
3005
3006Programming in which the orderly sequence of events can be determined;
3007that is, when things happen one after the other, not at the same time.
3008
3009=item syntactic sugar
3010
3011An alternative way of writing something more easily; a shortcut.
3012
3013=item syntax
3014
3015From Greek, "with-arrangement". How things (particularly symbols) are
3016put together with each other.
3017
3018=item syntax tree
3019
3020An internal representation of your program wherein lower-level
27ed30b8 3021L<constructs|/construct> dangle off the higher-level constructs
97a1d740
YST
3022enclosing them.
3023
3024=item syscall
3025
27ed30b8 3026A L</function> call directly to the L</operating system>. Many of the
97a1d740
YST
3027important subroutines and functions you use aren't direct system
3028calls, but are built up in one or more layers above the system call
3029level. In general, Perl programmers don't need to worry about the
3030distinction. However, if you do happen to know which Perl functions
3031are really syscalls, you can predict which of these will set the C<$!>
3032(C<$ERRNO>) variable on failure. Unfortunately, beginning programmers
3033often confusingly employ the term "system call" to mean what happens
3034when you call the Perl L<system|perlfunc/system> function, which
3035actually involves many syscalls. To avoid any confusion, we nearly
3036always use say "syscall" for something you could call indirectly via
3037Perl's L<syscall|perlfunc/syscall> function, and never for something
3038you would call with Perl's L<system|perlfunc/system> function.
3039
3040=back
3041
5bbd0522
YST
3042=head2 T
3043
97a1d740
YST
3044=over 4
3045
3046=item tainted
3047
3048Said of data derived from the grubby hands of a user and thus unsafe
3049for a secure program to rely on. Perl does taint checks if you run a
27ed30b8 3050L</setuid> (or L</setgid>) program, or if you use the B<-T> switch.
97a1d740
YST
3051
3052=item TCP
3053
3054Short for Transmission Control Protocol. A protocol wrapped around
3055the Internet Protocol to make an unreliable packet transmission
3056mechanism appear to the application program to be a reliable
27ed30b8 3057L</stream> of bytes. (Usually.)
97a1d740
YST
3058
3059=item term
3060
27ed30b8
YST
3061Short for a "terminal", that is, a leaf node of a L</syntax tree>. A
3062thing that functions grammatically as an L</operand> for the operators
97a1d740
YST
3063in an expression.
3064
3065=item terminator
3066
27ed30b8 3067A L</character> or L</string> that marks the end of another string.
97a1d740
YST
3068The C<$/> variable contains the string that terminates a
3069L<readline|perlfunc/readline> operation, which L<chomp|perlfunc/chomp>
3070deletes from the end. Not to be confused with
27ed30b8 3071L<delimiters|/delimiter> or L<separators|/separator>. The period at
97a1d740
YST
3072the end of this sentence is a terminator.
3073
3074=item ternary
3075
27ed30b8
YST
3076An L</operator> taking three L<operands|/operand>. Sometimes
3077pronounced L</trinary>.
97a1d740
YST
3078
3079=item text
3080
27ed30b8 3081A L</string> or L</file> containing primarily printable characters.
97a1d740
YST
3082
3083=item thread
3084
27ed30b8 3085Like a forked process, but without L</fork>'s inherent memory
97a1d740
YST
3086protection. A thread is lighter weight than a full process, in that a
3087process could have multiple threads running around in it, all fighting
3088over the same process's memory space unless steps are taken to protect
3089threads from each other. See L<threads>.
3090
3091=item tie
3092
3093The bond between a magical variable and its implementation class. See
3094L<perlfunc/tie> and L<perltie>.
3095
3096=item TMTOWTDI
3097
3098There's More Than One Way To Do It, the Perl Motto. The notion that
3099there can be more than one valid path to solving a programming problem
3100in context. (This doesn't mean that more ways are always better or
3101that all possible paths are equally desirable--just that there need
3102not be One True Way.) Pronounced TimToady.
3103
3104=item token
3105
3106A morpheme in a programming language, the smallest unit of text with
3107semantic significance.
3108
3109=item tokener
3110
3111A module that breaks a program text into a sequence of
27ed30b8 3112L<tokens|/token> for later analysis by a parser.
97a1d740
YST
3113
3114=item tokenizing
3115
27ed30b8 3116Splitting up a program text into L<tokens|/token>. Also known as
97a1d740
YST
3117"lexing", in which case you get "lexemes" instead of tokens.
3118
3119=item toolbox approach
3120
3121The notion that, with a complete set of simple tools that work well
3122together, you can build almost anything you want. Which is fine if
3123you're assembling a tricycle, but if you're building a defranishizing
3124comboflux regurgalator, you really want your own machine shop in which
3125to build special tools. Perl is sort of a machine shop.
3126
3127=item transliterate
3128
3129To turn one string representation into another by mapping each
3130character of the source string to its corresponding character in the
3131result string. See
3132L<perlop/trE<sol>SEARCHLISTE<sol>REPLACEMENTLISTE<sol>cds>.
3133
3134=item trigger
3135
27ed30b8 3136An event that causes a L</handler> to be run.
97a1d740
YST
3137
3138=item trinary
3139
27ed30b8
YST
3140Not a stellar system with three stars, but an L</operator> taking
3141three L<operands|/operand>. Sometimes pronounced L</ternary>.
97a1d740
YST
3142
3143=item troff
3144
3145A venerable typesetting language from which Perl derives the name of
3146its C<$%> variable and which is secretly used in the production of
3147Camel books.
3148
3149=item true
3150
3151Any scalar value that doesn't evaluate to 0 or C<"">.
3152
3153=item truncating
3154
3155Emptying a file of existing contents, either automatically when
3156opening a file for writing or explicitly via the
3157L<truncate|perlfunc/truncate> function.
3158
3159=item type
3160
27ed30b8 3161See L</data type> and L</class>.
97a1d740
YST
3162
3163=item type casting
3164
3165Converting data from one type to another. C permits this. Perl does
3166not need it. Nor want it.
3167
3168=item typed lexical
3169
27ed30b8 3170A L</lexical variable> that is declared with a L</class> type: C<my
97a1d740
YST
3171Pony $bill>.
3172
3173=item typedef
3174
3175A type definition in the C language.
3176
3177=item typeglob
3178
3179Use of a single identifier, prefixed with C<*>. For example, C<*name>
3180stands for any or all of C<$name>, C<@name>, C<%name>, C<&name>, or
3181just C<name>. How you use it determines whether it is interpreted as
3182all or only one of them. See L<perldata/Typeglobs and Filehandles>.
3183
3184=item typemap
3185
3186A description of how C types may be transformed to and from Perl types
27ed30b8 3187within an L</extension> module written in L</XS>.
97a1d740
YST
3188
3189=back
3190
5bbd0522
YST
3191=head2 U
3192
97a1d740
YST
3193=over 4
3194
3195=item UDP
3196
27ed30b8 3197User Datagram Protocol, the typical way to send L<datagrams|/datagram>
97a1d740
YST
3198over the Internet.
3199
3200=item UID
3201
27ed30b8 3202A user ID. Often used in the context of L</file> or L</process>
97a1d740
YST
3203ownership.
3204
3205=item umask
3206
27ed30b8 3207A mask of those L</permission bits> that should be forced off when
97a1d740
YST
3208creating files or directories, in order to establish a policy of whom
3209you'll ordinarily deny access to. See the L<umask|perlfunc/umask>
3210function.
3211
3212=item unary operator
3213
27ed30b8 3214An operator with only one L</operand>, like C<!> or
97a1d740
YST
3215L<chdir|perlfunc/chdir>. Unary operators are usually prefix
3216operators; that is, they precede their operand. The C<++> and C<-->
3217operators can be either prefix or postfix. (Their position I<does>
3218change their meanings.)
3219
3220=item Unicode
3221
3222A character set comprising all the major character sets of the world,
3223more or less. See L<http://www.unicode.org>.
3224
3225=item Unix
3226
3227A very large and constantly evolving language with several alternative
3228and largely incompatible syntaxes, in which anyone can define anything
3229any way they choose, and usually do. Speakers of this language think
3230it's easy to learn because it's so easily twisted to one's own ends,
3231but dialectical differences make tribal intercommunication nearly
3232impossible, and travelers are often reduced to a pidgin-like subset of
3233the language. To be universally understood, a Unix shell programmer
3234must spend years of study in the art. Many have abandoned this
3235discipline and now communicate via an Esperanto-like language called
3236Perl.
3237
3238In ancient times, Unix was also used to refer to some code that a
3239couple of people at Bell Labs wrote to make use of a PDP-7 computer
3240that wasn't doing much of anything else at the time.
3241
3242=back
3243
5bbd0522
YST
3244=head2 V
3245
97a1d740
YST
3246=over 4
3247
3248=item value
3249
3250An actual piece of data, in contrast to all the variables, references,
3251keys, indexes, operators, and whatnot that you need to access the
3252value.
3253
3254=item variable
3255
3256A named storage location that can hold any of various kinds of
27ed30b8 3257L</value>, as your program sees fit.
97a1d740
YST
3258
3259=item variable interpolation
3260
27ed30b8 3261The L</interpolation> of a scalar or array variable into a string.
97a1d740
YST
3262
3263=item variadic
3264
27ed30b8
YST
3265Said of a L</function> that happily receives an indeterminate number
3266of L</actual arguments>.
97a1d740
YST
3267
3268=item vector
3269
27ed30b8 3270Mathematical jargon for a list of L<scalar values|/scalar value>.
97a1d740
YST
3271
3272=item virtual
3273
3274Providing the appearance of something without the reality, as in:
27ed30b8 3275virtual memory is not real memory. (See also L</memory>.) The
97a1d740
YST
3276opposite of "virtual" is "transparent", which means providing the
3277reality of something without the appearance, as in: Perl handles the
3278variable-length UTF-8 character encoding transparently.
3279
3280=item void context
3281
27ed30b8
YST
3282A form of L</scalar context> in which an L</expression> is not
3283expected to return any L</value> at all and is evaluated for its
3284L</side effects> alone.
97a1d740
YST
3285
3286=item v-string
3287
27ed30b8 3288A "version" or "vector" L</string> specified with a C<v> followed by a
97a1d740 3289series of decimal integers in dot notation, for instance,
27ed30b8 3290C<v1.20.300.4000>. Each number turns into a L</character> with the
97a1d740
YST
3291specified ordinal value. (The C<v> is optional when there are at
3292least three integers.)
3293
3294=back
3295
5bbd0522
YST
3296=head2 W
3297
97a1d740
YST
3298=over 4
3299
3300=item warning
3301
27ed30b8 3302A message printed to the L</STDERR> stream to the effect that something
97a1d740
YST
3303might be wrong but isn't worth blowing up over. See L<perlfunc/warn>
3304and the L<warnings> pragma.
3305
3306=item watch expression
3307
3308An expression which, when its value changes, causes a breakpoint in
3309the Perl debugger.
3310
3311=item whitespace
3312
27ed30b8 3313A L</character> that moves your cursor but doesn't otherwise put
97a1d740
YST
3314anything on your screen. Typically refers to any of: space, tab, line
3315feed, carriage return, or form feed.
3316
3317=item word
3318
3319In normal "computerese", the piece of data of the size most
3320efficiently handled by your computer, typically 32 bits or so, give or
3321take a few powers of 2. In Perl culture, it more often refers to an
27ed30b8
YST
3322alphanumeric L</identifier> (including underscores), or to a string of
3323nonwhitespace L<characters|/character> bounded by whitespace or string
97a1d740
YST
3324boundaries.
3325
3326=item working directory
3327
27ed30b8
YST
3328Your current L</directory>, from which relative pathnames are
3329interpreted by the L</operating system>. The operating system knows
97a1d740
YST
3330your current directory because you told it with a
3331L<chdir|perlfunc/chdir> or because you started out in the place where
27ed30b8 3332your parent L</process> was when you were born.
97a1d740
YST
3333
3334=item wrapper
3335
3336A program or subroutine that runs some other program or subroutine for
3337you, modifying some of its input or output to better suit your
3338purposes.
3339
3340=item WYSIWYG
3341
3342What You See Is What You Get. Usually used when something that
3343appears on the screen matches how it will eventually look, like Perl's
3344L<format|perlfunc/format> declarations. Also used to mean the
3345opposite of magic because everything works exactly as it appears, as
3346in the three-argument form of L<open|perlfunc/open>.
3347
3348=back
3349
5bbd0522
YST
3350=head2 X
3351
97a1d740
YST
3352=over 4
3353
3354=item XS
3355
3356An extraordinarily exported, expeditiously excellent, expressly
3357eXternal Subroutine, executed in existing C or C++ or in an exciting
3358new extension language called (exasperatingly) XS. Examine L<perlxs>
3359for the exact explanation or L<perlxstut> for an exemplary unexacting
3360one.
3361
3362=item XSUB
3363
27ed30b8 3364An external L</subroutine> defined in L</XS>.
97a1d740
YST
3365
3366=back
3367
5bbd0522
YST
3368=head2 Y
3369
97a1d740
YST
3370=over 4
3371
3372=item yacc
3373
3374Yet Another Compiler Compiler. A parser generator without which Perl
3375probably would not have existed. See the file I<perly.y> in the Perl
3376source distribution.
3377
3378=back
3379
5bbd0522
YST
3380=head2 Z
3381
97a1d740
YST
3382=over 4
3383
3384=item zero width
3385
27ed30b8
YST
3386A subpattern L</assertion> matching the L</null string> between
3387L<characters|/character>.
97a1d740
YST
3388
3389=item zombie
3390
3391A process that has died (exited) but whose parent has not yet received
3392proper notification of its demise by virtue of having called
3393L<wait|perlfunc/wait> or L<waitpid|perlfunc/waitpid>. If you
3394L<fork|perlfunc/fork>, you must clean up after your child processes
3395when they exit, or else the process table will fill up and your system
3396administrator will Not Be Happy with you.
3397
3398=back
3399
3400=head1 AUTHOR AND COPYRIGHT
3401
3402Based on the Glossary of Programming Perl, Third Edition,
3403by Larry Wall, Tom Christiansen & Jon Orwant.
3404Copyright (c) 2000, 1996, 1991 O'Reilly Media, Inc.
20fd23ef 3405This document may be distributed under the same terms as Perl itself.