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1=head1 NAME
2X<subroutine> X<function>
3
4perlsub - Perl subroutines
5
6=head1 SYNOPSIS
7
8To declare subroutines:
9X<subroutine, declaration> X<sub>
10
11 sub NAME; # A "forward" declaration.
12 sub NAME(PROTO); # ditto, but with prototypes
13 sub NAME : ATTRS; # with attributes
14 sub NAME(PROTO) : ATTRS; # with attributes and prototypes
15
16 sub NAME BLOCK # A declaration and a definition.
17 sub NAME(PROTO) BLOCK # ditto, but with prototypes
18 sub NAME : ATTRS BLOCK # with attributes
19 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
20
21To define an anonymous subroutine at runtime:
22X<subroutine, anonymous>
23
24 $subref = sub BLOCK; # no proto
25 $subref = sub (PROTO) BLOCK; # with proto
26 $subref = sub : ATTRS BLOCK; # with attributes
27 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
28
29To import subroutines:
30X<import>
31
32 use MODULE qw(NAME1 NAME2 NAME3);
33
34To call subroutines:
35X<subroutine, call> X<call>
36
37 NAME(LIST); # & is optional with parentheses.
38 NAME LIST; # Parentheses optional if predeclared/imported.
39 &NAME(LIST); # Circumvent prototypes.
40 &NAME; # Makes current @_ visible to called subroutine.
41
42=head1 DESCRIPTION
43
44Like many languages, Perl provides for user-defined subroutines.
45These may be located anywhere in the main program, loaded in from
46other files via the C<do>, C<require>, or C<use> keywords, or
47generated on the fly using C<eval> or anonymous subroutines.
48You can even call a function indirectly using a variable containing
49its name or a CODE reference.
50
51The Perl model for function call and return values is simple: all
52functions are passed as parameters one single flat list of scalars, and
53all functions likewise return to their caller one single flat list of
54scalars. Any arrays or hashes in these call and return lists will
55collapse, losing their identities--but you may always use
56pass-by-reference instead to avoid this. Both call and return lists may
57contain as many or as few scalar elements as you'd like. (Often a
58function without an explicit return statement is called a subroutine, but
59there's really no difference from Perl's perspective.)
60X<subroutine, parameter> X<parameter>
61
62Any arguments passed in show up in the array C<@_>. Therefore, if
63you called a function with two arguments, those would be stored in
64C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
65elements are aliases for the actual scalar parameters. In particular,
66if an element C<$_[0]> is updated, the corresponding argument is
67updated (or an error occurs if it is not updatable). If an argument
68is an array or hash element which did not exist when the function
69was called, that element is created only when (and if) it is modified
70or a reference to it is taken. (Some earlier versions of Perl
71created the element whether or not the element was assigned to.)
72Assigning to the whole array C<@_> removes that aliasing, and does
73not update any arguments.
74X<subroutine, argument> X<argument> X<@_>
75
76A C<return> statement may be used to exit a subroutine, optionally
77specifying the returned value, which will be evaluated in the
78appropriate context (list, scalar, or void) depending on the context of
79the subroutine call. If you specify no return value, the subroutine
80returns an empty list in list context, the undefined value in scalar
81context, or nothing in void context. If you return one or more
82aggregates (arrays and hashes), these will be flattened together into
83one large indistinguishable list.
84
85If no C<return> is found and if the last statement is an expression, its
86value is returned. If the last statement is a loop control structure
87like a C<foreach> or a C<while>, the returned value is unspecified. The
88empty sub returns the empty list.
89X<subroutine, return value> X<return value> X<return>
90
91Perl does not have named formal parameters. In practice all you
92do is assign to a C<my()> list of these. Variables that aren't
93declared to be private are global variables. For gory details
94on creating private variables, see L<"Private Variables via my()">
95and L<"Temporary Values via local()">. To create protected
96environments for a set of functions in a separate package (and
97probably a separate file), see L<perlmod/"Packages">.
98X<formal parameter> X<parameter, formal>
99
100Example:
101
102 sub max {
103 my $max = shift(@_);
104 foreach $foo (@_) {
105 $max = $foo if $max < $foo;
106 }
107 return $max;
108 }
109 $bestday = max($mon,$tue,$wed,$thu,$fri);
110
111Example:
112
113 # get a line, combining continuation lines
114 # that start with whitespace
115
116 sub get_line {
117 $thisline = $lookahead; # global variables!
118 LINE: while (defined($lookahead = <STDIN>)) {
119 if ($lookahead =~ /^[ \t]/) {
120 $thisline .= $lookahead;
121 }
122 else {
123 last LINE;
124 }
125 }
126 return $thisline;
127 }
128
129 $lookahead = <STDIN>; # get first line
130 while (defined($line = get_line())) {
131 ...
132 }
133
134Assigning to a list of private variables to name your arguments:
135
136 sub maybeset {
137 my($key, $value) = @_;
138 $Foo{$key} = $value unless $Foo{$key};
139 }
140
141Because the assignment copies the values, this also has the effect
142of turning call-by-reference into call-by-value. Otherwise a
143function is free to do in-place modifications of C<@_> and change
144its caller's values.
145X<call-by-reference> X<call-by-value>
146
147 upcase_in($v1, $v2); # this changes $v1 and $v2
148 sub upcase_in {
149 for (@_) { tr/a-z/A-Z/ }
150 }
151
152You aren't allowed to modify constants in this way, of course. If an
153argument were actually literal and you tried to change it, you'd take a
154(presumably fatal) exception. For example, this won't work:
155X<call-by-reference> X<call-by-value>
156
157 upcase_in("frederick");
158
159It would be much safer if the C<upcase_in()> function
160were written to return a copy of its parameters instead
161of changing them in place:
162
163 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
164 sub upcase {
165 return unless defined wantarray; # void context, do nothing
166 my @parms = @_;
167 for (@parms) { tr/a-z/A-Z/ }
168 return wantarray ? @parms : $parms[0];
169 }
170
171Notice how this (unprototyped) function doesn't care whether it was
172passed real scalars or arrays. Perl sees all arguments as one big,
173long, flat parameter list in C<@_>. This is one area where
174Perl's simple argument-passing style shines. The C<upcase()>
175function would work perfectly well without changing the C<upcase()>
176definition even if we fed it things like this:
177
178 @newlist = upcase(@list1, @list2);
179 @newlist = upcase( split /:/, $var );
180
181Do not, however, be tempted to do this:
182
183 (@a, @b) = upcase(@list1, @list2);
184
185Like the flattened incoming parameter list, the return list is also
186flattened on return. So all you have managed to do here is stored
187everything in C<@a> and made C<@b> empty. See
188L<Pass by Reference> for alternatives.
189
190A subroutine may be called using an explicit C<&> prefix. The
191C<&> is optional in modern Perl, as are parentheses if the
192subroutine has been predeclared. The C<&> is I<not> optional
193when just naming the subroutine, such as when it's used as
194an argument to defined() or undef(). Nor is it optional when you
195want to do an indirect subroutine call with a subroutine name or
196reference using the C<&$subref()> or C<&{$subref}()> constructs,
197although the C<< $subref->() >> notation solves that problem.
198See L<perlref> for more about all that.
199X<&>
200
201Subroutines may be called recursively. If a subroutine is called
202using the C<&> form, the argument list is optional, and if omitted,
203no C<@_> array is set up for the subroutine: the C<@_> array at the
204time of the call is visible to subroutine instead. This is an
205efficiency mechanism that new users may wish to avoid.
206X<recursion>
207
208 &foo(1,2,3); # pass three arguments
209 foo(1,2,3); # the same
210
211 foo(); # pass a null list
212 &foo(); # the same
213
214 &foo; # foo() get current args, like foo(@_) !!
215 foo; # like foo() IFF sub foo predeclared, else "foo"
216
217Not only does the C<&> form make the argument list optional, it also
218disables any prototype checking on arguments you do provide. This
219is partly for historical reasons, and partly for having a convenient way
220to cheat if you know what you're doing. See L<Prototypes> below.
221X<&>
222
223Subroutines whose names are in all upper case are reserved to the Perl
224core, as are modules whose names are in all lower case. A subroutine in
225all capitals is a loosely-held convention meaning it will be called
226indirectly by the run-time system itself, usually due to a triggered event.
227Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
228C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
229
230The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
231are not so much subroutines as named special code blocks, of which you
232can have more than one in a package, and which you can B<not> call
233explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
234
235=head2 Private Variables via my()
236X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
237X<lexical scope> X<attributes, my>
238
239Synopsis:
240
241 my $foo; # declare $foo lexically local
242 my (@wid, %get); # declare list of variables local
243 my $foo = "flurp"; # declare $foo lexical, and init it
244 my @oof = @bar; # declare @oof lexical, and init it
245 my $x : Foo = $y; # similar, with an attribute applied
246
247B<WARNING>: The use of attribute lists on C<my> declarations is still
248evolving. The current semantics and interface are subject to change.
249See L<attributes> and L<Attribute::Handlers>.
250
251The C<my> operator declares the listed variables to be lexically
252confined to the enclosing block, conditional (C<if/unless/elsif/else>),
253loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
254or C<do/require/use>'d file. If more than one value is listed, the
255list must be placed in parentheses. All listed elements must be
256legal lvalues. Only alphanumeric identifiers may be lexically
257scoped--magical built-ins like C<$/> must currently be C<local>ized
258with C<local> instead.
259
260Unlike dynamic variables created by the C<local> operator, lexical
261variables declared with C<my> are totally hidden from the outside
262world, including any called subroutines. This is true if it's the
263same subroutine called from itself or elsewhere--every call gets
264its own copy.
265X<local>
266
267This doesn't mean that a C<my> variable declared in a statically
268enclosing lexical scope would be invisible. Only dynamic scopes
269are cut off. For example, the C<bumpx()> function below has access
270to the lexical $x variable because both the C<my> and the C<sub>
271occurred at the same scope, presumably file scope.
272
273 my $x = 10;
274 sub bumpx { $x++ }
275
276An C<eval()>, however, can see lexical variables of the scope it is
277being evaluated in, so long as the names aren't hidden by declarations within
278the C<eval()> itself. See L<perlref>.
279X<eval, scope of>
280
281The parameter list to my() may be assigned to if desired, which allows you
282to initialize your variables. (If no initializer is given for a
283particular variable, it is created with the undefined value.) Commonly
284this is used to name input parameters to a subroutine. Examples:
285
286 $arg = "fred"; # "global" variable
287 $n = cube_root(27);
288 print "$arg thinks the root is $n\n";
289 fred thinks the root is 3
290
291 sub cube_root {
292 my $arg = shift; # name doesn't matter
293 $arg **= 1/3;
294 return $arg;
295 }
296
297The C<my> is simply a modifier on something you might assign to. So when
298you do assign to variables in its argument list, C<my> doesn't
299change whether those variables are viewed as a scalar or an array. So
300
301 my ($foo) = <STDIN>; # WRONG?
302 my @FOO = <STDIN>;
303
304both supply a list context to the right-hand side, while
305
306 my $foo = <STDIN>;
307
308supplies a scalar context. But the following declares only one variable:
309
310 my $foo, $bar = 1; # WRONG
311
312That has the same effect as
313
314 my $foo;
315 $bar = 1;
316
317The declared variable is not introduced (is not visible) until after
318the current statement. Thus,
319
320 my $x = $x;
321
322can be used to initialize a new $x with the value of the old $x, and
323the expression
324
325 my $x = 123 and $x == 123
326
327is false unless the old $x happened to have the value C<123>.
328
329Lexical scopes of control structures are not bounded precisely by the
330braces that delimit their controlled blocks; control expressions are
331part of that scope, too. Thus in the loop
332
333 while (my $line = <>) {
334 $line = lc $line;
335 } continue {
336 print $line;
337 }
338
339the scope of $line extends from its declaration throughout the rest of
340the loop construct (including the C<continue> clause), but not beyond
341it. Similarly, in the conditional
342
343 if ((my $answer = <STDIN>) =~ /^yes$/i) {
344 user_agrees();
345 } elsif ($answer =~ /^no$/i) {
346 user_disagrees();
347 } else {
348 chomp $answer;
349 die "'$answer' is neither 'yes' nor 'no'";
350 }
351
352the scope of $answer extends from its declaration through the rest
353of that conditional, including any C<elsif> and C<else> clauses,
354but not beyond it. See L<perlsyn/"Simple statements"> for information
355on the scope of variables in statements with modifiers.
356
357The C<foreach> loop defaults to scoping its index variable dynamically
358in the manner of C<local>. However, if the index variable is
359prefixed with the keyword C<my>, or if there is already a lexical
360by that name in scope, then a new lexical is created instead. Thus
361in the loop
362X<foreach> X<for>
363
364 for my $i (1, 2, 3) {
365 some_function();
366 }
367
368the scope of $i extends to the end of the loop, but not beyond it,
369rendering the value of $i inaccessible within C<some_function()>.
370X<foreach> X<for>
371
372Some users may wish to encourage the use of lexically scoped variables.
373As an aid to catching implicit uses to package variables,
374which are always global, if you say
375
376 use strict 'vars';
377
378then any variable mentioned from there to the end of the enclosing
379block must either refer to a lexical variable, be predeclared via
380C<our> or C<use vars>, or else must be fully qualified with the package name.
381A compilation error results otherwise. An inner block may countermand
382this with C<no strict 'vars'>.
383
384A C<my> has both a compile-time and a run-time effect. At compile
385time, the compiler takes notice of it. The principal usefulness
386of this is to quiet C<use strict 'vars'>, but it is also essential
387for generation of closures as detailed in L<perlref>. Actual
388initialization is delayed until run time, though, so it gets executed
389at the appropriate time, such as each time through a loop, for
390example.
391
392Variables declared with C<my> are not part of any package and are therefore
393never fully qualified with the package name. In particular, you're not
394allowed to try to make a package variable (or other global) lexical:
395
396 my $pack::var; # ERROR! Illegal syntax
397
398In fact, a dynamic variable (also known as package or global variables)
399are still accessible using the fully qualified C<::> notation even while a
400lexical of the same name is also visible:
401
402 package main;
403 local $x = 10;
404 my $x = 20;
405 print "$x and $::x\n";
406
407That will print out C<20> and C<10>.
408
409You may declare C<my> variables at the outermost scope of a file
410to hide any such identifiers from the world outside that file. This
411is similar in spirit to C's static variables when they are used at
412the file level. To do this with a subroutine requires the use of
413a closure (an anonymous function that accesses enclosing lexicals).
414If you want to create a private subroutine that cannot be called
415from outside that block, it can declare a lexical variable containing
416an anonymous sub reference:
417
418 my $secret_version = '1.001-beta';
419 my $secret_sub = sub { print $secret_version };
420 &$secret_sub();
421
422As long as the reference is never returned by any function within the
423module, no outside module can see the subroutine, because its name is not in
424any package's symbol table. Remember that it's not I<REALLY> called
425C<$some_pack::secret_version> or anything; it's just $secret_version,
426unqualified and unqualifiable.
427
428This does not work with object methods, however; all object methods
429have to be in the symbol table of some package to be found. See
430L<perlref/"Function Templates"> for something of a work-around to
431this.
432
433=head2 Persistent Private Variables
434X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
435
436There are two ways to build persistent private variables in Perl 5.10.
437First, you can simply use the C<state> feature. Or, you can use closures,
438if you want to stay compatible with releases older than 5.10.
439
440=head3 Persistent variables via state()
441
442Beginning with perl 5.9.4, you can declare variables with the C<state>
443keyword in place of C<my>. For that to work, though, you must have
444enabled that feature beforehand, either by using the C<feature> pragma, or
445by using C<-E> on one-liners. (see L<feature>)
446
447For example, the following code maintains a private counter, incremented
448each time the gimme_another() function is called:
449
450 use feature 'state';
451 sub gimme_another { state $x; return ++$x }
452
453Also, since C<$x> is lexical, it can't be reached or modified by any Perl
454code outside.
455
456When combined with variable declaration, simple scalar assignment to C<state>
457variables (as in C<state $x = 42>) is executed only the first time. When such
458statements are evaluated subsequent times, the assignment is ignored. The
459behavior of this sort of assignment to non-scalar variables is undefined.
460
461=head3 Persistent variables with closures
462
463Just because a lexical variable is lexically (also called statically)
464scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
465within a function it works like a C static. It normally works more
466like a C auto, but with implicit garbage collection.
467
468Unlike local variables in C or C++, Perl's lexical variables don't
469necessarily get recycled just because their scope has exited.
470If something more permanent is still aware of the lexical, it will
471stick around. So long as something else references a lexical, that
472lexical won't be freed--which is as it should be. You wouldn't want
473memory being free until you were done using it, or kept around once you
474were done. Automatic garbage collection takes care of this for you.
475
476This means that you can pass back or save away references to lexical
477variables, whereas to return a pointer to a C auto is a grave error.
478It also gives us a way to simulate C's function statics. Here's a
479mechanism for giving a function private variables with both lexical
480scoping and a static lifetime. If you do want to create something like
481C's static variables, just enclose the whole function in an extra block,
482and put the static variable outside the function but in the block.
483
484 {
485 my $secret_val = 0;
486 sub gimme_another {
487 return ++$secret_val;
488 }
489 }
490 # $secret_val now becomes unreachable by the outside
491 # world, but retains its value between calls to gimme_another
492
493If this function is being sourced in from a separate file
494via C<require> or C<use>, then this is probably just fine. If it's
495all in the main program, you'll need to arrange for the C<my>
496to be executed early, either by putting the whole block above
497your main program, or more likely, placing merely a C<BEGIN>
498code block around it to make sure it gets executed before your program
499starts to run:
500
501 BEGIN {
502 my $secret_val = 0;
503 sub gimme_another {
504 return ++$secret_val;
505 }
506 }
507
508See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
509special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
510C<INIT> and C<END>.
511
512If declared at the outermost scope (the file scope), then lexicals
513work somewhat like C's file statics. They are available to all
514functions in that same file declared below them, but are inaccessible
515from outside that file. This strategy is sometimes used in modules
516to create private variables that the whole module can see.
517
518=head2 Temporary Values via local()
519X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
520X<variable, temporary>
521
522B<WARNING>: In general, you should be using C<my> instead of C<local>, because
523it's faster and safer. Exceptions to this include the global punctuation
524variables, global filehandles and formats, and direct manipulation of the
525Perl symbol table itself. C<local> is mostly used when the current value
526of a variable must be visible to called subroutines.
527
528Synopsis:
529
530 # localization of values
531
532 local $foo; # make $foo dynamically local
533 local (@wid, %get); # make list of variables local
534 local $foo = "flurp"; # make $foo dynamic, and init it
535 local @oof = @bar; # make @oof dynamic, and init it
536
537 local $hash{key} = "val"; # sets a local value for this hash entry
538 local ($cond ? $v1 : $v2); # several types of lvalues support
539 # localization
540
541 # localization of symbols
542
543 local *FH; # localize $FH, @FH, %FH, &FH ...
544 local *merlyn = *randal; # now $merlyn is really $randal, plus
545 # @merlyn is really @randal, etc
546 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
547 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
548
549A C<local> modifies its listed variables to be "local" to the
550enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
551called from within that block>. A C<local> just gives temporary
552values to global (meaning package) variables. It does I<not> create
553a local variable. This is known as dynamic scoping. Lexical scoping
554is done with C<my>, which works more like C's auto declarations.
555
556Some types of lvalues can be localized as well : hash and array elements
557and slices, conditionals (provided that their result is always
558localizable), and symbolic references. As for simple variables, this
559creates new, dynamically scoped values.
560
561If more than one variable or expression is given to C<local>, they must be
562placed in parentheses. This operator works
563by saving the current values of those variables in its argument list on a
564hidden stack and restoring them upon exiting the block, subroutine, or
565eval. This means that called subroutines can also reference the local
566variable, but not the global one. The argument list may be assigned to if
567desired, which allows you to initialize your local variables. (If no
568initializer is given for a particular variable, it is created with an
569undefined value.)
570
571Because C<local> is a run-time operator, it gets executed each time
572through a loop. Consequently, it's more efficient to localize your
573variables outside the loop.
574
575=head3 Grammatical note on local()
576X<local, context>
577
578A C<local> is simply a modifier on an lvalue expression. When you assign to
579a C<local>ized variable, the C<local> doesn't change whether its list is viewed
580as a scalar or an array. So
581
582 local($foo) = <STDIN>;
583 local @FOO = <STDIN>;
584
585both supply a list context to the right-hand side, while
586
587 local $foo = <STDIN>;
588
589supplies a scalar context.
590
591=head3 Localization of special variables
592X<local, special variable>
593
594If you localize a special variable, you'll be giving a new value to it,
595but its magic won't go away. That means that all side-effects related
596to this magic still work with the localized value.
597
598This feature allows code like this to work :
599
600 # Read the whole contents of FILE in $slurp
601 { local $/ = undef; $slurp = <FILE>; }
602
603Note, however, that this restricts localization of some values ; for
604example, the following statement dies, as of perl 5.9.0, with an error
605I<Modification of a read-only value attempted>, because the $1 variable is
606magical and read-only :
607
608 local $1 = 2;
609
610Similarly, but in a way more difficult to spot, the following snippet will
611die in perl 5.9.0 :
612
613 sub f { local $_ = "foo"; print }
614 for ($1) {
615 # now $_ is aliased to $1, thus is magic and readonly
616 f();
617 }
618
619See next section for an alternative to this situation.
620
621B<WARNING>: Localization of tied arrays and hashes does not currently
622work as described.
623This will be fixed in a future release of Perl; in the meantime, avoid
624code that relies on any particular behaviour of localising tied arrays
625or hashes (localising individual elements is still okay).
626See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
627details.
628X<local, tie>
629
630=head3 Localization of globs
631X<local, glob> X<glob>
632
633The construct
634
635 local *name;
636
637creates a whole new symbol table entry for the glob C<name> in the
638current package. That means that all variables in its glob slot ($name,
639@name, %name, &name, and the C<name> filehandle) are dynamically reset.
640
641This implies, among other things, that any magic eventually carried by
642those variables is locally lost. In other words, saying C<local */>
643will not have any effect on the internal value of the input record
644separator.
645
646Notably, if you want to work with a brand new value of the default scalar
647$_, and avoid the potential problem listed above about $_ previously
648carrying a magic value, you should use C<local *_> instead of C<local $_>.
649As of perl 5.9.1, you can also use the lexical form of C<$_> (declaring it
650with C<my $_>), which avoids completely this problem.
651
652=head3 Localization of elements of composite types
653X<local, composite type element> X<local, array element> X<local, hash element>
654
655It's also worth taking a moment to explain what happens when you
656C<local>ize a member of a composite type (i.e. an array or hash element).
657In this case, the element is C<local>ized I<by name>. This means that
658when the scope of the C<local()> ends, the saved value will be
659restored to the hash element whose key was named in the C<local()>, or
660the array element whose index was named in the C<local()>. If that
661element was deleted while the C<local()> was in effect (e.g. by a
662C<delete()> from a hash or a C<shift()> of an array), it will spring
663back into existence, possibly extending an array and filling in the
664skipped elements with C<undef>. For instance, if you say
665
666 %hash = ( 'This' => 'is', 'a' => 'test' );
667 @ary = ( 0..5 );
668 {
669 local($ary[5]) = 6;
670 local($hash{'a'}) = 'drill';
671 while (my $e = pop(@ary)) {
672 print "$e . . .\n";
673 last unless $e > 3;
674 }
675 if (@ary) {
676 $hash{'only a'} = 'test';
677 delete $hash{'a'};
678 }
679 }
680 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
681 print "The array has ",scalar(@ary)," elements: ",
682 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
683
684Perl will print
685
686 6 . . .
687 4 . . .
688 3 . . .
689 This is a test only a test.
690 The array has 6 elements: 0, 1, 2, undef, undef, 5
691
692The behavior of local() on non-existent members of composite
693types is subject to change in future.
694
695=head2 Lvalue subroutines
696X<lvalue> X<subroutine, lvalue>
697
698B<WARNING>: Lvalue subroutines are still experimental and the
699implementation may change in future versions of Perl.
700
701It is possible to return a modifiable value from a subroutine.
702To do this, you have to declare the subroutine to return an lvalue.
703
704 my $val;
705 sub canmod : lvalue {
706 # return $val; this doesn't work, don't say "return"
707 $val;
708 }
709 sub nomod {
710 $val;
711 }
712
713 canmod() = 5; # assigns to $val
714 nomod() = 5; # ERROR
715
716The scalar/list context for the subroutine and for the right-hand
717side of assignment is determined as if the subroutine call is replaced
718by a scalar. For example, consider:
719
720 data(2,3) = get_data(3,4);
721
722Both subroutines here are called in a scalar context, while in:
723
724 (data(2,3)) = get_data(3,4);
725
726and in:
727
728 (data(2),data(3)) = get_data(3,4);
729
730all the subroutines are called in a list context.
731
732=over 4
733
734=item Lvalue subroutines are EXPERIMENTAL
735
736They appear to be convenient, but there are several reasons to be
737circumspect.
738
739You can't use the return keyword, you must pass out the value before
740falling out of subroutine scope. (see comment in example above). This
741is usually not a problem, but it disallows an explicit return out of a
742deeply nested loop, which is sometimes a nice way out.
743
744They violate encapsulation. A normal mutator can check the supplied
745argument before setting the attribute it is protecting, an lvalue
746subroutine never gets that chance. Consider;
747
748 my $some_array_ref = []; # protected by mutators ??
749
750 sub set_arr { # normal mutator
751 my $val = shift;
752 die("expected array, you supplied ", ref $val)
753 unless ref $val eq 'ARRAY';
754 $some_array_ref = $val;
755 }
756 sub set_arr_lv : lvalue { # lvalue mutator
757 $some_array_ref;
758 }
759
760 # set_arr_lv cannot stop this !
761 set_arr_lv() = { a => 1 };
762
763=back
764
765=head2 Passing Symbol Table Entries (typeglobs)
766X<typeglob> X<*>
767
768B<WARNING>: The mechanism described in this section was originally
769the only way to simulate pass-by-reference in older versions of
770Perl. While it still works fine in modern versions, the new reference
771mechanism is generally easier to work with. See below.
772
773Sometimes you don't want to pass the value of an array to a subroutine
774but rather the name of it, so that the subroutine can modify the global
775copy of it rather than working with a local copy. In perl you can
776refer to all objects of a particular name by prefixing the name
777with a star: C<*foo>. This is often known as a "typeglob", because the
778star on the front can be thought of as a wildcard match for all the
779funny prefix characters on variables and subroutines and such.
780
781When evaluated, the typeglob produces a scalar value that represents
782all the objects of that name, including any filehandle, format, or
783subroutine. When assigned to, it causes the name mentioned to refer to
784whatever C<*> value was assigned to it. Example:
785
786 sub doubleary {
787 local(*someary) = @_;
788 foreach $elem (@someary) {
789 $elem *= 2;
790 }
791 }
792 doubleary(*foo);
793 doubleary(*bar);
794
795Scalars are already passed by reference, so you can modify
796scalar arguments without using this mechanism by referring explicitly
797to C<$_[0]> etc. You can modify all the elements of an array by passing
798all the elements as scalars, but you have to use the C<*> mechanism (or
799the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
800an array. It will certainly be faster to pass the typeglob (or reference).
801
802Even if you don't want to modify an array, this mechanism is useful for
803passing multiple arrays in a single LIST, because normally the LIST
804mechanism will merge all the array values so that you can't extract out
805the individual arrays. For more on typeglobs, see
806L<perldata/"Typeglobs and Filehandles">.
807
808=head2 When to Still Use local()
809X<local> X<variable, local>
810
811Despite the existence of C<my>, there are still three places where the
812C<local> operator still shines. In fact, in these three places, you
813I<must> use C<local> instead of C<my>.
814
815=over 4
816
817=item 1.
818
819You need to give a global variable a temporary value, especially $_.
820
821The global variables, like C<@ARGV> or the punctuation variables, must be
822C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
823it up into chunks separated by lines of equal signs, which are placed
824in C<@Fields>.
825
826 {
827 local @ARGV = ("/etc/motd");
828 local $/ = undef;
829 local $_ = <>;
830 @Fields = split /^\s*=+\s*$/;
831 }
832
833It particular, it's important to C<local>ize $_ in any routine that assigns
834to it. Look out for implicit assignments in C<while> conditionals.
835
836=item 2.
837
838You need to create a local file or directory handle or a local function.
839
840A function that needs a filehandle of its own must use
841C<local()> on a complete typeglob. This can be used to create new symbol
842table entries:
843
844 sub ioqueue {
845 local (*READER, *WRITER); # not my!
846 pipe (READER, WRITER) or die "pipe: $!";
847 return (*READER, *WRITER);
848 }
849 ($head, $tail) = ioqueue();
850
851See the Symbol module for a way to create anonymous symbol table
852entries.
853
854Because assignment of a reference to a typeglob creates an alias, this
855can be used to create what is effectively a local function, or at least,
856a local alias.
857
858 {
859 local *grow = \&shrink; # only until this block exists
860 grow(); # really calls shrink()
861 move(); # if move() grow()s, it shrink()s too
862 }
863 grow(); # get the real grow() again
864
865See L<perlref/"Function Templates"> for more about manipulating
866functions by name in this way.
867
868=item 3.
869
870You want to temporarily change just one element of an array or hash.
871
872You can C<local>ize just one element of an aggregate. Usually this
873is done on dynamics:
874
875 {
876 local $SIG{INT} = 'IGNORE';
877 funct(); # uninterruptible
878 }
879 # interruptibility automatically restored here
880
881But it also works on lexically declared aggregates. Prior to 5.005,
882this operation could on occasion misbehave.
883
884=back
885
886=head2 Pass by Reference
887X<pass by reference> X<pass-by-reference> X<reference>
888
889If you want to pass more than one array or hash into a function--or
890return them from it--and have them maintain their integrity, then
891you're going to have to use an explicit pass-by-reference. Before you
892do that, you need to understand references as detailed in L<perlref>.
893This section may not make much sense to you otherwise.
894
895Here are a few simple examples. First, let's pass in several arrays
896to a function and have it C<pop> all of then, returning a new list
897of all their former last elements:
898
899 @tailings = popmany ( \@a, \@b, \@c, \@d );
900
901 sub popmany {
902 my $aref;
903 my @retlist = ();
904 foreach $aref ( @_ ) {
905 push @retlist, pop @$aref;
906 }
907 return @retlist;
908 }
909
910Here's how you might write a function that returns a
911list of keys occurring in all the hashes passed to it:
912
913 @common = inter( \%foo, \%bar, \%joe );
914 sub inter {
915 my ($k, $href, %seen); # locals
916 foreach $href (@_) {
917 while ( $k = each %$href ) {
918 $seen{$k}++;
919 }
920 }
921 return grep { $seen{$_} == @_ } keys %seen;
922 }
923
924So far, we're using just the normal list return mechanism.
925What happens if you want to pass or return a hash? Well,
926if you're using only one of them, or you don't mind them
927concatenating, then the normal calling convention is ok, although
928a little expensive.
929
930Where people get into trouble is here:
931
932 (@a, @b) = func(@c, @d);
933or
934 (%a, %b) = func(%c, %d);
935
936That syntax simply won't work. It sets just C<@a> or C<%a> and
937clears the C<@b> or C<%b>. Plus the function didn't get passed
938into two separate arrays or hashes: it got one long list in C<@_>,
939as always.
940
941If you can arrange for everyone to deal with this through references, it's
942cleaner code, although not so nice to look at. Here's a function that
943takes two array references as arguments, returning the two array elements
944in order of how many elements they have in them:
945
946 ($aref, $bref) = func(\@c, \@d);
947 print "@$aref has more than @$bref\n";
948 sub func {
949 my ($cref, $dref) = @_;
950 if (@$cref > @$dref) {
951 return ($cref, $dref);
952 } else {
953 return ($dref, $cref);
954 }
955 }
956
957It turns out that you can actually do this also:
958
959 (*a, *b) = func(\@c, \@d);
960 print "@a has more than @b\n";
961 sub func {
962 local (*c, *d) = @_;
963 if (@c > @d) {
964 return (\@c, \@d);
965 } else {
966 return (\@d, \@c);
967 }
968 }
969
970Here we're using the typeglobs to do symbol table aliasing. It's
971a tad subtle, though, and also won't work if you're using C<my>
972variables, because only globals (even in disguise as C<local>s)
973are in the symbol table.
974
975If you're passing around filehandles, you could usually just use the bare
976typeglob, like C<*STDOUT>, but typeglobs references work, too.
977For example:
978
979 splutter(\*STDOUT);
980 sub splutter {
981 my $fh = shift;
982 print $fh "her um well a hmmm\n";
983 }
984
985 $rec = get_rec(\*STDIN);
986 sub get_rec {
987 my $fh = shift;
988 return scalar <$fh>;
989 }
990
991If you're planning on generating new filehandles, you could do this.
992Notice to pass back just the bare *FH, not its reference.
993
994 sub openit {
995 my $path = shift;
996 local *FH;
997 return open (FH, $path) ? *FH : undef;
998 }
999
1000=head2 Prototypes
1001X<prototype> X<subroutine, prototype>
1002
1003Perl supports a very limited kind of compile-time argument checking
1004using function prototyping. If you declare
1005
1006 sub mypush (\@@)
1007
1008then C<mypush()> takes arguments exactly like C<push()> does. The
1009function declaration must be visible at compile time. The prototype
1010affects only interpretation of new-style calls to the function,
1011where new-style is defined as not using the C<&> character. In
1012other words, if you call it like a built-in function, then it behaves
1013like a built-in function. If you call it like an old-fashioned
1014subroutine, then it behaves like an old-fashioned subroutine. It
1015naturally falls out from this rule that prototypes have no influence
1016on subroutine references like C<\&foo> or on indirect subroutine
1017calls like C<&{$subref}> or C<< $subref->() >>.
1018
1019Method calls are not influenced by prototypes either, because the
1020function to be called is indeterminate at compile time, since
1021the exact code called depends on inheritance.
1022
1023Because the intent of this feature is primarily to let you define
1024subroutines that work like built-in functions, here are prototypes
1025for some other functions that parse almost exactly like the
1026corresponding built-in.
1027
1028 Declared as Called as
1029
1030 sub mylink ($$) mylink $old, $new
1031 sub myvec ($$$) myvec $var, $offset, 1
1032 sub myindex ($$;$) myindex &getstring, "substr"
1033 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1034 sub myreverse (@) myreverse $a, $b, $c
1035 sub myjoin ($@) myjoin ":", $a, $b, $c
1036 sub mypop (\@) mypop @array
1037 sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
1038 sub mykeys (\%) mykeys %{$hashref}
1039 sub myopen (*;$) myopen HANDLE, $name
1040 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1041 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1042 sub myrand (;$) myrand 42
1043 sub mytime () mytime
1044
1045Any backslashed prototype character represents an actual argument
1046that absolutely must start with that character. The value passed
1047as part of C<@_> will be a reference to the actual argument given
1048in the subroutine call, obtained by applying C<\> to that argument.
1049
1050You can also backslash several argument types simultaneously by using
1051the C<\[]> notation:
1052
1053 sub myref (\[$@%&*])
1054
1055will allow calling myref() as
1056
1057 myref $var
1058 myref @array
1059 myref %hash
1060 myref &sub
1061 myref *glob
1062
1063and the first argument of myref() will be a reference to
1064a scalar, an array, a hash, a code, or a glob.
1065
1066Unbackslashed prototype characters have special meanings. Any
1067unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1068list context. An argument represented by C<$> forces scalar context. An
1069C<&> requires an anonymous subroutine, which, if passed as the first
1070argument, does not require the C<sub> keyword or a subsequent comma.
1071
1072A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1073typeglob, or a reference to a typeglob in that slot. The value will be
1074available to the subroutine either as a simple scalar, or (in the latter
1075two cases) as a reference to the typeglob. If you wish to always convert
1076such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1077follows:
1078
1079 use Symbol 'qualify_to_ref';
1080
1081 sub foo (*) {
1082 my $fh = qualify_to_ref(shift, caller);
1083 ...
1084 }
1085
1086A semicolon (C<;>) separates mandatory arguments from optional arguments.
1087It is redundant before C<@> or C<%>, which gobble up everything else.
1088
1089As the last character of a prototype, or just before a semicolon, you can
1090use C<_> in place of C<$>: if this argument is not provided, C<$_> will be
1091used instead.
1092
1093Note how the last three examples in the table above are treated
1094specially by the parser. C<mygrep()> is parsed as a true list
1095operator, C<myrand()> is parsed as a true unary operator with unary
1096precedence the same as C<rand()>, and C<mytime()> is truly without
1097arguments, just like C<time()>. That is, if you say
1098
1099 mytime +2;
1100
1101you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1102without a prototype.
1103
1104The interesting thing about C<&> is that you can generate new syntax with it,
1105provided it's in the initial position:
1106X<&>
1107
1108 sub try (&@) {
1109 my($try,$catch) = @_;
1110 eval { &$try };
1111 if ($@) {
1112 local $_ = $@;
1113 &$catch;
1114 }
1115 }
1116 sub catch (&) { $_[0] }
1117
1118 try {
1119 die "phooey";
1120 } catch {
1121 /phooey/ and print "unphooey\n";
1122 };
1123
1124That prints C<"unphooey">. (Yes, there are still unresolved
1125issues having to do with visibility of C<@_>. I'm ignoring that
1126question for the moment. (But note that if we make C<@_> lexically
1127scoped, those anonymous subroutines can act like closures... (Gee,
1128is this sounding a little Lispish? (Never mind.))))
1129
1130And here's a reimplementation of the Perl C<grep> operator:
1131X<grep>
1132
1133 sub mygrep (&@) {
1134 my $code = shift;
1135 my @result;
1136 foreach $_ (@_) {
1137 push(@result, $_) if &$code;
1138 }
1139 @result;
1140 }
1141
1142Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1143been intentionally left out of prototypes for the express purpose of
1144someday in the future adding named, formal parameters. The current
1145mechanism's main goal is to let module writers provide better diagnostics
1146for module users. Larry feels the notation quite understandable to Perl
1147programmers, and that it will not intrude greatly upon the meat of the
1148module, nor make it harder to read. The line noise is visually
1149encapsulated into a small pill that's easy to swallow.
1150
1151If you try to use an alphanumeric sequence in a prototype you will
1152generate an optional warning - "Illegal character in prototype...".
1153Unfortunately earlier versions of Perl allowed the prototype to be
1154used as long as its prefix was a valid prototype. The warning may be
1155upgraded to a fatal error in a future version of Perl once the
1156majority of offending code is fixed.
1157
1158It's probably best to prototype new functions, not retrofit prototyping
1159into older ones. That's because you must be especially careful about
1160silent impositions of differing list versus scalar contexts. For example,
1161if you decide that a function should take just one parameter, like this:
1162
1163 sub func ($) {
1164 my $n = shift;
1165 print "you gave me $n\n";
1166 }
1167
1168and someone has been calling it with an array or expression
1169returning a list:
1170
1171 func(@foo);
1172 func( split /:/ );
1173
1174Then you've just supplied an automatic C<scalar> in front of their
1175argument, which can be more than a bit surprising. The old C<@foo>
1176which used to hold one thing doesn't get passed in. Instead,
1177C<func()> now gets passed in a C<1>; that is, the number of elements
1178in C<@foo>. And the C<split> gets called in scalar context so it
1179starts scribbling on your C<@_> parameter list. Ouch!
1180
1181This is all very powerful, of course, and should be used only in moderation
1182to make the world a better place.
1183
1184=head2 Constant Functions
1185X<constant>
1186
1187Functions with a prototype of C<()> are potential candidates for
1188inlining. If the result after optimization and constant folding
1189is either a constant or a lexically-scoped scalar which has no other
1190references, then it will be used in place of function calls made
1191without C<&>. Calls made using C<&> are never inlined. (See
1192F<constant.pm> for an easy way to declare most constants.)
1193
1194The following functions would all be inlined:
1195
1196 sub pi () { 3.14159 } # Not exact, but close.
1197 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1198 # and it's inlined, too!
1199 sub ST_DEV () { 0 }
1200 sub ST_INO () { 1 }
1201
1202 sub FLAG_FOO () { 1 << 8 }
1203 sub FLAG_BAR () { 1 << 9 }
1204 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1205
1206 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1207
1208 sub N () { int(OPT_BAZ) / 3 }
1209
1210 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1211
1212Be aware that these will not be inlined; as they contain inner scopes,
1213the constant folding doesn't reduce them to a single constant:
1214
1215 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1216
1217 sub baz_val () {
1218 if (OPT_BAZ) {
1219 return 23;
1220 }
1221 else {
1222 return 42;
1223 }
1224 }
1225
1226If you redefine a subroutine that was eligible for inlining, you'll get
1227a mandatory warning. (You can use this warning to tell whether or not a
1228particular subroutine is considered constant.) The warning is
1229considered severe enough not to be optional because previously compiled
1230invocations of the function will still be using the old value of the
1231function. If you need to be able to redefine the subroutine, you need to
1232ensure that it isn't inlined, either by dropping the C<()> prototype
1233(which changes calling semantics, so beware) or by thwarting the
1234inlining mechanism in some other way, such as
1235
1236 sub not_inlined () {
1237 23 if $];
1238 }
1239
1240=head2 Overriding Built-in Functions
1241X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1242
1243Many built-in functions may be overridden, though this should be tried
1244only occasionally and for good reason. Typically this might be
1245done by a package attempting to emulate missing built-in functionality
1246on a non-Unix system.
1247
1248Overriding may be done only by importing the name from a module at
1249compile time--ordinary predeclaration isn't good enough. However, the
1250C<use subs> pragma lets you, in effect, predeclare subs
1251via the import syntax, and these names may then override built-in ones:
1252
1253 use subs 'chdir', 'chroot', 'chmod', 'chown';
1254 chdir $somewhere;
1255 sub chdir { ... }
1256
1257To unambiguously refer to the built-in form, precede the
1258built-in name with the special package qualifier C<CORE::>. For example,
1259saying C<CORE::open()> always refers to the built-in C<open()>, even
1260if the current package has imported some other subroutine called
1261C<&open()> from elsewhere. Even though it looks like a regular
1262function call, it isn't: you can't take a reference to it, such as
1263the incorrect C<\&CORE::open> might appear to produce.
1264
1265Library modules should not in general export built-in names like C<open>
1266or C<chdir> as part of their default C<@EXPORT> list, because these may
1267sneak into someone else's namespace and change the semantics unexpectedly.
1268Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1269possible for a user to import the name explicitly, but not implicitly.
1270That is, they could say
1271
1272 use Module 'open';
1273
1274and it would import the C<open> override. But if they said
1275
1276 use Module;
1277
1278they would get the default imports without overrides.
1279
1280The foregoing mechanism for overriding built-in is restricted, quite
1281deliberately, to the package that requests the import. There is a second
1282method that is sometimes applicable when you wish to override a built-in
1283everywhere, without regard to namespace boundaries. This is achieved by
1284importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1285example that quite brazenly replaces the C<glob> operator with something
1286that understands regular expressions.
1287
1288 package REGlob;
1289 require Exporter;
1290 @ISA = 'Exporter';
1291 @EXPORT_OK = 'glob';
1292
1293 sub import {
1294 my $pkg = shift;
1295 return unless @_;
1296 my $sym = shift;
1297 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1298 $pkg->export($where, $sym, @_);
1299 }
1300
1301 sub glob {
1302 my $pat = shift;
1303 my @got;
1304 if (opendir my $d, '.') {
1305 @got = grep /$pat/, readdir $d;
1306 closedir $d;
1307 }
1308 return @got;
1309 }
1310 1;
1311
1312And here's how it could be (ab)used:
1313
1314 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1315 package Foo;
1316 use REGlob 'glob'; # override glob() in Foo:: only
1317 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1318
1319The initial comment shows a contrived, even dangerous example.
1320By overriding C<glob> globally, you would be forcing the new (and
1321subversive) behavior for the C<glob> operator for I<every> namespace,
1322without the complete cognizance or cooperation of the modules that own
1323those namespaces. Naturally, this should be done with extreme caution--if
1324it must be done at all.
1325
1326The C<REGlob> example above does not implement all the support needed to
1327cleanly override perl's C<glob> operator. The built-in C<glob> has
1328different behaviors depending on whether it appears in a scalar or list
1329context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1330context sensitive behaviors, and these must be adequately supported by
1331a properly written override. For a fully functional example of overriding
1332C<glob>, study the implementation of C<File::DosGlob> in the standard
1333library.
1334
1335When you override a built-in, your replacement should be consistent (if
1336possible) with the built-in native syntax. You can achieve this by using
1337a suitable prototype. To get the prototype of an overridable built-in,
1338use the C<prototype> function with an argument of C<"CORE::builtin_name">
1339(see L<perlfunc/prototype>).
1340
1341Note however that some built-ins can't have their syntax expressed by a
1342prototype (such as C<system> or C<chomp>). If you override them you won't
1343be able to fully mimic their original syntax.
1344
1345The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1346to special magic, their original syntax is preserved, and you don't have
1347to define a prototype for their replacements. (You can't override the
1348C<do BLOCK> syntax, though).
1349
1350C<require> has special additional dark magic: if you invoke your
1351C<require> replacement as C<require Foo::Bar>, it will actually receive
1352the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1353
1354And, as you'll have noticed from the previous example, if you override
1355C<glob>, the C<< <*> >> glob operator is overridden as well.
1356
1357In a similar fashion, overriding the C<readline> function also overrides
1358the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1359C<readpipe> also overrides the operators C<``> and C<qx//>.
1360
1361Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1362
1363=head2 Autoloading
1364X<autoloading> X<AUTOLOAD>
1365
1366If you call a subroutine that is undefined, you would ordinarily
1367get an immediate, fatal error complaining that the subroutine doesn't
1368exist. (Likewise for subroutines being used as methods, when the
1369method doesn't exist in any base class of the class's package.)
1370However, if an C<AUTOLOAD> subroutine is defined in the package or
1371packages used to locate the original subroutine, then that
1372C<AUTOLOAD> subroutine is called with the arguments that would have
1373been passed to the original subroutine. The fully qualified name
1374of the original subroutine magically appears in the global $AUTOLOAD
1375variable of the same package as the C<AUTOLOAD> routine. The name
1376is not passed as an ordinary argument because, er, well, just
1377because, that's why. (As an exception, a method call to a nonexistent
1378C<import> or C<unimport> method is just skipped instead.)
1379
1380Many C<AUTOLOAD> routines load in a definition for the requested
1381subroutine using eval(), then execute that subroutine using a special
1382form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1383without a trace. (See the source to the standard module documented
1384in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1385also just emulate the routine and never define it. For example,
1386let's pretend that a function that wasn't defined should just invoke
1387C<system> with those arguments. All you'd do is:
1388
1389 sub AUTOLOAD {
1390 my $program = $AUTOLOAD;
1391 $program =~ s/.*:://;
1392 system($program, @_);
1393 }
1394 date();
1395 who('am', 'i');
1396 ls('-l');
1397
1398In fact, if you predeclare functions you want to call that way, you don't
1399even need parentheses:
1400
1401 use subs qw(date who ls);
1402 date;
1403 who "am", "i";
1404 ls '-l';
1405
1406A more complete example of this is the standard Shell module, which
1407can treat undefined subroutine calls as calls to external programs.
1408
1409Mechanisms are available to help modules writers split their modules
1410into autoloadable files. See the standard AutoLoader module
1411described in L<AutoLoader> and in L<AutoSplit>, the standard
1412SelfLoader modules in L<SelfLoader>, and the document on adding C
1413functions to Perl code in L<perlxs>.
1414
1415=head2 Subroutine Attributes
1416X<attribute> X<subroutine, attribute> X<attrs>
1417
1418A subroutine declaration or definition may have a list of attributes
1419associated with it. If such an attribute list is present, it is
1420broken up at space or colon boundaries and treated as though a
1421C<use attributes> had been seen. See L<attributes> for details
1422about what attributes are currently supported.
1423Unlike the limitation with the obsolescent C<use attrs>, the
1424C<sub : ATTRLIST> syntax works to associate the attributes with
1425a pre-declaration, and not just with a subroutine definition.
1426
1427The attributes must be valid as simple identifier names (without any
1428punctuation other than the '_' character). They may have a parameter
1429list appended, which is only checked for whether its parentheses ('(',')')
1430nest properly.
1431
1432Examples of valid syntax (even though the attributes are unknown):
1433
1434 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1435 sub plugh () : Ugly('\(") :Bad;
1436 sub xyzzy : _5x5 { ... }
1437
1438Examples of invalid syntax:
1439
1440 sub fnord : switch(10,foo(); # ()-string not balanced
1441 sub snoid : Ugly('('); # ()-string not balanced
1442 sub xyzzy : 5x5; # "5x5" not a valid identifier
1443 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1444 sub snurt : foo + bar; # "+" not a colon or space
1445
1446The attribute list is passed as a list of constant strings to the code
1447which associates them with the subroutine. In particular, the second example
1448of valid syntax above currently looks like this in terms of how it's
1449parsed and invoked:
1450
1451 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1452
1453For further details on attribute lists and their manipulation,
1454see L<attributes> and L<Attribute::Handlers>.
1455
1456=head1 SEE ALSO
1457
1458See L<perlref/"Function Templates"> for more about references and closures.
1459See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1460See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1461See L<perlmod> to learn about bundling up your functions in separate files.
1462See L<perlmodlib> to learn what library modules come standard on your system.
1463See L<perltoot> to learn how to make object method calls.