2 X<subroutine> X<function>
4 perlsub - Perl subroutines
8 To declare subroutines:
9 X<subroutine, declaration> X<sub>
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
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
21 To define an anonymous subroutine at runtime:
22 X<subroutine, anonymous>
24 $subref = sub BLOCK; # no proto
25 $subref = sub (PROTO) BLOCK; # with proto
26 $subref = sub : ATTRS BLOCK; # with attributes
27 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
29 To import subroutines:
32 use MODULE qw(NAME1 NAME2 NAME3);
35 X<subroutine, call> X<call>
37 NAME(LIST); # & is optional with parentheses.
38 NAME LIST; # Parentheses optional if predeclared/imported.
39 &NAME(LIST); # Circumvent prototypes.
40 &NAME; # Makes current @_ visible to called subroutine.
44 Like many languages, Perl provides for user-defined subroutines.
45 These may be located anywhere in the main program, loaded in from
46 other files via the C<do>, C<require>, or C<use> keywords, or
47 generated on the fly using C<eval> or anonymous subroutines.
48 You can even call a function indirectly using a variable containing
49 its name or a CODE reference.
51 The Perl model for function call and return values is simple: all
52 functions are passed as parameters one single flat list of scalars, and
53 all functions likewise return to their caller one single flat list of
54 scalars. Any arrays or hashes in these call and return lists will
55 collapse, losing their identities--but you may always use
56 pass-by-reference instead to avoid this. Both call and return lists may
57 contain as many or as few scalar elements as you'd like. (Often a
58 function without an explicit return statement is called a subroutine, but
59 there's really no difference from Perl's perspective.)
60 X<subroutine, parameter> X<parameter>
62 Any arguments passed in show up in the array C<@_>. Therefore, if
63 you called a function with two arguments, those would be stored in
64 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
65 elements are aliases for the actual scalar parameters. In particular,
66 if an element C<$_[0]> is updated, the corresponding argument is
67 updated (or an error occurs if it is not updatable). If an argument
68 is an array or hash element which did not exist when the function
69 was called, that element is created only when (and if) it is modified
70 or a reference to it is taken. (Some earlier versions of Perl
71 created the element whether or not the element was assigned to.)
72 Assigning to the whole array C<@_> removes that aliasing, and does
73 not update any arguments.
74 X<subroutine, argument> X<argument> X<@_>
76 A C<return> statement may be used to exit a subroutine, optionally
77 specifying the returned value, which will be evaluated in the
78 appropriate context (list, scalar, or void) depending on the context of
79 the subroutine call. If you specify no return value, the subroutine
80 returns an empty list in list context, the undefined value in scalar
81 context, or nothing in void context. If you return one or more
82 aggregates (arrays and hashes), these will be flattened together into
83 one large indistinguishable list.
85 If no C<return> is found and if the last statement is an expression, its
86 value is returned. If the last statement is a loop control structure
87 like a C<foreach> or a C<while>, the returned value is unspecified. The
88 empty sub returns the empty list.
89 X<subroutine, return value> X<return value> X<return>
91 Perl does not have named formal parameters. In practice all you
92 do is assign to a C<my()> list of these. Variables that aren't
93 declared to be private are global variables. For gory details
94 on creating private variables, see L<"Private Variables via my()">
95 and L<"Temporary Values via local()">. To create protected
96 environments for a set of functions in a separate package (and
97 probably a separate file), see L<perlmod/"Packages">.
98 X<formal parameter> X<parameter, formal>
105 $max = $foo if $max < $foo;
109 $bestday = max($mon,$tue,$wed,$thu,$fri);
113 # get a line, combining continuation lines
114 # that start with whitespace
117 $thisline = $lookahead; # global variables!
118 LINE: while (defined($lookahead = <STDIN>)) {
119 if ($lookahead =~ /^[ \t]/) {
120 $thisline .= $lookahead;
129 $lookahead = <STDIN>; # get first line
130 while (defined($line = get_line())) {
134 Assigning to a list of private variables to name your arguments:
137 my($key, $value) = @_;
138 $Foo{$key} = $value unless $Foo{$key};
141 Because the assignment copies the values, this also has the effect
142 of turning call-by-reference into call-by-value. Otherwise a
143 function is free to do in-place modifications of C<@_> and change
145 X<call-by-reference> X<call-by-value>
147 upcase_in($v1, $v2); # this changes $v1 and $v2
149 for (@_) { tr/a-z/A-Z/ }
152 You aren't allowed to modify constants in this way, of course. If an
153 argument were actually literal and you tried to change it, you'd take a
154 (presumably fatal) exception. For example, this won't work:
155 X<call-by-reference> X<call-by-value>
157 upcase_in("frederick");
159 It would be much safer if the C<upcase_in()> function
160 were written to return a copy of its parameters instead
161 of changing them in place:
163 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
165 return unless defined wantarray; # void context, do nothing
167 for (@parms) { tr/a-z/A-Z/ }
168 return wantarray ? @parms : $parms[0];
171 Notice how this (unprototyped) function doesn't care whether it was
172 passed real scalars or arrays. Perl sees all arguments as one big,
173 long, flat parameter list in C<@_>. This is one area where
174 Perl's simple argument-passing style shines. The C<upcase()>
175 function would work perfectly well without changing the C<upcase()>
176 definition even if we fed it things like this:
178 @newlist = upcase(@list1, @list2);
179 @newlist = upcase( split /:/, $var );
181 Do not, however, be tempted to do this:
183 (@a, @b) = upcase(@list1, @list2);
185 Like the flattened incoming parameter list, the return list is also
186 flattened on return. So all you have managed to do here is stored
187 everything in C<@a> and made C<@b> empty. See
188 L<Pass by Reference> for alternatives.
190 A subroutine may be called using an explicit C<&> prefix. The
191 C<&> is optional in modern Perl, as are parentheses if the
192 subroutine has been predeclared. The C<&> is I<not> optional
193 when just naming the subroutine, such as when it's used as
194 an argument to defined() or undef(). Nor is it optional when you
195 want to do an indirect subroutine call with a subroutine name or
196 reference using the C<&$subref()> or C<&{$subref}()> constructs,
197 although the C<< $subref->() >> notation solves that problem.
198 See L<perlref> for more about all that.
201 Subroutines may be called recursively. If a subroutine is called
202 using the C<&> form, the argument list is optional, and if omitted,
203 no C<@_> array is set up for the subroutine: the C<@_> array at the
204 time of the call is visible to subroutine instead. This is an
205 efficiency mechanism that new users may wish to avoid.
208 &foo(1,2,3); # pass three arguments
209 foo(1,2,3); # the same
211 foo(); # pass a null list
214 &foo; # foo() get current args, like foo(@_) !!
215 foo; # like foo() IFF sub foo predeclared, else "foo"
217 Not only does the C<&> form make the argument list optional, it also
218 disables any prototype checking on arguments you do provide. This
219 is partly for historical reasons, and partly for having a convenient way
220 to cheat if you know what you're doing. See L<Prototypes> below.
223 Subroutines whose names are in all upper case are reserved to the Perl
224 core, as are modules whose names are in all lower case. A subroutine in
225 all capitals is a loosely-held convention meaning it will be called
226 indirectly by the run-time system itself, usually due to a triggered event.
227 Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
228 C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
230 The C<BEGIN>, C<CHECK>, C<INIT> and C<END> subroutines are not so much
231 subroutines as named special code blocks, of which you can have more
232 than one in a package, and which you can B<not> call explicitly. See
233 L<perlmod/"BEGIN, CHECK, INIT and END">
235 =head2 Private Variables via my()
236 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
237 X<lexical scope> X<attributes, my>
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 = $y; # similar, with an attribute applied
247 B<WARNING>: The use of attribute lists on C<my> declarations is still
248 evolving. The current semantics and interface are subject to change.
249 See L<attributes> and L<Attribute::Handlers>.
251 The C<my> operator declares the listed variables to be lexically
252 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
253 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
254 or C<do/require/use>'d file. If more than one value is listed, the
255 list must be placed in parentheses. All listed elements must be
256 legal lvalues. Only alphanumeric identifiers may be lexically
257 scoped--magical built-ins like C<$/> must currently be C<local>ized
258 with C<local> instead.
260 Unlike dynamic variables created by the C<local> operator, lexical
261 variables declared with C<my> are totally hidden from the outside
262 world, including any called subroutines. This is true if it's the
263 same subroutine called from itself or elsewhere--every call gets
267 This doesn't mean that a C<my> variable declared in a statically
268 enclosing lexical scope would be invisible. Only dynamic scopes
269 are cut off. For example, the C<bumpx()> function below has access
270 to the lexical $x variable because both the C<my> and the C<sub>
271 occurred at the same scope, presumably file scope.
276 An C<eval()>, however, can see lexical variables of the scope it is
277 being evaluated in, so long as the names aren't hidden by declarations within
278 the C<eval()> itself. See L<perlref>.
281 The parameter list to my() may be assigned to if desired, which allows you
282 to initialize your variables. (If no initializer is given for a
283 particular variable, it is created with the undefined value.) Commonly
284 this is used to name input parameters to a subroutine. Examples:
286 $arg = "fred"; # "global" variable
288 print "$arg thinks the root is $n\n";
289 fred thinks the root is 3
292 my $arg = shift; # name doesn't matter
297 The C<my> is simply a modifier on something you might assign to. So when
298 you do assign to variables in its argument list, C<my> doesn't
299 change whether those variables are viewed as a scalar or an array. So
301 my ($foo) = <STDIN>; # WRONG?
304 both supply a list context to the right-hand side, while
308 supplies a scalar context. But the following declares only one variable:
310 my $foo, $bar = 1; # WRONG
312 That has the same effect as
317 The declared variable is not introduced (is not visible) until after
318 the current statement. Thus,
322 can be used to initialize a new $x with the value of the old $x, and
325 my $x = 123 and $x == 123
327 is false unless the old $x happened to have the value C<123>.
329 Lexical scopes of control structures are not bounded precisely by the
330 braces that delimit their controlled blocks; control expressions are
331 part of that scope, too. Thus in the loop
333 while (my $line = <>) {
339 the scope of $line extends from its declaration throughout the rest of
340 the loop construct (including the C<continue> clause), but not beyond
341 it. Similarly, in the conditional
343 if ((my $answer = <STDIN>) =~ /^yes$/i) {
345 } elsif ($answer =~ /^no$/i) {
349 die "'$answer' is neither 'yes' nor 'no'";
352 the scope of $answer extends from its declaration through the rest
353 of that conditional, including any C<elsif> and C<else> clauses,
354 but not beyond it. See L<perlsyn/"Simple statements"> for information
355 on the scope of variables in statements with modifiers.
357 The C<foreach> loop defaults to scoping its index variable dynamically
358 in the manner of C<local>. However, if the index variable is
359 prefixed with the keyword C<my>, or if there is already a lexical
360 by that name in scope, then a new lexical is created instead. Thus
364 for my $i (1, 2, 3) {
368 the scope of $i extends to the end of the loop, but not beyond it,
369 rendering the value of $i inaccessible within C<some_function()>.
372 Some users may wish to encourage the use of lexically scoped variables.
373 As an aid to catching implicit uses to package variables,
374 which are always global, if you say
378 then any variable mentioned from there to the end of the enclosing
379 block must either refer to a lexical variable, be predeclared via
380 C<our> or C<use vars>, or else must be fully qualified with the package name.
381 A compilation error results otherwise. An inner block may countermand
382 this with C<no strict 'vars'>.
384 A C<my> has both a compile-time and a run-time effect. At compile
385 time, the compiler takes notice of it. The principal usefulness
386 of this is to quiet C<use strict 'vars'>, but it is also essential
387 for generation of closures as detailed in L<perlref>. Actual
388 initialization is delayed until run time, though, so it gets executed
389 at the appropriate time, such as each time through a loop, for
392 Variables declared with C<my> are not part of any package and are therefore
393 never fully qualified with the package name. In particular, you're not
394 allowed to try to make a package variable (or other global) lexical:
396 my $pack::var; # ERROR! Illegal syntax
397 my $_; # also illegal (currently)
399 In fact, a dynamic variable (also known as package or global variables)
400 are still accessible using the fully qualified C<::> notation even while a
401 lexical of the same name is also visible:
406 print "$x and $::x\n";
408 That will print out C<20> and C<10>.
410 You may declare C<my> variables at the outermost scope of a file
411 to hide any such identifiers from the world outside that file. This
412 is similar in spirit to C's static variables when they are used at
413 the file level. To do this with a subroutine requires the use of
414 a closure (an anonymous function that accesses enclosing lexicals).
415 If you want to create a private subroutine that cannot be called
416 from outside that block, it can declare a lexical variable containing
417 an anonymous sub reference:
419 my $secret_version = '1.001-beta';
420 my $secret_sub = sub { print $secret_version };
423 As long as the reference is never returned by any function within the
424 module, no outside module can see the subroutine, because its name is not in
425 any package's symbol table. Remember that it's not I<REALLY> called
426 C<$some_pack::secret_version> or anything; it's just $secret_version,
427 unqualified and unqualifiable.
429 This does not work with object methods, however; all object methods
430 have to be in the symbol table of some package to be found. See
431 L<perlref/"Function Templates"> for something of a work-around to
434 =head2 Persistent Private Variables
435 X<static> X<variable, persistent> X<variable, static> X<closure>
437 Just because a lexical variable is lexically (also called statically)
438 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
439 within a function it works like a C static. It normally works more
440 like a C auto, but with implicit garbage collection.
442 Unlike local variables in C or C++, Perl's lexical variables don't
443 necessarily get recycled just because their scope has exited.
444 If something more permanent is still aware of the lexical, it will
445 stick around. So long as something else references a lexical, that
446 lexical won't be freed--which is as it should be. You wouldn't want
447 memory being free until you were done using it, or kept around once you
448 were done. Automatic garbage collection takes care of this for you.
450 This means that you can pass back or save away references to lexical
451 variables, whereas to return a pointer to a C auto is a grave error.
452 It also gives us a way to simulate C's function statics. Here's a
453 mechanism for giving a function private variables with both lexical
454 scoping and a static lifetime. If you do want to create something like
455 C's static variables, just enclose the whole function in an extra block,
456 and put the static variable outside the function but in the block.
461 return ++$secret_val;
464 # $secret_val now becomes unreachable by the outside
465 # world, but retains its value between calls to gimme_another
467 If this function is being sourced in from a separate file
468 via C<require> or C<use>, then this is probably just fine. If it's
469 all in the main program, you'll need to arrange for the C<my>
470 to be executed early, either by putting the whole block above
471 your main program, or more likely, placing merely a C<BEGIN>
472 code block around it to make sure it gets executed before your program
478 return ++$secret_val;
482 See L<perlmod/"BEGIN, CHECK, INIT and END"> about the
483 special triggered code blocks, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
485 If declared at the outermost scope (the file scope), then lexicals
486 work somewhat like C's file statics. They are available to all
487 functions in that same file declared below them, but are inaccessible
488 from outside that file. This strategy is sometimes used in modules
489 to create private variables that the whole module can see.
491 =head2 Temporary Values via local()
492 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
493 X<variable, temporary>
495 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
496 it's faster and safer. Exceptions to this include the global punctuation
497 variables, global filehandles and formats, and direct manipulation of the
498 Perl symbol table itself. C<local> is mostly used when the current value
499 of a variable must be visible to called subroutines.
503 # localization of values
505 local $foo; # make $foo dynamically local
506 local (@wid, %get); # make list of variables local
507 local $foo = "flurp"; # make $foo dynamic, and init it
508 local @oof = @bar; # make @oof dynamic, and init it
510 local $hash{key} = "val"; # sets a local value for this hash entry
511 local ($cond ? $v1 : $v2); # several types of lvalues support
514 # localization of symbols
516 local *FH; # localize $FH, @FH, %FH, &FH ...
517 local *merlyn = *randal; # now $merlyn is really $randal, plus
518 # @merlyn is really @randal, etc
519 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
520 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
522 A C<local> modifies its listed variables to be "local" to the
523 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
524 called from within that block>. A C<local> just gives temporary
525 values to global (meaning package) variables. It does I<not> create
526 a local variable. This is known as dynamic scoping. Lexical scoping
527 is done with C<my>, which works more like C's auto declarations.
529 Some types of lvalues can be localized as well : hash and array elements
530 and slices, conditionals (provided that their result is always
531 localizable), and symbolic references. As for simple variables, this
532 creates new, dynamically scoped values.
534 If more than one variable or expression is given to C<local>, they must be
535 placed in parentheses. This operator works
536 by saving the current values of those variables in its argument list on a
537 hidden stack and restoring them upon exiting the block, subroutine, or
538 eval. This means that called subroutines can also reference the local
539 variable, but not the global one. The argument list may be assigned to if
540 desired, which allows you to initialize your local variables. (If no
541 initializer is given for a particular variable, it is created with an
544 Because C<local> is a run-time operator, it gets executed each time
545 through a loop. Consequently, it's more efficient to localize your
546 variables outside the loop.
548 =head3 Grammatical note on local()
551 A C<local> is simply a modifier on an lvalue expression. When you assign to
552 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
553 as a scalar or an array. So
555 local($foo) = <STDIN>;
556 local @FOO = <STDIN>;
558 both supply a list context to the right-hand side, while
560 local $foo = <STDIN>;
562 supplies a scalar context.
564 =head3 Localization of special variables
565 X<local, special variable>
567 If you localize a special variable, you'll be giving a new value to it,
568 but its magic won't go away. That means that all side-effects related
569 to this magic still work with the localized value.
571 This feature allows code like this to work :
573 # Read the whole contents of FILE in $slurp
574 { local $/ = undef; $slurp = <FILE>; }
576 Note, however, that this restricts localization of some values ; for
577 example, the following statement dies, as of perl 5.9.0, with an error
578 I<Modification of a read-only value attempted>, because the $1 variable is
579 magical and read-only :
583 Similarly, but in a way more difficult to spot, the following snippet will
586 sub f { local $_ = "foo"; print }
588 # now $_ is aliased to $1, thus is magic and readonly
592 See next section for an alternative to this situation.
594 B<WARNING>: Localization of tied arrays and hashes does not currently
596 This will be fixed in a future release of Perl; in the meantime, avoid
597 code that relies on any particular behaviour of localising tied arrays
598 or hashes (localising individual elements is still okay).
599 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
603 =head3 Localization of globs
604 X<local, glob> X<glob>
610 creates a whole new symbol table entry for the glob C<name> in the
611 current package. That means that all variables in its glob slot ($name,
612 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
614 This implies, among other things, that any magic eventually carried by
615 those variables is locally lost. In other words, saying C<local */>
616 will not have any effect on the internal value of the input record
619 Notably, if you want to work with a brand new value of the default scalar
620 $_, and avoid the potential problem listed above about $_ previously
621 carrying a magic value, you should use C<local *_> instead of C<local $_>.
623 =head3 Localization of elements of composite types
624 X<local, composite type element> X<local, array element> X<local, hash element>
626 It's also worth taking a moment to explain what happens when you
627 C<local>ize a member of a composite type (i.e. an array or hash element).
628 In this case, the element is C<local>ized I<by name>. This means that
629 when the scope of the C<local()> ends, the saved value will be
630 restored to the hash element whose key was named in the C<local()>, or
631 the array element whose index was named in the C<local()>. If that
632 element was deleted while the C<local()> was in effect (e.g. by a
633 C<delete()> from a hash or a C<shift()> of an array), it will spring
634 back into existence, possibly extending an array and filling in the
635 skipped elements with C<undef>. For instance, if you say
637 %hash = ( 'This' => 'is', 'a' => 'test' );
641 local($hash{'a'}) = 'drill';
642 while (my $e = pop(@ary)) {
647 $hash{'only a'} = 'test';
651 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
652 print "The array has ",scalar(@ary)," elements: ",
653 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
660 This is a test only a test.
661 The array has 6 elements: 0, 1, 2, undef, undef, 5
663 The behavior of local() on non-existent members of composite
664 types is subject to change in future.
666 =head2 Lvalue subroutines
667 X<lvalue> X<subroutine, lvalue>
669 B<WARNING>: Lvalue subroutines are still experimental and the
670 implementation may change in future versions of Perl.
672 It is possible to return a modifiable value from a subroutine.
673 To do this, you have to declare the subroutine to return an lvalue.
676 sub canmod : lvalue {
677 # return $val; this doesn't work, don't say "return"
684 canmod() = 5; # assigns to $val
687 The scalar/list context for the subroutine and for the right-hand
688 side of assignment is determined as if the subroutine call is replaced
689 by a scalar. For example, consider:
691 data(2,3) = get_data(3,4);
693 Both subroutines here are called in a scalar context, while in:
695 (data(2,3)) = get_data(3,4);
699 (data(2),data(3)) = get_data(3,4);
701 all the subroutines are called in a list context.
705 =item Lvalue subroutines are EXPERIMENTAL
707 They appear to be convenient, but there are several reasons to be
710 You can't use the return keyword, you must pass out the value before
711 falling out of subroutine scope. (see comment in example above). This
712 is usually not a problem, but it disallows an explicit return out of a
713 deeply nested loop, which is sometimes a nice way out.
715 They violate encapsulation. A normal mutator can check the supplied
716 argument before setting the attribute it is protecting, an lvalue
717 subroutine never gets that chance. Consider;
719 my $some_array_ref = []; # protected by mutators ??
721 sub set_arr { # normal mutator
723 die("expected array, you supplied ", ref $val)
724 unless ref $val eq 'ARRAY';
725 $some_array_ref = $val;
727 sub set_arr_lv : lvalue { # lvalue mutator
731 # set_arr_lv cannot stop this !
732 set_arr_lv() = { a => 1 };
736 =head2 Passing Symbol Table Entries (typeglobs)
739 B<WARNING>: The mechanism described in this section was originally
740 the only way to simulate pass-by-reference in older versions of
741 Perl. While it still works fine in modern versions, the new reference
742 mechanism is generally easier to work with. See below.
744 Sometimes you don't want to pass the value of an array to a subroutine
745 but rather the name of it, so that the subroutine can modify the global
746 copy of it rather than working with a local copy. In perl you can
747 refer to all objects of a particular name by prefixing the name
748 with a star: C<*foo>. This is often known as a "typeglob", because the
749 star on the front can be thought of as a wildcard match for all the
750 funny prefix characters on variables and subroutines and such.
752 When evaluated, the typeglob produces a scalar value that represents
753 all the objects of that name, including any filehandle, format, or
754 subroutine. When assigned to, it causes the name mentioned to refer to
755 whatever C<*> value was assigned to it. Example:
758 local(*someary) = @_;
759 foreach $elem (@someary) {
766 Scalars are already passed by reference, so you can modify
767 scalar arguments without using this mechanism by referring explicitly
768 to C<$_[0]> etc. You can modify all the elements of an array by passing
769 all the elements as scalars, but you have to use the C<*> mechanism (or
770 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
771 an array. It will certainly be faster to pass the typeglob (or reference).
773 Even if you don't want to modify an array, this mechanism is useful for
774 passing multiple arrays in a single LIST, because normally the LIST
775 mechanism will merge all the array values so that you can't extract out
776 the individual arrays. For more on typeglobs, see
777 L<perldata/"Typeglobs and Filehandles">.
779 =head2 When to Still Use local()
780 X<local> X<variable, local>
782 Despite the existence of C<my>, there are still three places where the
783 C<local> operator still shines. In fact, in these three places, you
784 I<must> use C<local> instead of C<my>.
790 You need to give a global variable a temporary value, especially $_.
792 The global variables, like C<@ARGV> or the punctuation variables, must be
793 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
794 it up into chunks separated by lines of equal signs, which are placed
798 local @ARGV = ("/etc/motd");
801 @Fields = split /^\s*=+\s*$/;
804 It particular, it's important to C<local>ize $_ in any routine that assigns
805 to it. Look out for implicit assignments in C<while> conditionals.
809 You need to create a local file or directory handle or a local function.
811 A function that needs a filehandle of its own must use
812 C<local()> on a complete typeglob. This can be used to create new symbol
816 local (*READER, *WRITER); # not my!
817 pipe (READER, WRITER) or die "pipe: $!";
818 return (*READER, *WRITER);
820 ($head, $tail) = ioqueue();
822 See the Symbol module for a way to create anonymous symbol table
825 Because assignment of a reference to a typeglob creates an alias, this
826 can be used to create what is effectively a local function, or at least,
830 local *grow = \&shrink; # only until this block exists
831 grow(); # really calls shrink()
832 move(); # if move() grow()s, it shrink()s too
834 grow(); # get the real grow() again
836 See L<perlref/"Function Templates"> for more about manipulating
837 functions by name in this way.
841 You want to temporarily change just one element of an array or hash.
843 You can C<local>ize just one element of an aggregate. Usually this
847 local $SIG{INT} = 'IGNORE';
848 funct(); # uninterruptible
850 # interruptibility automatically restored here
852 But it also works on lexically declared aggregates. Prior to 5.005,
853 this operation could on occasion misbehave.
857 =head2 Pass by Reference
858 X<pass by reference> X<pass-by-reference> X<reference>
860 If you want to pass more than one array or hash into a function--or
861 return them from it--and have them maintain their integrity, then
862 you're going to have to use an explicit pass-by-reference. Before you
863 do that, you need to understand references as detailed in L<perlref>.
864 This section may not make much sense to you otherwise.
866 Here are a few simple examples. First, let's pass in several arrays
867 to a function and have it C<pop> all of then, returning a new list
868 of all their former last elements:
870 @tailings = popmany ( \@a, \@b, \@c, \@d );
875 foreach $aref ( @_ ) {
876 push @retlist, pop @$aref;
881 Here's how you might write a function that returns a
882 list of keys occurring in all the hashes passed to it:
884 @common = inter( \%foo, \%bar, \%joe );
886 my ($k, $href, %seen); # locals
888 while ( $k = each %$href ) {
892 return grep { $seen{$_} == @_ } keys %seen;
895 So far, we're using just the normal list return mechanism.
896 What happens if you want to pass or return a hash? Well,
897 if you're using only one of them, or you don't mind them
898 concatenating, then the normal calling convention is ok, although
901 Where people get into trouble is here:
903 (@a, @b) = func(@c, @d);
905 (%a, %b) = func(%c, %d);
907 That syntax simply won't work. It sets just C<@a> or C<%a> and
908 clears the C<@b> or C<%b>. Plus the function didn't get passed
909 into two separate arrays or hashes: it got one long list in C<@_>,
912 If you can arrange for everyone to deal with this through references, it's
913 cleaner code, although not so nice to look at. Here's a function that
914 takes two array references as arguments, returning the two array elements
915 in order of how many elements they have in them:
917 ($aref, $bref) = func(\@c, \@d);
918 print "@$aref has more than @$bref\n";
920 my ($cref, $dref) = @_;
921 if (@$cref > @$dref) {
922 return ($cref, $dref);
924 return ($dref, $cref);
928 It turns out that you can actually do this also:
930 (*a, *b) = func(\@c, \@d);
931 print "@a has more than @b\n";
941 Here we're using the typeglobs to do symbol table aliasing. It's
942 a tad subtle, though, and also won't work if you're using C<my>
943 variables, because only globals (even in disguise as C<local>s)
944 are in the symbol table.
946 If you're passing around filehandles, you could usually just use the bare
947 typeglob, like C<*STDOUT>, but typeglobs references work, too.
953 print $fh "her um well a hmmm\n";
956 $rec = get_rec(\*STDIN);
962 If you're planning on generating new filehandles, you could do this.
963 Notice to pass back just the bare *FH, not its reference.
968 return open (FH, $path) ? *FH : undef;
972 X<prototype> X<subroutine, prototype>
974 Perl supports a very limited kind of compile-time argument checking
975 using function prototyping. If you declare
979 then C<mypush()> takes arguments exactly like C<push()> does. The
980 function declaration must be visible at compile time. The prototype
981 affects only interpretation of new-style calls to the function,
982 where new-style is defined as not using the C<&> character. In
983 other words, if you call it like a built-in function, then it behaves
984 like a built-in function. If you call it like an old-fashioned
985 subroutine, then it behaves like an old-fashioned subroutine. It
986 naturally falls out from this rule that prototypes have no influence
987 on subroutine references like C<\&foo> or on indirect subroutine
988 calls like C<&{$subref}> or C<< $subref->() >>.
990 Method calls are not influenced by prototypes either, because the
991 function to be called is indeterminate at compile time, since
992 the exact code called depends on inheritance.
994 Because the intent of this feature is primarily to let you define
995 subroutines that work like built-in functions, here are prototypes
996 for some other functions that parse almost exactly like the
997 corresponding built-in.
999 Declared as Called as
1001 sub mylink ($$) mylink $old, $new
1002 sub myvec ($$$) myvec $var, $offset, 1
1003 sub myindex ($$;$) myindex &getstring, "substr"
1004 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1005 sub myreverse (@) myreverse $a, $b, $c
1006 sub myjoin ($@) myjoin ":", $a, $b, $c
1007 sub mypop (\@) mypop @array
1008 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
1009 sub mykeys (\%) mykeys %{$hashref}
1010 sub myopen (*;$) myopen HANDLE, $name
1011 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1012 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1013 sub myrand ($) myrand 42
1014 sub mytime () mytime
1016 Any backslashed prototype character represents an actual argument
1017 that absolutely must start with that character. The value passed
1018 as part of C<@_> will be a reference to the actual argument given
1019 in the subroutine call, obtained by applying C<\> to that argument.
1021 You can also backslash several argument types simultaneously by using
1022 the C<\[]> notation:
1024 sub myref (\[$@%&*])
1026 will allow calling myref() as
1034 and the first argument of myref() will be a reference to
1035 a scalar, an array, a hash, a code, or a glob.
1037 Unbackslashed prototype characters have special meanings. Any
1038 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1039 list context. An argument represented by C<$> forces scalar context. An
1040 C<&> requires an anonymous subroutine, which, if passed as the first
1041 argument, does not require the C<sub> keyword or a subsequent comma.
1043 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1044 typeglob, or a reference to a typeglob in that slot. The value will be
1045 available to the subroutine either as a simple scalar, or (in the latter
1046 two cases) as a reference to the typeglob. If you wish to always convert
1047 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1050 use Symbol 'qualify_to_ref';
1053 my $fh = qualify_to_ref(shift, caller);
1057 A semicolon separates mandatory arguments from optional arguments.
1058 It is redundant before C<@> or C<%>, which gobble up everything else.
1060 Note how the last three examples in the table above are treated
1061 specially by the parser. C<mygrep()> is parsed as a true list
1062 operator, C<myrand()> is parsed as a true unary operator with unary
1063 precedence the same as C<rand()>, and C<mytime()> is truly without
1064 arguments, just like C<time()>. That is, if you say
1068 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1069 without a prototype.
1071 The interesting thing about C<&> is that you can generate new syntax with it,
1072 provided it's in the initial position:
1076 my($try,$catch) = @_;
1083 sub catch (&) { $_[0] }
1088 /phooey/ and print "unphooey\n";
1091 That prints C<"unphooey">. (Yes, there are still unresolved
1092 issues having to do with visibility of C<@_>. I'm ignoring that
1093 question for the moment. (But note that if we make C<@_> lexically
1094 scoped, those anonymous subroutines can act like closures... (Gee,
1095 is this sounding a little Lispish? (Never mind.))))
1097 And here's a reimplementation of the Perl C<grep> operator:
1104 push(@result, $_) if &$code;
1109 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1110 been intentionally left out of prototypes for the express purpose of
1111 someday in the future adding named, formal parameters. The current
1112 mechanism's main goal is to let module writers provide better diagnostics
1113 for module users. Larry feels the notation quite understandable to Perl
1114 programmers, and that it will not intrude greatly upon the meat of the
1115 module, nor make it harder to read. The line noise is visually
1116 encapsulated into a small pill that's easy to swallow.
1118 If you try to use an alphanumeric sequence in a prototype you will
1119 generate an optional warning - "Illegal character in prototype...".
1120 Unfortunately earlier versions of Perl allowed the prototype to be
1121 used as long as its prefix was a valid prototype. The warning may be
1122 upgraded to a fatal error in a future version of Perl once the
1123 majority of offending code is fixed.
1125 It's probably best to prototype new functions, not retrofit prototyping
1126 into older ones. That's because you must be especially careful about
1127 silent impositions of differing list versus scalar contexts. For example,
1128 if you decide that a function should take just one parameter, like this:
1132 print "you gave me $n\n";
1135 and someone has been calling it with an array or expression
1141 Then you've just supplied an automatic C<scalar> in front of their
1142 argument, which can be more than a bit surprising. The old C<@foo>
1143 which used to hold one thing doesn't get passed in. Instead,
1144 C<func()> now gets passed in a C<1>; that is, the number of elements
1145 in C<@foo>. And the C<split> gets called in scalar context so it
1146 starts scribbling on your C<@_> parameter list. Ouch!
1148 This is all very powerful, of course, and should be used only in moderation
1149 to make the world a better place.
1151 =head2 Constant Functions
1154 Functions with a prototype of C<()> are potential candidates for
1155 inlining. If the result after optimization and constant folding
1156 is either a constant or a lexically-scoped scalar which has no other
1157 references, then it will be used in place of function calls made
1158 without C<&>. Calls made using C<&> are never inlined. (See
1159 F<constant.pm> for an easy way to declare most constants.)
1161 The following functions would all be inlined:
1163 sub pi () { 3.14159 } # Not exact, but close.
1164 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1165 # and it's inlined, too!
1169 sub FLAG_FOO () { 1 << 8 }
1170 sub FLAG_BAR () { 1 << 9 }
1171 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1173 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1175 sub N () { int(OPT_BAZ) / 3 }
1177 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1179 Be aware that these will not be inlined; as they contain inner scopes,
1180 the constant folding doesn't reduce them to a single constant:
1182 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1193 If you redefine a subroutine that was eligible for inlining, you'll get
1194 a mandatory warning. (You can use this warning to tell whether or not a
1195 particular subroutine is considered constant.) The warning is
1196 considered severe enough not to be optional because previously compiled
1197 invocations of the function will still be using the old value of the
1198 function. If you need to be able to redefine the subroutine, you need to
1199 ensure that it isn't inlined, either by dropping the C<()> prototype
1200 (which changes calling semantics, so beware) or by thwarting the
1201 inlining mechanism in some other way, such as
1203 sub not_inlined () {
1207 =head2 Overriding Built-in Functions
1208 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1210 Many built-in functions may be overridden, though this should be tried
1211 only occasionally and for good reason. Typically this might be
1212 done by a package attempting to emulate missing built-in functionality
1213 on a non-Unix system.
1215 Overriding may be done only by importing the name from a module at
1216 compile time--ordinary predeclaration isn't good enough. However, the
1217 C<use subs> pragma lets you, in effect, predeclare subs
1218 via the import syntax, and these names may then override built-in ones:
1220 use subs 'chdir', 'chroot', 'chmod', 'chown';
1224 To unambiguously refer to the built-in form, precede the
1225 built-in name with the special package qualifier C<CORE::>. For example,
1226 saying C<CORE::open()> always refers to the built-in C<open()>, even
1227 if the current package has imported some other subroutine called
1228 C<&open()> from elsewhere. Even though it looks like a regular
1229 function call, it isn't: you can't take a reference to it, such as
1230 the incorrect C<\&CORE::open> might appear to produce.
1232 Library modules should not in general export built-in names like C<open>
1233 or C<chdir> as part of their default C<@EXPORT> list, because these may
1234 sneak into someone else's namespace and change the semantics unexpectedly.
1235 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1236 possible for a user to import the name explicitly, but not implicitly.
1237 That is, they could say
1241 and it would import the C<open> override. But if they said
1245 they would get the default imports without overrides.
1247 The foregoing mechanism for overriding built-in is restricted, quite
1248 deliberately, to the package that requests the import. There is a second
1249 method that is sometimes applicable when you wish to override a built-in
1250 everywhere, without regard to namespace boundaries. This is achieved by
1251 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1252 example that quite brazenly replaces the C<glob> operator with something
1253 that understands regular expressions.
1258 @EXPORT_OK = 'glob';
1264 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1265 $pkg->export($where, $sym, @_);
1272 if (opendir D, '.') {
1273 @got = grep /$pat/, readdir D;
1280 And here's how it could be (ab)used:
1282 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1284 use REGlob 'glob'; # override glob() in Foo:: only
1285 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1287 The initial comment shows a contrived, even dangerous example.
1288 By overriding C<glob> globally, you would be forcing the new (and
1289 subversive) behavior for the C<glob> operator for I<every> namespace,
1290 without the complete cognizance or cooperation of the modules that own
1291 those namespaces. Naturally, this should be done with extreme caution--if
1292 it must be done at all.
1294 The C<REGlob> example above does not implement all the support needed to
1295 cleanly override perl's C<glob> operator. The built-in C<glob> has
1296 different behaviors depending on whether it appears in a scalar or list
1297 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1298 context sensitive behaviors, and these must be adequately supported by
1299 a properly written override. For a fully functional example of overriding
1300 C<glob>, study the implementation of C<File::DosGlob> in the standard
1303 When you override a built-in, your replacement should be consistent (if
1304 possible) with the built-in native syntax. You can achieve this by using
1305 a suitable prototype. To get the prototype of an overridable built-in,
1306 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1307 (see L<perlfunc/prototype>).
1309 Note however that some built-ins can't have their syntax expressed by a
1310 prototype (such as C<system> or C<chomp>). If you override them you won't
1311 be able to fully mimic their original syntax.
1313 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1314 to special magic, their original syntax is preserved, and you don't have
1315 to define a prototype for their replacements. (You can't override the
1316 C<do BLOCK> syntax, though).
1318 C<require> has special additional dark magic: if you invoke your
1319 C<require> replacement as C<require Foo::Bar>, it will actually receive
1320 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1322 And, as you'll have noticed from the previous example, if you override
1323 C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well.
1325 In a similar fashion, overriding the C<readline> function also overrides
1326 the equivalent I/O operator C<< <FILEHANDLE> >>.
1328 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1331 X<autoloading> X<AUTOLOAD>
1333 If you call a subroutine that is undefined, you would ordinarily
1334 get an immediate, fatal error complaining that the subroutine doesn't
1335 exist. (Likewise for subroutines being used as methods, when the
1336 method doesn't exist in any base class of the class's package.)
1337 However, if an C<AUTOLOAD> subroutine is defined in the package or
1338 packages used to locate the original subroutine, then that
1339 C<AUTOLOAD> subroutine is called with the arguments that would have
1340 been passed to the original subroutine. The fully qualified name
1341 of the original subroutine magically appears in the global $AUTOLOAD
1342 variable of the same package as the C<AUTOLOAD> routine. The name
1343 is not passed as an ordinary argument because, er, well, just
1344 because, that's why...
1346 Many C<AUTOLOAD> routines load in a definition for the requested
1347 subroutine using eval(), then execute that subroutine using a special
1348 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1349 without a trace. (See the source to the standard module documented
1350 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1351 also just emulate the routine and never define it. For example,
1352 let's pretend that a function that wasn't defined should just invoke
1353 C<system> with those arguments. All you'd do is:
1356 my $program = $AUTOLOAD;
1357 $program =~ s/.*:://;
1358 system($program, @_);
1364 In fact, if you predeclare functions you want to call that way, you don't
1365 even need parentheses:
1367 use subs qw(date who ls);
1372 A more complete example of this is the standard Shell module, which
1373 can treat undefined subroutine calls as calls to external programs.
1375 Mechanisms are available to help modules writers split their modules
1376 into autoloadable files. See the standard AutoLoader module
1377 described in L<AutoLoader> and in L<AutoSplit>, the standard
1378 SelfLoader modules in L<SelfLoader>, and the document on adding C
1379 functions to Perl code in L<perlxs>.
1381 =head2 Subroutine Attributes
1382 X<attribute> X<subroutine, attribute> X<attrs>
1384 A subroutine declaration or definition may have a list of attributes
1385 associated with it. If such an attribute list is present, it is
1386 broken up at space or colon boundaries and treated as though a
1387 C<use attributes> had been seen. See L<attributes> for details
1388 about what attributes are currently supported.
1389 Unlike the limitation with the obsolescent C<use attrs>, the
1390 C<sub : ATTRLIST> syntax works to associate the attributes with
1391 a pre-declaration, and not just with a subroutine definition.
1393 The attributes must be valid as simple identifier names (without any
1394 punctuation other than the '_' character). They may have a parameter
1395 list appended, which is only checked for whether its parentheses ('(',')')
1398 Examples of valid syntax (even though the attributes are unknown):
1400 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1401 sub plugh () : Ugly('\(") :Bad;
1402 sub xyzzy : _5x5 { ... }
1404 Examples of invalid syntax:
1406 sub fnord : switch(10,foo(); # ()-string not balanced
1407 sub snoid : Ugly('('); # ()-string not balanced
1408 sub xyzzy : 5x5; # "5x5" not a valid identifier
1409 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1410 sub snurt : foo + bar; # "+" not a colon or space
1412 The attribute list is passed as a list of constant strings to the code
1413 which associates them with the subroutine. In particular, the second example
1414 of valid syntax above currently looks like this in terms of how it's
1417 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1419 For further details on attribute lists and their manipulation,
1420 see L<attributes> and L<Attribute::Handlers>.
1424 See L<perlref/"Function Templates"> for more about references and closures.
1425 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1426 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1427 See L<perlmod> to learn about bundling up your functions in separate files.
1428 See L<perlmodlib> to learn what library modules come standard on your system.
1429 See L<perltoot> to learn how to make object method calls.