MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm
PERLSUB
Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index NAME
perlsub - Perl subroutines
SYNOPSIS
To declare subroutines:
sub NAME; # A "forward" declaration. sub NAME(PROTO); # ditto, but with prototypes
sub NAME BLOCK # A declaration and a definition. sub NAME(PROTO) BLOCK # ditto, but with prototypes
To define an anonymous subroutine at runtime:
$subref = sub BLOCK;
To import subroutines:
use PACKAGE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses. NAME LIST; # Parentheses optional if predeclared/imported. &NAME; # Passes current @_ to subroutine.
DESCRIPTION
Like many languages, Perl provides for user-defined subroutines. Thesemay be located anywhere in the main program, loaded in from other filesvia the
do,
require, or
use keywords, or even generated on thefly using
eval or anonymous subroutines (closures). You can even calla function indirectly using a variable containing its name or a CODE referenceto it, as in
$var = \&function.
The Perl model for function call and return values is simple: allfunctions are passed as parameters one single flat list of scalars, andall functions likewise return to their caller one single flat list ofscalars. Any arrays or hashes in these call and return lists willcollapse, losing their identities---but you may always usepass-by-reference instead to avoid this. Both call and return lists maycontain as many or as few scalar elements as you'd like. (Often afunction without an explicit return statement is called a subroutine, butthere's really no difference from the language's perspective.)
Any arguments passed to the routine come in as the array @_. Thus if youcalled a function with two arguments, those would be stored in $_[0]and $_[1]. The array @_ is a local array, but its elements arealiases for the actual scalar parameters. In particular, if an element$_[0] is updated, the corresponding argument is updated (or an erroroccurs if it is not updatable). If an argument is an array or hashelement which did not exist when the function was called, that element iscreated only when (and if) it is modified or if a reference to it istaken. (Some earlier versions of Perl created the element whether or notit was assigned to.) Note that assigning to the whole array @_ removesthe aliasing, and does not update any arguments.
The return value of the subroutine is the value of the last expressionevaluated. Alternatively, a return statement may be used to exit thesubroutine, optionally specifying the returned value, which will beevaluated in the appropriate context (list, scalar, or void) dependingon the context of the subroutine call. If you specify no return value,the subroutine will return an empty list in a list context, an undefinedvalue in a scalar context, or nothing in a void context. If you returnone or more arrays and/or hashes, these will be flattened together intoone large indistinguishable list.
Perl does not have named formal parameters, but in practice all you do isassign to a my() list of these. Any variables you use in the functionthat aren't declared private are global variables. For the gory detailson creating private variables, seethe section on Private Variables via my() and the section on Temporary Values via local().To create protected environments for a set of functions in a separatepackage (and probably a separate file), see the section on Packages in the perlmod manpage.
Example:
sub max { my $max = shift(@_); foreach $foo (@_) { $max = $foo if $max < $foo; } return $max; } $bestday = max($mon,$tue,$wed,$thu,$fri);Example:
# get a line, combining continuation lines # that start with whitespace
sub get_line { $thisline = $lookahead; # GLOBAL VARIABLES!! LINE: while (defined($lookahead = <STDIN>)) { if ($lookahead =~ /^[ \t]/) { $thisline .= $lookahead; } else { last LINE; } } $thisline; } $lookahead = <STDIN>; # get first line while ($_ = get_line()) { ... }Use array assignment to a local list to name your formal arguments:
sub maybeset { my($key, $value) = @_; $Foo{$key} = $value unless $Foo{$key}; }This also has the effect of turning call-by-reference into call-by-value,because the assignment copies the values. Otherwise a function is free todo in-place modifications of
@_ and change its caller's values.
upcase_in($v1, $v2); # this changes $v1 and $v2 sub upcase_in { for (@_) { tr/a-z/A-Z/ } }You aren't allowed to modify constants in this way, of course. If anargument were actually literal and you tried to change it, you'd take a(presumably fatal) exception. For example, this won't work:
upcase_in("frederick");It would be much safer if the
upcase_in() functionwere written to return a copy of its parameters insteadof changing them in place:
($v3, $v4) = upcase($v1, $v2); # this doesn't sub upcase { return unless defined wantarray; # void context, do nothing my @parms = @_; for (@parms) { tr/a-z/A-Z/ } return wantarray ? @parms : $parms[0]; }Notice how this (unprototyped) function doesn't care whether it was passedreal scalars or arrays. Perl will see everything as one big long flat
@_parameter list. This is one of the ways where Perl's simpleargument-passing style shines. The
upcase() function would work perfectlywell without changing the
upcase() definition even if we fed it thingslike this:
@newlist = upcase(@list1, @list2); @newlist = upcase( split /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Because like its flat incoming parameter list, the return list is alsoflat. So all you have managed to do here is stored everything in
@a andmade
@b an empty list. See the section on
/"Pass by Reference for alternatives.
A subroutine may be called using the ``&'' prefix. The ``&'' is optionalin modern Perls, and so are the parentheses if the subroutine has beenpredeclared. (Note, however, that the ``&'' is NOT optional whenyou're just naming the subroutine, such as when it's used as anargument to defined() or undef(). Nor is it optional when you want todo an indirect subroutine call with a subroutine name or referenceusing the &$subref() or &{$subref}() constructs. See the perlref manpagefor more on that.)
Subroutines may be called recursively. If a subroutine is called usingthe ``&'' form, the argument list is optional, and if omitted, no @_ array isset up for the subroutine: the @_ array at the time of the call isvisible to subroutine instead. This is an efficiency mechanism thatnew users may wish to avoid.
&foo(1,2,3); # pass three arguments foo(1,2,3); # the same
foo(); # pass a null list &foo(); # the same
&foo; # foo() get current args, like foo(@_) !! foo; # like foo() IFF sub foo predeclared, else "foo"
Not only does the ``&'' form make the argument list optional, but it alsodisables any prototype checking on the arguments you do provide. Thisis partly for historical reasons, and partly for having a convenient wayto cheat if you know what you're doing. See the section on Prototypes below.
Private Variables via my()
Synopsis:
my $foo; # declare $foo lexically local my (@wid, %get); # declare list of variables local my $foo = "flurp"; # declare $foo lexical, and init it my @oof = @bar; # declare @oof lexical, and init it
A ``my'' declares the listed variables to be confined (lexically) to theenclosing block, conditional (
if/unless/elsif/else), loop(
for/foreach/while/until/continue), subroutine,
eval, or
do/require/use'd file. If more than one value is listed, the listmust be placed in parentheses. All listed elements must be legal lvalues.Only alphanumeric identifiers may be lexically scoped---magicalbuiltins like $/ must currently be localized with ``local'' instead.
Unlike dynamic variables created by the ``local'' statement, lexicalvariables declared with ``my'' are totally hidden from the outside world,including any called subroutines (even if it's the same subroutine calledfrom itself or elsewhere---every call gets its own copy).
(An eval(), however, can see the lexical variables of the scope it isbeing evaluated in so long as the names aren't hidden by declarations withinthe eval() itself. See the perlref manpage.)
The parameter list to my() may be assigned to if desired, which allows youto initialize your variables. (If no initializer is given for aparticular variable, it is created with the undefined value.) Commonlythis is used to name the parameters to a subroutine. Examples:
$arg = "fred"; # "global" variable $n = cube_root(27); print "$arg thinks the root is $n\n"; fred thinks the root is 3
sub cube_root { my $arg = shift; # name doesn't matter $arg **= 1/3; return $arg; }The ``my'' is simply a modifier on something you might assign to. So whenyou do assign to the variables in its argument list, the ``my'' doesn'tchange whether those variables is viewed as a scalar or an array. So
my ($foo) = <STDIN>; my @FOO = <STDIN>;
both supply a list context to the right-hand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following declares only one variable:
my $foo, $bar = 1;
That has the same effect as
my $foo; $bar = 1;
The declared variable is not introduced (is not visible) until afterthe current statement. Thus,
my $x = $x;
can be used to initialize the new
$x with the value of the old
$x, andthe expression
my $x = 123 and $x == 123
is false unless the old
$x happened to have the value 123.
Lexical scopes of control structures are not bounded precisely by thebraces that delimit their controlled blocks; control expressions arepart of the scope, too. Thus in the loop
while (defined(my $line = <>)) { $line = lc $line; } continue { print $line; }the scope of
$line extends from its declaration throughout the rest ofthe loop construct (including the
continue clause), but not beyondit. Similarly, in the conditional
if ((my $answer = <STDIN>) =~ /^yes$/i) { user_agrees(); } elsif ($answer =~ /^no$/i) { user_disagrees(); } else { chomp $answer; die "'$answer' is neither 'yes' nor 'no'"; }the scope of
$answer extends from its declaration throughout the restof the conditional (including
elsif and
else clauses, if any),but not beyond it.
(None of the foregoing applies to if/unless or while/untilmodifiers appended to simple statements. Such modifiers are notcontrol structures and have no effect on scoping.)
The foreach loop defaults to scoping its index variable dynamically(in the manner of local; see below). However, if the indexvariable is prefixed with the keyword ``my'', then it is lexicallyscoped instead. Thus in the loop
for my $i (1, 2, 3) { some_function(); }the scope of
$i extends to the end of the loop, but not beyond it, andso the value of
$i is unavailable in
some_function().
Some users may wish to encourage the use of lexically scoped variables.As an aid to catching implicit references to package variables,if you say
use strict 'vars';
then any variable reference from there to the end of the enclosingblock must either refer to a lexical variable, or must be fullyqualified with the package name. A compilation error resultsotherwise. An inner block may countermand this with ``no strict `vars'''.
A my() has both a compile-time and a run-time effect. At compile time,the compiler takes notice of it; the principle usefulness of this is toquiet use strict 'vars'. The actual initialization is delayed untilrun time, so it gets executed appropriately; every time through a loop,for example.
Variables declared with ``my'' are not part of any package and are thereforenever fully qualified with the package name. In particular, you're notallowed to try to make a package variable (or other global) lexical:
my $pack::var; # ERROR! Illegal syntax my $_; # also illegal (currently)
In fact, a dynamic variable (also known as package or global variables)are still accessible using the fully qualified :: notation even while alexical of the same name is also visible:
package main; local $x = 10; my $x = 20; print "$x and $::x\n";
That will print out 20 and 10.
You may declare ``my'' variables at the outermost scope of a file tohide any such identifiers totally from the outside world. This is similarto C's static variables at the file level. To do this with a subroutinerequires the use of a closure (anonymous function). If a block (such asan eval(), function, or package) wants to create a private subroutinethat cannot be called from outside that block, it can declare a lexicalvariable containing an anonymous sub reference:
my $secret_version = '1.001-beta'; my $secret_sub = sub { print $secret_version }; &$secret_sub();As long as the reference is never returned by any function within themodule, no outside module can see the subroutine, because its name is not inany package's symbol table. Remember that it's not
REALLY called
$some_pack::secret_version or anything; it's just
$secret_version,unqualified and unqualifiable.
This does not work with object methods, however; all object methods haveto be in the symbol table of some package to be found.
Just because the lexical variable is lexically (also called statically)scoped doesn't mean that within a function it works like a C static. Itnormally works more like a C auto. But here's a mechanism for giving afunction private variables with both lexical scoping and a staticlifetime. If you do want to create something like C's static variables,just enclose the whole function in an extra block, and put thestatic variable outside the function but in the block.
{ my $secret_val = 0; sub gimme_another { return ++$secret_val; } } # $secret_val now becomes unreachable by the outside # world, but retains its value between calls to gimme_anotherIf this function is being sourced in from a separate filevia
require or
use, then this is probably just fine. If it'sall in the main program, you'll need to arrange for the
my()to be executed early, either by putting the whole block aboveyour main program, or more likely, placing merely a
BEGINsub around it to make sure it gets executed before your programstarts to run:
sub BEGIN { my $secret_val = 0; sub gimme_another { return ++$secret_val; } }See the
perlrun manpage about the
BEGIN function.
Temporary Values via local()
NOTE: In general, you should be using ``my'' instead of ``local'', becauseit's faster and safer. Exceptions to this include the global punctuationvariables, filehandles and formats, and direct manipulation of the Perlsymbol table itself. Format variables often use ``local'' though, as doother variables whose current value must be visible to calledsubroutines.
Synopsis:
local $foo; # declare $foo dynamically local local (@wid, %get); # declare list of variables local local $foo = "flurp"; # declare $foo dynamic, and init it local @oof = @bar; # declare @oof dynamic, and init it
local *FH; # localize $FH, @FH, %FH, &FH ... local *merlyn = *randal; # now $merlyn is really $randal, plus # @merlyn is really @randal, etc local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
A
local() modifies its listed variables to be local to the enclosingblock, (or subroutine,
eval{}, or
do) and
any called fromwithin that block. A
local() just gives temporary values to global(meaning package) variables. This is known as dynamic scoping. Lexicalscoping is done with ``my'', which works more like C's auto declarations.
If more than one variable is given to local(), they must be placed inparentheses. All listed elements must be legal lvalues. This operator worksby saving the current values of those variables in its argument list on ahidden stack and restoring them upon exiting the block, subroutine, oreval. This means that called subroutines can also reference the localvariable, but not the global one. The argument list may be assigned to ifdesired, which allows you to initialize your local variables. (If noinitializer is given for a particular variable, it is created with anundefined value.) Commonly this is used to name the parameters to asubroutine. Examples:
for $i ( 0 .. 9 ) { $digits{$i} = $i; } # assume this function uses global %digits hash parse_num(); # now temporarily add to %digits hash if ($base12) { # (NOTE: not claiming this is efficient!) local %digits = (%digits, 't' => 10, 'e' => 11); parse_num(); # parse_num gets this new %digits! } # old %digits restored hereBecause
local() is a run-time command, it gets executed every timethrough a loop. In releases of Perl previous to 5.0, this used more stackstorage each time until the loop was exited. Perl now reclaims the spaceeach time through, but it's still more efficient to declare your variablesoutside the loop.
A local is simply a modifier on an lvalue expression. When you assign toa localized variable, the local doesn't change whether its list is viewedas a scalar or an array. So
local($foo) = <STDIN>; local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
A note about local() and composite types is in order. Somethinglike local(%foo) works by temporarily placing a brand new hash inthe symbol table. The old hash is left alone, but is hidden ``behind''the new one.
This means the old variable is completely invisible via the symboltable (i.e. the hash entry in the *foo typeglob) for the durationof the dynamic scope within which the local() was seen. Thishas the effect of allowing one to temporarily occlude any magic oncomposite types. For instance, this will briefly alter a tiedhash to some other implementation:
tie %ahash, 'APackage'; [...] { local %ahash; tie %ahash, 'BPackage'; [..called code will see %ahash tied to 'BPackage'..] { local %ahash; [..%ahash is a normal (untied) hash here..] } } [..%ahash back to its initial tied self again..]As another example, a custom implementation of
%ENV might looklike this:
{ local %ENV; tie %ENV, 'MyOwnEnv'; [..do your own fancy %ENV manipulation here..] } [..normal %ENV behavior here..] Passing Symbol Table Entries (typeglobs)
[Note: The mechanism described in this section was originally the onlyway to simulate pass-by-reference in older versions of Perl. While itstill works fine in modern versions, the new reference mechanism isgenerally easier to work with. See below.]
Sometimes you don't want to pass the value of an array to a subroutinebut rather the name of it, so that the subroutine can modify the globalcopy of it rather than working with a local copy. In perl you canrefer to all objects of a particular name by prefixing the namewith a star: *foo. This is often known as a ``typeglob'', because thestar on the front can be thought of as a wildcard match for all thefunny prefix characters on variables and subroutines and such.
When evaluated, the typeglob produces a scalar value that representsall the objects of that name, including any filehandle, format, orsubroutine. When assigned to, it causes the name mentioned to refer towhatever ``*'' value was assigned to it. Example:
sub doubleary { local(*someary) = @_; foreach $elem (@someary) { $elem *= 2; } } doubleary(*foo); doubleary(*bar);Note that scalars are already passed by reference, so you can modifyscalar arguments without using this mechanism by referring explicitlyto
$_[0] etc. You can modify all the elements of an array by passingall the elements as scalars, but you have to use the * mechanism (orthe equivalent reference mechanism) to push, pop, or change the size ofan array. It will certainly be faster to pass the typeglob (or reference).
Even if you don't want to modify an array, this mechanism is useful forpassing multiple arrays in a single LIST, because normally the LISTmechanism will merge all the array values so that you can't extract outthe individual arrays. For more on typeglobs, seethe section on Typeglobs and Filehandles in the perldata manpage.
Pass by Reference
If you want to pass more than one array or hash into a function---orreturn them from it---and have them maintain their integrity, thenyou're going to have to use an explicit pass-by-reference. Before youdo that, you need to understand references as detailed in the
perlref manpage.This section may not make much sense to you otherwise.
Here are a few simple examples. First, let's pass in severalarrays to a function and have it pop all of then, return a newlist of all their former last elements:
@tailings = popmany ( \@a, \@b, \@c, \@d );
sub popmany { my $aref; my @retlist = (); foreach $aref ( @_ ) { push @retlist, pop @$aref; } return @retlist; }Here's how you might write a function that returns alist of keys occurring in all the hashes passed to it:
@common = inter( \%foo, \%bar, \%joe ); sub inter { my ($k, $href, %seen); # locals foreach $href (@_) { while ( $k = each %$href ) { $seen{$k}++; } } return grep { $seen{$_} == @_ } keys %seen; }So far, we're using just the normal list return mechanism.What happens if you want to pass or return a hash? Well,if you're using only one of them, or you don't mind themconcatenating, then the normal calling convention is ok, althougha little expensive.
Where people get into trouble is here:
(@a, @b) = func(@c, @d);or (%a, %b) = func(%c, %d);
That syntax simply won't work. It sets just
@a or
%a and clears the
@b or
%b. Plus the function didn't get passed into two separate arrays orhashes: it got one long list in
@_, as always.
If you can arrange for everyone to deal with this through references, it'scleaner code, although not so nice to look at. Here's a function thattakes two array references as arguments, returning the two array elementsin order of how many elements they have in them:
($aref, $bref) = func(\@c, \@d); print "@$aref has more than @$bref\n"; sub func { my ($cref, $dref) = @_; if (@$cref > @$dref) { return ($cref, $dref); } else { return ($dref, $cref); } }It turns out that you can actually do this also:
(*a, *b) = func(\@c, \@d); print "@a has more than @b\n"; sub func { local (*c, *d) = @_; if (@c > @d) { return (\@c, \@d); } else { return (\@d, \@c); } }Here we're using the typeglobs to do symbol table aliasing. It'sa tad subtle, though, and also won't work if you're using
my()variables, because only globals (well, and
local()s) are in the symbol table.
If you're passing around filehandles, you could usually just use the baretypeglob, like *STDOUT, but typeglobs references would be better becausethey'll still work properly under use strict 'refs'. For example:
splutter(\*STDOUT); sub splutter { my $fh = shift; print $fh "her um well a hmmm\n"; } $rec = get_rec(\*STDIN); sub get_rec { my $fh = shift; return scalar <$fh>; }Another way to do this is using *
HANDLE{
IO}, see the
perlref manpage for usageand caveats.
If you're planning on generating new filehandles, you could do this:
sub openit { my $name = shift; local *FH; return open (FH, $path) ? *FH : undef; }Although that will actually produce a small memory leak. See the bottomof the
open() entry in the
perlfunc manpage for a somewhat cleaner way using the
IO::Handlepackage.
Prototypes
As of the 5.002 release of perl, if you declare
sub mypush (\@@)
then
mypush() takes arguments exactly like
push() does. The declarationof the function to be called must be visible at compile time. The prototypeaffects only the interpretation of new-style calls to the function, wherenew-style is defined as not using the
& character. In other words,if you call it like a builtin function, then it behaves like a builtinfunction. If you call it like an old-fashioned subroutine, then itbehaves like an old-fashioned subroutine. It naturally falls out fromthis rule that prototypes have no influence on subroutine referenceslike
\&foo or on indirect subroutine calls like
&{$subref}.
Method calls are not influenced by prototypes either, because thefunction to be called is indeterminate at compile time, because it dependson inheritance.
Because the intent is primarily to let you define subroutines that worklike builtin commands, here are the prototypes for some other functionsthat parse almost exactly like the corresponding builtins.
Declared as Called as
sub mylink ($$) mylink $old, $new sub myvec ($$$) myvec $var, $offset, 1 sub myindex ($$;$) myindex &getstring, "substr" sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off sub myreverse (@) myreverse $a,$b,$c sub myjoin ($@) myjoin ":",$a,$b,$c sub mypop (\@) mypop @array sub mysplice (\@$$@) mysplice @array,@array,0,@pushme sub mykeys (\%) mykeys %{$hashref} sub myopen (*;$) myopen HANDLE, $name sub mypipe (**) mypipe READHANDLE, WRITEHANDLE sub mygrep (&@) mygrep { /foo/ } $a,$b,$c sub myrand ($) myrand 42 sub mytime () mytimeAny backslashed prototype character represents an actual argumentthat absolutely must start with that character. The value passedto the subroutine (as part of
@_) will be a reference to theactual argument given in the subroutine call, obtained by applying
\ to that argument.
Unbackslashed prototype characters have special meanings. Anyunbackslashed @ or % eats all the rest of the arguments, and forceslist context. An argument represented by $ forces scalar context. An& requires an anonymous subroutine, which, if passed as the firstargument, does not require the ``sub'' keyword or a subsequent comma. A* does whatever it has to do to turn the argument into a reference to asymbol table entry.
A semicolon separates mandatory arguments from optional arguments.(It is redundant before @ or %.)
Note how the last three examples above are treated specially by the parser.mygrep() is parsed as a true list operator, myrand() is parsed as atrue unary operator with unary precedence the same as rand(), andmytime() is truly without arguments, just like time(). That is, if yousay
mytime +2;
you'll get
mytime() + 2, not
mytime(2), which is how it would be parsedwithout the prototype.
The interesting thing about & is that you can generate new syntax with it:
sub try (&@) { my($try,$catch) = @_; eval { &$try }; if ($@) { local $_ = $@; &$catch; } } sub catch (&) { $_[0] } try { die "phooey"; } catch { /phooey/ and print "unphooey\n"; };That prints ``unphooey''. (Yes, there are still unresolvedissues having to do with the visibility of
@_. I'm ignoring thatquestion for the moment. (But note that if we make
@_ lexicallyscoped, those anonymous subroutines can act like closures... (Gee,is this sounding a little Lispish? (Never mind.))))
And here's a reimplementation of grep:
sub mygrep (&@) { my $code = shift; my @result; foreach $_ (@_) { push(@result, $_) if &$code; } @result; }Some folks would prefer full alphanumeric prototypes. Alphanumerics havebeen intentionally left out of prototypes for the express purpose ofsomeday in the future adding named, formal parameters. The currentmechanism's main goal is to let module writers provide better diagnosticsfor module users. Larry feels the notation quite understandable to Perlprogrammers, and that it will not intrude greatly upon the meat of themodule, nor make it harder to read. The line noise is visuallyencapsulated into a small pill that's easy to swallow.
It's probably best to prototype new functions, not retrofit prototypinginto older ones. That's because you must be especially careful aboutsilent impositions of differing list versus scalar contexts. For example,if you decide that a function should take just one parameter, like this:
sub func ($) { my $n = shift; print "you gave me $n\n"; }and someone has been calling it with an array or expressionreturning a list:
func(@foo); func( split /:/ );
Then you've just supplied an automatic
scalar() in front of theirargument, which can be more than a bit surprising. The old
@foowhich used to hold one thing doesn't get passed in. Instead,the
func() now gets passed in 1, that is, the number of elementsin
@foo. And the
split() gets called in a scalar context andstarts scribbling on your
@_ parameter list.
This is all very powerful, of course, and should be used only in moderationto make the world a better place.
Constant Functions
Functions with a prototype of
() are potential candidates forinlining. If the result after optimization and constant folding iseither a constant or a lexically-scoped scalar which has no otherreferences, then it will be used in place of function calls madewithout
& or
do. Calls made using
& or
do are neverinlined. (See constant.pm for an easy way to declare mostconstants.)
All of the following functions would be inlined.
sub pi () { 3.14159 } # Not exact, but close. sub PI () { 4 * atan2 1, 1 } # As good as it gets, # and it's inlined, too! sub ST_DEV () { 0 } sub ST_INO () { 1 } sub FLAG_FOO () { 1 << 8 } sub FLAG_BAR () { 1 << 9 } sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } sub BAZ_VAL () { if (OPT_BAZ) { return 23; } else { return 42; } } sub N () { int(BAZ_VAL) / 3 } BEGIN { my $prod = 1; for (1..N) { $prod *= $_ } sub N_FACTORIAL () { $prod } }If you redefine a subroutine which was eligible for inlining you'll geta mandatory warning. (You can use this warning to tell whether or not aparticular subroutine is considered constant.) The warning isconsidered severe enough not to be optional because previously compiledinvocations of the function will still be using the old value of thefunction. If you need to be able to redefine the subroutine you need toensure that it isn't inlined, either by dropping the
() prototype(which changes the calling semantics, so beware) or by thwarting theinlining mechanism in some other way, such as
sub not_inlined () { 23 if $]; } Overriding Builtin Functions
Many builtin functions may be overridden, though this should be triedonly occasionally and for good reason. Typically this might bedone by a package attempting to emulate missing builtin functionalityon a non-Unix system.
Overriding may be done only by importing the name from amodule---ordinary predeclaration isn't good enough. However, thesubs pragma (compiler directive) lets you, in effect, predeclare subsvia the import syntax, and these names may then override the builtin ones:
use subs 'chdir', 'chroot', 'chmod', 'chown'; chdir $somewhere; sub chdir { ... }To unambiguously refer to the builtin form, one may precede thebuiltin name with the special package qualifier
CORE::. For example,saying
CORE::open() will always refer to the builtin
open(), evenif the current package has imported some other subroutine called
&open() from elsewhere.
Library modules should not in general export builtin names like ``open''or ``chdir'' as part of their default @EXPORT list, because these maysneak into someone else's namespace and change the semantics unexpectedly.Instead, if the module adds the name to the @EXPORT_OK list, then it'spossible for a user to import the name explicitly, but not implicitly.That is, they could say
use Module 'open';
and it would import the open override, but if they said
use Module;
they would get the default imports without the overrides.
Note that such overriding is restricted to the package that requeststhe import. Some means of ``globally'' overriding builtins may becomeavailable in future.
Autoloading
If you call a subroutine that is undefined, you would ordinarily get animmediate fatal error complaining that the subroutine doesn't exist.(Likewise for subroutines being used as methods, when the methoddoesn't exist in any of the base classes of the class package.) If,however, there is an
AUTOLOAD subroutine defined in the package orpackages that were searched for the original subroutine, then that
AUTOLOAD subroutine is called with the arguments that would have beenpassed to the original subroutine. The fully qualified name of theoriginal subroutine magically appears in the
$AUTOLOAD variable in thesame package as the
AUTOLOAD routine. The name is not passed as anordinary argument because, er, well, just because, that's why...
Most AUTOLOAD routines will load in a definition for the subroutine inquestion using eval, and then execute that subroutine using a specialform of ``goto'' that erases the stack frame of the AUTOLOAD routinewithout a trace. (See the standard AutoLoader module, for example.)But an AUTOLOAD routine can also just emulate the routine and neverdefine it. For example, let's pretend that a function that wasn't definedshould just call system() with those arguments. All you'd do is this:
sub AUTOLOAD { my $program = $AUTOLOAD; $program =~ s/.*:://; system($program, @_); } date(); who('am', 'i'); ls('-l');In fact, if you predeclare the functions you want to call that way, you don'teven need the parentheses:
use subs qw(date who ls); date; who "am", "i"; ls -l;
A more complete example of this is the standard Shell module, whichcan treat undefined subroutine calls as calls to Unix programs.
Mechanisms are available for modules writers to help split the modulesup into autoloadable files. See the standard AutoLoader moduledescribed in the AutoLoader manpage and in the AutoSplit manpage, the standardSelfLoader modules in the SelfLoader manpage, and the document on adding Cfunctions to perl code in the perlxs manpage.
SEE ALSO
See the
perlref manpage for more on references. See the
perlxs manpage if you'dlike to learn about calling C subroutines from perl. Seethe
perlmod manpage to learn about bundling up your functions inseparate files.
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- Private Variables via my()
- Temporary Values via local()
- Passing Symbol Table Entries (typeglobs)
- Pass by Reference
- Prototypes
- Constant Functions
- Overriding Builtin Functions
- Autoloading
- SEE ALSO
This document was created byman2html,using the manual pages.
