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PERLSUB(1)			 Perl Programmers Reference Guide		       PERLSUB(1)

NAME
       perlsub - Perl subroutines

SYNOPSIS
       To declare subroutines:

	   sub NAME;			 # A "forward" declaration.
	   sub NAME(PROTO);		 #  ditto, but with prototypes
	   sub NAME : ATTRS;		 #  with attributes
	   sub NAME(PROTO) : ATTRS;	 #  with attributes and prototypes

	   sub NAME BLOCK		 # A declaration and a definition.
	   sub NAME(PROTO) BLOCK	 #  ditto, but with prototypes
	   sub NAME : ATTRS BLOCK	 #  with attributes
	   sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes

       To define an anonymous subroutine at runtime:

	   $subref = sub BLOCK; 		# no proto
	   $subref = sub (PROTO) BLOCK; 	# with proto
	   $subref = sub : ATTRS BLOCK; 	# with attributes
	   $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

       To import subroutines:

	   use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

	   NAME(LIST);	  # & is optional with parentheses.
	   NAME LIST;	  # Parentheses optional if predeclared/imported.
	   &NAME(LIST);   # Circumvent prototypes.
	   &NAME;	  # Makes current @_ visible to called subroutine.

DESCRIPTION
       Like many languages, Perl provides for user-defined subroutines.  These may be located
       anywhere in the main program, loaded in from other files via the "do", "require", or "use"
       keywords, or generated on the fly using "eval" or anonymous subroutines.  You can even
       call a function indirectly using a variable containing its name or a CODE reference.

       The Perl model for function call and return values is simple: all functions are passed as
       parameters one single flat list of scalars, and all functions likewise return to their
       caller one single flat list of scalars.	Any arrays or hashes in these call and return
       lists will collapse, losing their identities--but you may always use pass-by-reference
       instead to avoid this.  Both call and return lists may contain as many or as few scalar
       elements as you'd like.	(Often a function without an explicit return statement is called
       a subroutine, but there's really no difference from Perl's perspective.)

       Any arguments passed in show up in the array @_.  Therefore, if you called a function with
       two arguments, those would be stored in $_[0] and $_[1].  The array @_ is a local array,
       but its elements are aliases for the actual scalar parameters.  In particular, if an ele-
       ment $_[0] is updated, the corresponding argument is updated (or an error occurs if it is
       not updatable).	If an argument is an array or hash element which did not exist when the
       function was called, that element is created only when (and if) it is modified or a refer-
       ence to it is taken.  (Some earlier versions of Perl created the element whether or not
       the element was assigned to.)  Assigning to the whole array @_ removes that aliasing, and
       does not update any arguments.

       A "return" statement may be used to exit a subroutine, optionally specifying the returned
       value, which will be evaluated in the appropriate context (list, scalar, or void) depend-
       ing on the context of the subroutine call.  If you specify no return value, the subroutine
       returns an empty list in list context, the undefined value in scalar context, or nothing
       in void context.  If you return one or more aggregates (arrays and hashes), these will be
       flattened together into one large indistinguishable list.

       If no "return" is found and if the last statement is an expression, its value is returned.
       If the last statement is a loop control structure like a "foreach" or a "while", the
       returned value is unspecified. The empty sub returns the empty list.

       Perl does not have named formal parameters.  In practice all you do is assign to a "my()"
       list of these.  Variables that aren't declared to be private are global variables.  For
       gory details on creating private variables, see "Private Variables via my()" and "Tempo-
       rary Values via local()".  To create protected environments for a set of functions in a
       separate package (and probably a separate file), see "Packages" in perlmod.

       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;
		   }
	       }
	       return $thisline;
	   }

	   $lookahead = <STDIN>;       # get first line
	   while (defined($line = get_line())) {
	       ...
	   }

       Assigning to a list of private variables to name your arguments:

	   sub maybeset {
	       my($key, $value) = @_;
	       $Foo{$key} = $value unless $Foo{$key};
	   }

       Because the assignment copies the values, this also has the effect of turning call-by-ref-
       erence into call-by-value.  Otherwise a function is free to do 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 an argument were actu-
       ally 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()" function were written to return a copy of its
       parameters instead of changing them in place:

	   ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
	   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 passed real scalars or
       arrays.	Perl sees all arguments as one big, long, flat parameter list in @_.  This is one
       area where Perl's simple argument-passing style shines.	The "upcase()" function would
       work perfectly well without changing the "upcase()" definition even if we fed it things
       like this:

	   @newlist   = upcase(@list1, @list2);
	   @newlist   = upcase( split /:/, $var );

       Do not, however, be tempted to do this:

	   (@a, @b)   = upcase(@list1, @list2);

       Like the flattened incoming parameter list, the return list is also flattened on return.
       So all you have managed to do here is stored everything in @a and made @b empty.  See
       "Pass by Reference" for alternatives.

       A subroutine may be called using an explicit "&" prefix.  The "&" is optional in modern
       Perl, as are parentheses if the subroutine has been predeclared.  The "&" is not optional
       when just naming the subroutine, such as when it's used as an argument to defined() or
       undef().  Nor is it optional when you want to do an indirect subroutine call with a sub-
       routine name or reference using the "&$subref()" or "&{$subref}()" constructs, although
       the "$subref->()" notation solves that problem.	See perlref for more about all that.

       Subroutines may be called recursively.  If a subroutine is called using the "&" form, the
       argument list is optional, and if omitted, no @_ array is set up for the subroutine: the
       @_ array at the time of the call is visible to subroutine instead.  This is an efficiency
       mechanism that new 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, it also disables any prototype
       checking on arguments you do provide.  This is partly for historical reasons, and partly
       for having a convenient way to cheat if you know what you're doing.  See Prototypes below.

       Subroutines whose names are in all upper case are reserved to the Perl core, as are mod-
       ules whose names are in all lower case.	A subroutine in all capitals is a loosely-held
       convention meaning it will be called indirectly by the run-time system itself, usually due
       to a triggered event.  Subroutines that do special, pre-defined things include "AUTOLOAD",
       "CLONE", "DESTROY" plus all functions mentioned in perltie and PerlIO::via.

       The "BEGIN", "CHECK", "INIT" and "END" subroutines are not so much subroutines as named
       special code blocks, of which you can have more than one in a package, and which you can
       not call explicitly.  See "BEGIN, CHECK, INIT and END" in perlmod

       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
	   my $x : Foo = $y;   # similar, with an attribute applied

       WARNING: The use of attribute lists on "my" declarations is still evolving.  The current
       semantics and interface are subject to change.  See attributes and Attribute::Handlers.

       The "my" operator declares the listed variables to be lexically confined to the enclosing
       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
       list must be placed in parentheses.  All listed elements must be legal lvalues.	Only
       alphanumeric identifiers may be lexically scoped--magical built-ins like $/ must currently
       be "local"ized with "local" instead.

       Unlike dynamic variables created by the "local" operator, lexical variables declared with
       "my" are totally hidden from the outside world, including any called subroutines.  This is
       true if it's the same subroutine called from itself or elsewhere--every call gets its own
       copy.

       This doesn't mean that a "my" variable declared in a statically enclosing lexical scope
       would be invisible.  Only dynamic scopes are cut off.   For example, the "bumpx()" func-
       tion below has access to the lexical $x variable because both the "my" and the "sub"
       occurred at the same scope, presumably file scope.

	   my $x = 10;
	   sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the scope it is being evaluated in, so
       long as the names aren't hidden by declarations within the "eval()" itself.  See perlref.

       The parameter list to my() may be assigned to if desired, which allows you to initialize
       your variables.	(If no initializer is given for a particular variable, it is created with
       the undefined value.)  Commonly this is used to name input 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 when you do assign to
       variables in its argument list, "my" doesn't change whether those variables are viewed as
       a scalar or an array.  So

	   my ($foo) = <STDIN>; 	       # WRONG?
	   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;		       # WRONG

       That has the same effect as

	   my $foo;
	   $bar = 1;

       The declared variable is not introduced (is not visible) until after the current state-
       ment.  Thus,

	   my $x = $x;

       can be used to initialize a new $x with the value of the old $x, and the 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 the braces that delimit
       their controlled blocks; control expressions are part of that scope, too.  Thus in the
       loop

	   while (my $line = <>) {
	       $line = lc $line;
	   } continue {
	       print $line;
	   }

       the scope of $line extends from its declaration throughout the rest of the loop construct
       (including the "continue" clause), but not beyond it.  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 through the rest of that conditional,
       including any "elsif" and "else" clauses, but not beyond it.  See "Simple statements" in
       perlsyn for information on the scope of variables in statements with modifiers.

       The "foreach" loop defaults to scoping its index variable dynamically in the manner of
       "local".  However, if the index variable is prefixed with the keyword "my", or if there is
       already a lexical by that name in scope, then a new lexical is created 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, rendering the value of
       $i inaccessible within "some_function()".

       Some users may wish to encourage the use of lexically scoped variables.	As an aid to
       catching implicit uses to package variables, which are always global, if you say

	   use strict 'vars';

       then any variable mentioned from there to the end of the enclosing block must either refer
       to a lexical variable, be predeclared via "our" or "use vars", or else must be fully qual-
       ified with the package name.  A compilation error results otherwise.  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 principal usefulness of this is to quiet "use strict 'vars'", but it is
       also essential for generation of closures as detailed in perlref.  Actual initialization
       is delayed until run time, though, so it gets executed at the appropriate time, such as
       each time through a loop, for example.

       Variables declared with "my" are not part of any package and are therefore never fully
       qualified with the package name.  In particular, you're not allowed to try to make a pack-
       age 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 accessi-
       ble using the fully qualified "::" notation even while a lexical 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 to hide any such identi-
       fiers from the world outside that file.	This is similar in spirit to C's static variables
       when they are used at the file level.  To do this with a subroutine requires the use of a
       closure (an anonymous function that accesses enclosing lexicals).  If you want to create a
       private subroutine that cannot be called from outside that block, it can declare a lexical
       variable 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 the module, no outside
       module can see the subroutine, because its name is not in any 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 have to be in the sym-
       bol table of some package to be found.  See "Function Templates" in perlref for something
       of a work-around to this.

       Persistent Private Variables

       Just because a lexical variable is lexically (also called statically) scoped to its
       enclosing block, "eval", or "do" FILE, this doesn't mean that within a function it works
       like a C static.  It normally works more like a C auto, but with implicit garbage collec-
       tion.

       Unlike local variables in C or C++, Perl's lexical variables don't necessarily get recy-
       cled just because their scope has exited.  If something more permanent is still aware of
       the lexical, it will stick around.  So long as something else references a lexical, that
       lexical won't be freed--which is as it should be.  You wouldn't want memory being free
       until you were done using it, or kept around once you were done.  Automatic garbage col-
       lection takes care of this for you.

       This means that you can pass back or save away references to lexical variables, whereas to
       return a pointer to a C auto is a grave error.  It also gives us a way to simulate C's
       function statics.  Here's a mechanism for giving a function private variables with both
       lexical scoping and a static lifetime.  If you do want to create something like C's static
       variables, just enclose the whole function in an extra block, and put the static 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_another

       If this function is being sourced in from a separate file via "require" or "use", then
       this is probably just fine.  If it's all in the main program, you'll need to arrange for
       the "my" to be executed early, either by putting the whole block above your main program,
       or more likely, placing merely a "BEGIN" code block around it to make sure it gets exe-
       cuted before your program starts to run:

	   BEGIN {
	       my $secret_val = 0;
	       sub gimme_another {
		   return ++$secret_val;
	       }
	   }

       See "BEGIN, CHECK, INIT and END" in perlmod about the special triggered code blocks,
       "BEGIN", "CHECK", "INIT" and "END".

       If declared at the outermost scope (the file scope), then lexicals work somewhat like C's
       file statics.  They are available to all functions in that same file declared below them,
       but are inaccessible from outside that file.  This strategy is sometimes used in modules
       to create private variables that the whole module can see.

       Temporary Values via local()

       WARNING: In general, you should be using "my" instead of "local", because it's faster and
       safer.  Exceptions to this include the global punctuation variables, global filehandles
       and formats, and direct manipulation of the Perl symbol table itself.  "local" is mostly
       used when the current value of a variable must be visible to called subroutines.

       Synopsis:

	   # localization of values

	   local $foo;		       # make $foo dynamically local
	   local (@wid, %get);	       # make list of variables local
	   local $foo = "flurp";       # make $foo dynamic, and init it
	   local @oof = @bar;	       # make @oof dynamic, and init it

	   local $hash{key} = "val";   # sets a local value for this hash entry
	   local ($cond ? $v1 : $v2);  # several types of lvalues support
				       # localization

	   # localization of symbols

	   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 enclosing block, "eval", or
       "do FILE"--and to any subroutine called from within that block.	A "local" just gives tem-
       porary values to global (meaning package) variables.  It does not create a local variable.
       This is known as dynamic scoping.  Lexical scoping is done with "my", which works more
       like C's auto declarations.

       Some types of lvalues can be localized as well : hash and array elements and slices, con-
       ditionals (provided that their result is always localizable), and symbolic references.  As
       for simple variables, this creates new, dynamically scoped values.

       If more than one variable or expression is given to "local", they must be placed in paren-
       theses.	This operator works by saving the current values of those variables in its argu-
       ment list on a hidden stack and restoring them upon exiting the block, subroutine, or
       eval.  This means that called subroutines can also reference the local variable, but not
       the global one.	The argument list may be assigned to if desired, which allows you to ini-
       tialize your local variables.  (If no initializer is given for a particular variable, it
       is created with an undefined value.)

       Because "local" is a run-time operator, it gets executed each time through a loop.  Conse-
       quently, it's more efficient to localize your variables outside the loop.

       Grammatical note on local()

       A "local" is simply a modifier on an lvalue expression.	When you assign to a "local"ized
       variable, the "local" doesn't change whether its list is viewed as 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.

       Localization of special variables

       If you localize a special variable, you'll be giving a new value to it, but its magic
       won't go away.  That means that all side-effects related to this magic still work with the
       localized value.

       This feature allows code like this to work :

	   # Read the whole contents of FILE in $slurp
	   { local $/ = undef; $slurp = <FILE>; }

       Note, however, that this restricts localization of some values ; for example, the follow-
       ing statement dies, as of perl 5.9.0, with an error Modification of a read-only value
       attempted, because the $1 variable is magical and read-only :

	   local $1 = 2;

       Similarly, but in a way more difficult to spot, the following snippet will die in perl
       5.9.0 :

	   sub f { local $_ = "foo"; print }
	   for ($1) {
	       # now $_ is aliased to $1, thus is magic and readonly
	       f();
	   }

       See next section for an alternative to this situation.

       WARNING: Localization of tied arrays and hashes does not currently work as described.
       This will be fixed in a future release of Perl; in the meantime, avoid code that relies on
       any particular behaviour of localising tied arrays or hashes (localising individual ele-
       ments is still okay).  See "Localising Tied Arrays and Hashes Is Broken" in perl58delta
       for more details.

       Localization of globs

       The construct

	   local *name;

       creates a whole new symbol table entry for the glob "name" in the current package.  That
       means that all variables in its glob slot ($name, @name, %name, &name, and the "name"
       filehandle) are dynamically reset.

       This implies, among other things, that any magic eventually carried by those variables is
       locally lost.  In other words, saying "local */" will not have any effect on the internal
       value of the input record separator.

       Notably, if you want to work with a brand new value of the default scalar $_, and avoid
       the potential problem listed above about $_ previously carrying a magic value, you should
       use "local *_" instead of "local $_".

       Localization of elements of composite types

       It's also worth taking a moment to explain what happens when you "local"ize a member of a
       composite type (i.e. an array or hash element).	In this case, the element is "local"ized
       by name. This means that when the scope of the "local()" ends, the saved value will be
       restored to the hash element whose key was named in the "local()", or the array element
       whose index was named in the "local()".	If that element was deleted while the "local()"
       was in effect (e.g. by a "delete()" from a hash or a "shift()" of an array), it will
       spring back into existence, possibly extending an array and filling in the skipped ele-
       ments with "undef".  For instance, if you say

	   %hash = ( 'This' => 'is', 'a' => 'test' );
	   @ary  = ( 0..5 );
	   {
		local($ary[5]) = 6;
		local($hash{'a'}) = 'drill';
		while (my $e = pop(@ary)) {
		    print "$e . . .\n";
		    last unless $e > 3;
		}
		if (@ary) {
		    $hash{'only a'} = 'test';
		    delete $hash{'a'};
		}
	   }
	   print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
	   print "The array has ",scalar(@ary)," elements: ",
		 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

	   6 . . .
	   4 . . .
	   3 . . .
	   This is a test only a test.
	   The array has 6 elements: 0, 1, 2, undef, undef, 5

       The behavior of local() on non-existent members of composite types is subject to change in
       future.

       Lvalue subroutines

       WARNING: Lvalue subroutines are still experimental and the implementation may change in
       future versions of Perl.

       It is possible to return a modifiable value from a subroutine.  To do this, you have to
       declare the subroutine to return an lvalue.

	   my $val;
	   sub canmod : lvalue {
	       # return $val; this doesn't work, don't say "return"
	       $val;
	   }
	   sub nomod {
	       $val;
	   }

	   canmod() = 5;   # assigns to $val
	   nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the right-hand side of assignment is
       determined as if the subroutine call is replaced by a scalar. For example, consider:

	   data(2,3) = get_data(3,4);

       Both subroutines here are called in a scalar context, while in:

	   (data(2,3)) = get_data(3,4);

       and in:

	   (data(2),data(3)) = get_data(3,4);

       all the subroutines are called in a list context.

       Lvalue subroutines are EXPERIMENTAL
	   They appear to be convenient, but there are several reasons to be circumspect.

	   You can't use the return keyword, you must pass out the value before falling out of
	   subroutine scope. (see comment in example above).  This is usually not a problem, but
	   it disallows an explicit return out of a deeply nested loop, which is sometimes a nice
	   way out.

	   They violate encapsulation.	A normal mutator can check the supplied argument before
	   setting the attribute it is protecting, an lvalue subroutine never gets that chance.
	   Consider;

	       my $some_array_ref = [];    # protected by mutators ??

	       sub set_arr {		   # normal mutator
		   my $val = shift;
		   die("expected array, you supplied ", ref $val)
		      unless ref $val eq 'ARRAY';
		   $some_array_ref = $val;
	       }
	       sub set_arr_lv : lvalue {   # lvalue mutator
		   $some_array_ref;
	       }

	       # set_arr_lv cannot stop this !
	       set_arr_lv() = { a => 1 };

       Passing Symbol Table Entries (typeglobs)

       WARNING: The mechanism described in this section was originally the only way to simulate
       pass-by-reference in older versions of Perl.  While it still works fine in modern ver-
       sions, the new reference mechanism is generally easier to work with.  See below.

       Sometimes you don't want to pass the value of an array to a subroutine but rather the name
       of it, so that the subroutine can modify the global copy of it rather than working with a
       local copy.  In perl you can refer to all objects of a particular name by prefixing the
       name with a star: *foo.	This is often known as a "typeglob", because the star on the
       front can be thought of as a wildcard match for all the funny prefix characters on vari-
       ables and subroutines and such.

       When evaluated, the typeglob produces a scalar value that represents all the objects of
       that name, including any filehandle, format, or subroutine.  When assigned to, it causes
       the name mentioned to refer to whatever "*" value was assigned to it.  Example:

	   sub doubleary {
	       local(*someary) = @_;
	       foreach $elem (@someary) {
		   $elem *= 2;
	       }
	   }
	   doubleary(*foo);
	   doubleary(*bar);

       Scalars are already passed by reference, so you can modify scalar arguments without using
       this mechanism by referring explicitly to $_[0] etc.  You can modify all the elements of
       an array by passing all the elements as scalars, but you have to use the "*" mechanism (or
       the equivalent reference mechanism) to "push", "pop", or change the size of an 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 for passing multiple
       arrays in a single LIST, because normally the LIST mechanism will merge all the array val-
       ues so that you can't extract out the individual arrays.  For more on typeglobs, see
       "Typeglobs and Filehandles" in perldata.

       When to Still Use local()

       Despite the existence of "my", there are still three places where the "local" operator
       still shines.  In fact, in these three places, you must use "local" instead of "my".

       1.  You need to give a global variable a temporary value, especially $_.

	   The global variables, like @ARGV or the punctuation variables, must be "local"ized
	   with "local()".  This block reads in /etc/motd, and splits it up into chunks separated
	   by lines of equal signs, which are placed in @Fields.

	       {
		   local @ARGV = ("/etc/motd");
		   local $/ = undef;
		   local $_ = <>;
		   @Fields = split /^\s*=+\s*$/;
	       }

	   It particular, it's important to "local"ize $_ in any routine that assigns to it.
	   Look out for implicit assignments in "while" conditionals.

       2.  You need to create a local file or directory handle or a local function.

	   A function that needs a filehandle of its own must use "local()" on a complete type-
	   glob.   This can be used to create new symbol table entries:

	       sub ioqueue {
		   local  (*READER, *WRITER);	 # not my!
		   pipe    (READER,  WRITER)	 or die "pipe: $!";
		   return (*READER, *WRITER);
	       }
	       ($head, $tail) = ioqueue();

	   See the Symbol module for a way to create anonymous symbol table entries.

	   Because assignment of a reference to a typeglob creates an alias, this can be used to
	   create what is effectively a local function, or at least, a local alias.

	       {
		   local *grow = \&shrink; # only until this block exists
		   grow();		   # really calls shrink()
		   move();		   # if move() grow()s, it shrink()s too
	       }
	       grow();			   # get the real grow() again

	   See "Function Templates" in perlref for more about manipulating functions by name in
	   this way.

       3.  You want to temporarily change just one element of an array or hash.

	   You can "local"ize just one element of an aggregate.  Usually this is done on dynam-
	   ics:

	       {
		   local $SIG{INT} = 'IGNORE';
		   funct();			       # uninterruptible
	       }
	       # interruptibility automatically restored here

	   But it also works on lexically declared aggregates.	Prior to 5.005, this operation
	   could on occasion misbehave.

       Pass by Reference

       If you want to pass more than one array or hash into a function--or return them from
       it--and have them maintain their integrity, then you're going to have to use an explicit
       pass-by-reference.  Before you do that, you need to understand references as detailed in
       perlref.  This section may not make much sense to you otherwise.

       Here are a few simple examples.	First, let's pass in several arrays to a function and
       have it "pop" all of then, returning a new list 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 a list 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 them
       concatenating, then the normal calling convention is ok, although a 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 or hashes: it got one long list in @_,
       as always.

       If you can arrange for everyone to deal with this through references, it's cleaner code,
       although not so nice to look at.  Here's a function that takes two array references as
       arguments, returning the two array elements in 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's a tad subtle, though,
       and also won't work if you're using "my" variables, because only globals (even in disguise
       as "local"s) are in the symbol table.

       If you're passing around filehandles, you could usually just use the bare typeglob, like
       *STDOUT, but typeglobs references work, too.  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>;
	   }

       If you're planning on generating new filehandles, you could do this.  Notice to pass back
       just the bare *FH, not its reference.

	   sub openit {
	       my $path = shift;
	       local *FH;
	       return open (FH, $path) ? *FH : undef;
	   }

       Prototypes

       Perl supports a very limited kind of compile-time argument checking using function proto-
       typing.	If you declare

	   sub mypush (\@@)

       then "mypush()" takes arguments exactly like "push()" does.  The function declaration must
       be visible at compile time.  The prototype affects only interpretation of new-style calls
       to the function, where new-style is defined as not using the "&" character.  In other
       words, if you call it like a built-in function, then it behaves like a built-in function.
       If you call it like an old-fashioned subroutine, then it behaves like an old-fashioned
       subroutine.  It naturally falls out from this rule that prototypes have no influence on
       subroutine references like "\&foo" or on indirect subroutine calls like "&{$subref}" or
       "$subref->()".

       Method calls are not influenced by prototypes either, because the function to be called is
       indeterminate at compile time, since the exact code called depends on inheritance.

       Because the intent of this feature is primarily to let you define subroutines that work
       like built-in functions, here are prototypes for some other functions that parse almost
       exactly like the corresponding built-in.

	   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, 0, 2, @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 ()	    mytime

       Any backslashed prototype character represents an actual argument that absolutely must
       start with that character.  The value passed as part of @_ will be a reference to the
       actual argument given in the subroutine call, obtained by applying "\" to that argument.

       You can also backslash several argument types simultaneously by using the "\[]" notation:

	   sub myref (\[$@%&*])

       will allow calling myref() as

	   myref $var
	   myref @array
	   myref %hash
	   myref &sub
	   myref *glob

       and the first argument of myref() will be a reference to a scalar, an array, a hash, a
       code, or a glob.

       Unbackslashed prototype characters have special meanings.  Any unbackslashed "@" or "%"
       eats all remaining arguments, and forces list context.  An argument represented by "$"
       forces scalar context.  An "&" requires an anonymous subroutine, which, if passed as the
       first argument, does not require the "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept a bareword, constant, scalar expression, typeglob,
       or a reference to a typeglob in that slot.  The value will be available to the subroutine
       either as a simple scalar, or (in the latter two cases) as a reference to the typeglob.
       If you wish to always convert such arguments to a typeglob reference, use Symbol::qual-
       ify_to_ref() as follows:

	   use Symbol 'qualify_to_ref';

	   sub foo (*) {
	       my $fh = qualify_to_ref(shift, caller);
	       ...
	   }

       A semicolon separates mandatory arguments from optional arguments.  It is redundant before
       "@" or "%", which gobble up everything else.

       Note how the last three examples in the table above are treated specially by the parser.
       "mygrep()" is parsed as a true list operator, "myrand()" is parsed as a true unary opera-
       tor with unary precedence the same as "rand()", and "mytime()" is truly without arguments,
       just like "time()".  That is, if you say

	   mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it would be parsed without a proto-
       type.

       The interesting thing about "&" is that you can generate new syntax with it, provided it's
       in the initial position:

	   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 unresolved issues having to do with visi-
       bility of @_.  I'm ignoring that question for the moment.  (But note that if we make @_
       lexically scoped, those anonymous subroutines can act like closures... (Gee, is this
       sounding a little Lispish?  (Never mind.))))

       And here's a reimplementation of the Perl "grep" operator:

	   sub mygrep (&@) {
	       my $code = shift;
	       my @result;
	       foreach $_ (@_) {
		   push(@result, $_) if &$code;
	       }
	       @result;
	   }

       Some folks would prefer full alphanumeric prototypes.  Alphanumerics have been intention-
       ally left out of prototypes for the express purpose of someday in the future adding named,
       formal parameters.  The current mechanism's main goal is to let module writers provide
       better diagnostics for module users.  Larry feels the notation quite understandable to
       Perl programmers, and that it will not intrude greatly upon the meat of the module, nor
       make it harder to read.	The line noise is visually encapsulated into a small pill that's
       easy to swallow.

       If you try to use an alphanumeric sequence in a prototype you will generate an optional
       warning - "Illegal character in prototype...".  Unfortunately earlier versions of Perl
       allowed the prototype to be used as long as its prefix was a valid prototype.  The warning
       may be upgraded to a fatal error in a future version of Perl once the majority of offend-
       ing code is fixed.

       It's probably best to prototype new functions, not retrofit prototyping into older ones.
       That's because you must be especially careful about silent 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 expression returning a list:

	   func(@foo);
	   func( split /:/ );

       Then you've just supplied an automatic "scalar" in front of their argument, which can be
       more than a bit surprising.  The old @foo which used to hold one thing doesn't get passed
       in.  Instead, "func()" now gets passed in a 1; that is, the number of elements in @foo.
       And the "split" gets called in scalar context so it starts scribbling on your @_ parameter
       list.  Ouch!

       This is all very powerful, of course, and should be used only in moderation to make the
       world a better place.

       Constant Functions

       Functions with a prototype of "()" are potential candidates for inlining.  If the result
       after optimization and constant folding is either a constant or a lexically-scoped scalar
       which has no other references, then it will be used in place of function calls made with-
       out "&".  Calls made using "&" are never inlined.  (See constant.pm for an easy way to
       declare most constants.)

       The following functions would all 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 N () { int(OPT_BAZ) / 3 }

	   sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }

       Be aware that these will not be inlined; as they contain inner scopes, the constant fold-
       ing doesn't reduce them to a single constant:

	   sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }

	   sub baz_val () {
	       if (OPT_BAZ) {
		   return 23;
	       }
	       else {
		   return 42;
	       }
	   }

       If you redefine a subroutine that was eligible for inlining, you'll get a mandatory warn-
       ing.  (You can use this warning to tell whether or not a particular subroutine is consid-
       ered constant.)	The warning is considered severe enough not to be optional because previ-
       ously compiled invocations of the function will still be using the old value of the func-
       tion.  If you need to be able to redefine the subroutine, you need to ensure that it isn't
       inlined, either by dropping the "()" prototype (which changes calling semantics, so
       beware) or by thwarting the inlining mechanism in some other way, such as

	   sub not_inlined () {
	       23 if $];
	   }

       Overriding Built-in Functions

       Many built-in functions may be overridden, though this should be tried only occasionally
       and for good reason.  Typically this might be done by a package attempting to emulate
       missing built-in functionality on a non-Unix system.

       Overriding may be done only by importing the name from a module at compile time--ordinary
       predeclaration isn't good enough.  However, the "use subs" pragma lets you, in effect,
       predeclare subs via the import syntax, and these names may then override built-in ones:

	   use subs 'chdir', 'chroot', 'chmod', 'chown';
	   chdir $somewhere;
	   sub chdir { ... }

       To unambiguously refer to the built-in form, precede the built-in name with the special
       package qualifier "CORE::".  For example, saying "CORE::open()" always refers to the
       built-in "open()", even if the current package has imported some other subroutine called
       "&open()" from elsewhere.  Even though it looks like a regular function call, it isn't:
       you can't take a reference to it, such as the incorrect "\&CORE::open" might appear to
       produce.

       Library modules should not in general export built-in names like "open" or "chdir" as part
       of their default @EXPORT list, because these may sneak into someone else's namespace and
       change the semantics unexpectedly.  Instead, if the module adds that name to @EXPORT_OK,
       then it's possible 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 overrides.

       The foregoing mechanism for overriding built-in is restricted, quite deliberately, to the
       package that requests the import.  There is a second method that is sometimes applicable
       when you wish to override a built-in everywhere, without regard to namespace boundaries.
       This is achieved by importing a sub into the special namespace "CORE::GLOBAL::".  Here is
       an example that quite brazenly replaces the "glob" operator with something that under-
       stands regular expressions.

	   package REGlob;
	   require Exporter;
	   @ISA = 'Exporter';
	   @EXPORT_OK = 'glob';

	   sub import {
	       my $pkg = shift;
	       return unless @_;
	       my $sym = shift;
	       my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
	       $pkg->export($where, $sym, @_);
	   }

	   sub glob {
	       my $pat = shift;
	       my @got;
	       if (opendir my $d, '.') {
		   @got = grep /$pat/, readdir $d;
		   closedir $d;
	       }
	       return @got;
	   }
	   1;

       And here's how it could be (ab)used:

	   #use REGlob 'GLOBAL_glob';	   # override glob() in ALL namespaces
	   package Foo;
	   use REGlob 'glob';		   # override glob() in Foo:: only
	   print for <^[a-z_]+\.pm\$>;	   # show all pragmatic modules

       The initial comment shows a contrived, even dangerous example.  By overriding "glob" glob-
       ally, you would be forcing the new (and subversive) behavior for the "glob" operator for
       every namespace, without the complete cognizance or cooperation of the modules that own
       those namespaces.  Naturally, this should be done with extreme caution--if it must be done
       at all.

       The "REGlob" example above does not implement all the support needed to cleanly override
       perl's "glob" operator.	The built-in "glob" has different behaviors depending on whether
       it appears in a scalar or list context, but our "REGlob" doesn't.  Indeed, many perl
       built-in have such context sensitive behaviors, and these must be adequately supported by
       a properly written override.  For a fully functional example of overriding "glob", study
       the implementation of "File::DosGlob" in the standard library.

       When you override a built-in, your replacement should be consistent (if possible) with the
       built-in native syntax.	You can achieve this by using a suitable prototype.  To get the
       prototype of an overridable built-in, use the "prototype" function with an argument of
       "CORE::builtin_name" (see "prototype" in perlfunc).

       Note however that some built-ins can't have their syntax expressed by a prototype (such as
       "system" or "chomp").  If you override them you won't be able to fully mimic their origi-
       nal syntax.

       The built-ins "do", "require" and "glob" can also be overridden, but due to special magic,
       their original syntax is preserved, and you don't have to define a prototype for their
       replacements.  (You can't override the "do BLOCK" syntax, though).

       "require" has special additional dark magic: if you invoke your "require" replacement as
       "require Foo::Bar", it will actually receive the argument "Foo/Bar.pm" in @_.  See
       "require" in perlfunc.

       And, as you'll have noticed from the previous example, if you override "glob", the "<*>"
       glob operator is overridden as well.

       In a similar fashion, overriding the "readline" function also overrides the equivalent I/O
       operator "<FILEHANDLE>".

       Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.

       Autoloading

       If you call a subroutine that is undefined, you would ordinarily get an immediate, fatal
       error complaining that the subroutine doesn't exist.  (Likewise for subroutines being used
       as methods, when the method doesn't exist in any base class of the class's package.)  How-
       ever, if an "AUTOLOAD" subroutine is defined in the package or packages used to locate the
       original subroutine, then that "AUTOLOAD" subroutine is called with the arguments that
       would have been passed to the original subroutine.  The fully qualified name of the origi-
       nal subroutine magically appears in the global $AUTOLOAD variable of the same package as
       the "AUTOLOAD" routine.	The name is not passed as an ordinary argument because, er, well,
       just because, that's why...

       Many "AUTOLOAD" routines load in a definition for the requested subroutine using eval(),
       then execute that subroutine using a special form of goto() that erases the stack frame of
       the "AUTOLOAD" routine without a trace.	(See the source to the standard module documented
       in AutoLoader, for example.)  But an "AUTOLOAD" routine can also just emulate the routine
       and never define it.   For example, let's pretend that a function that wasn't defined
       should just invoke "system" with those arguments.  All you'd do is:

	   sub AUTOLOAD {
	       my $program = $AUTOLOAD;
	       $program =~ s/.*:://;
	       system($program, @_);
	   }
	   date();
	   who('am', 'i');
	   ls('-l');

       In fact, if you predeclare functions you want to call that way, you don't even need paren-
       theses:

	   use subs qw(date who ls);
	   date;
	   who "am", "i";
	   ls -l;

       A more complete example of this is the standard Shell module, which can treat undefined
       subroutine calls as calls to external programs.

       Mechanisms are available to help modules writers split their modules into autoloadable
       files.  See the standard AutoLoader module described in AutoLoader and in AutoSplit, the
       standard SelfLoader modules in SelfLoader, and the document on adding C functions to Perl
       code in perlxs.

       Subroutine Attributes

       A subroutine declaration or definition may have a list of attributes associated with it.
       If such an attribute list is present, it is broken up at space or colon boundaries and
       treated as though a "use attributes" had been seen.  See attributes for details about what
       attributes are currently supported.  Unlike the limitation with the obsolescent "use
       attrs", the "sub : ATTRLIST" syntax works to associate the attributes with a pre-declara-
       tion, and not just with a subroutine definition.

       The attributes must be valid as simple identifier names (without any punctuation other
       than the '_' character).  They may have a parameter list appended, which is only checked
       for whether its parentheses ('(',')') nest properly.

       Examples of valid syntax (even though the attributes are unknown):

	   sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
	   sub plugh () : Ugly('\(") :Bad;
	   sub xyzzy : _5x5 { ... }

       Examples of invalid syntax:

	   sub fnord : switch(10,foo(); # ()-string not balanced
	   sub snoid : Ugly('(');	 # ()-string not balanced
	   sub xyzzy : 5x5;		 # "5x5" not a valid identifier
	   sub plugh : Y2::north;	 # "Y2::north" not a simple identifier
	   sub snurt : foo + bar;	 # "+" not a colon or space

       The attribute list is passed as a list of constant strings to the code which associates
       them with the subroutine.  In particular, the second example of valid syntax above cur-
       rently looks like this in terms of how it's parsed and invoked:

	   use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

       For further details on attribute lists and their manipulation, see attributes and
       Attribute::Handlers.

SEE ALSO
       See "Function Templates" in perlref for more about references and closures.  See perlxs if
       you'd like to learn about calling C subroutines from Perl.  See perlembed if you'd like to
       learn about calling Perl subroutines from C.  See perlmod to learn about bundling up your
       functions in separate files.  See perlmodlib to learn what library modules come standard
       on your system.	See perltoot to learn how to make object method calls.

perl v5.8.9				    2007-11-17				       PERLSUB(1)
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