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perlsub ()
  • >> perlsub (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlsub (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    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 (closures).  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 element
         `$_[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 reference 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.
    
         The return value of a subroutine is the value of the last
         expression evaluated.  More explicitly, a `return' statement
         may be used to exit the subroutine, optionally specifying
         the returned value, which will be evaluated in the
         appropriate context (list, scalar, or void) depending 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.
    
         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 the 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 separate package
         (and probably a separate file), see the Packages entry 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;
                     }
                 }
                 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-reference 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 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()' 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
         arugments 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' an empty
         list.  See the Pass by Reference entry elsewhere in this
         document 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 subroutine name or reference using the `&$subref()'
         or `&{$subref}()' constructs, although the `$subref->()'
         notation solves that problem.  See the perlref manpage 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 the Prototypes manpage below.
    
         Functions whose names are in all upper case are reserved to
         the Perl core, as are modules whose names are in all lower
         case.  A function 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.
         Functions that do special, pre-defined things include
         `BEGIN', `CHECK', `INIT', `END', `AUTOLOAD', and
         `DESTROY'--plus all functions mentioned in the perltie
         manpage.
    
         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
         experimental.  This feature should not be relied upon.  It
         may change or disappear in future releases of Perl.  See the
         attributes manpage.
    
         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'ize 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()'
         function 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 the perlref
         manpage.
    
         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 statement.  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.
    
         None of the foregoing text applies to `if/unless' or
         `while/until' modifiers appended to simple statements.  Such
         modifiers are not control structures and have no effect on
         scoping.
    
         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
         qualified 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
         principle usefulness of this is to quiet `use strict
         'vars'', but it is also essential for generation of closures
         as detailed in the perlref manpage.  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 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 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 identifiers 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 symbol table of some package to be
         found.  See the Function Templates entry in the perlref
         manpage 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 collection.
    
         Unlike local variables in C or C++, Perl's lexical variables
         don't necessarily get recycled 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 collection 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' sub around it to make sure it gets executed before
         your program starts to run:
    
             sub BEGIN {
                 my $secret_val = 0;
                 sub gimme_another {
                     return ++$secret_val;
                 }
             }
    
         See the Package Constructors and Destructors entry in the
         perlmod manpage about the special triggered functions,
         `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, filehandles and
         formats, and direct manipulation of the Perl symbol table
         itself.  Format variables often use `local' though, as do
         other variables whose current value must be visible to
         called subroutines.
    
         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
         enclosing block, `eval', or `do FILE'--and to any subroutine
         called from within that block.  A `local' just gives
         temporary 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.
    
         If more than one variable is given to `local', they must be
         placed in parentheses.  All listed elements must be legal
         lvalues.  This operator works by saving the current values
         of those variables in its argument 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
         initialize your local variables.  (If no initializer is
         given for a particular variable, it is created with an
         undefined value.)  Commonly this is used to name the
         parameters to a subroutine.  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 here
    
         Because `local' is a run-time operator, it gets executed
         each time through a loop.  In releases of Perl previous to
         5.0, this used more stack storage each time until the loop
         was exited.  Perl now reclaims the space each time through,
         but it's still more efficient to declare your variables
         outside the loop.
    
         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.
    
         A note about `local()' and composite types is in order.
         Something like `local(%foo)' works by temporarily placing a
         brand new hash in the 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
         symbol table (i.e. the hash entry in the `*foo' typeglob)
         for the duration of the dynamic scope within which the
         `local()' was seen.  This has the effect of allowing one to
         temporarily occlude any magic on composite types.  For
         instance, this will briefly alter a tied hash 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
         look like this:
    
             {
                 local %ENV;
                 tie %ENV, 'MyOwnEnv';
                 [..do your own fancy %ENV manipulation here..]
             }
             [..normal %ENV behavior here..]
    
         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 elements 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 {
                 $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.
    
         The current implementation does not allow arrays and hashes
         to be returned from lvalue subroutines directly.  You may
         return a reference instead.  This restriction may be lifted
         in future.
    
         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
         versions, 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 variables 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 values
         so that you can't extract out the individual arrays.  For
         more on typeglobs, see the Typeglobs and Filehandles entry
         in the perldata manpage.
    
         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'.
    
    especially $_.
         1. You need to give a global variable a temporary value,
             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.
    
    function.
         2. You need to create a local file or directory handle or a local
             A function that needs a filehandle of its own must use
             `local()' on a complete typeglob.   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 the Function Templates entry in the perlref manpage
             for more about manipulating functions by name in this
             way.
    
    hash.
         3. You want to temporarily change just one element of an array or
             You can `local'ize just one element of an aggregate.
             Usually this is done on dynamics:
    
                 {
                     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 the perlref manpage.
         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 prototyping.  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, @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 ()            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.
    
         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::qualify_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
         operator 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 prototype.
    
         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 visibility 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 intentionally 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.
    
         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 without `&'.  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 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 that was eligible for inlining,
         you'll get a mandatory warning.  (You can use this warning
         to tell whether or not a particular subroutine is considered
         constant.)  The warning is considered severe enough not to
         be optional because previously compiled invocations of the
         function will still be using the old value of the function.
         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--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 understands 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 $pat = shift;
                 my @got;
                 local *D;
                 if (opendir D, '.') {
                     @got = grep /$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' globally, 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.
    
         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.)  However, 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 original 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 the
         AutoLoader manpage, 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 parentheses:
    
             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 the AutoLoader manpage and in
         the AutoSplit manpage, the standard SelfLoader modules in
         the SelfLoader manpage, and the document on adding C
         functions to Perl code in the perlxs manpage.
    
         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 the
         attributes manpage 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-declaration, 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
         currently 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 the attributes manpage.
    
    
    

    SEE ALSO

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


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