Perl provides a mechanism for alternative namespaces to protectpackages from stomping on each other's variables. In fact, there'sreally no such thing as a global variable in Perl. The packagestatement declares the compilation unit as being in the givennamespace. The scope of the package declaration is from thedeclaration itself through the end of the enclosing block, eval,or file, whichever comes first (the same scope as the my() andlocal() operators). Unqualified dynamic identifiers will be inthis namespace, except for those few identifiers that if unqualified,default to the main package instead of the current one as describedbelow. A package statement affects only dynamic variables--includingthose you've used local() on--but not lexical variables createdwith my(). Typically it would be the first declaration in a fileincluded by the do, require, or use operators. You canswitch into a package in more than one place; it merely influenceswhich symbol table is used by the compiler for the rest of thatblock. You can refer to variables and filehandles in other packagesby prefixing the identifier with the package name and a doublecolon: $Package::Variable. If the package name is null, themain package is assumed. That is, $::sail is equivalent to$main::sail.

The old package delimiter was a single quote, but double colon is now thepreferred delimiter, in part because it's more readable to humans, andin part because it's more readable to emacs macros. It also makes C++programmers feel like they know what's going on--as opposed to using thesingle quote as separator, which was there to make Ada programmers feellike they knew what was going on. Because the old-fashioned syntax is stillsupported for backwards compatibility, if you try to use a string like"This is $owner's house", you'll be accessing $owner::s; that is,the $s variable in package owner, which is probably not what you meant.Use braces to disambiguate, as in "This is ${owner}'s house".

Packages may themselves contain package separators, as in$OUTER::INNER::var. This implies nothing about the order ofname lookups, however. There are no relative packages: all symbolsare either local to the current package, or must be fully qualifiedfrom the outer package name down. For instance, there is nowherewithin package OUTER that $INNER::var refers to$OUTER::INNER::var. INNER refers to a totallyseparate global package.

Only identifiers starting with letters (or underscore) are storedin a package's symbol table. All other symbols are kept in packagemain, including all punctuation variables, like $_. In addition,when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,ARGVOUT, ENV, INC, and SIG are forced to be in package main,even when used for other purposes than their built-in ones. If youhave a package called m, s, or y, then you can't use thequalified form of an identifier because it would be instead interpretedas a pattern match, a substitution, or a transliteration.

Variables beginning with underscore used to be forced into packagemain, but we decided it was more useful for package writers to be ableto use leading underscore to indicate private variables and method names.However, variables and functions named with a single _, such as$_ and sub _, are still forced into the package main. See alsoThe Syntax of Variable Names in perlvar.

evaled strings are compiled in the package in which the eval() wascompiled. (Assignments to $SIG{}, however, assume the signalhandler specified is in the main package. Qualify the signal handlername if you wish to have a signal handler in a package.) For anexample, examine perldb.pl in the Perl library. It initially switchesto the DB package so that the debugger doesn't interfere with variablesin the program you are trying to debug. At various points, however, ittemporarily switches back to the main package to evaluate variousexpressions in the context of the main package (or wherever you camefrom). See perldebug.

The special symbol __PACKAGE__ contains the current package, but cannot(easily) be used to construct variable names.

See perlsub for other scoping issues related to my() and local(),and perlref regarding closures.

Symbol Tables

The symbol table for a package happens to be stored in the hash of thatname with two colons appended. The main symbol table's name is thus%main::, or %:: for short. Likewise the symbol table for the nestedpackage mentioned earlier is named %OUTER::INNER::.

The value in each entry of the hash is what you are referring to when youuse the *name typeglob notation.

 local *main::foo = *main::bar;

You can use this to print out all the variables in a package, forinstance. The standard but antiquated dumpvar.pl library andthe CPAN module Devel::Symdump make use of this.

The results of creating new symbol table entries directly or modifying anyentries that are not already typeglobs are undefined and subject to changebetween releases of perl.

Assignment to a typeglob performs an aliasing operation, i.e.,

 *dick = *richard;

causes variables, subroutines, formats, and file and directory handlesaccessible via the identifier richard also to be accessible via theidentifier dick. If you want to alias only a particular variable orsubroutine, assign a reference instead:

 *dick = \$richard;

Which makes $richard and $dick the same variable, but leaves@richard and @dick as separate arrays. Tricky, eh?

There is one subtle difference between the following statements:

 *foo = *bar;
  *foo = \$bar;

*foo = *bar makes the typeglobs themselves synonymous while*foo = \$bar makes the SCALAR portions of two distinct typeglobsrefer to the same scalar value. This means that the following code:

 $bar = 1;
  *foo = \$bar;   # Make $foo an alias for $bar
 
  {
  local $bar = 2; # Restrict changes to block
  print $foo; # Prints '1'!
  }

Would print '1', because $foo holds a reference to the original$bar. The one that was stuffed away by local() and which will berestored when the block ends. Because variables are accessed through thetypeglob, you can use *foo = *bar to create an alias which can belocalized. (But be aware that this means you can't have a separate@foo and @bar, etc.)

What makes all of this important is that the Exporter module uses globaliasing as the import/export mechanism. Whether or not you can properlylocalize a variable that has been exported from a module depends on howit was exported:

 @EXPORT = qw($FOO); # Usual form, can't be localized
  @EXPORT = qw(*FOO); # Can be localized

You can work around the first case by using the fully qualified name($Package::FOO) where you need a local value, or by overriding itby saying *FOO = *Package::FOO in your script.

The *x = \$y mechanism may be used to pass and return cheap referencesinto or from subroutines if you don't want to copy the wholething. It only works when assigning to dynamic variables, notlexicals.

 %some_hash = ();# can't be my()
  *some_hash = fn( \%another_hash );
  sub fn {
 local *hashsym = shift;
 # now use %hashsym normally, and you
 # will affect the caller's %another_hash
 my %nhash = (); # do what you want
 return \%nhash;
  }

On return, the reference will overwrite the hash slot in thesymbol table specified by the *some_hash typeglob. Thisis a somewhat tricky way of passing around references cheaplywhen you don't want to have to remember to dereference variablesexplicitly.

Another use of symbol tables is for making "constant" scalars.

 *PI = \3.14159265358979;

Now you cannot alter $PI, which is probably a good thing all in all.This isn't the same as a constant subroutine, which is subject tooptimization at compile-time. A constant subroutine is one prototypedto take no arguments and to return a constant expression. Seeperlsub for details on these. The use constant pragma is aconvenient shorthand for these.

You can say *foo{PACKAGE} and *foo{NAME} to find out what name andpackage the *foo symbol table entry comes from. This may be usefulin a subroutine that gets passed typeglobs as arguments:

 sub identify_typeglob {
  my $glob = shift;
  print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
  }
  identify_typeglob *foo;
  identify_typeglob *bar::baz;

This prints

 You gave me main::foo
  You gave me bar::baz

The *foo{THING} notation can also be used to obtain references to theindividual elements of *foo. See perlref.

Subroutine definitions (and declarations, for that matter) neednot necessarily be situated in the package whose symbol table theyoccupy. You can define a subroutine outside its package byexplicitly qualifying the name of the subroutine:

 package main;
  sub Some_package::foo { ... }   # &foo defined in Some_package

This is just a shorthand for a typeglob assignment at compile time:

 BEGIN { *Some_package::foo = sub { ... } }

and is not the same as writing:

 {
 package Some_package;
 sub foo { ... }
  }

In the first two versions, the body of the subroutine islexically in the main package, not in Some_package. Sosomething like this:

 package main;
 
  $Some_package::name = "fred";
  $main::name = "barney";
 
  sub Some_package::foo {
 print "in ", __PACKAGE__, ": \$name is '$name'\n";
  }
 
  Some_package::foo();

prints:

 in main: $name is 'barney'

rather than:

 in Some_package: $name is 'fred'

This also has implications for the use of the SUPER:: qualifier(see perlobj).

BEGIN, UNITCHECK, CHECK, INIT and END

Five specially named code blocks are executed at the beginning and atthe end of a running Perl program. These are the BEGIN,UNITCHECK, CHECK, INIT, and END blocks.

These code blocks can be prefixed with sub to give the appearance of asubroutine (although this is not considered good style). One should notethat these code blocks don't really exist as named subroutines (despitetheir appearance). The thing that gives this away is the fact that you canhave more than one of these code blocks in a program, and they will getall executed at the appropriate moment. So you can't execute any ofthese code blocks by name.

A BEGIN code block is executed as soon as possible, that is, the momentit is completely defined, even before the rest of the containing file (orstring) is parsed. You may have multiple BEGIN blocks within a file (oreval'ed string); they will execute in order of definition. Because a BEGINcode block executes immediately, it can pull in definitions of subroutinesand such from other files in time to be visible to the rest of the compileand run time. Once a BEGIN has run, it is immediately undefined and anycode it used is returned to Perl's memory pool.

An END code block is executed as late as possible, that is, afterperl has finished running the program and just before the interpreteris being exited, even if it is exiting as a result of a die() function.(But not if it's morphing into another program via exec, orbeing blown out of the water by a signal--you have to trap that yourself(if you can).) You may have multiple END blocks within a file--theywill execute in reverse order of definition; that is: last in, firstout (LIFO). END blocks are not executed when you run perl with the-c switch, or if compilation fails.

Note that END code blocks are not executed at the end of a stringeval(): if any END code blocks are created in a string eval(),they will be executed just as any other END code block of that packagein LIFO order just before the interpreter is being exited.

Inside an END code block, $? contains the value that the program isgoing to pass to exit(). You can modify $? to change the exitvalue of the program. Beware of changing $? by accident (e.g. byrunning something via system).

Inside of a END block, the value of ${^GLOBAL_PHASE} will be"END".

UNITCHECK, CHECK and INIT code blocks are useful to catch thetransition between the compilation phase and the execution phase ofthe main program.

UNITCHECK blocks are run just after the unit which defined them hasbeen compiled. The main program file and each module it loads arecompilation units, as are string evals, code compiled using the(?{ }) construct in a regex, calls to do FILE, require FILE,and code after the -e switch on the command line.

BEGIN and UNITCHECK blocks are not directly related to the phase ofthe interpreter. They can be created and executed during any phase.

CHECK code blocks are run just after the initial Perl compile phase endsand before the run time begins, in LIFO order. CHECK code blocks are usedin the Perl compiler suite to save the compiled state of the program.

Inside of a CHECK block, the value of ${^GLOBAL_PHASE} will be"CHECK".

INIT blocks are run just before the Perl runtime begins execution, in"first in, first out" (FIFO) order.

Inside of an INIT block, the value of ${^GLOBAL_PHASE} will be "INIT".

The CHECK and INIT blocks in code compiled by require, string do,or string eval will not be executed if they occur after the end of themain compilation phase; that can be a problem in mod_perl and other persistentenvironments which use those functions to load code at runtime.

When you use the -n and -p switches to Perl, BEGIN andEND work just as they do in awk, as a degenerate case.Both BEGIN and CHECK blocks are run when you use the -cswitch for a compile-only syntax check, although your main codeis not.

The begincheck program makes it all clear, eventually:

  #!/usr/bin/perl
 
   # begincheck
 
   print "10. Ordinary code runs at runtime.\n";
 
   END { print   "16.   So this is the end of the tale.\n" }
   INIT { print  " 7. INIT blocks run FIFO just before runtime.\n" }
   UNITCHECK {
  print   " 4.   And therefore before any CHECK blocks.\n"
   }
   CHECK { print " 6.   So this is the sixth line.\n" }
 
   print "11.   It runs in order, of course.\n";
 
   BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
   END { print   "15.   Read perlmod for the rest of the story.\n" }
   CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
   INIT { print  " 8.   Run this again, using Perl's -c switch.\n" }
 
   print "12.   This is anti-obfuscated code.\n";
 
   END { print   "14. END blocks run LIFO at quitting time.\n" }
   BEGIN { print " 2.   So this line comes out second.\n" }
   UNITCHECK {
    print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
   }
   INIT { print  " 9.   You'll see the difference right away.\n" }
 
   print "13.   It merely _looks_ like it should be confusing.\n";
 
   __END__

Perl Classes

There is no special class syntax in Perl, but a package may actas a class if it provides subroutines to act as methods. Such apackage may also derive some of its methods from another class (package)by listing the other package name(s) in its global @ISA array (whichmust be a package global, not a lexical).

For more on this, see perlootut and perlobj.

Perl Modules

A module is just a set of related functions in a library file, i.e.,a Perl package with the same name as the file. It is specificallydesigned to be reusable by other modules or programs. It may do thisby providing a mechanism for exporting some of its symbols into thesymbol table of any package using it, or it may function as a classdefinition and make its semantics available implicitly throughmethod calls on the class and its objects, without explicitlyexporting anything. Or it can do a little of both.

For example, to start a traditional, non-OO module called Some::Module,create a file called Some/Module.pm and start with this template:

 package Some::Module;  # assumes Some/Module.pm
 
  use strict;
  use warnings;
 
  BEGIN {
  require Exporter;
 
  # set the version for version checking
  our $VERSION = 1.00;
 
  # Inherit from Exporter to export functions and variables
  our @ISA = qw(Exporter);
 
  # Functions and variables which are exported by default
  our @EXPORT  = qw(func1 func2);
 
  # Functions and variables which can be optionally exported
  our @EXPORT_OK   = qw($Var1 %Hashit func3);
  }
 
  # exported package globals go here
  our $Var1 = '';
  our %Hashit  = ();
 
  # non-exported package globals go here
  # (they are still accessible as $Some::Module::stuff)
  our @more = ();
  our $stuff   = '';
 
  # file-private lexicals go here, before any functions which use them
  my $priv_var = '';
  my %secret_hash = ();
 
  # here's a file-private function as a closure,
  # callable as $priv_func->();
  my $priv_func = sub {
  ...
  };
 
  # make all your functions, whether exported or not;
  # remember to put something interesting in the {} stubs
  sub func1  { ... }
  sub func2  { ... }
 
  # this one isn't exported, but could be called directly
  # as Some::Module::func3()
  sub func3  { ... }
 
  END { ... }   # module clean-up code here (global destructor)
 
  1;  # don't forget to return a true value from the file

Then go on to declare and use your variables in functions withoutany qualifications. See Exporter and the perlmodlib fordetails on mechanics and style issues in module creation.

Perl modules are included into your program by saying

 use Module;

 use Module LIST;

This is exactly equivalent to

 BEGIN { require 'Module.pm'; 'Module'->import; }

 BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }

As a special case

 use Module ();

is exactly equivalent to

 BEGIN { require 'Module.pm'; }

All Perl module files have the extension .pm. The use operatorassumes this so you don't have to spell out "Module.pm" in quotes.This also helps to differentiate new modules from old .pl and.ph files. Module names are also capitalized unless they'refunctioning as pragmas; pragmas are in effect compiler directives,and are sometimes called "pragmatic modules" (or even "pragmata"if you're a classicist).

The two statements:

 require SomeModule;
  require "SomeModule.pm";

differ from each other in two ways. In the first case, any doublecolons in the module name, such as Some::Module, are translatedinto your system's directory separator, usually "/". The secondcase does not, and would have to be specified literally. The otherdifference is that seeing the first require clues in the compilerthat uses of indirect object notation involving "SomeModule", asin $ob = purge SomeModule, are method calls, not function calls.(Yes, this really can make a difference.)

Because the use statement implies a BEGIN block, the importingof semantics happens as soon as the use statement is compiled,before the rest of the file is compiled. This is how it is ableto function as a pragma mechanism, and also how modules are able todeclare subroutines that are then visible as list or unary operators forthe rest of the current file. This will not work if you use requireinstead of use. With require you can get into this problem:

 require Cwd;# make Cwd:: accessible
  $here = Cwd::getcwd();
 
  use Cwd;# import names from Cwd::
  $here = getcwd();
 
  require Cwd; # make Cwd:: accessible
  $here = getcwd(); # oops! no main::getcwd()

In general, use Module () is recommended over require Module,because it determines module availability at compile time, not in themiddle of your program's execution. An exception would be if two moduleseach tried to use each other, and each also called a function fromthat other module. In that case, it's easy to use require instead.

Perl packages may be nested inside other package names, so we can havepackage names containing ::. But if we used that package namedirectly as a filename it would make for unwieldy or impossiblefilenames on some systems. Therefore, if a module's name is, say,Text::Soundex, then its definition is actually found in the libraryfile Text/Soundex.pm.

Perl modules always have a .pm file, but there may also bedynamically linked executables (often ending in .so) or autoloadedsubroutine definitions (often ending in .al) associated with themodule. If so, these will be entirely transparent to the user ofthe module. It is the responsibility of the .pm file to load(or arrange to autoload) any additional functionality. For example,although the POSIX module happens to do both dynamic loading andautoloading, the user can say just use POSIX to get it all.

Making your module threadsafe

Since 5.6.0, Perl has had support for a new type of threads calledinterpreter threads (ithreads). These threads can be used explicitlyand implicitly.

Ithreads work by cloning the data tree so that no data is sharedbetween different threads. These threads can be used by using the threadsmodule or by doing fork() on win32 (fake fork() support). When athread is cloned all Perl data is cloned, however non-Perl data cannotbe cloned automatically. Perl after 5.7.2 has support for the CLONEspecial subroutine. In CLONE you can do whateveryou need to do,like for example handle the cloning of non-Perl data, if necessary.CLONE will be called once as a class method for every package that has itdefined (or inherits it). It will be called in the context of the new thread,so all modifications are made in the new area. Currently CLONE is called withno parameters other than the invocant package name, but code should not assumethat this will remain unchanged, as it is likely that in future extra parameterswill be passed in to give more information about the state of cloning.

If you want to CLONE all objects you will need to keep track of them perpackage. This is simply done using a hash and Scalar::Util::weaken().

Perl after 5.8.7 has support for the CLONE_SKIP special subroutine.Like CLONE, CLONE_SKIP is called once per package; however, it iscalled just before cloning starts, and in the context of the parentthread. If it returns a true value, then no objects of that class willbe cloned; or rather, they will be copied as unblessed, undef values.For example: if in the parent there are two references to a single blessedhash, then in the child there will be two references to a single undefinedscalar value instead.This provides a simple mechanism for making a module threadsafe; just addsub CLONE_SKIP { 1 } at the top of the class, and DESTROY() willnow only be called once per object. Of course, if the child thread needsto make use of the objects, then a more sophisticated approach isneeded.

Like CLONE, CLONE_SKIP is currently called with no parameters otherthan the invocant package name, although that may change. Similarly, toallow for future expansion, the return value should be a single 0 or1 value.

Perl modules (packages and symbol tables)

Daftar Isi

NAME

DESCRIPTION

Packages

Symbol Tables

BEGIN, UNITCHECK, CHECK, INIT and END

Perl Classes

Perl Modules

Making your module threadsafe

SEE ALSO