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PERLMOD(1)                            Perl Programmers Reference Guide                            PERLMOD(1)



NAME
       perlmod - Perl modules (packages and symbol tables)

DESCRIPTION
   Packages
       Perl provides a mechanism for alternative namespaces to protect packages from stomping on each
       other's variables.  In fact, there's really no such thing as a global variable in Perl.  The package
       statement declares the compilation unit as being in the given namespace.  The scope of the package
       declaration is from the declaration itself through the end of the enclosing block, "eval", or file,
       whichever comes first (the same scope as the my() and local() operators).  Unqualified dynamic
       identifiers will be in this namespace, except for those few identifiers that if unqualified, default
       to the main package instead of the current one as described below.  A package statement affects only
       dynamic variables--including those you've used local() on--but not lexical variables created with
       my().  Typically it would be the first declaration in a file included by the "do", "require", or
       "use" operators.  You can switch into a package in more than one place; it merely influences which
       symbol table is used by the compiler for the rest of that block.  You can refer to variables and
       filehandles in other packages by prefixing the identifier with the package name and a double colon:
       $Package::Variable.  If the package name is null, the "main" package is assumed.  That is, $::sail is
       equivalent to $main::sail.

       The old package delimiter was a single quote, but double colon is now the preferred delimiter, in
       part because it's more readable to humans, and in 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 the single
       quote as separator, which was there to make Ada programmers feel like they knew what was going on.
       Because the old-fashioned syntax is still supported 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 of name lookups, however.  There are no relative packages: all symbols are either
       local to the current package, or must be fully qualified from the outer package name down.  For
       instance, there is nowhere within package "OUTER" that $INNER::var refers to $OUTER::INNER::var.
       "INNER" refers to a totally separate global package.

       Only identifiers starting with letters (or underscore) are stored in a package's symbol table.  All
       other symbols are kept in package "main", 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 you have a
       package called "m", "s", or "y", then you can't use the qualified form of an identifier because it
       would be instead interpreted as a pattern match, a substitution, or a transliteration.

       Variables beginning with underscore used to be forced into package main, but we decided it was more
       useful for package writers to be able to 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 also "Technical Note on the Syntax of Variable Names" in
       perlvar.

       "eval"ed strings are compiled in the package in which the eval() was compiled.  (Assignments to
       $SIG{}, however, assume the signal handler specified is in the "main" package.  Qualify the signal
       handler name if you wish to have a signal handler in a package.)  For an example, examine perldb.pl
       in the Perl library.  It initially switches to the "DB" package so that the debugger doesn't
       interfere with variables in the program you are trying to debug.  At various points, however, it
       temporarily switches back to the "main" package to evaluate various expressions in the context of the
       "main" package (or wherever you came from).  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 that name with two colons
       appended.  The main symbol table's name is thus %main::, or %:: for short.  Likewise the symbol table
       for the nested package mentioned earlier is named %OUTER::INNER::.

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

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

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

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

           *dick = *richard;

       causes variables, subroutines, formats, and file and directory handles accessible via the identifier
       "richard" also to be accessible via the identifier "dick".  If you want to alias only a particular
       variable or subroutine, 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 typeglobs refer 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 be restored when the block ends. Because variables are accessed through
       the typeglob, you can use "*foo = *bar" to create an alias which can be localized. (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 glob aliasing as the import/export
       mechanism. Whether or not you can properly localize a variable that has been exported from a module
       depends on how it 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 it by saying "*FOO = *Package::FOO" in your script.

       The "*x = \$y" mechanism may be used to pass and return cheap references into or from subroutines if
       you don't want to copy the whole thing.  It only works when assigning to dynamic variables, not
       lexicals.

           %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 the symbol table specified by the *some_hash
       typeglob.  This is a somewhat tricky way of passing around references cheaply when you don't want to
       have to remember to dereference variables explicitly.

       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 to optimization at compile-time.  A constant subroutine is one
       prototyped to take no arguments and to return a constant expression.  See perlsub for details on
       these.  The "use constant" pragma is a convenient shorthand for these.

       You can say *foo{PACKAGE} and *foo{NAME} to find out what name and package the *foo symbol table
       entry comes from.  This may be useful in 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 the individual elements of *foo.
       See perlref.

       Subroutine definitions (and declarations, for that matter) need not necessarily be situated in the
       package whose symbol table they occupy.  You can define a subroutine outside its package by
       explicitly 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 is lexically in the main package, not in
       Some_package. So something 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 at the 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 a subroutine (although this is
       not considered good style).  One should note that these code blocks don't really exist as named
       subroutines (despite their appearance). The thing that gives this away is the fact that you can have
       more than one of these code blocks in a program, and they will get all executed at the appropriate
       moment.  So you can't execute any of these code blocks by name.

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

       An "END" code block is executed as late as possible, that is, after perl has finished running the
       program and just before the interpreter is 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", or being 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--they will execute in reverse order of definition; that is: last in, first out (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 string "eval()": if any "END" code
       blocks are created in a string "eval()", they will be executed just as any other "END" code block of
       that package in LIFO order just before the interpreter is being exited.

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

       "UNITCHECK", "CHECK" and "INIT" code blocks are useful to catch the transition between the
       compilation phase and the execution phase of the main program.

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

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

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

       The "CHECK" and "INIT" code blocks will not be executed inside a string eval(), if that eval()
       happens after the end of the main compilation phase; that can be a problem in mod_perl and other
       persistent environments which use "eval STRING" to load code at runtime.

       When you use the -n and -p switches to Perl, "BEGIN" and "END" work just as they do in awk, as a
       degenerate case.  Both "BEGIN" and "CHECK" blocks are run when you use the -c switch for a compile-only compileonly
       only syntax check, although your main code is 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 act as a class if it provides subroutines
       to act as methods.  Such a package may also derive some of its methods from another class (package)
       by listing the other package name(s) in its global @ISA array (which must be a package global, not a
       lexical).

       For more on this, see perltoot 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 specifically designed to be reusable by other modules or programs.  It may
       do this by providing a mechanism for exporting some of its symbols into the symbol table of any
       package using it, or it may function as a class definition and make its semantics available
       implicitly through method calls on the class and its objects, without explicitly exporting 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 {
               use Exporter   ();
               our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);

               # set the version for version checking
               $VERSION     = 1.00;
               # if using RCS/CVS, this may be preferred
               $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;

               @ISA         = qw(Exporter);
               @EXPORT      = qw(&func1 &func2 &func4);
               %EXPORT_TAGS = ( );     # eg: TAG => [ qw!name1 name2! ],

               # your exported package globals go here,
               # as well as any optionally exported functions
               @EXPORT_OK   = qw($Var1 %Hashit &func3);
           }
           our @EXPORT_OK;

           # exported package globals go here
           our $Var1;
           our %Hashit;

           # non-exported package globals go here
           our @more;
           our $stuff;

           # initialize package globals, first exported ones
           $Var1   = '';
           %Hashit = ();

           # then the others (which are still accessible as $Some::Module::stuff)
           $stuff  = '';
           @more   = ();

           # all file-scoped lexicals must be created before
           # the functions below that use them.

           # file-private lexicals go here
           my $priv_var    = '';
           my %secret_hash = ();

           # here's a file-private function as a closure,
           # callable as &$priv_func;  it cannot be prototyped.
           my $priv_func = sub {
               # stuff goes here.
           };

           # make all your functions, whether exported or not;
           # remember to put something interesting in the {} stubs
           sub func1      {}    # no prototype
           sub func2()    {}    # proto'd void
           sub func3($$)  {}    # proto'd to 2 scalars

           # this one isn't exported, but could be called!
           sub func4(\%)  {}    # proto'd to 1 hash ref

           END { }       # module clean-up code here (global destructor)

           ## YOUR CODE GOES HERE

           1;  # don't forget to return a true value from the file

       Then go on to declare and use your variables in functions without any qualifications.  See Exporter
       and the perlmodlib for details on mechanics and style issues in module creation.

       Perl modules are included into your program by saying

           use Module;

       or

           use Module LIST;

       This is exactly equivalent to

           BEGIN { require Module; import Module; }

       or

           BEGIN { require Module; import Module LIST; }

       As a special case

           use Module ();

       is exactly equivalent to

           BEGIN { require Module; }

       All Perl module files have the extension .pm.  The "use" operator assumes 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're functioning 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 double colons in the module name, such as
       "Some::Module", are translated into your system's directory separator, usually "/".   The second case
       does not, and would have to be specified literally.  The other difference is that seeing the first
       "require" clues in the compiler that uses of indirect object notation involving "SomeModule", as in
       "$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 importing of semantics happens as soon as
       the "use" statement is compiled, before the rest of the file is compiled.  This is how it is able to
       function as a pragma mechanism, and also how modules are able to declare subroutines that are then
       visible as list or unary operators for the rest of the current file.  This will not work if you use
       "require" instead 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 the middle of your program's execution.  An exception would be
       if two modules each tried to "use" each other, and each also called a function from that other
       module.  In that case, it's easy to use "require" instead.

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

       Perl modules always have a .pm file, but there may also be dynamically linked executables (often
       ending in .so) or autoloaded subroutine definitions (often ending in .al) associated with the module.
       If so, these will be entirely transparent to the user of the 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 and autoloading, 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 called interpreter threads (ithreads).
       These threads can be used explicitly and implicitly.

       Ithreads work by cloning the data tree so that no data is shared between different threads. These
       threads can be used by using the "threads" module or by doing fork() on win32 (fake fork() support).
       When a thread is cloned all Perl data is cloned, however non-Perl data cannot be cloned
       automatically.  Perl after 5.7.2 has support for the "CLONE" special subroutine.  In "CLONE" you can
       do whatever you 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 it defined (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 with no parameters other than the invocant package name, but code should
       not assume that this will remain unchanged, as it is likely that in future extra parameters will 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 per package. 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 is called just before cloning starts, and in the context of the
       parent thread. If it returns a true value, then no objects of that class will be cloned; or rather,
       they will be copied as unblessed, undef values.  For example: if in the parent there are two
       references to a single blessed hash, then in the child there will be two references to a single
       undefined scalar value instead.  This provides a simple mechanism for making a module threadsafe;
       just add "sub CLONE_SKIP { 1 }" at the top of the class, and "DESTROY()" will be now only be called
       once per object. Of course, if the child thread needs to make use of the objects, then a more
       sophisticated approach is needed.

       Like "CLONE", "CLONE_SKIP" is currently called with no parameters other than the invocant package
       name, although that may change. Similarly, to allow for future expansion, the return value should be
       a single 0 or 1 value.

SEE ALSO
       See perlmodlib for general style issues related to building Perl modules and classes, as well as
       descriptions of the standard library and CPAN, Exporter for how Perl's standard import/export
       mechanism works, perltoot and perltooc for an in-depth tutorial on creating classes, perlobj for a
       hard-core reference document on objects, perlsub for an explanation of functions and scoping, and
       perlxstut and perlguts for more information on writing extension modules.



perl v5.12.5                                     2012-11-03                                       PERLMOD(1)