


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



perl v5.10.1                2009-10-12                          1





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


       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



perl v5.10.1                2009-10-12                          2





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


       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



perl v5.10.1                2009-10-12                          3





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


       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





perl v5.10.1                2009-10-12                          4





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


           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).




perl v5.10.1                2009-10-12                          5





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


   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



perl v5.10.1                2009-10-12                          6





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


       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 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" }



perl v5.10.1                2009-10-12                          7





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


         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: 1.12 $ =~ /(\d+)/g;

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




perl v5.10.1                2009-10-12                          8





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


               # 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 v5.10.1                2009-10-12                          9





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


       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



perl v5.10.1                2009-10-12                         10





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


       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



perl v5.10.1                2009-10-12                         11





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


       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 v5.10.1                2009-10-12                         12


