perlmod - Perl modules (packages and symbol tables)


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   This doc
     Perl's packages, namespaces, and some info on classes.

     Tutorial on making a new module.

     Best practices for making a new module.

   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
   "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 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 library and the CPAN
   module Devel::Symdump make use of this.

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

   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

       %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";



       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

   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").

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

   "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, run-time 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 of
   the interpreter.  They can be created and executed during any phase.

   "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.

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

   "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

   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 the main compilation phase; that can be a problem in mod_perl and
   other persistent environments which use those functions to load code at

   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:


     # 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" }
       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" }
      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 only _looks_ like it should be confusing.\n";


   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 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 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/ and start with this template:

       package Some::Module;  # assumes Some/

       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 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;


       use Module LIST;

   This is exactly equivalent to

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


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

   As a special case

       use Module ();

   is exactly equivalent to

       BEGIN { require ''; }

   All Perl module files have the extension .pm.  The "use" operator
   assumes this so you don't have to spell out "" 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 "";

   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"

   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/

   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
   Perl supports a 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 5.8.0 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

   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 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


   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, perlootut and perlobj for in-depth information 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.


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