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RedHat 9 (Linux i386) - man page for perlmod (redhat section 1)

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 names-
       pace.  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 vari-
       ables--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 delim-
       iter, 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's 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 prob-
       ably 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.  It would treat package "INNER" as 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 one.  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 pri-
       vate 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 "Techni-
       cal Note on the Syntax of Variable Names" in perlvar.

       "eval"ed strings are compiled in the package in which the eval() was compiled.  (Assign-
       ments 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 try-
       ing 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 variables.

       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.  Like-
       wise 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.  In fact, the following have the same effect, though the first is more
       efficient because it does the symbol table lookups at compile time:

	   local *main::foo    = *main::bar;
	   local $main::{foo}  = $main::{bar};

       (Be sure to note the vast difference between the second line above and "local $main::foo =
       $main::bar". The former is accessing the hash %main::, which is the symbol table of pack-
       age "main". The latter is simply assigning scalar $bar in package "main" to scalar $foo of
       the same package.)

       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 sepa-
       rate 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 vari-
       ables 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 sub-
       routines 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 situ-
       ated 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).

       Package Constructors and Destructors

       Four special subroutines act as package constructors and destructors.  These are the
       "BEGIN", "CHECK", "INIT", and "END" routines.  The "sub" is optional for these routines.

       A "BEGIN" subroutine is executed as soon as possible, that is, the moment it is completely
       defined, even before the rest of the containing file is parsed.	You may have multiple
       "BEGIN" blocks within a file--they will execute in order of definition.	Because a "BEGIN"
       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 file.  Once a "BEGIN" has run, it is imme-
       diately undefined and any code it used is returned to Perl's memory pool.  This means you
       can't ever explicitly call a "BEGIN".

       An "END" subroutine is executed as late as possible, that is, after perl has finished run-
       ning 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 polymorphing 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.

       Inside an "END" subroutine, $? 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").

       Similar to "BEGIN" blocks, "INIT" blocks are run just before the Perl runtime begins exe-
       cution, in "first in, first out" (FIFO) order.  For example, the code generators docu-
       mented in perlcc make use of "INIT" blocks to initialize and resolve pointers to XSUBs.

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

       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.

       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
	       $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker

	       @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 mod-
       ules from old .pl and .ph files.  Module names are also capitalized unless they're func-
       tioning 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, usu-
       ally "/".   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"s instead.

       Perl packages may be nested inside other package names, so we can have package names con-
       taining "::".  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) associ-
       ated with the module.  If so, these will be entirely transparent to the user of the mod-
       ule.  It is the responsibility of the .pm file to load (or arrange to autoload) any addi-
       tional 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 has since 5.6.0 support for a new type of threads called interpreter threads. 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 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 executed once 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.

       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().

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 v5.8.0				    2003-02-18				       PERLMOD(1)


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