perlmod - Perl modules (packages and symbol tables)

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, CWeval, 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 variablesincluding those you've used local() onbut not lexical variables created with my(). Typically it would be the first declaration in a file included by the CWdo, CWrequire, or CWuse 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: CW$Package::Variable. If the package name is null, the CWmain package is assumed. That is, CW$::sail is equivalent to CW$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 programmers feel like they know what's going onas 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 CW"This is $owner's house", you'll be accessing CW$owner::s; that is, the CW$s variable in package CWowner, which is probably not what you meant. Use braces to disambiguate, as in CW"This is ${owner}'s house".

Packages may themselves contain package separators, as in CW$OUTER::INNER::var. This implies nothing about the order of name lookups, however. There are no relative packages: all symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere within package CWOUTER that CW$INNER::var refers to CW$OUTER::INNER::var. CWINNER 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 CWmain, including all punctuation variables, like CW$_. In addition, when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are forced to be in package CWmain, even when used for other purposes than their built-in ones. If you have a package called CWm, CWs, or CWy, 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 CW_, such as CW$_ and CWsub _, are still forced into the package CWmain. See also Technical Note on the Syntax of Variable Names in perlvar.

CWevaled strings are compiled in the package in which the eval() was compiled. (Assignments to CW$SIG{}, however, assume the signal handler specified is in the CWmain 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 CWDB 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 CWmain package to evaluate various expressions in the context of the CWmain package (or wherever you came from). See perldebug.

The special symbol CW__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.

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 CW%main::, or CW%:: for short. Likewise the symbol table for the nested package mentioned earlier is named CW%OUTER::INNER::.

The value in each entry of the hash is what you are referring to when you use the CW*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 CWlocal $main::foo = $main::bar. The former is accessing the hash CW%main::, which is the symbol table of package CWmain. The latter is simply assigning scalar CW$bar in package CWmain to scalar CW$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 CWrichard also to be accessible via the identifier CWdick. If you want to alias only a particular variable or subroutine, assign a reference instead:

    *dick = \$richard;

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

There is one subtle difference between the following statements:

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

CW*foo = *bar makes the typeglobs themselves synonymous while CW*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 CW$foo holds a reference to the original CW$bar the one that was stuffed away by CWlocal() and which will be restored when the block ends. Because variables are accessed through the typeglob, you can use CW*foo = *bar to create an alias which can be localized. (But be aware that this means you can't have a separate CW@foo and CW@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 (CW$Package::FOO) where you need a local value, or by overriding it by saying CW*FOO = *Package::FOO in your script.

The CW*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 CW$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 CWuse constant pragma is a convenient shorthand for these.

You can say CW*foo{PACKAGE} and CW*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 CW*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).

Four specially named code blocks are executed at the beginning and at the end of a running Perl program. These are the CWBEGIN, CWCHECK, CWINIT, and CWEND blocks.

These code blocks can be prefixed with CWsub 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 CWBEGIN 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 CWBEGIN blocks within a file (or eval'ed string) they will execute in order of definition. Because a CWBEGIN 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 CWBEGIN has run, it is immediately undefined and any code it used is returned to Perl's memory pool.

It should be noted that CWBEGIN code blocks are executed inside string CWeval()'s. The CWCHECK and CWINIT code blocks are not executed inside a string eval, which e.g. can be a problem in a mod_perl environment.

An CWEND 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 polymorphing into another program via CWexec, or being blown out of the water by a signalyou have to trap that yourself (if you can).) You may have multiple CWEND blocks within a filethey will execute in reverse order of definition; that is: last in, first out (LIFO). CWEND blocks are not executed when you run perl with the CW-c switch, or if compilation fails.

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

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

CWCHECK and CWINIT code blocks are useful to catch the transition between the compilation phase and the execution phase of the main program.

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

CWINIT blocks are run just before the Perl runtime begins execution, in first in, first out (FIFO) order. For example, the code generators documented in perlcc make use of CWINIT blocks to initialize and resolve pointers to XSUBs.

When you use the -n and -p switches to Perl, CWBEGIN and CWEND work just as they do in awk, as a degenerate case. Both CWBEGIN and CWCHECK 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         " 8. Ordinary code runs at runtime.\n";

  END { print   "14.   So this is the end of the tale.\n" }
  INIT { print  " 5. INIT blocks run FIFO just before runtime.\n" }
  CHECK { print " 4.   So this is the fourth line.\n" }

  print         " 9.   It runs in order, of course.\n";

  BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
  END { print   "13.   Read perlmod for the rest of the story.\n" }
  CHECK { print " 3. CHECK blocks run LIFO at compilation's end.\n" }
  INIT { print  " 6.   Run this again, using Perl's -c switch.\n" }

  print         "10.   This is anti-obfuscated code.\n";

  END { print   "12. END blocks run LIFO at quitting time.\n" }
  BEGIN { print " 2.   So this line comes out second.\n" }
  INIT { print  " 7.   You'll see the difference right away.\n" }

  print         "11.   It merely _looks_ like it should be confusing.\n";

  __END__

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 CW@ISA array (which must be a package global, not a lexical).

For more on this, see perltoot and perlobj.

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 CWuse 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 CWSome::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 CWrequire clues in the compiler that uses of indirect object notation involving SomeModule, as in CW$ob = purge SomeModule, are method calls, not function calls. (Yes, this really can make a difference.)

Because the CWuse statement implies a CWBEGIN block, the importing of semantics happens as soon as the CWuse 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 CWrequire instead of CWuse. With CWrequire 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, CWuse Module () is recommended over CWrequire 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 CWuse each other, and each also called a function from that other module. In that case, it's easy to use CWrequire instead.

Perl packages may be nested inside other package names, so we can have package names containing CW::. 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, CWText::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 CWuse POSIX to get it all.

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 CWthreads 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 CWCLONE and CWCLONE_SKIP special subroutines. In CWCLONE you can do whatever you need to do, like for example handle the cloning of non-Perl data, if necessary. CWCLONE 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().

Like CWCLONE, CWCLONE_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. This provides a simple mechanism for making a module threadsafe; just add CWsub CLONE_SKIP { 1 } at the top of the class, and CWDESTROY() 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 CWCLONE, CWCLONE_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 CW0 or CW1 value.

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.