PERLMOD(1) Perl Programmers Reference Guide PERLMOD(1)
NAME
perlmod - Perl modules (packages and symbol tables)
DESCRIPTION
Packages
Perl provides a mechanism for alternative namespaces to protect
packages from stomping on each other's variables. In fact, there's
really no such thing as a global variable in Perl. The package
statement declares the compilation unit as being in the given
namespace. The scope of the package declaration is from the
declaration itself through the end of the enclosing block, "eval", or
file, whichever comes first (the same scope as the my() and local()
operators). Unqualified dynamic identifiers will be in this namespace,
except for those few identifiers that if unqualified, default to the
main package instead of the current one as described below. A package
statement affects only dynamic variables--including those you've used
local() on--but not lexical variables created with my(). Typically it
would be the first declaration in a file included by the "do",
"require", or "use" operators. You can switch into a package in more
than one place; it merely influences which symbol table is used by the
compiler for the rest of that block. You can refer to variables and
filehandles in other packages by prefixing the identifier with the
package name and a double colon: $Package::Variable. If the package
name is null, the "main" package is assumed. That is, $::sail is
equivalent to $main::sail.
The old package delimiter was a single quote, but double colon is now
the preferred delimiter, in part because it's more readable to humans,
and in part because it's more readable to emacs macros. It also makes
C++ programmers feel like they know what's going on--as opposed to
using the single quote as separator, which was there to make Ada
programmers feel like they knew what was going on. Because the old-
fashioned syntax is still supported for backwards compatibility, if you
try to use a string like "This is $owner's house", you'll be accessing
$owner::s; that is, the $s variable in package "owner", which is
probably not what you meant. Use braces to disambiguate, as in "This
is ${owner}'s house".
Packages may themselves contain package separators, as in
$OUTER::INNER::var. This implies nothing about the order of name
lookups, however. There are no relative packages: all symbols are
either local to the current package, or must be fully qualified from
the outer package name down. For instance, there is nowhere within
package "OUTER" that $INNER::var refers to $OUTER::INNER::var. "INNER"
refers to a totally separate global package.
Only identifiers starting with letters (or underscore) are stored in a
package's symbol table. All other symbols are kept in package "main",
including all punctuation variables, like $_. In addition, when
unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV,
INC, and SIG are forced to be in package "main", even when used for
other purposes than their built-in ones. If you have a package called
"m", "s", or "y", then you can't use the qualified form of an
identifier because it would be instead interpreted as a pattern match,
a substitution, or a transliteration.
Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method
names. However, variables and functions named with a single "_", such
as $_ and "sub _", are still forced into the package "main". See also
"The Syntax of Variable Names" in perlvar.
"eval"ed strings are compiled in the package in which the eval() was
compiled. (Assignments to $SIG{}, however, assume the signal handler
specified is in the "main" package. Qualify the signal handler name if
you wish to have a signal handler in a package.) For an example,
examine perldb.pl in the Perl library. It initially switches to the
"DB" package so that the debugger doesn't interfere with variables in
the program you are trying to debug. At various points, however, it
temporarily switches back to the "main" package to evaluate various
expressions in the context of the "main" package (or wherever you came
from). See perldebug.
The special symbol "__PACKAGE__" contains the current package, but
cannot (easily) be used to construct variable names.
See perlsub for other scoping issues related to my() and local(), and
perlref regarding closures.
Symbol Tables
The symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
%main::, or %:: for short. Likewise the symbol table for the nested
package mentioned earlier is named %OUTER::INNER::.
The value in each entry of the hash is what you are referring to when
you use the *name typeglob notation.
local *main::foo = *main::bar;
You can use this to print out all the variables in a package, for
instance. The standard but antiquated dumpvar.pl library and the CPAN
module Devel::Symdump make use of this.
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
lexicals.
%some_hash = (); # can't be my()
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
# now use %hashsym normally, and you
# will affect the caller's %another_hash
my %nhash = (); # do what you want
return \%nhash;
}
On return, the reference will overwrite the hash slot in the symbol
table specified by the *some_hash typeglob. This is a somewhat tricky
way of passing around references cheaply when you don't want to have to
remember to dereference variables explicitly.
Another use of symbol tables is for making "constant" scalars.
*PI = \3.14159265358979;
Now you cannot alter $PI, which is probably a good thing all in all.
This isn't the same as a constant subroutine, which is subject to
optimization at compile-time. A constant subroutine is one prototyped
to take no arguments and to return a constant expression. See perlsub
for details on these. The "use constant" pragma is a convenient
shorthand for these.
You can say *foo{PACKAGE} and *foo{NAME} to find out what name and
package the *foo symbol table entry comes from. This may be useful in
a subroutine that gets passed typeglobs as arguments:
sub identify_typeglob {
my $glob = shift;
print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
}
identify_typeglob *foo;
identify_typeglob *bar::baz;
This prints
You gave me main::foo
You gave me bar::baz
The *foo{THING} notation can also be used to obtain references to the
individual elements of *foo. See perlref.
Subroutine definitions (and declarations, for that matter) need not
necessarily be situated in the package whose symbol table they occupy.
You can define a subroutine outside its package by explicitly
qualifying the name of the subroutine:
package main;
sub Some_package::foo { ... } # &foo defined in Some_package
This is just a shorthand for a typeglob assignment at compile time:
BEGIN { *Some_package::foo = sub { ... } }
and is not the same as writing:
{
package Some_package;
sub foo { ... }
}
In the first two versions, the body of the subroutine is lexically in
the main package, not in Some_package. So something like this:
package main;
$Some_package::name = "fred";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": \$name is '$name'\n";
}
Some_package::foo();
prints:
in main: $name is 'barney'
rather than:
in Some_package: $name is 'fred'
This also has implications for the use of the SUPER:: qualifier (see
perlobj).
BEGIN, UNITCHECK, CHECK, INIT and END
Five specially named code blocks are executed at the beginning and at
the end of a running Perl program. These are the "BEGIN", "UNITCHECK",
"CHECK", "INIT", and "END" blocks.
These code blocks can be prefixed with "sub" to give the appearance of
a subroutine (although this is not considered good style). One should
note that these code blocks don't really exist as named subroutines
(despite their appearance). The thing that gives this away is the fact
that you can have more than one of these code blocks in a program, and
they will get all executed at the appropriate moment. So you can't
execute any of these code blocks by name.
A "BEGIN" code block is executed as soon as possible, that is, the
moment it is completely defined, even before the rest of the containing
file (or string) is parsed. You may have multiple "BEGIN" blocks
within a file (or eval'ed string); they will execute in order of
definition. Because a "BEGIN" code block executes immediately, it can
pull in definitions of subroutines and such from other files in time to
be visible to the rest of the compile and run time. Once a "BEGIN" has
run, it is immediately undefined and any code it used is returned to
Perl's memory pool.
An "END" code block is executed as late as possible, that is, after
perl has finished running the program and just before the interpreter
is being exited, even if it is exiting as a result of a die() function.
(But not if it's morphing into another program via "exec", or being
blown out of the water by a signal--you have to trap that yourself (if
you can).) You may have multiple "END" blocks within a file--they will
execute in reverse order of definition; that is: last in, first out
(LIFO). "END" blocks are not executed when you run perl with the "-c"
switch, or if compilation fails.
Note that "END" code blocks are not executed at the end of a string
"eval()": if any "END" code blocks are created in a string "eval()",
they will be executed just as any other "END" code block of that
package in LIFO order just before the interpreter is being exited.
Inside an "END" code block, $? contains the value that the program is
going to pass to "exit()". You can modify $? to change the exit value
of the program. Beware of changing $? by accident (e.g. by running
something via "system").
Inside of a "END" block, the value of "${^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, 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 "${^GLOBAL_PHASE}" will be
"CHECK".
"INIT" blocks are run just before the Perl runtime begins execution, in
"first in, first out" (FIFO) order.
Inside of an "INIT" block, the value of "${^GLOBAL_PHASE}" will be
"INIT".
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
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" }
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 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/Module.pm and start with this template:
package Some::Module; # assumes Some/Module.pm
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 $priv_func = sub {
...
};
# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
sub func1 { ... }
sub func2 { ... }
# this one isn't exported, but could be called directly
# as Some::Module::func3()
sub func3 { ... }
END { ... } # module clean-up code here (global destructor)
1; # don't forget to return a true value from the file
Then go on to declare and use your variables in functions without any
qualifications. See Exporter and the perlmodlib for details on
mechanics and style issues in module creation.
Perl modules are included into your program by saying
use Module;
or
use Module LIST;
This is exactly equivalent to
BEGIN { require 'Module.pm'; 'Module'->import; }
or
BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }
As a special case
use Module ();
is exactly equivalent to
BEGIN { require 'Module.pm'; }
All Perl module files have the extension .pm. The "use" operator
assumes this so you don't have to spell out "Module.pm" in quotes.
This also helps to differentiate new modules from old .pl and .ph
files. Module names are also capitalized unless they're functioning as
pragmas; pragmas are in effect compiler directives, and are sometimes
called "pragmatic modules" (or even "pragmata" if you're a classicist).
The two statements:
require SomeModule;
require "SomeModule.pm";
differ from each other in two ways. In the first case, any double
colons in the module name, such as "Some::Module", are translated into
your system's directory separator, usually "/". The second case does
not, and would have to be specified literally. The other difference is
that seeing the first "require" clues in the compiler that uses of
indirect object notation involving "SomeModule", as in "$ob = purge
SomeModule", are method calls, not function calls. (Yes, this really
can make a difference.)
Because the "use" statement implies a "BEGIN" block, the importing of
semantics happens as soon as the "use" statement is compiled, before
the rest of the file is compiled. This is how it is able to function
as a pragma mechanism, and also how modules are able to declare
subroutines that are then visible as list or unary operators for the
rest of the current file. This will not work if you use "require"
instead of "use". With "require" you can get into this problem:
require Cwd; # make Cwd:: accessible
$here = Cwd::getcwd();
use Cwd; # import names from Cwd::
$here = getcwd();
require Cwd; # make Cwd:: accessible
$here = getcwd(); # oops! no main::getcwd()
In general, "use Module ()" is recommended over "require Module",
because it determines module availability at compile time, not in the
middle of your program's execution. An exception would be if two
modules each tried to "use" each other, and each also called a function
from that other module. In that case, it's easy to use "require"
instead.
Perl packages may be nested inside other package names, so we can have
package names containing "::". But if we used that package name
directly as a filename it would make for unwieldy or impossible
filenames on some systems. Therefore, if a module's name is, say,
"Text::Soundex", then its definition is actually found in the library
file Text/Soundex.pm.
Perl modules always have a .pm file, but there may also be dynamically
linked executables (often ending in .so) or autoloaded subroutine
definitions (often ending in .al) associated with the module. If so,
these will be entirely transparent to the user of the module. It is
the responsibility of the .pm file to load (or arrange to autoload) any
additional functionality. For example, although the POSIX module
happens to do both dynamic loading and autoloading, the user can say
just "use POSIX" to get it all.
Making your module threadsafe
Since 5.6.0, Perl has had support for a new type of threads called
interpreter threads (ithreads). These threads can be used explicitly
and implicitly.
Ithreads work by cloning the data tree so that no data is shared
between different threads. These threads can be used by using the
"threads" module or by doing fork() on win32 (fake fork() support).
When a thread is cloned all Perl data is cloned, however non-Perl data
cannot be cloned automatically. Perl after 5.7.2 has support for the
"CLONE" special subroutine. In "CLONE" you can do whatever you need to
do, like for example handle the cloning of non-Perl data, if necessary.
"CLONE" will be called once as a class method for every package that
has it defined (or inherits it). It will be called in the context of
the new thread, so all modifications are made in the new area.
Currently CLONE is called with no parameters other than the invocant
package name, but code should not assume that this will remain
unchanged, as it is likely that in future extra parameters will be
passed in to give more information about the state of cloning.
If you want to CLONE all objects you will need to keep track of them
per package. This is simply done using a hash and
Scalar::Util::weaken().
Perl after 5.8.7 has support for the "CLONE_SKIP" special subroutine.
Like "CLONE", "CLONE_SKIP" is called once per package; however, it is
called just before cloning starts, and in the context of the parent
thread. If it returns a true value, then no objects of that class will
be cloned; or rather, they will be copied as unblessed, undef values.
For example: if in the parent there are two references to a single
blessed hash, then in the child there will be two references to a
single undefined scalar value instead. This provides a simple
mechanism for making a module threadsafe; just add "sub CLONE_SKIP { 1
}" at the top of the class, and "DESTROY()" will 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, 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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