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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 pack-
ages 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 lex-
ical variables created with my(). Typically it would be the first dec-
laration in a file included by the "do", "require", or "use" operators.
You can switch into a package in more than one place; it merely influ-
ences 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 pro-
grammers feel like they knew what was going on. Because the old-fash-
ioned 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 proba-
bly 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 identi-
fier because it would be instead interpreted as a pattern match, a sub-
stitution, or a transliteration.
Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method
names. However, variables and functions named with a single "_", such
as $_ and "sub _", are still forced into the package "main". See also
"Technical Note on the Syntax of Variable Names" in perlvar.
"eval"ed strings are compiled in the package in which the eval() was
compiled. (Assignments to $SIG{}, however, assume the signal handler
specified is in the "main" package. Qualify the signal handler name if
you wish to have a signal handler in a package.) For an example, exam-
ine 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 tempo-
rarily switches back to the "main" package to evaluate various expres-
sions in the context of the "main" package (or wherever you came from).
See perldebug.
The special symbol "__PACKAGE__" contains the current package, but can-
not (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. 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 package "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 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 $bar --
the one that was stuffed away by "local()" and which will be restored
when the block ends. Because variables are accessed through the type-
glob, you can use "*foo = *bar" to create an alias which can be local-
ized. (But be aware that this means you can't have a separate @foo and
@bar, etc.)
What makes all of this important is that the Exporter module uses glob
aliasing as the import/export mechanism. Whether or not you can prop-
erly localize a variable that has been exported from a module depends
on how it was exported:
@EXPORT = qw($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 refer-
ences into or from subroutines if you don't want to copy the whole
thing. It only works when assigning to dynamic variables, not lexi-
cals.
%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 ta-
ble 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 opti-
mization 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 qualify-
ing 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, CHECK, INIT and END
Four specially named code blocks are executed at the beginning and at
the end of a running Perl program. These are the "BEGIN", "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 def-
inition. 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.
It should be noted that "BEGIN" code blocks are executed inside string
"eval()"'s. The "CHECK" and "INIT" code blocks are not executed inside
a string eval, which e.g. can be a problem in a mod_perl environment.
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 pack-
age 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").
"CHECK" and "INIT" code blocks are useful to catch the transition
between the compilation phase and the execution phase of the main pro-
gram.
"CHECK" code blocks are run just after the initial Perl compile phase
ends and before the run time begins, in LIFO order. "CHECK" code
blocks are used in the Perl compiler suite to save the compiled state
of the program.
"INIT" blocks are run just before the Perl runtime begins execution, in
"first in, first out" (FIFO) order. For example, the code generators
documented in perlcc make use of "INIT" blocks to initialize and
resolve pointers to XSUBs.
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 " 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__
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 any-
thing. 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 mechan-
ics 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 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 subrou-
tines 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 mod-
ules 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 file-
names 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 defi-
nitions (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 hap-
pens 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. Cur-
rently CLONE is called with no parameters other than the invocant pack-
age 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.
This provides a simple mechanism for making a module threadsafe; just
add "sub CLONE_SKIP { 1 }" at the top of the class, and "DESTROY()"
will be now only be called once per object. Of course, if the child
thread needs to make use of the objects, then a more sophisticated
approach is needed.
Like "CLONE", "CLONE_SKIP" is currently called with no parameters other
than the invocant package name, although that may change. Similarly, to
allow for future expansion, the return value should be a single 0 or 1
value.
SEE ALSO
See perlmodlib for general style issues related to building Perl mod-
ules 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 v5.8.8 2006-01-07 PERLMOD(1)
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