PERLMOD(1) Perl Programmers Reference Guide PERLMOD(1)NAMEperlmod - Perl modules (packages and symbol tables)
DESCRIPTION
Packages
Perl provides a mechanism for alternative namespaces to pro-
tect 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 pack-
age in more than one place; it merely influences which sym-
bol 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 read-
able to emacs macros. It also makes C++ programmers feel
like they know what's going on--as opposed to using the sin-
gle quote as separator, which was there to make Ada program-
mers feel like they knew what was going on. Because the
old-fashioned syntax is still supported for backwards compa-
tibility, if you try to use a string like "This is $owner's
house", you'll be accessing $owner::s; that is, the $s vari-
able 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.
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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 inter-
preted as a pattern match, a substitution, or a translitera-
tion.
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 "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. (Assignments to $SIG{}, however,
assume the signal handler specified is in the "main" pack-
age. 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 vari-
ous points, however, it temporarily switches back to the
"main" package to evaluate various expressions in the con-
text of the "main" package (or wherever you came from). See
perldebug.
The special symbol "__PACKAGE__" contains the current pack-
age, 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 refer-
ring 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
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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 pack-
age, 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 direc-
tory 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 state-
ments:
*foo = *bar;
*foo = \$bar;
"*foo = *bar" makes the typeglobs themselves synonymous
while "*foo = \$bar" makes the SCALAR portions of two dis-
tinct 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 ori-
ginal $bar -- the one that was stuffed away by "local()" and
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which will be restored when the block ends. Because vari-
ables are accessed through the typeglob, you can use "*foo =
*bar" to create an alias which can be localized. (But be
aware that this means you can't have a separate @foo and
@bar, etc.)
What makes all of this important is that the Exporter module
uses glob aliasing as the import/export mechanism. Whether
or not you can properly localize a variable that has been
exported from a module depends on how it was exported:
@EXPORT = qw($FOO); # Usual form, can't be localized
@EXPORT = qw(*FOO); # Can be localized
You can work around the first case by using the fully quali-
fied 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 vari-
ables 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 sub-
routine 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.
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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 refer-
ences 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 com-
pile 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 some-
thing like this:
package main;
$Some_package::name = "fred";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": \$name is '$name'\n";
}
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prints:
in main: $name is 'barney'
rather than:
in Some_package: $name is 'fred'
This also has implications for the use of the SUPER:: qual-
ifier (see perlobj).
BEGIN, CHECK, INIT and END
Four specially named code blocks are executed at the begin-
ning 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 appear-
ance). 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.
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 exit-
ing as a result of a die() function. (But not if it's morph-
ing 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
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is: last in, first out (LIFO). "END" blocks are not exe-
cuted when you run perl with the "-c" switch, or if compila-
tion 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").
"CHECK" and "INIT" code blocks are useful to catch the tran-
sition between the compilation phase and the execution phase
of the main program.
"CHECK" code blocks are run just after the initial Perl com-
pile 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 exam-
ple, 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";
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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 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);
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# set the version for version checking
$VERSION = 1.00;
# if using RCS/CVS, this may be preferred
$VERSION = sprintf "%d.%03d", q$Revision: 1.10 $ =~ /(\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
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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 crea-
tion.
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 classi-
cist).
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,
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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" state-
ment 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 com-
pile 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. There-
fore, 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 responsi-
bility 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.
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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 automati-
cally. 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 modifica-
tions 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. 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 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 explana-
tion of functions and scoping, and perlxstut and perlguts
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for more information on writing extension modules.
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