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.
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 perltoot and perlobj.
Perl Modules¶
A module is just a set of related functions in a library file, i.e., a Perl
package with the same name as the file. It is specifically designed to be
reusable by other modules or programs. It may do this by providing a mechanism
for exporting some of its symbols into the symbol table of any package using
it, or it may function as a class definition and make its semantics available
implicitly through method calls on the class and its objects, without
explicitly exporting anything. Or it can do a little of both.
For example, to start a traditional, non-OO module called Some::Module, create a
file called
Some/Module.pm and start with this template:
package Some::Module; # assumes Some/Module.pm
use strict;
use warnings;
BEGIN {
use Exporter ();
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
# set the version for version checking
$VERSION = 1.00;
# if using RCS/CVS, this may be preferred
$VERSION = sprintf "%d.%03d", q$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 "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, 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.