perlmod - Perl modules (packages and symbol tables)
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 "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, 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 [Toc] [Back]
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 = ichard;
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 = ar;
"*foo = *bar" makes the typeglobs themselves synonymous
while "*foo = ar" makes the SCALAR portions of two distinct
typeglobs refer to the same scalar value. This means
that the following code:
$bar = 1;
*foo = ar; # 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 = " 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}, "0;
}
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";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": ame is '$name'0;
}
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 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
exiting as a result of a die() function. (But not if it's
polymorphing 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").
"CHECK" and "INIT" code blocks are useful to catch the
transition between the compilation phase and the execution
phase of the main program.
"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.0;
END { print "14. So this is the end of the tale.0 }
INIT { print " 5. INIT blocks run FIFO just before runtime.0 }
CHECK { print " 4. So this is the fourth line.0 }
print " 9. It runs in order, of course.0;
BEGIN { print " 1. BEGIN blocks run FIFO during compilation.0 }
END { print "13. Read perlmod for the rest of the
story.0 }
CHECK { print " 3. CHECK blocks run LIFO at compilation's end.0 }
INIT { print " 6. Run this again, using Perl's -c
switch.0 }
print "10. This is anti-obfuscated code.0;
END { print "12. END blocks run LIFO at quitting
time.0 }
BEGIN { print " 2. So this line comes out second.0 }
INIT { print " 7. You'll see the difference right
away.0 }
print "11. It merely _looks_ like it should be
confusing.0;
__END__
Perl Classes [Toc] [Back]
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 [Toc] [Back]
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: 1.9 $ =~
/(+)/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 "Mod-
ule.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 [Toc] [Back]
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().
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.
perl v5.8.5 2002-11-06 12 [ Back ] |
