3 perlmod - Perl modules (packages and symbol tables)
9 Perl provides a mechanism for alternative namespaces to protect
10 packages from stomping on each other's variables. In fact, there's
11 really no such thing as a global variable in Perl. The package
12 statement declares the compilation unit as being in the given
13 namespace. The scope of the package declaration is from the
14 declaration itself through the end of the enclosing block, C<eval>,
15 or file, whichever comes first (the same scope as the my() and
16 local() operators). Unqualified dynamic identifiers will be in
17 this namespace, except for those few identifiers that if unqualified,
18 default to the main package instead of the current one as described
19 below. A package statement affects only dynamic variables--including
20 those you've used local() on--but I<not> lexical variables created
21 with my(). Typically it would be the first declaration in a file
22 included by the C<do>, C<require>, or C<use> operators. You can
23 switch into a package in more than one place; it merely influences
24 which symbol table is used by the compiler for the rest of that
25 block. You can refer to variables and filehandles in other packages
26 by prefixing the identifier with the package name and a double
27 colon: C<$Package::Variable>. If the package name is null, the
28 C<main> package is assumed. That is, C<$::sail> is equivalent to
31 The old package delimiter was a single quote, but double colon is now the
32 preferred delimiter, in part because it's more readable to humans, and
33 in part because it's more readable to B<emacs> macros. It also makes C++
34 programmers feel like they know what's going on--as opposed to using the
35 single quote as separator, which was there to make Ada programmers feel
36 like they knew what was going on. Because the old-fashioned syntax is still
37 supported for backwards compatibility, if you try to use a string like
38 C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
39 the $s variable in package C<owner>, which is probably not what you meant.
40 Use braces to disambiguate, as in C<"This is ${owner}'s house">.
42 Packages may themselves contain package separators, as in
43 C<$OUTER::INNER::var>. This implies nothing about the order of
44 name lookups, however. There are no relative packages: all symbols
45 are either local to the current package, or must be fully qualified
46 from the outer package name down. For instance, there is nowhere
47 within package C<OUTER> that C<$INNER::var> refers to
48 C<$OUTER::INNER::var>. C<INNER> refers to a totally
49 separate global package.
51 Only identifiers starting with letters (or underscore) are stored
52 in a package's symbol table. All other symbols are kept in package
53 C<main>, including all punctuation variables, like $_. In addition,
54 when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
55 ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
56 even when used for other purposes than their built-in ones. If you
57 have a package called C<m>, C<s>, or C<y>, then you can't use the
58 qualified form of an identifier because it would be instead interpreted
59 as a pattern match, a substitution, or a transliteration.
61 Variables beginning with underscore used to be forced into package
62 main, but we decided it was more useful for package writers to be able
63 to use leading underscore to indicate private variables and method names.
64 However, variables and functions named with a single C<_>, such as
65 $_ and C<sub _>, are still forced into the package C<main>. See also
66 L<perlvar/"Technical Note on the Syntax of Variable Names">.
68 C<eval>ed strings are compiled in the package in which the eval() was
69 compiled. (Assignments to C<$SIG{}>, however, assume the signal
70 handler specified is in the C<main> package. Qualify the signal handler
71 name if you wish to have a signal handler in a package.) For an
72 example, examine F<perldb.pl> in the Perl library. It initially switches
73 to the C<DB> package so that the debugger doesn't interfere with variables
74 in the program you are trying to debug. At various points, however, it
75 temporarily switches back to the C<main> package to evaluate various
76 expressions in the context of the C<main> package (or wherever you came
77 from). See L<perldebug>.
79 The special symbol C<__PACKAGE__> contains the current package, but cannot
80 (easily) be used to construct variable names.
82 See L<perlsub> for other scoping issues related to my() and local(),
83 and L<perlref> regarding closures.
87 The symbol table for a package happens to be stored in the hash of that
88 name with two colons appended. The main symbol table's name is thus
89 C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
90 package mentioned earlier is named C<%OUTER::INNER::>.
92 The value in each entry of the hash is what you are referring to when you
93 use the C<*name> typeglob notation. In fact, the following have the same
94 effect, though the first is more efficient because it does the symbol
95 table lookups at compile time:
97 local *main::foo = *main::bar;
98 local $main::{foo} = $main::{bar};
100 (Be sure to note the B<vast> difference between the second line above
101 and C<local $main::foo = $main::bar>. The former is accessing the hash
102 C<%main::>, which is the symbol table of package C<main>. The latter is
103 simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
106 You can use this to print out all the variables in a package, for
107 instance. The standard but antiquated F<dumpvar.pl> library and
108 the CPAN module Devel::Symdump make use of this.
110 Assignment to a typeglob performs an aliasing operation, i.e.,
114 causes variables, subroutines, formats, and file and directory handles
115 accessible via the identifier C<richard> also to be accessible via the
116 identifier C<dick>. If you want to alias only a particular variable or
117 subroutine, assign a reference instead:
121 Which makes $richard and $dick the same variable, but leaves
122 @richard and @dick as separate arrays. Tricky, eh?
124 There is one subtle difference between the following statements:
129 C<*foo = *bar> makes the typeglobs themselves synonymous while
130 C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
131 refer to the same scalar value. This means that the following code:
134 *foo = \$bar; # Make $foo an alias for $bar
137 local $bar = 2; # Restrict changes to block
138 print $foo; # Prints '1'!
141 Would print '1', because C<$foo> holds a reference to the I<original>
142 C<$bar> -- the one that was stuffed away by C<local()> and which will be
143 restored when the block ends. Because variables are accessed through the
144 typeglob, you can use C<*foo = *bar> to create an alias which can be
145 localized. (But be aware that this means you can't have a separate
146 C<@foo> and C<@bar>, etc.)
148 What makes all of this important is that the Exporter module uses glob
149 aliasing as the import/export mechanism. Whether or not you can properly
150 localize a variable that has been exported from a module depends on how
153 @EXPORT = qw($FOO); # Usual form, can't be localized
154 @EXPORT = qw(*FOO); # Can be localized
156 You can work around the first case by using the fully qualified name
157 (C<$Package::FOO>) where you need a local value, or by overriding it
158 by saying C<*FOO = *Package::FOO> in your script.
160 The C<*x = \$y> mechanism may be used to pass and return cheap references
161 into or from subroutines if you don't want to copy the whole
162 thing. It only works when assigning to dynamic variables, not
165 %some_hash = (); # can't be my()
166 *some_hash = fn( \%another_hash );
168 local *hashsym = shift;
169 # now use %hashsym normally, and you
170 # will affect the caller's %another_hash
171 my %nhash = (); # do what you want
175 On return, the reference will overwrite the hash slot in the
176 symbol table specified by the *some_hash typeglob. This
177 is a somewhat tricky way of passing around references cheaply
178 when you don't want to have to remember to dereference variables
181 Another use of symbol tables is for making "constant" scalars.
183 *PI = \3.14159265358979;
185 Now you cannot alter C<$PI>, which is probably a good thing all in all.
186 This isn't the same as a constant subroutine, which is subject to
187 optimization at compile-time. A constant subroutine is one prototyped
188 to take no arguments and to return a constant expression. See
189 L<perlsub> for details on these. The C<use constant> pragma is a
190 convenient shorthand for these.
192 You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
193 package the *foo symbol table entry comes from. This may be useful
194 in a subroutine that gets passed typeglobs as arguments:
196 sub identify_typeglob {
198 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
200 identify_typeglob *foo;
201 identify_typeglob *bar::baz;
205 You gave me main::foo
208 The C<*foo{THING}> notation can also be used to obtain references to the
209 individual elements of *foo. See L<perlref>.
211 Subroutine definitions (and declarations, for that matter) need
212 not necessarily be situated in the package whose symbol table they
213 occupy. You can define a subroutine outside its package by
214 explicitly qualifying the name of the subroutine:
217 sub Some_package::foo { ... } # &foo defined in Some_package
219 This is just a shorthand for a typeglob assignment at compile time:
221 BEGIN { *Some_package::foo = sub { ... } }
223 and is I<not> the same as writing:
226 package Some_package;
230 In the first two versions, the body of the subroutine is
231 lexically in the main package, I<not> in Some_package. So
236 $Some_package::name = "fred";
237 $main::name = "barney";
239 sub Some_package::foo {
240 print "in ", __PACKAGE__, ": \$name is '$name'\n";
247 in main: $name is 'barney'
251 in Some_package: $name is 'fred'
253 This also has implications for the use of the SUPER:: qualifier
256 =head2 Package Constructors and Destructors
258 Four special subroutines act as package constructors and destructors.
259 These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The
260 C<sub> is optional for these routines.
262 A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
263 it is completely defined, even before the rest of the containing file
264 is parsed. You may have multiple C<BEGIN> blocks within a file--they
265 will execute in order of definition. Because a C<BEGIN> block executes
266 immediately, it can pull in definitions of subroutines and such from other
267 files in time to be visible to the rest of the file. Once a C<BEGIN>
268 has run, it is immediately undefined and any code it used is returned to
269 Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
271 An C<END> subroutine is executed as late as possible, that is, after
272 perl has finished running the program and just before the interpreter
273 is being exited, even if it is exiting as a result of a die() function.
274 (But not if it's polymorphing into another program via C<exec>, or
275 being blown out of the water by a signal--you have to trap that yourself
276 (if you can).) You may have multiple C<END> blocks within a file--they
277 will execute in reverse order of definition; that is: last in, first
278 out (LIFO). C<END> blocks are not executed when you run perl with the
279 C<-c> switch, or if compilation fails.
281 Inside an C<END> subroutine, C<$?> contains the value that the program is
282 going to pass to C<exit()>. You can modify C<$?> to change the exit
283 value of the program. Beware of changing C<$?> by accident (e.g. by
284 running something via C<system>).
286 C<CHECK> and C<INIT> blocks are useful to catch the transition between
287 the compilation phase and the execution phase of the main program.
289 C<CHECK> blocks are run just after the Perl compile phase ends and before
290 the run time begins, in LIFO order. C<CHECK> blocks are used in
291 the Perl compiler suite to save the compiled state of the program.
293 C<INIT> blocks are run just before the Perl runtime begins execution, in
294 "first in, first out" (FIFO) order. For example, the code generators
295 documented in L<perlcc> make use of C<INIT> blocks to initialize and
296 resolve pointers to XSUBs.
298 When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
299 C<END> work just as they do in B<awk>, as a degenerate case.
300 Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>
301 switch for a compile-only syntax check, although your main code
306 There is no special class syntax in Perl, but a package may act
307 as a class if it provides subroutines to act as methods. Such a
308 package may also derive some of its methods from another class (package)
309 by listing the other package name(s) in its global @ISA array (which
310 must be a package global, not a lexical).
312 For more on this, see L<perltoot> and L<perlobj>.
316 A module is just a set of related functions in a library file, i.e.,
317 a Perl package with the same name as the file. It is specifically
318 designed to be reusable by other modules or programs. It may do this
319 by providing a mechanism for exporting some of its symbols into the
320 symbol table of any package using it, or it may function as a class
321 definition and make its semantics available implicitly through
322 method calls on the class and its objects, without explicitly
323 exporting anything. Or it can do a little of both.
325 For example, to start a traditional, non-OO module called Some::Module,
326 create a file called F<Some/Module.pm> and start with this template:
328 package Some::Module; # assumes Some/Module.pm
335 our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
337 # set the version for version checking
339 # if using RCS/CVS, this may be preferred
340 $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
343 @EXPORT = qw(&func1 &func2 &func4);
344 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
346 # your exported package globals go here,
347 # as well as any optionally exported functions
348 @EXPORT_OK = qw($Var1 %Hashit &func3);
352 # exported package globals go here
356 # non-exported package globals go here
360 # initialize package globals, first exported ones
364 # then the others (which are still accessible as $Some::Module::stuff)
368 # all file-scoped lexicals must be created before
369 # the functions below that use them.
371 # file-private lexicals go here
373 my %secret_hash = ();
375 # here's a file-private function as a closure,
376 # callable as &$priv_func; it cannot be prototyped.
377 my $priv_func = sub {
381 # make all your functions, whether exported or not;
382 # remember to put something interesting in the {} stubs
383 sub func1 {} # no prototype
384 sub func2() {} # proto'd void
385 sub func3($$) {} # proto'd to 2 scalars
387 # this one isn't exported, but could be called!
388 sub func4(\%) {} # proto'd to 1 hash ref
390 END { } # module clean-up code here (global destructor)
392 ## YOUR CODE GOES HERE
394 1; # don't forget to return a true value from the file
396 Then go on to declare and use your variables in functions without
397 any qualifications. See L<Exporter> and the L<perlmodlib> for
398 details on mechanics and style issues in module creation.
400 Perl modules are included into your program by saying
408 This is exactly equivalent to
410 BEGIN { require Module; import Module; }
414 BEGIN { require Module; import Module LIST; }
420 is exactly equivalent to
422 BEGIN { require Module; }
424 All Perl module files have the extension F<.pm>. The C<use> operator
425 assumes this so you don't have to spell out "F<Module.pm>" in quotes.
426 This also helps to differentiate new modules from old F<.pl> and
427 F<.ph> files. Module names are also capitalized unless they're
428 functioning as pragmas; pragmas are in effect compiler directives,
429 and are sometimes called "pragmatic modules" (or even "pragmata"
430 if you're a classicist).
435 require "SomeModule.pm";
437 differ from each other in two ways. In the first case, any double
438 colons in the module name, such as C<Some::Module>, are translated
439 into your system's directory separator, usually "/". The second
440 case does not, and would have to be specified literally. The other
441 difference is that seeing the first C<require> clues in the compiler
442 that uses of indirect object notation involving "SomeModule", as
443 in C<$ob = purge SomeModule>, are method calls, not function calls.
444 (Yes, this really can make a difference.)
446 Because the C<use> statement implies a C<BEGIN> block, the importing
447 of semantics happens as soon as the C<use> statement is compiled,
448 before the rest of the file is compiled. This is how it is able
449 to function as a pragma mechanism, and also how modules are able to
450 declare subroutines that are then visible as list or unary operators for
451 the rest of the current file. This will not work if you use C<require>
452 instead of C<use>. With C<require> you can get into this problem:
454 require Cwd; # make Cwd:: accessible
455 $here = Cwd::getcwd();
457 use Cwd; # import names from Cwd::
460 require Cwd; # make Cwd:: accessible
461 $here = getcwd(); # oops! no main::getcwd()
463 In general, C<use Module ()> is recommended over C<require Module>,
464 because it determines module availability at compile time, not in the
465 middle of your program's execution. An exception would be if two modules
466 each tried to C<use> each other, and each also called a function from
467 that other module. In that case, it's easy to use C<require> instead.
469 Perl packages may be nested inside other package names, so we can have
470 package names containing C<::>. But if we used that package name
471 directly as a filename it would make for unwieldy or impossible
472 filenames on some systems. Therefore, if a module's name is, say,
473 C<Text::Soundex>, then its definition is actually found in the library
474 file F<Text/Soundex.pm>.
476 Perl modules always have a F<.pm> file, but there may also be
477 dynamically linked executables (often ending in F<.so>) or autoloaded
478 subroutine definitions (often ending in F<.al>) associated with the
479 module. If so, these will be entirely transparent to the user of
480 the module. It is the responsibility of the F<.pm> file to load
481 (or arrange to autoload) any additional functionality. For example,
482 although the POSIX module happens to do both dynamic loading and
483 autoloading, the user can say just C<use POSIX> to get it all.
485 =head2 Making your module threadsafe
487 Since 5.6.0, Perl has had support for a new type of threads called
488 interpreter threads (ithreads). These threads can be used explicitly
491 Ithreads work by cloning the data tree so that no data is shared
492 between different threads. These threads can be used by using the C<threads>
493 module or by doing fork() on win32 (fake fork() support). When a
494 thread is cloned all Perl data is cloned, however non-Perl data cannot
495 be cloned automatically. Perl after 5.7.2 has support for the C<CLONE>
496 special subroutine . In C<CLONE> you can do whatever you need to do,
497 like for example handle the cloning of non-Perl data, if necessary.
498 C<CLONE> will be executed once for every package that has it defined
499 (or inherits it). It will be called in the context of the new thread,
500 so all modifications are made in the new area.
502 If you want to CLONE all objects you will need to keep track of them per
503 package. This is simply done using a hash and Scalar::Util::weaken().
507 See L<perlmodlib> for general style issues related to building Perl
508 modules and classes, as well as descriptions of the standard library
509 and CPAN, L<Exporter> for how Perl's standard import/export mechanism
510 works, L<perltoot> and L<perltooc> for an in-depth tutorial on
511 creating classes, L<perlobj> for a hard-core reference document on
512 objects, L<perlsub> for an explanation of functions and scoping,
513 and L<perlxstut> and L<perlguts> for more information on writing