3 perlmod - Perl modules (packages and symbol tables)
8 X<package> X<namespace> X<variable, global> X<global variable> X<global>
10 Perl provides a mechanism for alternative namespaces to protect
11 packages from stomping on each other's variables. In fact, there's
12 really no such thing as a global variable in Perl. The package
13 statement declares the compilation unit as being in the given
14 namespace. The scope of the package declaration is from the
15 declaration itself through the end of the enclosing block, C<eval>,
16 or file, whichever comes first (the same scope as the my() and
17 local() operators). Unqualified dynamic identifiers will be in
18 this namespace, except for those few identifiers that if unqualified,
19 default to the main package instead of the current one as described
20 below. A package statement affects only dynamic variables--including
21 those you've used local() on--but I<not> lexical variables created
22 with my(). Typically it would be the first declaration in a file
23 included by the C<do>, C<require>, or C<use> operators. You can
24 switch into a package in more than one place; it merely influences
25 which symbol table is used by the compiler for the rest of that
26 block. You can refer to variables and filehandles in other packages
27 by prefixing the identifier with the package name and a double
28 colon: C<$Package::Variable>. If the package name is null, the
29 C<main> package is assumed. That is, C<$::sail> is equivalent to
32 The old package delimiter was a single quote, but double colon is now the
33 preferred delimiter, in part because it's more readable to humans, and
34 in part because it's more readable to B<emacs> macros. It also makes C++
35 programmers feel like they know what's going on--as opposed to using the
36 single quote as separator, which was there to make Ada programmers feel
37 like they knew what was going on. Because the old-fashioned syntax is still
38 supported for backwards compatibility, if you try to use a string like
39 C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
40 the $s variable in package C<owner>, which is probably not what you meant.
41 Use braces to disambiguate, as in C<"This is ${owner}'s house">.
44 Packages may themselves contain package separators, as in
45 C<$OUTER::INNER::var>. This implies nothing about the order of
46 name lookups, however. There are no relative packages: all symbols
47 are either local to the current package, or must be fully qualified
48 from the outer package name down. For instance, there is nowhere
49 within package C<OUTER> that C<$INNER::var> refers to
50 C<$OUTER::INNER::var>. C<INNER> refers to a totally
51 separate global package.
53 Only identifiers starting with letters (or underscore) are stored
54 in a package's symbol table. All other symbols are kept in package
55 C<main>, including all punctuation variables, like $_. In addition,
56 when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
57 ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
58 even when used for other purposes than their built-in ones. If you
59 have a package called C<m>, C<s>, or C<y>, then you can't use the
60 qualified form of an identifier because it would be instead interpreted
61 as a pattern match, a substitution, or a transliteration.
62 X<variable, punctuation>
64 Variables beginning with underscore used to be forced into package
65 main, but we decided it was more useful for package writers to be able
66 to use leading underscore to indicate private variables and method names.
67 However, variables and functions named with a single C<_>, such as
68 $_ and C<sub _>, are still forced into the package C<main>. See also
69 L<perlvar/"Technical Note on the Syntax of Variable Names">.
71 C<eval>ed strings are compiled in the package in which the eval() was
72 compiled. (Assignments to C<$SIG{}>, however, assume the signal
73 handler specified is in the C<main> package. Qualify the signal handler
74 name if you wish to have a signal handler in a package.) For an
75 example, examine F<perldb.pl> in the Perl library. It initially switches
76 to the C<DB> package so that the debugger doesn't interfere with variables
77 in the program you are trying to debug. At various points, however, it
78 temporarily switches back to the C<main> package to evaluate various
79 expressions in the context of the C<main> package (or wherever you came
80 from). See L<perldebug>.
82 The special symbol C<__PACKAGE__> contains the current package, but cannot
83 (easily) be used to construct variable names.
85 See L<perlsub> for other scoping issues related to my() and local(),
86 and L<perlref> regarding closures.
89 X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias>
91 The symbol table for a package happens to be stored in the hash of that
92 name with two colons appended. The main symbol table's name is thus
93 C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
94 package mentioned earlier is named C<%OUTER::INNER::>.
96 The value in each entry of the hash is what you are referring to when you
97 use the C<*name> typeglob notation. In fact, the following have the same
98 effect, though the first is more efficient because it does the symbol
99 table lookups at compile time:
101 local *main::foo = *main::bar;
102 local $main::{foo} = $main::{bar};
104 (Be sure to note the B<vast> difference between the second line above
105 and C<local $main::foo = $main::bar>. The former is accessing the hash
106 C<%main::>, which is the symbol table of package C<main>. The latter is
107 simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
110 You can use this to print out all the variables in a package, for
111 instance. The standard but antiquated F<dumpvar.pl> library and
112 the CPAN module Devel::Symdump make use of this.
114 Assignment to a typeglob performs an aliasing operation, i.e.,
118 causes variables, subroutines, formats, and file and directory handles
119 accessible via the identifier C<richard> also to be accessible via the
120 identifier C<dick>. If you want to alias only a particular variable or
121 subroutine, assign a reference instead:
125 Which makes $richard and $dick the same variable, but leaves
126 @richard and @dick as separate arrays. Tricky, eh?
128 There is one subtle difference between the following statements:
133 C<*foo = *bar> makes the typeglobs themselves synonymous while
134 C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
135 refer to the same scalar value. This means that the following code:
138 *foo = \$bar; # Make $foo an alias for $bar
141 local $bar = 2; # Restrict changes to block
142 print $foo; # Prints '1'!
145 Would print '1', because C<$foo> holds a reference to the I<original>
146 C<$bar> -- the one that was stuffed away by C<local()> and which will be
147 restored when the block ends. Because variables are accessed through the
148 typeglob, you can use C<*foo = *bar> to create an alias which can be
149 localized. (But be aware that this means you can't have a separate
150 C<@foo> and C<@bar>, etc.)
152 What makes all of this important is that the Exporter module uses glob
153 aliasing as the import/export mechanism. Whether or not you can properly
154 localize a variable that has been exported from a module depends on how
157 @EXPORT = qw($FOO); # Usual form, can't be localized
158 @EXPORT = qw(*FOO); # Can be localized
160 You can work around the first case by using the fully qualified name
161 (C<$Package::FOO>) where you need a local value, or by overriding it
162 by saying C<*FOO = *Package::FOO> in your script.
164 The C<*x = \$y> mechanism may be used to pass and return cheap references
165 into or from subroutines if you don't want to copy the whole
166 thing. It only works when assigning to dynamic variables, not
169 %some_hash = (); # can't be my()
170 *some_hash = fn( \%another_hash );
172 local *hashsym = shift;
173 # now use %hashsym normally, and you
174 # will affect the caller's %another_hash
175 my %nhash = (); # do what you want
179 On return, the reference will overwrite the hash slot in the
180 symbol table specified by the *some_hash typeglob. This
181 is a somewhat tricky way of passing around references cheaply
182 when you don't want to have to remember to dereference variables
185 Another use of symbol tables is for making "constant" scalars.
186 X<constant> X<scalar, constant>
188 *PI = \3.14159265358979;
190 Now you cannot alter C<$PI>, which is probably a good thing all in all.
191 This isn't the same as a constant subroutine, which is subject to
192 optimization at compile-time. A constant subroutine is one prototyped
193 to take no arguments and to return a constant expression. See
194 L<perlsub> for details on these. The C<use constant> pragma is a
195 convenient shorthand for these.
197 You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
198 package the *foo symbol table entry comes from. This may be useful
199 in a subroutine that gets passed typeglobs as arguments:
201 sub identify_typeglob {
203 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
205 identify_typeglob *foo;
206 identify_typeglob *bar::baz;
210 You gave me main::foo
213 The C<*foo{THING}> notation can also be used to obtain references to the
214 individual elements of *foo. See L<perlref>.
216 Subroutine definitions (and declarations, for that matter) need
217 not necessarily be situated in the package whose symbol table they
218 occupy. You can define a subroutine outside its package by
219 explicitly qualifying the name of the subroutine:
222 sub Some_package::foo { ... } # &foo defined in Some_package
224 This is just a shorthand for a typeglob assignment at compile time:
226 BEGIN { *Some_package::foo = sub { ... } }
228 and is I<not> the same as writing:
231 package Some_package;
235 In the first two versions, the body of the subroutine is
236 lexically in the main package, I<not> in Some_package. So
241 $Some_package::name = "fred";
242 $main::name = "barney";
244 sub Some_package::foo {
245 print "in ", __PACKAGE__, ": \$name is '$name'\n";
252 in main: $name is 'barney'
256 in Some_package: $name is 'fred'
258 This also has implications for the use of the SUPER:: qualifier
261 =head2 BEGIN, UNITCHECK, CHECK, INIT and END
262 X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END>
264 Five specially named code blocks are executed at the beginning and at
265 the end of a running Perl program. These are the C<BEGIN>,
266 C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks.
268 These code blocks can be prefixed with C<sub> to give the appearance of a
269 subroutine (although this is not considered good style). One should note
270 that these code blocks don't really exist as named subroutines (despite
271 their appearance). The thing that gives this away is the fact that you can
272 have B<more than one> of these code blocks in a program, and they will get
273 B<all> executed at the appropriate moment. So you can't execute any of
274 these code blocks by name.
276 A C<BEGIN> code block is executed as soon as possible, that is, the moment
277 it is completely defined, even before the rest of the containing file (or
278 string) is parsed. You may have multiple C<BEGIN> blocks within a file (or
279 eval'ed string) -- they will execute in order of definition. Because a C<BEGIN>
280 code block executes immediately, it can pull in definitions of subroutines
281 and such from other files in time to be visible to the rest of the compile
282 and run time. Once a C<BEGIN> has run, it is immediately undefined and any
283 code it used is returned to Perl's memory pool.
285 It should be noted that C<BEGIN> and C<UNITCHECK> code blocks B<are>
286 executed inside string C<eval()>'s. The C<CHECK> and C<INIT> code
287 blocks are B<not> executed inside a string eval, which e.g. can be a
288 problem in a mod_perl environment.
290 An C<END> code block is executed as late as possible, that is, after
291 perl has finished running the program and just before the interpreter
292 is being exited, even if it is exiting as a result of a die() function.
293 (But not if it's morphing into another program via C<exec>, or
294 being blown out of the water by a signal--you have to trap that yourself
295 (if you can).) You may have multiple C<END> blocks within a file--they
296 will execute in reverse order of definition; that is: last in, first
297 out (LIFO). C<END> blocks are not executed when you run perl with the
298 C<-c> switch, or if compilation fails.
300 Note that C<END> code blocks are B<not> executed at the end of a string
301 C<eval()>: if any C<END> code blocks are created in a string C<eval()>,
302 they will be executed just as any other C<END> code block of that package
303 in LIFO order just before the interpreter is being exited.
305 Inside an C<END> code block, C<$?> contains the value that the program is
306 going to pass to C<exit()>. You can modify C<$?> to change the exit
307 value of the program. Beware of changing C<$?> by accident (e.g. by
308 running something via C<system>).
311 C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the
312 transition between the compilation phase and the execution phase of
315 C<UNITCHECK> blocks are run just after the unit which defined them has
316 been compiled. The main program file and each module it loads are
317 compilation units, as are string C<eval>s, code compiled using the
318 C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>,
319 and code after the C<-e> switch on the command line.
321 C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends
322 and before the run time begins, in LIFO order. C<CHECK> code blocks are used
323 in the Perl compiler suite to save the compiled state of the program.
325 C<INIT> blocks are run just before the Perl runtime begins execution, in
326 "first in, first out" (FIFO) order.
328 When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
329 C<END> work just as they do in B<awk>, as a degenerate case.
330 Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>
331 switch for a compile-only syntax check, although your main code
334 The B<begincheck> program makes it all clear, eventually:
340 print "10. Ordinary code runs at runtime.\n";
342 END { print "16. So this is the end of the tale.\n" }
343 INIT { print " 7. INIT blocks run FIFO just before runtime.\n" }
345 print " 4. And therefore before any CHECK blocks.\n"
347 CHECK { print " 6. So this is the sixth line.\n" }
349 print "11. It runs in order, of course.\n";
351 BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
352 END { print "15. Read perlmod for the rest of the story.\n" }
353 CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
354 INIT { print " 8. Run this again, using Perl's -c switch.\n" }
356 print "12. This is anti-obfuscated code.\n";
358 END { print "14. END blocks run LIFO at quitting time.\n" }
359 BEGIN { print " 2. So this line comes out second.\n" }
361 print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
363 INIT { print " 9. You'll see the difference right away.\n" }
365 print "13. It merely _looks_ like it should be confusing.\n";
372 There is no special class syntax in Perl, but a package may act
373 as a class if it provides subroutines to act as methods. Such a
374 package may also derive some of its methods from another class (package)
375 by listing the other package name(s) in its global @ISA array (which
376 must be a package global, not a lexical).
378 For more on this, see L<perltoot> and L<perlobj>.
383 A module is just a set of related functions in a library file, i.e.,
384 a Perl package with the same name as the file. It is specifically
385 designed to be reusable by other modules or programs. It may do this
386 by providing a mechanism for exporting some of its symbols into the
387 symbol table of any package using it, or it may function as a class
388 definition and make its semantics available implicitly through
389 method calls on the class and its objects, without explicitly
390 exporting anything. Or it can do a little of both.
392 For example, to start a traditional, non-OO module called Some::Module,
393 create a file called F<Some/Module.pm> and start with this template:
395 package Some::Module; # assumes Some/Module.pm
402 our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
404 # set the version for version checking
406 # if using RCS/CVS, this may be preferred
407 $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
410 @EXPORT = qw(&func1 &func2 &func4);
411 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
413 # your exported package globals go here,
414 # as well as any optionally exported functions
415 @EXPORT_OK = qw($Var1 %Hashit &func3);
419 # exported package globals go here
423 # non-exported package globals go here
427 # initialize package globals, first exported ones
431 # then the others (which are still accessible as $Some::Module::stuff)
435 # all file-scoped lexicals must be created before
436 # the functions below that use them.
438 # file-private lexicals go here
440 my %secret_hash = ();
442 # here's a file-private function as a closure,
443 # callable as &$priv_func; it cannot be prototyped.
444 my $priv_func = sub {
448 # make all your functions, whether exported or not;
449 # remember to put something interesting in the {} stubs
450 sub func1 {} # no prototype
451 sub func2() {} # proto'd void
452 sub func3($$) {} # proto'd to 2 scalars
454 # this one isn't exported, but could be called!
455 sub func4(\%) {} # proto'd to 1 hash ref
457 END { } # module clean-up code here (global destructor)
459 ## YOUR CODE GOES HERE
461 1; # don't forget to return a true value from the file
463 Then go on to declare and use your variables in functions without
464 any qualifications. See L<Exporter> and the L<perlmodlib> for
465 details on mechanics and style issues in module creation.
467 Perl modules are included into your program by saying
475 This is exactly equivalent to
477 BEGIN { require Module; import Module; }
481 BEGIN { require Module; import Module LIST; }
487 is exactly equivalent to
489 BEGIN { require Module; }
491 All Perl module files have the extension F<.pm>. The C<use> operator
492 assumes this so you don't have to spell out "F<Module.pm>" in quotes.
493 This also helps to differentiate new modules from old F<.pl> and
494 F<.ph> files. Module names are also capitalized unless they're
495 functioning as pragmas; pragmas are in effect compiler directives,
496 and are sometimes called "pragmatic modules" (or even "pragmata"
497 if you're a classicist).
502 require "SomeModule.pm";
504 differ from each other in two ways. In the first case, any double
505 colons in the module name, such as C<Some::Module>, are translated
506 into your system's directory separator, usually "/". The second
507 case does not, and would have to be specified literally. The other
508 difference is that seeing the first C<require> clues in the compiler
509 that uses of indirect object notation involving "SomeModule", as
510 in C<$ob = purge SomeModule>, are method calls, not function calls.
511 (Yes, this really can make a difference.)
513 Because the C<use> statement implies a C<BEGIN> block, the importing
514 of semantics happens as soon as the C<use> statement is compiled,
515 before the rest of the file is compiled. This is how it is able
516 to function as a pragma mechanism, and also how modules are able to
517 declare subroutines that are then visible as list or unary operators for
518 the rest of the current file. This will not work if you use C<require>
519 instead of C<use>. With C<require> you can get into this problem:
521 require Cwd; # make Cwd:: accessible
522 $here = Cwd::getcwd();
524 use Cwd; # import names from Cwd::
527 require Cwd; # make Cwd:: accessible
528 $here = getcwd(); # oops! no main::getcwd()
530 In general, C<use Module ()> is recommended over C<require Module>,
531 because it determines module availability at compile time, not in the
532 middle of your program's execution. An exception would be if two modules
533 each tried to C<use> each other, and each also called a function from
534 that other module. In that case, it's easy to use C<require> instead.
536 Perl packages may be nested inside other package names, so we can have
537 package names containing C<::>. But if we used that package name
538 directly as a filename it would make for unwieldy or impossible
539 filenames on some systems. Therefore, if a module's name is, say,
540 C<Text::Soundex>, then its definition is actually found in the library
541 file F<Text/Soundex.pm>.
543 Perl modules always have a F<.pm> file, but there may also be
544 dynamically linked executables (often ending in F<.so>) or autoloaded
545 subroutine definitions (often ending in F<.al>) associated with the
546 module. If so, these will be entirely transparent to the user of
547 the module. It is the responsibility of the F<.pm> file to load
548 (or arrange to autoload) any additional functionality. For example,
549 although the POSIX module happens to do both dynamic loading and
550 autoloading, the user can say just C<use POSIX> to get it all.
552 =head2 Making your module threadsafe
553 X<threadsafe> X<thread safe>
554 X<module, threadsafe> X<module, thread safe>
555 X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread>
557 Since 5.6.0, Perl has had support for a new type of threads called
558 interpreter threads (ithreads). These threads can be used explicitly
561 Ithreads work by cloning the data tree so that no data is shared
562 between different threads. These threads can be used by using the C<threads>
563 module or by doing fork() on win32 (fake fork() support). When a
564 thread is cloned all Perl data is cloned, however non-Perl data cannot
565 be cloned automatically. Perl after 5.7.2 has support for the C<CLONE>
566 special subroutine. In C<CLONE> you can do whatever
568 like for example handle the cloning of non-Perl data, if necessary.
569 C<CLONE> will be called once as a class method for every package that has it
570 defined (or inherits it). It will be called in the context of the new thread,
571 so all modifications are made in the new area. Currently CLONE is called with
572 no parameters other than the invocant package name, but code should not assume
573 that this will remain unchanged, as it is likely that in future extra parameters
574 will be passed in to give more information about the state of cloning.
576 If you want to CLONE all objects you will need to keep track of them per
577 package. This is simply done using a hash and Scalar::Util::weaken().
579 Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine.
580 Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is
581 called just before cloning starts, and in the context of the parent
582 thread. If it returns a true value, then no objects of that class will
583 be cloned; or rather, they will be copied as unblessed, undef values.
584 This provides a simple mechanism for making a module threadsafe; just add
585 C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be
586 now only be called once per object. Of course, if the child thread needs
587 to make use of the objects, then a more sophisticated approach is
590 Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other
591 than the invocant package name, although that may change. Similarly, to
592 allow for future expansion, the return value should be a single C<0> or
597 See L<perlmodlib> for general style issues related to building Perl
598 modules and classes, as well as descriptions of the standard library
599 and CPAN, L<Exporter> for how Perl's standard import/export mechanism
600 works, L<perltoot> and L<perltooc> for an in-depth tutorial on
601 creating classes, L<perlobj> for a hard-core reference document on
602 objects, L<perlsub> for an explanation of functions and scoping,
603 and L<perlxstut> and L<perlguts> for more information on writing