1PERLMOD(1)             Perl Programmers Reference Guide             PERLMOD(1)
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NAME

6       perlmod - Perl modules (packages and symbol tables)
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DESCRIPTION

9   Packages
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, "eval", or
16       file, whichever comes first (the same scope as the my() and local()
17       operators).  Unqualified dynamic identifiers will be in this namespace,
18       except for those few identifiers that if unqualified, default to the
19       main package instead of the current one as described below.  A package
20       statement affects only dynamic variables--including those you've used
21       local() on--but not lexical variables created with my().  Typically it
22       would be the first declaration in a file included by the "do",
23       "require", or "use" operators.  You can switch into a package in more
24       than one place; it merely influences which symbol table is used by the
25       compiler for the rest of that block.  You can refer to variables and
26       filehandles in other packages by prefixing the identifier with the
27       package name and a double colon: $Package::Variable.  If the package
28       name is null, the "main" package is assumed.  That is, $::sail is
29       equivalent to $main::sail.
30
31       The old package delimiter was a single quote, but double colon is now
32       the preferred delimiter, in part because it's more readable to humans,
33       and in part because it's more readable to emacs macros.  It also makes
34       C++ programmers feel like they know what's going on--as opposed to
35       using the single quote as separator, which was there to make Ada
36       programmers feel like they knew what was going on.  Because the old-
37       fashioned syntax is still supported for backwards compatibility, if you
38       try to use a string like "This is $owner's house", you'll be accessing
39       $owner::s; that is, the $s variable in package "owner", which is
40       probably not what you meant.  Use braces to disambiguate, as in "This
41       is ${owner}'s house".
42
43       Packages may themselves contain package separators, as in
44       $OUTER::INNER::var.  This implies nothing about the order of name
45       lookups, however.  There are no relative packages: all symbols are
46       either local to the current package, or must be fully qualified from
47       the outer package name down.  For instance, there is nowhere within
48       package "OUTER" that $INNER::var refers to $OUTER::INNER::var.  "INNER"
49       refers to a totally separate global package.
50
51       Only identifiers starting with letters (or underscore) are stored in a
52       package's symbol table.  All other symbols are kept in package "main",
53       including all punctuation variables, like $_.  In addition, when
54       unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV,
55       INC, and SIG are forced to be in package "main", even when used for
56       other purposes than their built-in ones.  If you have a package called
57       "m", "s", or "y", then you can't use the qualified form of an
58       identifier because it would be instead interpreted as a pattern match,
59       a substitution, or a transliteration.
60
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
64       names.  However, variables and functions named with a single "_", such
65       as $_ and "sub _", are still forced into the package "main".  See also
66       "Technical Note on the Syntax of Variable Names" in perlvar.
67
68       "eval"ed strings are compiled in the package in which the eval() was
69       compiled.  (Assignments to $SIG{}, however, assume the signal handler
70       specified is in the "main" package.  Qualify the signal handler name if
71       you wish to have a signal handler in a package.)  For an example,
72       examine perldb.pl in the Perl library.  It initially switches to the
73       "DB" package so that the debugger doesn't interfere with variables in
74       the program you are trying to debug.  At various points, however, it
75       temporarily switches back to the "main" package to evaluate various
76       expressions in the context of the "main" package (or wherever you came
77       from).  See perldebug.
78
79       The special symbol "__PACKAGE__" contains the current package, but
80       cannot (easily) be used to construct variable names.
81
82       See perlsub for other scoping issues related to my() and local(), and
83       perlref regarding closures.
84
85   Symbol Tables
86       The symbol table for a package happens to be stored in the hash of that
87       name with two colons appended.  The main symbol table's name is thus
88       %main::, or %:: for short.  Likewise the symbol table for the nested
89       package mentioned earlier is named %OUTER::INNER::.
90
91       The value in each entry of the hash is what you are referring to when
92       you use the *name typeglob notation.
93
94           local *main::foo    = *main::bar;
95
96       You can use this to print out all the variables in a package, for
97       instance.  The standard but antiquated dumpvar.pl library and the CPAN
98       module Devel::Symdump make use of this.
99
100       Assignment to a typeglob performs an aliasing operation, i.e.,
101
102           *dick = *richard;
103
104       causes variables, subroutines, formats, and file and directory handles
105       accessible via the identifier "richard" also to be accessible via the
106       identifier "dick".  If you want to alias only a particular variable or
107       subroutine, assign a reference instead:
108
109           *dick = \$richard;
110
111       Which makes $richard and $dick the same variable, but leaves @richard
112       and @dick as separate arrays.  Tricky, eh?
113
114       There is one subtle difference between the following statements:
115
116           *foo = *bar;
117           *foo = \$bar;
118
119       "*foo = *bar" makes the typeglobs themselves synonymous while "*foo =
120       \$bar" makes the SCALAR portions of two distinct typeglobs refer to the
121       same scalar value. This means that the following code:
122
123           $bar = 1;
124           *foo = \$bar;       # Make $foo an alias for $bar
125
126           {
127               local $bar = 2; # Restrict changes to block
128               print $foo;     # Prints '1'!
129           }
130
131       Would print '1', because $foo holds a reference to the original $bar --
132       the one that was stuffed away by "local()" and which will be restored
133       when the block ends. Because variables are accessed through the
134       typeglob, you can use "*foo = *bar" to create an alias which can be
135       localized. (But be aware that this means you can't have a separate @foo
136       and @bar, etc.)
137
138       What makes all of this important is that the Exporter module uses glob
139       aliasing as the import/export mechanism. Whether or not you can
140       properly localize a variable that has been exported from a module
141       depends on how it was exported:
142
143           @EXPORT = qw($FOO); # Usual form, can't be localized
144           @EXPORT = qw(*FOO); # Can be localized
145
146       You can work around the first case by using the fully qualified name
147       ($Package::FOO) where you need a local value, or by overriding it by
148       saying "*FOO = *Package::FOO" in your script.
149
150       The "*x = \$y" mechanism may be used to pass and return cheap
151       references into or from subroutines if you don't want to copy the whole
152       thing.  It only works when assigning to dynamic variables, not
153       lexicals.
154
155           %some_hash = ();                    # can't be my()
156           *some_hash = fn( \%another_hash );
157           sub fn {
158               local *hashsym = shift;
159               # now use %hashsym normally, and you
160               # will affect the caller's %another_hash
161               my %nhash = (); # do what you want
162               return \%nhash;
163           }
164
165       On return, the reference will overwrite the hash slot in the symbol
166       table specified by the *some_hash typeglob.  This is a somewhat tricky
167       way of passing around references cheaply when you don't want to have to
168       remember to dereference variables explicitly.
169
170       Another use of symbol tables is for making "constant" scalars.
171
172           *PI = \3.14159265358979;
173
174       Now you cannot alter $PI, which is probably a good thing all in all.
175       This isn't the same as a constant subroutine, which is subject to
176       optimization at compile-time.  A constant subroutine is one prototyped
177       to take no arguments and to return a constant expression.  See perlsub
178       for details on these.  The "use constant" pragma is a convenient
179       shorthand for these.
180
181       You can say *foo{PACKAGE} and *foo{NAME} to find out what name and
182       package the *foo symbol table entry comes from.  This may be useful in
183       a subroutine that gets passed typeglobs as arguments:
184
185           sub identify_typeglob {
186               my $glob = shift;
187               print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
188           }
189           identify_typeglob *foo;
190           identify_typeglob *bar::baz;
191
192       This prints
193
194           You gave me main::foo
195           You gave me bar::baz
196
197       The *foo{THING} notation can also be used to obtain references to the
198       individual elements of *foo.  See perlref.
199
200       Subroutine definitions (and declarations, for that matter) need not
201       necessarily be situated in the package whose symbol table they occupy.
202       You can define a subroutine outside its package by explicitly
203       qualifying the name of the subroutine:
204
205           package main;
206           sub Some_package::foo { ... }   # &foo defined in Some_package
207
208       This is just a shorthand for a typeglob assignment at compile time:
209
210           BEGIN { *Some_package::foo = sub { ... } }
211
212       and is not the same as writing:
213
214           {
215               package Some_package;
216               sub foo { ... }
217           }
218
219       In the first two versions, the body of the subroutine is lexically in
220       the main package, not in Some_package. So something like this:
221
222           package main;
223
224           $Some_package::name = "fred";
225           $main::name = "barney";
226
227           sub Some_package::foo {
228               print "in ", __PACKAGE__, ": \$name is '$name'\n";
229           }
230
231           Some_package::foo();
232
233       prints:
234
235           in main: $name is 'barney'
236
237       rather than:
238
239           in Some_package: $name is 'fred'
240
241       This also has implications for the use of the SUPER:: qualifier (see
242       perlobj).
243
244   BEGIN, UNITCHECK, CHECK, INIT and END
245       Five specially named code blocks are executed at the beginning and at
246       the end of a running Perl program.  These are the "BEGIN", "UNITCHECK",
247       "CHECK", "INIT", and "END" blocks.
248
249       These code blocks can be prefixed with "sub" to give the appearance of
250       a subroutine (although this is not considered good style).  One should
251       note that these code blocks don't really exist as named subroutines
252       (despite their appearance). The thing that gives this away is the fact
253       that you can have more than one of these code blocks in a program, and
254       they will get all executed at the appropriate moment.  So you can't
255       execute any of these code blocks by name.
256
257       A "BEGIN" code block is executed as soon as possible, that is, the
258       moment it is completely defined, even before the rest of the containing
259       file (or string) is parsed.  You may have multiple "BEGIN" blocks
260       within a file (or eval'ed string) -- they will execute in order of
261       definition.  Because a "BEGIN" code block executes immediately, it can
262       pull in definitions of subroutines and such from other files in time to
263       be visible to the rest of the compile and run time.  Once a "BEGIN" has
264       run, it is immediately undefined and any code it used is returned to
265       Perl's memory pool.
266
267       An "END" code block is executed as late as possible, that is, after
268       perl has finished running the program and just before the interpreter
269       is being exited, even if it is exiting as a result of a die() function.
270       (But not if it's morphing into another program via "exec", or being
271       blown out of the water by a signal--you have to trap that yourself (if
272       you can).)  You may have multiple "END" blocks within a file--they will
273       execute in reverse order of definition; that is: last in, first out
274       (LIFO).  "END" blocks are not executed when you run perl with the "-c"
275       switch, or if compilation fails.
276
277       Note that "END" code blocks are not executed at the end of a string
278       "eval()": if any "END" code blocks are created in a string "eval()",
279       they will be executed just as any other "END" code block of that
280       package in LIFO order just before the interpreter is being exited.
281
282       Inside an "END" code block, $? contains the value that the program is
283       going to pass to "exit()".  You can modify $? to change the exit value
284       of the program.  Beware of changing $? by accident (e.g. by running
285       something via "system").
286
287       "UNITCHECK", "CHECK" and "INIT" code blocks are useful to catch the
288       transition between the compilation phase and the execution phase of the
289       main program.
290
291       "UNITCHECK" blocks are run just after the unit which defined them has
292       been compiled.  The main program file and each module it loads are
293       compilation units, as are string "eval"s, code compiled using the "(?{
294       })" construct in a regex, calls to "do FILE", "require FILE", and code
295       after the "-e" switch on the command line.
296
297       "CHECK" code blocks are run just after the initial Perl compile phase
298       ends and before the run time begins, in LIFO order.  "CHECK" code
299       blocks are used in the Perl compiler suite to save the compiled state
300       of the program.
301
302       "INIT" blocks are run just before the Perl runtime begins execution, in
303       "first in, first out" (FIFO) order.
304
305       The "CHECK" and "INIT" code blocks will not be executed inside a string
306       eval(), if that eval() happens after the end of the main compilation
307       phase; that can be a problem in mod_perl and other persistent
308       environments which use "eval STRING" to load code at runtime.
309
310       When you use the -n and -p switches to Perl, "BEGIN" and "END" work
311       just as they do in awk, as a degenerate case.  Both "BEGIN" and "CHECK"
312       blocks are run when you use the -c switch for a compile-only syntax
313       check, although your main code is not.
314
315       The begincheck program makes it all clear, eventually:
316
317         #!/usr/bin/perl
318
319         # begincheck
320
321         print         "10. Ordinary code runs at runtime.\n";
322
323         END { print   "16.   So this is the end of the tale.\n" }
324         INIT { print  " 7. INIT blocks run FIFO just before runtime.\n" }
325         UNITCHECK {
326           print       " 4.   And therefore before any CHECK blocks.\n"
327         }
328         CHECK { print " 6.   So this is the sixth line.\n" }
329
330         print         "11.   It runs in order, of course.\n";
331
332         BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
333         END { print   "15.   Read perlmod for the rest of the story.\n" }
334         CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
335         INIT { print  " 8.   Run this again, using Perl's -c switch.\n" }
336
337         print         "12.   This is anti-obfuscated code.\n";
338
339         END { print   "14. END blocks run LIFO at quitting time.\n" }
340         BEGIN { print " 2.   So this line comes out second.\n" }
341         UNITCHECK {
342          print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
343         }
344         INIT { print  " 9.   You'll see the difference right away.\n" }
345
346         print         "13.   It merely _looks_ like it should be confusing.\n";
347
348         __END__
349
350   Perl Classes
351       There is no special class syntax in Perl, but a package may act as a
352       class if it provides subroutines to act as methods.  Such a package may
353       also derive some of its methods from another class (package) by listing
354       the other package name(s) in its global @ISA array (which must be a
355       package global, not a lexical).
356
357       For more on this, see perltoot and perlobj.
358
359   Perl Modules
360       A module is just a set of related functions in a library file, i.e., a
361       Perl package with the same name as the file.  It is specifically
362       designed to be reusable by other modules or programs.  It may do this
363       by providing a mechanism for exporting some of its symbols into the
364       symbol table of any package using it, or it may function as a class
365       definition and make its semantics available implicitly through method
366       calls on the class and its objects, without explicitly exporting
367       anything.  Or it can do a little of both.
368
369       For example, to start a traditional, non-OO module called Some::Module,
370       create a file called Some/Module.pm and start with this template:
371
372           package Some::Module;  # assumes Some/Module.pm
373
374           use strict;
375           use warnings;
376
377           BEGIN {
378               use Exporter   ();
379               our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
380
381               # set the version for version checking
382               $VERSION     = 1.00;
383               # if using RCS/CVS, this may be preferred
384               $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
385
386               @ISA         = qw(Exporter);
387               @EXPORT      = qw(&func1 &func2 &func4);
388               %EXPORT_TAGS = ( );     # eg: TAG => [ qw!name1 name2! ],
389
390               # your exported package globals go here,
391               # as well as any optionally exported functions
392               @EXPORT_OK   = qw($Var1 %Hashit &func3);
393           }
394           our @EXPORT_OK;
395
396           # exported package globals go here
397           our $Var1;
398           our %Hashit;
399
400           # non-exported package globals go here
401           our @more;
402           our $stuff;
403
404           # initialize package globals, first exported ones
405           $Var1   = '';
406           %Hashit = ();
407
408           # then the others (which are still accessible as $Some::Module::stuff)
409           $stuff  = '';
410           @more   = ();
411
412           # all file-scoped lexicals must be created before
413           # the functions below that use them.
414
415           # file-private lexicals go here
416           my $priv_var    = '';
417           my %secret_hash = ();
418
419           # here's a file-private function as a closure,
420           # callable as &$priv_func;  it cannot be prototyped.
421           my $priv_func = sub {
422               # stuff goes here.
423           };
424
425           # make all your functions, whether exported or not;
426           # remember to put something interesting in the {} stubs
427           sub func1      {}    # no prototype
428           sub func2()    {}    # proto'd void
429           sub func3($$)  {}    # proto'd to 2 scalars
430
431           # this one isn't exported, but could be called!
432           sub func4(\%)  {}    # proto'd to 1 hash ref
433
434           END { }       # module clean-up code here (global destructor)
435
436           ## YOUR CODE GOES HERE
437
438           1;  # don't forget to return a true value from the file
439
440       Then go on to declare and use your variables in functions without any
441       qualifications.  See Exporter and the perlmodlib for details on
442       mechanics and style issues in module creation.
443
444       Perl modules are included into your program by saying
445
446           use Module;
447
448       or
449
450           use Module LIST;
451
452       This is exactly equivalent to
453
454           BEGIN { require Module; import Module; }
455
456       or
457
458           BEGIN { require Module; import Module LIST; }
459
460       As a special case
461
462           use Module ();
463
464       is exactly equivalent to
465
466           BEGIN { require Module; }
467
468       All Perl module files have the extension .pm.  The "use" operator
469       assumes this so you don't have to spell out "Module.pm" in quotes.
470       This also helps to differentiate new modules from old .pl and .ph
471       files.  Module names are also capitalized unless they're functioning as
472       pragmas; pragmas are in effect compiler directives, and are sometimes
473       called "pragmatic modules" (or even "pragmata" if you're a classicist).
474
475       The two statements:
476
477           require SomeModule;
478           require "SomeModule.pm";
479
480       differ from each other in two ways.  In the first case, any double
481       colons in the module name, such as "Some::Module", are translated into
482       your system's directory separator, usually "/".   The second case does
483       not, and would have to be specified literally.  The other difference is
484       that seeing the first "require" clues in the compiler that uses of
485       indirect object notation involving "SomeModule", as in "$ob = purge
486       SomeModule", are method calls, not function calls.  (Yes, this really
487       can make a difference.)
488
489       Because the "use" statement implies a "BEGIN" block, the importing of
490       semantics happens as soon as the "use" statement is compiled, before
491       the rest of the file is compiled.  This is how it is able to function
492       as a pragma mechanism, and also how modules are able to declare
493       subroutines that are then visible as list or unary operators for the
494       rest of the current file.  This will not work if you use "require"
495       instead of "use".  With "require" you can get into this problem:
496
497           require Cwd;                # make Cwd:: accessible
498           $here = Cwd::getcwd();
499
500           use Cwd;                    # import names from Cwd::
501           $here = getcwd();
502
503           require Cwd;                # make Cwd:: accessible
504           $here = getcwd();           # oops! no main::getcwd()
505
506       In general, "use Module ()" is recommended over "require Module",
507       because it determines module availability at compile time, not in the
508       middle of your program's execution.  An exception would be if two
509       modules each tried to "use" each other, and each also called a function
510       from that other module.  In that case, it's easy to use "require"
511       instead.
512
513       Perl packages may be nested inside other package names, so we can have
514       package names containing "::".  But if we used that package name
515       directly as a filename it would make for unwieldy or impossible
516       filenames on some systems.  Therefore, if a module's name is, say,
517       "Text::Soundex", then its definition is actually found in the library
518       file Text/Soundex.pm.
519
520       Perl modules always have a .pm file, but there may also be dynamically
521       linked executables (often ending in .so) or autoloaded subroutine
522       definitions (often ending in .al) associated with the module.  If so,
523       these will be entirely transparent to the user of the module.  It is
524       the responsibility of the .pm file to load (or arrange to autoload) any
525       additional functionality.  For example, although the POSIX module
526       happens to do both dynamic loading and autoloading, the user can say
527       just "use POSIX" to get it all.
528
529   Making your module threadsafe
530       Since 5.6.0, Perl has had support for a new type of threads called
531       interpreter threads (ithreads). These threads can be used explicitly
532       and implicitly.
533
534       Ithreads work by cloning the data tree so that no data is shared
535       between different threads. These threads can be used by using the
536       "threads" module or by doing fork() on win32 (fake fork() support).
537       When a thread is cloned all Perl data is cloned, however non-Perl data
538       cannot be cloned automatically.  Perl after 5.7.2 has support for the
539       "CLONE" special subroutine.  In "CLONE" you can do whatever you need to
540       do, like for example handle the cloning of non-Perl data, if necessary.
541       "CLONE" will be called once as a class method for every package that
542       has it defined (or inherits it).  It will be called in the context of
543       the new thread, so all modifications are made in the new area.
544       Currently CLONE is called with no parameters other than the invocant
545       package name, but code should not assume that this will remain
546       unchanged, as it is likely that in future extra parameters will be
547       passed in to give more information about the state of cloning.
548
549       If you want to CLONE all objects you will need to keep track of them
550       per package. This is simply done using a hash and
551       Scalar::Util::weaken().
552
553       Perl after 5.8.7 has support for the "CLONE_SKIP" special subroutine.
554       Like "CLONE", "CLONE_SKIP" is called once per package; however, it is
555       called just before cloning starts, and in the context of the parent
556       thread. If it returns a true value, then no objects of that class will
557       be cloned; or rather, they will be copied as unblessed, undef values.
558       For example: if in the parent there are two references to a single
559       blessed hash, then in the child there will be two references to a
560       single undefined scalar value instead.  This provides a simple
561       mechanism for making a module threadsafe; just add "sub CLONE_SKIP { 1
562       }" at the top of the class, and "DESTROY()" will be now only be called
563       once per object. Of course, if the child thread needs to make use of
564       the objects, then a more sophisticated approach is needed.
565
566       Like "CLONE", "CLONE_SKIP" is currently called with no parameters other
567       than the invocant package name, although that may change. Similarly, to
568       allow for future expansion, the return value should be a single 0 or 1
569       value.
570

SEE ALSO

572       See perlmodlib for general style issues related to building Perl
573       modules and classes, as well as descriptions of the standard library
574       and CPAN, Exporter for how Perl's standard import/export mechanism
575       works, perltoot and perltooc for an in-depth tutorial on creating
576       classes, perlobj for a hard-core reference document on objects, perlsub
577       for an explanation of functions and scoping, and perlxstut and perlguts
578       for more information on writing extension modules.
579
580
581
582perl v5.10.1                      2009-07-24                        PERLMOD(1)
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