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1 | =head1 NAME |
2 | ||
3 | perlmod - Perl modules (packages and symbol tables) | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Packages | |
8 | X<package> X<namespace> X<variable, global> X<global variable> X<global> | |
9 | ||
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 | |
30 | C<$main::sail>. | |
31 | ||
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 ${owner}'s house">. | |
42 | X<::> X<'> | |
43 | ||
44 | Packages may themselves contain package separators, as in | |
45 | C<$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. | |
52 | ||
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> | |
63 | ||
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 | $_ and C<sub _>, are still forced into the package C<main>. See also | |
69 | L<perlvar/"Technical Note on the Syntax of Variable Names">. | |
70 | ||
71 | C<eval>ed strings are compiled in the package in which the eval() was | |
72 | compiled. (Assignments to C<$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>. | |
81 | ||
82 | The special symbol C<__PACKAGE__> contains the current package, but cannot | |
83 | (easily) be used to construct variable names. | |
84 | ||
85 | See L<perlsub> for other scoping issues related to my() and local(), | |
86 | and L<perlref> regarding closures. | |
87 | ||
88 | =head2 Symbol Tables | |
89 | X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias> | |
90 | ||
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::>. | |
95 | ||
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: | |
100 | ||
101 | local *main::foo = *main::bar; | |
102 | local $main::{foo} = $main::{bar}; | |
103 | ||
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 | |
108 | the same package.) | |
109 | ||
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. | |
113 | ||
114 | Assignment to a typeglob performs an aliasing operation, i.e., | |
115 | ||
116 | *dick = *richard; | |
117 | ||
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: | |
122 | ||
123 | *dick = \$richard; | |
124 | ||
125 | Which makes $richard and $dick the same variable, but leaves | |
126 | @richard and @dick as separate arrays. Tricky, eh? | |
127 | ||
128 | There is one subtle difference between the following statements: | |
129 | ||
130 | *foo = *bar; | |
131 | *foo = \$bar; | |
132 | ||
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: | |
136 | ||
137 | $bar = 1; | |
138 | *foo = \$bar; # Make $foo an alias for $bar | |
139 | ||
140 | { | |
141 | local $bar = 2; # Restrict changes to block | |
142 | print $foo; # Prints '1'! | |
143 | } | |
144 | ||
145 | Would print '1', because C<$foo> holds a reference to the I<original> | |
146 | C<$bar> -- the one that was stuffed away by C<local()> and which will be | |
147 | restored when the block ends. Because variables are accessed through the | |
148 | typeglob, you can use C<*foo = *bar> to create an alias which can be | |
149 | localized. (But be aware that this means you can't have a separate | |
150 | C<@foo> and C<@bar>, etc.) | |
151 | ||
152 | What makes all of this important is that the Exporter module uses glob | |
153 | aliasing as the import/export mechanism. Whether or not you can properly | |
154 | localize a variable that has been exported from a module depends on how | |
155 | it was exported: | |
156 | ||
157 | @EXPORT = qw($FOO); # Usual form, can't be localized | |
158 | @EXPORT = qw(*FOO); # Can be localized | |
159 | ||
160 | You can work around the first case by using the fully qualified name | |
161 | (C<$Package::FOO>) where you need a local value, or by overriding it | |
162 | by saying C<*FOO = *Package::FOO> in your script. | |
163 | ||
164 | The C<*x = \$y> mechanism may be used to pass and return cheap references | |
165 | into or from subroutines if you don't want to copy the whole | |
166 | thing. It only works when assigning to dynamic variables, not | |
167 | lexicals. | |
168 | ||
169 | %some_hash = (); # can't be my() | |
170 | *some_hash = fn( \%another_hash ); | |
171 | sub fn { | |
172 | local *hashsym = shift; | |
173 | # now use %hashsym normally, and you | |
174 | # will affect the caller's %another_hash | |
175 | my %nhash = (); # do what you want | |
176 | return \%nhash; | |
177 | } | |
178 | ||
179 | On return, the reference will overwrite the hash slot in the | |
180 | symbol table specified by the *some_hash typeglob. This | |
181 | is a somewhat tricky way of passing around references cheaply | |
182 | when you don't want to have to remember to dereference variables | |
183 | explicitly. | |
184 | ||
185 | Another use of symbol tables is for making "constant" scalars. | |
186 | X<constant> X<scalar, constant> | |
187 | ||
188 | *PI = \3.14159265358979; | |
189 | ||
190 | Now you cannot alter C<$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. | |
196 | ||
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: | |
200 | ||
201 | sub identify_typeglob { | |
202 | my $glob = shift; | |
203 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; | |
204 | } | |
205 | identify_typeglob *foo; | |
206 | identify_typeglob *bar::baz; | |
207 | ||
208 | This prints | |
209 | ||
210 | You gave me main::foo | |
211 | You gave me bar::baz | |
212 | ||
213 | The C<*foo{THING}> notation can also be used to obtain references to the | |
214 | individual elements of *foo. See L<perlref>. | |
215 | ||
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: | |
220 | ||
221 | package main; | |
222 | sub Some_package::foo { ... } # &foo defined in Some_package | |
223 | ||
224 | This is just a shorthand for a typeglob assignment at compile time: | |
225 | ||
226 | BEGIN { *Some_package::foo = sub { ... } } | |
227 | ||
228 | and is I<not> the same as writing: | |
229 | ||
230 | { | |
231 | package Some_package; | |
232 | sub foo { ... } | |
233 | } | |
234 | ||
235 | In the first two versions, the body of the subroutine is | |
236 | lexically in the main package, I<not> in Some_package. So | |
237 | something like this: | |
238 | ||
239 | package main; | |
240 | ||
241 | $Some_package::name = "fred"; | |
242 | $main::name = "barney"; | |
243 | ||
244 | sub Some_package::foo { | |
245 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
246 | } | |
247 | ||
248 | Some_package::foo(); | |
249 | ||
250 | prints: | |
251 | ||
252 | in main: $name is 'barney' | |
253 | ||
254 | rather than: | |
255 | ||
256 | in Some_package: $name is 'fred' | |
257 | ||
258 | This also has implications for the use of the SUPER:: qualifier | |
259 | (see L<perlobj>). | |
260 | ||
261 | =head2 BEGIN, CHECK, INIT and END | |
262 | X<BEGIN> X<CHECK> X<INIT> X<END> | |
263 | ||
264 | Four specially named code blocks are executed at the beginning and at the end | |
265 | of a running Perl program. These are the C<BEGIN>, C<CHECK>, C<INIT>, and | |
266 | C<END> blocks. | |
267 | ||
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. | |
275 | ||
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. | |
284 | ||
285 | It should be noted that C<BEGIN> code blocks B<are> executed inside string | |
286 | C<eval()>'s. The C<CHECK> and C<INIT> code blocks are B<not> executed inside | |
287 | a string eval, which e.g. can be a problem in a mod_perl environment. | |
288 | ||
289 | An C<END> code block is executed as late as possible, that is, after | |
290 | perl has finished running the program and just before the interpreter | |
291 | is being exited, even if it is exiting as a result of a die() function. | |
292 | (But not if it's morphing into another program via C<exec>, or | |
293 | being blown out of the water by a signal--you have to trap that yourself | |
294 | (if you can).) You may have multiple C<END> blocks within a file--they | |
295 | will execute in reverse order of definition; that is: last in, first | |
296 | out (LIFO). C<END> blocks are not executed when you run perl with the | |
297 | C<-c> switch, or if compilation fails. | |
298 | ||
299 | Note that C<END> code blocks are B<not> executed at the end of a string | |
300 | C<eval()>: if any C<END> code blocks are created in a string C<eval()>, | |
301 | they will be executed just as any other C<END> code block of that package | |
302 | in LIFO order just before the interpreter is being exited. | |
303 | ||
304 | Inside an C<END> code block, C<$?> contains the value that the program is | |
305 | going to pass to C<exit()>. You can modify C<$?> to change the exit | |
306 | value of the program. Beware of changing C<$?> by accident (e.g. by | |
307 | running something via C<system>). | |
308 | X<$?> | |
309 | ||
310 | C<CHECK> and C<INIT> code blocks are useful to catch the transition between | |
311 | the compilation phase and the execution phase of the main program. | |
312 | ||
313 | C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends | |
314 | and before the run time begins, in LIFO order. C<CHECK> code blocks are used | |
315 | in the Perl compiler suite to save the compiled state of the program. | |
316 | ||
317 | C<INIT> blocks are run just before the Perl runtime begins execution, in | |
318 | "first in, first out" (FIFO) order. For example, the code generators | |
319 | documented in L<perlcc> make use of C<INIT> blocks to initialize and | |
320 | resolve pointers to XSUBs. | |
321 | ||
322 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and | |
323 | C<END> work just as they do in B<awk>, as a degenerate case. | |
324 | Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c> | |
325 | switch for a compile-only syntax check, although your main code | |
326 | is not. | |
327 | ||
328 | The B<begincheck> program makes it all clear, eventually: | |
329 | ||
330 | #!/usr/bin/perl | |
331 | ||
332 | # begincheck | |
333 | ||
334 | print " 8. Ordinary code runs at runtime.\n"; | |
335 | ||
336 | END { print "14. So this is the end of the tale.\n" } | |
337 | INIT { print " 5. INIT blocks run FIFO just before runtime.\n" } | |
338 | CHECK { print " 4. So this is the fourth line.\n" } | |
339 | ||
340 | print " 9. It runs in order, of course.\n"; | |
341 | ||
342 | BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" } | |
343 | END { print "13. Read perlmod for the rest of the story.\n" } | |
344 | CHECK { print " 3. CHECK blocks run LIFO at compilation's end.\n" } | |
345 | INIT { print " 6. Run this again, using Perl's -c switch.\n" } | |
346 | ||
347 | print "10. This is anti-obfuscated code.\n"; | |
348 | ||
349 | END { print "12. END blocks run LIFO at quitting time.\n" } | |
350 | BEGIN { print " 2. So this line comes out second.\n" } | |
351 | INIT { print " 7. You'll see the difference right away.\n" } | |
352 | ||
353 | print "11. It merely _looks_ like it should be confusing.\n"; | |
354 | ||
355 | __END__ | |
356 | ||
357 | =head2 Perl Classes | |
358 | X<class> X<@ISA> | |
359 | ||
360 | There is no special class syntax in Perl, but a package may act | |
361 | as a class if it provides subroutines to act as methods. Such a | |
362 | package may also derive some of its methods from another class (package) | |
363 | by listing the other package name(s) in its global @ISA array (which | |
364 | must be a package global, not a lexical). | |
365 | ||
366 | For more on this, see L<perltoot> and L<perlobj>. | |
367 | ||
368 | =head2 Perl Modules | |
369 | X<module> | |
370 | ||
371 | A module is just a set of related functions in a library file, i.e., | |
372 | a Perl package with the same name as the file. It is specifically | |
373 | designed to be reusable by other modules or programs. It may do this | |
374 | by providing a mechanism for exporting some of its symbols into the | |
375 | symbol table of any package using it, or it may function as a class | |
376 | definition and make its semantics available implicitly through | |
377 | method calls on the class and its objects, without explicitly | |
378 | exporting anything. Or it can do a little of both. | |
379 | ||
380 | For example, to start a traditional, non-OO module called Some::Module, | |
381 | create a file called F<Some/Module.pm> and start with this template: | |
382 | ||
383 | package Some::Module; # assumes Some/Module.pm | |
384 | ||
385 | use strict; | |
386 | use warnings; | |
387 | ||
388 | BEGIN { | |
389 | use Exporter (); | |
390 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); | |
391 | ||
392 | # set the version for version checking | |
393 | $VERSION = 1.00; | |
394 | # if using RCS/CVS, this may be preferred | |
395 | $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g; | |
396 | ||
397 | @ISA = qw(Exporter); | |
398 | @EXPORT = qw(&func1 &func2 &func4); | |
399 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], | |
400 | ||
401 | # your exported package globals go here, | |
402 | # as well as any optionally exported functions | |
403 | @EXPORT_OK = qw($Var1 %Hashit &func3); | |
404 | } | |
405 | our @EXPORT_OK; | |
406 | ||
407 | # exported package globals go here | |
408 | our $Var1; | |
409 | our %Hashit; | |
410 | ||
411 | # non-exported package globals go here | |
412 | our @more; | |
413 | our $stuff; | |
414 | ||
415 | # initialize package globals, first exported ones | |
416 | $Var1 = ''; | |
417 | %Hashit = (); | |
418 | ||
419 | # then the others (which are still accessible as $Some::Module::stuff) | |
420 | $stuff = ''; | |
421 | @more = (); | |
422 | ||
423 | # all file-scoped lexicals must be created before | |
424 | # the functions below that use them. | |
425 | ||
426 | # file-private lexicals go here | |
427 | my $priv_var = ''; | |
428 | my %secret_hash = (); | |
429 | ||
430 | # here's a file-private function as a closure, | |
431 | # callable as &$priv_func; it cannot be prototyped. | |
432 | my $priv_func = sub { | |
433 | # stuff goes here. | |
434 | }; | |
435 | ||
436 | # make all your functions, whether exported or not; | |
437 | # remember to put something interesting in the {} stubs | |
438 | sub func1 {} # no prototype | |
439 | sub func2() {} # proto'd void | |
440 | sub func3($$) {} # proto'd to 2 scalars | |
441 | ||
442 | # this one isn't exported, but could be called! | |
443 | sub func4(\%) {} # proto'd to 1 hash ref | |
444 | ||
445 | END { } # module clean-up code here (global destructor) | |
446 | ||
447 | ## YOUR CODE GOES HERE | |
448 | ||
449 | 1; # don't forget to return a true value from the file | |
450 | ||
451 | Then go on to declare and use your variables in functions without | |
452 | any qualifications. See L<Exporter> and the L<perlmodlib> for | |
453 | details on mechanics and style issues in module creation. | |
454 | ||
455 | Perl modules are included into your program by saying | |
456 | ||
457 | use Module; | |
458 | ||
459 | or | |
460 | ||
461 | use Module LIST; | |
462 | ||
463 | This is exactly equivalent to | |
464 | ||
465 | BEGIN { require Module; import Module; } | |
466 | ||
467 | or | |
468 | ||
469 | BEGIN { require Module; import Module LIST; } | |
470 | ||
471 | As a special case | |
472 | ||
473 | use Module (); | |
474 | ||
475 | is exactly equivalent to | |
476 | ||
477 | BEGIN { require Module; } | |
478 | ||
479 | All Perl module files have the extension F<.pm>. The C<use> operator | |
480 | assumes this so you don't have to spell out "F<Module.pm>" in quotes. | |
481 | This also helps to differentiate new modules from old F<.pl> and | |
482 | F<.ph> files. Module names are also capitalized unless they're | |
483 | functioning as pragmas; pragmas are in effect compiler directives, | |
484 | and are sometimes called "pragmatic modules" (or even "pragmata" | |
485 | if you're a classicist). | |
486 | ||
487 | The two statements: | |
488 | ||
489 | require SomeModule; | |
490 | require "SomeModule.pm"; | |
491 | ||
492 | differ from each other in two ways. In the first case, any double | |
493 | colons in the module name, such as C<Some::Module>, are translated | |
494 | into your system's directory separator, usually "/". The second | |
495 | case does not, and would have to be specified literally. The other | |
496 | difference is that seeing the first C<require> clues in the compiler | |
497 | that uses of indirect object notation involving "SomeModule", as | |
498 | in C<$ob = purge SomeModule>, are method calls, not function calls. | |
499 | (Yes, this really can make a difference.) | |
500 | ||
501 | Because the C<use> statement implies a C<BEGIN> block, the importing | |
502 | of semantics happens as soon as the C<use> statement is compiled, | |
503 | before the rest of the file is compiled. This is how it is able | |
504 | to function as a pragma mechanism, and also how modules are able to | |
505 | declare subroutines that are then visible as list or unary operators for | |
506 | the rest of the current file. This will not work if you use C<require> | |
507 | instead of C<use>. With C<require> you can get into this problem: | |
508 | ||
509 | require Cwd; # make Cwd:: accessible | |
510 | $here = Cwd::getcwd(); | |
511 | ||
512 | use Cwd; # import names from Cwd:: | |
513 | $here = getcwd(); | |
514 | ||
515 | require Cwd; # make Cwd:: accessible | |
516 | $here = getcwd(); # oops! no main::getcwd() | |
517 | ||
518 | In general, C<use Module ()> is recommended over C<require Module>, | |
519 | because it determines module availability at compile time, not in the | |
520 | middle of your program's execution. An exception would be if two modules | |
521 | each tried to C<use> each other, and each also called a function from | |
522 | that other module. In that case, it's easy to use C<require> instead. | |
523 | ||
524 | Perl packages may be nested inside other package names, so we can have | |
525 | package names containing C<::>. But if we used that package name | |
526 | directly as a filename it would make for unwieldy or impossible | |
527 | filenames on some systems. Therefore, if a module's name is, say, | |
528 | C<Text::Soundex>, then its definition is actually found in the library | |
529 | file F<Text/Soundex.pm>. | |
530 | ||
531 | Perl modules always have a F<.pm> file, but there may also be | |
532 | dynamically linked executables (often ending in F<.so>) or autoloaded | |
533 | subroutine definitions (often ending in F<.al>) associated with the | |
534 | module. If so, these will be entirely transparent to the user of | |
535 | the module. It is the responsibility of the F<.pm> file to load | |
536 | (or arrange to autoload) any additional functionality. For example, | |
537 | although the POSIX module happens to do both dynamic loading and | |
538 | autoloading, the user can say just C<use POSIX> to get it all. | |
539 | ||
540 | =head2 Making your module threadsafe | |
541 | X<threadsafe> X<thread safe> | |
542 | X<module, threadsafe> X<module, thread safe> | |
543 | X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread> | |
544 | ||
545 | Since 5.6.0, Perl has had support for a new type of threads called | |
546 | interpreter threads (ithreads). These threads can be used explicitly | |
547 | and implicitly. | |
548 | ||
549 | Ithreads work by cloning the data tree so that no data is shared | |
550 | between different threads. These threads can be used by using the C<threads> | |
551 | module or by doing fork() on win32 (fake fork() support). When a | |
552 | thread is cloned all Perl data is cloned, however non-Perl data cannot | |
553 | be cloned automatically. Perl after 5.7.2 has support for the C<CLONE> | |
554 | special subroutine. In C<CLONE> you can do whatever | |
555 | you need to do, | |
556 | like for example handle the cloning of non-Perl data, if necessary. | |
557 | C<CLONE> will be called once as a class method for every package that has it | |
558 | defined (or inherits it). It will be called in the context of the new thread, | |
559 | so all modifications are made in the new area. Currently CLONE is called with | |
560 | no parameters other than the invocant package name, but code should not assume | |
561 | that this will remain unchanged, as it is likely that in future extra parameters | |
562 | will be passed in to give more information about the state of cloning. | |
563 | ||
564 | If you want to CLONE all objects you will need to keep track of them per | |
565 | package. This is simply done using a hash and Scalar::Util::weaken(). | |
566 | ||
567 | Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine. | |
568 | Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is | |
569 | called just before cloning starts, and in the context of the parent | |
570 | thread. If it returns a true value, then no objects of that class will | |
571 | be cloned; or rather, they will be copied as unblessed, undef values. | |
572 | This provides a simple mechanism for making a module threadsafe; just add | |
573 | C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be | |
574 | now only be called once per object. Of course, if the child thread needs | |
575 | to make use of the objects, then a more sophisticated approach is | |
576 | needed. | |
577 | ||
578 | Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other | |
579 | than the invocant package name, although that may change. Similarly, to | |
580 | allow for future expansion, the return value should be a single C<0> or | |
581 | C<1> value. | |
582 | ||
583 | =head1 SEE ALSO | |
584 | ||
585 | See L<perlmodlib> for general style issues related to building Perl | |
586 | modules and classes, as well as descriptions of the standard library | |
587 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism | |
588 | works, L<perltoot> and L<perltooc> for an in-depth tutorial on | |
589 | creating classes, L<perlobj> for a hard-core reference document on | |
590 | objects, L<perlsub> for an explanation of functions and scoping, | |
591 | and L<perlxstut> and L<perlguts> for more information on writing | |
592 | extension modules. |