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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
f102b883 | 3 | perlmod - Perl modules (packages and symbol tables) |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Packages | |
8 | ||
19799a22 GS |
9 | Perl provides a mechanism for alternative namespaces to protect |
10 | packages from stomping on each other's variables. In fact, there's | |
bc8df162 | 11 | really no such thing as a global variable in Perl. The package |
19799a22 GS |
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 | |
29 | C<$main::sail>. | |
a0d0e21e | 30 | |
d3ebb66b GS |
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 | |
14c715f4 | 36 | like they knew what was going on. Because the old-fashioned syntax is still |
d3ebb66b GS |
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 ${owner}'s house">. | |
a0d0e21e | 41 | |
19799a22 GS |
42 | Packages may themselves contain package separators, as in |
43 | C<$OUTER::INNER::var>. This implies nothing about the order of | |
44 | name lookups, however. There are no relative packages: all symbols | |
a0d0e21e LW |
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 | |
19799a22 | 47 | within package C<OUTER> that C<$INNER::var> refers to |
14c715f4 | 48 | C<$OUTER::INNER::var>. C<INNER> refers to a totally |
19799a22 GS |
49 | separate global package. |
50 | ||
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>, | |
14c715f4 | 56 | even when used for other purposes than their built-in ones. If you |
19799a22 GS |
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. | |
60 | ||
61 | Variables beginning with underscore used to be forced into package | |
a0d0e21e | 62 | main, but we decided it was more useful for package writers to be able |
cb1a09d0 | 63 | to use leading underscore to indicate private variables and method names. |
b58b0d99 AT |
64 | However, variables and functions named with a single C<_>, such as |
65 | $_ and C<sub _>, are still forced into the package C<main>. See also | |
cea6626f | 66 | L<perlvar/"Technical Note on the Syntax of Variable Names">. |
a0d0e21e | 67 | |
19799a22 | 68 | C<eval>ed strings are compiled in the package in which the eval() was |
a0d0e21e | 69 | compiled. (Assignments to C<$SIG{}>, however, assume the signal |
748a9306 | 70 | handler specified is in the C<main> package. Qualify the signal handler |
a0d0e21e LW |
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 | |
19799a22 | 74 | in the program you are trying to debug. At various points, however, it |
a0d0e21e LW |
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>. | |
78 | ||
f102b883 | 79 | The special symbol C<__PACKAGE__> contains the current package, but cannot |
14c715f4 | 80 | (easily) be used to construct variable names. |
f102b883 | 81 | |
5f05dabc | 82 | See L<perlsub> for other scoping issues related to my() and local(), |
f102b883 | 83 | and L<perlref> regarding closures. |
cb1a09d0 | 84 | |
a0d0e21e LW |
85 | =head2 Symbol Tables |
86 | ||
aa689395 | 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 | |
5803be0d | 89 | C<%main::>, or C<%::> for short. Likewise the symbol table for the nested |
aa689395 | 90 | package mentioned earlier is named C<%OUTER::INNER::>. |
91 | ||
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: | |
a0d0e21e | 96 | |
f102b883 TC |
97 | local *main::foo = *main::bar; |
98 | local $main::{foo} = $main::{bar}; | |
a0d0e21e | 99 | |
bc8df162 DC |
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 | |
104 | the same package.) | |
105 | ||
a0d0e21e | 106 | You can use this to print out all the variables in a package, for |
4375e838 | 107 | instance. The standard but antiquated F<dumpvar.pl> library and |
19799a22 | 108 | the CPAN module Devel::Symdump make use of this. |
a0d0e21e | 109 | |
cb1a09d0 | 110 | Assignment to a typeglob performs an aliasing operation, i.e., |
a0d0e21e LW |
111 | |
112 | *dick = *richard; | |
113 | ||
5a964f20 TC |
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 | |
19799a22 | 117 | subroutine, assign a reference instead: |
a0d0e21e LW |
118 | |
119 | *dick = \$richard; | |
120 | ||
5a964f20 | 121 | Which makes $richard and $dick the same variable, but leaves |
a0d0e21e LW |
122 | @richard and @dick as separate arrays. Tricky, eh? |
123 | ||
5e76a0e2 MC |
124 | There is one subtle difference between the following statements: |
125 | ||
126 | *foo = *bar; | |
127 | *foo = \$bar; | |
128 | ||
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: | |
132 | ||
133 | $bar = 1; | |
134 | *foo = \$bar; # Make $foo an alias for $bar | |
135 | ||
136 | { | |
137 | local $bar = 2; # Restrict changes to block | |
138 | print $foo; # Prints '1'! | |
139 | } | |
140 | ||
141 | Would print '1', because C<$foo> holds a reference to the I<original> | |
142 | C<$bar> -- the one that was stuffed away by C<local()> and which will be | |
143 | restored when the block ends. Because variables are accessed through the | |
144 | typeglob, you can use C<*foo = *bar> to create an alias which can be | |
145 | localized. (But be aware that this means you can't have a separate | |
146 | C<@foo> and C<@bar>, etc.) | |
147 | ||
148 | What makes all of this important is that the Exporter module uses glob | |
149 | aliasing as the import/export mechanism. Whether or not you can properly | |
150 | localize a variable that has been exported from a module depends on how | |
151 | it was exported: | |
152 | ||
153 | @EXPORT = qw($FOO); # Usual form, can't be localized | |
154 | @EXPORT = qw(*FOO); # Can be localized | |
155 | ||
14c715f4 | 156 | You can work around the first case by using the fully qualified name |
5e76a0e2 MC |
157 | (C<$Package::FOO>) where you need a local value, or by overriding it |
158 | by saying C<*FOO = *Package::FOO> in your script. | |
159 | ||
160 | The C<*x = \$y> mechanism may be used to pass and return cheap references | |
5803be0d | 161 | into or from subroutines if you don't want to copy the whole |
5a964f20 TC |
162 | thing. It only works when assigning to dynamic variables, not |
163 | lexicals. | |
cb1a09d0 | 164 | |
5a964f20 | 165 | %some_hash = (); # can't be my() |
cb1a09d0 AD |
166 | *some_hash = fn( \%another_hash ); |
167 | sub fn { | |
168 | local *hashsym = shift; | |
169 | # now use %hashsym normally, and you | |
170 | # will affect the caller's %another_hash | |
171 | my %nhash = (); # do what you want | |
5f05dabc | 172 | return \%nhash; |
cb1a09d0 AD |
173 | } |
174 | ||
5f05dabc | 175 | On return, the reference will overwrite the hash slot in the |
cb1a09d0 | 176 | symbol table specified by the *some_hash typeglob. This |
c36e9b62 | 177 | is a somewhat tricky way of passing around references cheaply |
5803be0d | 178 | when you don't want to have to remember to dereference variables |
cb1a09d0 AD |
179 | explicitly. |
180 | ||
19799a22 | 181 | Another use of symbol tables is for making "constant" scalars. |
cb1a09d0 AD |
182 | |
183 | *PI = \3.14159265358979; | |
184 | ||
bc8df162 | 185 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
5a964f20 | 186 | This isn't the same as a constant subroutine, which is subject to |
5803be0d | 187 | optimization at compile-time. A constant subroutine is one prototyped |
14c715f4 | 188 | to take no arguments and to return a constant expression. See |
5803be0d | 189 | L<perlsub> for details on these. The C<use constant> pragma is a |
5a964f20 | 190 | convenient shorthand for these. |
cb1a09d0 | 191 | |
55497cff | 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 | |
5a964f20 | 194 | in a subroutine that gets passed typeglobs as arguments: |
55497cff | 195 | |
196 | sub identify_typeglob { | |
197 | my $glob = shift; | |
198 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; | |
199 | } | |
200 | identify_typeglob *foo; | |
201 | identify_typeglob *bar::baz; | |
202 | ||
203 | This prints | |
204 | ||
205 | You gave me main::foo | |
206 | You gave me bar::baz | |
207 | ||
19799a22 | 208 | The C<*foo{THING}> notation can also be used to obtain references to the |
5803be0d | 209 | individual elements of *foo. See L<perlref>. |
55497cff | 210 | |
9263d47b GS |
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: | |
215 | ||
216 | package main; | |
217 | sub Some_package::foo { ... } # &foo defined in Some_package | |
218 | ||
219 | This is just a shorthand for a typeglob assignment at compile time: | |
220 | ||
221 | BEGIN { *Some_package::foo = sub { ... } } | |
222 | ||
223 | and is I<not> the same as writing: | |
224 | ||
225 | { | |
226 | package Some_package; | |
227 | sub foo { ... } | |
228 | } | |
229 | ||
230 | In the first two versions, the body of the subroutine is | |
231 | lexically in the main package, I<not> in Some_package. So | |
232 | something like this: | |
233 | ||
234 | package main; | |
235 | ||
236 | $Some_package::name = "fred"; | |
237 | $main::name = "barney"; | |
238 | ||
239 | sub Some_package::foo { | |
240 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
241 | } | |
242 | ||
243 | Some_package::foo(); | |
244 | ||
245 | prints: | |
246 | ||
247 | in main: $name is 'barney' | |
248 | ||
249 | rather than: | |
250 | ||
251 | in Some_package: $name is 'fred' | |
252 | ||
253 | This also has implications for the use of the SUPER:: qualifier | |
254 | (see L<perlobj>). | |
255 | ||
a0d0e21e LW |
256 | =head2 Package Constructors and Destructors |
257 | ||
7d981616 | 258 | Four special subroutines act as package constructors and destructors. |
7d30b5c4 | 259 | These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The |
7d981616 | 260 | C<sub> is optional for these routines. |
a0d0e21e | 261 | |
f102b883 TC |
262 | A C<BEGIN> subroutine is executed as soon as possible, that is, the moment |
263 | it is completely defined, even before the rest of the containing file | |
264 | is parsed. You may have multiple C<BEGIN> blocks within a file--they | |
265 | will execute in order of definition. Because a C<BEGIN> block executes | |
266 | immediately, it can pull in definitions of subroutines and such from other | |
267 | files in time to be visible to the rest of the file. Once a C<BEGIN> | |
268 | has run, it is immediately undefined and any code it used is returned to | |
269 | Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>. | |
a0d0e21e | 270 | |
4f25aa18 GS |
271 | An C<END> subroutine is executed as late as possible, that is, after |
272 | perl has finished running the program and just before the interpreter | |
273 | is being exited, even if it is exiting as a result of a die() function. | |
274 | (But not if it's polymorphing into another program via C<exec>, or | |
275 | being blown out of the water by a signal--you have to trap that yourself | |
276 | (if you can).) You may have multiple C<END> blocks within a file--they | |
277 | will execute in reverse order of definition; that is: last in, first | |
278 | out (LIFO). C<END> blocks are not executed when you run perl with the | |
db517d64 | 279 | C<-c> switch, or if compilation fails. |
a0d0e21e | 280 | |
19799a22 | 281 | Inside an C<END> subroutine, C<$?> contains the value that the program is |
c36e9b62 | 282 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
19799a22 | 283 | value of the program. Beware of changing C<$?> by accident (e.g. by |
c36e9b62 | 284 | running something via C<system>). |
285 | ||
ca62f0fc RGS |
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. | |
288 | ||
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. | |
292 | ||
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. | |
4f25aa18 | 297 | |
19799a22 | 298 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
4375e838 GS |
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 | |
302 | is not. | |
a0d0e21e LW |
303 | |
304 | =head2 Perl Classes | |
305 | ||
19799a22 | 306 | There is no special class syntax in Perl, but a package may act |
5a964f20 TC |
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) | |
14c715f4 | 309 | by listing the other package name(s) in its global @ISA array (which |
5a964f20 | 310 | must be a package global, not a lexical). |
4633a7c4 | 311 | |
f102b883 | 312 | For more on this, see L<perltoot> and L<perlobj>. |
a0d0e21e LW |
313 | |
314 | =head2 Perl Modules | |
315 | ||
5803be0d | 316 | A module is just a set of related functions in a library file, i.e., |
14c715f4 | 317 | a Perl package with the same name as the file. It is specifically |
5803be0d GS |
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 | |
14c715f4 | 320 | symbol table of any package using it, or it may function as a class |
19799a22 GS |
321 | definition and make its semantics available implicitly through |
322 | method calls on the class and its objects, without explicitly | |
4375e838 | 323 | exporting anything. Or it can do a little of both. |
a0d0e21e | 324 | |
19799a22 GS |
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: | |
9607fc9c | 327 | |
328 | package Some::Module; # assumes Some/Module.pm | |
329 | ||
330 | use strict; | |
9f1b1f2d | 331 | use warnings; |
9607fc9c | 332 | |
333 | BEGIN { | |
334 | use Exporter (); | |
77ca0c92 | 335 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
9607fc9c | 336 | |
337 | # set the version for version checking | |
338 | $VERSION = 1.00; | |
339 | # if using RCS/CVS, this may be preferred | |
328fc025 | 340 | $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g; |
9607fc9c | 341 | |
342 | @ISA = qw(Exporter); | |
343 | @EXPORT = qw(&func1 &func2 &func4); | |
344 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], | |
345 | ||
346 | # your exported package globals go here, | |
347 | # as well as any optionally exported functions | |
348 | @EXPORT_OK = qw($Var1 %Hashit &func3); | |
349 | } | |
77ca0c92 | 350 | our @EXPORT_OK; |
9607fc9c | 351 | |
3da4c8f2 DB |
352 | # exported package globals go here |
353 | our $Var1; | |
354 | our %Hashit; | |
355 | ||
9607fc9c | 356 | # non-exported package globals go here |
77ca0c92 LW |
357 | our @more; |
358 | our $stuff; | |
9607fc9c | 359 | |
c2611fb3 | 360 | # initialize package globals, first exported ones |
9607fc9c | 361 | $Var1 = ''; |
362 | %Hashit = (); | |
363 | ||
364 | # then the others (which are still accessible as $Some::Module::stuff) | |
365 | $stuff = ''; | |
366 | @more = (); | |
367 | ||
368 | # all file-scoped lexicals must be created before | |
369 | # the functions below that use them. | |
370 | ||
371 | # file-private lexicals go here | |
372 | my $priv_var = ''; | |
373 | my %secret_hash = (); | |
374 | ||
375 | # here's a file-private function as a closure, | |
376 | # callable as &$priv_func; it cannot be prototyped. | |
377 | my $priv_func = sub { | |
378 | # stuff goes here. | |
379 | }; | |
380 | ||
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 | |
386 | ||
387 | # this one isn't exported, but could be called! | |
388 | sub func4(\%) {} # proto'd to 1 hash ref | |
389 | ||
390 | END { } # module clean-up code here (global destructor) | |
4633a7c4 | 391 | |
19799a22 GS |
392 | ## YOUR CODE GOES HERE |
393 | ||
394 | 1; # don't forget to return a true value from the file | |
395 | ||
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. | |
4633a7c4 LW |
399 | |
400 | Perl modules are included into your program by saying | |
a0d0e21e LW |
401 | |
402 | use Module; | |
403 | ||
404 | or | |
405 | ||
406 | use Module LIST; | |
407 | ||
408 | This is exactly equivalent to | |
409 | ||
5a964f20 | 410 | BEGIN { require Module; import Module; } |
a0d0e21e LW |
411 | |
412 | or | |
413 | ||
5a964f20 | 414 | BEGIN { require Module; import Module LIST; } |
a0d0e21e | 415 | |
cb1a09d0 AD |
416 | As a special case |
417 | ||
418 | use Module (); | |
419 | ||
420 | is exactly equivalent to | |
421 | ||
5a964f20 | 422 | BEGIN { require Module; } |
cb1a09d0 | 423 | |
19799a22 GS |
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). | |
a0d0e21e | 431 | |
5a964f20 TC |
432 | The two statements: |
433 | ||
434 | require SomeModule; | |
14c715f4 | 435 | require "SomeModule.pm"; |
5a964f20 TC |
436 | |
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 | |
19799a22 GS |
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.) | |
445 | ||
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, | |
a0d0e21e LW |
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 | |
19799a22 | 450 | declare subroutines that are then visible as list or unary operators for |
a0d0e21e | 451 | the rest of the current file. This will not work if you use C<require> |
19799a22 | 452 | instead of C<use>. With C<require> you can get into this problem: |
a0d0e21e LW |
453 | |
454 | require Cwd; # make Cwd:: accessible | |
54310121 | 455 | $here = Cwd::getcwd(); |
a0d0e21e | 456 | |
5f05dabc | 457 | use Cwd; # import names from Cwd:: |
a0d0e21e LW |
458 | $here = getcwd(); |
459 | ||
460 | require Cwd; # make Cwd:: accessible | |
461 | $here = getcwd(); # oops! no main::getcwd() | |
462 | ||
5a964f20 TC |
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 | |
14c715f4 | 467 | that other module. In that case, it's easy to use C<require> instead. |
cb1a09d0 | 468 | |
a0d0e21e LW |
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 | |
5803be0d | 471 | directly as a filename it would make for unwieldy or impossible |
a0d0e21e LW |
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>. | |
475 | ||
19799a22 GS |
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 | |
5803be0d | 478 | subroutine definitions (often ending in F<.al>) associated with the |
19799a22 GS |
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 | |
5803be0d | 483 | autoloading, the user can say just C<use POSIX> to get it all. |
a0d0e21e | 484 | |
f2fc0a40 AB |
485 | =head2 Making your module threadsafe |
486 | ||
14c715f4 CP |
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 | |
489 | and implicitly. | |
f2fc0a40 AB |
490 | |
491 | Ithreads work by cloning the data tree so that no data is shared | |
14c715f4 | 492 | between different threads. These threads can be used by using the C<threads> |
4ebc451b JH |
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 | |
14c715f4 | 495 | be cloned automatically. Perl after 5.7.2 has support for the C<CLONE> |
cd0db8e5 | 496 | special subroutine . In C<CLONE> you can do whatever you need to do, |
4ebc451b JH |
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. | |
f2fc0a40 AB |
501 | |
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(). | |
504 | ||
f102b883 | 505 | =head1 SEE ALSO |
cb1a09d0 | 506 | |
f102b883 | 507 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 GS |
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 | |
890a53b9 | 510 | works, L<perltoot> and L<perltooc> for an in-depth tutorial on |
19799a22 GS |
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 | |
514 | extension modules. |