Commit | Line | Data |
---|---|---|
a0d0e21e LW |
1 | =head1 NAME |
2 | ||
f102b883 | 3 | perlmod - Perl modules (packages and symbol tables) |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
69520e41 E |
7 | =head2 Is this the document you were after? |
8 | ||
9 | There are other documents which might contain the information that you're | |
10 | looking for: | |
11 | ||
12 | =over 2 | |
13 | ||
14 | =item This doc | |
15 | ||
16 | Perl's packages, namespaces, and some info on classes. | |
17 | ||
18 | =item L<perlnewmod> | |
19 | ||
20 | Tutorial on making a new module. | |
21 | ||
22 | =item L<perlmodstyle> | |
23 | ||
24 | Best practices for making a new module. | |
25 | ||
26 | =back | |
27 | ||
a0d0e21e | 28 | =head2 Packages |
d74e8afc | 29 | X<package> X<namespace> X<variable, global> X<global variable> X<global> |
a0d0e21e | 30 | |
19799a22 GS |
31 | Perl provides a mechanism for alternative namespaces to protect |
32 | packages from stomping on each other's variables. In fact, there's | |
bc8df162 | 33 | really no such thing as a global variable in Perl. The package |
19799a22 GS |
34 | statement declares the compilation unit as being in the given |
35 | namespace. The scope of the package declaration is from the | |
36 | declaration itself through the end of the enclosing block, C<eval>, | |
37 | or file, whichever comes first (the same scope as the my() and | |
38 | local() operators). Unqualified dynamic identifiers will be in | |
39 | this namespace, except for those few identifiers that if unqualified, | |
40 | default to the main package instead of the current one as described | |
41 | below. A package statement affects only dynamic variables--including | |
42 | those you've used local() on--but I<not> lexical variables created | |
43 | with my(). Typically it would be the first declaration in a file | |
44 | included by the C<do>, C<require>, or C<use> operators. You can | |
45 | switch into a package in more than one place; it merely influences | |
46 | which symbol table is used by the compiler for the rest of that | |
47 | block. You can refer to variables and filehandles in other packages | |
48 | by prefixing the identifier with the package name and a double | |
49 | colon: C<$Package::Variable>. If the package name is null, the | |
50 | C<main> package is assumed. That is, C<$::sail> is equivalent to | |
51 | C<$main::sail>. | |
a0d0e21e | 52 | |
d3ebb66b GS |
53 | The old package delimiter was a single quote, but double colon is now the |
54 | preferred delimiter, in part because it's more readable to humans, and | |
55 | in part because it's more readable to B<emacs> macros. It also makes C++ | |
56 | programmers feel like they know what's going on--as opposed to using the | |
57 | single quote as separator, which was there to make Ada programmers feel | |
14c715f4 | 58 | like they knew what was going on. Because the old-fashioned syntax is still |
d3ebb66b GS |
59 | supported for backwards compatibility, if you try to use a string like |
60 | C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is, | |
61 | the $s variable in package C<owner>, which is probably not what you meant. | |
62 | Use braces to disambiguate, as in C<"This is ${owner}'s house">. | |
d74e8afc | 63 | X<::> X<'> |
a0d0e21e | 64 | |
19799a22 GS |
65 | Packages may themselves contain package separators, as in |
66 | C<$OUTER::INNER::var>. This implies nothing about the order of | |
67 | name lookups, however. There are no relative packages: all symbols | |
a0d0e21e LW |
68 | are either local to the current package, or must be fully qualified |
69 | from the outer package name down. For instance, there is nowhere | |
19799a22 | 70 | within package C<OUTER> that C<$INNER::var> refers to |
14c715f4 | 71 | C<$OUTER::INNER::var>. C<INNER> refers to a totally |
19799a22 GS |
72 | separate global package. |
73 | ||
74 | Only identifiers starting with letters (or underscore) are stored | |
75 | in a package's symbol table. All other symbols are kept in package | |
76 | C<main>, including all punctuation variables, like $_. In addition, | |
77 | when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, | |
78 | ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>, | |
14c715f4 | 79 | even when used for other purposes than their built-in ones. If you |
19799a22 GS |
80 | have a package called C<m>, C<s>, or C<y>, then you can't use the |
81 | qualified form of an identifier because it would be instead interpreted | |
82 | as a pattern match, a substitution, or a transliteration. | |
d74e8afc | 83 | X<variable, punctuation> |
19799a22 GS |
84 | |
85 | Variables beginning with underscore used to be forced into package | |
a0d0e21e | 86 | main, but we decided it was more useful for package writers to be able |
cb1a09d0 | 87 | to use leading underscore to indicate private variables and method names. |
b58b0d99 AT |
88 | However, variables and functions named with a single C<_>, such as |
89 | $_ and C<sub _>, are still forced into the package C<main>. See also | |
96090e4f | 90 | L<perlvar/"The Syntax of Variable Names">. |
a0d0e21e | 91 | |
19799a22 | 92 | C<eval>ed strings are compiled in the package in which the eval() was |
a0d0e21e | 93 | compiled. (Assignments to C<$SIG{}>, however, assume the signal |
748a9306 | 94 | handler specified is in the C<main> package. Qualify the signal handler |
a0d0e21e LW |
95 | name if you wish to have a signal handler in a package.) For an |
96 | example, examine F<perldb.pl> in the Perl library. It initially switches | |
97 | to the C<DB> package so that the debugger doesn't interfere with variables | |
19799a22 | 98 | in the program you are trying to debug. At various points, however, it |
a0d0e21e LW |
99 | temporarily switches back to the C<main> package to evaluate various |
100 | expressions in the context of the C<main> package (or wherever you came | |
101 | from). See L<perldebug>. | |
102 | ||
f102b883 | 103 | The special symbol C<__PACKAGE__> contains the current package, but cannot |
14c715f4 | 104 | (easily) be used to construct variable names. |
f102b883 | 105 | |
5f05dabc | 106 | See L<perlsub> for other scoping issues related to my() and local(), |
f102b883 | 107 | and L<perlref> regarding closures. |
cb1a09d0 | 108 | |
a0d0e21e | 109 | =head2 Symbol Tables |
d74e8afc | 110 | X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias> |
a0d0e21e | 111 | |
aa689395 | 112 | The symbol table for a package happens to be stored in the hash of that |
113 | name with two colons appended. The main symbol table's name is thus | |
5803be0d | 114 | C<%main::>, or C<%::> for short. Likewise the symbol table for the nested |
aa689395 | 115 | package mentioned earlier is named C<%OUTER::INNER::>. |
116 | ||
117 | The value in each entry of the hash is what you are referring to when you | |
8c44bff1 | 118 | use the C<*name> typeglob notation. |
a0d0e21e | 119 | |
f102b883 | 120 | local *main::foo = *main::bar; |
bc8df162 | 121 | |
a0d0e21e | 122 | You can use this to print out all the variables in a package, for |
4375e838 | 123 | instance. The standard but antiquated F<dumpvar.pl> library and |
19799a22 | 124 | the CPAN module Devel::Symdump make use of this. |
a0d0e21e | 125 | |
993e39b1 | 126 | The results of creating new symbol table entries directly or modifying any |
fa4ec284 | 127 | entries that are not already typeglobs are undefined and subject to change |
993e39b1 FC |
128 | between releases of perl. |
129 | ||
cb1a09d0 | 130 | Assignment to a typeglob performs an aliasing operation, i.e., |
a0d0e21e LW |
131 | |
132 | *dick = *richard; | |
133 | ||
5a964f20 TC |
134 | causes variables, subroutines, formats, and file and directory handles |
135 | accessible via the identifier C<richard> also to be accessible via the | |
136 | identifier C<dick>. If you want to alias only a particular variable or | |
19799a22 | 137 | subroutine, assign a reference instead: |
a0d0e21e LW |
138 | |
139 | *dick = \$richard; | |
140 | ||
5a964f20 | 141 | Which makes $richard and $dick the same variable, but leaves |
a0d0e21e LW |
142 | @richard and @dick as separate arrays. Tricky, eh? |
143 | ||
5e76a0e2 MC |
144 | There is one subtle difference between the following statements: |
145 | ||
146 | *foo = *bar; | |
147 | *foo = \$bar; | |
148 | ||
149 | C<*foo = *bar> makes the typeglobs themselves synonymous while | |
150 | C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs | |
151 | refer to the same scalar value. This means that the following code: | |
152 | ||
153 | $bar = 1; | |
154 | *foo = \$bar; # Make $foo an alias for $bar | |
155 | ||
156 | { | |
157 | local $bar = 2; # Restrict changes to block | |
158 | print $foo; # Prints '1'! | |
159 | } | |
160 | ||
161 | Would print '1', because C<$foo> holds a reference to the I<original> | |
ac036724 | 162 | C<$bar>. The one that was stuffed away by C<local()> and which will be |
5e76a0e2 MC |
163 | restored when the block ends. Because variables are accessed through the |
164 | typeglob, you can use C<*foo = *bar> to create an alias which can be | |
165 | localized. (But be aware that this means you can't have a separate | |
166 | C<@foo> and C<@bar>, etc.) | |
167 | ||
168 | What makes all of this important is that the Exporter module uses glob | |
169 | aliasing as the import/export mechanism. Whether or not you can properly | |
170 | localize a variable that has been exported from a module depends on how | |
171 | it was exported: | |
172 | ||
173 | @EXPORT = qw($FOO); # Usual form, can't be localized | |
174 | @EXPORT = qw(*FOO); # Can be localized | |
175 | ||
14c715f4 | 176 | You can work around the first case by using the fully qualified name |
5e76a0e2 MC |
177 | (C<$Package::FOO>) where you need a local value, or by overriding it |
178 | by saying C<*FOO = *Package::FOO> in your script. | |
179 | ||
180 | The C<*x = \$y> mechanism may be used to pass and return cheap references | |
5803be0d | 181 | into or from subroutines if you don't want to copy the whole |
5a964f20 TC |
182 | thing. It only works when assigning to dynamic variables, not |
183 | lexicals. | |
cb1a09d0 | 184 | |
5a964f20 | 185 | %some_hash = (); # can't be my() |
cb1a09d0 AD |
186 | *some_hash = fn( \%another_hash ); |
187 | sub fn { | |
188 | local *hashsym = shift; | |
189 | # now use %hashsym normally, and you | |
190 | # will affect the caller's %another_hash | |
191 | my %nhash = (); # do what you want | |
5f05dabc | 192 | return \%nhash; |
cb1a09d0 AD |
193 | } |
194 | ||
5f05dabc | 195 | On return, the reference will overwrite the hash slot in the |
cb1a09d0 | 196 | symbol table specified by the *some_hash typeglob. This |
c36e9b62 | 197 | is a somewhat tricky way of passing around references cheaply |
5803be0d | 198 | when you don't want to have to remember to dereference variables |
cb1a09d0 AD |
199 | explicitly. |
200 | ||
19799a22 | 201 | Another use of symbol tables is for making "constant" scalars. |
d74e8afc | 202 | X<constant> X<scalar, constant> |
cb1a09d0 AD |
203 | |
204 | *PI = \3.14159265358979; | |
205 | ||
bc8df162 | 206 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
5a964f20 | 207 | This isn't the same as a constant subroutine, which is subject to |
5803be0d | 208 | optimization at compile-time. A constant subroutine is one prototyped |
14c715f4 | 209 | to take no arguments and to return a constant expression. See |
5803be0d | 210 | L<perlsub> for details on these. The C<use constant> pragma is a |
5a964f20 | 211 | convenient shorthand for these. |
cb1a09d0 | 212 | |
55497cff | 213 | You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and |
214 | package the *foo symbol table entry comes from. This may be useful | |
5a964f20 | 215 | in a subroutine that gets passed typeglobs as arguments: |
55497cff | 216 | |
217 | sub identify_typeglob { | |
218 | my $glob = shift; | |
555bd962 BG |
219 | print 'You gave me ', *{$glob}{PACKAGE}, |
220 | '::', *{$glob}{NAME}, "\n"; | |
55497cff | 221 | } |
222 | identify_typeglob *foo; | |
223 | identify_typeglob *bar::baz; | |
224 | ||
225 | This prints | |
226 | ||
227 | You gave me main::foo | |
228 | You gave me bar::baz | |
229 | ||
19799a22 | 230 | The C<*foo{THING}> notation can also be used to obtain references to the |
5803be0d | 231 | individual elements of *foo. See L<perlref>. |
55497cff | 232 | |
9263d47b GS |
233 | Subroutine definitions (and declarations, for that matter) need |
234 | not necessarily be situated in the package whose symbol table they | |
235 | occupy. You can define a subroutine outside its package by | |
236 | explicitly qualifying the name of the subroutine: | |
237 | ||
238 | package main; | |
239 | sub Some_package::foo { ... } # &foo defined in Some_package | |
240 | ||
241 | This is just a shorthand for a typeglob assignment at compile time: | |
242 | ||
243 | BEGIN { *Some_package::foo = sub { ... } } | |
244 | ||
245 | and is I<not> the same as writing: | |
246 | ||
247 | { | |
248 | package Some_package; | |
249 | sub foo { ... } | |
250 | } | |
251 | ||
252 | In the first two versions, the body of the subroutine is | |
253 | lexically in the main package, I<not> in Some_package. So | |
254 | something like this: | |
255 | ||
256 | package main; | |
257 | ||
258 | $Some_package::name = "fred"; | |
259 | $main::name = "barney"; | |
260 | ||
261 | sub Some_package::foo { | |
262 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
263 | } | |
264 | ||
265 | Some_package::foo(); | |
266 | ||
267 | prints: | |
268 | ||
269 | in main: $name is 'barney' | |
270 | ||
271 | rather than: | |
272 | ||
273 | in Some_package: $name is 'fred' | |
274 | ||
275 | This also has implications for the use of the SUPER:: qualifier | |
276 | (see L<perlobj>). | |
277 | ||
3c10abe3 AG |
278 | =head2 BEGIN, UNITCHECK, CHECK, INIT and END |
279 | X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END> | |
ac90fb77 | 280 | |
3c10abe3 AG |
281 | Five specially named code blocks are executed at the beginning and at |
282 | the end of a running Perl program. These are the C<BEGIN>, | |
283 | C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks. | |
ac90fb77 EM |
284 | |
285 | These code blocks can be prefixed with C<sub> to give the appearance of a | |
286 | subroutine (although this is not considered good style). One should note | |
287 | that these code blocks don't really exist as named subroutines (despite | |
288 | their appearance). The thing that gives this away is the fact that you can | |
289 | have B<more than one> of these code blocks in a program, and they will get | |
290 | B<all> executed at the appropriate moment. So you can't execute any of | |
291 | these code blocks by name. | |
292 | ||
293 | A C<BEGIN> code block is executed as soon as possible, that is, the moment | |
294 | it is completely defined, even before the rest of the containing file (or | |
295 | string) is parsed. You may have multiple C<BEGIN> blocks within a file (or | |
ac036724 | 296 | eval'ed string); they will execute in order of definition. Because a C<BEGIN> |
ac90fb77 EM |
297 | code block executes immediately, it can pull in definitions of subroutines |
298 | and such from other files in time to be visible to the rest of the compile | |
299 | and run time. Once a C<BEGIN> has run, it is immediately undefined and any | |
300 | code it used is returned to Perl's memory pool. | |
301 | ||
ac90fb77 | 302 | An C<END> code block is executed as late as possible, that is, after |
4f25aa18 GS |
303 | perl has finished running the program and just before the interpreter |
304 | is being exited, even if it is exiting as a result of a die() function. | |
3bf5301d | 305 | (But not if it's morphing into another program via C<exec>, or |
4f25aa18 GS |
306 | being blown out of the water by a signal--you have to trap that yourself |
307 | (if you can).) You may have multiple C<END> blocks within a file--they | |
308 | will execute in reverse order of definition; that is: last in, first | |
309 | out (LIFO). C<END> blocks are not executed when you run perl with the | |
db517d64 | 310 | C<-c> switch, or if compilation fails. |
a0d0e21e | 311 | |
ac90fb77 EM |
312 | Note that C<END> code blocks are B<not> executed at the end of a string |
313 | C<eval()>: if any C<END> code blocks are created in a string C<eval()>, | |
314 | they will be executed just as any other C<END> code block of that package | |
315 | in LIFO order just before the interpreter is being exited. | |
316 | ||
317 | Inside an C<END> code block, C<$?> contains the value that the program is | |
c36e9b62 | 318 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
19799a22 | 319 | value of the program. Beware of changing C<$?> by accident (e.g. by |
c36e9b62 | 320 | running something via C<system>). |
d74e8afc | 321 | X<$?> |
c36e9b62 | 322 | |
191f4b8c CO |
323 | Inside of a C<END> block, the value of C<${^GLOBAL_PHASE}> will be |
324 | C<"END">. | |
325 | ||
3c10abe3 AG |
326 | C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the |
327 | transition between the compilation phase and the execution phase of | |
328 | the main program. | |
329 | ||
330 | C<UNITCHECK> blocks are run just after the unit which defined them has | |
331 | been compiled. The main program file and each module it loads are | |
68e2671b | 332 | compilation units, as are string C<eval>s, run-time code compiled using the |
3c10abe3 AG |
333 | C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>, |
334 | and code after the C<-e> switch on the command line. | |
ca62f0fc | 335 | |
191f4b8c CO |
336 | C<BEGIN> and C<UNITCHECK> blocks are not directly related to the phase of |
337 | the interpreter. They can be created and executed during any phase. | |
338 | ||
ac90fb77 EM |
339 | C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends |
340 | and before the run time begins, in LIFO order. C<CHECK> code blocks are used | |
341 | in the Perl compiler suite to save the compiled state of the program. | |
ca62f0fc | 342 | |
191f4b8c CO |
343 | Inside of a C<CHECK> block, the value of C<${^GLOBAL_PHASE}> will be |
344 | C<"CHECK">. | |
345 | ||
ca62f0fc | 346 | C<INIT> blocks are run just before the Perl runtime begins execution, in |
59f521f4 | 347 | "first in, first out" (FIFO) order. |
4f25aa18 | 348 | |
191f4b8c CO |
349 | Inside of an C<INIT> block, the value of C<${^GLOBAL_PHASE}> will be C<"INIT">. |
350 | ||
9e923162 CO |
351 | The C<CHECK> and C<INIT> blocks in code compiled by C<require>, string C<do>, |
352 | or string C<eval> will not be executed if they occur after the end of the | |
353 | main compilation phase; that can be a problem in mod_perl and other persistent | |
354 | environments which use those functions to load code at runtime. | |
98107fc7 | 355 | |
19799a22 | 356 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
4375e838 GS |
357 | C<END> work just as they do in B<awk>, as a degenerate case. |
358 | Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c> | |
359 | switch for a compile-only syntax check, although your main code | |
360 | is not. | |
a0d0e21e | 361 | |
055634da TP |
362 | The B<begincheck> program makes it all clear, eventually: |
363 | ||
364 | #!/usr/bin/perl | |
365 | ||
366 | # begincheck | |
367 | ||
3c10abe3 | 368 | print "10. Ordinary code runs at runtime.\n"; |
055634da | 369 | |
3c10abe3 AG |
370 | END { print "16. So this is the end of the tale.\n" } |
371 | INIT { print " 7. INIT blocks run FIFO just before runtime.\n" } | |
372 | UNITCHECK { | |
373 | print " 4. And therefore before any CHECK blocks.\n" | |
374 | } | |
375 | CHECK { print " 6. So this is the sixth line.\n" } | |
055634da | 376 | |
3c10abe3 | 377 | print "11. It runs in order, of course.\n"; |
055634da TP |
378 | |
379 | BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" } | |
3c10abe3 AG |
380 | END { print "15. Read perlmod for the rest of the story.\n" } |
381 | CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" } | |
382 | INIT { print " 8. Run this again, using Perl's -c switch.\n" } | |
055634da | 383 | |
3c10abe3 | 384 | print "12. This is anti-obfuscated code.\n"; |
055634da | 385 | |
3c10abe3 | 386 | END { print "14. END blocks run LIFO at quitting time.\n" } |
055634da | 387 | BEGIN { print " 2. So this line comes out second.\n" } |
3c10abe3 AG |
388 | UNITCHECK { |
389 | print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n" | |
390 | } | |
391 | INIT { print " 9. You'll see the difference right away.\n" } | |
055634da | 392 | |
555bd962 | 393 | print "13. It only _looks_ like it should be confusing.\n"; |
055634da TP |
394 | |
395 | __END__ | |
396 | ||
a0d0e21e | 397 | =head2 Perl Classes |
d74e8afc | 398 | X<class> X<@ISA> |
a0d0e21e | 399 | |
19799a22 | 400 | There is no special class syntax in Perl, but a package may act |
5a964f20 TC |
401 | as a class if it provides subroutines to act as methods. Such a |
402 | package may also derive some of its methods from another class (package) | |
14c715f4 | 403 | by listing the other package name(s) in its global @ISA array (which |
5a964f20 | 404 | must be a package global, not a lexical). |
4633a7c4 | 405 | |
82e1c0d9 | 406 | For more on this, see L<perlootut> and L<perlobj>. |
a0d0e21e LW |
407 | |
408 | =head2 Perl Modules | |
d74e8afc | 409 | X<module> |
a0d0e21e | 410 | |
5803be0d | 411 | A module is just a set of related functions in a library file, i.e., |
14c715f4 | 412 | a Perl package with the same name as the file. It is specifically |
5803be0d GS |
413 | designed to be reusable by other modules or programs. It may do this |
414 | by providing a mechanism for exporting some of its symbols into the | |
14c715f4 | 415 | symbol table of any package using it, or it may function as a class |
19799a22 GS |
416 | definition and make its semantics available implicitly through |
417 | method calls on the class and its objects, without explicitly | |
4375e838 | 418 | exporting anything. Or it can do a little of both. |
a0d0e21e | 419 | |
19799a22 GS |
420 | For example, to start a traditional, non-OO module called Some::Module, |
421 | create a file called F<Some/Module.pm> and start with this template: | |
9607fc9c | 422 | |
423 | package Some::Module; # assumes Some/Module.pm | |
424 | ||
425 | use strict; | |
9f1b1f2d | 426 | use warnings; |
9607fc9c | 427 | |
428 | BEGIN { | |
01d915c0 | 429 | require Exporter; |
9607fc9c | 430 | |
431 | # set the version for version checking | |
01d915c0 | 432 | our $VERSION = 1.00; |
9607fc9c | 433 | |
01d915c0 MS |
434 | # Inherit from Exporter to export functions and variables |
435 | our @ISA = qw(Exporter); | |
9607fc9c | 436 | |
01d915c0 MS |
437 | # Functions and variables which are exported by default |
438 | our @EXPORT = qw(func1 func2); | |
439 | ||
440 | # Functions and variables which can be optionally exported | |
441 | our @EXPORT_OK = qw($Var1 %Hashit func3); | |
9607fc9c | 442 | } |
9607fc9c | 443 | |
3da4c8f2 | 444 | # exported package globals go here |
01d915c0 MS |
445 | our $Var1 = ''; |
446 | our %Hashit = (); | |
3da4c8f2 | 447 | |
9607fc9c | 448 | # non-exported package globals go here |
01d915c0 MS |
449 | # (they are still accessible as $Some::Module::stuff) |
450 | our @more = (); | |
451 | our $stuff = ''; | |
9607fc9c | 452 | |
01d915c0 | 453 | # file-private lexicals go here, before any functions which use them |
9607fc9c | 454 | my $priv_var = ''; |
455 | my %secret_hash = (); | |
456 | ||
457 | # here's a file-private function as a closure, | |
01d915c0 | 458 | # callable as $priv_func->(); |
9607fc9c | 459 | my $priv_func = sub { |
01d915c0 | 460 | ... |
9607fc9c | 461 | }; |
462 | ||
463 | # make all your functions, whether exported or not; | |
464 | # remember to put something interesting in the {} stubs | |
01d915c0 MS |
465 | sub func1 { ... } |
466 | sub func2 { ... } | |
9607fc9c | 467 | |
01d915c0 MS |
468 | # this one isn't exported, but could be called directly |
469 | # as Some::Module::func3() | |
470 | sub func3 { ... } | |
4633a7c4 | 471 | |
01d915c0 | 472 | END { ... } # module clean-up code here (global destructor) |
19799a22 GS |
473 | |
474 | 1; # don't forget to return a true value from the file | |
475 | ||
476 | Then go on to declare and use your variables in functions without | |
477 | any qualifications. See L<Exporter> and the L<perlmodlib> for | |
478 | details on mechanics and style issues in module creation. | |
4633a7c4 LW |
479 | |
480 | Perl modules are included into your program by saying | |
a0d0e21e LW |
481 | |
482 | use Module; | |
483 | ||
484 | or | |
485 | ||
486 | use Module LIST; | |
487 | ||
488 | This is exactly equivalent to | |
489 | ||
76503c97 | 490 | BEGIN { require 'Module.pm'; 'Module'->import; } |
a0d0e21e LW |
491 | |
492 | or | |
493 | ||
76503c97 | 494 | BEGIN { require 'Module.pm'; 'Module'->import( LIST ); } |
a0d0e21e | 495 | |
cb1a09d0 AD |
496 | As a special case |
497 | ||
498 | use Module (); | |
499 | ||
500 | is exactly equivalent to | |
501 | ||
76503c97 | 502 | BEGIN { require 'Module.pm'; } |
cb1a09d0 | 503 | |
19799a22 GS |
504 | All Perl module files have the extension F<.pm>. The C<use> operator |
505 | assumes this so you don't have to spell out "F<Module.pm>" in quotes. | |
506 | This also helps to differentiate new modules from old F<.pl> and | |
507 | F<.ph> files. Module names are also capitalized unless they're | |
508 | functioning as pragmas; pragmas are in effect compiler directives, | |
509 | and are sometimes called "pragmatic modules" (or even "pragmata" | |
510 | if you're a classicist). | |
a0d0e21e | 511 | |
5a964f20 TC |
512 | The two statements: |
513 | ||
514 | require SomeModule; | |
14c715f4 | 515 | require "SomeModule.pm"; |
5a964f20 TC |
516 | |
517 | differ from each other in two ways. In the first case, any double | |
518 | colons in the module name, such as C<Some::Module>, are translated | |
519 | into your system's directory separator, usually "/". The second | |
19799a22 GS |
520 | case does not, and would have to be specified literally. The other |
521 | difference is that seeing the first C<require> clues in the compiler | |
522 | that uses of indirect object notation involving "SomeModule", as | |
523 | in C<$ob = purge SomeModule>, are method calls, not function calls. | |
524 | (Yes, this really can make a difference.) | |
525 | ||
526 | Because the C<use> statement implies a C<BEGIN> block, the importing | |
527 | of semantics happens as soon as the C<use> statement is compiled, | |
a0d0e21e LW |
528 | before the rest of the file is compiled. This is how it is able |
529 | to function as a pragma mechanism, and also how modules are able to | |
19799a22 | 530 | declare subroutines that are then visible as list or unary operators for |
a0d0e21e | 531 | the rest of the current file. This will not work if you use C<require> |
19799a22 | 532 | instead of C<use>. With C<require> you can get into this problem: |
a0d0e21e LW |
533 | |
534 | require Cwd; # make Cwd:: accessible | |
54310121 | 535 | $here = Cwd::getcwd(); |
a0d0e21e | 536 | |
5f05dabc | 537 | use Cwd; # import names from Cwd:: |
a0d0e21e LW |
538 | $here = getcwd(); |
539 | ||
540 | require Cwd; # make Cwd:: accessible | |
541 | $here = getcwd(); # oops! no main::getcwd() | |
542 | ||
5a964f20 TC |
543 | In general, C<use Module ()> is recommended over C<require Module>, |
544 | because it determines module availability at compile time, not in the | |
545 | middle of your program's execution. An exception would be if two modules | |
546 | each tried to C<use> each other, and each also called a function from | |
14c715f4 | 547 | that other module. In that case, it's easy to use C<require> instead. |
cb1a09d0 | 548 | |
a0d0e21e LW |
549 | Perl packages may be nested inside other package names, so we can have |
550 | package names containing C<::>. But if we used that package name | |
5803be0d | 551 | directly as a filename it would make for unwieldy or impossible |
a0d0e21e LW |
552 | filenames on some systems. Therefore, if a module's name is, say, |
553 | C<Text::Soundex>, then its definition is actually found in the library | |
554 | file F<Text/Soundex.pm>. | |
555 | ||
19799a22 GS |
556 | Perl modules always have a F<.pm> file, but there may also be |
557 | dynamically linked executables (often ending in F<.so>) or autoloaded | |
5803be0d | 558 | subroutine definitions (often ending in F<.al>) associated with the |
19799a22 GS |
559 | module. If so, these will be entirely transparent to the user of |
560 | the module. It is the responsibility of the F<.pm> file to load | |
561 | (or arrange to autoload) any additional functionality. For example, | |
562 | although the POSIX module happens to do both dynamic loading and | |
5803be0d | 563 | autoloading, the user can say just C<use POSIX> to get it all. |
a0d0e21e | 564 | |
f2fc0a40 | 565 | =head2 Making your module threadsafe |
d74e8afc ITB |
566 | X<threadsafe> X<thread safe> |
567 | X<module, threadsafe> X<module, thread safe> | |
568 | X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread> | |
f2fc0a40 | 569 | |
8f416bb0 BF |
570 | Perl supports a type of threads called interpreter threads (ithreads). |
571 | These threads can be used explicitly and implicitly. | |
f2fc0a40 AB |
572 | |
573 | Ithreads work by cloning the data tree so that no data is shared | |
14c715f4 | 574 | between different threads. These threads can be used by using the C<threads> |
4ebc451b JH |
575 | module or by doing fork() on win32 (fake fork() support). When a |
576 | thread is cloned all Perl data is cloned, however non-Perl data cannot | |
8f416bb0 | 577 | be cloned automatically. Perl after 5.8.0 has support for the C<CLONE> |
4d5ff0dd | 578 | special subroutine. In C<CLONE> you can do whatever |
9660f481 | 579 | you need to do, |
4ebc451b | 580 | like for example handle the cloning of non-Perl data, if necessary. |
38e4e52d NC |
581 | C<CLONE> will be called once as a class method for every package that has it |
582 | defined (or inherits it). It will be called in the context of the new thread, | |
583 | so all modifications are made in the new area. Currently CLONE is called with | |
7698aede | 584 | no parameters other than the invocant package name, but code should not assume |
38e4e52d NC |
585 | that this will remain unchanged, as it is likely that in future extra parameters |
586 | will be passed in to give more information about the state of cloning. | |
f2fc0a40 AB |
587 | |
588 | If you want to CLONE all objects you will need to keep track of them per | |
589 | package. This is simply done using a hash and Scalar::Util::weaken(). | |
590 | ||
4d5ff0dd | 591 | Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine. |
9660f481 DM |
592 | Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is |
593 | called just before cloning starts, and in the context of the parent | |
594 | thread. If it returns a true value, then no objects of that class will | |
595 | be cloned; or rather, they will be copied as unblessed, undef values. | |
33de8e4a DM |
596 | For example: if in the parent there are two references to a single blessed |
597 | hash, then in the child there will be two references to a single undefined | |
598 | scalar value instead. | |
9660f481 | 599 | This provides a simple mechanism for making a module threadsafe; just add |
bca52ca1 | 600 | C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will |
9660f481 DM |
601 | now only be called once per object. Of course, if the child thread needs |
602 | to make use of the objects, then a more sophisticated approach is | |
603 | needed. | |
604 | ||
605 | Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other | |
7698aede | 606 | than the invocant package name, although that may change. Similarly, to |
9660f481 DM |
607 | allow for future expansion, the return value should be a single C<0> or |
608 | C<1> value. | |
609 | ||
f102b883 | 610 | =head1 SEE ALSO |
cb1a09d0 | 611 | |
f102b883 | 612 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 GS |
613 | modules and classes, as well as descriptions of the standard library |
614 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism | |
82e1c0d9 | 615 | works, L<perlootut> and L<perlobj> for in-depth information on |
19799a22 GS |
616 | creating classes, L<perlobj> for a hard-core reference document on |
617 | objects, L<perlsub> for an explanation of functions and scoping, | |
618 | and L<perlxstut> and L<perlguts> for more information on writing | |
619 | extension modules. |