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