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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 | $_ and C<sub _>, are still forced into the package C<main>. See also | |
cea6626f | 69 | L<perlvar/"Technical Note on the Syntax of Variable Names">. |
a0d0e21e | 70 | |
19799a22 | 71 | C<eval>ed strings are compiled in the package in which the eval() was |
a0d0e21e | 72 | compiled. (Assignments to C<$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 | |
cb1a09d0 | 105 | Assignment to a typeglob performs an aliasing operation, i.e., |
a0d0e21e LW |
106 | |
107 | *dick = *richard; | |
108 | ||
5a964f20 TC |
109 | causes variables, subroutines, formats, and file and directory handles |
110 | accessible via the identifier C<richard> also to be accessible via the | |
111 | identifier C<dick>. If you want to alias only a particular variable or | |
19799a22 | 112 | subroutine, assign a reference instead: |
a0d0e21e LW |
113 | |
114 | *dick = \$richard; | |
115 | ||
5a964f20 | 116 | Which makes $richard and $dick the same variable, but leaves |
a0d0e21e LW |
117 | @richard and @dick as separate arrays. Tricky, eh? |
118 | ||
5e76a0e2 MC |
119 | There is one subtle difference between the following statements: |
120 | ||
121 | *foo = *bar; | |
122 | *foo = \$bar; | |
123 | ||
124 | C<*foo = *bar> makes the typeglobs themselves synonymous while | |
125 | C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs | |
126 | refer to the same scalar value. This means that the following code: | |
127 | ||
128 | $bar = 1; | |
129 | *foo = \$bar; # Make $foo an alias for $bar | |
130 | ||
131 | { | |
132 | local $bar = 2; # Restrict changes to block | |
133 | print $foo; # Prints '1'! | |
134 | } | |
135 | ||
136 | Would print '1', because C<$foo> holds a reference to the I<original> | |
ac036724 | 137 | C<$bar>. The one that was stuffed away by C<local()> and which will be |
5e76a0e2 MC |
138 | restored when the block ends. Because variables are accessed through the |
139 | typeglob, you can use C<*foo = *bar> to create an alias which can be | |
140 | localized. (But be aware that this means you can't have a separate | |
141 | C<@foo> and C<@bar>, etc.) | |
142 | ||
143 | What makes all of this important is that the Exporter module uses glob | |
144 | aliasing as the import/export mechanism. Whether or not you can properly | |
145 | localize a variable that has been exported from a module depends on how | |
146 | it was exported: | |
147 | ||
148 | @EXPORT = qw($FOO); # Usual form, can't be localized | |
149 | @EXPORT = qw(*FOO); # Can be localized | |
150 | ||
14c715f4 | 151 | You can work around the first case by using the fully qualified name |
5e76a0e2 MC |
152 | (C<$Package::FOO>) where you need a local value, or by overriding it |
153 | by saying C<*FOO = *Package::FOO> in your script. | |
154 | ||
155 | The C<*x = \$y> mechanism may be used to pass and return cheap references | |
5803be0d | 156 | into or from subroutines if you don't want to copy the whole |
5a964f20 TC |
157 | thing. It only works when assigning to dynamic variables, not |
158 | lexicals. | |
cb1a09d0 | 159 | |
5a964f20 | 160 | %some_hash = (); # can't be my() |
cb1a09d0 AD |
161 | *some_hash = fn( \%another_hash ); |
162 | sub fn { | |
163 | local *hashsym = shift; | |
164 | # now use %hashsym normally, and you | |
165 | # will affect the caller's %another_hash | |
166 | my %nhash = (); # do what you want | |
5f05dabc | 167 | return \%nhash; |
cb1a09d0 AD |
168 | } |
169 | ||
5f05dabc | 170 | On return, the reference will overwrite the hash slot in the |
cb1a09d0 | 171 | symbol table specified by the *some_hash typeglob. This |
c36e9b62 | 172 | is a somewhat tricky way of passing around references cheaply |
5803be0d | 173 | when you don't want to have to remember to dereference variables |
cb1a09d0 AD |
174 | explicitly. |
175 | ||
19799a22 | 176 | Another use of symbol tables is for making "constant" scalars. |
d74e8afc | 177 | X<constant> X<scalar, constant> |
cb1a09d0 AD |
178 | |
179 | *PI = \3.14159265358979; | |
180 | ||
bc8df162 | 181 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
5a964f20 | 182 | This isn't the same as a constant subroutine, which is subject to |
5803be0d | 183 | optimization at compile-time. A constant subroutine is one prototyped |
14c715f4 | 184 | to take no arguments and to return a constant expression. See |
5803be0d | 185 | L<perlsub> for details on these. The C<use constant> pragma is a |
5a964f20 | 186 | convenient shorthand for these. |
cb1a09d0 | 187 | |
55497cff | 188 | You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and |
189 | package the *foo symbol table entry comes from. This may be useful | |
5a964f20 | 190 | in a subroutine that gets passed typeglobs as arguments: |
55497cff | 191 | |
192 | sub identify_typeglob { | |
193 | my $glob = shift; | |
194 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; | |
195 | } | |
196 | identify_typeglob *foo; | |
197 | identify_typeglob *bar::baz; | |
198 | ||
199 | This prints | |
200 | ||
201 | You gave me main::foo | |
202 | You gave me bar::baz | |
203 | ||
19799a22 | 204 | The C<*foo{THING}> notation can also be used to obtain references to the |
5803be0d | 205 | individual elements of *foo. See L<perlref>. |
55497cff | 206 | |
9263d47b GS |
207 | Subroutine definitions (and declarations, for that matter) need |
208 | not necessarily be situated in the package whose symbol table they | |
209 | occupy. You can define a subroutine outside its package by | |
210 | explicitly qualifying the name of the subroutine: | |
211 | ||
212 | package main; | |
213 | sub Some_package::foo { ... } # &foo defined in Some_package | |
214 | ||
215 | This is just a shorthand for a typeglob assignment at compile time: | |
216 | ||
217 | BEGIN { *Some_package::foo = sub { ... } } | |
218 | ||
219 | and is I<not> the same as writing: | |
220 | ||
221 | { | |
222 | package Some_package; | |
223 | sub foo { ... } | |
224 | } | |
225 | ||
226 | In the first two versions, the body of the subroutine is | |
227 | lexically in the main package, I<not> in Some_package. So | |
228 | something like this: | |
229 | ||
230 | package main; | |
231 | ||
232 | $Some_package::name = "fred"; | |
233 | $main::name = "barney"; | |
234 | ||
235 | sub Some_package::foo { | |
236 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
237 | } | |
238 | ||
239 | Some_package::foo(); | |
240 | ||
241 | prints: | |
242 | ||
243 | in main: $name is 'barney' | |
244 | ||
245 | rather than: | |
246 | ||
247 | in Some_package: $name is 'fred' | |
248 | ||
249 | This also has implications for the use of the SUPER:: qualifier | |
250 | (see L<perlobj>). | |
251 | ||
3c10abe3 AG |
252 | =head2 BEGIN, UNITCHECK, CHECK, INIT and END |
253 | X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END> | |
ac90fb77 | 254 | |
3c10abe3 AG |
255 | Five specially named code blocks are executed at the beginning and at |
256 | the end of a running Perl program. These are the C<BEGIN>, | |
257 | C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks. | |
ac90fb77 EM |
258 | |
259 | These code blocks can be prefixed with C<sub> to give the appearance of a | |
260 | subroutine (although this is not considered good style). One should note | |
261 | that these code blocks don't really exist as named subroutines (despite | |
262 | their appearance). The thing that gives this away is the fact that you can | |
263 | have B<more than one> of these code blocks in a program, and they will get | |
264 | B<all> executed at the appropriate moment. So you can't execute any of | |
265 | these code blocks by name. | |
266 | ||
267 | A C<BEGIN> code block is executed as soon as possible, that is, the moment | |
268 | it is completely defined, even before the rest of the containing file (or | |
269 | string) is parsed. You may have multiple C<BEGIN> blocks within a file (or | |
ac036724 | 270 | eval'ed string); they will execute in order of definition. Because a C<BEGIN> |
ac90fb77 EM |
271 | code block executes immediately, it can pull in definitions of subroutines |
272 | and such from other files in time to be visible to the rest of the compile | |
273 | and run time. Once a C<BEGIN> has run, it is immediately undefined and any | |
274 | code it used is returned to Perl's memory pool. | |
275 | ||
ac90fb77 | 276 | An C<END> code block is executed as late as possible, that is, after |
4f25aa18 GS |
277 | perl has finished running the program and just before the interpreter |
278 | is being exited, even if it is exiting as a result of a die() function. | |
3bf5301d | 279 | (But not if it's morphing into another program via C<exec>, or |
4f25aa18 GS |
280 | being blown out of the water by a signal--you have to trap that yourself |
281 | (if you can).) You may have multiple C<END> blocks within a file--they | |
282 | will execute in reverse order of definition; that is: last in, first | |
283 | out (LIFO). C<END> blocks are not executed when you run perl with the | |
db517d64 | 284 | C<-c> switch, or if compilation fails. |
a0d0e21e | 285 | |
ac90fb77 EM |
286 | Note that C<END> code blocks are B<not> executed at the end of a string |
287 | C<eval()>: if any C<END> code blocks are created in a string C<eval()>, | |
288 | they will be executed just as any other C<END> code block of that package | |
289 | in LIFO order just before the interpreter is being exited. | |
290 | ||
291 | Inside an C<END> code block, C<$?> contains the value that the program is | |
c36e9b62 | 292 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
19799a22 | 293 | value of the program. Beware of changing C<$?> by accident (e.g. by |
c36e9b62 | 294 | running something via C<system>). |
d74e8afc | 295 | X<$?> |
c36e9b62 | 296 | |
3c10abe3 AG |
297 | C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the |
298 | transition between the compilation phase and the execution phase of | |
299 | the main program. | |
300 | ||
301 | C<UNITCHECK> blocks are run just after the unit which defined them has | |
302 | been compiled. The main program file and each module it loads are | |
303 | compilation units, as are string C<eval>s, code compiled using the | |
304 | C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>, | |
305 | and code after the C<-e> switch on the command line. | |
ca62f0fc | 306 | |
ac90fb77 EM |
307 | C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends |
308 | and before the run time begins, in LIFO order. C<CHECK> code blocks are used | |
309 | in the Perl compiler suite to save the compiled state of the program. | |
ca62f0fc RGS |
310 | |
311 | C<INIT> blocks are run just before the Perl runtime begins execution, in | |
59f521f4 | 312 | "first in, first out" (FIFO) order. |
4f25aa18 | 313 | |
98107fc7 RGS |
314 | The C<CHECK> and C<INIT> code blocks will not be executed inside a string |
315 | eval(), if that eval() happens after the end of the main compilation | |
316 | phase; that can be a problem in mod_perl and other persistent environments | |
317 | which use C<eval STRING> to load code at runtime. | |
318 | ||
19799a22 | 319 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
4375e838 GS |
320 | C<END> work just as they do in B<awk>, as a degenerate case. |
321 | Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c> | |
322 | switch for a compile-only syntax check, although your main code | |
323 | is not. | |
a0d0e21e | 324 | |
055634da TP |
325 | The B<begincheck> program makes it all clear, eventually: |
326 | ||
327 | #!/usr/bin/perl | |
328 | ||
329 | # begincheck | |
330 | ||
3c10abe3 | 331 | print "10. Ordinary code runs at runtime.\n"; |
055634da | 332 | |
3c10abe3 AG |
333 | END { print "16. So this is the end of the tale.\n" } |
334 | INIT { print " 7. INIT blocks run FIFO just before runtime.\n" } | |
335 | UNITCHECK { | |
336 | print " 4. And therefore before any CHECK blocks.\n" | |
337 | } | |
338 | CHECK { print " 6. So this is the sixth line.\n" } | |
055634da | 339 | |
3c10abe3 | 340 | print "11. It runs in order, of course.\n"; |
055634da TP |
341 | |
342 | BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" } | |
3c10abe3 AG |
343 | END { print "15. Read perlmod for the rest of the story.\n" } |
344 | CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" } | |
345 | INIT { print " 8. Run this again, using Perl's -c switch.\n" } | |
055634da | 346 | |
3c10abe3 | 347 | print "12. This is anti-obfuscated code.\n"; |
055634da | 348 | |
3c10abe3 | 349 | END { print "14. END blocks run LIFO at quitting time.\n" } |
055634da | 350 | BEGIN { print " 2. So this line comes out second.\n" } |
3c10abe3 AG |
351 | UNITCHECK { |
352 | print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n" | |
353 | } | |
354 | INIT { print " 9. You'll see the difference right away.\n" } | |
055634da | 355 | |
3c10abe3 | 356 | print "13. It merely _looks_ like it should be confusing.\n"; |
055634da TP |
357 | |
358 | __END__ | |
359 | ||
a0d0e21e | 360 | =head2 Perl Classes |
d74e8afc | 361 | X<class> X<@ISA> |
a0d0e21e | 362 | |
19799a22 | 363 | There is no special class syntax in Perl, but a package may act |
5a964f20 TC |
364 | as a class if it provides subroutines to act as methods. Such a |
365 | package may also derive some of its methods from another class (package) | |
14c715f4 | 366 | by listing the other package name(s) in its global @ISA array (which |
5a964f20 | 367 | must be a package global, not a lexical). |
4633a7c4 | 368 | |
f102b883 | 369 | For more on this, see L<perltoot> and L<perlobj>. |
a0d0e21e LW |
370 | |
371 | =head2 Perl Modules | |
d74e8afc | 372 | X<module> |
a0d0e21e | 373 | |
5803be0d | 374 | A module is just a set of related functions in a library file, i.e., |
14c715f4 | 375 | a Perl package with the same name as the file. It is specifically |
5803be0d GS |
376 | designed to be reusable by other modules or programs. It may do this |
377 | by providing a mechanism for exporting some of its symbols into the | |
14c715f4 | 378 | symbol table of any package using it, or it may function as a class |
19799a22 GS |
379 | definition and make its semantics available implicitly through |
380 | method calls on the class and its objects, without explicitly | |
4375e838 | 381 | exporting anything. Or it can do a little of both. |
a0d0e21e | 382 | |
19799a22 GS |
383 | For example, to start a traditional, non-OO module called Some::Module, |
384 | create a file called F<Some/Module.pm> and start with this template: | |
9607fc9c | 385 | |
386 | package Some::Module; # assumes Some/Module.pm | |
387 | ||
388 | use strict; | |
9f1b1f2d | 389 | use warnings; |
9607fc9c | 390 | |
391 | BEGIN { | |
392 | use Exporter (); | |
77ca0c92 | 393 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
9607fc9c | 394 | |
395 | # set the version for version checking | |
396 | $VERSION = 1.00; | |
397 | # if using RCS/CVS, this may be preferred | |
328fc025 | 398 | $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g; |
9607fc9c | 399 | |
400 | @ISA = qw(Exporter); | |
401 | @EXPORT = qw(&func1 &func2 &func4); | |
402 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], | |
403 | ||
404 | # your exported package globals go here, | |
405 | # as well as any optionally exported functions | |
406 | @EXPORT_OK = qw($Var1 %Hashit &func3); | |
407 | } | |
77ca0c92 | 408 | our @EXPORT_OK; |
9607fc9c | 409 | |
3da4c8f2 DB |
410 | # exported package globals go here |
411 | our $Var1; | |
412 | our %Hashit; | |
413 | ||
9607fc9c | 414 | # non-exported package globals go here |
77ca0c92 LW |
415 | our @more; |
416 | our $stuff; | |
9607fc9c | 417 | |
c2611fb3 | 418 | # initialize package globals, first exported ones |
9607fc9c | 419 | $Var1 = ''; |
420 | %Hashit = (); | |
421 | ||
422 | # then the others (which are still accessible as $Some::Module::stuff) | |
423 | $stuff = ''; | |
424 | @more = (); | |
425 | ||
426 | # all file-scoped lexicals must be created before | |
427 | # the functions below that use them. | |
428 | ||
429 | # file-private lexicals go here | |
430 | my $priv_var = ''; | |
431 | my %secret_hash = (); | |
432 | ||
433 | # here's a file-private function as a closure, | |
434 | # callable as &$priv_func; it cannot be prototyped. | |
435 | my $priv_func = sub { | |
436 | # stuff goes here. | |
437 | }; | |
438 | ||
439 | # make all your functions, whether exported or not; | |
440 | # remember to put something interesting in the {} stubs | |
441 | sub func1 {} # no prototype | |
442 | sub func2() {} # proto'd void | |
443 | sub func3($$) {} # proto'd to 2 scalars | |
444 | ||
445 | # this one isn't exported, but could be called! | |
446 | sub func4(\%) {} # proto'd to 1 hash ref | |
447 | ||
448 | END { } # module clean-up code here (global destructor) | |
4633a7c4 | 449 | |
19799a22 GS |
450 | ## YOUR CODE GOES HERE |
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 | ||
5a964f20 | 468 | BEGIN { require Module; import Module; } |
a0d0e21e LW |
469 | |
470 | or | |
471 | ||
5a964f20 | 472 | BEGIN { require Module; import Module LIST; } |
a0d0e21e | 473 | |
cb1a09d0 AD |
474 | As a special case |
475 | ||
476 | use Module (); | |
477 | ||
478 | is exactly equivalent to | |
479 | ||
5a964f20 | 480 | BEGIN { require Module; } |
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 | |
14c715f4 CP |
548 | Since 5.6.0, Perl has had support for a new type of threads called |
549 | interpreter threads (ithreads). These threads can be used explicitly | |
550 | and implicitly. | |
f2fc0a40 AB |
551 | |
552 | Ithreads work by cloning the data tree so that no data is shared | |
14c715f4 | 553 | between different threads. These threads can be used by using the C<threads> |
4ebc451b JH |
554 | module or by doing fork() on win32 (fake fork() support). When a |
555 | thread is cloned all Perl data is cloned, however non-Perl data cannot | |
14c715f4 | 556 | be cloned automatically. Perl after 5.7.2 has support for the C<CLONE> |
4d5ff0dd | 557 | special subroutine. In C<CLONE> you can do whatever |
9660f481 | 558 | you need to do, |
4ebc451b | 559 | like for example handle the cloning of non-Perl data, if necessary. |
38e4e52d NC |
560 | C<CLONE> will be called once as a class method for every package that has it |
561 | defined (or inherits it). It will be called in the context of the new thread, | |
562 | so all modifications are made in the new area. Currently CLONE is called with | |
563 | no parameters other than the invocant package name, but code should not assume | |
564 | that this will remain unchanged, as it is likely that in future extra parameters | |
565 | will be passed in to give more information about the state of cloning. | |
f2fc0a40 AB |
566 | |
567 | If you want to CLONE all objects you will need to keep track of them per | |
568 | package. This is simply done using a hash and Scalar::Util::weaken(). | |
569 | ||
4d5ff0dd | 570 | Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine. |
9660f481 DM |
571 | Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is |
572 | called just before cloning starts, and in the context of the parent | |
573 | thread. If it returns a true value, then no objects of that class will | |
574 | be cloned; or rather, they will be copied as unblessed, undef values. | |
33de8e4a DM |
575 | For example: if in the parent there are two references to a single blessed |
576 | hash, then in the child there will be two references to a single undefined | |
577 | scalar value instead. | |
9660f481 DM |
578 | This provides a simple mechanism for making a module threadsafe; just add |
579 | C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be | |
580 | now only be called once per object. Of course, if the child thread needs | |
581 | to make use of the objects, then a more sophisticated approach is | |
582 | needed. | |
583 | ||
584 | Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other | |
585 | than the invocant package name, although that may change. Similarly, to | |
586 | allow for future expansion, the return value should be a single C<0> or | |
587 | C<1> value. | |
588 | ||
f102b883 | 589 | =head1 SEE ALSO |
cb1a09d0 | 590 | |
f102b883 | 591 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 GS |
592 | modules and classes, as well as descriptions of the standard library |
593 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism | |
890a53b9 | 594 | works, L<perltoot> and L<perltooc> for an in-depth tutorial on |
19799a22 GS |
595 | creating classes, L<perlobj> for a hard-core reference document on |
596 | objects, L<perlsub> for an explanation of functions and scoping, | |
597 | and L<perlxstut> and L<perlguts> for more information on writing | |
598 | extension modules. |