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1 | =head1 NAME |
2 | |
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3 | perlmod - Perl modules (packages and symbol tables) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Packages |
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8 | X<package> X<namespace> X<variable, global> X<global variable> X<global> |
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9 | |
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10 | Perl provides a mechanism for alternative namespaces to protect |
11 | packages from stomping on each other's variables. In fact, there's |
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12 | really no such thing as a global variable in Perl. The package |
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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>. |
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31 | |
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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 |
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37 | like they knew what was going on. Because the old-fashioned syntax is still |
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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 ${owner}'s house">. |
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42 | X<::> X<'> |
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43 | |
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44 | Packages may themselves contain package separators, as in |
45 | C<$OUTER::INNER::var>. This implies nothing about the order of |
46 | name lookups, however. There are no relative packages: all symbols |
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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 |
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49 | within package C<OUTER> that C<$INNER::var> refers to |
14c715f4 |
50 | C<$OUTER::INNER::var>. C<INNER> refers to a totally |
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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>, |
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58 | even when used for other purposes than their built-in ones. If you |
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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. |
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62 | X<variable, punctuation> |
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63 | |
64 | Variables beginning with underscore used to be forced into package |
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65 | main, but we decided it was more useful for package writers to be able |
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66 | to use leading underscore to indicate private variables and method names. |
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67 | However, variables and functions named with a single C<_>, such as |
68 | $_ and C<sub _>, are still forced into the package C<main>. See also |
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69 | L<perlvar/"Technical Note on the Syntax of Variable Names">. |
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70 | |
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71 | C<eval>ed strings are compiled in the package in which the eval() was |
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72 | compiled. (Assignments to C<$SIG{}>, however, assume the signal |
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73 | handler specified is in the C<main> package. Qualify the signal handler |
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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 |
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77 | in the program you are trying to debug. At various points, however, it |
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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 | |
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82 | The special symbol C<__PACKAGE__> contains the current package, but cannot |
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83 | (easily) be used to construct variable names. |
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84 | |
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85 | See L<perlsub> for other scoping issues related to my() and local(), |
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86 | and L<perlref> regarding closures. |
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87 | |
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88 | =head2 Symbol Tables |
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89 | X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias> |
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90 | |
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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 |
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93 | C<%main::>, or C<%::> for short. Likewise the symbol table for the nested |
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94 | package mentioned earlier is named C<%OUTER::INNER::>. |
95 | |
96 | The value in each entry of the hash is what you are referring to when you |
97 | use the C<*name> typeglob notation. In fact, the following have the same |
98 | effect, though the first is more efficient because it does the symbol |
99 | table lookups at compile time: |
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100 | |
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101 | local *main::foo = *main::bar; |
102 | local $main::{foo} = $main::{bar}; |
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103 | |
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104 | (Be sure to note the B<vast> difference between the second line above |
105 | and C<local $main::foo = $main::bar>. The former is accessing the hash |
106 | C<%main::>, which is the symbol table of package C<main>. The latter is |
107 | simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of |
108 | the same package.) |
109 | |
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110 | You can use this to print out all the variables in a package, for |
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111 | instance. The standard but antiquated F<dumpvar.pl> library and |
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112 | the CPAN module Devel::Symdump make use of this. |
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113 | |
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114 | Assignment to a typeglob performs an aliasing operation, i.e., |
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115 | |
116 | *dick = *richard; |
117 | |
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118 | causes variables, subroutines, formats, and file and directory handles |
119 | accessible via the identifier C<richard> also to be accessible via the |
120 | identifier C<dick>. If you want to alias only a particular variable or |
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121 | subroutine, assign a reference instead: |
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122 | |
123 | *dick = \$richard; |
124 | |
5a964f20 |
125 | Which makes $richard and $dick the same variable, but leaves |
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126 | @richard and @dick as separate arrays. Tricky, eh? |
127 | |
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128 | There is one subtle difference between the following statements: |
129 | |
130 | *foo = *bar; |
131 | *foo = \$bar; |
132 | |
133 | C<*foo = *bar> makes the typeglobs themselves synonymous while |
134 | C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs |
135 | refer to the same scalar value. This means that the following code: |
136 | |
137 | $bar = 1; |
138 | *foo = \$bar; # Make $foo an alias for $bar |
139 | |
140 | { |
141 | local $bar = 2; # Restrict changes to block |
142 | print $foo; # Prints '1'! |
143 | } |
144 | |
145 | Would print '1', because C<$foo> holds a reference to the I<original> |
146 | C<$bar> -- the one that was stuffed away by C<local()> and which will be |
147 | restored when the block ends. Because variables are accessed through the |
148 | typeglob, you can use C<*foo = *bar> to create an alias which can be |
149 | localized. (But be aware that this means you can't have a separate |
150 | C<@foo> and C<@bar>, etc.) |
151 | |
152 | What makes all of this important is that the Exporter module uses glob |
153 | aliasing as the import/export mechanism. Whether or not you can properly |
154 | localize a variable that has been exported from a module depends on how |
155 | it was exported: |
156 | |
157 | @EXPORT = qw($FOO); # Usual form, can't be localized |
158 | @EXPORT = qw(*FOO); # Can be localized |
159 | |
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160 | You can work around the first case by using the fully qualified name |
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161 | (C<$Package::FOO>) where you need a local value, or by overriding it |
162 | by saying C<*FOO = *Package::FOO> in your script. |
163 | |
164 | The C<*x = \$y> mechanism may be used to pass and return cheap references |
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165 | into or from subroutines if you don't want to copy the whole |
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166 | thing. It only works when assigning to dynamic variables, not |
167 | lexicals. |
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168 | |
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169 | %some_hash = (); # can't be my() |
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170 | *some_hash = fn( \%another_hash ); |
171 | sub fn { |
172 | local *hashsym = shift; |
173 | # now use %hashsym normally, and you |
174 | # will affect the caller's %another_hash |
175 | my %nhash = (); # do what you want |
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176 | return \%nhash; |
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177 | } |
178 | |
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179 | On return, the reference will overwrite the hash slot in the |
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180 | symbol table specified by the *some_hash typeglob. This |
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181 | is a somewhat tricky way of passing around references cheaply |
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182 | when you don't want to have to remember to dereference variables |
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183 | explicitly. |
184 | |
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185 | Another use of symbol tables is for making "constant" scalars. |
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186 | X<constant> X<scalar, constant> |
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187 | |
188 | *PI = \3.14159265358979; |
189 | |
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190 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
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191 | This isn't the same as a constant subroutine, which is subject to |
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192 | optimization at compile-time. A constant subroutine is one prototyped |
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193 | to take no arguments and to return a constant expression. See |
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194 | L<perlsub> for details on these. The C<use constant> pragma is a |
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195 | convenient shorthand for these. |
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196 | |
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197 | You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and |
198 | package the *foo symbol table entry comes from. This may be useful |
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199 | in a subroutine that gets passed typeglobs as arguments: |
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200 | |
201 | sub identify_typeglob { |
202 | my $glob = shift; |
203 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; |
204 | } |
205 | identify_typeglob *foo; |
206 | identify_typeglob *bar::baz; |
207 | |
208 | This prints |
209 | |
210 | You gave me main::foo |
211 | You gave me bar::baz |
212 | |
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213 | The C<*foo{THING}> notation can also be used to obtain references to the |
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214 | individual elements of *foo. See L<perlref>. |
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215 | |
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216 | Subroutine definitions (and declarations, for that matter) need |
217 | not necessarily be situated in the package whose symbol table they |
218 | occupy. You can define a subroutine outside its package by |
219 | explicitly qualifying the name of the subroutine: |
220 | |
221 | package main; |
222 | sub Some_package::foo { ... } # &foo defined in Some_package |
223 | |
224 | This is just a shorthand for a typeglob assignment at compile time: |
225 | |
226 | BEGIN { *Some_package::foo = sub { ... } } |
227 | |
228 | and is I<not> the same as writing: |
229 | |
230 | { |
231 | package Some_package; |
232 | sub foo { ... } |
233 | } |
234 | |
235 | In the first two versions, the body of the subroutine is |
236 | lexically in the main package, I<not> in Some_package. So |
237 | something like this: |
238 | |
239 | package main; |
240 | |
241 | $Some_package::name = "fred"; |
242 | $main::name = "barney"; |
243 | |
244 | sub Some_package::foo { |
245 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; |
246 | } |
247 | |
248 | Some_package::foo(); |
249 | |
250 | prints: |
251 | |
252 | in main: $name is 'barney' |
253 | |
254 | rather than: |
255 | |
256 | in Some_package: $name is 'fred' |
257 | |
258 | This also has implications for the use of the SUPER:: qualifier |
259 | (see L<perlobj>). |
260 | |
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261 | =head2 BEGIN, UNITCHECK, CHECK, INIT and END |
262 | X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END> |
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263 | |
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264 | Five specially named code blocks are executed at the beginning and at |
265 | the end of a running Perl program. These are the C<BEGIN>, |
266 | C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks. |
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267 | |
268 | These code blocks can be prefixed with C<sub> to give the appearance of a |
269 | subroutine (although this is not considered good style). One should note |
270 | that these code blocks don't really exist as named subroutines (despite |
271 | their appearance). The thing that gives this away is the fact that you can |
272 | have B<more than one> of these code blocks in a program, and they will get |
273 | B<all> executed at the appropriate moment. So you can't execute any of |
274 | these code blocks by name. |
275 | |
276 | A C<BEGIN> code block is executed as soon as possible, that is, the moment |
277 | it is completely defined, even before the rest of the containing file (or |
278 | string) is parsed. You may have multiple C<BEGIN> blocks within a file (or |
279 | eval'ed string) -- they will execute in order of definition. Because a C<BEGIN> |
280 | code block executes immediately, it can pull in definitions of subroutines |
281 | and such from other files in time to be visible to the rest of the compile |
282 | and run time. Once a C<BEGIN> has run, it is immediately undefined and any |
283 | code it used is returned to Perl's memory pool. |
284 | |
3c10abe3 |
285 | It should be noted that C<BEGIN> and C<UNITCHECK> code blocks B<are> |
286 | executed inside string C<eval()>'s. The C<CHECK> and C<INIT> code |
287 | blocks are B<not> executed inside a string eval, which e.g. can be a |
288 | problem in a mod_perl environment. |
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289 | |
290 | An C<END> code block is executed as late as possible, that is, after |
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291 | perl has finished running the program and just before the interpreter |
292 | is being exited, even if it is exiting as a result of a die() function. |
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293 | (But not if it's morphing into another program via C<exec>, or |
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294 | being blown out of the water by a signal--you have to trap that yourself |
295 | (if you can).) You may have multiple C<END> blocks within a file--they |
296 | will execute in reverse order of definition; that is: last in, first |
297 | out (LIFO). C<END> blocks are not executed when you run perl with the |
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298 | C<-c> switch, or if compilation fails. |
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299 | |
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300 | Note that C<END> code blocks are B<not> executed at the end of a string |
301 | C<eval()>: if any C<END> code blocks are created in a string C<eval()>, |
302 | they will be executed just as any other C<END> code block of that package |
303 | in LIFO order just before the interpreter is being exited. |
304 | |
305 | Inside an C<END> code block, C<$?> contains the value that the program is |
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306 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
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307 | value of the program. Beware of changing C<$?> by accident (e.g. by |
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308 | running something via C<system>). |
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309 | X<$?> |
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310 | |
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311 | C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the |
312 | transition between the compilation phase and the execution phase of |
313 | the main program. |
314 | |
315 | C<UNITCHECK> blocks are run just after the unit which defined them has |
316 | been compiled. The main program file and each module it loads are |
317 | compilation units, as are string C<eval>s, code compiled using the |
318 | C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>, |
319 | and code after the C<-e> switch on the command line. |
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320 | |
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321 | C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends |
322 | and before the run time begins, in LIFO order. C<CHECK> code blocks are used |
323 | in the Perl compiler suite to save the compiled state of the program. |
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324 | |
325 | C<INIT> blocks are run just before the Perl runtime begins execution, in |
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326 | "first in, first out" (FIFO) order. |
4f25aa18 |
327 | |
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328 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
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329 | C<END> work just as they do in B<awk>, as a degenerate case. |
330 | Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c> |
331 | switch for a compile-only syntax check, although your main code |
332 | is not. |
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333 | |
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334 | The B<begincheck> program makes it all clear, eventually: |
335 | |
336 | #!/usr/bin/perl |
337 | |
338 | # begincheck |
339 | |
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340 | print "10. Ordinary code runs at runtime.\n"; |
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341 | |
3c10abe3 |
342 | END { print "16. So this is the end of the tale.\n" } |
343 | INIT { print " 7. INIT blocks run FIFO just before runtime.\n" } |
344 | UNITCHECK { |
345 | print " 4. And therefore before any CHECK blocks.\n" |
346 | } |
347 | CHECK { print " 6. So this is the sixth line.\n" } |
055634da |
348 | |
3c10abe3 |
349 | print "11. It runs in order, of course.\n"; |
055634da |
350 | |
351 | BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" } |
3c10abe3 |
352 | END { print "15. Read perlmod for the rest of the story.\n" } |
353 | CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" } |
354 | INIT { print " 8. Run this again, using Perl's -c switch.\n" } |
055634da |
355 | |
3c10abe3 |
356 | print "12. This is anti-obfuscated code.\n"; |
055634da |
357 | |
3c10abe3 |
358 | END { print "14. END blocks run LIFO at quitting time.\n" } |
055634da |
359 | BEGIN { print " 2. So this line comes out second.\n" } |
3c10abe3 |
360 | UNITCHECK { |
361 | print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n" |
362 | } |
363 | INIT { print " 9. You'll see the difference right away.\n" } |
055634da |
364 | |
3c10abe3 |
365 | print "13. It merely _looks_ like it should be confusing.\n"; |
055634da |
366 | |
367 | __END__ |
368 | |
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369 | =head2 Perl Classes |
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370 | X<class> X<@ISA> |
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371 | |
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372 | There is no special class syntax in Perl, but a package may act |
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373 | as a class if it provides subroutines to act as methods. Such a |
374 | package may also derive some of its methods from another class (package) |
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375 | by listing the other package name(s) in its global @ISA array (which |
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376 | must be a package global, not a lexical). |
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377 | |
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378 | For more on this, see L<perltoot> and L<perlobj>. |
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379 | |
380 | =head2 Perl Modules |
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381 | X<module> |
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382 | |
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383 | A module is just a set of related functions in a library file, i.e., |
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384 | a Perl package with the same name as the file. It is specifically |
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385 | designed to be reusable by other modules or programs. It may do this |
386 | by providing a mechanism for exporting some of its symbols into the |
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387 | symbol table of any package using it, or it may function as a class |
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388 | definition and make its semantics available implicitly through |
389 | method calls on the class and its objects, without explicitly |
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390 | exporting anything. Or it can do a little of both. |
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391 | |
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392 | For example, to start a traditional, non-OO module called Some::Module, |
393 | create a file called F<Some/Module.pm> and start with this template: |
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394 | |
395 | package Some::Module; # assumes Some/Module.pm |
396 | |
397 | use strict; |
9f1b1f2d |
398 | use warnings; |
9607fc9c |
399 | |
400 | BEGIN { |
401 | use Exporter (); |
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402 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
9607fc9c |
403 | |
404 | # set the version for version checking |
405 | $VERSION = 1.00; |
406 | # if using RCS/CVS, this may be preferred |
328fc025 |
407 | $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g; |
9607fc9c |
408 | |
409 | @ISA = qw(Exporter); |
410 | @EXPORT = qw(&func1 &func2 &func4); |
411 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
412 | |
413 | # your exported package globals go here, |
414 | # as well as any optionally exported functions |
415 | @EXPORT_OK = qw($Var1 %Hashit &func3); |
416 | } |
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417 | our @EXPORT_OK; |
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418 | |
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419 | # exported package globals go here |
420 | our $Var1; |
421 | our %Hashit; |
422 | |
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423 | # non-exported package globals go here |
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424 | our @more; |
425 | our $stuff; |
9607fc9c |
426 | |
c2611fb3 |
427 | # initialize package globals, first exported ones |
9607fc9c |
428 | $Var1 = ''; |
429 | %Hashit = (); |
430 | |
431 | # then the others (which are still accessible as $Some::Module::stuff) |
432 | $stuff = ''; |
433 | @more = (); |
434 | |
435 | # all file-scoped lexicals must be created before |
436 | # the functions below that use them. |
437 | |
438 | # file-private lexicals go here |
439 | my $priv_var = ''; |
440 | my %secret_hash = (); |
441 | |
442 | # here's a file-private function as a closure, |
443 | # callable as &$priv_func; it cannot be prototyped. |
444 | my $priv_func = sub { |
445 | # stuff goes here. |
446 | }; |
447 | |
448 | # make all your functions, whether exported or not; |
449 | # remember to put something interesting in the {} stubs |
450 | sub func1 {} # no prototype |
451 | sub func2() {} # proto'd void |
452 | sub func3($$) {} # proto'd to 2 scalars |
453 | |
454 | # this one isn't exported, but could be called! |
455 | sub func4(\%) {} # proto'd to 1 hash ref |
456 | |
457 | END { } # module clean-up code here (global destructor) |
4633a7c4 |
458 | |
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459 | ## YOUR CODE GOES HERE |
460 | |
461 | 1; # don't forget to return a true value from the file |
462 | |
463 | Then go on to declare and use your variables in functions without |
464 | any qualifications. See L<Exporter> and the L<perlmodlib> for |
465 | details on mechanics and style issues in module creation. |
4633a7c4 |
466 | |
467 | Perl modules are included into your program by saying |
a0d0e21e |
468 | |
469 | use Module; |
470 | |
471 | or |
472 | |
473 | use Module LIST; |
474 | |
475 | This is exactly equivalent to |
476 | |
5a964f20 |
477 | BEGIN { require Module; import Module; } |
a0d0e21e |
478 | |
479 | or |
480 | |
5a964f20 |
481 | BEGIN { require Module; import Module LIST; } |
a0d0e21e |
482 | |
cb1a09d0 |
483 | As a special case |
484 | |
485 | use Module (); |
486 | |
487 | is exactly equivalent to |
488 | |
5a964f20 |
489 | BEGIN { require Module; } |
cb1a09d0 |
490 | |
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491 | All Perl module files have the extension F<.pm>. The C<use> operator |
492 | assumes this so you don't have to spell out "F<Module.pm>" in quotes. |
493 | This also helps to differentiate new modules from old F<.pl> and |
494 | F<.ph> files. Module names are also capitalized unless they're |
495 | functioning as pragmas; pragmas are in effect compiler directives, |
496 | and are sometimes called "pragmatic modules" (or even "pragmata" |
497 | if you're a classicist). |
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498 | |
5a964f20 |
499 | The two statements: |
500 | |
501 | require SomeModule; |
14c715f4 |
502 | require "SomeModule.pm"; |
5a964f20 |
503 | |
504 | differ from each other in two ways. In the first case, any double |
505 | colons in the module name, such as C<Some::Module>, are translated |
506 | into your system's directory separator, usually "/". The second |
19799a22 |
507 | case does not, and would have to be specified literally. The other |
508 | difference is that seeing the first C<require> clues in the compiler |
509 | that uses of indirect object notation involving "SomeModule", as |
510 | in C<$ob = purge SomeModule>, are method calls, not function calls. |
511 | (Yes, this really can make a difference.) |
512 | |
513 | Because the C<use> statement implies a C<BEGIN> block, the importing |
514 | of semantics happens as soon as the C<use> statement is compiled, |
a0d0e21e |
515 | before the rest of the file is compiled. This is how it is able |
516 | to function as a pragma mechanism, and also how modules are able to |
19799a22 |
517 | declare subroutines that are then visible as list or unary operators for |
a0d0e21e |
518 | the rest of the current file. This will not work if you use C<require> |
19799a22 |
519 | instead of C<use>. With C<require> you can get into this problem: |
a0d0e21e |
520 | |
521 | require Cwd; # make Cwd:: accessible |
54310121 |
522 | $here = Cwd::getcwd(); |
a0d0e21e |
523 | |
5f05dabc |
524 | use Cwd; # import names from Cwd:: |
a0d0e21e |
525 | $here = getcwd(); |
526 | |
527 | require Cwd; # make Cwd:: accessible |
528 | $here = getcwd(); # oops! no main::getcwd() |
529 | |
5a964f20 |
530 | In general, C<use Module ()> is recommended over C<require Module>, |
531 | because it determines module availability at compile time, not in the |
532 | middle of your program's execution. An exception would be if two modules |
533 | each tried to C<use> each other, and each also called a function from |
14c715f4 |
534 | that other module. In that case, it's easy to use C<require> instead. |
cb1a09d0 |
535 | |
a0d0e21e |
536 | Perl packages may be nested inside other package names, so we can have |
537 | package names containing C<::>. But if we used that package name |
5803be0d |
538 | directly as a filename it would make for unwieldy or impossible |
a0d0e21e |
539 | filenames on some systems. Therefore, if a module's name is, say, |
540 | C<Text::Soundex>, then its definition is actually found in the library |
541 | file F<Text/Soundex.pm>. |
542 | |
19799a22 |
543 | Perl modules always have a F<.pm> file, but there may also be |
544 | dynamically linked executables (often ending in F<.so>) or autoloaded |
5803be0d |
545 | subroutine definitions (often ending in F<.al>) associated with the |
19799a22 |
546 | module. If so, these will be entirely transparent to the user of |
547 | the module. It is the responsibility of the F<.pm> file to load |
548 | (or arrange to autoload) any additional functionality. For example, |
549 | although the POSIX module happens to do both dynamic loading and |
5803be0d |
550 | autoloading, the user can say just C<use POSIX> to get it all. |
a0d0e21e |
551 | |
f2fc0a40 |
552 | =head2 Making your module threadsafe |
d74e8afc |
553 | X<threadsafe> X<thread safe> |
554 | X<module, threadsafe> X<module, thread safe> |
555 | X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread> |
f2fc0a40 |
556 | |
14c715f4 |
557 | Since 5.6.0, Perl has had support for a new type of threads called |
558 | interpreter threads (ithreads). These threads can be used explicitly |
559 | and implicitly. |
f2fc0a40 |
560 | |
561 | Ithreads work by cloning the data tree so that no data is shared |
14c715f4 |
562 | between different threads. These threads can be used by using the C<threads> |
4ebc451b |
563 | module or by doing fork() on win32 (fake fork() support). When a |
564 | thread is cloned all Perl data is cloned, however non-Perl data cannot |
14c715f4 |
565 | be cloned automatically. Perl after 5.7.2 has support for the C<CLONE> |
4d5ff0dd |
566 | special subroutine. In C<CLONE> you can do whatever |
9660f481 |
567 | you need to do, |
4ebc451b |
568 | like for example handle the cloning of non-Perl data, if necessary. |
38e4e52d |
569 | C<CLONE> will be called once as a class method for every package that has it |
570 | defined (or inherits it). It will be called in the context of the new thread, |
571 | so all modifications are made in the new area. Currently CLONE is called with |
572 | no parameters other than the invocant package name, but code should not assume |
573 | that this will remain unchanged, as it is likely that in future extra parameters |
574 | will be passed in to give more information about the state of cloning. |
f2fc0a40 |
575 | |
576 | If you want to CLONE all objects you will need to keep track of them per |
577 | package. This is simply done using a hash and Scalar::Util::weaken(). |
578 | |
4d5ff0dd |
579 | Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine. |
9660f481 |
580 | Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is |
581 | called just before cloning starts, and in the context of the parent |
582 | thread. If it returns a true value, then no objects of that class will |
583 | be cloned; or rather, they will be copied as unblessed, undef values. |
584 | This provides a simple mechanism for making a module threadsafe; just add |
585 | C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be |
586 | now only be called once per object. Of course, if the child thread needs |
587 | to make use of the objects, then a more sophisticated approach is |
588 | needed. |
589 | |
590 | Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other |
591 | than the invocant package name, although that may change. Similarly, to |
592 | allow for future expansion, the return value should be a single C<0> or |
593 | C<1> value. |
594 | |
f102b883 |
595 | =head1 SEE ALSO |
cb1a09d0 |
596 | |
f102b883 |
597 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 |
598 | modules and classes, as well as descriptions of the standard library |
599 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism |
890a53b9 |
600 | works, L<perltoot> and L<perltooc> for an in-depth tutorial on |
19799a22 |
601 | creating classes, L<perlobj> for a hard-core reference document on |
602 | objects, L<perlsub> for an explanation of functions and scoping, |
603 | and L<perlxstut> and L<perlguts> for more information on writing |
604 | extension modules. |