=head1 DESCRIPTION
=head2 Packages
+X<package> X<namespace> X<variable, global> X<global variable> X<global>
Perl provides a mechanism for alternative namespaces to protect
packages from stomping on each other's variables. In fact, there's
C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
the $s variable in package C<owner>, which is probably not what you meant.
Use braces to disambiguate, as in C<"This is ${owner}'s house">.
+X<::> X<'>
Packages may themselves contain package separators, as in
C<$OUTER::INNER::var>. This implies nothing about the order of
have a package called C<m>, C<s>, or C<y>, then you can't use the
qualified form of an identifier because it would be instead interpreted
as a pattern match, a substitution, or a transliteration.
+X<variable, punctuation>
Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
However, variables and functions named with a single C<_>, such as
$_ and C<sub _>, are still forced into the package C<main>. See also
-L<perlvar/"Technical Note on the Syntax of Variable Names">.
+L<perlvar/"The Syntax of Variable Names">.
C<eval>ed strings are compiled in the package in which the eval() was
compiled. (Assignments to C<$SIG{}>, however, assume the signal
and L<perlref> regarding closures.
=head2 Symbol Tables
+X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias>
The symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
package mentioned earlier is named C<%OUTER::INNER::>.
The value in each entry of the hash is what you are referring to when you
-use the C<*name> typeglob notation. In fact, the following have the same
-effect, though the first is more efficient because it does the symbol
-table lookups at compile time:
+use the C<*name> typeglob notation.
local *main::foo = *main::bar;
- local $main::{foo} = $main::{bar};
-
-(Be sure to note the B<vast> difference between the second line above
-and C<local $main::foo = $main::bar>. The former is accessing the hash
-C<%main::>, which is the symbol table of package C<main>. The latter is
-simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
-the same package.)
You can use this to print out all the variables in a package, for
instance. The standard but antiquated F<dumpvar.pl> library and
the CPAN module Devel::Symdump make use of this.
+The results of creating new symbol table entries directly or modifying any
+entries that are not already typeglobs are undefined and subject to change
+between releases of perl.
+
Assignment to a typeglob performs an aliasing operation, i.e.,
*dick = *richard;
}
Would print '1', because C<$foo> holds a reference to the I<original>
-C<$bar> -- the one that was stuffed away by C<local()> and which will be
+C<$bar>. The one that was stuffed away by C<local()> and which will be
restored when the block ends. Because variables are accessed through the
typeglob, you can use C<*foo = *bar> to create an alias which can be
localized. (But be aware that this means you can't have a separate
explicitly.
Another use of symbol tables is for making "constant" scalars.
+X<constant> X<scalar, constant>
*PI = \3.14159265358979;
This also has implications for the use of the SUPER:: qualifier
(see L<perlobj>).
-=head2 BEGIN, CHECK, INIT and END
+=head2 BEGIN, UNITCHECK, CHECK, INIT and END
+X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END>
-Four specially named code blocks are executed at the beginning and at the end
-of a running Perl program. These are the C<BEGIN>, C<CHECK>, C<INIT>, and
-C<END> blocks.
+Five specially named code blocks are executed at the beginning and at
+the end of a running Perl program. These are the C<BEGIN>,
+C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks.
These code blocks can be prefixed with C<sub> to give the appearance of a
subroutine (although this is not considered good style). One should note
A C<BEGIN> code block is executed as soon as possible, that is, the moment
it is completely defined, even before the rest of the containing file (or
string) is parsed. You may have multiple C<BEGIN> blocks within a file (or
-eval'ed string) -- they will execute in order of definition. Because a C<BEGIN>
+eval'ed string); they will execute in order of definition. Because a C<BEGIN>
code block executes immediately, it can pull in definitions of subroutines
and such from other files in time to be visible to the rest of the compile
and run time. Once a C<BEGIN> has run, it is immediately undefined and any
code it used is returned to Perl's memory pool.
-It should be noted that C<BEGIN> code blocks B<are> executed inside string
-C<eval()>'s. The C<CHECK> and C<INIT> code blocks are B<not> executed inside
-a string eval, which e.g. can be a problem in a mod_perl environment.
-
An C<END> code block is executed as late as possible, that is, after
perl has finished running the program and just before the interpreter
is being exited, even if it is exiting as a result of a die() function.
-(But not if it's polymorphing into another program via C<exec>, or
+(But not if it's morphing into another program via C<exec>, or
being blown out of the water by a signal--you have to trap that yourself
(if you can).) You may have multiple C<END> blocks within a file--they
will execute in reverse order of definition; that is: last in, first
going to pass to C<exit()>. You can modify C<$?> to change the exit
value of the program. Beware of changing C<$?> by accident (e.g. by
running something via C<system>).
+X<$?>
+
+Inside of a C<END> block, the value of C<${^GLOBAL_PHASE}> will be
+C<"END">.
-C<CHECK> and C<INIT> code blocks are useful to catch the transition between
-the compilation phase and the execution phase of the main program.
+C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the
+transition between the compilation phase and the execution phase of
+the main program.
+
+C<UNITCHECK> blocks are run just after the unit which defined them has
+been compiled. The main program file and each module it loads are
+compilation units, as are string C<eval>s, run-time code compiled using the
+C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>,
+and code after the C<-e> switch on the command line.
+
+C<BEGIN> and C<UNITCHECK> blocks are not directly related to the phase of
+the interpreter. They can be created and executed during any phase.
C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends
and before the run time begins, in LIFO order. C<CHECK> code blocks are used
in the Perl compiler suite to save the compiled state of the program.
+Inside of a C<CHECK> block, the value of C<${^GLOBAL_PHASE}> will be
+C<"CHECK">.
+
C<INIT> blocks are run just before the Perl runtime begins execution, in
-"first in, first out" (FIFO) order. For example, the code generators
-documented in L<perlcc> make use of C<INIT> blocks to initialize and
-resolve pointers to XSUBs.
+"first in, first out" (FIFO) order.
+
+Inside of an C<INIT> block, the value of C<${^GLOBAL_PHASE}> will be C<"INIT">.
+
+The C<CHECK> and C<INIT> blocks in code compiled by C<require>, string C<do>,
+or string C<eval> will not be executed if they occur after the end of the
+main compilation phase; that can be a problem in mod_perl and other persistent
+environments which use those functions to load code at runtime.
When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
C<END> work just as they do in B<awk>, as a degenerate case.
# begincheck
- print " 8. Ordinary code runs at runtime.\n";
+ print "10. Ordinary code runs at runtime.\n";
- END { print "14. So this is the end of the tale.\n" }
- INIT { print " 5. INIT blocks run FIFO just before runtime.\n" }
- CHECK { print " 4. So this is the fourth line.\n" }
+ END { print "16. So this is the end of the tale.\n" }
+ INIT { print " 7. INIT blocks run FIFO just before runtime.\n" }
+ UNITCHECK {
+ print " 4. And therefore before any CHECK blocks.\n"
+ }
+ CHECK { print " 6. So this is the sixth line.\n" }
- print " 9. It runs in order, of course.\n";
+ print "11. It runs in order, of course.\n";
BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
- END { print "13. Read perlmod for the rest of the story.\n" }
- CHECK { print " 3. CHECK blocks run LIFO at compilation's end.\n" }
- INIT { print " 6. Run this again, using Perl's -c switch.\n" }
+ END { print "15. Read perlmod for the rest of the story.\n" }
+ CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
+ INIT { print " 8. Run this again, using Perl's -c switch.\n" }
- print "10. This is anti-obfuscated code.\n";
+ print "12. This is anti-obfuscated code.\n";
- END { print "12. END blocks run LIFO at quitting time.\n" }
+ END { print "14. END blocks run LIFO at quitting time.\n" }
BEGIN { print " 2. So this line comes out second.\n" }
- INIT { print " 7. You'll see the difference right away.\n" }
+ UNITCHECK {
+ print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
+ }
+ INIT { print " 9. You'll see the difference right away.\n" }
- print "11. It merely _looks_ like it should be confusing.\n";
+ print "13. It merely _looks_ like it should be confusing.\n";
__END__
=head2 Perl Classes
+X<class> X<@ISA>
There is no special class syntax in Perl, but a package may act
as a class if it provides subroutines to act as methods. Such a
by listing the other package name(s) in its global @ISA array (which
must be a package global, not a lexical).
-For more on this, see L<perl
toot> and L<perlobj>.
+For more on this, see L<perl
ootut> and L<perlobj>.
=head2 Perl Modules
+X<module>
A module is just a set of related functions in a library file, i.e.,
a Perl package with the same name as the file. It is specifically
use warnings;
BEGIN {
- use Exporter ();
- our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
+ require Exporter;
# set the version for version checking
- $VERSION = 1.00;
- # if using RCS/CVS, this may be preferred
- $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
+ our $VERSION = 1.00;
- @ISA = qw(Exporter);
- @EXPORT = qw(&func1 &func2 &func4);
- %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
+ # Inherit from Exporter to export functions and variables
+ our @ISA = qw(Exporter);
- # your exported package globals go here,
- # as well as any optionally exported functions
- @EXPORT_OK = qw($Var1 %Hashit &func3);
+ # Functions and variables which are exported by default
+ our @EXPORT = qw(func1 func2);
+
+ # Functions and variables which can be optionally exported
+ our @EXPORT_OK = qw($Var1 %Hashit func3);
}
- our @EXPORT_OK;
# exported package globals go here
- our $Var1;
- our %Hashit;
+ our $Var1 = '';
+ our %Hashit = ();
# non-exported package globals go here
- our @more;
- our $stuff;
-
- # initialize package globals, first exported ones
- $Var1 = '';
- %Hashit = ();
+ # (they are still accessible as $Some::Module::stuff)
+ our @more = ();
+ our $stuff = '';
- # then the others (which are still accessible as $Some::Module::stuff)
- $stuff = '';
- @more = ();
-
- # all file-scoped lexicals must be created before
- # the functions below that use them.
-
- # file-private lexicals go here
+ # file-private lexicals go here, before any functions which use them
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
- # callable as &$priv_func; it cannot be prototyped.
+ # callable as $priv_func->();
my $priv_func = sub {
- # stuff goes here.
+ ...
};
# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
- sub func1 {} # no prototype
- sub func2() {} # proto'd void
- sub func3($$) {} # proto'd to 2 scalars
-
- # this one isn't exported, but could be called!
- sub func4(\%) {} # proto'd to 1 hash ref
+ sub func1 { ... }
+ sub func2 { ... }
- END { } # module clean-up code here (global destructor)
+ # this one isn't exported, but could be called directly
+ # as Some::Module::func3()
+ sub func3 { ... }
- ## YOUR CODE GOES HERE
+ END { ... } # module clean-up code here (global destructor)
1; # don't forget to return a true value from the file
This is exactly equivalent to
- BEGIN { require Module; import Module; }
+ BEGIN { require 'Module.pm'; 'Module'->import; }
or
- BEGIN { require Module; import Module LIST; }
+ BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }
As a special case
is exactly equivalent to
- BEGIN { require Module; }
+ BEGIN { require 'Module.pm'; }
All Perl module files have the extension F<.pm>. The C<use> operator
assumes this so you don't have to spell out "F<Module.pm>" in quotes.
autoloading, the user can say just C<use POSIX> to get it all.
=head2 Making your module threadsafe
+X<threadsafe> X<thread safe>
+X<module, threadsafe> X<module, thread safe>
+X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread>
-Since 5.6.0, Perl has had support for a new type of threads called
-interpreter threads (ithreads). These threads can be used explicitly
-and implicitly.
+Perl supports a type of threads called interpreter threads (ithreads).
+These threads can be used explicitly and implicitly.
Ithreads work by cloning the data tree so that no data is shared
between different threads. These threads can be used by using the C<threads>
module or by doing fork() on win32 (fake fork() support). When a
thread is cloned all Perl data is cloned, however non-Perl data cannot
-be cloned automatically. Perl after 5.7.2 has support for the C<CLONE>
+be cloned automatically. Perl after 5.8.0 has support for the C<CLONE>
special subroutine. In C<CLONE> you can do whatever
you need to do,
like for example handle the cloning of non-Perl data, if necessary.
called just before cloning starts, and in the context of the parent
thread. If it returns a true value, then no objects of that class will
be cloned; or rather, they will be copied as unblessed, undef values.
+For example: if in the parent there are two references to a single blessed
+hash, then in the child there will be two references to a single undefined
+scalar value instead.
This provides a simple mechanism for making a module threadsafe; just add
-C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will be
+C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will
now only be called once per object. Of course, if the child thread needs
to make use of the objects, then a more sophisticated approach is
needed.
See L<perlmodlib> for general style issues related to building Perl
modules and classes, as well as descriptions of the standard library
and CPAN, L<Exporter> for how Perl's standard import/export mechanism
-works, L<perltoot> and L<perltooc> for an in-depth tutorial on
+works, L<perlootut> and L<perlobj> for in-depth information on
creating classes, L<perlobj> for a hard-core reference document on
objects, L<perlsub> for an explanation of functions and scoping,
and L<perlxstut> and L<perlguts> for more information on writing
https://perl5.git.perl.org / perl5.git / blobdiff