perlmod
(1)
名称
perlmod - Perl modules (packages and symbol tables)
用法概要
Please see following description for synopsis
描述
Perl Programmers Reference Guide PERLMOD(1)
NAME
perlmod - Perl modules (packages and symbol tables)
DESCRIPTION
Packages
Perl provides a mechanism for alternative namespaces to
protect packages from stomping on each other's variables.
In fact, there's really no such thing as a global variable
in Perl. The package statement declares the compilation
unit as being in the given namespace. The scope of the
package declaration is from the declaration itself through
the end of the enclosing block, "eval", or file, whichever
comes first (the same scope as the my() and local()
operators). Unqualified dynamic identifiers will be in this
namespace, except for those few identifiers that if
unqualified, default to the main package instead of the
current one as described below. A package statement affects
only dynamic variables--including those you've used local()
on--but not lexical variables created with my(). Typically
it would be the first declaration in a file included by the
"do", "require", or "use" operators. You can switch into a
package in more than one place; it merely influences which
symbol table is used by the compiler for the rest of that
block. You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name
and a double colon: $Package::Variable. If the package name
is null, the "main" package is assumed. That is, $::sail is
equivalent to $main::sail.
The old package delimiter was a single quote, but double
colon is now the preferred delimiter, in part because it's
more readable to humans, and in part because it's more
readable to emacs macros. It also makes C++ programmers
feel like they know what's going on--as opposed to using the
single quote as separator, which was there to make Ada
programmers feel like they knew what was going on. Because
the old-fashioned syntax is still supported for backwards
compatibility, if you try to use a string like "This is
$owner's house", you'll be accessing $owner::s; that is, the
$s variable in package "owner", which is probably not what
you meant. Use braces to disambiguate, as in "This is
${owner}'s house".
Packages may themselves contain package separators, as in
$OUTER::INNER::var. This implies nothing about the order of
name lookups, however. There are no relative packages: all
symbols are either local to the current package, or must be
fully qualified from the outer package name down. For
instance, there is nowhere within package "OUTER" that
$INNER::var refers to $OUTER::INNER::var. "INNER" refers to
a totally separate global package.
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Only identifiers starting with letters (or underscore) are
stored in a package's symbol table. All other symbols are
kept in package "main", including all punctuation variables,
like $_. In addition, when unqualified, the identifiers
STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are
forced to be in package "main", even when used for other
purposes than their built-in ones. If you have a package
called "m", "s", or "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.
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 "_", such as $_ and "sub _",
are still forced into the package "main". See also
"Technical Note on the Syntax of Variable Names" in perlvar.
"eval"ed strings are compiled in the package in which the
eval() was compiled. (Assignments to $SIG{}, however,
assume the signal handler specified is in the "main"
package. Qualify the signal handler name if you wish to
have a signal handler in a package.) For an example,
examine perldb.pl in the Perl library. It initially
switches to the "DB" package so that the debugger doesn't
interfere with variables in the program you are trying to
debug. At various points, however, it temporarily switches
back to the "main" package to evaluate various expressions
in the context of the "main" package (or wherever you came
from). See perldebug.
The special symbol "__PACKAGE__" contains the current
package, but cannot (easily) be used to construct variable
names.
See perlsub for other scoping issues related to my() and
local(), and perlref regarding closures.
Symbol Tables
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 %main::, or %:: for short. Likewise
the symbol table for the nested package mentioned earlier is
named %OUTER::INNER::.
The value in each entry of the hash is what you are
referring to when you use the *name typeglob notation.
local *main::foo = *main::bar;
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You can use this to print out all the variables in a
package, for instance. The standard but antiquated
dumpvar.pl library and the CPAN module Devel::Symdump make
use of this.
Assignment to a typeglob performs an aliasing operation,
i.e.,
*dick = *richard;
causes variables, subroutines, formats, and file and
directory handles accessible via the identifier "richard"
also to be accessible via the identifier "dick". If you
want to alias only a particular variable or subroutine,
assign a reference instead:
*dick = \$richard;
Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays. Tricky, eh?
There is one subtle difference between the following
statements:
*foo = *bar;
*foo = \$bar;
"*foo = *bar" makes the typeglobs themselves synonymous
while "*foo = \$bar" makes the SCALAR portions of two
distinct typeglobs refer to the same scalar value. This
means that the following code:
$bar = 1;
*foo = \$bar; # Make $foo an alias for $bar
{
local $bar = 2; # Restrict changes to block
print $foo; # Prints '1'!
}
Would print '1', because $foo holds a reference to the
original $bar. The one that was stuffed away by "local()"
and which will be restored when the block ends. Because
variables are accessed through the typeglob, you can use
"*foo = *bar" to create an alias which can be localized.
(But be aware that this means you can't have a separate @foo
and @bar, etc.)
What makes all of this important is that the Exporter module
uses glob aliasing as the import/export mechanism. Whether
or not you can properly localize a variable that has been
exported from a module depends on how it was exported:
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@EXPORT = qw($FOO); # Usual form, can't be localized
@EXPORT = qw(*FOO); # Can be localized
You can work around the first case by using the fully
qualified name ($Package::FOO) where you need a local value,
or by overriding it by saying "*FOO = *Package::FOO" in your
script.
The "*x = \$y" mechanism may be used to pass and return
cheap references into or from subroutines if you don't want
to copy the whole thing. It only works when assigning to
dynamic variables, not lexicals.
%some_hash = (); # can't be my()
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
# now use %hashsym normally, and you
# will affect the caller's %another_hash
my %nhash = (); # do what you want
return \%nhash;
}
On return, the reference will overwrite the hash slot in the
symbol table specified by the *some_hash typeglob. This is
a somewhat tricky way of passing around references cheaply
when you don't want to have to remember to dereference
variables explicitly.
Another use of symbol tables is for making "constant"
scalars.
*PI = \3.14159265358979;
Now you cannot alter $PI, which is probably a good thing all
in all. This isn't the same as a constant subroutine, which
is subject to optimization at compile-time. A constant
subroutine is one prototyped to take no arguments and to
return a constant expression. See perlsub for details on
these. The "use constant" pragma is a convenient shorthand
for these.
You can say *foo{PACKAGE} and *foo{NAME} to find out what
name and package the *foo symbol table entry comes from.
This may be useful in a subroutine that gets passed
typeglobs as arguments:
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sub identify_typeglob {
my $glob = shift;
print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
}
identify_typeglob *foo;
identify_typeglob *bar::baz;
This prints
You gave me main::foo
You gave me bar::baz
The *foo{THING} notation can also be used to obtain
references to the individual elements of *foo. See perlref.
Subroutine definitions (and declarations, for that matter)
need not necessarily be situated in the package whose symbol
table they occupy. You can define a subroutine outside its
package by explicitly qualifying the name of the subroutine:
package main;
sub Some_package::foo { ... } # &foo defined in Some_package
This is just a shorthand for a typeglob assignment at
compile time:
BEGIN { *Some_package::foo = sub { ... } }
and is not the same as writing:
{
package Some_package;
sub foo { ... }
}
In the first two versions, the body of the subroutine is
lexically in the main package, not in Some_package. So
something like this:
package main;
$Some_package::name = "fred";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": \$name is '$name'\n";
}
Some_package::foo();
prints:
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in main: $name is 'barney'
rather than:
in Some_package: $name is 'fred'
This also has implications for the use of the SUPER::
qualifier (see perlobj).
BEGIN, UNITCHECK, CHECK, INIT and END
Five specially named code blocks are executed at the
beginning and at the end of a running Perl program. These
are the "BEGIN", "UNITCHECK", "CHECK", "INIT", and "END"
blocks.
These code blocks can be prefixed with "sub" to give the
appearance of a subroutine (although this is not considered
good style). One should note that these code blocks don't
really exist as named subroutines (despite their
appearance). The thing that gives this away is the fact that
you can have more than one of these code blocks in a
program, and they will get all executed at the appropriate
moment. So you can't execute any of these code blocks by
name.
A "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 "BEGIN" blocks within a file (or eval'ed
string); they will execute in order of definition. Because
a "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 "BEGIN" has run, it is immediately undefined and any code
it used is returned to Perl's memory pool.
An "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
morphing into another program via "exec", or being blown out
of the water by a signal--you have to trap that yourself (if
you can).) You may have multiple "END" blocks within a
file--they will execute in reverse order of definition; that
is: last in, first out (LIFO). "END" blocks are not
executed when you run perl with the "-c" switch, or if
compilation fails.
Note that "END" code blocks are not executed at the end of a
string "eval()": if any "END" code blocks are created in a
string "eval()", they will be executed just as any other
"END" code block of that package in LIFO order just before
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the interpreter is being exited.
Inside an "END" code block, $? contains the value that the
program is going to pass to "exit()". You can modify $? to
change the exit value of the program. Beware of changing $?
by accident (e.g. by running something via "system").
"UNITCHECK", "CHECK" and "INIT" code blocks are useful to
catch the transition between the compilation phase and the
execution phase of the main program.
"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
"eval"s, code compiled using the "(?{ })" construct in a
regex, calls to "do FILE", "require FILE", and code after
the "-e" switch on the command line.
"CHECK" code blocks are run just after the initial Perl
compile phase ends and before the run time begins, in LIFO
order. "CHECK" code blocks are used in the Perl compiler
suite to save the compiled state of the program.
"INIT" blocks are run just before the Perl runtime begins
execution, in "first in, first out" (FIFO) order.
The "CHECK" and "INIT" code blocks will not be executed
inside a string eval(), if that eval() happens after the end
of the main compilation phase; that can be a problem in
mod_perl and other persistent environments which use "eval
STRING" to load code at runtime.
When you use the -n and -p switches to Perl, "BEGIN" and
"END" work just as they do in awk, as a degenerate case.
Both "BEGIN" and "CHECK" blocks are run when you use the -c
switch for a compile-only syntax check, although your main
code is not.
The begincheck program makes it all clear, eventually:
#!/usr/bin/perl
# begincheck
print "10. Ordinary code runs at runtime.\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" }
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print "11. It runs in order, of course.\n";
BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\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 "12. This is anti-obfuscated code.\n";
END { print "14. END blocks run LIFO at quitting time.\n" }
BEGIN { print " 2. So this line comes out second.\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 "13. It merely _looks_ like it should be confusing.\n";
__END__
Perl Classes
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 package may also derive some of its methods from
another class (package) 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 perltoot and perlobj.
Perl Modules
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 designed to be reusable by other modules
or programs. It may do this by providing a mechanism for
exporting some of its symbols into the symbol table of any
package using it, or it may function as a class definition
and make its semantics available implicitly through method
calls on the class and its objects, without explicitly
exporting anything. Or it can do a little of both.
For example, to start a traditional, non-OO module called
Some::Module, create a file called Some/Module.pm and start
with this template:
package Some::Module; # assumes Some/Module.pm
use strict;
use warnings;
BEGIN {
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use Exporter ();
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
# 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;
@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func4);
%EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit &func3);
}
our @EXPORT_OK;
# exported package globals go here
our $Var1;
our %Hashit;
# non-exported package globals go here
our @more;
our $stuff;
# initialize package globals, first exported ones
$Var1 = '';
%Hashit = ();
# 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
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
# callable as &$priv_func; it cannot be prototyped.
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
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# this one isn't exported, but could be called!
sub func4(\%) {} # proto'd to 1 hash ref
END { } # module clean-up code here (global destructor)
## YOUR CODE GOES HERE
1; # don't forget to return a true value from the file
Then go on to declare and use your variables in functions
without any qualifications. See Exporter and the perlmodlib
for details on mechanics and style issues in module
creation.
Perl modules are included into your program by saying
use Module;
or
use Module LIST;
This is exactly equivalent to
BEGIN { require Module; import Module; }
or
BEGIN { require Module; import Module LIST; }
As a special case
use Module ();
is exactly equivalent to
BEGIN { require Module; }
All Perl module files have the extension .pm. The "use"
operator assumes this so you don't have to spell out
"Module.pm" in quotes. This also helps to differentiate new
modules from old .pl and .ph files. Module names are also
capitalized unless they're functioning as pragmas; pragmas
are in effect compiler directives, and are sometimes called
"pragmatic modules" (or even "pragmata" if you're a
classicist).
The two statements:
require SomeModule;
require "SomeModule.pm";
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differ from each other in two ways. In the first case, any
double colons in the module name, such as "Some::Module",
are translated into your system's directory separator,
usually "/". The second case does not, and would have to
be specified literally. The other difference is that seeing
the first "require" clues in the compiler that uses of
indirect object notation involving "SomeModule", as in "$ob
= purge SomeModule", are method calls, not function calls.
(Yes, this really can make a difference.)
Because the "use" statement implies a "BEGIN" block, the
importing of semantics happens as soon as the "use"
statement is compiled, before the rest of the file is
compiled. This is how it is able to function as a pragma
mechanism, and also how modules are able to declare
subroutines that are then visible as list or unary operators
for the rest of the current file. This will not work if you
use "require" instead of "use". With "require" you can get
into this problem:
require Cwd; # make Cwd:: accessible
$here = Cwd::getcwd();
use Cwd; # import names from Cwd::
$here = getcwd();
require Cwd; # make Cwd:: accessible
$here = getcwd(); # oops! no main::getcwd()
In general, "use Module ()" is recommended over "require
Module", because it determines module availability at
compile time, not in the middle of your program's execution.
An exception would be if two modules each tried to "use"
each other, and each also called a function from that other
module. In that case, it's easy to use "require" instead.
Perl packages may be nested inside other package names, so
we can have package names containing "::". But if we used
that package name directly as a filename it would make for
unwieldy or impossible filenames on some systems.
Therefore, if a module's name is, say, "Text::Soundex", then
its definition is actually found in the library file
Text/Soundex.pm.
Perl modules always have a .pm file, but there may also be
dynamically linked executables (often ending in .so) or
autoloaded subroutine definitions (often ending in .al)
associated with the module. If so, these will be entirely
transparent to the user of the module. It is the
responsibility of the .pm file to load (or arrange to
autoload) any additional functionality. For example,
although the POSIX module happens to do both dynamic loading
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and autoloading, the user can say just "use POSIX" to get it
all.
Making your module threadsafe
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.
Ithreads work by cloning the data tree so that no data is
shared between different threads. These threads can be used
by using the "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 "CLONE"
special subroutine. In "CLONE" you can do whatever you need
to do, like for example handle the cloning of non-Perl data,
if necessary. "CLONE" will be called once as a class method
for every package that has it defined (or inherits it). It
will be called in the context of the new thread, so all
modifications are made in the new area. Currently CLONE is
called with no parameters other than the invocant package
name, but code should not assume that this will remain
unchanged, as it is likely that in future extra parameters
will be passed in to give more information about the state
of cloning.
If you want to CLONE all objects you will need to keep track
of them per package. This is simply done using a hash and
Scalar::Util::weaken().
Perl after 5.8.7 has support for the "CLONE_SKIP" special
subroutine. Like "CLONE", "CLONE_SKIP" is called once per
package; however, it is 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 "sub CLONE_SKIP { 1 }" at the top of the class, and
"DESTROY()" will be 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.
Like "CLONE", "CLONE_SKIP" is currently called with no
parameters other than the invocant package name, although
that may change. Similarly, to allow for future expansion,
the return value should be a single 0 or 1 value.
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ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
See perlmodlib for general style issues related to building
Perl modules and classes, as well as descriptions of the
standard library and CPAN, Exporter for how Perl's standard
import/export mechanism works, perltoot and perltooc for an
in-depth tutorial on creating classes, perlobj for a hard-
core reference document on objects, perlsub for an
explanation of functions and scoping, and perlxstut and
perlguts for more information on writing extension modules.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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