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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() opera tors). 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 com piler for the rest of that block. You can refer to vari ables 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. Only identifiers starting with letters (or underscore) are cate private variables and method names. However, vari ables 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" pack age. 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 sym bol table's name is thus %main::, or %:: for short. Like wise 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 refer ring to when you use the *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: local *main::foo = *main::bar; local $main::{foo} = $main::{bar}; (Be sure to note the vast difference between the second line above and "local $main::foo = $main::bar". The former is accessing the hash %main::, which is the symbol table of package "main". The latter is simply assigning scalar $bar in package "main" to scalar $foo of the same pack age.) You can use this to print out all the variables in a pack age, for instance. The standard but antiquated dumpvar.pl *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 dis tinct 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 $bar -- the one that was stuffed away by "local()" and which will be restored when the block ends. Because variables are accessed through the typeglob, you can use "*foo = *bar" to create an alias which can be localized. (But be aware that this means you can't have a separate @foo and @bar, etc.) What makes all of this important is that the Exporter mod ule uses glob aliasing as the import/export mechanism. Whether or not you can properly localize a variable that has been exported from a module depends on how it was exported: @EXPORT = qw($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 = *Pack age::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 assign ing to dynamic variables, not lexicals. 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 deref erence 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 con stant 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 conve nient 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 type globs as arguments: 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 refer ences to the individual elements of *foo. See perlref. Subroutine definitions (and declarations, for that matter) need not necessarily be situated in the package whose sym bol table they occupy. You can define a subroutine out side 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 com 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: 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). Package Constructors and Destructors Four special subroutines act as package constructors and destructors. These are the "BEGIN", "CHECK", "INIT", and "END" routines. The "sub" is optional for these routines. A "BEGIN" subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have mul tiple "BEGIN" blocks within a file--they will execute in order of definition. Because a "BEGIN" 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 file. Once a "BEGIN" has run, it is immediately unde fined and any code it used is returned to Perl's memory pool. This means you can't ever explicitly call a "BEGIN". An "END" subroutine 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 "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" "CHECK" 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. For example, the code generators documented in perlcc make use of "INIT" blocks to initialize and resolve pointers to XSUBs. 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. 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 meth ods 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 mod ules 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 defi nition and make its semantics available implicitly through method calls on the class and its objects, without explic itly 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 { use Exporter (); our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); # set the version for version checking # 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 # 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 perlmod lib for details on mechanics and style issues in module creation. Perl modules are included into your program by saying use Module; 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 "Mod ule.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"; differ from each other in two ways. In the first case, any double colons in the module name, such as "Some::Mod ule", are translated into your system's directory separa tor, 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" state ment 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 com 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 respon sibility 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 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 executed once 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. 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().

perlmod

DESCRIPTION

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