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Storable(3pm)			 Perl Programmers Reference Guide		    Storable(3pm)

       Storable - persistence for Perl data structures

	use Storable;
	store \%table, 'file';
	$hashref = retrieve('file');

	use Storable qw(nstore store_fd nstore_fd freeze thaw dclone);

	# Network order
	nstore \%table, 'file';
	$hashref = retrieve('file');   # There is NO nretrieve()

	# Storing to and retrieving from an already opened file
	store_fd \@array, \*STDOUT;
	nstore_fd \%table, \*STDOUT;
	$aryref = fd_retrieve(\*SOCKET);
	$hashref = fd_retrieve(\*SOCKET);

	# Serializing to memory
	$serialized = freeze \%table;
	%table_clone = %{ thaw($serialized) };

	# Deep (recursive) cloning
	$cloneref = dclone($ref);

	# Advisory locking
	use Storable qw(lock_store lock_nstore lock_retrieve)
	lock_store \%table, 'file';
	lock_nstore \%table, 'file';
	$hashref = lock_retrieve('file');

       The Storable package brings persistence to your Perl data structures containing SCALAR,
       ARRAY, HASH or REF objects, i.e. anything that can be conveniently stored to disk and
       retrieved at a later time.

       It can be used in the regular procedural way by calling "store" with a reference to the
       object to be stored, along with the file name where the image should be written.

       The routine returns "undef" for I/O problems or other internal error, a true value other-
       wise. Serious errors are propagated as a "die" exception.

       To retrieve data stored to disk, use "retrieve" with a file name.  The objects stored into
       that file are recreated into memory for you, and a reference to the root object is
       returned. In case an I/O error occurs while reading, "undef" is returned instead. Other
       serious errors are propagated via "die".

       Since storage is performed recursively, you might want to stuff references to objects that
       share a lot of common data into a single array or hash table, and then store that object.
       That way, when you retrieve back the whole thing, the objects will continue to share what
       they originally shared.

       At the cost of a slight header overhead, you may store to an already opened file descrip-
       tor using the "store_fd" routine, and retrieve from a file via "fd_retrieve". Those names
       aren't imported by default, so you will have to do that explicitly if you need those rou-
       tines.  The file descriptor you supply must be already opened, for read if you're going to
       retrieve and for write if you wish to store.

	       store_fd(\%table, *STDOUT) || die "can't store to stdout\n";
	       $hashref = fd_retrieve(*STDIN);

       You can also store data in network order to allow easy sharing across multiple platforms,
       or when storing on a socket known to be remotely connected. The routines to call have an
       initial "n" prefix for network, as in "nstore" and "nstore_fd". At retrieval time, your
       data will be correctly restored so you don't have to know whether you're restoring from
       native or network ordered data.	Double values are stored stringified to ensure portabil-
       ity as well, at the slight risk of loosing some precision in the last decimals.

       When using "fd_retrieve", objects are retrieved in sequence, one object (i.e. one recur-
       sive tree) per associated "store_fd".

       If you're more from the object-oriented camp, you can inherit from Storable and directly
       store your objects by invoking "store" as a method. The fact that the root of the to-be-
       stored tree is a blessed reference (i.e. an object) is special-cased so that the retrieve
       does not provide a reference to that object but rather the blessed object reference
       itself. (Otherwise, you'd get a reference to that blessed object).

       The Storable engine can also store data into a Perl scalar instead, to later retrieve
       them. This is mainly used to freeze a complex structure in some safe compact memory place
       (where it can possibly be sent to another process via some IPC, since freezing the struc-
       ture also serializes it in effect). Later on, and maybe somewhere else, you can thaw the
       Perl scalar out and recreate the original complex structure in memory.

       Surprisingly, the routines to be called are named "freeze" and "thaw".  If you wish to
       send out the frozen scalar to another machine, use "nfreeze" instead to get a portable

       Note that freezing an object structure and immediately thawing it actually achieves a deep
       cloning of that structure:

	   dclone(.) = thaw(freeze(.))

       Storable provides you with a "dclone" interface which does not create that intermediary
       scalar but instead freezes the structure in some internal memory space and then immedi-
       ately thaws it out.

       The "lock_store" and "lock_nstore" routine are equivalent to "store" and "nstore", except
       that they get an exclusive lock on the file before writing.  Likewise, "lock_retrieve"
       does the same as "retrieve", but also gets a shared lock on the file before reading.

       As with any advisory locking scheme, the protection only works if you systematically use
       "lock_store" and "lock_retrieve".  If one side of your application uses "store" whilst the
       other uses "lock_retrieve", you will get no protection at all.

       The internal advisory locking is implemented using Perl's flock() routine.  If your system
       does not support any form of flock(), or if you share your files across NFS, you might
       wish to use other forms of locking by using modules such as LockFile::Simple which lock a
       file using a filesystem entry, instead of locking the file descriptor.

       The heart of Storable is written in C for decent speed. Extra low-level optimizations have
       been made when manipulating perl internals, to sacrifice encapsulation for the benefit of
       greater speed.

       Normally, Storable stores elements of hashes in the order they are stored internally by
       Perl, i.e. pseudo-randomly.  If you set $Storable::canonical to some "TRUE" value,
       Storable will store hashes with the elements sorted by their key.  This allows you to com-
       pare data structures by comparing their frozen representations (or even the compressed
       frozen representations), which can be useful for creating lookup tables for complicated

       Canonical order does not imply network order; those are two orthogonal settings.

       Since Storable version 2.05, CODE references may be serialized with the help of
       B::Deparse. To enable this feature, set $Storable::Deparse to a true value. To enable
       deserializazion, $Storable::Eval should be set to a true value. Be aware that deserializa-
       tion is done through "eval", which is dangerous if the Storable file contains malicious
       data. You can set $Storable::Eval to a subroutine reference which would be used instead of
       "eval". See below for an example using a Safe compartment for deserialization of CODE ref-

       If $Storable::Deparse and/or $Storable::Eval are set to false values, then the value of
       $Storable::forgive_me (see below) is respected while serializing and deserializing.

       This release of Storable can be used on a newer version of Perl to serialize data which is
       not supported by earlier Perls.	By default, Storable will attempt to do the right thing,
       by "croak()"ing if it encounters data that it cannot deserialize.  However, the defaults
       can be changed as follows:

       utf8 data
	   Perl 5.6 added support for Unicode characters with code points > 255, and Perl 5.8 has
	   full support for Unicode characters in hash keys.  Perl internally encodes strings
	   with these characters using utf8, and Storable serializes them as utf8.  By default,
	   if an older version of Perl encounters a utf8 value it cannot represent, it will
	   "croak()".  To change this behaviour so that Storable deserializes utf8 encoded values
	   as the string of bytes (effectively dropping the is_utf8 flag) set
	   $Storable::drop_utf8 to some "TRUE" value.  This is a form of data loss, because with
	   $drop_utf8 true, it becomes impossible to tell whether the original data was the Uni-
	   code string, or a series of bytes that happen to be valid utf8.

       restricted hashes
	   Perl 5.8 adds support for restricted hashes, which have keys restricted to a given
	   set, and can have values locked to be read only.  By default, when Storable encounters
	   a restricted hash on a perl that doesn't support them, it will deserialize it as a
	   normal hash, silently discarding any placeholder keys and leaving the keys and all
	   values unlocked.  To make Storable "croak()" instead, set $Storable::down-
	   grade_restricted to a "FALSE" value.  To restore the default set it back to some
	   "TRUE" value.

       files from future versions of Storable
	   Earlier versions of Storable would immediately croak if they encountered a file with a
	   higher internal version number than the reading Storable knew about.  Internal version
	   numbers are increased each time new data types (such as restricted hashes) are added
	   to the vocabulary of the file format.  This meant that a newer Storable module had no
	   way of writing a file readable by an older Storable, even if the writer didn't store
	   newer data types.

	   This version of Storable will defer croaking until it encounters a data type in the
	   file that it does not recognize.  This means that it will continue to read files gen-
	   erated by newer Storable modules which are careful in what they write out, making it
	   easier to upgrade Storable modules in a mixed environment.

	   The old behaviour of immediate croaking can be re-instated by setting
	   $Storable::accept_future_minor to some "FALSE" value.

       All these variables have no effect on a newer Perl which supports the relevant feature.

       Storable uses the "exception" paradigm, in that it does not try to workaround failures: if
       something bad happens, an exception is generated from the caller's perspective (see Carp
       and "croak()").	Use eval {} to trap those exceptions.

       When Storable croaks, it tries to report the error via the "logcroak()" routine from the
       "Log::Agent" package, if it is available.

       Normal errors are reported by having store() or retrieve() return "undef".  Such errors
       are usually I/O errors (or truncated stream errors at retrieval).


       Any class may define hooks that will be called during the serialization and deserializa-
       tion process on objects that are instances of that class.  Those hooks can redefine the
       way serialization is performed (and therefore, how the symmetrical deserialization should
       be conducted).

       Since we said earlier:

	   dclone(.) = thaw(freeze(.))

       everything we say about hooks should also hold for deep cloning. However, hooks get to
       know whether the operation is a mere serialization, or a cloning.

       Therefore, when serializing hooks are involved,

	   dclone(.) <> thaw(freeze(.))

       Well, you could keep them in sync, but there's no guarantee it will always hold on classes
       somebody else wrote.  Besides, there is little to gain in doing so: a serializing hook
       could keep only one attribute of an object, which is probably not what should happen dur-
       ing a deep cloning of that same object.

       Here is the hooking interface:

       "STORABLE_freeze" obj, cloning
	   The serializing hook, called on the object during serialization.  It can be inherited,
	   or defined in the class itself, like any other method.

	   Arguments: obj is the object to serialize, cloning is a flag indicating whether we're
	   in a dclone() or a regular serialization via store() or freeze().

	   Returned value: A LIST "($serialized, $ref1, $ref2, ...)" where $serialized is the
	   serialized form to be used, and the optional $ref1, $ref2, etc... are extra references
	   that you wish to let the Storable engine serialize.

	   At deserialization time, you will be given back the same LIST, but all the extra ref-
	   erences will be pointing into the deserialized structure.

	   The first time the hook is hit in a serialization flow, you may have it return an
	   empty list.	That will signal the Storable engine to further discard that hook for
	   this class and to therefore revert to the default serialization of the underlying Perl
	   data.  The hook will again be normally processed in the next serialization.

	   Unless you know better, serializing hook should always say:

	       sub STORABLE_freeze {
		   my ($self, $cloning) = @_;
		   return if $cloning;	       # Regular default serialization

	   in order to keep reasonable dclone() semantics.

       "STORABLE_thaw" obj, cloning, serialized, ...
	   The deserializing hook called on the object during deserialization.	But wait: if
	   we're deserializing, there's no object yet... right?

	   Wrong: the Storable engine creates an empty one for you.  If you know Eiffel, you can
	   view "STORABLE_thaw" as an alternate creation routine.

	   This means the hook can be inherited like any other method, and that obj is your
	   blessed reference for this particular instance.

	   The other arguments should look familiar if you know "STORABLE_freeze": cloning is
	   true when we're part of a deep clone operation, serialized is the serialized string
	   you returned to the engine in "STORABLE_freeze", and there may be an optional list of
	   references, in the same order you gave them at serialization time, pointing to the
	   deserialized objects (which have been processed courtesy of the Storable engine).

	   When the Storable engine does not find any "STORABLE_thaw" hook routine, it tries to
	   load the class by requiring the package dynamically (using the blessed package name),
	   and then re-attempts the lookup.  If at that time the hook cannot be located, the
	   engine croaks.  Note that this mechanism will fail if you define several classes in
	   the same file, but perlmod warned you.

	   It is up to you to use this information to populate obj the way you want.

	   Returned value: none.


       Predicates are not exportable.  They must be called by explicitly prefixing them with the
       Storable package name.

	   The "Storable::last_op_in_netorder()" predicate will tell you whether network order
	   was used in the last store or retrieve operation.  If you don't know how to use this,
	   just forget about it.

	   Returns true if within a store operation (via STORABLE_freeze hook).

	   Returns true if within a retrieve operation (via STORABLE_thaw hook).


       With hooks comes the ability to recurse back to the Storable engine.  Indeed, hooks are
       regular Perl code, and Storable is convenient when it comes to serializing and deserializ-
       ing things, so why not use it to handle the serialization string?

       There are a few things you need to know, however:

       o   You can create endless loops if the things you serialize via freeze() (for instance)
	   point back to the object we're trying to serialize in the hook.

       o   Shared references among objects will not stay shared: if we're serializing the list of
	   object [A, C] where both object A and C refer to the SAME object B, and if there is a
	   serializing hook in A that says freeze(B), then when deserializing, we'll get [A', C']
	   where A' refers to B', but C' refers to D, a deep clone of B'.  The topology was not

       That's why "STORABLE_freeze" lets you provide a list of references to serialize.  The
       engine guarantees that those will be serialized in the same context as the other objects,
       and therefore that shared objects will stay shared.

       In the above [A, C] example, the "STORABLE_freeze" hook could return:

	       ("something", $self->{B})

       and the B part would be serialized by the engine.  In "STORABLE_thaw", you would get back
       the reference to the B' object, deserialized for you.

       Therefore, recursion should normally be avoided, but is nonetheless supported.

       Deep Cloning

       There is a Clone module available on CPAN which implements deep cloning natively, i.e.
       without freezing to memory and thawing the result.  It is aimed to replace Storable's
       dclone() some day.  However, it does not currently support Storable hooks to redefine the
       way deep cloning is performed.

Storable magic
       Yes, there's a lot of that :-) But more precisely, in UNIX systems there's a utility
       called "file", which recognizes data files based on their contents (usually their first
       few bytes).  For this to work, a certain file called magic needs to taught about the sig-
       nature of the data.  Where that configuration file lives depends on the UNIX flavour;
       often it's something like /usr/share/misc/magic or /etc/magic.  Your system administrator
       needs to do the updating of the magic file.  The necessary signature information is output
       to STDOUT by invoking Storable::show_file_magic().  Note that the GNU implementation of
       the "file" utility, version 3.38 or later, is expected to contain support for recognising
       Storable files out-of-the-box, in addition to other kinds of Perl files.

       Here are some code samples showing a possible usage of Storable:

	       use Storable qw(store retrieve freeze thaw dclone);

	       %color = ('Blue' => 0.1, 'Red' => 0.8, 'Black' => 0, 'White' => 1);

	       store(\%color, '/tmp/colors') or die "Can't store %a in /tmp/colors!\n";

	       $colref = retrieve('/tmp/colors');
	       die "Unable to retrieve from /tmp/colors!\n" unless defined $colref;
	       printf "Blue is still %lf\n", $colref->{'Blue'};

	       $colref2 = dclone(\%color);

	       $str = freeze(\%color);
	       printf "Serialization of %%color is %d bytes long.\n", length($str);
	       $colref3 = thaw($str);

       which prints (on my machine):

	       Blue is still 0.100000
	       Serialization of %color is 102 bytes long.

       Serialization of CODE references and deserialization in a safe compartment:

	       use Storable qw(freeze thaw);
	       use Safe;
	       use strict;
	       my $safe = new Safe;
	       # because of opcodes used in "use strict":
	       $safe->permit(qw(:default require));
	       local $Storable::Deparse = 1;
	       local $Storable::Eval = sub { $safe->reval($_[0]) };
	       my $serialized = freeze(sub { 42 });
	       my $code = thaw($serialized);
	       $code->() == 42;

       If you're using references as keys within your hash tables, you're bound to be disap-
       pointed when retrieving your data. Indeed, Perl stringifies references used as hash table
       keys. If you later wish to access the items via another reference stringification (i.e.
       using the same reference that was used for the key originally to record the value into the
       hash table), it will work because both references stringify to the same string.

       It won't work across a sequence of "store" and "retrieve" operations, however, because the
       addresses in the retrieved objects, which are part of the stringified references, will
       probably differ from the original addresses. The topology of your structure is preserved,
       but not hidden semantics like those.

       On platforms where it matters, be sure to call "binmode()" on the descriptors that you
       pass to Storable functions.

       Storing data canonically that contains large hashes can be significantly slower than stor-
       ing the same data normally, as temporary arrays to hold the keys for each hash have to be
       allocated, populated, sorted and freed.	Some tests have shown a halving of the speed of
       storing -- the exact penalty will depend on the complexity of your data.  There is no
       slowdown on retrieval.

       You can't store GLOB, FORMLINE, etc.... If you can define semantics for those operations,
       feel free to enhance Storable so that it can deal with them.

       The store functions will "croak" if they run into such references unless you set
       $Storable::forgive_me to some "TRUE" value. In that case, the fatal message is turned in a
       warning and some meaningless string is stored instead.

       Setting $Storable::canonical may not yield frozen strings that compare equal due to possi-
       ble stringification of numbers. When the string version of a scalar exists, it is the form
       stored; therefore, if you happen to use your numbers as strings between two freezing oper-
       ations on the same data structures, you will get different results.

       When storing doubles in network order, their value is stored as text.  However, you should
       also not expect non-numeric floating-point values such as infinity and "not a number" to
       pass successfully through a nstore()/retrieve() pair.

       As Storable neither knows nor cares about character sets (although it does know that char-
       acters may be more than eight bits wide), any difference in the interpretation of charac-
       ter codes between a host and a target system is your problem.  In particular, if host and
       target use different code points to represent the characters used in the text representa-
       tion of floating-point numbers, you will not be able be able to exchange floating-point
       data, even with nstore().

       "Storable::drop_utf8" is a blunt tool.  There is no facility either to return all strings
       as utf8 sequences, or to attempt to convert utf8 data back to 8 bit and "croak()" if the
       conversion fails.

       Prior to Storable 2.01, no distinction was made between signed and unsigned integers on
       storing.  By default Storable prefers to store a scalars string representation (if it has
       one) so this would only cause problems when storing large unsigned integers that had never
       been coverted to string or floating point.  In other words values that had been generated
       by integer operations such as logic ops and then not used in any string or arithmetic con-
       text before storing.

       64 bit data in perl 5.6.0 and 5.6.1

       This section only applies to you if you have existing data written out by Storable 2.02 or
       earlier on perl 5.6.0 or 5.6.1 on Unix or Linux which has been configured with 64 bit
       integer support (not the default) If you got a precompiled perl, rather than running Con-
       figure to build your own perl from source, then it almost certainly does not affect you,
       and you can stop reading now (unless you're curious). If you're using perl on Windows it
       does not affect you.

       Storable writes a file header which contains the sizes of various C language types for the
       C compiler that built Storable (when not writing in network order), and will refuse to
       load files written by a Storable not on the same (or compatible) architecture.  This check
       and a check on machine byteorder is needed because the size of various fields in the file
       are given by the sizes of the C language types, and so files written on different archi-
       tectures are incompatible.  This is done for increased speed.  (When writing in network
       order, all fields are written out as standard lengths, which allows full interworking, but
       takes longer to read and write)

       Perl 5.6.x introduced the ability to optional configure the perl interpreter to use C's
       "long long" type to allow scalars to store 64 bit integers on 32 bit systems.  However,
       due to the way the Perl configuration system generated the C configuration files on non-
       Windows platforms, and the way Storable generates its header, nothing in the Storable file
       header reflected whether the perl writing was using 32 or 64 bit integers, despite the
       fact that Storable was storing some data differently in the file.  Hence Storable running
       on perl with 64 bit integers will read the header from a file written by a 32 bit perl,
       not realise that the data is actually in a subtly incompatible format, and then go horri-
       bly wrong (possibly crashing) if it encountered a stored integer.  This is a design fail-

       Storable has now been changed to write out and read in a file header with information
       about the size of integers.  It's impossible to detect whether an old file being read in
       was written with 32 or 64 bit integers (they have the same header) so it's impossible to
       automatically switch to a correct backwards compatibility mode.	Hence this Storable
       defaults to the new, correct behaviour.

       What this means is that if you have data written by Storable 1.x running on perl 5.6.0 or
       5.6.1 configured with 64 bit integers on Unix or Linux then by default this Storable will
       refuse to read it, giving the error Byte order is not compatible.  If you have such data
       then you you should set $Storable::interwork_56_64bit to a true value to make this
       Storable read and write files with the old header.  You should also migrate your data, or
       any older perl you are communicating with, to this current version of Storable.

       If you don't have data written with specific configuration of perl described above, then
       you do not and should not do anything.  Don't set the flag - not only will Storable on an
       identically configured perl refuse to load them, but Storable a differently configured
       perl will load them believing them to be correct for it, and then may well fail or crash
       part way through reading them.

       Thank you to (in chronological order):

	       Jarkko Hietaniemi <jhi@iki.fi>
	       Ulrich Pfeifer <pfeifer@charly.informatik.uni-dortmund.de>
	       Benjamin A. Holzman <bah@ecnvantage.com>
	       Andrew Ford <A.Ford@ford-mason.co.uk>
	       Gisle Aas <gisle@aas.no>
	       Jeff Gresham <gresham_jeffrey@jpmorgan.com>
	       Murray Nesbitt <murray@activestate.com>
	       Marc Lehmann <pcg@opengroup.org>
	       Justin Banks <justinb@wamnet.com>
	       Jarkko Hietaniemi <jhi@iki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
	       Salvador Ortiz Garcia <sog@msg.com.mx>
	       Dominic Dunlop <domo@computer.org>
	       Erik Haugan <erik@solbors.no>

       for their bug reports, suggestions and contributions.

       Benjamin Holzman contributed the tied variable support, Andrew Ford contributed the canon-
       ical order for hashes, and Gisle Aas fixed a few misunderstandings of mine regarding the
       perl internals, and optimized the emission of "tags" in the output streams by simply
       counting the objects instead of tagging them (leading to a binary incompatibility for the
       Storable image starting at version 0.6--older images are, of course, still properly under-
       stood).	Murray Nesbitt made Storable thread-safe.  Marc Lehmann added overloading and
       references to tied items support.

       Storable was written by Raphael Manfredi <Raphael_Manfredi@pobox.com> Maintenance is now
       done by the perl5-porters <perl5-porters@perl.org>

       Please e-mail us with problems, bug fixes, comments and complaints, although if you have
       complements you should send them to Raphael.  Please don't e-mail Raphael with problems,
       as he no longer works on Storable, and your message will be delayed while he forwards it
       to us.


perl v5.8.0				    2002-06-01				    Storable(3pm)
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