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



NAME
       Storable - persistence for Perl data structures

SYNOPSIS
        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');

DESCRIPTION
       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 otherwise. 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 recre-
       ated 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 com-
       mon 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 descriptor 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 routines.  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
       portability 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 recursive tree) per associ-
       ated "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).

MEMORY STORE
       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 pro-
       cess via some IPC, since freezing the structure 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 image.

       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 immediately thaws it out.

ADVISORY LOCKING
       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.

SPEED
       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.

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

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

CODE REFERENCES
       Since Storable version 2.05, CODE references may be serialized with the help of B::Deparse. To enable this fea-
       ture, set $Storable::Deparse to a true value. To enable deserializazion, $Storable::Eval should be set to a
       true value. Be aware that deserialization is done through "eval", which is dangerous if the Storable file con-
       tains 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 references.

       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.

FORWARD COMPATIBILITY
       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 can-
           not represent, it will "croak()".  To change this behaviour so that Storable deserializes utf8 encoded val-
           ues 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 Unicode 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 sup-
           port 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::downgrade_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 ver-
           sion 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 generated 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.

ERROR REPORTING
       Storable uses the "exception" paradigm, in that it does not try to workaround failures: if something bad hap-
       pens, 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).

WIZARDS ONLY
       Hooks

       Any class may define hooks that will be called during the serialization and deserialization 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 opera-
       tion 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 during 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 references will be point-
           ing 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.

       "STORABLE_attach" class, cloning, serialized
           While "STORABLE_freeze" and "STORABLE_thaw" are useful for classes where each instance is independant, this
           mechanism has difficulty (or is incompatible) with objects that exist as common process-level or system-
           level resources, such as singleton objects, database pools, caches or memoized objects.

           The alternative "STORABLE_attach" method provides a solution for these shared objects. Instead of
           "STORABLE_freeze" --E<GT> "STORABLE_thaw", you implement "STORABLE_freeze" --E<GT> "STORABLE_attach"
           instead.

           Arguments: class is the class we are attaching to, cloning is a flag indicating whether we're in a dclone()
           or a regular de-serialization via thaw(), and serialized is the stored string for the resource object.

           Because these resource objects are considered to be owned by the entire process/system, and not the
           "property" of whatever is being serialized, no references underneath the object should be included in the
           serialized string. Thus, in any class that implements "STORABLE_attach", the "STORABLE_freeze" method can-
           not return any references, and "Storable" will throw an error if "STORABLE_freeze" tries to return refer-
           ences.

           All information required to "attach" back to the shared resource object must be contained only in the
           "STORABLE_freeze" return string.  Otherwise, "STORABLE_freeze" behaves as normal for "STORABLE_attach"
           classes.

           Because "STORABLE_attach" is passed the class (rather than an object), it also returns the object directly,
           rather than modifying the passed object.

           Returned value: object of type "class"

       Predicates

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

       "Storable::last_op_in_netorder"
           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.

       "Storable::is_storing"
           Returns true if within a store operation (via STORABLE_freeze hook).

       "Storable::is_retrieving"
           Returns true if within a retrieve operation (via STORABLE_thaw hook).

       Recursion

       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 deserializing things, so why not use it to handle the
       serialization string?

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

       ?   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.

       ?   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 preserved.

       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 mem-
       ory and thawing the result.  It is aimed to replace Storable's dclone() some day.  However, it does not cur-
       rently 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 rec-
       ognizes 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 signature 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 invok-
       ing 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.

EXAMPLES
       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, 'mycolors') or die "Can't store %a in mycolors!\n";

               $colref = retrieve('mycolors');
               die "Unable to retrieve from mycolors!\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;

WARNING
       If you're using references as keys within your hash tables, you're bound to be disappointed 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 func-
       tions.

       Storing data canonically that contains large hashes can be significantly slower than storing the same data nor-
       mally, 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.

BUGS
       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 possible 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 operations 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 characters may be more
       than eight bits wide), any difference in the interpretation of character codes between a host and a target sys-
       tem is your problem.  In particular, if host and target use different code points to represent the characters
       used in the text representation 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 context 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 Configure to build your own perl from source, then it almost cer-
       tainly does not affect you, and you can stop reading now (unless you're curious). If you're using perl on Win-
       dows 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
       architectures 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 sys-
       tem 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 horribly wrong (possibly crashing) if it encountered a stored
       integer.  This is a design failure.

       Storable has now been changed to write out and read in a file header with information about the size of inte-
       gers.  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.

CREDITS
       Thank you to (in chronological order):

               Jarkko Hietaniemi <jhiATiki.fi>
               Ulrich Pfeifer <pfeiferATcharly.de>
               Benjamin A. Holzman <bahATecnvantage.com>
               Andrew Ford <A.FordATford-mason.uk>
               Gisle Aas <gisleATaas.no>
               Jeff Gresham <gresham_jeffreyATjpmorgan.com>
               Murray Nesbitt <murrayATactivestate.com>
               Marc Lehmann <pcgATopengroup.org>
               Justin Banks <justinbATwamnet.com>
               Jarkko Hietaniemi <jhiATiki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
               Salvador Ortiz Garcia <sogATmsg.mx>
               Dominic Dunlop <domoATcomputer.org>
               Erik Haugan <erikATsolbors.no>

       for their bug reports, suggestions and contributions.

       Benjamin Holzman contributed the tied variable support, Andrew Ford contributed the canonical 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 incom-
       patibility 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.

AUTHOR
       Storable was written by Raphael Manfredi <Raphael_ManfrediATpobox.com> Maintenance is now done by the
       perl5-porters <perl5-portersATperl.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.

SEE ALSO
       Clone.



perl v5.8.8                       2001-09-21                       Storable(3)