Storable(3) - phpMan

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

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 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
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 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).

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 process 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-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
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 feature, set $Storable::Deparse to a true value. To enable
deserialization, $Storable::Eval should be set to a true value. Be aware that
deserialization 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 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 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
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 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::downgrade_restricted to a "FALSE" value. To restore the default set it
back to some "TRUE" value.

The cperl PERL_PERTURB_KEYS_TOP hash strategy has a known problem with restricted
hashes.

huge objects
On 64bit systems some data structures may exceed the 2G (i.e. I32_MAX) limit. On 32bit
systems also strings between I32 and U32 (2G-4G). Since Storable 3.00 (not in perl5
core) we are able to store and retrieve these objects, even if perl5 itself is not
able to handle them. These are strings longer then 4G, arrays with more then 2G
elements and hashes with more then 2G elements. cperl forbids hashes with more than 2G
elements, but this fail in cperl then. perl5 itself at least until 5.26 allows it, but
cannot iterate over them. Note that creating those objects might cause out of memory
exceptions by the operating system before perl has a chance to abort.

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
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 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).

When Storable throws the "Max. recursion depth with nested structures exceeded" error we
are already out of stack space. Unfortunately on some earlier perl versions cleaning up a
recursive data structure recurses into the free calls, which will lead to stack overflows
in the cleanup. This data structure is not properly cleaned up then, it will only be
destroyed during global destruction.

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 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
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 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.

"STORABLE_attach" class, cloning, serialized
While "STORABLE_freeze" and "STORABLE_thaw" are useful for classes where each instance
is independent, 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" --> "STORABLE_thaw", you implement "STORABLE_freeze" -->
"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 cannot return any
references, and "Storable" will throw an error if "STORABLE_freeze" tries to return
references.

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:

o From Storable 3.05 to 3.13 we probed for the stack recursion limit for references,
arrays and hashes to a maximal depth of ~1200-35000, otherwise we might fall into a
stack-overflow. On JSON::XS this limit is 512 btw. With references not immediately
referencing each other there's no such limit yet, so you might fall into such a stack-
overflow segfault.

This probing and the checks we performed have some limitations:

o the stack size at build time might be different at run time, eg. the stack size
may have been modified with ulimit(1). If it's larger at run time Storable may
fail the freeze() or thaw() unnecessarily. If it's larger at build time Storable
may segmentation fault when processing a deep structure at run time.

o the stack size might be different in a thread.

o array and hash recursion limits are checked separately against the same recursion
depth, a frozen structure with a large sequence of nested arrays within many
nested hashes may exhaust the processor stack without triggering Storable's
recursion protection.

So these now have simple defaults rather than probing at build-time.

You can control the maximum array and hash recursion depths by modifying
$Storable::recursion_limit and $Storable::recursion_limit_hash respectively. Either
can be set to "-1" to prevent any depth checks, though this isn't recommended.

If you want to test what the limits are, the stacksize tool is included in the
"Storable" distribution.

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
preserved.

o The maximal stack recursion limit for your system is returned by "stack_depth()" and
"stack_depth_hash()". The hash limit is usually half the size of the array and ref
limit, as the Perl hash API is not optimal.

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
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 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.

You can also use the following functions to extract the file header information from
Storable images:

$info = Storable::file_magic( $filename )
If the given file is a Storable image return a hash describing it. If the file is
readable, but not a Storable image return "undef". If the file does not exist or is
unreadable then croak.

The hash returned has the following elements:

"version"
This returns the file format version. It is a string like "2.7".

Note that this version number is not the same as the version number of the
Storable module itself. For instance Storable v0.7 create files in format v2.0
and Storable v2.15 create files in format v2.7. The file format version number
only increment when additional features that would confuse older versions of the
module are added.

Files older than v2.0 will have the one of the version numbers "-1", "0" or "1".
No minor number was used at that time.

"version_nv"
This returns the file format version as number. It is a string like "2.007".
This value is suitable for numeric comparisons.

The constant function "Storable::BIN_VERSION_NV" returns a comparable number that
represents the highest file version number that this version of Storable fully
supports (but see discussion of $Storable::accept_future_minor above). The
constant "Storable::BIN_WRITE_VERSION_NV" function returns what file version is
written and might be less than "Storable::BIN_VERSION_NV" in some configurations.

"major", "minor"
This also returns the file format version. If the version is "2.7" then major
would be 2 and minor would be 7. The minor element is missing for when major is
less than 2.

"hdrsize"
The is the number of bytes that the Storable header occupies.

"netorder"
This is TRUE if the image store data in network order. This means that it was
created with nstore() or similar.

"byteorder"
This is only present when "netorder" is FALSE. It is the $Config{byteorder}
string of the perl that created this image. It is a string like "1234" (32 bit
little endian) or "87654321" (64 bit big endian). This must match the current
perl for the image to be readable by Storable.

"intsize", "longsize", "ptrsize", "nvsize"
These are only present when "netorder" is FALSE. These are the sizes of various C
datatypes of the perl that created this image. These must match the current perl
for the image to be readable by Storable.

The "nvsize" element is only present for file format v2.2 and higher.

"file"
The name of the file.

$info = Storable::read_magic( $buffer )
$info = Storable::read_magic( $buffer, $must_be_file )
The $buffer should be a Storable image or the first few bytes of it. If $buffer
starts with a Storable header, then a hash describing the image is returned, otherwise
"undef" is returned.

The hash has the same structure as the one returned by Storable::file_magic(). The
"file" element is true if the image is a file image.

If the $must_be_file argument is provided and is TRUE, then return "undef" unless the
image looks like it belongs to a file dump.

The maximum size of a Storable header is currently 21 bytes. If the provided $buffer
is only the first part of a Storable image it should at least be this long to ensure
that read_magic() will recognize it as such.

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;

SECURITY WARNING
Do not accept Storable documents from untrusted sources!

Some features of Storable can lead to security vulnerabilities if you accept Storable
documents from untrusted sources with the default flags. Most obviously, the optional (off
by default) CODE reference serialization feature allows transfer of code to the
deserializing process. Furthermore, any serialized object will cause Storable to helpfully
load the module corresponding to the class of the object in the deserializing module. For
manipulated module names, this can load almost arbitrary code. Finally, the deserialized
object's destructors will be invoked when the objects get destroyed in the deserializing
process. Maliciously crafted Storable documents may put such objects in the value of a
hash key that is overridden by another key/value pair in the same hash, thus causing
immediate destructor execution.

To disable blessing objects while thawing/retrieving remove the flag "BLESS_OK" = 2 from
$Storable::flags or set the 2nd argument for thaw/retrieve to 0.

To disable tieing data while thawing/retrieving remove the flag "TIE_OK" = 4 from
$Storable::flags or set the 2nd argument for thaw/retrieve to 0.

With the default setting of $Storable::flags = 6, creating or destroying random objects,
even renamed objects can be controlled by an attacker. See CVE-2015-1592 and its
metasploit module.

If your application requires accepting data from untrusted sources, you are best off with
a less powerful and more-likely safe serialization format and implementation. If your data
is sufficiently simple, Cpanel::JSON::XS, Data::MessagePack or Sereal are the best choices
and offer maximum interoperability, but note that Sereal is unsafe by default.

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 functions.

Storing data canonically that contains large hashes can be significantly slower than
storing 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.

REGULAR EXPRESSIONS
Storable now has experimental support for storing regular expressions, but there are
significant limitations:

o perl 5.8 or later is required.

o regular expressions with code blocks, ie "/(?{ ... })/" or "/(??{ ... })/" will throw
an exception when thawed.

o regular expression syntax and flags have changed over the history of perl, so a
regular expression that you freeze in one version of perl may fail to thaw or behave
differently in another version of perl.

o depending on the version of perl, regular expressions can change in behaviour
depending on the context, but later perls will bake that behaviour into the regexp.

Storable will throw an exception if a frozen regular expression cannot be thawed.

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 converted
to 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 system 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 converted 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 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
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 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
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 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 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 <jhi AT iki.fi>
Ulrich Pfeifer <pfeifer AT charly.de>
Benjamin A. Holzman <bholzman AT earthlink.net>
Andrew Ford <A.Ford AT ford-mason.uk>
Gisle Aas <gisle AT aas.no>
Jeff Gresham <gresham_jeffrey AT jpmorgan.com>
Murray Nesbitt <murray AT activestate.com>
Marc Lehmann <pcg AT opengroup.org>
Justin Banks <justinb AT wamnet.com>
Jarkko Hietaniemi <jhi AT iki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
Salvador Ortiz Garcia <sog AT msg.mx>
Dominic Dunlop <domo AT computer.org>
Erik Haugan <erik AT solbors.no>
Benjamin A. Holzman <ben.holzman AT grantstreet.com>
Reini Urban <rurban AT cpan.org>
Todd Rinaldo <toddr AT cpanel.net>
Aaron Crane <arc AT cpan.org>

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 incompatibility for the
Storable image starting at version 0.6--older images are, of course, still properly
understood). Murray Nesbitt made Storable thread-safe. Marc Lehmann added overloading
and references to tied items support. Benjamin Holzman added a performance improvement
for overloaded classes; thanks to Grant Street Group for footing the bill. Reini Urban
took over maintenance from p5p, and added security fixes and huge object support.

AUTHOR
Storable was written by Raphael Manfredi <Raphael_Manfredi AT pobox.com> Maintenance is now
done by cperl <http://perl11.org/cperl>

Please e-mail us with problems, bug fixes, comments and complaints, although if you have
compliments 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.