Storable
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" excep-
tion.
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 refer-
ences 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 stringi-
fied 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 serial-
izes it in effect). Later on, and maybe somewhere else, you can thaw
the Perl scalar out and recreate the original complex structure in mem-
ory.
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 actu-
ally 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() rou-
tine. 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 lock-
ing 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 com-
pressed 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 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 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 dese-
rializes 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.
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 set-
ting $Storable::accept_future_minor to some "FALSE" value.
All these variables have no effect on a newer Perl which supports the
relevant feature.
ERROR REPORTING
Storable uses the "exception" paradigm, in that it does not try to
workaround failures: if something bad happens, an exception is gener-
ated from the caller's perspective (see Carp and "croak()"). Use eval
{} to trap those exceptions.
When Storable croaks, it tries to report the error via the "logcroak()"
routine from the "Log::Agent" package, if it is available.
Normal errors are reported by having store() or retrieve() return
"undef". Such errors are usually I/O errors (or truncated stream
errors at retrieval).
WIZARDS ONLY
Hooks
Any class may define hooks that will be called during the serialization
and deserialization process on objects that are instances of that
class. Those hooks can redefine the way serialization is performed
(and therefore, how the symmetrical deserialization should be con-
ducted).
Since we said earlier:
dclone(.) = thaw(freeze(.))
everything we say about hooks should also hold for deep cloning. How-
ever, hooks get to know whether the operation is a mere serialization,
or a cloning.
Therefore, when serializing hooks are involved,
dclone(.) <> thaw(freeze(.))
Well, you could keep them in sync, but there's no guarantee it will
always hold on classes somebody else wrote. Besides, there is little
to gain in doing so: a serializing hook could keep only one attribute
of an object, which is probably not what should happen during a deep
cloning of that same object.
Here is the hooking interface:
"STORABLE_freeze" obj, cloning
The serializing hook, called on the object during serialization.
It can be inherited, or defined in the class itself, like any other
method.
Arguments: obj is the object to serialize, cloning is a flag indi-
cating whether we're in a dclone() or a regular serialization via
store() or freeze().
Returned value: A LIST "($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 dynam-
ically (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
Predicates are not exportable. They must be called by explicitly pre-
fixing 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 opera-
tion. 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 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.
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.
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 compart-
ment:
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 functions.
Storing data canonically that contains large hashes can be signifi-
cantly 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.
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 ver-
sion of a scalar exists, it is the form stored; therefore, if you hap-
pen 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 dif-
ference 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 rep-
resentation 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 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 writ-
ten 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 inter-
preter 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 configu-
ration system generated the C configuration files on non-Windows plat-
forms, 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 inte-
gers (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 run-
ning 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 <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 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.
AUTHOR
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.
SEE ALSO
Clone.
perl v5.8.6 2001-09-21 Storable(3)
