ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)NAMEStorable - 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 inter-
nal error, a true value otherwise. Serious errors are pro-
pagated 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
perl v5.8.8 2005-02-05 1
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
"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 shar-
ing 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 refer-
ence 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 struc-
ture in memory.
perl v5.8.8 2005-02-05 2
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 thaw-
ing 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 manipulat-
ing perl internals, to sacrifice encapsulation for the bene-
fit 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, Stor-
able will store hashes with the elements sorted by their
key. This allows you to compare data structures by compar-
ing their frozen representations (or even the compressed
frozen representations), which can be useful for creating
lookup tables for complicated queries.
perl v5.8.8 2005-02-05 3
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
Canonical order does not imply network order; those are two
orthogonal settings.
CODE REFERENCES
Since Storable version 2.05, CODE references may be serial-
ized with the help of B::Deparse. To enable this feature,
set $Storable::Deparse to a true value. To enable deserial-
izazion, $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 refer-
ences.
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 fol-
lows:
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 serial-
izes 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 Stor-
able 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 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
perl v5.8.8 2005-02-05 4
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 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 REPORTINGStorable 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 trun-
cated 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 sym-
metrical deserialization should be conducted).
perl v5.8.8 2005-02-05 5
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 seri-
alization. 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, $ref2, etc... are extra
references that you wish to let the Storable engine
serialize.
At deserialization time, you will be given back the same
LIST, but all the extra references will be pointing into
the deserialized structure.
The first time the hook is hit in a serialization flow,
you may have it return an empty list. That will signal
the Storable engine to further discard that hook for
this class and to therefore revert to the default seri-
alization of the underlying Perl data. The hook will
again be normally processed in the next serialization.
Unless you know better, serializing hook should always
say:
perl v5.8.8 2005-02-05 6
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
sub STORABLE_freeze {
my ($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
perl v5.8.8 2005-02-05 7
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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, clon-
ing 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 "pro-
perty" of whatever is being serialized, no references
underneath the object should be included in the serial-
ized 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).
perl v5.8.8 2005-02-05 8
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 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 per-
formed.
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
perl v5.8.8 2005-02-05 9
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
bytes). For this to work, a certain file called magic needs
to taught about the signature of the data. Where that con-
figuration 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 recognis-
ing 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 Stor-
able:
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;
perl v5.8.8 2005-02-05 10
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)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.
BUGS
You can't store GLOB, FORMLINE, etc.... If you can define
semantics for those operations, feel free to enhance Stor-
able 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
perl v5.8.8 2005-02-05 11
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 Stor-
able 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 Stor-
able (when not writing in network order), and will refuse to
load files written by a Storable not on the same (or compa-
tible) 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 incompati-
ble. 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. How-
ever, due to the way the Perl configuration system generated
the C configuration files on non-Windows platforms, and the
perl v5.8.8 2005-02-05 12
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 stor-
ing some data differently in the file. Hence Storable run-
ning 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 compa-
tible. 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 Stor-
able 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>
perl v5.8.8 2005-02-05 13
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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.
AUTHORStorable 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 com-
plaints, 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 2005-02-05 14
