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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 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.
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.
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
$Storable::accept_future_minor to some "FALSE" value.
All these variables have no effect on a newer Perl which supports the
relevant feature.
ERROR REPORTING
Storable uses the "exception" paradigm, in that it does not try to
workaround failures: if something bad happens, an exception is generated
from the caller's perspective (see Carp and "croak()"). Use eval {} to
trap those exceptions.
When Storable croaks, it tries to report the error via the "logcroak()"
routine from the "Log::Agent" package, if it is available.
Normal errors are reported by having store() or retrieve() return "undef".
Such errors are usually I/O errors (or truncated stream errors at
retrieval).
WIZARDS ONLY
Hooks
Any class may define hooks that will be called during the serialization and
deserialization process on objects that are instances of that class. Those
hooks can redefine the way serialization is performed (and therefore, how
the symmetrical deserialization should be conducted).
Since we said earlier:
dclone(.) = thaw(freeze(.))
everything we say about hooks should also hold for deep cloning. However,
hooks get to know whether the operation is a mere serialization, or a
cloning.
Therefore, when serializing hooks are involved,
dclone(.) <> thaw(freeze(.))
Well, you could keep them in sync, but there's no guarantee it will always
hold on classes somebody else wrote. Besides, there is little to gain in
doing so: a serializing hook could keep only one attribute of an object,
which is probably not what should happen during a deep cloning of that same
object.
Here is the hooking interface:
"STORABLE_freeze" obj, cloning
The serializing hook, called on the object during serialization. It
can be inherited, or defined in the class itself, like any other
method.
Arguments: obj is the object to serialize, cloning is a flag indicating
whether we're in a dclone() or a regular serialization via store() or
freeze().
Returned value: A LIST "($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.
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 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, '/tmp/colors') or die "Can't store %a in /tmp/colors!\n";
$colref = retrieve('/tmp/colors');
die "Unable to retrieve from /tmp/colors!\n" unless defined $colref;
printf "Blue is still %lf\n", $colref->{'Blue'};
$colref2 = dclone(\%color);
$str = freeze(\%color);
printf "Serialization of %%color is %d bytes long.\n", length($str);
$colref3 = thaw($str);
which prints (on my machine):
Blue is still 0.100000
Serialization of %color is 102 bytes long.
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, CODE, 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 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 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 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 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.
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