| |
man : Storable(3p)
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
NAME
Storable - persistence for Perl data structures
SYNOPSIS
use Storable;
store \%table, 'file';
$hashref = retrieve('file');
use Storable qw(nstore store_fd nstore_fd freeze thaw dclone);
# Network order
nstore \%table, 'file';
$hashref = retrieve('file'); # There is NO nretrieve()
# Storing to and retrieving from an already opened file
store_fd \@array, \*STDOUT;
nstore_fd \%table, \*STDOUT;
$aryref = fd_retrieve(\*SOCKET);
$hashref = fd_retrieve(\*SOCKET);
# Serializing to memory
$serialized = freeze \%table;
%table_clone = %{ thaw($serialized) };
# Deep (recursive) cloning
$cloneref = dclone($ref);
# Advisory locking
use Storable qw(lock_store lock_nstore lock_retrieve)
lock_store \%table, 'file';
lock_nstore \%table, 'file';
$hashref = lock_retrieve('file');
DESCRIPTION
The Storable package brings persistence to your Perl data
structures containing SCALAR, ARRAY, HASH or REF objects,
i.e. anything that can be conveniently stored to disk and
retrieved at a later time.
It can be used in the regular procedural way by calling
"store" with a reference to the object to be stored, along
with the file name where the image should be written.
The routine returns "undef" for I/O problems or other
internal error, a true value otherwise. Serious errors are
propagated as a "die" exception.
To retrieve data stored to disk, use "retrieve" with a
file name. The objects stored into that file are recre-
ated into memory for you, and a reference to the root
object is returned. In case an I/O error occurs while
reading, "undef" is returned instead. Other serious errors
are propagated via "die".
perl v5.8.8 2002-10-27 1
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 origi-
nally shared.
At the cost of a slight header overhead, you may store to
an already opened file descriptor using the "store_fd"
routine, and retrieve from a file via "fd_retrieve". Those
names aren't imported by default, so you will have to do
that explicitly if you need those routines. The file
descriptor you supply must be already opened, for read if
you're going to retrieve and for write if you wish to
store.
store_fd(\%table, *STDOUT) || die "can't store to stdout\n";
$hashref = fd_retrieve(*STDIN);
You can also store data in network order to allow easy
sharing across multiple platforms, or when storing on a
socket known to be remotely connected. The routines to
call have an initial "n" prefix for network, as in
"nstore" and "nstore_fd". At retrieval time, your data
will be correctly restored so you don't have to know
whether you're restoring from native or network ordered
data. Double values are stored stringified to ensure
portability as well, at the slight risk of loosing some
precision in the last decimals.
When using "fd_retrieve", objects are retrieved in
sequence, one object (i.e. one recursive tree) per associ-
ated "store_fd".
If you're more from the object-oriented camp, you can
inherit from Storable and directly store your objects by
invoking "store" as a method. The fact that the root of
the to-be-stored tree is a blessed reference (i.e. an
object) is special-cased so that the retrieve does not
provide a reference to that object but rather the blessed
object reference itself. (Otherwise, you'd get a reference
to that blessed object).
MEMORY STORE
The Storable engine can also store data into a Perl scalar
instead, to later retrieve them. This is mainly used to
freeze a complex structure in some safe compact memory
place (where it can possibly be sent to another 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"
perl v5.8.8 2002-10-27 2
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 struc-
ture:
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 imme-
diately 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 manipu-
lating 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.
perl v5.8.8 2002-10-27 3
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
CODE REFERENCES
Since Storable version 2.05, CODE references may be seri-
alized with the help of B::Deparse. To enable this fea-
ture, set $Storable::Deparse to a true value. To enable
deserializazion, $Storable::Eval should be set to a true
value. Be aware that deserialization is done through
"eval", which is dangerous if the Storable file contains
malicious data. You can set $Storable::Eval to a subrou-
tine reference which would be used instead of "eval". See
below for an example using a Safe compartment for deseri-
alization 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 can-
not deserialize. However, the defaults can be changed as
follows:
utf8 data
Perl 5.6 added support for Unicode characters with
code points > 255, and Perl 5.8 has full support for
Unicode characters in hash keys. Perl internally
encodes strings with these characters using utf8, and
Storable serializes them as utf8. By default, if an
older version of Perl encounters a utf8 value it can-
not represent, it will "croak()". To change this
behaviour so that Storable deserializes utf8 encoded
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 val-
ues 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
perl v5.8.8 2002-10-27 4
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
if they encountered a file with a higher internal ver-
sion number than the reading Storable knew about.
Internal version numbers are increased each time new
data types (such as restricted hashes) are added to
the vocabulary of the file format. This meant that a
newer Storable module had no way of writing a file
readable by an older Storable, even if the writer
didn't store newer data types.
This version of Storable will defer croaking until it
encounters a data type in the file that it does not
recognize. This means that it will continue to read
files generated by newer Storable modules which are
careful in what they write out, making it easier to
upgrade Storable modules in a mixed environment.
The old behaviour of immediate croaking can be re-
instated by setting $Storable::accept_future_minor to
some "FALSE" value.
All these variables have no effect on a newer Perl which
supports the relevant feature.
ERROR REPORTING
Storable uses the "exception" paradigm, in that it does
not try to workaround failures: if something bad 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.
perl v5.8.8 2002-10-27 5
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
Therefore, when serializing hooks are involved,
dclone(.) <> thaw(freeze(.))
Well, you could keep them in sync, but there's no guaran-
tee it will always hold on classes somebody else wrote.
Besides, there is little to gain in doing so: a serializ-
ing hook could keep only one attribute of an object, which
is probably not what should happen during a deep cloning
of that same object.
Here is the hooking interface:
"STORABLE_freeze" obj, cloning
The serializing hook, called on the object during
serialization. It can be inherited, or defined in the
class itself, like any other method.
Arguments: obj is the object to serialize, cloning is
a flag indicating whether we're in a dclone() or a
regular serialization via store() or freeze().
Returned value: A LIST "($serialized, $ref1, $ref2,
...)" where $serialized is the serialized form to be
used, and the optional $ref1, $ref2, etc... are extra
references that you wish to let the Storable engine
serialize.
At deserialization time, you will be given back the
same LIST, but all the extra references will be point-
ing into the deserialized structure.
The first time the hook is hit in a serialization
flow, you may have it return an empty list. That will
signal the Storable engine to further discard that
hook for this class and to therefore revert to the
default serialization of the underlying Perl data.
The hook will again be normally processed in the next
serialization.
Unless you know better, serializing hook should always
say:
sub STORABLE_freeze {
my ($self, $cloning) = @_;
return if $cloning; # Regular default serialization
....
}
in order to keep reasonable dclone() semantics.
"STORABLE_thaw" obj, cloning, serialized, ...
The deserializing hook called on the object during
deserialization. But wait: if we're deserializing,
perl v5.8.8 2002-10-27 6
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
there's no object yet... right?
Wrong: the Storable engine creates an empty one for
you. If you know Eiffel, you can view "STORABLE_thaw"
as an alternate creation routine.
This means the hook can be inherited like any other
method, and that obj is your blessed reference for
this particular instance.
The other arguments should look familiar if you know
"STORABLE_freeze": cloning is true when we're part of
a deep clone operation, serialized is the serialized
string you returned to the engine in
"STORABLE_freeze", and there may be an optional list
of references, in the same order you gave them at
serialization time, pointing to the deserialized
objects (which have been processed courtesy of the
Storable engine).
When the Storable engine does not find any
"STORABLE_thaw" hook routine, it tries to load the
class by requiring the package dynamically (using the
blessed package name), and then re-attempts the
lookup. If at that time the hook cannot be located,
the engine croaks. Note that this mechanism will fail
if you define several classes in the same file, but
perlmod warned you.
It is up to you to use this information to populate
obj the way you want.
Returned value: none.
"STORABLE_attach" class, cloning, serialized
While "STORABLE_freeze" and "STORABLE_thaw" are useful
for classes where each instance is independant, this
mechanism has difficulty (or is incompatible) with
objects that exist as common process-level or system-
level resources, such as singleton objects, database
pools, caches or memoized objects.
The alternative "STORABLE_attach" method provides a
solution for these shared objects. Instead of
"STORABLE_freeze" --> "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
perl v5.8.8 2002-10-27 7
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
owned by the entire process/system, and not the "prop-
erty" of whatever is being serialized, no references
underneath the object should be included in the seri-
alized 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 You can create endless loops if the things you serial-
ize 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:
perl v5.8.8 2002-10-27 8
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 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 implemen-
tation 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:
perl v5.8.8 2002-10-27 9
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
use Storable qw(store retrieve freeze thaw dclone);
%color = ('Blue' => 0.1, 'Red' => 0.8, 'Black' => 0, 'White' => 1);
store(\%color, 'mycolors') or die "Can't store %a in mycolors!\n";
$colref = retrieve('mycolors');
die "Unable to retrieve from mycolors!\n" unless defined $colref;
printf "Blue is still %lf\n", $colref->{'Blue'};
$colref2 = dclone(\%color);
$str = freeze(\%color);
printf "Serialization of %%color is %d bytes long.\n", length($str);
$colref3 = thaw($str);
which prints (on my machine):
Blue is still 0.100000
Serialization of %color is 102 bytes long.
Serialization of CODE references and deserialization in a
safe compartment:
use Storable qw(freeze thaw);
use Safe;
use strict;
my $safe = new Safe;
# because of opcodes used in "use strict":
$safe->permit(qw(:default require));
local $Storable::Deparse = 1;
local $Storable::Eval = sub { $safe->reval($_[0]) };
my $serialized = freeze(sub { 42 });
my $code = thaw($serialized);
$code->() == 42;
WARNING
If you're using references as keys within your hash
tables, you're bound to be disappointed when retrieving
your data. Indeed, Perl stringifies references used as
hash table keys. If you later wish to access the items via
another reference stringification (i.e. using the same
reference that was used for the key originally to record
the value into the hash table), it will work because both
references stringify to the same string.
It won't work across a sequence of "store" and "retrieve"
operations, however, because the addresses in the
retrieved objects, which are part of the stringified ref-
erences, will probably differ from the original addresses.
The topology of your structure is preserved, but not hid-
den semantics like those.
On platforms where it matters, be sure to call "binmode()"
perl v5.8.8 2002-10-27 10
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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
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 num-
bers. When the string version of a scalar exists, it is
the form stored; therefore, if you happen to use your num-
bers 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 facil-
ity 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
perl v5.8.8 2002-10-27 11
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 precom-
piled 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 lan-
guage types, and so files written on different architec-
tures 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. How-
ever, due to the way the Perl configuration system gener-
ated 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 fail-
ure.
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
perl v5.8.8 2002-10-27 12
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
Storable 1.x running on perl 5.6.0 or 5.6.1 configured
with 64 bit integers on Unix or Linux then by default this
Storable will refuse to read it, giving the error Byte
order is not compatible. If you have such data then you
you should set $Storable::interwork_56_64bit to a true
value to make this Storable read and write files with the
old header. You should also migrate your data, or any
older perl you are communicating with, to this current
version of Storable.
If you don't have data written with specific configuration
of perl described above, then you do not and should not do
anything. Don't set the flag - not only will Storable on
an identically configured perl refuse to load them, but
Storable a differently configured perl will load them
believing them to be correct for it, and then may well
fail or crash part way through reading them.
CREDITS
Thank you to (in chronological order):
Jarkko Hietaniemi <jhiATiki.fi>
Ulrich Pfeifer <pfeiferATcharly.de>
Benjamin A. Holzman <bahATecnvantage.com>
Andrew Ford <A.FordATford-mason.uk>
Gisle Aas <gisleATaas.no>
Jeff Gresham <gresham_jeffreyATjpmorgan.com>
Murray Nesbitt <murrayATactivestate.com>
Marc Lehmann <pcgATopengroup.org>
Justin Banks <justinbATwamnet.com>
Jarkko Hietaniemi <jhiATiki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
Salvador Ortiz Garcia <sogATmsg.mx>
Dominic Dunlop <domoATcomputer.org>
Erik Haugan <erikATsolbors.no>
for their bug reports, suggestions and contributions.
Benjamin Holzman contributed the tied variable support,
Andrew Ford contributed the canonical order for hashes,
and Gisle Aas fixed a few misunderstandings of mine
regarding the perl internals, and optimized the emission
of "tags" in the output streams by simply counting the
objects instead of tagging them (leading to a binary
incompatibility for the Storable image starting at version
0.6--older images are, of course, still properly under-
stood). Murray Nesbitt made Storable thread-safe. Marc
Lehmann added overloading and references to tied items
support.
AUTHOR
Storable was written by Raphael Manfredi <Raphael_Man-
frediATpobox.com> Maintenance is now done by the
perl5-porters <perl5-portersATperl.org>
perl v5.8.8 2002-10-27 13
ext::Storable::StPerllProgrammers Refeext::Storable::Storable(3p)
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 mes-
sage will be delayed while he forwards it to us.
SEE ALSO
Clone.
perl v5.8.8 2002-10-27 14
|
