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Re: Change 16457: [PATCH] Re: [PATCH] Storable (Re: perl@16433)
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596596d5 1;# $Id: Storable.pm,v 1.0.1.13 2001/12/01 13:34:49 ram Exp $
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2;#
3;# Copyright (c) 1995-2000, Raphael Manfredi
4;#
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5;# You may redistribute only under the same terms as Perl 5, as specified
6;# in the README file that comes with the distribution.
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7;#
8;# $Log: Storable.pm,v $
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9;# Revision 1.0.1.13 2001/12/01 13:34:49 ram
10;# patch14: avoid requiring Fcntl upfront, useful to embedded runtimes
11;# patch14: store_fd() will now correctly autoflush file if needed
12;#
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13;# Revision 1.0.1.12 2001/08/28 21:51:51 ram
14;# patch13: fixed truncation race with lock_retrieve() in lock_store()
15;#
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16;# Revision 1.0.1.11 2001/07/01 11:22:14 ram
17;# patch12: systematically use "=over 4" for POD linters
18;# patch12: updated version number
19;#
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20;# Revision 1.0.1.10 2001/03/15 00:20:25 ram
21;# patch11: updated version number
22;#
23;# Revision 1.0.1.9 2001/02/17 12:37:32 ram
24;# patch10: forgot to increase version number at previous patch
25;#
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26;# Revision 1.0.1.8 2001/02/17 12:24:37 ram
27;# patch8: fixed incorrect error message
28;#
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29;# Revision 1.0.1.7 2001/01/03 09:39:02 ram
30;# patch7: added CAN_FLOCK to determine whether we can flock() or not
31;#
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32;# Revision 1.0.1.6 2000/11/05 17:20:25 ram
33;# patch6: increased version number
34;#
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35;# Revision 1.0.1.5 2000/10/26 17:10:18 ram
36;# patch5: documented that store() and retrieve() can return undef
37;# patch5: added paragraph explaining the auto require for thaw hooks
38;#
39;# Revision 1.0.1.4 2000/10/23 18:02:57 ram
40;# patch4: protected calls to flock() for dos platform
41;# patch4: added logcarp emulation if they don't have Log::Agent
42;#
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43;# Revision 1.0.1.3 2000/09/29 19:49:01 ram
44;# patch3: updated version number
45;#
46;# Revision 1.0.1.2 2000/09/28 21:42:51 ram
47;# patch2: added lock_store lock_nstore lock_retrieve
48;#
49;# Revision 1.0.1.1 2000/09/17 16:46:21 ram
50;# patch1: documented that doubles are stringified by nstore()
51;# patch1: added Salvador Ortiz Garcia in CREDITS section
52;#
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53;# Revision 1.0 2000/09/01 19:40:41 ram
54;# Baseline for first official release.
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55;#
56
57require DynaLoader;
58require Exporter;
59package Storable; @ISA = qw(Exporter DynaLoader);
60
61@EXPORT = qw(store retrieve);
62@EXPORT_OK = qw(
9e21b3d0 63 nstore store_fd nstore_fd fd_retrieve
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64 freeze nfreeze thaw
65 dclone
9e21b3d0 66 retrieve_fd
dd19458b 67 lock_store lock_nstore lock_retrieve
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68);
69
70use AutoLoader;
71use vars qw($forgive_me $VERSION);
72
caa9d880 73$VERSION = '2.0';
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74*AUTOLOAD = \&AutoLoader::AUTOLOAD; # Grrr...
75
76#
77# Use of Log::Agent is optional
78#
79
80eval "use Log::Agent";
81
530b72ba 82require Carp;
7a6a85bf 83
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84#
85# They might miss :flock in Fcntl
86#
87
88BEGIN {
596596d5 89 if (eval { require Fcntl; 1 } && exists $Fcntl::EXPORT_TAGS{'flock'}) {
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90 Fcntl->import(':flock');
91 } else {
92 eval q{
93 sub LOCK_SH () {1}
94 sub LOCK_EX () {2}
95 };
96 }
97}
98
b8778c7c 99# Can't Autoload cleanly as this clashes 8.3 with &retrieve
9e21b3d0 100sub retrieve_fd { &fd_retrieve } # Backward compatibility
cb3d9de5 101
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102# By default restricted hashes are downgraded on earlier perls.
103
104$Storable::downgrade_restricted = 1;
e8189732 105$Storable::accept_future_minor = 1;
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106bootstrap Storable;
1071;
108__END__
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109#
110# Use of Log::Agent is optional. If it hasn't imported these subs then
111# Autoloader will kindly supply our fallback implementation.
112#
113
114sub logcroak {
115 Carp::croak(@_);
116}
117
118sub logcarp {
119 Carp::carp(@_);
120}
b8778c7c 121
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122#
123# Determine whether locking is possible, but only when needed.
124#
125
530b72ba 126sub CAN_FLOCK; my $CAN_FLOCK; sub CAN_FLOCK {
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127 return $CAN_FLOCK if defined $CAN_FLOCK;
128 require Config; import Config;
129 return $CAN_FLOCK =
130 $Config{'d_flock'} ||
131 $Config{'d_fcntl_can_lock'} ||
132 $Config{'d_lockf'};
133}
134
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135sub show_file_magic {
136 print <<EOM;
137#
138# To recognize the data files of the Perl module Storable,
139# the following lines need to be added to the local magic(5) file,
140# usually either /usr/share/misc/magic or /etc/magic.
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141#
1420 string perl-store perl Storable(v0.6) data
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143>4 byte >0 (net-order %d)
144>>4 byte &01 (network-ordered)
145>>4 byte =3 (major 1)
146>>4 byte =2 (major 1)
147
0a0da639 1480 string pst0 perl Storable(v0.7) data
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149>4 byte >0
150>>4 byte &01 (network-ordered)
151>>4 byte =5 (major 2)
152>>4 byte =4 (major 2)
153>>5 byte >0 (minor %d)
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154EOM
155}
156
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157sub read_magic {
158 my $header = shift;
159 return unless defined $header and length $header > 11;
160 my $result;
161 if ($header =~ s/^perl-store//) {
162 die "Can't deal with version 0 headers";
163 } elsif ($header =~ s/^pst0//) {
164 $result->{file} = 1;
165 }
166 # Assume it's a string.
167 my ($major, $minor, $bytelen) = unpack "C3", $header;
168
169 my $net_order = $major & 1;
170 $major >>= 1;
171 @$result{qw(major minor netorder)} = ($major, $minor, $net_order);
172
173 return $result if $net_order;
174
175 # I assume that it is rare to find v1 files, so this is an intentionally
176 # inefficient way of doing it, to make the rest of the code constant.
177 if ($major < 2) {
178 delete $result->{minor};
179 $header = '.' . $header;
180 $bytelen = $minor;
181 }
182
183 @$result{qw(byteorder intsize longsize ptrsize)} =
184 unpack "x3 A$bytelen C3", $header;
185
186 if ($major >= 2 and $minor >= 2) {
187 $result->{nvsize} = unpack "x6 x$bytelen C", $header;
188 }
189 $result;
190}
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191
192#
193# store
194#
195# Store target object hierarchy, identified by a reference to its root.
196# The stored object tree may later be retrieved to memory via retrieve.
197# Returns undef if an I/O error occurred, in which case the file is
198# removed.
199#
200sub store {
dd19458b 201 return _store(\&pstore, @_, 0);
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202}
203
204#
205# nstore
206#
207# Same as store, but in network order.
208#
209sub nstore {
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210 return _store(\&net_pstore, @_, 0);
211}
212
213#
214# lock_store
215#
216# Same as store, but flock the file first (advisory locking).
217#
218sub lock_store {
219 return _store(\&pstore, @_, 1);
220}
221
222#
223# lock_nstore
224#
225# Same as nstore, but flock the file first (advisory locking).
226#
227sub lock_nstore {
228 return _store(\&net_pstore, @_, 1);
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229}
230
231# Internal store to file routine
232sub _store {
233 my $xsptr = shift;
234 my $self = shift;
dd19458b 235 my ($file, $use_locking) = @_;
7a6a85bf 236 logcroak "not a reference" unless ref($self);
b12202d0 237 logcroak "wrong argument number" unless @_ == 2; # No @foo in arglist
7a6a85bf 238 local *FILE;
dd19458b 239 if ($use_locking) {
6e0ac6f5 240 open(FILE, ">>$file") || logcroak "can't write into $file: $!";
862382c7 241 unless (&CAN_FLOCK) {
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242 logcarp "Storable::lock_store: fcntl/flock emulation broken on $^O";
243 return undef;
f567092b 244 }
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245 flock(FILE, LOCK_EX) ||
246 logcroak "can't get exclusive lock on $file: $!";
247 truncate FILE, 0;
248 # Unlocking will happen when FILE is closed
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249 } else {
250 open(FILE, ">$file") || logcroak "can't create $file: $!";
dd19458b 251 }
6e0ac6f5 252 binmode FILE; # Archaic systems...
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253 my $da = $@; # Don't mess if called from exception handler
254 my $ret;
255 # Call C routine nstore or pstore, depending on network order
256 eval { $ret = &$xsptr(*FILE, $self) };
257 close(FILE) or $ret = undef;
258 unlink($file) or warn "Can't unlink $file: $!\n" if $@ || !defined $ret;
259 logcroak $@ if $@ =~ s/\.?\n$/,/;
260 $@ = $da;
261 return $ret ? $ret : undef;
262}
263
264#
265# store_fd
266#
267# Same as store, but perform on an already opened file descriptor instead.
268# Returns undef if an I/O error occurred.
269#
270sub store_fd {
271 return _store_fd(\&pstore, @_);
272}
273
274#
275# nstore_fd
276#
277# Same as store_fd, but in network order.
278#
279sub nstore_fd {
280 my ($self, $file) = @_;
281 return _store_fd(\&net_pstore, @_);
282}
283
284# Internal store routine on opened file descriptor
285sub _store_fd {
286 my $xsptr = shift;
287 my $self = shift;
288 my ($file) = @_;
289 logcroak "not a reference" unless ref($self);
290 logcroak "too many arguments" unless @_ == 1; # No @foo in arglist
291 my $fd = fileno($file);
292 logcroak "not a valid file descriptor" unless defined $fd;
293 my $da = $@; # Don't mess if called from exception handler
294 my $ret;
295 # Call C routine nstore or pstore, depending on network order
296 eval { $ret = &$xsptr($file, $self) };
297 logcroak $@ if $@ =~ s/\.?\n$/,/;
596596d5 298 local $\; print $file ''; # Autoflush the file if wanted
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299 $@ = $da;
300 return $ret ? $ret : undef;
301}
302
303#
304# freeze
305#
306# Store oject and its hierarchy in memory and return a scalar
307# containing the result.
308#
309sub freeze {
310 _freeze(\&mstore, @_);
311}
312
313#
314# nfreeze
315#
316# Same as freeze but in network order.
317#
318sub nfreeze {
319 _freeze(\&net_mstore, @_);
320}
321
322# Internal freeze routine
323sub _freeze {
324 my $xsptr = shift;
325 my $self = shift;
326 logcroak "not a reference" unless ref($self);
327 logcroak "too many arguments" unless @_ == 0; # No @foo in arglist
328 my $da = $@; # Don't mess if called from exception handler
329 my $ret;
330 # Call C routine mstore or net_mstore, depending on network order
331 eval { $ret = &$xsptr($self) };
332 logcroak $@ if $@ =~ s/\.?\n$/,/;
333 $@ = $da;
334 return $ret ? $ret : undef;
335}
336
337#
338# retrieve
339#
340# Retrieve object hierarchy from disk, returning a reference to the root
341# object of that tree.
342#
343sub retrieve {
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344 _retrieve($_[0], 0);
345}
346
347#
348# lock_retrieve
349#
350# Same as retrieve, but with advisory locking.
351#
352sub lock_retrieve {
353 _retrieve($_[0], 1);
354}
355
356# Internal retrieve routine
357sub _retrieve {
358 my ($file, $use_locking) = @_;
7a6a85bf 359 local *FILE;
dd19458b 360 open(FILE, $file) || logcroak "can't open $file: $!";
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361 binmode FILE; # Archaic systems...
362 my $self;
363 my $da = $@; # Could be from exception handler
dd19458b 364 if ($use_locking) {
862382c7 365 unless (&CAN_FLOCK) {
8be2b38b 366 logcarp "Storable::lock_store: fcntl/flock emulation broken on $^O";
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367 return undef;
368 }
8be2b38b 369 flock(FILE, LOCK_SH) || logcroak "can't get shared lock on $file: $!";
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370 # Unlocking will happen when FILE is closed
371 }
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372 eval { $self = pretrieve(*FILE) }; # Call C routine
373 close(FILE);
374 logcroak $@ if $@ =~ s/\.?\n$/,/;
375 $@ = $da;
376 return $self;
377}
378
379#
9e21b3d0 380# fd_retrieve
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381#
382# Same as retrieve, but perform from an already opened file descriptor instead.
383#
9e21b3d0 384sub fd_retrieve {
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385 my ($file) = @_;
386 my $fd = fileno($file);
387 logcroak "not a valid file descriptor" unless defined $fd;
388 my $self;
389 my $da = $@; # Could be from exception handler
390 eval { $self = pretrieve($file) }; # Call C routine
391 logcroak $@ if $@ =~ s/\.?\n$/,/;
392 $@ = $da;
393 return $self;
394}
395
396#
397# thaw
398#
399# Recreate objects in memory from an existing frozen image created
400# by freeze. If the frozen image passed is undef, return undef.
401#
402sub thaw {
403 my ($frozen) = @_;
404 return undef unless defined $frozen;
405 my $self;
406 my $da = $@; # Could be from exception handler
407 eval { $self = mretrieve($frozen) }; # Call C routine
408 logcroak $@ if $@ =~ s/\.?\n$/,/;
409 $@ = $da;
410 return $self;
411}
412
413=head1 NAME
414
415Storable - persistency for perl data structures
416
417=head1 SYNOPSIS
418
419 use Storable;
420 store \%table, 'file';
421 $hashref = retrieve('file');
422
423 use Storable qw(nstore store_fd nstore_fd freeze thaw dclone);
424
425 # Network order
426 nstore \%table, 'file';
427 $hashref = retrieve('file'); # There is NO nretrieve()
428
429 # Storing to and retrieving from an already opened file
430 store_fd \@array, \*STDOUT;
431 nstore_fd \%table, \*STDOUT;
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432 $aryref = fd_retrieve(\*SOCKET);
433 $hashref = fd_retrieve(\*SOCKET);
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434
435 # Serializing to memory
436 $serialized = freeze \%table;
437 %table_clone = %{ thaw($serialized) };
438
439 # Deep (recursive) cloning
440 $cloneref = dclone($ref);
441
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442 # Advisory locking
443 use Storable qw(lock_store lock_nstore lock_retrieve)
444 lock_store \%table, 'file';
445 lock_nstore \%table, 'file';
446 $hashref = lock_retrieve('file');
447
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448=head1 DESCRIPTION
449
c261f00e 450The Storable package brings persistence to your perl data structures
7a6a85bf 451containing SCALAR, ARRAY, HASH or REF objects, i.e. anything that can be
c261f00e 452conveniently stored to disk and retrieved at a later time.
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453
454It can be used in the regular procedural way by calling C<store> with
455a reference to the object to be stored, along with the file name where
456the image should be written.
775ecd75 457
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458The routine returns C<undef> for I/O problems or other internal error,
459a true value otherwise. Serious errors are propagated as a C<die> exception.
460
461To retrieve data stored to disk, use C<retrieve> with a file name,
462and the objects stored into that file are recreated into memory for you,
463a I<reference> to the root object being returned. In case an I/O error
464occurs while reading, C<undef> is returned instead. Other serious
465errors are propagated via C<die>.
466
467Since storage is performed recursively, you might want to stuff references
468to objects that share a lot of common data into a single array or hash
469table, and then store that object. That way, when you retrieve back the
470whole thing, the objects will continue to share what they originally shared.
471
472At the cost of a slight header overhead, you may store to an already
473opened file descriptor using the C<store_fd> routine, and retrieve
9e21b3d0 474from a file via C<fd_retrieve>. Those names aren't imported by default,
c261f00e 475so you will have to do that explicitly if you need those routines.
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476The file descriptor you supply must be already opened, for read
477if you're going to retrieve and for write if you wish to store.
478
479 store_fd(\%table, *STDOUT) || die "can't store to stdout\n";
9e21b3d0 480 $hashref = fd_retrieve(*STDIN);
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481
482You can also store data in network order to allow easy sharing across
483multiple platforms, or when storing on a socket known to be remotely
484connected. The routines to call have an initial C<n> prefix for I<network>,
485as in C<nstore> and C<nstore_fd>. At retrieval time, your data will be
486correctly restored so you don't have to know whether you're restoring
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487from native or network ordered data. Double values are stored stringified
488to ensure portability as well, at the slight risk of loosing some precision
489in the last decimals.
7a6a85bf 490
9e21b3d0 491When using C<fd_retrieve>, objects are retrieved in sequence, one
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492object (i.e. one recursive tree) per associated C<store_fd>.
493
494If you're more from the object-oriented camp, you can inherit from
495Storable and directly store your objects by invoking C<store> as
496a method. The fact that the root of the to-be-stored tree is a
497blessed reference (i.e. an object) is special-cased so that the
498retrieve does not provide a reference to that object but rather the
499blessed object reference itself. (Otherwise, you'd get a reference
500to that blessed object).
501
502=head1 MEMORY STORE
503
504The Storable engine can also store data into a Perl scalar instead, to
505later retrieve them. This is mainly used to freeze a complex structure in
506some safe compact memory place (where it can possibly be sent to another
507process via some IPC, since freezing the structure also serializes it in
508effect). Later on, and maybe somewhere else, you can thaw the Perl scalar
509out and recreate the original complex structure in memory.
510
511Surprisingly, the routines to be called are named C<freeze> and C<thaw>.
512If you wish to send out the frozen scalar to another machine, use
513C<nfreeze> instead to get a portable image.
514
515Note that freezing an object structure and immediately thawing it
516actually achieves a deep cloning of that structure:
517
518 dclone(.) = thaw(freeze(.))
519
520Storable provides you with a C<dclone> interface which does not create
521that intermediary scalar but instead freezes the structure in some
c261f00e 522internal memory space and then immediately thaws it out.
7a6a85bf 523
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524=head1 ADVISORY LOCKING
525
526The C<lock_store> and C<lock_nstore> routine are equivalent to C<store>
527and C<nstore>, only they get an exclusive lock on the file before
528writing. Likewise, C<lock_retrieve> performs as C<retrieve>, but also
529gets a shared lock on the file before reading.
530
531Like with any advisory locking scheme, the protection only works if
532you systematically use C<lock_store> and C<lock_retrieve>. If one
533side of your application uses C<store> whilst the other uses C<lock_retrieve>,
534you will get no protection at all.
535
536The internal advisory locking is implemented using Perl's flock() routine.
537If your system does not support any form of flock(), or if you share
538your files across NFS, you might wish to use other forms of locking by
539using modules like LockFile::Simple which lock a file using a filesystem
540entry, instead of locking the file descriptor.
541
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542=head1 SPEED
543
544The heart of Storable is written in C for decent speed. Extra low-level
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545optimizations have been made when manipulating perl internals, to
546sacrifice encapsulation for the benefit of greater speed.
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547
548=head1 CANONICAL REPRESENTATION
549
550Normally Storable stores elements of hashes in the order they are
551stored internally by Perl, i.e. pseudo-randomly. If you set
552C<$Storable::canonical> to some C<TRUE> value, Storable will store
553hashes with the elements sorted by their key. This allows you to
554compare data structures by comparing their frozen representations (or
555even the compressed frozen representations), which can be useful for
556creating lookup tables for complicated queries.
557
558Canonical order does not imply network order, those are two orthogonal
559settings.
560
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561=head1 FORWARD COMPATIBILITY
562
563This release of Storable can be used on a newer version of Perl to
564serialize data which is not supported by earlier Perls. By default
565Storable will attempt to do the right thing, by C<croak()>ing if it
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566encounters data that it cannot deserialize. However, the defaults
567can be changed as follows
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568
569=over 4
570
571=item utf8 data
572
573Perl 5.6 added support for Unicode characters with code points > 255,
574and Perl 5.8 has full support for Unicode characters in hash keys.
575Perl internally encodes strings with these characters using utf8, and
576Storable serializes them as utf8. By default, if an older version of
577Perl encounters a utf8 value it cannot represent, it will C<croak()>.
578To change this behaviour so that Storable deserializes utf8 encoded
579values as the string of bytes (effectively dropping the I<is_utf8> flag)
580set C<$Storable::drop_utf8> to some C<TRUE> value. This is a form of
581data loss, because with C<$drop_utf8> true, it becomes impossible to tell
582whether the original data was the Unicode string, or a series of bytes
583that happen to be valid utf8.
584
585=item restricted hashes
586
587Perl 5.8 adds support for restricted hashes, which have keys restricted to
588a given set, and can have values locked to be read only. By default
589when Storable encounters a restricted hash on a perl that doesn't support
590them, it will deserialize it as a normal hash, silently discarding any
591placeholder keys and leaving the keys and all values unlocked. To make
592Storable C<croak()> instead, set C<$Storable::downgrade_restricted> to
593a false value. To restore the default set it back to some C<TRUE> value.
594
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595=item files from future versions of Storable
596
597Earlier versions of Storable would immediately croak if they encountered
598a file with a higher internal version number than the reading Storable
599knew about. Internal version numbers are increased each time new data
600types (such as restricted hashes) are added to the vocabulary of the file
601format. This meant that a newer Storable module had no way of writing a
602file readable by an older Storable, even if writer didn't store newer
603data types.
604
605This version of Storable will defer croaking until it encounters a data
606type in the file that it does not recognize. This means that it will
607continue to read files generated by newer Storable modules which are careful
608in what they write out, making it easier to upgrade Storable modules in a
609mixed environment.
610
611The old behaviour of immediate croaking can be re-instated by setting
612C<$Storable::accept_future_minor> to false.
613
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614=back
615
616Both these variables have no effect on a newer Perl which supports the
617relevant feature.
618
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619=head1 ERROR REPORTING
620
621Storable uses the "exception" paradigm, in that it does not try to workaround
622failures: if something bad happens, an exception is generated from the
623caller's perspective (see L<Carp> and C<croak()>). Use eval {} to trap
624those exceptions.
625
626When Storable croaks, it tries to report the error via the C<logcroak()>
627routine from the C<Log::Agent> package, if it is available.
628
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629Normal errors are reported by having store() or retrieve() return C<undef>.
630Such errors are usually I/O errors (or truncated stream errors at retrieval).
631
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632=head1 WIZARDS ONLY
633
634=head2 Hooks
635
636Any class may define hooks that will be called during the serialization
637and deserialization process on objects that are instances of that class.
638Those hooks can redefine the way serialization is performed (and therefore,
c261f00e 639how the symmetrical deserialization should be conducted).
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640
641Since we said earlier:
642
643 dclone(.) = thaw(freeze(.))
644
645everything we say about hooks should also hold for deep cloning. However,
646hooks get to know whether the operation is a mere serialization, or a cloning.
647
648Therefore, when serializing hooks are involved,
649
650 dclone(.) <> thaw(freeze(.))
651
652Well, you could keep them in sync, but there's no guarantee it will always
653hold on classes somebody else wrote. Besides, there is little to gain in
654doing so: a serializing hook could only keep one attribute of an object,
655which is probably not what should happen during a deep cloning of that
656same object.
657
658Here is the hooking interface:
659
bbc7dcd2 660=over 4
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661
662=item C<STORABLE_freeze> I<obj>, I<cloning>
663
664The serializing hook, called on the object during serialization. It can be
665inherited, or defined in the class itself, like any other method.
666
667Arguments: I<obj> is the object to serialize, I<cloning> is a flag indicating
668whether we're in a dclone() or a regular serialization via store() or freeze().
669
670Returned value: A LIST C<($serialized, $ref1, $ref2, ...)> where $serialized
671is the serialized form to be used, and the optional $ref1, $ref2, etc... are
672extra references that you wish to let the Storable engine serialize.
673
674At deserialization time, you will be given back the same LIST, but all the
675extra references will be pointing into the deserialized structure.
676
677The B<first time> the hook is hit in a serialization flow, you may have it
678return an empty list. That will signal the Storable engine to further
679discard that hook for this class and to therefore revert to the default
680serialization of the underlying Perl data. The hook will again be normally
681processed in the next serialization.
682
683Unless you know better, serializing hook should always say:
684
685 sub STORABLE_freeze {
686 my ($self, $cloning) = @_;
687 return if $cloning; # Regular default serialization
688 ....
689 }
690
691in order to keep reasonable dclone() semantics.
692
693=item C<STORABLE_thaw> I<obj>, I<cloning>, I<serialized>, ...
694
695The deserializing hook called on the object during deserialization.
696But wait. If we're deserializing, there's no object yet... right?
697
698Wrong: the Storable engine creates an empty one for you. If you know Eiffel,
699you can view C<STORABLE_thaw> as an alternate creation routine.
700
701This means the hook can be inherited like any other method, and that
702I<obj> is your blessed reference for this particular instance.
703
704The other arguments should look familiar if you know C<STORABLE_freeze>:
705I<cloning> is true when we're part of a deep clone operation, I<serialized>
706is the serialized string you returned to the engine in C<STORABLE_freeze>,
707and there may be an optional list of references, in the same order you gave
708them at serialization time, pointing to the deserialized objects (which
709have been processed courtesy of the Storable engine).
710
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711When the Storable engine does not find any C<STORABLE_thaw> hook routine,
712it tries to load the class by requiring the package dynamically (using
713the blessed package name), and then re-attempts the lookup. If at that
714time the hook cannot be located, the engine croaks. Note that this mechanism
c261f00e 715will fail if you define several classes in the same file, but L<perlmod>
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716warned you.
717
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718It is up to you to use these information to populate I<obj> the way you want.
719
720Returned value: none.
721
722=back
723
724=head2 Predicates
725
c261f00e 726Predicates are not exportable. They must be called by explicitly prefixing
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727them with the Storable package name.
728
bbc7dcd2 729=over 4
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730
731=item C<Storable::last_op_in_netorder>
732
733The C<Storable::last_op_in_netorder()> predicate will tell you whether
734network order was used in the last store or retrieve operation. If you
735don't know how to use this, just forget about it.
736
737=item C<Storable::is_storing>
738
739Returns true if within a store operation (via STORABLE_freeze hook).
740
741=item C<Storable::is_retrieving>
742
743Returns true if within a retrieve operation, (via STORABLE_thaw hook).
744
745=back
746
747=head2 Recursion
748
749With hooks comes the ability to recurse back to the Storable engine. Indeed,
750hooks are regular Perl code, and Storable is convenient when it comes to
751serialize and deserialize things, so why not use it to handle the
752serialization string?
753
754There are a few things you need to know however:
755
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757
758=item *
759
760You can create endless loops if the things you serialize via freeze()
761(for instance) point back to the object we're trying to serialize in the hook.
762
763=item *
764
765Shared references among objects will not stay shared: if we're serializing
766the list of object [A, C] where both object A and C refer to the SAME object
767B, and if there is a serializing hook in A that says freeze(B), then when
768deserializing, we'll get [A', C'] where A' refers to B', but C' refers to D,
769a deep clone of B'. The topology was not preserved.
770
771=back
772
773That's why C<STORABLE_freeze> lets you provide a list of references
774to serialize. The engine guarantees that those will be serialized in the
775same context as the other objects, and therefore that shared objects will
776stay shared.
777
778In the above [A, C] example, the C<STORABLE_freeze> hook could return:
779
780 ("something", $self->{B})
781
782and the B part would be serialized by the engine. In C<STORABLE_thaw>, you
783would get back the reference to the B' object, deserialized for you.
784
785Therefore, recursion should normally be avoided, but is nonetheless supported.
786
787=head2 Deep Cloning
788
789There is a new Clone module available on CPAN which implements deep cloning
790natively, i.e. without freezing to memory and thawing the result. It is
791aimed to replace Storable's dclone() some day. However, it does not currently
792support Storable hooks to redefine the way deep cloning is performed.
793
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794=head1 Storable magic
795
796Yes, there's a lot of that :-) But more precisely, in UNIX systems
797there's a utility called C<file>, which recognizes data files based on
798their contents (usually their first few bytes). For this to work,
8b793558 799a certain file called F<magic> needs to taught about the I<signature>
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800of the data. Where that configuration file lives depends on the UNIX
801flavour, often it's something like F</usr/share/misc/magic> or
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802F</etc/magic>. Your system administrator needs to do the updating of
803the F<magic> file. The necessary signature information is output to
c261f00e 804STDOUT by invoking Storable::show_file_magic(). Note that the open
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805source implementation of the C<file> utility 3.38 (or later)
806is expected to contain the support for recognising Storable files,
807in addition to other kinds of Perl files.
0a0da639 808
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809=head1 EXAMPLES
810
811Here are some code samples showing a possible usage of Storable:
812
813 use Storable qw(store retrieve freeze thaw dclone);
814
815 %color = ('Blue' => 0.1, 'Red' => 0.8, 'Black' => 0, 'White' => 1);
816
817 store(\%color, '/tmp/colors') or die "Can't store %a in /tmp/colors!\n";
818
819 $colref = retrieve('/tmp/colors');
820 die "Unable to retrieve from /tmp/colors!\n" unless defined $colref;
821 printf "Blue is still %lf\n", $colref->{'Blue'};
822
823 $colref2 = dclone(\%color);
824
825 $str = freeze(\%color);
826 printf "Serialization of %%color is %d bytes long.\n", length($str);
827 $colref3 = thaw($str);
828
829which prints (on my machine):
830
831 Blue is still 0.100000
832 Serialization of %color is 102 bytes long.
833
834=head1 WARNING
835
836If you're using references as keys within your hash tables, you're bound
c261f00e 837to disappointment when retrieving your data. Indeed, Perl stringifies
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838references used as hash table keys. If you later wish to access the
839items via another reference stringification (i.e. using the same
840reference that was used for the key originally to record the value into
841the hash table), it will work because both references stringify to the
842same string.
843
844It won't work across a C<store> and C<retrieve> operations however, because
845the addresses in the retrieved objects, which are part of the stringified
846references, will probably differ from the original addresses. The
847topology of your structure is preserved, but not hidden semantics
848like those.
849
850On platforms where it matters, be sure to call C<binmode()> on the
851descriptors that you pass to Storable functions.
852
853Storing data canonically that contains large hashes can be
854significantly slower than storing the same data normally, as
c261f00e 855temporary arrays to hold the keys for each hash have to be allocated,
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856populated, sorted and freed. Some tests have shown a halving of the
857speed of storing -- the exact penalty will depend on the complexity of
858your data. There is no slowdown on retrieval.
859
860=head1 BUGS
861
862You can't store GLOB, CODE, FORMLINE, etc... If you can define
863semantics for those operations, feel free to enhance Storable so that
864it can deal with them.
865
866The store functions will C<croak> if they run into such references
867unless you set C<$Storable::forgive_me> to some C<TRUE> value. In that
868case, the fatal message is turned in a warning and some
869meaningless string is stored instead.
870
871Setting C<$Storable::canonical> may not yield frozen strings that
872compare equal due to possible stringification of numbers. When the
873string version of a scalar exists, it is the form stored, therefore
874if you happen to use your numbers as strings between two freezing
875operations on the same data structures, you will get different
876results.
877
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878When storing doubles in network order, their value is stored as text.
879However, you should also not expect non-numeric floating-point values
880such as infinity and "not a number" to pass successfully through a
881nstore()/retrieve() pair.
882
883As Storable neither knows nor cares about character sets (although it
884does know that characters may be more than eight bits wide), any difference
885in the interpretation of character codes between a host and a target
886system is your problem. In particular, if host and target use different
887code points to represent the characters used in the text representation
888of floating-point numbers, you will not be able be able to exchange
889floating-point data, even with nstore().
890
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891C<Storable::drop_utf8> is a blunt tool. There is no facility either to
892return B<all> strings as utf8 sequences, or to attempt to convert utf8
893data back to 8 bit and C<croak()> if the conversion fails.
894
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895=head1 CREDITS
896
897Thank you to (in chronological order):
898
899 Jarkko Hietaniemi <jhi@iki.fi>
900 Ulrich Pfeifer <pfeifer@charly.informatik.uni-dortmund.de>
901 Benjamin A. Holzman <bah@ecnvantage.com>
902 Andrew Ford <A.Ford@ford-mason.co.uk>
903 Gisle Aas <gisle@aas.no>
904 Jeff Gresham <gresham_jeffrey@jpmorgan.com>
905 Murray Nesbitt <murray@activestate.com>
906 Marc Lehmann <pcg@opengroup.org>
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907 Justin Banks <justinb@wamnet.com>
908 Jarkko Hietaniemi <jhi@iki.fi> (AGAIN, as perl 5.7.0 Pumpkin!)
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909 Salvador Ortiz Garcia <sog@msg.com.mx>
910 Dominic Dunlop <domo@computer.org>
911 Erik Haugan <erik@solbors.no>
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912
913for their bug reports, suggestions and contributions.
914
915Benjamin Holzman contributed the tied variable support, Andrew Ford
916contributed the canonical order for hashes, and Gisle Aas fixed
917a few misunderstandings of mine regarding the Perl internals,
918and optimized the emission of "tags" in the output streams by
919simply counting the objects instead of tagging them (leading to
920a binary incompatibility for the Storable image starting at version
9210.6--older images are of course still properly understood).
922Murray Nesbitt made Storable thread-safe. Marc Lehmann added overloading
923and reference to tied items support.
924
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925=head1 AUTHOR
926
0ba8809e 927Storable was written by Raphael Manfredi F<E<lt>Raphael_Manfredi@pobox.comE<gt>>
775ecd75 928Maintenance is now done by the perl5-porters F<E<lt>perl5-porters@perl.orgE<gt>>
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929
930Please e-mail us with problems, bug fixes, comments and complaints,
931although if you have complements you should send them to Raphael.
932Please don't e-mail Raphael with problems, as he no longer works on
933Storable, and your message will be delayed while he forwards it to us.
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934
935=head1 SEE ALSO
936
c261f00e 937L<Clone>.
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938
939=cut
940