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a0d0e21e LW |
1 | # DB_File.pm -- Perl 5 interface to Berkeley DB |
2 | # | |
6ca2e664 | 3 | # written by Paul Marquess (Paul.Marquess@btinternet.com) |
ee8c7f54 CB |
4 | # last modified 26th April 2000 |
5 | # version 1.73 | |
36477c24 | 6 | # |
cb50131a | 7 | # Copyright (c) 1995-2000 Paul Marquess. All rights reserved. |
36477c24 | 8 | # This program is free software; you can redistribute it and/or |
9 | # modify it under the same terms as Perl itself. | |
10 | ||
8e07c86e AD |
11 | |
12 | package DB_File::HASHINFO ; | |
785da04d | 13 | |
610ab055 PM |
14 | require 5.003 ; |
15 | ||
785da04d | 16 | use strict; |
8e07c86e | 17 | use Carp; |
88108326 | 18 | require Tie::Hash; |
19 | @DB_File::HASHINFO::ISA = qw(Tie::Hash); | |
8e07c86e | 20 | |
88108326 | 21 | sub new |
8e07c86e | 22 | { |
88108326 | 23 | my $pkg = shift ; |
24 | my %x ; | |
25 | tie %x, $pkg ; | |
26 | bless \%x, $pkg ; | |
8e07c86e AD |
27 | } |
28 | ||
610ab055 | 29 | |
88108326 | 30 | sub TIEHASH |
31 | { | |
32 | my $pkg = shift ; | |
33 | ||
36477c24 | 34 | bless { VALID => { map {$_, 1} |
35 | qw( bsize ffactor nelem cachesize hash lorder) | |
36 | }, | |
37 | GOT => {} | |
38 | }, $pkg ; | |
88108326 | 39 | } |
8e07c86e | 40 | |
610ab055 | 41 | |
8e07c86e AD |
42 | sub FETCH |
43 | { | |
88108326 | 44 | my $self = shift ; |
45 | my $key = shift ; | |
8e07c86e | 46 | |
36477c24 | 47 | return $self->{GOT}{$key} if exists $self->{VALID}{$key} ; |
88108326 | 48 | |
49 | my $pkg = ref $self ; | |
50 | croak "${pkg}::FETCH - Unknown element '$key'" ; | |
8e07c86e AD |
51 | } |
52 | ||
53 | ||
54 | sub STORE | |
55 | { | |
88108326 | 56 | my $self = shift ; |
57 | my $key = shift ; | |
58 | my $value = shift ; | |
59 | ||
36477c24 | 60 | if ( exists $self->{VALID}{$key} ) |
8e07c86e | 61 | { |
36477c24 | 62 | $self->{GOT}{$key} = $value ; |
8e07c86e AD |
63 | return ; |
64 | } | |
65 | ||
88108326 | 66 | my $pkg = ref $self ; |
67 | croak "${pkg}::STORE - Unknown element '$key'" ; | |
8e07c86e AD |
68 | } |
69 | ||
70 | sub DELETE | |
71 | { | |
88108326 | 72 | my $self = shift ; |
73 | my $key = shift ; | |
74 | ||
36477c24 | 75 | if ( exists $self->{VALID}{$key} ) |
8e07c86e | 76 | { |
36477c24 | 77 | delete $self->{GOT}{$key} ; |
8e07c86e AD |
78 | return ; |
79 | } | |
80 | ||
88108326 | 81 | my $pkg = ref $self ; |
82 | croak "DB_File::HASHINFO::DELETE - Unknown element '$key'" ; | |
8e07c86e AD |
83 | } |
84 | ||
88108326 | 85 | sub EXISTS |
8e07c86e | 86 | { |
88108326 | 87 | my $self = shift ; |
88 | my $key = shift ; | |
8e07c86e | 89 | |
36477c24 | 90 | exists $self->{VALID}{$key} ; |
8e07c86e AD |
91 | } |
92 | ||
88108326 | 93 | sub NotHere |
8e07c86e | 94 | { |
18d2dc8c | 95 | my $self = shift ; |
88108326 | 96 | my $method = shift ; |
8e07c86e | 97 | |
18d2dc8c | 98 | croak ref($self) . " does not define the method ${method}" ; |
8e07c86e AD |
99 | } |
100 | ||
18d2dc8c PM |
101 | sub FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") } |
102 | sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") } | |
103 | sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") } | |
8e07c86e AD |
104 | |
105 | package DB_File::RECNOINFO ; | |
785da04d | 106 | |
88108326 | 107 | use strict ; |
108 | ||
045291aa | 109 | @DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ; |
8e07c86e AD |
110 | |
111 | sub TIEHASH | |
112 | { | |
88108326 | 113 | my $pkg = shift ; |
114 | ||
36477c24 | 115 | bless { VALID => { map {$_, 1} |
116 | qw( bval cachesize psize flags lorder reclen bfname ) | |
117 | }, | |
118 | GOT => {}, | |
119 | }, $pkg ; | |
8e07c86e AD |
120 | } |
121 | ||
88108326 | 122 | package DB_File::BTREEINFO ; |
8e07c86e | 123 | |
88108326 | 124 | use strict ; |
8e07c86e | 125 | |
88108326 | 126 | @DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ; |
8e07c86e | 127 | |
88108326 | 128 | sub TIEHASH |
8e07c86e | 129 | { |
88108326 | 130 | my $pkg = shift ; |
131 | ||
36477c24 | 132 | bless { VALID => { map {$_, 1} |
133 | qw( flags cachesize maxkeypage minkeypage psize | |
134 | compare prefix lorder ) | |
135 | }, | |
136 | GOT => {}, | |
137 | }, $pkg ; | |
8e07c86e AD |
138 | } |
139 | ||
140 | ||
8e07c86e | 141 | package DB_File ; |
785da04d PM |
142 | |
143 | use strict; | |
cb50131a CB |
144 | use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO |
145 | $db_version $use_XSLoader | |
146 | ) ; | |
8e07c86e AD |
147 | use Carp; |
148 | ||
785da04d | 149 | |
ee8c7f54 | 150 | $VERSION = "1.73" ; |
8e07c86e AD |
151 | |
152 | #typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE; | |
88108326 | 153 | $DB_BTREE = new DB_File::BTREEINFO ; |
154 | $DB_HASH = new DB_File::HASHINFO ; | |
155 | $DB_RECNO = new DB_File::RECNOINFO ; | |
8e07c86e | 156 | |
785da04d | 157 | require Tie::Hash; |
8e07c86e AD |
158 | require Exporter; |
159 | use AutoLoader; | |
cb50131a CB |
160 | BEGIN { |
161 | $use_XSLoader = 1 ; | |
162 | eval { require XSLoader } ; | |
163 | ||
164 | if ($@) { | |
165 | $use_XSLoader = 0 ; | |
166 | require DynaLoader; | |
167 | @ISA = qw(DynaLoader); | |
168 | } | |
169 | } | |
170 | ||
171 | push @ISA, qw(Tie::Hash Exporter); | |
8e07c86e AD |
172 | @EXPORT = qw( |
173 | $DB_BTREE $DB_HASH $DB_RECNO | |
88108326 | 174 | |
8e07c86e AD |
175 | BTREEMAGIC |
176 | BTREEVERSION | |
177 | DB_LOCK | |
178 | DB_SHMEM | |
179 | DB_TXN | |
180 | HASHMAGIC | |
181 | HASHVERSION | |
182 | MAX_PAGE_NUMBER | |
183 | MAX_PAGE_OFFSET | |
184 | MAX_REC_NUMBER | |
185 | RET_ERROR | |
186 | RET_SPECIAL | |
187 | RET_SUCCESS | |
188 | R_CURSOR | |
189 | R_DUP | |
190 | R_FIRST | |
191 | R_FIXEDLEN | |
192 | R_IAFTER | |
193 | R_IBEFORE | |
194 | R_LAST | |
195 | R_NEXT | |
196 | R_NOKEY | |
197 | R_NOOVERWRITE | |
198 | R_PREV | |
199 | R_RECNOSYNC | |
200 | R_SETCURSOR | |
201 | R_SNAPSHOT | |
202 | __R_UNUSED | |
88108326 | 203 | |
045291aa | 204 | ); |
8e07c86e AD |
205 | |
206 | sub AUTOLOAD { | |
785da04d | 207 | my($constname); |
8e07c86e | 208 | ($constname = $AUTOLOAD) =~ s/.*:://; |
785da04d | 209 | my $val = constant($constname, @_ ? $_[0] : 0); |
8e07c86e | 210 | if ($! != 0) { |
265f5c4a | 211 | if ($! =~ /Invalid/ || $!{EINVAL}) { |
8e07c86e AD |
212 | $AutoLoader::AUTOLOAD = $AUTOLOAD; |
213 | goto &AutoLoader::AUTOLOAD; | |
214 | } | |
215 | else { | |
785da04d | 216 | my($pack,$file,$line) = caller; |
8e07c86e AD |
217 | croak "Your vendor has not defined DB macro $constname, used at $file line $line. |
218 | "; | |
219 | } | |
220 | } | |
221 | eval "sub $AUTOLOAD { $val }"; | |
222 | goto &$AUTOLOAD; | |
223 | } | |
224 | ||
f6b705ef | 225 | |
a6ed719b | 226 | eval { |
1f70e1ea PM |
227 | # Make all Fcntl O_XXX constants available for importing |
228 | require Fcntl; | |
229 | my @O = grep /^O_/, @Fcntl::EXPORT; | |
230 | Fcntl->import(@O); # first we import what we want to export | |
231 | push(@EXPORT, @O); | |
a6ed719b | 232 | }; |
f6b705ef | 233 | |
cb50131a CB |
234 | if ($use_XSLoader) |
235 | { XSLoader::load("DB_File", $VERSION)} | |
236 | else | |
237 | { bootstrap DB_File $VERSION } | |
8e07c86e AD |
238 | |
239 | # Preloaded methods go here. Autoload methods go after __END__, and are | |
240 | # processed by the autosplit program. | |
241 | ||
05475680 | 242 | sub tie_hash_or_array |
610ab055 PM |
243 | { |
244 | my (@arg) = @_ ; | |
05475680 | 245 | my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ; |
610ab055 PM |
246 | |
247 | $arg[4] = tied %{ $arg[4] } | |
248 | if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ; | |
249 | ||
1f70e1ea PM |
250 | # make recno in Berkeley DB version 2 work like recno in version 1. |
251 | if ($db_version > 1 and defined $arg[4] and $arg[4] =~ /RECNO/ and | |
252 | $arg[1] and ! -e $arg[1]) { | |
253 | open(FH, ">$arg[1]") or return undef ; | |
254 | close FH ; | |
255 | chmod $arg[3] ? $arg[3] : 0666 , $arg[1] ; | |
256 | } | |
257 | ||
05475680 | 258 | DoTie_($tieHASH, @arg) ; |
610ab055 PM |
259 | } |
260 | ||
05475680 PM |
261 | sub TIEHASH |
262 | { | |
263 | tie_hash_or_array(@_) ; | |
264 | } | |
265 | ||
266 | sub TIEARRAY | |
267 | { | |
268 | tie_hash_or_array(@_) ; | |
269 | } | |
88108326 | 270 | |
045291aa PM |
271 | sub CLEAR |
272 | { | |
1f70e1ea PM |
273 | my $self = shift; |
274 | my $key = "" ; | |
275 | my $value = "" ; | |
276 | my $status = $self->seq($key, $value, R_FIRST()); | |
277 | my @keys; | |
278 | ||
279 | while ($status == 0) { | |
280 | push @keys, $key; | |
281 | $status = $self->seq($key, $value, R_NEXT()); | |
282 | } | |
283 | foreach $key (reverse @keys) { | |
284 | my $s = $self->del($key); | |
285 | } | |
286 | } | |
287 | ||
045291aa PM |
288 | sub EXTEND { } |
289 | ||
290 | sub STORESIZE | |
291 | { | |
292 | my $self = shift; | |
293 | my $length = shift ; | |
294 | my $current_length = $self->length() ; | |
295 | ||
296 | if ($length < $current_length) { | |
297 | my $key ; | |
298 | for ($key = $current_length - 1 ; $key >= $length ; -- $key) | |
299 | { $self->del($key) } | |
300 | } | |
a9fd575d PM |
301 | elsif ($length > $current_length) { |
302 | $self->put($length-1, "") ; | |
303 | } | |
045291aa PM |
304 | } |
305 | ||
6ca2e664 PM |
306 | sub find_dup |
307 | { | |
308 | croak "Usage: \$db->find_dup(key,value)\n" | |
309 | unless @_ == 3 ; | |
310 | ||
311 | my $db = shift ; | |
312 | my ($origkey, $value_wanted) = @_ ; | |
313 | my ($key, $value) = ($origkey, 0); | |
314 | my ($status) = 0 ; | |
315 | ||
316 | for ($status = $db->seq($key, $value, R_CURSOR() ) ; | |
317 | $status == 0 ; | |
318 | $status = $db->seq($key, $value, R_NEXT() ) ) { | |
319 | ||
320 | return 0 if $key eq $origkey and $value eq $value_wanted ; | |
321 | } | |
322 | ||
323 | return $status ; | |
324 | } | |
325 | ||
326 | sub del_dup | |
327 | { | |
328 | croak "Usage: \$db->del_dup(key,value)\n" | |
329 | unless @_ == 3 ; | |
330 | ||
331 | my $db = shift ; | |
332 | my ($key, $value) = @_ ; | |
333 | my ($status) = $db->find_dup($key, $value) ; | |
334 | return $status if $status != 0 ; | |
335 | ||
336 | $status = $db->del($key, R_CURSOR() ) ; | |
337 | return $status ; | |
338 | } | |
339 | ||
88108326 | 340 | sub get_dup |
341 | { | |
342 | croak "Usage: \$db->get_dup(key [,flag])\n" | |
343 | unless @_ == 2 or @_ == 3 ; | |
344 | ||
345 | my $db = shift ; | |
346 | my $key = shift ; | |
347 | my $flag = shift ; | |
f6b705ef | 348 | my $value = 0 ; |
88108326 | 349 | my $origkey = $key ; |
350 | my $wantarray = wantarray ; | |
f6b705ef | 351 | my %values = () ; |
88108326 | 352 | my @values = () ; |
353 | my $counter = 0 ; | |
f6b705ef | 354 | my $status = 0 ; |
88108326 | 355 | |
f6b705ef | 356 | # iterate through the database until either EOF ($status == 0) |
357 | # or a different key is encountered ($key ne $origkey). | |
358 | for ($status = $db->seq($key, $value, R_CURSOR()) ; | |
359 | $status == 0 and $key eq $origkey ; | |
360 | $status = $db->seq($key, $value, R_NEXT()) ) { | |
88108326 | 361 | |
f6b705ef | 362 | # save the value or count number of matches |
363 | if ($wantarray) { | |
364 | if ($flag) | |
365 | { ++ $values{$value} } | |
366 | else | |
367 | { push (@values, $value) } | |
368 | } | |
369 | else | |
370 | { ++ $counter } | |
88108326 | 371 | |
88108326 | 372 | } |
373 | ||
f6b705ef | 374 | return ($wantarray ? ($flag ? %values : @values) : $counter) ; |
88108326 | 375 | } |
376 | ||
377 | ||
8e07c86e AD |
378 | 1; |
379 | __END__ | |
380 | ||
3b35bae3 AD |
381 | =head1 NAME |
382 | ||
1f70e1ea | 383 | DB_File - Perl5 access to Berkeley DB version 1.x |
3b35bae3 AD |
384 | |
385 | =head1 SYNOPSIS | |
386 | ||
387 | use DB_File ; | |
88108326 | 388 | |
389 | [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ; | |
390 | [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ; | |
391 | [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ; | |
760ac839 | 392 | |
3b35bae3 AD |
393 | $status = $X->del($key [, $flags]) ; |
394 | $status = $X->put($key, $value [, $flags]) ; | |
395 | $status = $X->get($key, $value [, $flags]) ; | |
760ac839 | 396 | $status = $X->seq($key, $value, $flags) ; |
3b35bae3 AD |
397 | $status = $X->sync([$flags]) ; |
398 | $status = $X->fd ; | |
760ac839 | 399 | |
f6b705ef | 400 | # BTREE only |
88108326 | 401 | $count = $X->get_dup($key) ; |
402 | @list = $X->get_dup($key) ; | |
403 | %list = $X->get_dup($key, 1) ; | |
6ca2e664 PM |
404 | $status = $X->find_dup($key, $value) ; |
405 | $status = $X->del_dup($key, $value) ; | |
88108326 | 406 | |
f6b705ef | 407 | # RECNO only |
408 | $a = $X->length; | |
409 | $a = $X->pop ; | |
410 | $X->push(list); | |
411 | $a = $X->shift; | |
412 | $X->unshift(list); | |
413 | ||
cad2e5aa JH |
414 | # DBM Filters |
415 | $old_filter = $db->filter_store_key ( sub { ... } ) ; | |
416 | $old_filter = $db->filter_store_value( sub { ... } ) ; | |
417 | $old_filter = $db->filter_fetch_key ( sub { ... } ) ; | |
418 | $old_filter = $db->filter_fetch_value( sub { ... } ) ; | |
419 | ||
3b35bae3 AD |
420 | untie %hash ; |
421 | untie @array ; | |
422 | ||
423 | =head1 DESCRIPTION | |
424 | ||
8e07c86e | 425 | B<DB_File> is a module which allows Perl programs to make use of the |
1f70e1ea | 426 | facilities provided by Berkeley DB version 1.x (if you have a newer |
c529f79d CB |
427 | version of DB, see L<Using DB_File with Berkeley DB version 2 or 3>). |
428 | It is assumed that you have a copy of the Berkeley DB manual pages at | |
429 | hand when reading this documentation. The interface defined here | |
430 | mirrors the Berkeley DB interface closely. | |
68dc0745 | 431 | |
8e07c86e AD |
432 | Berkeley DB is a C library which provides a consistent interface to a |
433 | number of database formats. B<DB_File> provides an interface to all | |
434 | three of the database types currently supported by Berkeley DB. | |
3b35bae3 AD |
435 | |
436 | The file types are: | |
437 | ||
438 | =over 5 | |
439 | ||
88108326 | 440 | =item B<DB_HASH> |
3b35bae3 | 441 | |
88108326 | 442 | This database type allows arbitrary key/value pairs to be stored in data |
8e07c86e AD |
443 | files. This is equivalent to the functionality provided by other |
444 | hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though, | |
445 | the files created using DB_HASH are not compatible with any of the | |
446 | other packages mentioned. | |
3b35bae3 | 447 | |
8e07c86e AD |
448 | A default hashing algorithm, which will be adequate for most |
449 | applications, is built into Berkeley DB. If you do need to use your own | |
450 | hashing algorithm it is possible to write your own in Perl and have | |
451 | B<DB_File> use it instead. | |
3b35bae3 | 452 | |
88108326 | 453 | =item B<DB_BTREE> |
454 | ||
455 | The btree format allows arbitrary key/value pairs to be stored in a | |
8e07c86e | 456 | sorted, balanced binary tree. |
3b35bae3 | 457 | |
8e07c86e AD |
458 | As with the DB_HASH format, it is possible to provide a user defined |
459 | Perl routine to perform the comparison of keys. By default, though, the | |
460 | keys are stored in lexical order. | |
3b35bae3 | 461 | |
88108326 | 462 | =item B<DB_RECNO> |
3b35bae3 | 463 | |
8e07c86e AD |
464 | DB_RECNO allows both fixed-length and variable-length flat text files |
465 | to be manipulated using the same key/value pair interface as in DB_HASH | |
466 | and DB_BTREE. In this case the key will consist of a record (line) | |
467 | number. | |
3b35bae3 AD |
468 | |
469 | =back | |
470 | ||
c529f79d | 471 | =head2 Using DB_File with Berkeley DB version 2 or 3 |
1f70e1ea PM |
472 | |
473 | Although B<DB_File> is intended to be used with Berkeley DB version 1, | |
c529f79d | 474 | it can also be used with version 2.or 3 In this case the interface is |
1f70e1ea | 475 | limited to the functionality provided by Berkeley DB 1.x. Anywhere the |
c529f79d CB |
476 | version 2 or 3 interface differs, B<DB_File> arranges for it to work |
477 | like version 1. This feature allows B<DB_File> scripts that were built | |
478 | with version 1 to be migrated to version 2 or 3 without any changes. | |
1f70e1ea PM |
479 | |
480 | If you want to make use of the new features available in Berkeley DB | |
cb50131a | 481 | 2.x or greater, use the Perl module B<BerkeleyDB> instead. |
1f70e1ea | 482 | |
c529f79d CB |
483 | B<Note:> The database file format has changed in both Berkeley DB |
484 | version 2 and 3. If you cannot recreate your databases, you must dump | |
485 | any existing databases with the C<db_dump185> utility that comes with | |
486 | Berkeley DB. | |
487 | Once you have rebuilt DB_File to use Berkeley DB version 2 or 3, your | |
1f70e1ea PM |
488 | databases can be recreated using C<db_load>. Refer to the Berkeley DB |
489 | documentation for further details. | |
490 | ||
c529f79d CB |
491 | Please read L<"COPYRIGHT"> before using version 2.x or 3.x of Berkeley |
492 | DB with DB_File. | |
1f70e1ea | 493 | |
68dc0745 | 494 | =head2 Interface to Berkeley DB |
3b35bae3 AD |
495 | |
496 | B<DB_File> allows access to Berkeley DB files using the tie() mechanism | |
8e07c86e AD |
497 | in Perl 5 (for full details, see L<perlfunc/tie()>). This facility |
498 | allows B<DB_File> to access Berkeley DB files using either an | |
499 | associative array (for DB_HASH & DB_BTREE file types) or an ordinary | |
500 | array (for the DB_RECNO file type). | |
3b35bae3 | 501 | |
88108326 | 502 | In addition to the tie() interface, it is also possible to access most |
503 | of the functions provided in the Berkeley DB API directly. | |
f6b705ef | 504 | See L<THE API INTERFACE>. |
3b35bae3 | 505 | |
88108326 | 506 | =head2 Opening a Berkeley DB Database File |
3b35bae3 | 507 | |
8e07c86e | 508 | Berkeley DB uses the function dbopen() to open or create a database. |
f6b705ef | 509 | Here is the C prototype for dbopen(): |
3b35bae3 AD |
510 | |
511 | DB* | |
512 | dbopen (const char * file, int flags, int mode, | |
513 | DBTYPE type, const void * openinfo) | |
514 | ||
515 | The parameter C<type> is an enumeration which specifies which of the 3 | |
516 | interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used. | |
517 | Depending on which of these is actually chosen, the final parameter, | |
518 | I<openinfo> points to a data structure which allows tailoring of the | |
519 | specific interface method. | |
520 | ||
8e07c86e | 521 | This interface is handled slightly differently in B<DB_File>. Here is |
88108326 | 522 | an equivalent call using B<DB_File>: |
3b35bae3 | 523 | |
88108326 | 524 | tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ; |
3b35bae3 | 525 | |
8e07c86e AD |
526 | The C<filename>, C<flags> and C<mode> parameters are the direct |
527 | equivalent of their dbopen() counterparts. The final parameter $DB_HASH | |
528 | performs the function of both the C<type> and C<openinfo> parameters in | |
529 | dbopen(). | |
3b35bae3 | 530 | |
88108326 | 531 | In the example above $DB_HASH is actually a pre-defined reference to a |
532 | hash object. B<DB_File> has three of these pre-defined references. | |
533 | Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO. | |
3b35bae3 | 534 | |
8e07c86e AD |
535 | The keys allowed in each of these pre-defined references is limited to |
536 | the names used in the equivalent C structure. So, for example, the | |
537 | $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>, | |
88108326 | 538 | C<ffactor>, C<hash>, C<lorder> and C<nelem>. |
539 | ||
540 | To change one of these elements, just assign to it like this: | |
541 | ||
542 | $DB_HASH->{'cachesize'} = 10000 ; | |
543 | ||
544 | The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are | |
545 | usually adequate for most applications. If you do need to create extra | |
546 | instances of these objects, constructors are available for each file | |
547 | type. | |
548 | ||
549 | Here are examples of the constructors and the valid options available | |
550 | for DB_HASH, DB_BTREE and DB_RECNO respectively. | |
551 | ||
552 | $a = new DB_File::HASHINFO ; | |
553 | $a->{'bsize'} ; | |
554 | $a->{'cachesize'} ; | |
555 | $a->{'ffactor'}; | |
556 | $a->{'hash'} ; | |
557 | $a->{'lorder'} ; | |
558 | $a->{'nelem'} ; | |
559 | ||
560 | $b = new DB_File::BTREEINFO ; | |
561 | $b->{'flags'} ; | |
562 | $b->{'cachesize'} ; | |
563 | $b->{'maxkeypage'} ; | |
564 | $b->{'minkeypage'} ; | |
565 | $b->{'psize'} ; | |
566 | $b->{'compare'} ; | |
567 | $b->{'prefix'} ; | |
568 | $b->{'lorder'} ; | |
569 | ||
570 | $c = new DB_File::RECNOINFO ; | |
571 | $c->{'bval'} ; | |
572 | $c->{'cachesize'} ; | |
573 | $c->{'psize'} ; | |
574 | $c->{'flags'} ; | |
575 | $c->{'lorder'} ; | |
576 | $c->{'reclen'} ; | |
577 | $c->{'bfname'} ; | |
578 | ||
579 | The values stored in the hashes above are mostly the direct equivalent | |
580 | of their C counterpart. Like their C counterparts, all are set to a | |
f6b705ef | 581 | default values - that means you don't have to set I<all> of the |
88108326 | 582 | values when you only want to change one. Here is an example: |
583 | ||
584 | $a = new DB_File::HASHINFO ; | |
585 | $a->{'cachesize'} = 12345 ; | |
586 | tie %y, 'DB_File', "filename", $flags, 0777, $a ; | |
587 | ||
36477c24 | 588 | A few of the options need extra discussion here. When used, the C |
88108326 | 589 | equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers |
590 | to C functions. In B<DB_File> these keys are used to store references | |
591 | to Perl subs. Below are templates for each of the subs: | |
592 | ||
593 | sub hash | |
594 | { | |
595 | my ($data) = @_ ; | |
596 | ... | |
597 | # return the hash value for $data | |
598 | return $hash ; | |
599 | } | |
3b35bae3 | 600 | |
88108326 | 601 | sub compare |
602 | { | |
603 | my ($key, $key2) = @_ ; | |
604 | ... | |
605 | # return 0 if $key1 eq $key2 | |
606 | # -1 if $key1 lt $key2 | |
607 | # 1 if $key1 gt $key2 | |
608 | return (-1 , 0 or 1) ; | |
609 | } | |
3b35bae3 | 610 | |
88108326 | 611 | sub prefix |
612 | { | |
613 | my ($key, $key2) = @_ ; | |
614 | ... | |
615 | # return number of bytes of $key2 which are | |
616 | # necessary to determine that it is greater than $key1 | |
617 | return $bytes ; | |
618 | } | |
3b35bae3 | 619 | |
f6b705ef | 620 | See L<Changing the BTREE sort order> for an example of using the |
621 | C<compare> template. | |
88108326 | 622 | |
36477c24 | 623 | If you are using the DB_RECNO interface and you intend making use of |
9a2c4ce3 | 624 | C<bval>, you should check out L<The 'bval' Option>. |
36477c24 | 625 | |
88108326 | 626 | =head2 Default Parameters |
627 | ||
628 | It is possible to omit some or all of the final 4 parameters in the | |
629 | call to C<tie> and let them take default values. As DB_HASH is the most | |
630 | common file format used, the call: | |
631 | ||
632 | tie %A, "DB_File", "filename" ; | |
633 | ||
634 | is equivalent to: | |
635 | ||
18d2dc8c | 636 | tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ; |
88108326 | 637 | |
638 | It is also possible to omit the filename parameter as well, so the | |
639 | call: | |
640 | ||
641 | tie %A, "DB_File" ; | |
642 | ||
643 | is equivalent to: | |
644 | ||
18d2dc8c | 645 | tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ; |
88108326 | 646 | |
f6b705ef | 647 | See L<In Memory Databases> for a discussion on the use of C<undef> |
88108326 | 648 | in place of a filename. |
649 | ||
f6b705ef | 650 | =head2 In Memory Databases |
651 | ||
652 | Berkeley DB allows the creation of in-memory databases by using NULL | |
653 | (that is, a C<(char *)0> in C) in place of the filename. B<DB_File> | |
654 | uses C<undef> instead of NULL to provide this functionality. | |
655 | ||
656 | =head1 DB_HASH | |
657 | ||
658 | The DB_HASH file format is probably the most commonly used of the three | |
659 | file formats that B<DB_File> supports. It is also very straightforward | |
660 | to use. | |
661 | ||
68dc0745 | 662 | =head2 A Simple Example |
f6b705ef | 663 | |
664 | This example shows how to create a database, add key/value pairs to the | |
665 | database, delete keys/value pairs and finally how to enumerate the | |
666 | contents of the database. | |
667 | ||
610ab055 | 668 | use strict ; |
f6b705ef | 669 | use DB_File ; |
610ab055 | 670 | use vars qw( %h $k $v ) ; |
f6b705ef | 671 | |
2c2d71f5 | 672 | unlink "fruit" ; |
f6b705ef | 673 | tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH |
674 | or die "Cannot open file 'fruit': $!\n"; | |
675 | ||
676 | # Add a few key/value pairs to the file | |
677 | $h{"apple"} = "red" ; | |
678 | $h{"orange"} = "orange" ; | |
679 | $h{"banana"} = "yellow" ; | |
680 | $h{"tomato"} = "red" ; | |
681 | ||
682 | # Check for existence of a key | |
683 | print "Banana Exists\n\n" if $h{"banana"} ; | |
684 | ||
685 | # Delete a key/value pair. | |
686 | delete $h{"apple"} ; | |
687 | ||
688 | # print the contents of the file | |
689 | while (($k, $v) = each %h) | |
690 | { print "$k -> $v\n" } | |
691 | ||
692 | untie %h ; | |
693 | ||
694 | here is the output: | |
695 | ||
696 | Banana Exists | |
697 | ||
698 | orange -> orange | |
699 | tomato -> red | |
700 | banana -> yellow | |
701 | ||
702 | Note that the like ordinary associative arrays, the order of the keys | |
703 | retrieved is in an apparently random order. | |
704 | ||
705 | =head1 DB_BTREE | |
706 | ||
707 | The DB_BTREE format is useful when you want to store data in a given | |
708 | order. By default the keys will be stored in lexical order, but as you | |
709 | will see from the example shown in the next section, it is very easy to | |
710 | define your own sorting function. | |
711 | ||
712 | =head2 Changing the BTREE sort order | |
713 | ||
714 | This script shows how to override the default sorting algorithm that | |
715 | BTREE uses. Instead of using the normal lexical ordering, a case | |
716 | insensitive compare function will be used. | |
88108326 | 717 | |
610ab055 | 718 | use strict ; |
f6b705ef | 719 | use DB_File ; |
610ab055 PM |
720 | |
721 | my %h ; | |
f6b705ef | 722 | |
723 | sub Compare | |
724 | { | |
725 | my ($key1, $key2) = @_ ; | |
726 | "\L$key1" cmp "\L$key2" ; | |
727 | } | |
728 | ||
729 | # specify the Perl sub that will do the comparison | |
730 | $DB_BTREE->{'compare'} = \&Compare ; | |
731 | ||
2c2d71f5 | 732 | unlink "tree" ; |
f6b705ef | 733 | tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE |
734 | or die "Cannot open file 'tree': $!\n" ; | |
735 | ||
736 | # Add a key/value pair to the file | |
737 | $h{'Wall'} = 'Larry' ; | |
738 | $h{'Smith'} = 'John' ; | |
739 | $h{'mouse'} = 'mickey' ; | |
740 | $h{'duck'} = 'donald' ; | |
741 | ||
742 | # Delete | |
743 | delete $h{"duck"} ; | |
744 | ||
745 | # Cycle through the keys printing them in order. | |
746 | # Note it is not necessary to sort the keys as | |
747 | # the btree will have kept them in order automatically. | |
748 | foreach (keys %h) | |
749 | { print "$_\n" } | |
750 | ||
751 | untie %h ; | |
752 | ||
753 | Here is the output from the code above. | |
754 | ||
755 | mouse | |
756 | Smith | |
757 | Wall | |
758 | ||
759 | There are a few point to bear in mind if you want to change the | |
760 | ordering in a BTREE database: | |
761 | ||
762 | =over 5 | |
763 | ||
764 | =item 1. | |
765 | ||
766 | The new compare function must be specified when you create the database. | |
767 | ||
768 | =item 2. | |
769 | ||
770 | You cannot change the ordering once the database has been created. Thus | |
771 | you must use the same compare function every time you access the | |
88108326 | 772 | database. |
773 | ||
f6b705ef | 774 | =back |
775 | ||
68dc0745 | 776 | =head2 Handling Duplicate Keys |
f6b705ef | 777 | |
778 | The BTREE file type optionally allows a single key to be associated | |
779 | with an arbitrary number of values. This option is enabled by setting | |
780 | the flags element of C<$DB_BTREE> to R_DUP when creating the database. | |
781 | ||
88108326 | 782 | There are some difficulties in using the tied hash interface if you |
783 | want to manipulate a BTREE database with duplicate keys. Consider this | |
784 | code: | |
785 | ||
610ab055 | 786 | use strict ; |
88108326 | 787 | use DB_File ; |
610ab055 PM |
788 | |
789 | use vars qw($filename %h ) ; | |
790 | ||
88108326 | 791 | $filename = "tree" ; |
792 | unlink $filename ; | |
793 | ||
794 | # Enable duplicate records | |
795 | $DB_BTREE->{'flags'} = R_DUP ; | |
796 | ||
797 | tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
798 | or die "Cannot open $filename: $!\n"; | |
799 | ||
800 | # Add some key/value pairs to the file | |
801 | $h{'Wall'} = 'Larry' ; | |
802 | $h{'Wall'} = 'Brick' ; # Note the duplicate key | |
f6b705ef | 803 | $h{'Wall'} = 'Brick' ; # Note the duplicate key and value |
88108326 | 804 | $h{'Smith'} = 'John' ; |
805 | $h{'mouse'} = 'mickey' ; | |
806 | ||
807 | # iterate through the associative array | |
808 | # and print each key/value pair. | |
2c2d71f5 | 809 | foreach (sort keys %h) |
88108326 | 810 | { print "$_ -> $h{$_}\n" } |
811 | ||
f6b705ef | 812 | untie %h ; |
813 | ||
88108326 | 814 | Here is the output: |
815 | ||
816 | Smith -> John | |
817 | Wall -> Larry | |
818 | Wall -> Larry | |
f6b705ef | 819 | Wall -> Larry |
88108326 | 820 | mouse -> mickey |
821 | ||
f6b705ef | 822 | As you can see 3 records have been successfully created with key C<Wall> |
88108326 | 823 | - the only thing is, when they are retrieved from the database they |
f6b705ef | 824 | I<seem> to have the same value, namely C<Larry>. The problem is caused |
825 | by the way that the associative array interface works. Basically, when | |
826 | the associative array interface is used to fetch the value associated | |
827 | with a given key, it will only ever retrieve the first value. | |
88108326 | 828 | |
829 | Although it may not be immediately obvious from the code above, the | |
830 | associative array interface can be used to write values with duplicate | |
831 | keys, but it cannot be used to read them back from the database. | |
832 | ||
833 | The way to get around this problem is to use the Berkeley DB API method | |
834 | called C<seq>. This method allows sequential access to key/value | |
f6b705ef | 835 | pairs. See L<THE API INTERFACE> for details of both the C<seq> method |
836 | and the API in general. | |
88108326 | 837 | |
838 | Here is the script above rewritten using the C<seq> API method. | |
839 | ||
610ab055 | 840 | use strict ; |
88108326 | 841 | use DB_File ; |
88108326 | 842 | |
610ab055 PM |
843 | use vars qw($filename $x %h $status $key $value) ; |
844 | ||
88108326 | 845 | $filename = "tree" ; |
846 | unlink $filename ; | |
847 | ||
848 | # Enable duplicate records | |
849 | $DB_BTREE->{'flags'} = R_DUP ; | |
850 | ||
851 | $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
852 | or die "Cannot open $filename: $!\n"; | |
853 | ||
854 | # Add some key/value pairs to the file | |
855 | $h{'Wall'} = 'Larry' ; | |
856 | $h{'Wall'} = 'Brick' ; # Note the duplicate key | |
f6b705ef | 857 | $h{'Wall'} = 'Brick' ; # Note the duplicate key and value |
88108326 | 858 | $h{'Smith'} = 'John' ; |
859 | $h{'mouse'} = 'mickey' ; | |
860 | ||
f6b705ef | 861 | # iterate through the btree using seq |
88108326 | 862 | # and print each key/value pair. |
610ab055 | 863 | $key = $value = 0 ; |
f6b705ef | 864 | for ($status = $x->seq($key, $value, R_FIRST) ; |
865 | $status == 0 ; | |
866 | $status = $x->seq($key, $value, R_NEXT) ) | |
88108326 | 867 | { print "$key -> $value\n" } |
868 | ||
869 | undef $x ; | |
870 | untie %h ; | |
871 | ||
872 | that prints: | |
873 | ||
874 | Smith -> John | |
875 | Wall -> Brick | |
f6b705ef | 876 | Wall -> Brick |
88108326 | 877 | Wall -> Larry |
878 | mouse -> mickey | |
879 | ||
f6b705ef | 880 | This time we have got all the key/value pairs, including the multiple |
88108326 | 881 | values associated with the key C<Wall>. |
882 | ||
6ca2e664 PM |
883 | To make life easier when dealing with duplicate keys, B<DB_File> comes with |
884 | a few utility methods. | |
885 | ||
68dc0745 | 886 | =head2 The get_dup() Method |
f6b705ef | 887 | |
6ca2e664 | 888 | The C<get_dup> method assists in |
88108326 | 889 | reading duplicate values from BTREE databases. The method can take the |
890 | following forms: | |
891 | ||
892 | $count = $x->get_dup($key) ; | |
893 | @list = $x->get_dup($key) ; | |
894 | %list = $x->get_dup($key, 1) ; | |
895 | ||
896 | In a scalar context the method returns the number of values associated | |
897 | with the key, C<$key>. | |
898 | ||
899 | In list context, it returns all the values which match C<$key>. Note | |
f6b705ef | 900 | that the values will be returned in an apparently random order. |
88108326 | 901 | |
7a2e2cd6 | 902 | In list context, if the second parameter is present and evaluates |
903 | TRUE, the method returns an associative array. The keys of the | |
904 | associative array correspond to the values that matched in the BTREE | |
905 | and the values of the array are a count of the number of times that | |
906 | particular value occurred in the BTREE. | |
88108326 | 907 | |
f6b705ef | 908 | So assuming the database created above, we can use C<get_dup> like |
88108326 | 909 | this: |
910 | ||
2c2d71f5 JH |
911 | use strict ; |
912 | use DB_File ; | |
913 | ||
914 | use vars qw($filename $x %h ) ; | |
915 | ||
916 | $filename = "tree" ; | |
917 | ||
918 | # Enable duplicate records | |
919 | $DB_BTREE->{'flags'} = R_DUP ; | |
920 | ||
921 | $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
922 | or die "Cannot open $filename: $!\n"; | |
923 | ||
610ab055 | 924 | my $cnt = $x->get_dup("Wall") ; |
88108326 | 925 | print "Wall occurred $cnt times\n" ; |
926 | ||
610ab055 | 927 | my %hash = $x->get_dup("Wall", 1) ; |
88108326 | 928 | print "Larry is there\n" if $hash{'Larry'} ; |
f6b705ef | 929 | print "There are $hash{'Brick'} Brick Walls\n" ; |
88108326 | 930 | |
2c2d71f5 | 931 | my @list = sort $x->get_dup("Wall") ; |
88108326 | 932 | print "Wall => [@list]\n" ; |
933 | ||
f6b705ef | 934 | @list = $x->get_dup("Smith") ; |
88108326 | 935 | print "Smith => [@list]\n" ; |
936 | ||
f6b705ef | 937 | @list = $x->get_dup("Dog") ; |
88108326 | 938 | print "Dog => [@list]\n" ; |
939 | ||
940 | ||
941 | and it will print: | |
942 | ||
f6b705ef | 943 | Wall occurred 3 times |
88108326 | 944 | Larry is there |
f6b705ef | 945 | There are 2 Brick Walls |
946 | Wall => [Brick Brick Larry] | |
88108326 | 947 | Smith => [John] |
948 | Dog => [] | |
3b35bae3 | 949 | |
6ca2e664 PM |
950 | =head2 The find_dup() Method |
951 | ||
952 | $status = $X->find_dup($key, $value) ; | |
953 | ||
cb50131a | 954 | This method checks for the existence of a specific key/value pair. If the |
6ca2e664 PM |
955 | pair exists, the cursor is left pointing to the pair and the method |
956 | returns 0. Otherwise the method returns a non-zero value. | |
957 | ||
958 | Assuming the database from the previous example: | |
959 | ||
960 | use strict ; | |
961 | use DB_File ; | |
962 | ||
963 | use vars qw($filename $x %h $found) ; | |
964 | ||
965 | my $filename = "tree" ; | |
966 | ||
967 | # Enable duplicate records | |
968 | $DB_BTREE->{'flags'} = R_DUP ; | |
969 | ||
970 | $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
971 | or die "Cannot open $filename: $!\n"; | |
972 | ||
973 | $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ; | |
974 | print "Larry Wall is $found there\n" ; | |
975 | ||
976 | $found = ( $x->find_dup("Wall", "Harry") == 0 ? "" : "not") ; | |
977 | print "Harry Wall is $found there\n" ; | |
978 | ||
979 | undef $x ; | |
980 | untie %h ; | |
981 | ||
982 | prints this | |
983 | ||
2c2d71f5 | 984 | Larry Wall is there |
6ca2e664 PM |
985 | Harry Wall is not there |
986 | ||
987 | ||
988 | =head2 The del_dup() Method | |
989 | ||
990 | $status = $X->del_dup($key, $value) ; | |
991 | ||
992 | This method deletes a specific key/value pair. It returns | |
993 | 0 if they exist and have been deleted successfully. | |
994 | Otherwise the method returns a non-zero value. | |
995 | ||
cb50131a | 996 | Again assuming the existence of the C<tree> database |
6ca2e664 PM |
997 | |
998 | use strict ; | |
999 | use DB_File ; | |
1000 | ||
1001 | use vars qw($filename $x %h $found) ; | |
1002 | ||
1003 | my $filename = "tree" ; | |
1004 | ||
1005 | # Enable duplicate records | |
1006 | $DB_BTREE->{'flags'} = R_DUP ; | |
1007 | ||
1008 | $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
1009 | or die "Cannot open $filename: $!\n"; | |
1010 | ||
1011 | $x->del_dup("Wall", "Larry") ; | |
1012 | ||
1013 | $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ; | |
1014 | print "Larry Wall is $found there\n" ; | |
1015 | ||
1016 | undef $x ; | |
1017 | untie %h ; | |
1018 | ||
1019 | prints this | |
1020 | ||
1021 | Larry Wall is not there | |
1022 | ||
f6b705ef | 1023 | =head2 Matching Partial Keys |
1024 | ||
1025 | The BTREE interface has a feature which allows partial keys to be | |
1026 | matched. This functionality is I<only> available when the C<seq> method | |
1027 | is used along with the R_CURSOR flag. | |
1028 | ||
1029 | $x->seq($key, $value, R_CURSOR) ; | |
1030 | ||
1031 | Here is the relevant quote from the dbopen man page where it defines | |
1032 | the use of the R_CURSOR flag with seq: | |
1033 | ||
f6b705ef | 1034 | Note, for the DB_BTREE access method, the returned key is not |
1035 | necessarily an exact match for the specified key. The returned key | |
1036 | is the smallest key greater than or equal to the specified key, | |
1037 | permitting partial key matches and range searches. | |
1038 | ||
f6b705ef | 1039 | In the example script below, the C<match> sub uses this feature to find |
1040 | and print the first matching key/value pair given a partial key. | |
1041 | ||
610ab055 | 1042 | use strict ; |
f6b705ef | 1043 | use DB_File ; |
1044 | use Fcntl ; | |
610ab055 PM |
1045 | |
1046 | use vars qw($filename $x %h $st $key $value) ; | |
f6b705ef | 1047 | |
1048 | sub match | |
1049 | { | |
1050 | my $key = shift ; | |
610ab055 | 1051 | my $value = 0; |
f6b705ef | 1052 | my $orig_key = $key ; |
1053 | $x->seq($key, $value, R_CURSOR) ; | |
1054 | print "$orig_key\t-> $key\t-> $value\n" ; | |
1055 | } | |
1056 | ||
1057 | $filename = "tree" ; | |
1058 | unlink $filename ; | |
1059 | ||
1060 | $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE | |
1061 | or die "Cannot open $filename: $!\n"; | |
1062 | ||
1063 | # Add some key/value pairs to the file | |
1064 | $h{'mouse'} = 'mickey' ; | |
1065 | $h{'Wall'} = 'Larry' ; | |
1066 | $h{'Walls'} = 'Brick' ; | |
1067 | $h{'Smith'} = 'John' ; | |
1068 | ||
1069 | ||
610ab055 | 1070 | $key = $value = 0 ; |
f6b705ef | 1071 | print "IN ORDER\n" ; |
1072 | for ($st = $x->seq($key, $value, R_FIRST) ; | |
1073 | $st == 0 ; | |
1074 | $st = $x->seq($key, $value, R_NEXT) ) | |
1075 | ||
2c2d71f5 | 1076 | { print "$key -> $value\n" } |
f6b705ef | 1077 | |
1078 | print "\nPARTIAL MATCH\n" ; | |
1079 | ||
1080 | match "Wa" ; | |
1081 | match "A" ; | |
1082 | match "a" ; | |
1083 | ||
1084 | undef $x ; | |
1085 | untie %h ; | |
1086 | ||
1087 | Here is the output: | |
1088 | ||
1089 | IN ORDER | |
1090 | Smith -> John | |
1091 | Wall -> Larry | |
1092 | Walls -> Brick | |
1093 | mouse -> mickey | |
1094 | ||
1095 | PARTIAL MATCH | |
1096 | Wa -> Wall -> Larry | |
1097 | A -> Smith -> John | |
1098 | a -> mouse -> mickey | |
1099 | ||
1100 | =head1 DB_RECNO | |
1101 | ||
1102 | DB_RECNO provides an interface to flat text files. Both variable and | |
1103 | fixed length records are supported. | |
3b35bae3 | 1104 | |
6ca2e664 | 1105 | In order to make RECNO more compatible with Perl, the array offset for |
88108326 | 1106 | all RECNO arrays begins at 0 rather than 1 as in Berkeley DB. |
3b35bae3 | 1107 | |
88108326 | 1108 | As with normal Perl arrays, a RECNO array can be accessed using |
1109 | negative indexes. The index -1 refers to the last element of the array, | |
1110 | -2 the second last, and so on. Attempting to access an element before | |
1111 | the start of the array will raise a fatal run-time error. | |
3b35bae3 | 1112 | |
68dc0745 | 1113 | =head2 The 'bval' Option |
36477c24 | 1114 | |
1115 | The operation of the bval option warrants some discussion. Here is the | |
1116 | definition of bval from the Berkeley DB 1.85 recno manual page: | |
1117 | ||
1118 | The delimiting byte to be used to mark the end of a | |
1119 | record for variable-length records, and the pad charac- | |
1120 | ter for fixed-length records. If no value is speci- | |
1121 | fied, newlines (``\n'') are used to mark the end of | |
1122 | variable-length records and fixed-length records are | |
1123 | padded with spaces. | |
1124 | ||
1125 | The second sentence is wrong. In actual fact bval will only default to | |
1126 | C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL | |
1127 | openinfo parameter is used at all, the value that happens to be in bval | |
1128 | will be used. That means you always have to specify bval when making | |
1129 | use of any of the options in the openinfo parameter. This documentation | |
1130 | error will be fixed in the next release of Berkeley DB. | |
1131 | ||
1132 | That clarifies the situation with regards Berkeley DB itself. What | |
1133 | about B<DB_File>? Well, the behavior defined in the quote above is | |
6ca2e664 | 1134 | quite useful, so B<DB_File> conforms to it. |
36477c24 | 1135 | |
1136 | That means that you can specify other options (e.g. cachesize) and | |
1137 | still have bval default to C<"\n"> for variable length records, and | |
1138 | space for fixed length records. | |
1139 | ||
f6b705ef | 1140 | =head2 A Simple Example |
3b35bae3 | 1141 | |
6ca2e664 PM |
1142 | Here is a simple example that uses RECNO (if you are using a version |
1143 | of Perl earlier than 5.004_57 this example won't work -- see | |
1144 | L<Extra RECNO Methods> for a workaround). | |
f6b705ef | 1145 | |
610ab055 | 1146 | use strict ; |
f6b705ef | 1147 | use DB_File ; |
f6b705ef | 1148 | |
2c2d71f5 JH |
1149 | my $filename = "text" ; |
1150 | unlink $filename ; | |
1151 | ||
610ab055 | 1152 | my @h ; |
2c2d71f5 | 1153 | tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_RECNO |
f6b705ef | 1154 | or die "Cannot open file 'text': $!\n" ; |
1155 | ||
1156 | # Add a few key/value pairs to the file | |
1157 | $h[0] = "orange" ; | |
1158 | $h[1] = "blue" ; | |
1159 | $h[2] = "yellow" ; | |
1160 | ||
6ca2e664 PM |
1161 | push @h, "green", "black" ; |
1162 | ||
1163 | my $elements = scalar @h ; | |
1164 | print "The array contains $elements entries\n" ; | |
1165 | ||
1166 | my $last = pop @h ; | |
1167 | print "popped $last\n" ; | |
1168 | ||
1169 | unshift @h, "white" ; | |
1170 | my $first = shift @h ; | |
1171 | print "shifted $first\n" ; | |
1172 | ||
f6b705ef | 1173 | # Check for existence of a key |
1174 | print "Element 1 Exists with value $h[1]\n" if $h[1] ; | |
1175 | ||
1176 | # use a negative index | |
1177 | print "The last element is $h[-1]\n" ; | |
1178 | print "The 2nd last element is $h[-2]\n" ; | |
1179 | ||
1180 | untie @h ; | |
3b35bae3 | 1181 | |
f6b705ef | 1182 | Here is the output from the script: |
1183 | ||
6ca2e664 PM |
1184 | The array contains 5 entries |
1185 | popped black | |
2c2d71f5 | 1186 | shifted white |
f6b705ef | 1187 | Element 1 Exists with value blue |
6ca2e664 PM |
1188 | The last element is green |
1189 | The 2nd last element is yellow | |
f6b705ef | 1190 | |
6ca2e664 | 1191 | =head2 Extra RECNO Methods |
f6b705ef | 1192 | |
045291aa | 1193 | If you are using a version of Perl earlier than 5.004_57, the tied |
6ca2e664 PM |
1194 | array interface is quite limited. In the example script above |
1195 | C<push>, C<pop>, C<shift>, C<unshift> | |
1196 | or determining the array length will not work with a tied array. | |
045291aa PM |
1197 | |
1198 | To make the interface more useful for older versions of Perl, a number | |
1199 | of methods are supplied with B<DB_File> to simulate the missing array | |
1200 | operations. All these methods are accessed via the object returned from | |
1201 | the tie call. | |
f6b705ef | 1202 | |
1203 | Here are the methods: | |
1204 | ||
1205 | =over 5 | |
3b35bae3 | 1206 | |
f6b705ef | 1207 | =item B<$X-E<gt>push(list) ;> |
1208 | ||
1209 | Pushes the elements of C<list> to the end of the array. | |
1210 | ||
1211 | =item B<$value = $X-E<gt>pop ;> | |
1212 | ||
1213 | Removes and returns the last element of the array. | |
1214 | ||
1215 | =item B<$X-E<gt>shift> | |
1216 | ||
1217 | Removes and returns the first element of the array. | |
1218 | ||
1219 | =item B<$X-E<gt>unshift(list) ;> | |
1220 | ||
1221 | Pushes the elements of C<list> to the start of the array. | |
1222 | ||
1223 | =item B<$X-E<gt>length> | |
1224 | ||
1225 | Returns the number of elements in the array. | |
1226 | ||
1227 | =back | |
1228 | ||
1229 | =head2 Another Example | |
1230 | ||
1231 | Here is a more complete example that makes use of some of the methods | |
1232 | described above. It also makes use of the API interface directly (see | |
1233 | L<THE API INTERFACE>). | |
1234 | ||
1235 | use strict ; | |
1236 | use vars qw(@h $H $file $i) ; | |
1237 | use DB_File ; | |
1238 | use Fcntl ; | |
1239 | ||
1240 | $file = "text" ; | |
1241 | ||
1242 | unlink $file ; | |
1243 | ||
1244 | $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO | |
1245 | or die "Cannot open file $file: $!\n" ; | |
1246 | ||
1247 | # first create a text file to play with | |
1248 | $h[0] = "zero" ; | |
1249 | $h[1] = "one" ; | |
1250 | $h[2] = "two" ; | |
1251 | $h[3] = "three" ; | |
1252 | $h[4] = "four" ; | |
1253 | ||
1254 | ||
1255 | # Print the records in order. | |
1256 | # | |
1257 | # The length method is needed here because evaluating a tied | |
1258 | # array in a scalar context does not return the number of | |
1259 | # elements in the array. | |
1260 | ||
1261 | print "\nORIGINAL\n" ; | |
1262 | foreach $i (0 .. $H->length - 1) { | |
1263 | print "$i: $h[$i]\n" ; | |
1264 | } | |
1265 | ||
1266 | # use the push & pop methods | |
1267 | $a = $H->pop ; | |
1268 | $H->push("last") ; | |
1269 | print "\nThe last record was [$a]\n" ; | |
1270 | ||
1271 | # and the shift & unshift methods | |
1272 | $a = $H->shift ; | |
1273 | $H->unshift("first") ; | |
1274 | print "The first record was [$a]\n" ; | |
1275 | ||
1276 | # Use the API to add a new record after record 2. | |
1277 | $i = 2 ; | |
1278 | $H->put($i, "Newbie", R_IAFTER) ; | |
1279 | ||
1280 | # and a new record before record 1. | |
1281 | $i = 1 ; | |
1282 | $H->put($i, "New One", R_IBEFORE) ; | |
1283 | ||
1284 | # delete record 3 | |
1285 | $H->del(3) ; | |
1286 | ||
1287 | # now print the records in reverse order | |
1288 | print "\nREVERSE\n" ; | |
1289 | for ($i = $H->length - 1 ; $i >= 0 ; -- $i) | |
1290 | { print "$i: $h[$i]\n" } | |
1291 | ||
1292 | # same again, but use the API functions instead | |
1293 | print "\nREVERSE again\n" ; | |
610ab055 | 1294 | my ($s, $k, $v) = (0, 0, 0) ; |
f6b705ef | 1295 | for ($s = $H->seq($k, $v, R_LAST) ; |
1296 | $s == 0 ; | |
1297 | $s = $H->seq($k, $v, R_PREV)) | |
1298 | { print "$k: $v\n" } | |
1299 | ||
1300 | undef $H ; | |
1301 | untie @h ; | |
1302 | ||
1303 | and this is what it outputs: | |
1304 | ||
1305 | ORIGINAL | |
1306 | 0: zero | |
1307 | 1: one | |
1308 | 2: two | |
1309 | 3: three | |
1310 | 4: four | |
1311 | ||
1312 | The last record was [four] | |
1313 | The first record was [zero] | |
1314 | ||
1315 | REVERSE | |
1316 | 5: last | |
1317 | 4: three | |
1318 | 3: Newbie | |
1319 | 2: one | |
1320 | 1: New One | |
1321 | 0: first | |
1322 | ||
1323 | REVERSE again | |
1324 | 5: last | |
1325 | 4: three | |
1326 | 3: Newbie | |
1327 | 2: one | |
1328 | 1: New One | |
1329 | 0: first | |
1330 | ||
1331 | Notes: | |
1332 | ||
1333 | =over 5 | |
1334 | ||
1335 | =item 1. | |
1336 | ||
1337 | Rather than iterating through the array, C<@h> like this: | |
1338 | ||
1339 | foreach $i (@h) | |
1340 | ||
1341 | it is necessary to use either this: | |
1342 | ||
1343 | foreach $i (0 .. $H->length - 1) | |
1344 | ||
1345 | or this: | |
1346 | ||
1347 | for ($a = $H->get($k, $v, R_FIRST) ; | |
1348 | $a == 0 ; | |
1349 | $a = $H->get($k, $v, R_NEXT) ) | |
1350 | ||
1351 | =item 2. | |
1352 | ||
1353 | Notice that both times the C<put> method was used the record index was | |
1354 | specified using a variable, C<$i>, rather than the literal value | |
1355 | itself. This is because C<put> will return the record number of the | |
1356 | inserted line via that parameter. | |
1357 | ||
1358 | =back | |
1359 | ||
1360 | =head1 THE API INTERFACE | |
3b35bae3 AD |
1361 | |
1362 | As well as accessing Berkeley DB using a tied hash or array, it is also | |
88108326 | 1363 | possible to make direct use of most of the API functions defined in the |
8e07c86e | 1364 | Berkeley DB documentation. |
3b35bae3 | 1365 | |
88108326 | 1366 | To do this you need to store a copy of the object returned from the tie. |
3b35bae3 | 1367 | |
88108326 | 1368 | $db = tie %hash, "DB_File", "filename" ; |
3b35bae3 | 1369 | |
8e07c86e | 1370 | Once you have done that, you can access the Berkeley DB API functions |
88108326 | 1371 | as B<DB_File> methods directly like this: |
3b35bae3 AD |
1372 | |
1373 | $db->put($key, $value, R_NOOVERWRITE) ; | |
1374 | ||
88108326 | 1375 | B<Important:> If you have saved a copy of the object returned from |
1376 | C<tie>, the underlying database file will I<not> be closed until both | |
1377 | the tied variable is untied and all copies of the saved object are | |
610ab055 | 1378 | destroyed. |
88108326 | 1379 | |
1380 | use DB_File ; | |
1381 | $db = tie %hash, "DB_File", "filename" | |
1382 | or die "Cannot tie filename: $!" ; | |
1383 | ... | |
1384 | undef $db ; | |
1385 | untie %hash ; | |
1386 | ||
9a2c4ce3 | 1387 | See L<The untie() Gotcha> for more details. |
778183f3 | 1388 | |
88108326 | 1389 | All the functions defined in L<dbopen> are available except for |
1390 | close() and dbopen() itself. The B<DB_File> method interface to the | |
1391 | supported functions have been implemented to mirror the way Berkeley DB | |
1392 | works whenever possible. In particular note that: | |
1393 | ||
1394 | =over 5 | |
1395 | ||
1396 | =item * | |
1397 | ||
1398 | The methods return a status value. All return 0 on success. | |
1399 | All return -1 to signify an error and set C<$!> to the exact | |
1400 | error code. The return code 1 generally (but not always) means that the | |
1401 | key specified did not exist in the database. | |
1402 | ||
1403 | Other return codes are defined. See below and in the Berkeley DB | |
1404 | documentation for details. The Berkeley DB documentation should be used | |
1405 | as the definitive source. | |
1406 | ||
1407 | =item * | |
3b35bae3 | 1408 | |
88108326 | 1409 | Whenever a Berkeley DB function returns data via one of its parameters, |
1410 | the equivalent B<DB_File> method does exactly the same. | |
3b35bae3 | 1411 | |
88108326 | 1412 | =item * |
1413 | ||
1414 | If you are careful, it is possible to mix API calls with the tied | |
1415 | hash/array interface in the same piece of code. Although only a few of | |
1416 | the methods used to implement the tied interface currently make use of | |
1417 | the cursor, you should always assume that the cursor has been changed | |
1418 | any time the tied hash/array interface is used. As an example, this | |
1419 | code will probably not do what you expect: | |
1420 | ||
1421 | $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE | |
1422 | or die "Cannot tie $filename: $!" ; | |
1423 | ||
1424 | # Get the first key/value pair and set the cursor | |
1425 | $X->seq($key, $value, R_FIRST) ; | |
1426 | ||
1427 | # this line will modify the cursor | |
1428 | $count = scalar keys %x ; | |
1429 | ||
1430 | # Get the second key/value pair. | |
1431 | # oops, it didn't, it got the last key/value pair! | |
1432 | $X->seq($key, $value, R_NEXT) ; | |
1433 | ||
1434 | The code above can be rearranged to get around the problem, like this: | |
1435 | ||
1436 | $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE | |
1437 | or die "Cannot tie $filename: $!" ; | |
1438 | ||
1439 | # this line will modify the cursor | |
1440 | $count = scalar keys %x ; | |
1441 | ||
1442 | # Get the first key/value pair and set the cursor | |
1443 | $X->seq($key, $value, R_FIRST) ; | |
1444 | ||
1445 | # Get the second key/value pair. | |
1446 | # worked this time. | |
1447 | $X->seq($key, $value, R_NEXT) ; | |
1448 | ||
1449 | =back | |
1450 | ||
1451 | All the constants defined in L<dbopen> for use in the flags parameters | |
1452 | in the methods defined below are also available. Refer to the Berkeley | |
1453 | DB documentation for the precise meaning of the flags values. | |
1454 | ||
1455 | Below is a list of the methods available. | |
3b35bae3 AD |
1456 | |
1457 | =over 5 | |
1458 | ||
f6b705ef | 1459 | =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;> |
88108326 | 1460 | |
1461 | Given a key (C<$key>) this method reads the value associated with it | |
1462 | from the database. The value read from the database is returned in the | |
1463 | C<$value> parameter. | |
3b35bae3 | 1464 | |
88108326 | 1465 | If the key does not exist the method returns 1. |
3b35bae3 | 1466 | |
88108326 | 1467 | No flags are currently defined for this method. |
3b35bae3 | 1468 | |
f6b705ef | 1469 | =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;> |
3b35bae3 | 1470 | |
88108326 | 1471 | Stores the key/value pair in the database. |
1472 | ||
1473 | If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter | |
8e07c86e | 1474 | will have the record number of the inserted key/value pair set. |
3b35bae3 | 1475 | |
88108326 | 1476 | Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and |
1477 | R_SETCURSOR. | |
1478 | ||
f6b705ef | 1479 | =item B<$status = $X-E<gt>del($key [, $flags]) ;> |
3b35bae3 | 1480 | |
88108326 | 1481 | Removes all key/value pairs with key C<$key> from the database. |
3b35bae3 | 1482 | |
88108326 | 1483 | A return code of 1 means that the requested key was not in the |
1484 | database. | |
3b35bae3 | 1485 | |
88108326 | 1486 | R_CURSOR is the only valid flag at present. |
3b35bae3 | 1487 | |
f6b705ef | 1488 | =item B<$status = $X-E<gt>fd ;> |
3b35bae3 | 1489 | |
88108326 | 1490 | Returns the file descriptor for the underlying database. |
3b35bae3 | 1491 | |
cb50131a CB |
1492 | See L<Locking: The Trouble with fd> for an explanation for why you should |
1493 | not use C<fd> to lock your database. | |
3b35bae3 | 1494 | |
f6b705ef | 1495 | =item B<$status = $X-E<gt>seq($key, $value, $flags) ;> |
3b35bae3 | 1496 | |
88108326 | 1497 | This interface allows sequential retrieval from the database. See |
1498 | L<dbopen> for full details. | |
1499 | ||
1500 | Both the C<$key> and C<$value> parameters will be set to the key/value | |
1501 | pair read from the database. | |
1502 | ||
1503 | The flags parameter is mandatory. The valid flag values are R_CURSOR, | |
1504 | R_FIRST, R_LAST, R_NEXT and R_PREV. | |
1505 | ||
f6b705ef | 1506 | =item B<$status = $X-E<gt>sync([$flags]) ;> |
88108326 | 1507 | |
1508 | Flushes any cached buffers to disk. | |
1509 | ||
1510 | R_RECNOSYNC is the only valid flag at present. | |
3b35bae3 AD |
1511 | |
1512 | =back | |
1513 | ||
cad2e5aa JH |
1514 | =head1 DBM FILTERS |
1515 | ||
1516 | A DBM Filter is a piece of code that is be used when you I<always> | |
1517 | want to make the same transformation to all keys and/or values in a | |
1518 | DBM database. | |
1519 | ||
1520 | There are four methods associated with DBM Filters. All work identically, | |
1521 | and each is used to install (or uninstall) a single DBM Filter. Each | |
1522 | expects a single parameter, namely a reference to a sub. The only | |
1523 | difference between them is the place that the filter is installed. | |
1524 | ||
1525 | To summarise: | |
1526 | ||
1527 | =over 5 | |
1528 | ||
1529 | =item B<filter_store_key> | |
1530 | ||
1531 | If a filter has been installed with this method, it will be invoked | |
1532 | every time you write a key to a DBM database. | |
1533 | ||
1534 | =item B<filter_store_value> | |
1535 | ||
1536 | If a filter has been installed with this method, it will be invoked | |
1537 | every time you write a value to a DBM database. | |
1538 | ||
1539 | ||
1540 | =item B<filter_fetch_key> | |
1541 | ||
1542 | If a filter has been installed with this method, it will be invoked | |
1543 | every time you read a key from a DBM database. | |
1544 | ||
1545 | =item B<filter_fetch_value> | |
1546 | ||
1547 | If a filter has been installed with this method, it will be invoked | |
1548 | every time you read a value from a DBM database. | |
1549 | ||
1550 | =back | |
1551 | ||
1552 | You can use any combination of the methods, from none, to all four. | |
1553 | ||
1554 | All filter methods return the existing filter, if present, or C<undef> | |
1555 | in not. | |
1556 | ||
1557 | To delete a filter pass C<undef> to it. | |
1558 | ||
1559 | =head2 The Filter | |
1560 | ||
1561 | When each filter is called by Perl, a local copy of C<$_> will contain | |
1562 | the key or value to be filtered. Filtering is achieved by modifying | |
1563 | the contents of C<$_>. The return code from the filter is ignored. | |
1564 | ||
1565 | =head2 An Example -- the NULL termination problem. | |
1566 | ||
1567 | Consider the following scenario. You have a DBM database | |
1568 | that you need to share with a third-party C application. The C application | |
1569 | assumes that I<all> keys and values are NULL terminated. Unfortunately | |
1570 | when Perl writes to DBM databases it doesn't use NULL termination, so | |
1571 | your Perl application will have to manage NULL termination itself. When | |
1572 | you write to the database you will have to use something like this: | |
1573 | ||
1574 | $hash{"$key\0"} = "$value\0" ; | |
1575 | ||
1576 | Similarly the NULL needs to be taken into account when you are considering | |
1577 | the length of existing keys/values. | |
1578 | ||
1579 | It would be much better if you could ignore the NULL terminations issue | |
1580 | in the main application code and have a mechanism that automatically | |
1581 | added the terminating NULL to all keys and values whenever you write to | |
1582 | the database and have them removed when you read from the database. As I'm | |
1583 | sure you have already guessed, this is a problem that DBM Filters can | |
1584 | fix very easily. | |
1585 | ||
1586 | use strict ; | |
1587 | use DB_File ; | |
1588 | ||
1589 | my %hash ; | |
1590 | my $filename = "/tmp/filt" ; | |
1591 | unlink $filename ; | |
1592 | ||
1593 | my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH | |
1594 | or die "Cannot open $filename: $!\n" ; | |
1595 | ||
1596 | # Install DBM Filters | |
1597 | $db->filter_fetch_key ( sub { s/\0$// } ) ; | |
1598 | $db->filter_store_key ( sub { $_ .= "\0" } ) ; | |
1599 | $db->filter_fetch_value( sub { s/\0$// } ) ; | |
1600 | $db->filter_store_value( sub { $_ .= "\0" } ) ; | |
1601 | ||
1602 | $hash{"abc"} = "def" ; | |
1603 | my $a = $hash{"ABC"} ; | |
1604 | # ... | |
1605 | undef $db ; | |
1606 | untie %hash ; | |
1607 | ||
1608 | Hopefully the contents of each of the filters should be | |
1609 | self-explanatory. Both "fetch" filters remove the terminating NULL, | |
1610 | and both "store" filters add a terminating NULL. | |
1611 | ||
1612 | ||
1613 | =head2 Another Example -- Key is a C int. | |
1614 | ||
1615 | Here is another real-life example. By default, whenever Perl writes to | |
1616 | a DBM database it always writes the key and value as strings. So when | |
1617 | you use this: | |
1618 | ||
1619 | $hash{12345} = "soemthing" ; | |
1620 | ||
1621 | the key 12345 will get stored in the DBM database as the 5 byte string | |
1622 | "12345". If you actually want the key to be stored in the DBM database | |
1623 | as a C int, you will have to use C<pack> when writing, and C<unpack> | |
1624 | when reading. | |
1625 | ||
1626 | Here is a DBM Filter that does it: | |
1627 | ||
1628 | use strict ; | |
1629 | use DB_File ; | |
1630 | my %hash ; | |
1631 | my $filename = "/tmp/filt" ; | |
1632 | unlink $filename ; | |
1633 | ||
1634 | ||
1635 | my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH | |
1636 | or die "Cannot open $filename: $!\n" ; | |
1637 | ||
1638 | $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } ) ; | |
1639 | $db->filter_store_key ( sub { $_ = pack ("i", $_) } ) ; | |
1640 | $hash{123} = "def" ; | |
1641 | # ... | |
1642 | undef $db ; | |
1643 | untie %hash ; | |
1644 | ||
1645 | This time only two filters have been used -- we only need to manipulate | |
1646 | the contents of the key, so it wasn't necessary to install any value | |
1647 | filters. | |
1648 | ||
f6b705ef | 1649 | =head1 HINTS AND TIPS |
3b35bae3 | 1650 | |
3b35bae3 | 1651 | |
cb50131a | 1652 | =head2 Locking: The Trouble with fd |
3b35bae3 | 1653 | |
cb50131a CB |
1654 | Until version 1.72 of this module, the recommended technique for locking |
1655 | B<DB_File> databases was to flock the filehandle returned from the "fd" | |
1656 | function. Unfortunately this technique has been shown to be fundamentally | |
1657 | flawed (Kudos to David Harris for tracking this down). Use it at your own | |
1658 | peril! | |
3b35bae3 | 1659 | |
cb50131a | 1660 | The locking technique went like this. |
cb1a09d0 | 1661 | |
cb50131a CB |
1662 | $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644) |
1663 | || die "dbcreat /tmp/foo.db $!"; | |
1664 | $fd = $db->fd; | |
1665 | open(DB_FH, "+<&=$fd") || die "dup $!"; | |
1666 | flock (DB_FH, LOCK_EX) || die "flock: $!"; | |
1667 | ... | |
1668 | $db{"Tom"} = "Jerry" ; | |
1669 | ... | |
1670 | flock(DB_FH, LOCK_UN); | |
1671 | undef $db; | |
1672 | untie %db; | |
1673 | close(DB_FH); | |
cb1a09d0 | 1674 | |
cb50131a | 1675 | In simple terms, this is what happens: |
cb1a09d0 | 1676 | |
cb50131a | 1677 | =over 5 |
cb1a09d0 | 1678 | |
cb50131a | 1679 | =item 1. |
cb1a09d0 | 1680 | |
cb50131a | 1681 | Use "tie" to open the database. |
cb1a09d0 | 1682 | |
cb50131a | 1683 | =item 2. |
cb1a09d0 | 1684 | |
cb50131a | 1685 | Lock the database with fd & flock. |
cb1a09d0 | 1686 | |
cb50131a | 1687 | =item 3. |
cb1a09d0 | 1688 | |
cb50131a | 1689 | Read & Write to the database. |
cb1a09d0 | 1690 | |
cb50131a | 1691 | =item 4. |
cb1a09d0 | 1692 | |
cb50131a | 1693 | Unlock and close the database. |
cb1a09d0 | 1694 | |
cb50131a CB |
1695 | =back |
1696 | ||
1697 | Here is the crux of the problem. A side-effect of opening the B<DB_File> | |
1698 | database in step 2 is that an initial block from the database will get | |
1699 | read from disk and cached in memory. | |
1700 | ||
1701 | To see why this is a problem, consider what can happen when two processes, | |
1702 | say "A" and "B", both want to update the same B<DB_File> database | |
1703 | using the locking steps outlined above. Assume process "A" has already | |
1704 | opened the database and has a write lock, but it hasn't actually updated | |
1705 | the database yet (it has finished step 2, but not started step 3 yet). Now | |
1706 | process "B" tries to open the same database - step 1 will succeed, | |
1707 | but it will block on step 2 until process "A" releases the lock. The | |
1708 | important thing to notice here is that at this point in time both | |
1709 | processes will have cached identical initial blocks from the database. | |
1710 | ||
1711 | Now process "A" updates the database and happens to change some of the | |
1712 | data held in the initial buffer. Process "A" terminates, flushing | |
1713 | all cached data to disk and releasing the database lock. At this point | |
1714 | the database on disk will correctly reflect the changes made by process | |
1715 | "A". | |
1716 | ||
1717 | With the lock released, process "B" can now continue. It also updates the | |
1718 | database and unfortunately it too modifies the data that was in its | |
1719 | initial buffer. Once that data gets flushed to disk it will overwrite | |
1720 | some/all of the changes process "A" made to the database. | |
1721 | ||
1722 | The result of this scenario is at best a database that doesn't contain | |
1723 | what you expect. At worst the database will corrupt. | |
1724 | ||
1725 | The above won't happen every time competing process update the same | |
1726 | B<DB_File> database, but it does illustrate why the technique should | |
1727 | not be used. | |
1728 | ||
1729 | =head2 Safe ways to lock a database | |
1730 | ||
1731 | Starting with version 2.x, Berkeley DB has internal support for locking. | |
1732 | The companion module to this one, B<BerkeleyDB>, provides an interface | |
1733 | to this locking functionality. If you are serious about locking | |
1734 | Berkeley DB databases, I strongly recommend using B<BerkeleyDB>. | |
1735 | ||
1736 | If using B<BerkeleyDB> isn't an option, there are a number of modules | |
1737 | available on CPAN that can be used to implement locking. Each one | |
1738 | implements locking differently and has different goals in mind. It is | |
1739 | therefore worth knowing the difference, so that you can pick the right | |
1740 | one for your application. Here are the three locking wrappers: | |
1741 | ||
1742 | =over 5 | |
1743 | ||
1744 | =item B<Tie::DB_Lock> | |
1745 | ||
1746 | A B<DB_File> wrapper which creates copies of the database file for | |
1747 | read access, so that you have a kind of a multiversioning concurrent read | |
1748 | system. However, updates are still serial. Use for databases where reads | |
1749 | may be lengthy and consistency problems may occur. | |
1750 | ||
1751 | =item B<Tie::DB_LockFile> | |
1752 | ||
1753 | A B<DB_File> wrapper that has the ability to lock and unlock the database | |
1754 | while it is being used. Avoids the tie-before-flock problem by simply | |
1755 | re-tie-ing the database when you get or drop a lock. Because of the | |
1756 | flexibility in dropping and re-acquiring the lock in the middle of a | |
1757 | session, this can be massaged into a system that will work with long | |
1758 | updates and/or reads if the application follows the hints in the POD | |
1759 | documentation. | |
1760 | ||
1761 | =item B<DB_File::Lock> | |
1762 | ||
1763 | An extremely lightweight B<DB_File> wrapper that simply flocks a lockfile | |
1764 | before tie-ing the database and drops the lock after the untie. Allows | |
1765 | one to use the same lockfile for multiple databases to avoid deadlock | |
1766 | problems, if desired. Use for databases where updates are reads are | |
1767 | quick and simple flock locking semantics are enough. | |
1768 | ||
1769 | =back | |
cb1a09d0 | 1770 | |
68dc0745 | 1771 | =head2 Sharing Databases With C Applications |
f6b705ef | 1772 | |
1773 | There is no technical reason why a Berkeley DB database cannot be | |
1774 | shared by both a Perl and a C application. | |
1775 | ||
1776 | The vast majority of problems that are reported in this area boil down | |
1777 | to the fact that C strings are NULL terminated, whilst Perl strings are | |
cad2e5aa | 1778 | not. See L<DBM FILTERS> for a generic way to work around this problem. |
f6b705ef | 1779 | |
1780 | Here is a real example. Netscape 2.0 keeps a record of the locations you | |
1781 | visit along with the time you last visited them in a DB_HASH database. | |
1782 | This is usually stored in the file F<~/.netscape/history.db>. The key | |
1783 | field in the database is the location string and the value field is the | |
1784 | time the location was last visited stored as a 4 byte binary value. | |
1785 | ||
1786 | If you haven't already guessed, the location string is stored with a | |
1787 | terminating NULL. This means you need to be careful when accessing the | |
1788 | database. | |
1789 | ||
1790 | Here is a snippet of code that is loosely based on Tom Christiansen's | |
1791 | I<ggh> script (available from your nearest CPAN archive in | |
1792 | F<authors/id/TOMC/scripts/nshist.gz>). | |
1793 | ||
610ab055 | 1794 | use strict ; |
f6b705ef | 1795 | use DB_File ; |
1796 | use Fcntl ; | |
f6b705ef | 1797 | |
610ab055 | 1798 | use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ; |
f6b705ef | 1799 | $dotdir = $ENV{HOME} || $ENV{LOGNAME}; |
1800 | ||
1801 | $HISTORY = "$dotdir/.netscape/history.db"; | |
1802 | ||
1803 | tie %hist_db, 'DB_File', $HISTORY | |
1804 | or die "Cannot open $HISTORY: $!\n" ;; | |
1805 | ||
1806 | # Dump the complete database | |
1807 | while ( ($href, $binary_time) = each %hist_db ) { | |
1808 | ||
1809 | # remove the terminating NULL | |
1810 | $href =~ s/\x00$// ; | |
1811 | ||
1812 | # convert the binary time into a user friendly string | |
1813 | $date = localtime unpack("V", $binary_time); | |
1814 | print "$date $href\n" ; | |
1815 | } | |
1816 | ||
1817 | # check for the existence of a specific key | |
1818 | # remember to add the NULL | |
1819 | if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) { | |
1820 | $date = localtime unpack("V", $binary_time) ; | |
1821 | print "Last visited mox.perl.com on $date\n" ; | |
1822 | } | |
1823 | else { | |
1824 | print "Never visited mox.perl.com\n" | |
1825 | } | |
1826 | ||
1827 | untie %hist_db ; | |
1828 | ||
68dc0745 | 1829 | =head2 The untie() Gotcha |
778183f3 | 1830 | |
7a2e2cd6 | 1831 | If you make use of the Berkeley DB API, it is I<very> strongly |
68dc0745 | 1832 | recommended that you read L<perltie/The untie Gotcha>. |
778183f3 PM |
1833 | |
1834 | Even if you don't currently make use of the API interface, it is still | |
1835 | worth reading it. | |
1836 | ||
1837 | Here is an example which illustrates the problem from a B<DB_File> | |
1838 | perspective: | |
1839 | ||
1840 | use DB_File ; | |
1841 | use Fcntl ; | |
1842 | ||
1843 | my %x ; | |
1844 | my $X ; | |
1845 | ||
1846 | $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC | |
1847 | or die "Cannot tie first time: $!" ; | |
1848 | ||
1849 | $x{123} = 456 ; | |
1850 | ||
1851 | untie %x ; | |
1852 | ||
1853 | tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT | |
1854 | or die "Cannot tie second time: $!" ; | |
1855 | ||
1856 | untie %x ; | |
1857 | ||
1858 | When run, the script will produce this error message: | |
1859 | ||
1860 | Cannot tie second time: Invalid argument at bad.file line 14. | |
1861 | ||
1862 | Although the error message above refers to the second tie() statement | |
1863 | in the script, the source of the problem is really with the untie() | |
1864 | statement that precedes it. | |
1865 | ||
1866 | Having read L<perltie> you will probably have already guessed that the | |
1867 | error is caused by the extra copy of the tied object stored in C<$X>. | |
1868 | If you haven't, then the problem boils down to the fact that the | |
1869 | B<DB_File> destructor, DESTROY, will not be called until I<all> | |
1870 | references to the tied object are destroyed. Both the tied variable, | |
1871 | C<%x>, and C<$X> above hold a reference to the object. The call to | |
1872 | untie() will destroy the first, but C<$X> still holds a valid | |
1873 | reference, so the destructor will not get called and the database file | |
1874 | F<tst.fil> will remain open. The fact that Berkeley DB then reports the | |
cb50131a | 1875 | attempt to open a database that is already open via the catch-all |
778183f3 PM |
1876 | "Invalid argument" doesn't help. |
1877 | ||
1878 | If you run the script with the C<-w> flag the error message becomes: | |
1879 | ||
1880 | untie attempted while 1 inner references still exist at bad.file line 12. | |
1881 | Cannot tie second time: Invalid argument at bad.file line 14. | |
1882 | ||
1883 | which pinpoints the real problem. Finally the script can now be | |
1884 | modified to fix the original problem by destroying the API object | |
1885 | before the untie: | |
1886 | ||
1887 | ... | |
1888 | $x{123} = 456 ; | |
1889 | ||
1890 | undef $X ; | |
1891 | untie %x ; | |
1892 | ||
1893 | $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT | |
1894 | ... | |
1895 | ||
f6b705ef | 1896 | |
1897 | =head1 COMMON QUESTIONS | |
1898 | ||
1899 | =head2 Why is there Perl source in my database? | |
1900 | ||
1901 | If you look at the contents of a database file created by DB_File, | |
1902 | there can sometimes be part of a Perl script included in it. | |
1903 | ||
1904 | This happens because Berkeley DB uses dynamic memory to allocate | |
1905 | buffers which will subsequently be written to the database file. Being | |
1906 | dynamic, the memory could have been used for anything before DB | |
1907 | malloced it. As Berkeley DB doesn't clear the memory once it has been | |
1908 | allocated, the unused portions will contain random junk. In the case | |
1909 | where a Perl script gets written to the database, the random junk will | |
1910 | correspond to an area of dynamic memory that happened to be used during | |
1911 | the compilation of the script. | |
1912 | ||
1913 | Unless you don't like the possibility of there being part of your Perl | |
1914 | scripts embedded in a database file, this is nothing to worry about. | |
1915 | ||
1916 | =head2 How do I store complex data structures with DB_File? | |
1917 | ||
1918 | Although B<DB_File> cannot do this directly, there is a module which | |
1919 | can layer transparently over B<DB_File> to accomplish this feat. | |
1920 | ||
1921 | Check out the MLDBM module, available on CPAN in the directory | |
1922 | F<modules/by-module/MLDBM>. | |
1923 | ||
1924 | =head2 What does "Invalid Argument" mean? | |
1925 | ||
1926 | You will get this error message when one of the parameters in the | |
1927 | C<tie> call is wrong. Unfortunately there are quite a few parameters to | |
1928 | get wrong, so it can be difficult to figure out which one it is. | |
1929 | ||
1930 | Here are a couple of possibilities: | |
1931 | ||
1932 | =over 5 | |
1933 | ||
1934 | =item 1. | |
1935 | ||
610ab055 | 1936 | Attempting to reopen a database without closing it. |
f6b705ef | 1937 | |
1938 | =item 2. | |
1939 | ||
1940 | Using the O_WRONLY flag. | |
1941 | ||
1942 | =back | |
1943 | ||
1944 | =head2 What does "Bareword 'DB_File' not allowed" mean? | |
1945 | ||
1946 | You will encounter this particular error message when you have the | |
1947 | C<strict 'subs'> pragma (or the full strict pragma) in your script. | |
1948 | Consider this script: | |
1949 | ||
1950 | use strict ; | |
1951 | use DB_File ; | |
1952 | use vars qw(%x) ; | |
1953 | tie %x, DB_File, "filename" ; | |
1954 | ||
1955 | Running it produces the error in question: | |
1956 | ||
1957 | Bareword "DB_File" not allowed while "strict subs" in use | |
1958 | ||
1959 | To get around the error, place the word C<DB_File> in either single or | |
1960 | double quotes, like this: | |
1961 | ||
1962 | tie %x, "DB_File", "filename" ; | |
1963 | ||
1964 | Although it might seem like a real pain, it is really worth the effort | |
1965 | of having a C<use strict> in all your scripts. | |
1966 | ||
cad2e5aa JH |
1967 | =head1 REFERENCES |
1968 | ||
1969 | Articles that are either about B<DB_File> or make use of it. | |
1970 | ||
1971 | =over 5 | |
1972 | ||
1973 | =item 1. | |
1974 | ||
1975 | I<Full-Text Searching in Perl>, Tim Kientzle (tkientzle@ddj.com), | |
1976 | Dr. Dobb's Journal, Issue 295, January 1999, pp 34-41 | |
1977 | ||
1978 | =back | |
1979 | ||
cb1a09d0 AD |
1980 | =head1 HISTORY |
1981 | ||
1f70e1ea | 1982 | Moved to the Changes file. |
610ab055 | 1983 | |
1f70e1ea | 1984 | =head1 BUGS |
05475680 | 1985 | |
1f70e1ea PM |
1986 | Some older versions of Berkeley DB had problems with fixed length |
1987 | records using the RECNO file format. This problem has been fixed since | |
1988 | version 1.85 of Berkeley DB. | |
e858de61 | 1989 | |
1f70e1ea PM |
1990 | I am sure there are bugs in the code. If you do find any, or can |
1991 | suggest any enhancements, I would welcome your comments. | |
a6ed719b | 1992 | |
1f70e1ea | 1993 | =head1 AVAILABILITY |
a6ed719b | 1994 | |
1f70e1ea PM |
1995 | B<DB_File> comes with the standard Perl source distribution. Look in |
1996 | the directory F<ext/DB_File>. Given the amount of time between releases | |
1997 | of Perl the version that ships with Perl is quite likely to be out of | |
1998 | date, so the most recent version can always be found on CPAN (see | |
1999 | L<perlmod/CPAN> for details), in the directory | |
2000 | F<modules/by-module/DB_File>. | |
a6ed719b | 2001 | |
c529f79d CB |
2002 | This version of B<DB_File> will work with either version 1.x, 2.x or |
2003 | 3.x of Berkeley DB, but is limited to the functionality provided by | |
2004 | version 1. | |
a6ed719b | 2005 | |
cad2e5aa | 2006 | The official web site for Berkeley DB is F<http://www.sleepycat.com>. |
c529f79d | 2007 | All versions of Berkeley DB are available there. |
93af7a87 | 2008 | |
1f70e1ea PM |
2009 | Alternatively, Berkeley DB version 1 is available at your nearest CPAN |
2010 | archive in F<src/misc/db.1.85.tar.gz>. | |
e858de61 | 2011 | |
1f70e1ea PM |
2012 | If you are running IRIX, then get Berkeley DB version 1 from |
2013 | F<http://reality.sgi.com/ariel>. It has the patches necessary to | |
2014 | compile properly on IRIX 5.3. | |
610ab055 | 2015 | |
1f70e1ea | 2016 | =head1 COPYRIGHT |
3b35bae3 | 2017 | |
cad2e5aa | 2018 | Copyright (c) 1995-1999 Paul Marquess. All rights reserved. This program |
a9fd575d PM |
2019 | is free software; you can redistribute it and/or modify it under the |
2020 | same terms as Perl itself. | |
3b35bae3 | 2021 | |
1f70e1ea PM |
2022 | Although B<DB_File> is covered by the Perl license, the library it |
2023 | makes use of, namely Berkeley DB, is not. Berkeley DB has its own | |
2024 | copyright and its own license. Please take the time to read it. | |
3b35bae3 | 2025 | |
a9fd575d | 2026 | Here are are few words taken from the Berkeley DB FAQ (at |
cb50131a | 2027 | F<http://www.sleepycat.com>) regarding the license: |
68dc0745 | 2028 | |
a9fd575d | 2029 | Do I have to license DB to use it in Perl scripts? |
3b35bae3 | 2030 | |
a9fd575d PM |
2031 | No. The Berkeley DB license requires that software that uses |
2032 | Berkeley DB be freely redistributable. In the case of Perl, that | |
2033 | software is Perl, and not your scripts. Any Perl scripts that you | |
2034 | write are your property, including scripts that make use of | |
2035 | Berkeley DB. Neither the Perl license nor the Berkeley DB license | |
2036 | place any restriction on what you may do with them. | |
88108326 | 2037 | |
1f70e1ea PM |
2038 | If you are in any doubt about the license situation, contact either the |
2039 | Berkeley DB authors or the author of DB_File. See L<"AUTHOR"> for details. | |
a0b8c8c1 PM |
2040 | |
2041 | ||
3b35bae3 AD |
2042 | =head1 SEE ALSO |
2043 | ||
9fe6733a PM |
2044 | L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>, |
2045 | L<dbmfilter> | |
3b35bae3 | 2046 | |
3b35bae3 AD |
2047 | =head1 AUTHOR |
2048 | ||
8e07c86e | 2049 | The DB_File interface was written by Paul Marquess |
6ca2e664 | 2050 | E<lt>Paul.Marquess@btinternet.comE<gt>. |
d3ef3b8a PM |
2051 | Questions about the DB system itself may be addressed to |
2052 | E<lt>db@sleepycat.com<gt>. | |
3b35bae3 AD |
2053 | |
2054 | =cut |