1 # DB_File.pm -- Perl 5 interface to Berkeley DB
3 # written by Paul Marquess (Paul.Marquess@btinternet.com)
4 # last modified 26th April 2001
7 # Copyright (c) 1995-2001 Paul Marquess. All rights reserved.
8 # This program is free software; you can redistribute it and/or
9 # modify it under the same terms as Perl itself.
12 package DB_File::HASHINFO ;
20 @DB_File::HASHINFO::ISA = qw(Tie::Hash);
35 bless { VALID => { map {$_, 1}
36 qw( bsize ffactor nelem cachesize hash lorder)
48 return $self->{GOT}{$key} if exists $self->{VALID}{$key} ;
51 croak "${pkg}::FETCH - Unknown element '$key'" ;
61 if ( exists $self->{VALID}{$key} )
63 $self->{GOT}{$key} = $value ;
68 croak "${pkg}::STORE - Unknown element '$key'" ;
76 if ( exists $self->{VALID}{$key} )
78 delete $self->{GOT}{$key} ;
83 croak "DB_File::HASHINFO::DELETE - Unknown element '$key'" ;
91 exists $self->{VALID}{$key} ;
99 croak ref($self) . " does not define the method ${method}" ;
102 sub FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") }
103 sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") }
104 sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") }
106 package DB_File::RECNOINFO ;
111 @DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
117 bless { VALID => { map {$_, 1}
118 qw( bval cachesize psize flags lorder reclen bfname )
124 package DB_File::BTREEINFO ;
129 @DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
135 bless { VALID => { map {$_, 1}
136 qw( flags cachesize maxkeypage minkeypage psize
137 compare prefix lorder )
148 use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO
149 $db_version $use_XSLoader
156 #typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
157 $DB_BTREE = new DB_File::BTREEINFO ;
158 $DB_HASH = new DB_File::HASHINFO ;
159 $DB_RECNO = new DB_File::RECNOINFO ;
166 eval { require XSLoader } ;
171 @ISA = qw(DynaLoader);
175 push @ISA, qw(Tie::Hash Exporter);
177 $DB_BTREE $DB_HASH $DB_RECNO
212 ($constname = $AUTOLOAD) =~ s/.*:://;
213 my $val = constant($constname, @_ ? $_[0] : 0);
215 if ($! =~ /Invalid/ || $!{EINVAL}) {
216 $AutoLoader::AUTOLOAD = $AUTOLOAD;
217 goto &AutoLoader::AUTOLOAD;
220 my($pack,$file,$line) = caller;
221 croak "Your vendor has not defined DB macro $constname, used at $file line $line.
225 eval "sub $AUTOLOAD { $val }";
231 # Make all Fcntl O_XXX constants available for importing
233 my @O = grep /^O_/, @Fcntl::EXPORT;
234 Fcntl->import(@O); # first we import what we want to export
239 { XSLoader::load("DB_File", $VERSION)}
241 { bootstrap DB_File $VERSION }
243 # Preloaded methods go here. Autoload methods go after __END__, and are
244 # processed by the autosplit program.
246 sub tie_hash_or_array
249 my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ;
251 $arg[4] = tied %{ $arg[4] }
252 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
254 # make recno in Berkeley DB version 2 work like recno in version 1.
255 if ($db_version > 1 and defined $arg[4] and $arg[4] =~ /RECNO/ and
256 $arg[1] and ! -e $arg[1]) {
257 open(FH, ">$arg[1]") or return undef ;
259 chmod $arg[3] ? $arg[3] : 0666 , $arg[1] ;
262 DoTie_($tieHASH, @arg) ;
267 tie_hash_or_array(@_) ;
272 tie_hash_or_array(@_) ;
280 my $status = $self->seq($key, $value, R_FIRST());
283 while ($status == 0) {
285 $status = $self->seq($key, $value, R_NEXT());
287 foreach $key (reverse @keys) {
288 my $s = $self->del($key);
298 my $current_length = $self->length() ;
300 if ($length < $current_length) {
302 for ($key = $current_length - 1 ; $key >= $length ; -- $key)
305 elsif ($length > $current_length) {
306 $self->put($length-1, "") ;
315 if (not defined $offset) {
316 carp 'Use of uninitialized value in splice';
320 my $length = @_ ? shift : 0;
321 # Carping about definedness comes _after_ the OFFSET sanity check.
322 # This is so we get the same error messages as Perl's splice().
327 my $size = $self->FETCHSIZE();
329 # 'If OFFSET is negative then it start that far from the end of
333 my $new_offset = $size + $offset;
334 if ($new_offset < 0) {
335 die "Modification of non-creatable array value attempted, "
336 . "subscript $offset";
338 $offset = $new_offset;
341 if ($offset > $size) {
345 if (not defined $length) {
346 carp 'Use of uninitialized value in splice';
350 # 'If LENGTH is omitted, removes everything from OFFSET onward.'
351 if (not defined $length) {
352 $length = $size - $offset;
355 # 'If LENGTH is negative, leave that many elements off the end of
359 $length = $size - $offset + $length;
362 # The user must have specified a length bigger than the
363 # length of the array passed in. But perl's splice()
364 # doesn't catch this, it just behaves as for length=0.
370 if ($length > $size - $offset) {
371 $length = $size - $offset;
374 # $num_elems holds the current number of elements in the database.
375 my $num_elems = $size;
377 # 'Removes the elements designated by OFFSET and LENGTH from an
381 foreach (0 .. $length - 1) {
383 my $status = $self->get($offset, $old);
385 my $msg = "error from Berkeley DB on get($offset, \$old)";
387 $msg .= ' (no such element?)';
390 $msg .= ": error status $status";
391 if (defined $! and $! ne '') {
392 $msg .= ", message $!";
399 $status = $self->del($offset);
401 my $msg = "error from Berkeley DB on del($offset)";
403 $msg .= ' (no such element?)';
406 $msg .= ": error status $status";
407 if (defined $! and $! ne '') {
408 $msg .= ", message $!";
417 # ...'and replaces them with the elements of LIST, if any.'
419 while (defined (my $elem = shift @list)) {
422 if ($pos >= $num_elems) {
423 $status = $self->put($pos, $elem);
426 $status = $self->put($pos, $elem, $self->R_IBEFORE);
430 my $msg = "error from Berkeley DB on put($pos, $elem, ...)";
432 $msg .= ' (no such element?)';
435 $msg .= ", error status $status";
436 if (defined $! and $! ne '') {
437 $msg .= ", message $!";
443 die "pos unexpectedly changed from $old_pos to $pos with R_IBEFORE"
451 # 'In list context, returns the elements removed from the
456 elsif (defined wantarray and not wantarray) {
457 # 'In scalar context, returns the last element removed, or
458 # undef if no elements are removed.'
461 my $last = pop @removed;
468 elsif (not defined wantarray) {
473 sub ::DB_File::splice { &SPLICE }
477 croak "Usage: \$db->find_dup(key,value)\n"
481 my ($origkey, $value_wanted) = @_ ;
482 my ($key, $value) = ($origkey, 0);
485 for ($status = $db->seq($key, $value, R_CURSOR() ) ;
487 $status = $db->seq($key, $value, R_NEXT() ) ) {
489 return 0 if $key eq $origkey and $value eq $value_wanted ;
497 croak "Usage: \$db->del_dup(key,value)\n"
501 my ($key, $value) = @_ ;
502 my ($status) = $db->find_dup($key, $value) ;
503 return $status if $status != 0 ;
505 $status = $db->del($key, R_CURSOR() ) ;
511 croak "Usage: \$db->get_dup(key [,flag])\n"
512 unless @_ == 2 or @_ == 3 ;
519 my $wantarray = wantarray ;
525 # iterate through the database until either EOF ($status == 0)
526 # or a different key is encountered ($key ne $origkey).
527 for ($status = $db->seq($key, $value, R_CURSOR()) ;
528 $status == 0 and $key eq $origkey ;
529 $status = $db->seq($key, $value, R_NEXT()) ) {
531 # save the value or count number of matches
534 { ++ $values{$value} }
536 { push (@values, $value) }
543 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
552 DB_File - Perl5 access to Berkeley DB version 1.x
558 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
559 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
560 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
562 $status = $X->del($key [, $flags]) ;
563 $status = $X->put($key, $value [, $flags]) ;
564 $status = $X->get($key, $value [, $flags]) ;
565 $status = $X->seq($key, $value, $flags) ;
566 $status = $X->sync([$flags]) ;
570 $count = $X->get_dup($key) ;
571 @list = $X->get_dup($key) ;
572 %list = $X->get_dup($key, 1) ;
573 $status = $X->find_dup($key, $value) ;
574 $status = $X->del_dup($key, $value) ;
582 @r = $X->splice(offset, length, elements);
585 $old_filter = $db->filter_store_key ( sub { ... } ) ;
586 $old_filter = $db->filter_store_value( sub { ... } ) ;
587 $old_filter = $db->filter_fetch_key ( sub { ... } ) ;
588 $old_filter = $db->filter_fetch_value( sub { ... } ) ;
595 B<DB_File> is a module which allows Perl programs to make use of the
596 facilities provided by Berkeley DB version 1.x (if you have a newer
597 version of DB, see L<Using DB_File with Berkeley DB version 2 or 3>).
598 It is assumed that you have a copy of the Berkeley DB manual pages at
599 hand when reading this documentation. The interface defined here
600 mirrors the Berkeley DB interface closely.
602 Berkeley DB is a C library which provides a consistent interface to a
603 number of database formats. B<DB_File> provides an interface to all
604 three of the database types currently supported by Berkeley DB.
612 This database type allows arbitrary key/value pairs to be stored in data
613 files. This is equivalent to the functionality provided by other
614 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
615 the files created using DB_HASH are not compatible with any of the
616 other packages mentioned.
618 A default hashing algorithm, which will be adequate for most
619 applications, is built into Berkeley DB. If you do need to use your own
620 hashing algorithm it is possible to write your own in Perl and have
621 B<DB_File> use it instead.
625 The btree format allows arbitrary key/value pairs to be stored in a
626 sorted, balanced binary tree.
628 As with the DB_HASH format, it is possible to provide a user defined
629 Perl routine to perform the comparison of keys. By default, though, the
630 keys are stored in lexical order.
634 DB_RECNO allows both fixed-length and variable-length flat text files
635 to be manipulated using the same key/value pair interface as in DB_HASH
636 and DB_BTREE. In this case the key will consist of a record (line)
641 =head2 Using DB_File with Berkeley DB version 2 or 3
643 Although B<DB_File> is intended to be used with Berkeley DB version 1,
644 it can also be used with version 2 or 3. In this case the interface is
645 limited to the functionality provided by Berkeley DB 1.x. Anywhere the
646 version 2 or 3 interface differs, B<DB_File> arranges for it to work
647 like version 1. This feature allows B<DB_File> scripts that were built
648 with version 1 to be migrated to version 2 or 3 without any changes.
650 If you want to make use of the new features available in Berkeley DB
651 2.x or greater, use the Perl module B<BerkeleyDB> instead.
653 B<Note:> The database file format has changed in both Berkeley DB
654 version 2 and 3. If you cannot recreate your databases, you must dump
655 any existing databases with either the C<db_dump> or the C<db_dump185>
656 utility that comes with Berkeley DB.
657 Once you have rebuilt DB_File to use Berkeley DB version 2 or 3, your
658 databases can be recreated using C<db_load>. Refer to the Berkeley DB
659 documentation for further details.
661 Please read L<"COPYRIGHT"> before using version 2.x or 3.x of Berkeley
664 =head2 Interface to Berkeley DB
666 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
667 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
668 allows B<DB_File> to access Berkeley DB files using either an
669 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
670 array (for the DB_RECNO file type).
672 In addition to the tie() interface, it is also possible to access most
673 of the functions provided in the Berkeley DB API directly.
674 See L<THE API INTERFACE>.
676 =head2 Opening a Berkeley DB Database File
678 Berkeley DB uses the function dbopen() to open or create a database.
679 Here is the C prototype for dbopen():
682 dbopen (const char * file, int flags, int mode,
683 DBTYPE type, const void * openinfo)
685 The parameter C<type> is an enumeration which specifies which of the 3
686 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
687 Depending on which of these is actually chosen, the final parameter,
688 I<openinfo> points to a data structure which allows tailoring of the
689 specific interface method.
691 This interface is handled slightly differently in B<DB_File>. Here is
692 an equivalent call using B<DB_File>:
694 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
696 The C<filename>, C<flags> and C<mode> parameters are the direct
697 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
698 performs the function of both the C<type> and C<openinfo> parameters in
701 In the example above $DB_HASH is actually a pre-defined reference to a
702 hash object. B<DB_File> has three of these pre-defined references.
703 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
705 The keys allowed in each of these pre-defined references is limited to
706 the names used in the equivalent C structure. So, for example, the
707 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
708 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
710 To change one of these elements, just assign to it like this:
712 $DB_HASH->{'cachesize'} = 10000 ;
714 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
715 usually adequate for most applications. If you do need to create extra
716 instances of these objects, constructors are available for each file
719 Here are examples of the constructors and the valid options available
720 for DB_HASH, DB_BTREE and DB_RECNO respectively.
722 $a = new DB_File::HASHINFO ;
730 $b = new DB_File::BTREEINFO ;
740 $c = new DB_File::RECNOINFO ;
749 The values stored in the hashes above are mostly the direct equivalent
750 of their C counterpart. Like their C counterparts, all are set to a
751 default values - that means you don't have to set I<all> of the
752 values when you only want to change one. Here is an example:
754 $a = new DB_File::HASHINFO ;
755 $a->{'cachesize'} = 12345 ;
756 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
758 A few of the options need extra discussion here. When used, the C
759 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
760 to C functions. In B<DB_File> these keys are used to store references
761 to Perl subs. Below are templates for each of the subs:
767 # return the hash value for $data
773 my ($key, $key2) = @_ ;
775 # return 0 if $key1 eq $key2
776 # -1 if $key1 lt $key2
777 # 1 if $key1 gt $key2
778 return (-1 , 0 or 1) ;
783 my ($key, $key2) = @_ ;
785 # return number of bytes of $key2 which are
786 # necessary to determine that it is greater than $key1
790 See L<Changing the BTREE sort order> for an example of using the
793 If you are using the DB_RECNO interface and you intend making use of
794 C<bval>, you should check out L<The 'bval' Option>.
796 =head2 Default Parameters
798 It is possible to omit some or all of the final 4 parameters in the
799 call to C<tie> and let them take default values. As DB_HASH is the most
800 common file format used, the call:
802 tie %A, "DB_File", "filename" ;
806 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
808 It is also possible to omit the filename parameter as well, so the
815 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
817 See L<In Memory Databases> for a discussion on the use of C<undef>
818 in place of a filename.
820 =head2 In Memory Databases
822 Berkeley DB allows the creation of in-memory databases by using NULL
823 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
824 uses C<undef> instead of NULL to provide this functionality.
828 The DB_HASH file format is probably the most commonly used of the three
829 file formats that B<DB_File> supports. It is also very straightforward
832 =head2 A Simple Example
834 This example shows how to create a database, add key/value pairs to the
835 database, delete keys/value pairs and finally how to enumerate the
836 contents of the database.
841 use vars qw( %h $k $v ) ;
844 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
845 or die "Cannot open file 'fruit': $!\n";
847 # Add a few key/value pairs to the file
848 $h{"apple"} = "red" ;
849 $h{"orange"} = "orange" ;
850 $h{"banana"} = "yellow" ;
851 $h{"tomato"} = "red" ;
853 # Check for existence of a key
854 print "Banana Exists\n\n" if $h{"banana"} ;
856 # Delete a key/value pair.
859 # print the contents of the file
860 while (($k, $v) = each %h)
861 { print "$k -> $v\n" }
873 Note that the like ordinary associative arrays, the order of the keys
874 retrieved is in an apparently random order.
878 The DB_BTREE format is useful when you want to store data in a given
879 order. By default the keys will be stored in lexical order, but as you
880 will see from the example shown in the next section, it is very easy to
881 define your own sorting function.
883 =head2 Changing the BTREE sort order
885 This script shows how to override the default sorting algorithm that
886 BTREE uses. Instead of using the normal lexical ordering, a case
887 insensitive compare function will be used.
897 my ($key1, $key2) = @_ ;
898 "\L$key1" cmp "\L$key2" ;
901 # specify the Perl sub that will do the comparison
902 $DB_BTREE->{'compare'} = \&Compare ;
905 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
906 or die "Cannot open file 'tree': $!\n" ;
908 # Add a key/value pair to the file
909 $h{'Wall'} = 'Larry' ;
910 $h{'Smith'} = 'John' ;
911 $h{'mouse'} = 'mickey' ;
912 $h{'duck'} = 'donald' ;
917 # Cycle through the keys printing them in order.
918 # Note it is not necessary to sort the keys as
919 # the btree will have kept them in order automatically.
925 Here is the output from the code above.
931 There are a few point to bear in mind if you want to change the
932 ordering in a BTREE database:
938 The new compare function must be specified when you create the database.
942 You cannot change the ordering once the database has been created. Thus
943 you must use the same compare function every time you access the
948 =head2 Handling Duplicate Keys
950 The BTREE file type optionally allows a single key to be associated
951 with an arbitrary number of values. This option is enabled by setting
952 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
954 There are some difficulties in using the tied hash interface if you
955 want to manipulate a BTREE database with duplicate keys. Consider this
962 use vars qw($filename %h ) ;
967 # Enable duplicate records
968 $DB_BTREE->{'flags'} = R_DUP ;
970 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
971 or die "Cannot open $filename: $!\n";
973 # Add some key/value pairs to the file
974 $h{'Wall'} = 'Larry' ;
975 $h{'Wall'} = 'Brick' ; # Note the duplicate key
976 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
977 $h{'Smith'} = 'John' ;
978 $h{'mouse'} = 'mickey' ;
980 # iterate through the associative array
981 # and print each key/value pair.
982 foreach (sort keys %h)
983 { print "$_ -> $h{$_}\n" }
995 As you can see 3 records have been successfully created with key C<Wall>
996 - the only thing is, when they are retrieved from the database they
997 I<seem> to have the same value, namely C<Larry>. The problem is caused
998 by the way that the associative array interface works. Basically, when
999 the associative array interface is used to fetch the value associated
1000 with a given key, it will only ever retrieve the first value.
1002 Although it may not be immediately obvious from the code above, the
1003 associative array interface can be used to write values with duplicate
1004 keys, but it cannot be used to read them back from the database.
1006 The way to get around this problem is to use the Berkeley DB API method
1007 called C<seq>. This method allows sequential access to key/value
1008 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
1009 and the API in general.
1011 Here is the script above rewritten using the C<seq> API method.
1017 use vars qw($filename $x %h $status $key $value) ;
1019 $filename = "tree" ;
1022 # Enable duplicate records
1023 $DB_BTREE->{'flags'} = R_DUP ;
1025 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
1026 or die "Cannot open $filename: $!\n";
1028 # Add some key/value pairs to the file
1029 $h{'Wall'} = 'Larry' ;
1030 $h{'Wall'} = 'Brick' ; # Note the duplicate key
1031 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
1032 $h{'Smith'} = 'John' ;
1033 $h{'mouse'} = 'mickey' ;
1035 # iterate through the btree using seq
1036 # and print each key/value pair.
1038 for ($status = $x->seq($key, $value, R_FIRST) ;
1040 $status = $x->seq($key, $value, R_NEXT) )
1041 { print "$key -> $value\n" }
1054 This time we have got all the key/value pairs, including the multiple
1055 values associated with the key C<Wall>.
1057 To make life easier when dealing with duplicate keys, B<DB_File> comes with
1058 a few utility methods.
1060 =head2 The get_dup() Method
1062 The C<get_dup> method assists in
1063 reading duplicate values from BTREE databases. The method can take the
1066 $count = $x->get_dup($key) ;
1067 @list = $x->get_dup($key) ;
1068 %list = $x->get_dup($key, 1) ;
1070 In a scalar context the method returns the number of values associated
1071 with the key, C<$key>.
1073 In list context, it returns all the values which match C<$key>. Note
1074 that the values will be returned in an apparently random order.
1076 In list context, if the second parameter is present and evaluates
1077 TRUE, the method returns an associative array. The keys of the
1078 associative array correspond to the values that matched in the BTREE
1079 and the values of the array are a count of the number of times that
1080 particular value occurred in the BTREE.
1082 So assuming the database created above, we can use C<get_dup> like
1089 use vars qw($filename $x %h ) ;
1091 $filename = "tree" ;
1093 # Enable duplicate records
1094 $DB_BTREE->{'flags'} = R_DUP ;
1096 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
1097 or die "Cannot open $filename: $!\n";
1099 my $cnt = $x->get_dup("Wall") ;
1100 print "Wall occurred $cnt times\n" ;
1102 my %hash = $x->get_dup("Wall", 1) ;
1103 print "Larry is there\n" if $hash{'Larry'} ;
1104 print "There are $hash{'Brick'} Brick Walls\n" ;
1106 my @list = sort $x->get_dup("Wall") ;
1107 print "Wall => [@list]\n" ;
1109 @list = $x->get_dup("Smith") ;
1110 print "Smith => [@list]\n" ;
1112 @list = $x->get_dup("Dog") ;
1113 print "Dog => [@list]\n" ;
1118 Wall occurred 3 times
1120 There are 2 Brick Walls
1121 Wall => [Brick Brick Larry]
1125 =head2 The find_dup() Method
1127 $status = $X->find_dup($key, $value) ;
1129 This method checks for the existence of a specific key/value pair. If the
1130 pair exists, the cursor is left pointing to the pair and the method
1131 returns 0. Otherwise the method returns a non-zero value.
1133 Assuming the database from the previous example:
1139 use vars qw($filename $x %h $found) ;
1141 my $filename = "tree" ;
1143 # Enable duplicate records
1144 $DB_BTREE->{'flags'} = R_DUP ;
1146 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
1147 or die "Cannot open $filename: $!\n";
1149 $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ;
1150 print "Larry Wall is $found there\n" ;
1152 $found = ( $x->find_dup("Wall", "Harry") == 0 ? "" : "not") ;
1153 print "Harry Wall is $found there\n" ;
1161 Harry Wall is not there
1164 =head2 The del_dup() Method
1166 $status = $X->del_dup($key, $value) ;
1168 This method deletes a specific key/value pair. It returns
1169 0 if they exist and have been deleted successfully.
1170 Otherwise the method returns a non-zero value.
1172 Again assuming the existence of the C<tree> database
1178 use vars qw($filename $x %h $found) ;
1180 my $filename = "tree" ;
1182 # Enable duplicate records
1183 $DB_BTREE->{'flags'} = R_DUP ;
1185 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
1186 or die "Cannot open $filename: $!\n";
1188 $x->del_dup("Wall", "Larry") ;
1190 $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ;
1191 print "Larry Wall is $found there\n" ;
1198 Larry Wall is not there
1200 =head2 Matching Partial Keys
1202 The BTREE interface has a feature which allows partial keys to be
1203 matched. This functionality is I<only> available when the C<seq> method
1204 is used along with the R_CURSOR flag.
1206 $x->seq($key, $value, R_CURSOR) ;
1208 Here is the relevant quote from the dbopen man page where it defines
1209 the use of the R_CURSOR flag with seq:
1211 Note, for the DB_BTREE access method, the returned key is not
1212 necessarily an exact match for the specified key. The returned key
1213 is the smallest key greater than or equal to the specified key,
1214 permitting partial key matches and range searches.
1216 In the example script below, the C<match> sub uses this feature to find
1217 and print the first matching key/value pair given a partial key.
1224 use vars qw($filename $x %h $st $key $value) ;
1230 my $orig_key = $key ;
1231 $x->seq($key, $value, R_CURSOR) ;
1232 print "$orig_key\t-> $key\t-> $value\n" ;
1235 $filename = "tree" ;
1238 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
1239 or die "Cannot open $filename: $!\n";
1241 # Add some key/value pairs to the file
1242 $h{'mouse'} = 'mickey' ;
1243 $h{'Wall'} = 'Larry' ;
1244 $h{'Walls'} = 'Brick' ;
1245 $h{'Smith'} = 'John' ;
1249 print "IN ORDER\n" ;
1250 for ($st = $x->seq($key, $value, R_FIRST) ;
1252 $st = $x->seq($key, $value, R_NEXT) )
1254 { print "$key -> $value\n" }
1256 print "\nPARTIAL MATCH\n" ;
1276 a -> mouse -> mickey
1280 DB_RECNO provides an interface to flat text files. Both variable and
1281 fixed length records are supported.
1283 In order to make RECNO more compatible with Perl, the array offset for
1284 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
1286 As with normal Perl arrays, a RECNO array can be accessed using
1287 negative indexes. The index -1 refers to the last element of the array,
1288 -2 the second last, and so on. Attempting to access an element before
1289 the start of the array will raise a fatal run-time error.
1291 =head2 The 'bval' Option
1293 The operation of the bval option warrants some discussion. Here is the
1294 definition of bval from the Berkeley DB 1.85 recno manual page:
1296 The delimiting byte to be used to mark the end of a
1297 record for variable-length records, and the pad charac-
1298 ter for fixed-length records. If no value is speci-
1299 fied, newlines (``\n'') are used to mark the end of
1300 variable-length records and fixed-length records are
1303 The second sentence is wrong. In actual fact bval will only default to
1304 C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
1305 openinfo parameter is used at all, the value that happens to be in bval
1306 will be used. That means you always have to specify bval when making
1307 use of any of the options in the openinfo parameter. This documentation
1308 error will be fixed in the next release of Berkeley DB.
1310 That clarifies the situation with regards Berkeley DB itself. What
1311 about B<DB_File>? Well, the behavior defined in the quote above is
1312 quite useful, so B<DB_File> conforms to it.
1314 That means that you can specify other options (e.g. cachesize) and
1315 still have bval default to C<"\n"> for variable length records, and
1316 space for fixed length records.
1318 Also note that the bval option only allows you to specify a single byte
1321 =head2 A Simple Example
1323 Here is a simple example that uses RECNO (if you are using a version
1324 of Perl earlier than 5.004_57 this example won't work -- see
1325 L<Extra RECNO Methods> for a workaround).
1331 my $filename = "text" ;
1335 tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_RECNO
1336 or die "Cannot open file 'text': $!\n" ;
1338 # Add a few key/value pairs to the file
1343 push @h, "green", "black" ;
1345 my $elements = scalar @h ;
1346 print "The array contains $elements entries\n" ;
1349 print "popped $last\n" ;
1351 unshift @h, "white" ;
1352 my $first = shift @h ;
1353 print "shifted $first\n" ;
1355 # Check for existence of a key
1356 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
1358 # use a negative index
1359 print "The last element is $h[-1]\n" ;
1360 print "The 2nd last element is $h[-2]\n" ;
1364 Here is the output from the script:
1366 The array contains 5 entries
1369 Element 1 Exists with value blue
1370 The last element is green
1371 The 2nd last element is yellow
1373 =head2 Extra RECNO Methods
1375 If you are using a version of Perl earlier than 5.004_57, the tied
1376 array interface is quite limited. In the example script above
1377 C<push>, C<pop>, C<shift>, C<unshift>
1378 or determining the array length will not work with a tied array.
1380 To make the interface more useful for older versions of Perl, a number
1381 of methods are supplied with B<DB_File> to simulate the missing array
1382 operations. All these methods are accessed via the object returned from
1385 Here are the methods:
1389 =item B<$X-E<gt>push(list) ;>
1391 Pushes the elements of C<list> to the end of the array.
1393 =item B<$value = $X-E<gt>pop ;>
1395 Removes and returns the last element of the array.
1397 =item B<$X-E<gt>shift>
1399 Removes and returns the first element of the array.
1401 =item B<$X-E<gt>unshift(list) ;>
1403 Pushes the elements of C<list> to the start of the array.
1405 =item B<$X-E<gt>length>
1407 Returns the number of elements in the array.
1409 =item B<$X-E<gt>splice(offset, length, elements);>
1411 Returns a splice of the the array.
1415 =head2 Another Example
1417 Here is a more complete example that makes use of some of the methods
1418 described above. It also makes use of the API interface directly (see
1419 L<THE API INTERFACE>).
1423 use vars qw(@h $H $file $i) ;
1431 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
1432 or die "Cannot open file $file: $!\n" ;
1434 # first create a text file to play with
1442 # Print the records in order.
1444 # The length method is needed here because evaluating a tied
1445 # array in a scalar context does not return the number of
1446 # elements in the array.
1448 print "\nORIGINAL\n" ;
1449 foreach $i (0 .. $H->length - 1) {
1450 print "$i: $h[$i]\n" ;
1453 # use the push & pop methods
1456 print "\nThe last record was [$a]\n" ;
1458 # and the shift & unshift methods
1460 $H->unshift("first") ;
1461 print "The first record was [$a]\n" ;
1463 # Use the API to add a new record after record 2.
1465 $H->put($i, "Newbie", R_IAFTER) ;
1467 # and a new record before record 1.
1469 $H->put($i, "New One", R_IBEFORE) ;
1474 # now print the records in reverse order
1475 print "\nREVERSE\n" ;
1476 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1477 { print "$i: $h[$i]\n" }
1479 # same again, but use the API functions instead
1480 print "\nREVERSE again\n" ;
1481 my ($s, $k, $v) = (0, 0, 0) ;
1482 for ($s = $H->seq($k, $v, R_LAST) ;
1484 $s = $H->seq($k, $v, R_PREV))
1485 { print "$k: $v\n" }
1490 and this is what it outputs:
1499 The last record was [four]
1500 The first record was [zero]
1524 Rather than iterating through the array, C<@h> like this:
1528 it is necessary to use either this:
1530 foreach $i (0 .. $H->length - 1)
1534 for ($a = $H->get($k, $v, R_FIRST) ;
1536 $a = $H->get($k, $v, R_NEXT) )
1540 Notice that both times the C<put> method was used the record index was
1541 specified using a variable, C<$i>, rather than the literal value
1542 itself. This is because C<put> will return the record number of the
1543 inserted line via that parameter.
1547 =head1 THE API INTERFACE
1549 As well as accessing Berkeley DB using a tied hash or array, it is also
1550 possible to make direct use of most of the API functions defined in the
1551 Berkeley DB documentation.
1553 To do this you need to store a copy of the object returned from the tie.
1555 $db = tie %hash, "DB_File", "filename" ;
1557 Once you have done that, you can access the Berkeley DB API functions
1558 as B<DB_File> methods directly like this:
1560 $db->put($key, $value, R_NOOVERWRITE) ;
1562 B<Important:> If you have saved a copy of the object returned from
1563 C<tie>, the underlying database file will I<not> be closed until both
1564 the tied variable is untied and all copies of the saved object are
1568 $db = tie %hash, "DB_File", "filename"
1569 or die "Cannot tie filename: $!" ;
1574 See L<The untie() Gotcha> for more details.
1576 All the functions defined in L<dbopen> are available except for
1577 close() and dbopen() itself. The B<DB_File> method interface to the
1578 supported functions have been implemented to mirror the way Berkeley DB
1579 works whenever possible. In particular note that:
1585 The methods return a status value. All return 0 on success.
1586 All return -1 to signify an error and set C<$!> to the exact
1587 error code. The return code 1 generally (but not always) means that the
1588 key specified did not exist in the database.
1590 Other return codes are defined. See below and in the Berkeley DB
1591 documentation for details. The Berkeley DB documentation should be used
1592 as the definitive source.
1596 Whenever a Berkeley DB function returns data via one of its parameters,
1597 the equivalent B<DB_File> method does exactly the same.
1601 If you are careful, it is possible to mix API calls with the tied
1602 hash/array interface in the same piece of code. Although only a few of
1603 the methods used to implement the tied interface currently make use of
1604 the cursor, you should always assume that the cursor has been changed
1605 any time the tied hash/array interface is used. As an example, this
1606 code will probably not do what you expect:
1608 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1609 or die "Cannot tie $filename: $!" ;
1611 # Get the first key/value pair and set the cursor
1612 $X->seq($key, $value, R_FIRST) ;
1614 # this line will modify the cursor
1615 $count = scalar keys %x ;
1617 # Get the second key/value pair.
1618 # oops, it didn't, it got the last key/value pair!
1619 $X->seq($key, $value, R_NEXT) ;
1621 The code above can be rearranged to get around the problem, like this:
1623 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1624 or die "Cannot tie $filename: $!" ;
1626 # this line will modify the cursor
1627 $count = scalar keys %x ;
1629 # Get the first key/value pair and set the cursor
1630 $X->seq($key, $value, R_FIRST) ;
1632 # Get the second key/value pair.
1634 $X->seq($key, $value, R_NEXT) ;
1638 All the constants defined in L<dbopen> for use in the flags parameters
1639 in the methods defined below are also available. Refer to the Berkeley
1640 DB documentation for the precise meaning of the flags values.
1642 Below is a list of the methods available.
1646 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1648 Given a key (C<$key>) this method reads the value associated with it
1649 from the database. The value read from the database is returned in the
1650 C<$value> parameter.
1652 If the key does not exist the method returns 1.
1654 No flags are currently defined for this method.
1656 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1658 Stores the key/value pair in the database.
1660 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1661 will have the record number of the inserted key/value pair set.
1663 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1666 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1668 Removes all key/value pairs with key C<$key> from the database.
1670 A return code of 1 means that the requested key was not in the
1673 R_CURSOR is the only valid flag at present.
1675 =item B<$status = $X-E<gt>fd ;>
1677 Returns the file descriptor for the underlying database.
1679 See L<Locking: The Trouble with fd> for an explanation for why you should
1680 not use C<fd> to lock your database.
1682 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1684 This interface allows sequential retrieval from the database. See
1685 L<dbopen> for full details.
1687 Both the C<$key> and C<$value> parameters will be set to the key/value
1688 pair read from the database.
1690 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1691 R_FIRST, R_LAST, R_NEXT and R_PREV.
1693 =item B<$status = $X-E<gt>sync([$flags]) ;>
1695 Flushes any cached buffers to disk.
1697 R_RECNOSYNC is the only valid flag at present.
1703 A DBM Filter is a piece of code that is be used when you I<always>
1704 want to make the same transformation to all keys and/or values in a
1707 There are four methods associated with DBM Filters. All work identically,
1708 and each is used to install (or uninstall) a single DBM Filter. Each
1709 expects a single parameter, namely a reference to a sub. The only
1710 difference between them is the place that the filter is installed.
1716 =item B<filter_store_key>
1718 If a filter has been installed with this method, it will be invoked
1719 every time you write a key to a DBM database.
1721 =item B<filter_store_value>
1723 If a filter has been installed with this method, it will be invoked
1724 every time you write a value to a DBM database.
1727 =item B<filter_fetch_key>
1729 If a filter has been installed with this method, it will be invoked
1730 every time you read a key from a DBM database.
1732 =item B<filter_fetch_value>
1734 If a filter has been installed with this method, it will be invoked
1735 every time you read a value from a DBM database.
1739 You can use any combination of the methods, from none, to all four.
1741 All filter methods return the existing filter, if present, or C<undef>
1744 To delete a filter pass C<undef> to it.
1748 When each filter is called by Perl, a local copy of C<$_> will contain
1749 the key or value to be filtered. Filtering is achieved by modifying
1750 the contents of C<$_>. The return code from the filter is ignored.
1752 =head2 An Example -- the NULL termination problem.
1754 Consider the following scenario. You have a DBM database
1755 that you need to share with a third-party C application. The C application
1756 assumes that I<all> keys and values are NULL terminated. Unfortunately
1757 when Perl writes to DBM databases it doesn't use NULL termination, so
1758 your Perl application will have to manage NULL termination itself. When
1759 you write to the database you will have to use something like this:
1761 $hash{"$key\0"} = "$value\0" ;
1763 Similarly the NULL needs to be taken into account when you are considering
1764 the length of existing keys/values.
1766 It would be much better if you could ignore the NULL terminations issue
1767 in the main application code and have a mechanism that automatically
1768 added the terminating NULL to all keys and values whenever you write to
1769 the database and have them removed when you read from the database. As I'm
1770 sure you have already guessed, this is a problem that DBM Filters can
1778 my $filename = "/tmp/filt" ;
1781 my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
1782 or die "Cannot open $filename: $!\n" ;
1784 # Install DBM Filters
1785 $db->filter_fetch_key ( sub { s/\0$// } ) ;
1786 $db->filter_store_key ( sub { $_ .= "\0" } ) ;
1787 $db->filter_fetch_value( sub { s/\0$// } ) ;
1788 $db->filter_store_value( sub { $_ .= "\0" } ) ;
1790 $hash{"abc"} = "def" ;
1791 my $a = $hash{"ABC"} ;
1796 Hopefully the contents of each of the filters should be
1797 self-explanatory. Both "fetch" filters remove the terminating NULL,
1798 and both "store" filters add a terminating NULL.
1801 =head2 Another Example -- Key is a C int.
1803 Here is another real-life example. By default, whenever Perl writes to
1804 a DBM database it always writes the key and value as strings. So when
1807 $hash{12345} = "soemthing" ;
1809 the key 12345 will get stored in the DBM database as the 5 byte string
1810 "12345". If you actually want the key to be stored in the DBM database
1811 as a C int, you will have to use C<pack> when writing, and C<unpack>
1814 Here is a DBM Filter that does it:
1820 my $filename = "/tmp/filt" ;
1824 my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
1825 or die "Cannot open $filename: $!\n" ;
1827 $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } ) ;
1828 $db->filter_store_key ( sub { $_ = pack ("i", $_) } ) ;
1829 $hash{123} = "def" ;
1834 This time only two filters have been used -- we only need to manipulate
1835 the contents of the key, so it wasn't necessary to install any value
1838 =head1 HINTS AND TIPS
1841 =head2 Locking: The Trouble with fd
1843 Until version 1.72 of this module, the recommended technique for locking
1844 B<DB_File> databases was to flock the filehandle returned from the "fd"
1845 function. Unfortunately this technique has been shown to be fundamentally
1846 flawed (Kudos to David Harris for tracking this down). Use it at your own
1849 The locking technique went like this.
1851 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1852 || die "dbcreat /tmp/foo.db $!";
1854 open(DB_FH, "+<&=$fd") || die "dup $!";
1855 flock (DB_FH, LOCK_EX) || die "flock: $!";
1857 $db{"Tom"} = "Jerry" ;
1859 flock(DB_FH, LOCK_UN);
1864 In simple terms, this is what happens:
1870 Use "tie" to open the database.
1874 Lock the database with fd & flock.
1878 Read & Write to the database.
1882 Unlock and close the database.
1886 Here is the crux of the problem. A side-effect of opening the B<DB_File>
1887 database in step 2 is that an initial block from the database will get
1888 read from disk and cached in memory.
1890 To see why this is a problem, consider what can happen when two processes,
1891 say "A" and "B", both want to update the same B<DB_File> database
1892 using the locking steps outlined above. Assume process "A" has already
1893 opened the database and has a write lock, but it hasn't actually updated
1894 the database yet (it has finished step 2, but not started step 3 yet). Now
1895 process "B" tries to open the same database - step 1 will succeed,
1896 but it will block on step 2 until process "A" releases the lock. The
1897 important thing to notice here is that at this point in time both
1898 processes will have cached identical initial blocks from the database.
1900 Now process "A" updates the database and happens to change some of the
1901 data held in the initial buffer. Process "A" terminates, flushing
1902 all cached data to disk and releasing the database lock. At this point
1903 the database on disk will correctly reflect the changes made by process
1906 With the lock released, process "B" can now continue. It also updates the
1907 database and unfortunately it too modifies the data that was in its
1908 initial buffer. Once that data gets flushed to disk it will overwrite
1909 some/all of the changes process "A" made to the database.
1911 The result of this scenario is at best a database that doesn't contain
1912 what you expect. At worst the database will corrupt.
1914 The above won't happen every time competing process update the same
1915 B<DB_File> database, but it does illustrate why the technique should
1918 =head2 Safe ways to lock a database
1920 Starting with version 2.x, Berkeley DB has internal support for locking.
1921 The companion module to this one, B<BerkeleyDB>, provides an interface
1922 to this locking functionality. If you are serious about locking
1923 Berkeley DB databases, I strongly recommend using B<BerkeleyDB>.
1925 If using B<BerkeleyDB> isn't an option, there are a number of modules
1926 available on CPAN that can be used to implement locking. Each one
1927 implements locking differently and has different goals in mind. It is
1928 therefore worth knowing the difference, so that you can pick the right
1929 one for your application. Here are the three locking wrappers:
1933 =item B<Tie::DB_Lock>
1935 A B<DB_File> wrapper which creates copies of the database file for
1936 read access, so that you have a kind of a multiversioning concurrent read
1937 system. However, updates are still serial. Use for databases where reads
1938 may be lengthy and consistency problems may occur.
1940 =item B<Tie::DB_LockFile>
1942 A B<DB_File> wrapper that has the ability to lock and unlock the database
1943 while it is being used. Avoids the tie-before-flock problem by simply
1944 re-tie-ing the database when you get or drop a lock. Because of the
1945 flexibility in dropping and re-acquiring the lock in the middle of a
1946 session, this can be massaged into a system that will work with long
1947 updates and/or reads if the application follows the hints in the POD
1950 =item B<DB_File::Lock>
1952 An extremely lightweight B<DB_File> wrapper that simply flocks a lockfile
1953 before tie-ing the database and drops the lock after the untie. Allows
1954 one to use the same lockfile for multiple databases to avoid deadlock
1955 problems, if desired. Use for databases where updates are reads are
1956 quick and simple flock locking semantics are enough.
1960 =head2 Sharing Databases With C Applications
1962 There is no technical reason why a Berkeley DB database cannot be
1963 shared by both a Perl and a C application.
1965 The vast majority of problems that are reported in this area boil down
1966 to the fact that C strings are NULL terminated, whilst Perl strings are
1967 not. See L<DBM FILTERS> for a generic way to work around this problem.
1969 Here is a real example. Netscape 2.0 keeps a record of the locations you
1970 visit along with the time you last visited them in a DB_HASH database.
1971 This is usually stored in the file F<~/.netscape/history.db>. The key
1972 field in the database is the location string and the value field is the
1973 time the location was last visited stored as a 4 byte binary value.
1975 If you haven't already guessed, the location string is stored with a
1976 terminating NULL. This means you need to be careful when accessing the
1979 Here is a snippet of code that is loosely based on Tom Christiansen's
1980 I<ggh> script (available from your nearest CPAN archive in
1981 F<authors/id/TOMC/scripts/nshist.gz>).
1988 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1989 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1991 $HISTORY = "$dotdir/.netscape/history.db";
1993 tie %hist_db, 'DB_File', $HISTORY
1994 or die "Cannot open $HISTORY: $!\n" ;;
1996 # Dump the complete database
1997 while ( ($href, $binary_time) = each %hist_db ) {
1999 # remove the terminating NULL
2000 $href =~ s/\x00$// ;
2002 # convert the binary time into a user friendly string
2003 $date = localtime unpack("V", $binary_time);
2004 print "$date $href\n" ;
2007 # check for the existence of a specific key
2008 # remember to add the NULL
2009 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
2010 $date = localtime unpack("V", $binary_time) ;
2011 print "Last visited mox.perl.com on $date\n" ;
2014 print "Never visited mox.perl.com\n"
2019 =head2 The untie() Gotcha
2021 If you make use of the Berkeley DB API, it is I<very> strongly
2022 recommended that you read L<perltie/The untie Gotcha>.
2024 Even if you don't currently make use of the API interface, it is still
2027 Here is an example which illustrates the problem from a B<DB_File>
2036 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
2037 or die "Cannot tie first time: $!" ;
2043 tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
2044 or die "Cannot tie second time: $!" ;
2048 When run, the script will produce this error message:
2050 Cannot tie second time: Invalid argument at bad.file line 14.
2052 Although the error message above refers to the second tie() statement
2053 in the script, the source of the problem is really with the untie()
2054 statement that precedes it.
2056 Having read L<perltie> you will probably have already guessed that the
2057 error is caused by the extra copy of the tied object stored in C<$X>.
2058 If you haven't, then the problem boils down to the fact that the
2059 B<DB_File> destructor, DESTROY, will not be called until I<all>
2060 references to the tied object are destroyed. Both the tied variable,
2061 C<%x>, and C<$X> above hold a reference to the object. The call to
2062 untie() will destroy the first, but C<$X> still holds a valid
2063 reference, so the destructor will not get called and the database file
2064 F<tst.fil> will remain open. The fact that Berkeley DB then reports the
2065 attempt to open a database that is already open via the catch-all
2066 "Invalid argument" doesn't help.
2068 If you run the script with the C<-w> flag the error message becomes:
2070 untie attempted while 1 inner references still exist at bad.file line 12.
2071 Cannot tie second time: Invalid argument at bad.file line 14.
2073 which pinpoints the real problem. Finally the script can now be
2074 modified to fix the original problem by destroying the API object
2083 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
2087 =head1 COMMON QUESTIONS
2089 =head2 Why is there Perl source in my database?
2091 If you look at the contents of a database file created by DB_File,
2092 there can sometimes be part of a Perl script included in it.
2094 This happens because Berkeley DB uses dynamic memory to allocate
2095 buffers which will subsequently be written to the database file. Being
2096 dynamic, the memory could have been used for anything before DB
2097 malloced it. As Berkeley DB doesn't clear the memory once it has been
2098 allocated, the unused portions will contain random junk. In the case
2099 where a Perl script gets written to the database, the random junk will
2100 correspond to an area of dynamic memory that happened to be used during
2101 the compilation of the script.
2103 Unless you don't like the possibility of there being part of your Perl
2104 scripts embedded in a database file, this is nothing to worry about.
2106 =head2 How do I store complex data structures with DB_File?
2108 Although B<DB_File> cannot do this directly, there is a module which
2109 can layer transparently over B<DB_File> to accomplish this feat.
2111 Check out the MLDBM module, available on CPAN in the directory
2112 F<modules/by-module/MLDBM>.
2114 =head2 What does "Invalid Argument" mean?
2116 You will get this error message when one of the parameters in the
2117 C<tie> call is wrong. Unfortunately there are quite a few parameters to
2118 get wrong, so it can be difficult to figure out which one it is.
2120 Here are a couple of possibilities:
2126 Attempting to reopen a database without closing it.
2130 Using the O_WRONLY flag.
2134 =head2 What does "Bareword 'DB_File' not allowed" mean?
2136 You will encounter this particular error message when you have the
2137 C<strict 'subs'> pragma (or the full strict pragma) in your script.
2138 Consider this script:
2144 tie %x, DB_File, "filename" ;
2146 Running it produces the error in question:
2148 Bareword "DB_File" not allowed while "strict subs" in use
2150 To get around the error, place the word C<DB_File> in either single or
2151 double quotes, like this:
2153 tie %x, "DB_File", "filename" ;
2155 Although it might seem like a real pain, it is really worth the effort
2156 of having a C<use strict> in all your scripts.
2160 Articles that are either about B<DB_File> or make use of it.
2166 I<Full-Text Searching in Perl>, Tim Kientzle (tkientzle@ddj.com),
2167 Dr. Dobb's Journal, Issue 295, January 1999, pp 34-41
2173 Moved to the Changes file.
2177 Some older versions of Berkeley DB had problems with fixed length
2178 records using the RECNO file format. This problem has been fixed since
2179 version 1.85 of Berkeley DB.
2181 I am sure there are bugs in the code. If you do find any, or can
2182 suggest any enhancements, I would welcome your comments.
2186 B<DB_File> comes with the standard Perl source distribution. Look in
2187 the directory F<ext/DB_File>. Given the amount of time between releases
2188 of Perl the version that ships with Perl is quite likely to be out of
2189 date, so the most recent version can always be found on CPAN (see
2190 L<perlmod/CPAN> for details), in the directory
2191 F<modules/by-module/DB_File>.
2193 This version of B<DB_File> will work with either version 1.x, 2.x or
2194 3.x of Berkeley DB, but is limited to the functionality provided by
2197 The official web site for Berkeley DB is F<http://www.sleepycat.com>.
2198 All versions of Berkeley DB are available there.
2200 Alternatively, Berkeley DB version 1 is available at your nearest CPAN
2201 archive in F<src/misc/db.1.85.tar.gz>.
2203 If you are running IRIX, then get Berkeley DB version 1 from
2204 F<http://reality.sgi.com/ariel>. It has the patches necessary to
2205 compile properly on IRIX 5.3.
2209 Copyright (c) 1995-2001 Paul Marquess. All rights reserved. This program
2210 is free software; you can redistribute it and/or modify it under the
2211 same terms as Perl itself.
2213 Although B<DB_File> is covered by the Perl license, the library it
2214 makes use of, namely Berkeley DB, is not. Berkeley DB has its own
2215 copyright and its own license. Please take the time to read it.
2217 Here are are few words taken from the Berkeley DB FAQ (at
2218 F<http://www.sleepycat.com>) regarding the license:
2220 Do I have to license DB to use it in Perl scripts?
2222 No. The Berkeley DB license requires that software that uses
2223 Berkeley DB be freely redistributable. In the case of Perl, that
2224 software is Perl, and not your scripts. Any Perl scripts that you
2225 write are your property, including scripts that make use of
2226 Berkeley DB. Neither the Perl license nor the Berkeley DB license
2227 place any restriction on what you may do with them.
2229 If you are in any doubt about the license situation, contact either the
2230 Berkeley DB authors or the author of DB_File. See L<"AUTHOR"> for details.
2235 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>,
2240 The DB_File interface was written by Paul Marquess
2241 E<lt>Paul.Marquess@btinternet.comE<gt>.
2242 Questions about the DB system itself may be addressed to
2243 E<lt>db@sleepycat.com<gt>.