1 # DB_File.pm -- Perl 5 interface to Berkeley DB
3 # written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
4 # last modified 30th Apr 1997
7 # Copyright (c) 1995, 1996, 1997 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 ;
19 @DB_File::HASHINFO::ISA = qw(Tie::Hash);
34 bless { VALID => { map {$_, 1}
35 qw( bsize ffactor nelem cachesize hash lorder)
47 return $self->{GOT}{$key} if exists $self->{VALID}{$key} ;
50 croak "${pkg}::FETCH - Unknown element '$key'" ;
60 if ( exists $self->{VALID}{$key} )
62 $self->{GOT}{$key} = $value ;
67 croak "${pkg}::STORE - Unknown element '$key'" ;
75 if ( exists $self->{VALID}{$key} )
77 delete $self->{GOT}{$key} ;
82 croak "DB_File::HASHINFO::DELETE - Unknown element '$key'" ;
90 exists $self->{VALID}{$key} ;
98 croak ref($self) . " does not define the method ${method}" ;
101 sub DESTROY { undef %{$_[0]} }
102 sub FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") }
103 sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") }
104 sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") }
106 package DB_File::RECNOINFO ;
110 @DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
116 bless { VALID => { map {$_, 1}
117 qw( bval cachesize psize flags lorder reclen bfname )
123 package DB_File::BTREEINFO ;
127 @DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
133 bless { VALID => { map {$_, 1}
134 qw( flags cachesize maxkeypage minkeypage psize
135 compare prefix lorder )
145 use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO) ;
151 #typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
152 $DB_BTREE = new DB_File::BTREEINFO ;
153 $DB_HASH = new DB_File::HASHINFO ;
154 $DB_RECNO = new DB_File::RECNOINFO ;
160 @ISA = qw(Tie::Hash Exporter DynaLoader);
162 $DB_BTREE $DB_HASH $DB_RECNO
197 ($constname = $AUTOLOAD) =~ s/.*:://;
198 my $val = constant($constname, @_ ? $_[0] : 0);
200 if ($! =~ /Invalid/) {
201 $AutoLoader::AUTOLOAD = $AUTOLOAD;
202 goto &AutoLoader::AUTOLOAD;
205 my($pack,$file,$line) = caller;
206 croak "Your vendor has not defined DB macro $constname, used at $file line $line.
210 eval "sub $AUTOLOAD { $val }";
215 # import borrowed from IO::File
216 # exports Fcntl constants if available.
219 my $callpkg = caller;
220 Exporter::export $pkg, $callpkg, @_;
223 Exporter::export 'Fcntl', $callpkg, '/^O_/';
227 bootstrap DB_File $VERSION;
229 # Preloaded methods go here. Autoload methods go after __END__, and are
230 # processed by the autosplit program.
232 sub tie_hash_or_array
235 my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ;
237 $arg[4] = tied %{ $arg[4] }
238 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
240 DoTie_($tieHASH, @arg) ;
245 tie_hash_or_array(@_) ;
250 tie_hash_or_array(@_) ;
255 croak "Usage: \$db->get_dup(key [,flag])\n"
256 unless @_ == 2 or @_ == 3 ;
263 my $wantarray = wantarray ;
269 # iterate through the database until either EOF ($status == 0)
270 # or a different key is encountered ($key ne $origkey).
271 for ($status = $db->seq($key, $value, R_CURSOR()) ;
272 $status == 0 and $key eq $origkey ;
273 $status = $db->seq($key, $value, R_NEXT()) ) {
275 # save the value or count number of matches
278 { ++ $values{$value} }
280 { push (@values, $value) }
287 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
296 DB_File - Perl5 access to Berkeley DB
302 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
303 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
304 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
306 $status = $X->del($key [, $flags]) ;
307 $status = $X->put($key, $value [, $flags]) ;
308 $status = $X->get($key, $value [, $flags]) ;
309 $status = $X->seq($key, $value, $flags) ;
310 $status = $X->sync([$flags]) ;
314 $count = $X->get_dup($key) ;
315 @list = $X->get_dup($key) ;
316 %list = $X->get_dup($key, 1) ;
330 B<DB_File> is a module which allows Perl programs to make use of the
331 facilities provided by Berkeley DB. If you intend to use this
332 module you should really have a copy of the Berkeley DB manual pages at
333 hand. The interface defined here mirrors the Berkeley DB interface
336 Please note that this module will only work with version 1.x of
337 Berkeley DB. Once Berkeley DB version 2 is released, B<DB_File> will be
338 upgraded to work with it.
340 Berkeley DB is a C library which provides a consistent interface to a
341 number of database formats. B<DB_File> provides an interface to all
342 three of the database types currently supported by Berkeley DB.
350 This database type allows arbitrary key/value pairs to be stored in data
351 files. This is equivalent to the functionality provided by other
352 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
353 the files created using DB_HASH are not compatible with any of the
354 other packages mentioned.
356 A default hashing algorithm, which will be adequate for most
357 applications, is built into Berkeley DB. If you do need to use your own
358 hashing algorithm it is possible to write your own in Perl and have
359 B<DB_File> use it instead.
363 The btree format allows arbitrary key/value pairs to be stored in a
364 sorted, balanced binary tree.
366 As with the DB_HASH format, it is possible to provide a user defined
367 Perl routine to perform the comparison of keys. By default, though, the
368 keys are stored in lexical order.
372 DB_RECNO allows both fixed-length and variable-length flat text files
373 to be manipulated using the same key/value pair interface as in DB_HASH
374 and DB_BTREE. In this case the key will consist of a record (line)
379 =head2 Interface to Berkeley DB
381 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
382 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
383 allows B<DB_File> to access Berkeley DB files using either an
384 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
385 array (for the DB_RECNO file type).
387 In addition to the tie() interface, it is also possible to access most
388 of the functions provided in the Berkeley DB API directly.
389 See L<THE API INTERFACE>.
391 =head2 Opening a Berkeley DB Database File
393 Berkeley DB uses the function dbopen() to open or create a database.
394 Here is the C prototype for dbopen():
397 dbopen (const char * file, int flags, int mode,
398 DBTYPE type, const void * openinfo)
400 The parameter C<type> is an enumeration which specifies which of the 3
401 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
402 Depending on which of these is actually chosen, the final parameter,
403 I<openinfo> points to a data structure which allows tailoring of the
404 specific interface method.
406 This interface is handled slightly differently in B<DB_File>. Here is
407 an equivalent call using B<DB_File>:
409 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
411 The C<filename>, C<flags> and C<mode> parameters are the direct
412 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
413 performs the function of both the C<type> and C<openinfo> parameters in
416 In the example above $DB_HASH is actually a pre-defined reference to a
417 hash object. B<DB_File> has three of these pre-defined references.
418 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
420 The keys allowed in each of these pre-defined references is limited to
421 the names used in the equivalent C structure. So, for example, the
422 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
423 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
425 To change one of these elements, just assign to it like this:
427 $DB_HASH->{'cachesize'} = 10000 ;
429 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
430 usually adequate for most applications. If you do need to create extra
431 instances of these objects, constructors are available for each file
434 Here are examples of the constructors and the valid options available
435 for DB_HASH, DB_BTREE and DB_RECNO respectively.
437 $a = new DB_File::HASHINFO ;
445 $b = new DB_File::BTREEINFO ;
455 $c = new DB_File::RECNOINFO ;
464 The values stored in the hashes above are mostly the direct equivalent
465 of their C counterpart. Like their C counterparts, all are set to a
466 default values - that means you don't have to set I<all> of the
467 values when you only want to change one. Here is an example:
469 $a = new DB_File::HASHINFO ;
470 $a->{'cachesize'} = 12345 ;
471 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
473 A few of the options need extra discussion here. When used, the C
474 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
475 to C functions. In B<DB_File> these keys are used to store references
476 to Perl subs. Below are templates for each of the subs:
482 # return the hash value for $data
488 my ($key, $key2) = @_ ;
490 # return 0 if $key1 eq $key2
491 # -1 if $key1 lt $key2
492 # 1 if $key1 gt $key2
493 return (-1 , 0 or 1) ;
498 my ($key, $key2) = @_ ;
500 # return number of bytes of $key2 which are
501 # necessary to determine that it is greater than $key1
505 See L<Changing the BTREE sort order> for an example of using the
508 If you are using the DB_RECNO interface and you intend making use of
509 C<bval>, you should check out L<The 'bval' Option>.
511 =head2 Default Parameters
513 It is possible to omit some or all of the final 4 parameters in the
514 call to C<tie> and let them take default values. As DB_HASH is the most
515 common file format used, the call:
517 tie %A, "DB_File", "filename" ;
521 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
523 It is also possible to omit the filename parameter as well, so the
530 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
532 See L<In Memory Databases> for a discussion on the use of C<undef>
533 in place of a filename.
535 =head2 In Memory Databases
537 Berkeley DB allows the creation of in-memory databases by using NULL
538 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
539 uses C<undef> instead of NULL to provide this functionality.
543 The DB_HASH file format is probably the most commonly used of the three
544 file formats that B<DB_File> supports. It is also very straightforward
547 =head2 A Simple Example
549 This example shows how to create a database, add key/value pairs to the
550 database, delete keys/value pairs and finally how to enumerate the
551 contents of the database.
555 use vars qw( %h $k $v ) ;
557 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
558 or die "Cannot open file 'fruit': $!\n";
560 # Add a few key/value pairs to the file
561 $h{"apple"} = "red" ;
562 $h{"orange"} = "orange" ;
563 $h{"banana"} = "yellow" ;
564 $h{"tomato"} = "red" ;
566 # Check for existence of a key
567 print "Banana Exists\n\n" if $h{"banana"} ;
569 # Delete a key/value pair.
572 # print the contents of the file
573 while (($k, $v) = each %h)
574 { print "$k -> $v\n" }
586 Note that the like ordinary associative arrays, the order of the keys
587 retrieved is in an apparently random order.
591 The DB_BTREE format is useful when you want to store data in a given
592 order. By default the keys will be stored in lexical order, but as you
593 will see from the example shown in the next section, it is very easy to
594 define your own sorting function.
596 =head2 Changing the BTREE sort order
598 This script shows how to override the default sorting algorithm that
599 BTREE uses. Instead of using the normal lexical ordering, a case
600 insensitive compare function will be used.
609 my ($key1, $key2) = @_ ;
610 "\L$key1" cmp "\L$key2" ;
613 # specify the Perl sub that will do the comparison
614 $DB_BTREE->{'compare'} = \&Compare ;
616 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
617 or die "Cannot open file 'tree': $!\n" ;
619 # Add a key/value pair to the file
620 $h{'Wall'} = 'Larry' ;
621 $h{'Smith'} = 'John' ;
622 $h{'mouse'} = 'mickey' ;
623 $h{'duck'} = 'donald' ;
628 # Cycle through the keys printing them in order.
629 # Note it is not necessary to sort the keys as
630 # the btree will have kept them in order automatically.
636 Here is the output from the code above.
642 There are a few point to bear in mind if you want to change the
643 ordering in a BTREE database:
649 The new compare function must be specified when you create the database.
653 You cannot change the ordering once the database has been created. Thus
654 you must use the same compare function every time you access the
659 =head2 Handling Duplicate Keys
661 The BTREE file type optionally allows a single key to be associated
662 with an arbitrary number of values. This option is enabled by setting
663 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
665 There are some difficulties in using the tied hash interface if you
666 want to manipulate a BTREE database with duplicate keys. Consider this
672 use vars qw($filename %h ) ;
677 # Enable duplicate records
678 $DB_BTREE->{'flags'} = R_DUP ;
680 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
681 or die "Cannot open $filename: $!\n";
683 # Add some key/value pairs to the file
684 $h{'Wall'} = 'Larry' ;
685 $h{'Wall'} = 'Brick' ; # Note the duplicate key
686 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
687 $h{'Smith'} = 'John' ;
688 $h{'mouse'} = 'mickey' ;
690 # iterate through the associative array
691 # and print each key/value pair.
693 { print "$_ -> $h{$_}\n" }
705 As you can see 3 records have been successfully created with key C<Wall>
706 - the only thing is, when they are retrieved from the database they
707 I<seem> to have the same value, namely C<Larry>. The problem is caused
708 by the way that the associative array interface works. Basically, when
709 the associative array interface is used to fetch the value associated
710 with a given key, it will only ever retrieve the first value.
712 Although it may not be immediately obvious from the code above, the
713 associative array interface can be used to write values with duplicate
714 keys, but it cannot be used to read them back from the database.
716 The way to get around this problem is to use the Berkeley DB API method
717 called C<seq>. This method allows sequential access to key/value
718 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
719 and the API in general.
721 Here is the script above rewritten using the C<seq> API method.
726 use vars qw($filename $x %h $status $key $value) ;
731 # Enable duplicate records
732 $DB_BTREE->{'flags'} = R_DUP ;
734 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
735 or die "Cannot open $filename: $!\n";
737 # Add some key/value pairs to the file
738 $h{'Wall'} = 'Larry' ;
739 $h{'Wall'} = 'Brick' ; # Note the duplicate key
740 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
741 $h{'Smith'} = 'John' ;
742 $h{'mouse'} = 'mickey' ;
744 # iterate through the btree using seq
745 # and print each key/value pair.
747 for ($status = $x->seq($key, $value, R_FIRST) ;
749 $status = $x->seq($key, $value, R_NEXT) )
750 { print "$key -> $value\n" }
763 This time we have got all the key/value pairs, including the multiple
764 values associated with the key C<Wall>.
766 =head2 The get_dup() Method
768 B<DB_File> comes with a utility method, called C<get_dup>, to assist in
769 reading duplicate values from BTREE databases. The method can take the
772 $count = $x->get_dup($key) ;
773 @list = $x->get_dup($key) ;
774 %list = $x->get_dup($key, 1) ;
776 In a scalar context the method returns the number of values associated
777 with the key, C<$key>.
779 In list context, it returns all the values which match C<$key>. Note
780 that the values will be returned in an apparently random order.
782 In list context, if the second parameter is present and evaluates
783 TRUE, the method returns an associative array. The keys of the
784 associative array correspond to the values that matched in the BTREE
785 and the values of the array are a count of the number of times that
786 particular value occurred in the BTREE.
788 So assuming the database created above, we can use C<get_dup> like
791 my $cnt = $x->get_dup("Wall") ;
792 print "Wall occurred $cnt times\n" ;
794 my %hash = $x->get_dup("Wall", 1) ;
795 print "Larry is there\n" if $hash{'Larry'} ;
796 print "There are $hash{'Brick'} Brick Walls\n" ;
798 my @list = $x->get_dup("Wall") ;
799 print "Wall => [@list]\n" ;
801 @list = $x->get_dup("Smith") ;
802 print "Smith => [@list]\n" ;
804 @list = $x->get_dup("Dog") ;
805 print "Dog => [@list]\n" ;
810 Wall occurred 3 times
812 There are 2 Brick Walls
813 Wall => [Brick Brick Larry]
817 =head2 Matching Partial Keys
819 The BTREE interface has a feature which allows partial keys to be
820 matched. This functionality is I<only> available when the C<seq> method
821 is used along with the R_CURSOR flag.
823 $x->seq($key, $value, R_CURSOR) ;
825 Here is the relevant quote from the dbopen man page where it defines
826 the use of the R_CURSOR flag with seq:
828 Note, for the DB_BTREE access method, the returned key is not
829 necessarily an exact match for the specified key. The returned key
830 is the smallest key greater than or equal to the specified key,
831 permitting partial key matches and range searches.
833 In the example script below, the C<match> sub uses this feature to find
834 and print the first matching key/value pair given a partial key.
840 use vars qw($filename $x %h $st $key $value) ;
846 my $orig_key = $key ;
847 $x->seq($key, $value, R_CURSOR) ;
848 print "$orig_key\t-> $key\t-> $value\n" ;
854 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
855 or die "Cannot open $filename: $!\n";
857 # Add some key/value pairs to the file
858 $h{'mouse'} = 'mickey' ;
859 $h{'Wall'} = 'Larry' ;
860 $h{'Walls'} = 'Brick' ;
861 $h{'Smith'} = 'John' ;
866 for ($st = $x->seq($key, $value, R_FIRST) ;
868 $st = $x->seq($key, $value, R_NEXT) )
870 { print "$key -> $value\n" }
872 print "\nPARTIAL MATCH\n" ;
896 DB_RECNO provides an interface to flat text files. Both variable and
897 fixed length records are supported.
899 In order to make RECNO more compatible with Perl the array offset for
900 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
902 As with normal Perl arrays, a RECNO array can be accessed using
903 negative indexes. The index -1 refers to the last element of the array,
904 -2 the second last, and so on. Attempting to access an element before
905 the start of the array will raise a fatal run-time error.
907 =head2 The 'bval' Option
909 The operation of the bval option warrants some discussion. Here is the
910 definition of bval from the Berkeley DB 1.85 recno manual page:
912 The delimiting byte to be used to mark the end of a
913 record for variable-length records, and the pad charac-
914 ter for fixed-length records. If no value is speci-
915 fied, newlines (``\n'') are used to mark the end of
916 variable-length records and fixed-length records are
919 The second sentence is wrong. In actual fact bval will only default to
920 C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
921 openinfo parameter is used at all, the value that happens to be in bval
922 will be used. That means you always have to specify bval when making
923 use of any of the options in the openinfo parameter. This documentation
924 error will be fixed in the next release of Berkeley DB.
926 That clarifies the situation with regards Berkeley DB itself. What
927 about B<DB_File>? Well, the behavior defined in the quote above is
928 quite useful, so B<DB_File> conforms it.
930 That means that you can specify other options (e.g. cachesize) and
931 still have bval default to C<"\n"> for variable length records, and
932 space for fixed length records.
934 =head2 A Simple Example
936 Here is a simple example that uses RECNO.
942 tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
943 or die "Cannot open file 'text': $!\n" ;
945 # Add a few key/value pairs to the file
950 # Check for existence of a key
951 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
953 # use a negative index
954 print "The last element is $h[-1]\n" ;
955 print "The 2nd last element is $h[-2]\n" ;
959 Here is the output from the script:
962 Element 1 Exists with value blue
963 The last element is yellow
964 The 2nd last element is blue
968 As you can see from the example above, the tied array interface is
969 quite limited. To make the interface more useful, a number of methods
970 are supplied with B<DB_File> to simulate the standard array operations
971 that are not currently implemented in Perl's tied array interface. All
972 these methods are accessed via the object returned from the tie call.
974 Here are the methods:
978 =item B<$X-E<gt>push(list) ;>
980 Pushes the elements of C<list> to the end of the array.
982 =item B<$value = $X-E<gt>pop ;>
984 Removes and returns the last element of the array.
986 =item B<$X-E<gt>shift>
988 Removes and returns the first element of the array.
990 =item B<$X-E<gt>unshift(list) ;>
992 Pushes the elements of C<list> to the start of the array.
994 =item B<$X-E<gt>length>
996 Returns the number of elements in the array.
1000 =head2 Another Example
1002 Here is a more complete example that makes use of some of the methods
1003 described above. It also makes use of the API interface directly (see
1004 L<THE API INTERFACE>).
1007 use vars qw(@h $H $file $i) ;
1015 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
1016 or die "Cannot open file $file: $!\n" ;
1018 # first create a text file to play with
1026 # Print the records in order.
1028 # The length method is needed here because evaluating a tied
1029 # array in a scalar context does not return the number of
1030 # elements in the array.
1032 print "\nORIGINAL\n" ;
1033 foreach $i (0 .. $H->length - 1) {
1034 print "$i: $h[$i]\n" ;
1037 # use the push & pop methods
1040 print "\nThe last record was [$a]\n" ;
1042 # and the shift & unshift methods
1044 $H->unshift("first") ;
1045 print "The first record was [$a]\n" ;
1047 # Use the API to add a new record after record 2.
1049 $H->put($i, "Newbie", R_IAFTER) ;
1051 # and a new record before record 1.
1053 $H->put($i, "New One", R_IBEFORE) ;
1058 # now print the records in reverse order
1059 print "\nREVERSE\n" ;
1060 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1061 { print "$i: $h[$i]\n" }
1063 # same again, but use the API functions instead
1064 print "\nREVERSE again\n" ;
1065 my ($s, $k, $v) = (0, 0, 0) ;
1066 for ($s = $H->seq($k, $v, R_LAST) ;
1068 $s = $H->seq($k, $v, R_PREV))
1069 { print "$k: $v\n" }
1074 and this is what it outputs:
1083 The last record was [four]
1084 The first record was [zero]
1108 Rather than iterating through the array, C<@h> like this:
1112 it is necessary to use either this:
1114 foreach $i (0 .. $H->length - 1)
1118 for ($a = $H->get($k, $v, R_FIRST) ;
1120 $a = $H->get($k, $v, R_NEXT) )
1124 Notice that both times the C<put> method was used the record index was
1125 specified using a variable, C<$i>, rather than the literal value
1126 itself. This is because C<put> will return the record number of the
1127 inserted line via that parameter.
1131 =head1 THE API INTERFACE
1133 As well as accessing Berkeley DB using a tied hash or array, it is also
1134 possible to make direct use of most of the API functions defined in the
1135 Berkeley DB documentation.
1137 To do this you need to store a copy of the object returned from the tie.
1139 $db = tie %hash, "DB_File", "filename" ;
1141 Once you have done that, you can access the Berkeley DB API functions
1142 as B<DB_File> methods directly like this:
1144 $db->put($key, $value, R_NOOVERWRITE) ;
1146 B<Important:> If you have saved a copy of the object returned from
1147 C<tie>, the underlying database file will I<not> be closed until both
1148 the tied variable is untied and all copies of the saved object are
1152 $db = tie %hash, "DB_File", "filename"
1153 or die "Cannot tie filename: $!" ;
1158 See L<The untie() Gotcha> for more details.
1160 All the functions defined in L<dbopen> are available except for
1161 close() and dbopen() itself. The B<DB_File> method interface to the
1162 supported functions have been implemented to mirror the way Berkeley DB
1163 works whenever possible. In particular note that:
1169 The methods return a status value. All return 0 on success.
1170 All return -1 to signify an error and set C<$!> to the exact
1171 error code. The return code 1 generally (but not always) means that the
1172 key specified did not exist in the database.
1174 Other return codes are defined. See below and in the Berkeley DB
1175 documentation for details. The Berkeley DB documentation should be used
1176 as the definitive source.
1180 Whenever a Berkeley DB function returns data via one of its parameters,
1181 the equivalent B<DB_File> method does exactly the same.
1185 If you are careful, it is possible to mix API calls with the tied
1186 hash/array interface in the same piece of code. Although only a few of
1187 the methods used to implement the tied interface currently make use of
1188 the cursor, you should always assume that the cursor has been changed
1189 any time the tied hash/array interface is used. As an example, this
1190 code will probably not do what you expect:
1192 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1193 or die "Cannot tie $filename: $!" ;
1195 # Get the first key/value pair and set the cursor
1196 $X->seq($key, $value, R_FIRST) ;
1198 # this line will modify the cursor
1199 $count = scalar keys %x ;
1201 # Get the second key/value pair.
1202 # oops, it didn't, it got the last key/value pair!
1203 $X->seq($key, $value, R_NEXT) ;
1205 The code above can be rearranged to get around the problem, like this:
1207 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1208 or die "Cannot tie $filename: $!" ;
1210 # this line will modify the cursor
1211 $count = scalar keys %x ;
1213 # Get the first key/value pair and set the cursor
1214 $X->seq($key, $value, R_FIRST) ;
1216 # Get the second key/value pair.
1218 $X->seq($key, $value, R_NEXT) ;
1222 All the constants defined in L<dbopen> for use in the flags parameters
1223 in the methods defined below are also available. Refer to the Berkeley
1224 DB documentation for the precise meaning of the flags values.
1226 Below is a list of the methods available.
1230 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1232 Given a key (C<$key>) this method reads the value associated with it
1233 from the database. The value read from the database is returned in the
1234 C<$value> parameter.
1236 If the key does not exist the method returns 1.
1238 No flags are currently defined for this method.
1240 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1242 Stores the key/value pair in the database.
1244 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1245 will have the record number of the inserted key/value pair set.
1247 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1250 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1252 Removes all key/value pairs with key C<$key> from the database.
1254 A return code of 1 means that the requested key was not in the
1257 R_CURSOR is the only valid flag at present.
1259 =item B<$status = $X-E<gt>fd ;>
1261 Returns the file descriptor for the underlying database.
1263 See L<Locking Databases> for an example of how to make use of the
1264 C<fd> method to lock your database.
1266 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1268 This interface allows sequential retrieval from the database. See
1269 L<dbopen> for full details.
1271 Both the C<$key> and C<$value> parameters will be set to the key/value
1272 pair read from the database.
1274 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1275 R_FIRST, R_LAST, R_NEXT and R_PREV.
1277 =item B<$status = $X-E<gt>sync([$flags]) ;>
1279 Flushes any cached buffers to disk.
1281 R_RECNOSYNC is the only valid flag at present.
1285 =head1 HINTS AND TIPS
1288 =head2 Locking Databases
1290 Concurrent access of a read-write database by several parties requires
1291 them all to use some kind of locking. Here's an example of Tom's that
1292 uses the I<fd> method to get the file descriptor, and then a careful
1293 open() to give something Perl will flock() for you. Run this repeatedly
1294 in the background to watch the locks granted in proper order.
1305 my($oldval, $fd, $db, %db, $value, $key);
1307 $key = shift || 'default';
1308 $value = shift || 'magic';
1312 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1313 || die "dbcreat /tmp/foo.db $!";
1315 print "$$: db fd is $fd\n";
1316 open(DB_FH, "+<&=$fd") || die "dup $!";
1319 unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
1320 print "$$: CONTENTION; can't read during write update!
1321 Waiting for read lock ($!) ....";
1322 unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
1324 print "$$: Read lock granted\n";
1326 $oldval = $db{$key};
1327 print "$$: Old value was $oldval\n";
1328 flock(DB_FH, LOCK_UN);
1330 unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
1331 print "$$: CONTENTION; must have exclusive lock!
1332 Waiting for write lock ($!) ....";
1333 unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
1336 print "$$: Write lock granted\n";
1338 $db->sync; # to flush
1341 flock(DB_FH, LOCK_UN);
1345 print "$$: Updated db to $key=$value\n";
1347 =head2 Sharing Databases With C Applications
1349 There is no technical reason why a Berkeley DB database cannot be
1350 shared by both a Perl and a C application.
1352 The vast majority of problems that are reported in this area boil down
1353 to the fact that C strings are NULL terminated, whilst Perl strings are
1356 Here is a real example. Netscape 2.0 keeps a record of the locations you
1357 visit along with the time you last visited them in a DB_HASH database.
1358 This is usually stored in the file F<~/.netscape/history.db>. The key
1359 field in the database is the location string and the value field is the
1360 time the location was last visited stored as a 4 byte binary value.
1362 If you haven't already guessed, the location string is stored with a
1363 terminating NULL. This means you need to be careful when accessing the
1366 Here is a snippet of code that is loosely based on Tom Christiansen's
1367 I<ggh> script (available from your nearest CPAN archive in
1368 F<authors/id/TOMC/scripts/nshist.gz>).
1374 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1375 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1377 $HISTORY = "$dotdir/.netscape/history.db";
1379 tie %hist_db, 'DB_File', $HISTORY
1380 or die "Cannot open $HISTORY: $!\n" ;;
1382 # Dump the complete database
1383 while ( ($href, $binary_time) = each %hist_db ) {
1385 # remove the terminating NULL
1386 $href =~ s/\x00$// ;
1388 # convert the binary time into a user friendly string
1389 $date = localtime unpack("V", $binary_time);
1390 print "$date $href\n" ;
1393 # check for the existence of a specific key
1394 # remember to add the NULL
1395 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
1396 $date = localtime unpack("V", $binary_time) ;
1397 print "Last visited mox.perl.com on $date\n" ;
1400 print "Never visited mox.perl.com\n"
1405 =head2 The untie() Gotcha
1407 If you make use of the Berkeley DB API, it is I<very> strongly
1408 recommended that you read L<perltie/The untie Gotcha>.
1410 Even if you don't currently make use of the API interface, it is still
1413 Here is an example which illustrates the problem from a B<DB_File>
1422 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
1423 or die "Cannot tie first time: $!" ;
1429 tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1430 or die "Cannot tie second time: $!" ;
1434 When run, the script will produce this error message:
1436 Cannot tie second time: Invalid argument at bad.file line 14.
1438 Although the error message above refers to the second tie() statement
1439 in the script, the source of the problem is really with the untie()
1440 statement that precedes it.
1442 Having read L<perltie> you will probably have already guessed that the
1443 error is caused by the extra copy of the tied object stored in C<$X>.
1444 If you haven't, then the problem boils down to the fact that the
1445 B<DB_File> destructor, DESTROY, will not be called until I<all>
1446 references to the tied object are destroyed. Both the tied variable,
1447 C<%x>, and C<$X> above hold a reference to the object. The call to
1448 untie() will destroy the first, but C<$X> still holds a valid
1449 reference, so the destructor will not get called and the database file
1450 F<tst.fil> will remain open. The fact that Berkeley DB then reports the
1451 attempt to open a database that is alreday open via the catch-all
1452 "Invalid argument" doesn't help.
1454 If you run the script with the C<-w> flag the error message becomes:
1456 untie attempted while 1 inner references still exist at bad.file line 12.
1457 Cannot tie second time: Invalid argument at bad.file line 14.
1459 which pinpoints the real problem. Finally the script can now be
1460 modified to fix the original problem by destroying the API object
1469 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1473 =head1 COMMON QUESTIONS
1475 =head2 Why is there Perl source in my database?
1477 If you look at the contents of a database file created by DB_File,
1478 there can sometimes be part of a Perl script included in it.
1480 This happens because Berkeley DB uses dynamic memory to allocate
1481 buffers which will subsequently be written to the database file. Being
1482 dynamic, the memory could have been used for anything before DB
1483 malloced it. As Berkeley DB doesn't clear the memory once it has been
1484 allocated, the unused portions will contain random junk. In the case
1485 where a Perl script gets written to the database, the random junk will
1486 correspond to an area of dynamic memory that happened to be used during
1487 the compilation of the script.
1489 Unless you don't like the possibility of there being part of your Perl
1490 scripts embedded in a database file, this is nothing to worry about.
1492 =head2 How do I store complex data structures with DB_File?
1494 Although B<DB_File> cannot do this directly, there is a module which
1495 can layer transparently over B<DB_File> to accomplish this feat.
1497 Check out the MLDBM module, available on CPAN in the directory
1498 F<modules/by-module/MLDBM>.
1500 =head2 What does "Invalid Argument" mean?
1502 You will get this error message when one of the parameters in the
1503 C<tie> call is wrong. Unfortunately there are quite a few parameters to
1504 get wrong, so it can be difficult to figure out which one it is.
1506 Here are a couple of possibilities:
1512 Attempting to reopen a database without closing it.
1516 Using the O_WRONLY flag.
1520 =head2 What does "Bareword 'DB_File' not allowed" mean?
1522 You will encounter this particular error message when you have the
1523 C<strict 'subs'> pragma (or the full strict pragma) in your script.
1524 Consider this script:
1529 tie %x, DB_File, "filename" ;
1531 Running it produces the error in question:
1533 Bareword "DB_File" not allowed while "strict subs" in use
1535 To get around the error, place the word C<DB_File> in either single or
1536 double quotes, like this:
1538 tie %x, "DB_File", "filename" ;
1540 Although it might seem like a real pain, it is really worth the effort
1541 of having a C<use strict> in all your scripts.
1553 When B<DB_File> is opening a database file it no longer terminates the
1554 process if I<dbopen> returned an error. This allows file protection
1555 errors to be caught at run time. Thanks to Judith Grass
1556 E<lt>grass@cybercash.comE<gt> for spotting the bug.
1560 Added prototype support for multiple btree compare callbacks.
1564 B<DB_File> has been in use for over a year. To reflect that, the
1565 version number has been incremented to 1.0.
1567 Added complete support for multiple concurrent callbacks.
1569 Using the I<push> method on an empty list didn't work properly. This
1574 Fixed a core dump problem with SunOS.
1576 The return value from TIEHASH wasn't set to NULL when dbopen returned
1581 Merged OS/2 specific code into DB_File.xs
1583 Removed some redundant code in DB_File.xs.
1585 Documentation update.
1587 Allow negative subscripts with RECNO interface.
1589 Changed the default flags from O_RDWR to O_CREAT|O_RDWR.
1591 The example code which showed how to lock a database needed a call to
1592 C<sync> added. Without it the resultant database file was empty.
1594 Added get_dup method.
1598 Documentation update.
1600 B<DB_File> now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl
1603 The standard hash function C<exists> is now supported.
1605 Modified the behavior of get_dup. When it returns an associative
1606 array, the value is the count of the number of matching BTREE values.
1610 Minor documentation changes.
1612 Fixed a bug in hash_cb. Patches supplied by Dave Hammen,
1613 E<lt>hammen@gothamcity.jsc.nasa.govE<gt>.
1615 Fixed a bug with the constructors for DB_File::HASHINFO,
1616 DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up the
1617 constructors to make them C<-w> clean.
1619 Reworked part of the test harness to be more locale friendly.
1623 Made all scripts in the documentation C<strict> and C<-w> clean.
1625 Added logic to F<DB_File.xs> to allow the module to be built after Perl
1630 Minor namespace cleanup: Localized C<PrintBtree>.
1634 Fixed bug with RECNO, where bval wasn't defaulting to "\n".
1638 Documented operation of bval.
1642 Minor bug fix in DB_File::HASHINFO, DB_File::RECNOINFO and
1645 Changed default mode to 0666.
1649 Fixed fd method so that it still returns -1 for in-memory files when db
1654 Documented the untie gotcha.
1658 Documented the incompatibility with version 2 of Berkeley DB.
1662 Minor changes to DB_FIle.xs and DB_File.pm
1666 Made it illegal to tie an associative array to a RECNO database and an
1667 ordinary array to a HASH or BTREE database.
1673 Some older versions of Berkeley DB had problems with fixed length
1674 records using the RECNO file format. The newest version at the time of
1675 writing was 1.85 - this seems to have fixed the problems with RECNO.
1677 I am sure there are bugs in the code. If you do find any, or can
1678 suggest any enhancements, I would welcome your comments.
1682 B<DB_File> comes with the standard Perl source distribution. Look in
1683 the directory F<ext/DB_File>.
1685 This version of B<DB_File> will only work with version 1.x of Berkeley
1686 DB. It is I<not> yet compatible with version 2.
1688 Version 1 of Berkeley DB is available at your nearest CPAN archive (see
1689 L<perlmod/"CPAN"> for a list) in F<src/misc/db.1.85.tar.gz>, or via the
1690 host F<ftp.cs.berkeley.edu> in F</ucb/4bsd/db.tar.gz>. Alternatively,
1691 check out the Berkeley DB home page at F<http://www.bostic.com/db>. It
1692 is I<not> under the GPL.
1694 If you are running IRIX, then get Berkeley DB from
1695 F<http://reality.sgi.com/ariel>. It has the patches necessary to
1696 compile properly on IRIX 5.3.
1698 As of January 1997, version 1.86 of Berkeley DB is available from the
1699 Berkeley DB home page. Although this release does fix a number of bugs
1700 that were present in 1.85 you should be aware of the following
1701 information (taken from the Berkeley DB home page) before you consider
1704 DB version 1.86 includes a new implementation of the hash access
1705 method that fixes a variety of hashing problems found in DB version
1706 1.85. We are making it available as an interim solution until DB
1709 PLEASE NOTE: the underlying file format for the hash access method
1710 changed between version 1.85 and version 1.86, so you will have to
1711 dump and reload all of your databases to convert from version 1.85
1712 to version 1.86. If you do not absolutely require the fixes from
1713 version 1.86, we strongly urge you to wait until DB 2.0 is released
1714 before upgrading from 1.85.
1719 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>
1723 The DB_File interface was written by Paul Marquess
1724 E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
1725 Questions about the DB system itself may be addressed to
1726 E<lt>db@sleepycat.com<gt>.