1 # DB_File.pm -- Perl 5 interface to Berkeley DB
3 # written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
4 # last modified 20th Nov 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 FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") }
102 sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") }
103 sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") }
105 package DB_File::RECNOINFO ;
109 @DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
115 bless { VALID => { map {$_, 1}
116 qw( bval cachesize psize flags lorder reclen bfname )
122 package DB_File::BTREEINFO ;
126 @DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
132 bless { VALID => { map {$_, 1}
133 qw( flags cachesize maxkeypage minkeypage psize
134 compare prefix lorder )
144 use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO $db_version) ;
150 #typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
151 $DB_BTREE = new DB_File::BTREEINFO ;
152 $DB_HASH = new DB_File::HASHINFO ;
153 $DB_RECNO = new DB_File::RECNOINFO ;
159 @ISA = qw(Tie::Hash Exporter DynaLoader);
161 $DB_BTREE $DB_HASH $DB_RECNO
196 ($constname = $AUTOLOAD) =~ s/.*:://;
197 my $val = constant($constname, @_ ? $_[0] : 0);
199 if ($! =~ /Invalid/) {
200 $AutoLoader::AUTOLOAD = $AUTOLOAD;
201 goto &AutoLoader::AUTOLOAD;
204 my($pack,$file,$line) = caller;
205 croak "Your vendor has not defined DB macro $constname, used at $file line $line.
209 eval "sub $AUTOLOAD { $val }";
215 # Make all Fcntl O_XXX constants available for importing
217 my @O = grep /^O_/, @Fcntl::EXPORT;
218 Fcntl->import(@O); # first we import what we want to export
222 ## import borrowed from IO::File
223 ## exports Fcntl constants if available.
226 # my $callpkg = caller;
227 # Exporter::export $pkg, $callpkg, @_;
230 # Exporter::export 'Fcntl', $callpkg, '/^O_/';
234 bootstrap DB_File $VERSION;
236 # Preloaded methods go here. Autoload methods go after __END__, and are
237 # processed by the autosplit program.
239 sub tie_hash_or_array
242 my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ;
244 $arg[4] = tied %{ $arg[4] }
245 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
247 # make recno in Berkeley DB version 2 work like recno in version 1.
248 if ($db_version > 1 and defined $arg[4] and $arg[4] =~ /RECNO/ and
249 $arg[1] and ! -e $arg[1]) {
250 open(FH, ">$arg[1]") or return undef ;
252 chmod $arg[3] ? $arg[3] : 0666 , $arg[1] ;
255 DoTie_($tieHASH, @arg) ;
260 tie_hash_or_array(@_) ;
265 tie_hash_or_array(@_) ;
272 my $status = $self->seq($key, $value, R_FIRST());
275 while ($status == 0) {
277 $status = $self->seq($key, $value, R_NEXT());
279 foreach $key (reverse @keys) {
280 my $s = $self->del($key);
286 croak "Usage: \$db->get_dup(key [,flag])\n"
287 unless @_ == 2 or @_ == 3 ;
294 my $wantarray = wantarray ;
300 # iterate through the database until either EOF ($status == 0)
301 # or a different key is encountered ($key ne $origkey).
302 for ($status = $db->seq($key, $value, R_CURSOR()) ;
303 $status == 0 and $key eq $origkey ;
304 $status = $db->seq($key, $value, R_NEXT()) ) {
306 # save the value or count number of matches
309 { ++ $values{$value} }
311 { push (@values, $value) }
318 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
327 DB_File - Perl5 access to Berkeley DB version 1.x
333 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
334 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
335 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
337 $status = $X->del($key [, $flags]) ;
338 $status = $X->put($key, $value [, $flags]) ;
339 $status = $X->get($key, $value [, $flags]) ;
340 $status = $X->seq($key, $value, $flags) ;
341 $status = $X->sync([$flags]) ;
345 $count = $X->get_dup($key) ;
346 @list = $X->get_dup($key) ;
347 %list = $X->get_dup($key, 1) ;
361 B<DB_File> is a module which allows Perl programs to make use of the
362 facilities provided by Berkeley DB version 1.x (if you have a newer
363 version of DB, see L<Using DB_File with Berkeley DB version 2>). It is
364 assumed that you have a copy of the Berkeley DB manual pages at hand
365 when reading this documentation. The interface defined here mirrors the
366 Berkeley DB interface closely.
368 Berkeley DB is a C library which provides a consistent interface to a
369 number of database formats. B<DB_File> provides an interface to all
370 three of the database types currently supported by Berkeley DB.
378 This database type allows arbitrary key/value pairs to be stored in data
379 files. This is equivalent to the functionality provided by other
380 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
381 the files created using DB_HASH are not compatible with any of the
382 other packages mentioned.
384 A default hashing algorithm, which will be adequate for most
385 applications, is built into Berkeley DB. If you do need to use your own
386 hashing algorithm it is possible to write your own in Perl and have
387 B<DB_File> use it instead.
391 The btree format allows arbitrary key/value pairs to be stored in a
392 sorted, balanced binary tree.
394 As with the DB_HASH format, it is possible to provide a user defined
395 Perl routine to perform the comparison of keys. By default, though, the
396 keys are stored in lexical order.
400 DB_RECNO allows both fixed-length and variable-length flat text files
401 to be manipulated using the same key/value pair interface as in DB_HASH
402 and DB_BTREE. In this case the key will consist of a record (line)
407 =head2 Using DB_File with Berkeley DB version 2
409 Although B<DB_File> is intended to be used with Berkeley DB version 1,
410 it can also be used with version 2. In this case the interface is
411 limited to the functionality provided by Berkeley DB 1.x. Anywhere the
412 version 2 interface differs, B<DB_File> arranges for it to work like
413 version 1. This feature allows B<DB_File> scripts that were built with
414 version 1 to be migrated to version 2 without any changes.
416 If you want to make use of the new features available in Berkeley DB
417 2.x, use the Perl module B<BerkeleyDB> instead.
419 At the time of writing this document the B<BerkeleyDB> module is still
420 alpha quality (the version number is < 1.0), and so unsuitable for use
421 in any serious development work. Once its version number is >= 1.0, it
422 is considered stable enough for real work.
424 B<Note:> The database file format has changed in Berkeley DB version 2.
425 If you cannot recreate your databases, you must dump any existing
426 databases with the C<db_dump185> utility that comes with Berkeley DB.
427 Once you have upgraded DB_File to use Berkeley DB version 2, your
428 databases can be recreated using C<db_load>. Refer to the Berkeley DB
429 documentation for further details.
431 Please read L<COPYRIGHT> before using version 2.x of Berkeley DB with
434 =head2 Interface to Berkeley DB
436 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
437 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
438 allows B<DB_File> to access Berkeley DB files using either an
439 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
440 array (for the DB_RECNO file type).
442 In addition to the tie() interface, it is also possible to access most
443 of the functions provided in the Berkeley DB API directly.
444 See L<THE API INTERFACE>.
446 =head2 Opening a Berkeley DB Database File
448 Berkeley DB uses the function dbopen() to open or create a database.
449 Here is the C prototype for dbopen():
452 dbopen (const char * file, int flags, int mode,
453 DBTYPE type, const void * openinfo)
455 The parameter C<type> is an enumeration which specifies which of the 3
456 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
457 Depending on which of these is actually chosen, the final parameter,
458 I<openinfo> points to a data structure which allows tailoring of the
459 specific interface method.
461 This interface is handled slightly differently in B<DB_File>. Here is
462 an equivalent call using B<DB_File>:
464 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
466 The C<filename>, C<flags> and C<mode> parameters are the direct
467 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
468 performs the function of both the C<type> and C<openinfo> parameters in
471 In the example above $DB_HASH is actually a pre-defined reference to a
472 hash object. B<DB_File> has three of these pre-defined references.
473 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
475 The keys allowed in each of these pre-defined references is limited to
476 the names used in the equivalent C structure. So, for example, the
477 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
478 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
480 To change one of these elements, just assign to it like this:
482 $DB_HASH->{'cachesize'} = 10000 ;
484 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
485 usually adequate for most applications. If you do need to create extra
486 instances of these objects, constructors are available for each file
489 Here are examples of the constructors and the valid options available
490 for DB_HASH, DB_BTREE and DB_RECNO respectively.
492 $a = new DB_File::HASHINFO ;
500 $b = new DB_File::BTREEINFO ;
510 $c = new DB_File::RECNOINFO ;
519 The values stored in the hashes above are mostly the direct equivalent
520 of their C counterpart. Like their C counterparts, all are set to a
521 default values - that means you don't have to set I<all> of the
522 values when you only want to change one. Here is an example:
524 $a = new DB_File::HASHINFO ;
525 $a->{'cachesize'} = 12345 ;
526 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
528 A few of the options need extra discussion here. When used, the C
529 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
530 to C functions. In B<DB_File> these keys are used to store references
531 to Perl subs. Below are templates for each of the subs:
537 # return the hash value for $data
543 my ($key, $key2) = @_ ;
545 # return 0 if $key1 eq $key2
546 # -1 if $key1 lt $key2
547 # 1 if $key1 gt $key2
548 return (-1 , 0 or 1) ;
553 my ($key, $key2) = @_ ;
555 # return number of bytes of $key2 which are
556 # necessary to determine that it is greater than $key1
560 See L<Changing the BTREE sort order> for an example of using the
563 If you are using the DB_RECNO interface and you intend making use of
564 C<bval>, you should check out L<The 'bval' Option>.
566 =head2 Default Parameters
568 It is possible to omit some or all of the final 4 parameters in the
569 call to C<tie> and let them take default values. As DB_HASH is the most
570 common file format used, the call:
572 tie %A, "DB_File", "filename" ;
576 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
578 It is also possible to omit the filename parameter as well, so the
585 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
587 See L<In Memory Databases> for a discussion on the use of C<undef>
588 in place of a filename.
590 =head2 In Memory Databases
592 Berkeley DB allows the creation of in-memory databases by using NULL
593 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
594 uses C<undef> instead of NULL to provide this functionality.
598 The DB_HASH file format is probably the most commonly used of the three
599 file formats that B<DB_File> supports. It is also very straightforward
602 =head2 A Simple Example
604 This example shows how to create a database, add key/value pairs to the
605 database, delete keys/value pairs and finally how to enumerate the
606 contents of the database.
610 use vars qw( %h $k $v ) ;
612 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
613 or die "Cannot open file 'fruit': $!\n";
615 # Add a few key/value pairs to the file
616 $h{"apple"} = "red" ;
617 $h{"orange"} = "orange" ;
618 $h{"banana"} = "yellow" ;
619 $h{"tomato"} = "red" ;
621 # Check for existence of a key
622 print "Banana Exists\n\n" if $h{"banana"} ;
624 # Delete a key/value pair.
627 # print the contents of the file
628 while (($k, $v) = each %h)
629 { print "$k -> $v\n" }
641 Note that the like ordinary associative arrays, the order of the keys
642 retrieved is in an apparently random order.
646 The DB_BTREE format is useful when you want to store data in a given
647 order. By default the keys will be stored in lexical order, but as you
648 will see from the example shown in the next section, it is very easy to
649 define your own sorting function.
651 =head2 Changing the BTREE sort order
653 This script shows how to override the default sorting algorithm that
654 BTREE uses. Instead of using the normal lexical ordering, a case
655 insensitive compare function will be used.
664 my ($key1, $key2) = @_ ;
665 "\L$key1" cmp "\L$key2" ;
668 # specify the Perl sub that will do the comparison
669 $DB_BTREE->{'compare'} = \&Compare ;
671 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
672 or die "Cannot open file 'tree': $!\n" ;
674 # Add a key/value pair to the file
675 $h{'Wall'} = 'Larry' ;
676 $h{'Smith'} = 'John' ;
677 $h{'mouse'} = 'mickey' ;
678 $h{'duck'} = 'donald' ;
683 # Cycle through the keys printing them in order.
684 # Note it is not necessary to sort the keys as
685 # the btree will have kept them in order automatically.
691 Here is the output from the code above.
697 There are a few point to bear in mind if you want to change the
698 ordering in a BTREE database:
704 The new compare function must be specified when you create the database.
708 You cannot change the ordering once the database has been created. Thus
709 you must use the same compare function every time you access the
714 =head2 Handling Duplicate Keys
716 The BTREE file type optionally allows a single key to be associated
717 with an arbitrary number of values. This option is enabled by setting
718 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
720 There are some difficulties in using the tied hash interface if you
721 want to manipulate a BTREE database with duplicate keys. Consider this
727 use vars qw($filename %h ) ;
732 # Enable duplicate records
733 $DB_BTREE->{'flags'} = R_DUP ;
735 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
736 or die "Cannot open $filename: $!\n";
738 # Add some key/value pairs to the file
739 $h{'Wall'} = 'Larry' ;
740 $h{'Wall'} = 'Brick' ; # Note the duplicate key
741 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
742 $h{'Smith'} = 'John' ;
743 $h{'mouse'} = 'mickey' ;
745 # iterate through the associative array
746 # and print each key/value pair.
748 { print "$_ -> $h{$_}\n" }
760 As you can see 3 records have been successfully created with key C<Wall>
761 - the only thing is, when they are retrieved from the database they
762 I<seem> to have the same value, namely C<Larry>. The problem is caused
763 by the way that the associative array interface works. Basically, when
764 the associative array interface is used to fetch the value associated
765 with a given key, it will only ever retrieve the first value.
767 Although it may not be immediately obvious from the code above, the
768 associative array interface can be used to write values with duplicate
769 keys, but it cannot be used to read them back from the database.
771 The way to get around this problem is to use the Berkeley DB API method
772 called C<seq>. This method allows sequential access to key/value
773 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
774 and the API in general.
776 Here is the script above rewritten using the C<seq> API method.
781 use vars qw($filename $x %h $status $key $value) ;
786 # Enable duplicate records
787 $DB_BTREE->{'flags'} = R_DUP ;
789 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
790 or die "Cannot open $filename: $!\n";
792 # Add some key/value pairs to the file
793 $h{'Wall'} = 'Larry' ;
794 $h{'Wall'} = 'Brick' ; # Note the duplicate key
795 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
796 $h{'Smith'} = 'John' ;
797 $h{'mouse'} = 'mickey' ;
799 # iterate through the btree using seq
800 # and print each key/value pair.
802 for ($status = $x->seq($key, $value, R_FIRST) ;
804 $status = $x->seq($key, $value, R_NEXT) )
805 { print "$key -> $value\n" }
818 This time we have got all the key/value pairs, including the multiple
819 values associated with the key C<Wall>.
821 =head2 The get_dup() Method
823 B<DB_File> comes with a utility method, called C<get_dup>, to assist in
824 reading duplicate values from BTREE databases. The method can take the
827 $count = $x->get_dup($key) ;
828 @list = $x->get_dup($key) ;
829 %list = $x->get_dup($key, 1) ;
831 In a scalar context the method returns the number of values associated
832 with the key, C<$key>.
834 In list context, it returns all the values which match C<$key>. Note
835 that the values will be returned in an apparently random order.
837 In list context, if the second parameter is present and evaluates
838 TRUE, the method returns an associative array. The keys of the
839 associative array correspond to the values that matched in the BTREE
840 and the values of the array are a count of the number of times that
841 particular value occurred in the BTREE.
843 So assuming the database created above, we can use C<get_dup> like
846 my $cnt = $x->get_dup("Wall") ;
847 print "Wall occurred $cnt times\n" ;
849 my %hash = $x->get_dup("Wall", 1) ;
850 print "Larry is there\n" if $hash{'Larry'} ;
851 print "There are $hash{'Brick'} Brick Walls\n" ;
853 my @list = $x->get_dup("Wall") ;
854 print "Wall => [@list]\n" ;
856 @list = $x->get_dup("Smith") ;
857 print "Smith => [@list]\n" ;
859 @list = $x->get_dup("Dog") ;
860 print "Dog => [@list]\n" ;
865 Wall occurred 3 times
867 There are 2 Brick Walls
868 Wall => [Brick Brick Larry]
872 =head2 Matching Partial Keys
874 The BTREE interface has a feature which allows partial keys to be
875 matched. This functionality is I<only> available when the C<seq> method
876 is used along with the R_CURSOR flag.
878 $x->seq($key, $value, R_CURSOR) ;
880 Here is the relevant quote from the dbopen man page where it defines
881 the use of the R_CURSOR flag with seq:
883 Note, for the DB_BTREE access method, the returned key is not
884 necessarily an exact match for the specified key. The returned key
885 is the smallest key greater than or equal to the specified key,
886 permitting partial key matches and range searches.
888 In the example script below, the C<match> sub uses this feature to find
889 and print the first matching key/value pair given a partial key.
895 use vars qw($filename $x %h $st $key $value) ;
901 my $orig_key = $key ;
902 $x->seq($key, $value, R_CURSOR) ;
903 print "$orig_key\t-> $key\t-> $value\n" ;
909 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
910 or die "Cannot open $filename: $!\n";
912 # Add some key/value pairs to the file
913 $h{'mouse'} = 'mickey' ;
914 $h{'Wall'} = 'Larry' ;
915 $h{'Walls'} = 'Brick' ;
916 $h{'Smith'} = 'John' ;
921 for ($st = $x->seq($key, $value, R_FIRST) ;
923 $st = $x->seq($key, $value, R_NEXT) )
925 { print "$key -> $value\n" }
927 print "\nPARTIAL MATCH\n" ;
951 DB_RECNO provides an interface to flat text files. Both variable and
952 fixed length records are supported.
954 In order to make RECNO more compatible with Perl the array offset for
955 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
957 As with normal Perl arrays, a RECNO array can be accessed using
958 negative indexes. The index -1 refers to the last element of the array,
959 -2 the second last, and so on. Attempting to access an element before
960 the start of the array will raise a fatal run-time error.
962 =head2 The 'bval' Option
964 The operation of the bval option warrants some discussion. Here is the
965 definition of bval from the Berkeley DB 1.85 recno manual page:
967 The delimiting byte to be used to mark the end of a
968 record for variable-length records, and the pad charac-
969 ter for fixed-length records. If no value is speci-
970 fied, newlines (``\n'') are used to mark the end of
971 variable-length records and fixed-length records are
974 The second sentence is wrong. In actual fact bval will only default to
975 C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
976 openinfo parameter is used at all, the value that happens to be in bval
977 will be used. That means you always have to specify bval when making
978 use of any of the options in the openinfo parameter. This documentation
979 error will be fixed in the next release of Berkeley DB.
981 That clarifies the situation with regards Berkeley DB itself. What
982 about B<DB_File>? Well, the behavior defined in the quote above is
983 quite useful, so B<DB_File> conforms it.
985 That means that you can specify other options (e.g. cachesize) and
986 still have bval default to C<"\n"> for variable length records, and
987 space for fixed length records.
989 =head2 A Simple Example
991 Here is a simple example that uses RECNO.
997 tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
998 or die "Cannot open file 'text': $!\n" ;
1000 # Add a few key/value pairs to the file
1005 # Check for existence of a key
1006 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
1008 # use a negative index
1009 print "The last element is $h[-1]\n" ;
1010 print "The 2nd last element is $h[-2]\n" ;
1014 Here is the output from the script:
1017 Element 1 Exists with value blue
1018 The last element is yellow
1019 The 2nd last element is blue
1021 =head2 Extra Methods
1023 As you can see from the example above, the tied array interface is
1024 quite limited. To make the interface more useful, a number of methods
1025 are supplied with B<DB_File> to simulate the standard array operations
1026 that are not currently implemented in Perl's tied array interface. All
1027 these methods are accessed via the object returned from the tie call.
1029 Here are the methods:
1033 =item B<$X-E<gt>push(list) ;>
1035 Pushes the elements of C<list> to the end of the array.
1037 =item B<$value = $X-E<gt>pop ;>
1039 Removes and returns the last element of the array.
1041 =item B<$X-E<gt>shift>
1043 Removes and returns the first element of the array.
1045 =item B<$X-E<gt>unshift(list) ;>
1047 Pushes the elements of C<list> to the start of the array.
1049 =item B<$X-E<gt>length>
1051 Returns the number of elements in the array.
1055 =head2 Another Example
1057 Here is a more complete example that makes use of some of the methods
1058 described above. It also makes use of the API interface directly (see
1059 L<THE API INTERFACE>).
1062 use vars qw(@h $H $file $i) ;
1070 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
1071 or die "Cannot open file $file: $!\n" ;
1073 # first create a text file to play with
1081 # Print the records in order.
1083 # The length method is needed here because evaluating a tied
1084 # array in a scalar context does not return the number of
1085 # elements in the array.
1087 print "\nORIGINAL\n" ;
1088 foreach $i (0 .. $H->length - 1) {
1089 print "$i: $h[$i]\n" ;
1092 # use the push & pop methods
1095 print "\nThe last record was [$a]\n" ;
1097 # and the shift & unshift methods
1099 $H->unshift("first") ;
1100 print "The first record was [$a]\n" ;
1102 # Use the API to add a new record after record 2.
1104 $H->put($i, "Newbie", R_IAFTER) ;
1106 # and a new record before record 1.
1108 $H->put($i, "New One", R_IBEFORE) ;
1113 # now print the records in reverse order
1114 print "\nREVERSE\n" ;
1115 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1116 { print "$i: $h[$i]\n" }
1118 # same again, but use the API functions instead
1119 print "\nREVERSE again\n" ;
1120 my ($s, $k, $v) = (0, 0, 0) ;
1121 for ($s = $H->seq($k, $v, R_LAST) ;
1123 $s = $H->seq($k, $v, R_PREV))
1124 { print "$k: $v\n" }
1129 and this is what it outputs:
1138 The last record was [four]
1139 The first record was [zero]
1163 Rather than iterating through the array, C<@h> like this:
1167 it is necessary to use either this:
1169 foreach $i (0 .. $H->length - 1)
1173 for ($a = $H->get($k, $v, R_FIRST) ;
1175 $a = $H->get($k, $v, R_NEXT) )
1179 Notice that both times the C<put> method was used the record index was
1180 specified using a variable, C<$i>, rather than the literal value
1181 itself. This is because C<put> will return the record number of the
1182 inserted line via that parameter.
1186 =head1 THE API INTERFACE
1188 As well as accessing Berkeley DB using a tied hash or array, it is also
1189 possible to make direct use of most of the API functions defined in the
1190 Berkeley DB documentation.
1192 To do this you need to store a copy of the object returned from the tie.
1194 $db = tie %hash, "DB_File", "filename" ;
1196 Once you have done that, you can access the Berkeley DB API functions
1197 as B<DB_File> methods directly like this:
1199 $db->put($key, $value, R_NOOVERWRITE) ;
1201 B<Important:> If you have saved a copy of the object returned from
1202 C<tie>, the underlying database file will I<not> be closed until both
1203 the tied variable is untied and all copies of the saved object are
1207 $db = tie %hash, "DB_File", "filename"
1208 or die "Cannot tie filename: $!" ;
1213 See L<The untie() Gotcha> for more details.
1215 All the functions defined in L<dbopen> are available except for
1216 close() and dbopen() itself. The B<DB_File> method interface to the
1217 supported functions have been implemented to mirror the way Berkeley DB
1218 works whenever possible. In particular note that:
1224 The methods return a status value. All return 0 on success.
1225 All return -1 to signify an error and set C<$!> to the exact
1226 error code. The return code 1 generally (but not always) means that the
1227 key specified did not exist in the database.
1229 Other return codes are defined. See below and in the Berkeley DB
1230 documentation for details. The Berkeley DB documentation should be used
1231 as the definitive source.
1235 Whenever a Berkeley DB function returns data via one of its parameters,
1236 the equivalent B<DB_File> method does exactly the same.
1240 If you are careful, it is possible to mix API calls with the tied
1241 hash/array interface in the same piece of code. Although only a few of
1242 the methods used to implement the tied interface currently make use of
1243 the cursor, you should always assume that the cursor has been changed
1244 any time the tied hash/array interface is used. As an example, this
1245 code will probably not do what you expect:
1247 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1248 or die "Cannot tie $filename: $!" ;
1250 # Get the first key/value pair and set the cursor
1251 $X->seq($key, $value, R_FIRST) ;
1253 # this line will modify the cursor
1254 $count = scalar keys %x ;
1256 # Get the second key/value pair.
1257 # oops, it didn't, it got the last key/value pair!
1258 $X->seq($key, $value, R_NEXT) ;
1260 The code above can be rearranged to get around the problem, like this:
1262 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1263 or die "Cannot tie $filename: $!" ;
1265 # this line will modify the cursor
1266 $count = scalar keys %x ;
1268 # Get the first key/value pair and set the cursor
1269 $X->seq($key, $value, R_FIRST) ;
1271 # Get the second key/value pair.
1273 $X->seq($key, $value, R_NEXT) ;
1277 All the constants defined in L<dbopen> for use in the flags parameters
1278 in the methods defined below are also available. Refer to the Berkeley
1279 DB documentation for the precise meaning of the flags values.
1281 Below is a list of the methods available.
1285 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1287 Given a key (C<$key>) this method reads the value associated with it
1288 from the database. The value read from the database is returned in the
1289 C<$value> parameter.
1291 If the key does not exist the method returns 1.
1293 No flags are currently defined for this method.
1295 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1297 Stores the key/value pair in the database.
1299 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1300 will have the record number of the inserted key/value pair set.
1302 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1305 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1307 Removes all key/value pairs with key C<$key> from the database.
1309 A return code of 1 means that the requested key was not in the
1312 R_CURSOR is the only valid flag at present.
1314 =item B<$status = $X-E<gt>fd ;>
1316 Returns the file descriptor for the underlying database.
1318 See L<Locking Databases> for an example of how to make use of the
1319 C<fd> method to lock your database.
1321 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1323 This interface allows sequential retrieval from the database. See
1324 L<dbopen> for full details.
1326 Both the C<$key> and C<$value> parameters will be set to the key/value
1327 pair read from the database.
1329 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1330 R_FIRST, R_LAST, R_NEXT and R_PREV.
1332 =item B<$status = $X-E<gt>sync([$flags]) ;>
1334 Flushes any cached buffers to disk.
1336 R_RECNOSYNC is the only valid flag at present.
1340 =head1 HINTS AND TIPS
1343 =head2 Locking Databases
1345 Concurrent access of a read-write database by several parties requires
1346 them all to use some kind of locking. Here's an example of Tom's that
1347 uses the I<fd> method to get the file descriptor, and then a careful
1348 open() to give something Perl will flock() for you. Run this repeatedly
1349 in the background to watch the locks granted in proper order.
1360 my($oldval, $fd, $db, %db, $value, $key);
1362 $key = shift || 'default';
1363 $value = shift || 'magic';
1367 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1368 || die "dbcreat /tmp/foo.db $!";
1370 print "$$: db fd is $fd\n";
1371 open(DB_FH, "+<&=$fd") || die "dup $!";
1374 unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
1375 print "$$: CONTENTION; can't read during write update!
1376 Waiting for read lock ($!) ....";
1377 unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
1379 print "$$: Read lock granted\n";
1381 $oldval = $db{$key};
1382 print "$$: Old value was $oldval\n";
1383 flock(DB_FH, LOCK_UN);
1385 unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
1386 print "$$: CONTENTION; must have exclusive lock!
1387 Waiting for write lock ($!) ....";
1388 unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
1391 print "$$: Write lock granted\n";
1393 $db->sync; # to flush
1396 flock(DB_FH, LOCK_UN);
1400 print "$$: Updated db to $key=$value\n";
1402 =head2 Sharing Databases With C Applications
1404 There is no technical reason why a Berkeley DB database cannot be
1405 shared by both a Perl and a C application.
1407 The vast majority of problems that are reported in this area boil down
1408 to the fact that C strings are NULL terminated, whilst Perl strings are
1411 Here is a real example. Netscape 2.0 keeps a record of the locations you
1412 visit along with the time you last visited them in a DB_HASH database.
1413 This is usually stored in the file F<~/.netscape/history.db>. The key
1414 field in the database is the location string and the value field is the
1415 time the location was last visited stored as a 4 byte binary value.
1417 If you haven't already guessed, the location string is stored with a
1418 terminating NULL. This means you need to be careful when accessing the
1421 Here is a snippet of code that is loosely based on Tom Christiansen's
1422 I<ggh> script (available from your nearest CPAN archive in
1423 F<authors/id/TOMC/scripts/nshist.gz>).
1429 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1430 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1432 $HISTORY = "$dotdir/.netscape/history.db";
1434 tie %hist_db, 'DB_File', $HISTORY
1435 or die "Cannot open $HISTORY: $!\n" ;;
1437 # Dump the complete database
1438 while ( ($href, $binary_time) = each %hist_db ) {
1440 # remove the terminating NULL
1441 $href =~ s/\x00$// ;
1443 # convert the binary time into a user friendly string
1444 $date = localtime unpack("V", $binary_time);
1445 print "$date $href\n" ;
1448 # check for the existence of a specific key
1449 # remember to add the NULL
1450 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
1451 $date = localtime unpack("V", $binary_time) ;
1452 print "Last visited mox.perl.com on $date\n" ;
1455 print "Never visited mox.perl.com\n"
1460 =head2 The untie() Gotcha
1462 If you make use of the Berkeley DB API, it is I<very> strongly
1463 recommended that you read L<perltie/The untie Gotcha>.
1465 Even if you don't currently make use of the API interface, it is still
1468 Here is an example which illustrates the problem from a B<DB_File>
1477 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
1478 or die "Cannot tie first time: $!" ;
1484 tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1485 or die "Cannot tie second time: $!" ;
1489 When run, the script will produce this error message:
1491 Cannot tie second time: Invalid argument at bad.file line 14.
1493 Although the error message above refers to the second tie() statement
1494 in the script, the source of the problem is really with the untie()
1495 statement that precedes it.
1497 Having read L<perltie> you will probably have already guessed that the
1498 error is caused by the extra copy of the tied object stored in C<$X>.
1499 If you haven't, then the problem boils down to the fact that the
1500 B<DB_File> destructor, DESTROY, will not be called until I<all>
1501 references to the tied object are destroyed. Both the tied variable,
1502 C<%x>, and C<$X> above hold a reference to the object. The call to
1503 untie() will destroy the first, but C<$X> still holds a valid
1504 reference, so the destructor will not get called and the database file
1505 F<tst.fil> will remain open. The fact that Berkeley DB then reports the
1506 attempt to open a database that is alreday open via the catch-all
1507 "Invalid argument" doesn't help.
1509 If you run the script with the C<-w> flag the error message becomes:
1511 untie attempted while 1 inner references still exist at bad.file line 12.
1512 Cannot tie second time: Invalid argument at bad.file line 14.
1514 which pinpoints the real problem. Finally the script can now be
1515 modified to fix the original problem by destroying the API object
1524 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1528 =head1 COMMON QUESTIONS
1530 =head2 Why is there Perl source in my database?
1532 If you look at the contents of a database file created by DB_File,
1533 there can sometimes be part of a Perl script included in it.
1535 This happens because Berkeley DB uses dynamic memory to allocate
1536 buffers which will subsequently be written to the database file. Being
1537 dynamic, the memory could have been used for anything before DB
1538 malloced it. As Berkeley DB doesn't clear the memory once it has been
1539 allocated, the unused portions will contain random junk. In the case
1540 where a Perl script gets written to the database, the random junk will
1541 correspond to an area of dynamic memory that happened to be used during
1542 the compilation of the script.
1544 Unless you don't like the possibility of there being part of your Perl
1545 scripts embedded in a database file, this is nothing to worry about.
1547 =head2 How do I store complex data structures with DB_File?
1549 Although B<DB_File> cannot do this directly, there is a module which
1550 can layer transparently over B<DB_File> to accomplish this feat.
1552 Check out the MLDBM module, available on CPAN in the directory
1553 F<modules/by-module/MLDBM>.
1555 =head2 What does "Invalid Argument" mean?
1557 You will get this error message when one of the parameters in the
1558 C<tie> call is wrong. Unfortunately there are quite a few parameters to
1559 get wrong, so it can be difficult to figure out which one it is.
1561 Here are a couple of possibilities:
1567 Attempting to reopen a database without closing it.
1571 Using the O_WRONLY flag.
1575 =head2 What does "Bareword 'DB_File' not allowed" mean?
1577 You will encounter this particular error message when you have the
1578 C<strict 'subs'> pragma (or the full strict pragma) in your script.
1579 Consider this script:
1584 tie %x, DB_File, "filename" ;
1586 Running it produces the error in question:
1588 Bareword "DB_File" not allowed while "strict subs" in use
1590 To get around the error, place the word C<DB_File> in either single or
1591 double quotes, like this:
1593 tie %x, "DB_File", "filename" ;
1595 Although it might seem like a real pain, it is really worth the effort
1596 of having a C<use strict> in all your scripts.
1600 Moved to the Changes file.
1604 Some older versions of Berkeley DB had problems with fixed length
1605 records using the RECNO file format. This problem has been fixed since
1606 version 1.85 of Berkeley DB.
1608 I am sure there are bugs in the code. If you do find any, or can
1609 suggest any enhancements, I would welcome your comments.
1613 B<DB_File> comes with the standard Perl source distribution. Look in
1614 the directory F<ext/DB_File>. Given the amount of time between releases
1615 of Perl the version that ships with Perl is quite likely to be out of
1616 date, so the most recent version can always be found on CPAN (see
1617 L<perlmod/CPAN> for details), in the directory
1618 F<modules/by-module/DB_File>.
1620 This version of B<DB_File> will work with either version 1.x or 2.x of
1621 Berkeley DB, but is limited to the functionality provided by version 1.
1623 The official web site for Berkeley DB is
1624 F<http://www.sleepycat.com/db>. The ftp equivalent is
1625 F<ftp.sleepycat.com:/pub>. Both versions 1 and 2 of Berkeley DB are
1628 Alternatively, Berkeley DB version 1 is available at your nearest CPAN
1629 archive in F<src/misc/db.1.85.tar.gz>.
1631 If you are running IRIX, then get Berkeley DB version 1 from
1632 F<http://reality.sgi.com/ariel>. It has the patches necessary to
1633 compile properly on IRIX 5.3.
1637 Copyright (c) 1997 Paul Marquess. All rights reserved. This program is
1638 free software; you can redistribute it and/or modify it under the same
1639 terms as Perl itself.
1641 Although B<DB_File> is covered by the Perl license, the library it
1642 makes use of, namely Berkeley DB, is not. Berkeley DB has its own
1643 copyright and its own license. Please take the time to read it.
1645 The license for Berkeley DB version 2, and how it relates to DB_File
1646 does need some extra clarification. Here are are few words taken from
1647 the Berkeley DB FAQ regarding the version 2 license:
1649 The major difference is that the license for DB 2.0, when
1650 downloaded from the net, requires that the software that
1651 uses DB 2.0 be freely redistributable.
1653 That means that if you want to use DB_File, and you have changed either
1654 the source for Berkeley DB or Perl, then the changes must be freely
1657 In the case of Perl, the term source refers to the complete source
1658 code for Perl (e.g. sv.c, toke.c, perl.h) and any external modules that
1659 you are using (e.g. DB_File, Tk).
1661 Note that any Perl scripts that you write are your property - this
1662 includes scripts that make use of DB_File. Neither the Perl license or
1663 the Berkeley DB license place any restriction on what you have to do
1666 If you are in any doubt about the license situation, contact either the
1667 Berkeley DB authors or the author of DB_File. See L<"AUTHOR"> for details.
1672 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>
1676 The DB_File interface was written by Paul Marquess
1677 E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
1678 Questions about the DB system itself may be addressed to
1679 E<lt>db@sleepycat.com<gt>.