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
197 return $self->length - 1;
202 ($constname = $AUTOLOAD) =~ s/.*:://;
203 my $val = constant($constname, @_ ? $_[0] : 0);
205 if ($! =~ /Invalid/) {
206 $AutoLoader::AUTOLOAD = $AUTOLOAD;
207 goto &AutoLoader::AUTOLOAD;
210 my($pack,$file,$line) = caller;
211 croak "Your vendor has not defined DB macro $constname, used at $file line $line.
215 eval "sub $AUTOLOAD { $val }";
221 # Make all Fcntl O_XXX constants available for importing
223 my @O = grep /^O_/, @Fcntl::EXPORT;
224 Fcntl->import(@O); # first we import what we want to export
228 ## import borrowed from IO::File
229 ## exports Fcntl constants if available.
232 # my $callpkg = caller;
233 # Exporter::export $pkg, $callpkg, @_;
236 # Exporter::export 'Fcntl', $callpkg, '/^O_/';
240 bootstrap DB_File $VERSION;
242 # Preloaded methods go here. Autoload methods go after __END__, and are
243 # processed by the autosplit program.
245 sub tie_hash_or_array
248 my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ;
250 $arg[4] = tied %{ $arg[4] }
251 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
253 # make recno in Berkeley DB version 2 work like recno in version 1.
254 if ($db_version > 1 and defined $arg[4] and $arg[4] =~ /RECNO/ and
255 $arg[1] and ! -e $arg[1]) {
256 open(FH, ">$arg[1]") or return undef ;
258 chmod $arg[3] ? $arg[3] : 0666 , $arg[1] ;
261 DoTie_($tieHASH, @arg) ;
266 tie_hash_or_array(@_) ;
271 tie_hash_or_array(@_) ;
278 my $status = $self->seq($key, $value, R_FIRST());
281 while ($status == 0) {
283 $status = $self->seq($key, $value, R_NEXT());
285 foreach $key (reverse @keys) {
286 my $s = $self->del($key);
292 croak "Usage: \$db->get_dup(key [,flag])\n"
293 unless @_ == 2 or @_ == 3 ;
300 my $wantarray = wantarray ;
306 # iterate through the database until either EOF ($status == 0)
307 # or a different key is encountered ($key ne $origkey).
308 for ($status = $db->seq($key, $value, R_CURSOR()) ;
309 $status == 0 and $key eq $origkey ;
310 $status = $db->seq($key, $value, R_NEXT()) ) {
312 # save the value or count number of matches
315 { ++ $values{$value} }
317 { push (@values, $value) }
324 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
333 DB_File - Perl5 access to Berkeley DB version 1.x
339 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
340 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
341 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
343 $status = $X->del($key [, $flags]) ;
344 $status = $X->put($key, $value [, $flags]) ;
345 $status = $X->get($key, $value [, $flags]) ;
346 $status = $X->seq($key, $value, $flags) ;
347 $status = $X->sync([$flags]) ;
351 $count = $X->get_dup($key) ;
352 @list = $X->get_dup($key) ;
353 %list = $X->get_dup($key, 1) ;
367 B<DB_File> is a module which allows Perl programs to make use of the
368 facilities provided by Berkeley DB version 1.x (if you have a newer
369 version of DB, see L<Using DB_File with Berkeley DB version 2>). It is
370 assumed that you have a copy of the Berkeley DB manual pages at hand
371 when reading this documentation. The interface defined here mirrors the
372 Berkeley DB interface closely.
374 Berkeley DB is a C library which provides a consistent interface to a
375 number of database formats. B<DB_File> provides an interface to all
376 three of the database types currently supported by Berkeley DB.
384 This database type allows arbitrary key/value pairs to be stored in data
385 files. This is equivalent to the functionality provided by other
386 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
387 the files created using DB_HASH are not compatible with any of the
388 other packages mentioned.
390 A default hashing algorithm, which will be adequate for most
391 applications, is built into Berkeley DB. If you do need to use your own
392 hashing algorithm it is possible to write your own in Perl and have
393 B<DB_File> use it instead.
397 The btree format allows arbitrary key/value pairs to be stored in a
398 sorted, balanced binary tree.
400 As with the DB_HASH format, it is possible to provide a user defined
401 Perl routine to perform the comparison of keys. By default, though, the
402 keys are stored in lexical order.
406 DB_RECNO allows both fixed-length and variable-length flat text files
407 to be manipulated using the same key/value pair interface as in DB_HASH
408 and DB_BTREE. In this case the key will consist of a record (line)
413 =head2 Using DB_File with Berkeley DB version 2
415 Although B<DB_File> is intended to be used with Berkeley DB version 1,
416 it can also be used with version 2. In this case the interface is
417 limited to the functionality provided by Berkeley DB 1.x. Anywhere the
418 version 2 interface differs, B<DB_File> arranges for it to work like
419 version 1. This feature allows B<DB_File> scripts that were built with
420 version 1 to be migrated to version 2 without any changes.
422 If you want to make use of the new features available in Berkeley DB
423 2.x, use the Perl module B<BerkeleyDB> instead.
425 At the time of writing this document the B<BerkeleyDB> module is still
426 alpha quality (the version number is < 1.0), and so unsuitable for use
427 in any serious development work. Once its version number is >= 1.0, it
428 is considered stable enough for real work.
430 B<Note:> The database file format has changed in Berkeley DB version 2.
431 If you cannot recreate your databases, you must dump any existing
432 databases with the C<db_dump185> utility that comes with Berkeley DB.
433 Once you have upgraded DB_File to use Berkeley DB version 2, your
434 databases can be recreated using C<db_load>. Refer to the Berkeley DB
435 documentation for further details.
437 Please read L<COPYRIGHT> before using version 2.x of Berkeley DB with
440 =head2 Interface to Berkeley DB
442 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
443 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
444 allows B<DB_File> to access Berkeley DB files using either an
445 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
446 array (for the DB_RECNO file type).
448 In addition to the tie() interface, it is also possible to access most
449 of the functions provided in the Berkeley DB API directly.
450 See L<THE API INTERFACE>.
452 =head2 Opening a Berkeley DB Database File
454 Berkeley DB uses the function dbopen() to open or create a database.
455 Here is the C prototype for dbopen():
458 dbopen (const char * file, int flags, int mode,
459 DBTYPE type, const void * openinfo)
461 The parameter C<type> is an enumeration which specifies which of the 3
462 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
463 Depending on which of these is actually chosen, the final parameter,
464 I<openinfo> points to a data structure which allows tailoring of the
465 specific interface method.
467 This interface is handled slightly differently in B<DB_File>. Here is
468 an equivalent call using B<DB_File>:
470 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
472 The C<filename>, C<flags> and C<mode> parameters are the direct
473 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
474 performs the function of both the C<type> and C<openinfo> parameters in
477 In the example above $DB_HASH is actually a pre-defined reference to a
478 hash object. B<DB_File> has three of these pre-defined references.
479 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
481 The keys allowed in each of these pre-defined references is limited to
482 the names used in the equivalent C structure. So, for example, the
483 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
484 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
486 To change one of these elements, just assign to it like this:
488 $DB_HASH->{'cachesize'} = 10000 ;
490 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
491 usually adequate for most applications. If you do need to create extra
492 instances of these objects, constructors are available for each file
495 Here are examples of the constructors and the valid options available
496 for DB_HASH, DB_BTREE and DB_RECNO respectively.
498 $a = new DB_File::HASHINFO ;
506 $b = new DB_File::BTREEINFO ;
516 $c = new DB_File::RECNOINFO ;
525 The values stored in the hashes above are mostly the direct equivalent
526 of their C counterpart. Like their C counterparts, all are set to a
527 default values - that means you don't have to set I<all> of the
528 values when you only want to change one. Here is an example:
530 $a = new DB_File::HASHINFO ;
531 $a->{'cachesize'} = 12345 ;
532 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
534 A few of the options need extra discussion here. When used, the C
535 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
536 to C functions. In B<DB_File> these keys are used to store references
537 to Perl subs. Below are templates for each of the subs:
543 # return the hash value for $data
549 my ($key, $key2) = @_ ;
551 # return 0 if $key1 eq $key2
552 # -1 if $key1 lt $key2
553 # 1 if $key1 gt $key2
554 return (-1 , 0 or 1) ;
559 my ($key, $key2) = @_ ;
561 # return number of bytes of $key2 which are
562 # necessary to determine that it is greater than $key1
566 See L<Changing the BTREE sort order> for an example of using the
569 If you are using the DB_RECNO interface and you intend making use of
570 C<bval>, you should check out L<The 'bval' Option>.
572 =head2 Default Parameters
574 It is possible to omit some or all of the final 4 parameters in the
575 call to C<tie> and let them take default values. As DB_HASH is the most
576 common file format used, the call:
578 tie %A, "DB_File", "filename" ;
582 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
584 It is also possible to omit the filename parameter as well, so the
591 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
593 See L<In Memory Databases> for a discussion on the use of C<undef>
594 in place of a filename.
596 =head2 In Memory Databases
598 Berkeley DB allows the creation of in-memory databases by using NULL
599 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
600 uses C<undef> instead of NULL to provide this functionality.
604 The DB_HASH file format is probably the most commonly used of the three
605 file formats that B<DB_File> supports. It is also very straightforward
608 =head2 A Simple Example
610 This example shows how to create a database, add key/value pairs to the
611 database, delete keys/value pairs and finally how to enumerate the
612 contents of the database.
616 use vars qw( %h $k $v ) ;
618 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
619 or die "Cannot open file 'fruit': $!\n";
621 # Add a few key/value pairs to the file
622 $h{"apple"} = "red" ;
623 $h{"orange"} = "orange" ;
624 $h{"banana"} = "yellow" ;
625 $h{"tomato"} = "red" ;
627 # Check for existence of a key
628 print "Banana Exists\n\n" if $h{"banana"} ;
630 # Delete a key/value pair.
633 # print the contents of the file
634 while (($k, $v) = each %h)
635 { print "$k -> $v\n" }
647 Note that the like ordinary associative arrays, the order of the keys
648 retrieved is in an apparently random order.
652 The DB_BTREE format is useful when you want to store data in a given
653 order. By default the keys will be stored in lexical order, but as you
654 will see from the example shown in the next section, it is very easy to
655 define your own sorting function.
657 =head2 Changing the BTREE sort order
659 This script shows how to override the default sorting algorithm that
660 BTREE uses. Instead of using the normal lexical ordering, a case
661 insensitive compare function will be used.
670 my ($key1, $key2) = @_ ;
671 "\L$key1" cmp "\L$key2" ;
674 # specify the Perl sub that will do the comparison
675 $DB_BTREE->{'compare'} = \&Compare ;
677 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
678 or die "Cannot open file 'tree': $!\n" ;
680 # Add a key/value pair to the file
681 $h{'Wall'} = 'Larry' ;
682 $h{'Smith'} = 'John' ;
683 $h{'mouse'} = 'mickey' ;
684 $h{'duck'} = 'donald' ;
689 # Cycle through the keys printing them in order.
690 # Note it is not necessary to sort the keys as
691 # the btree will have kept them in order automatically.
697 Here is the output from the code above.
703 There are a few point to bear in mind if you want to change the
704 ordering in a BTREE database:
710 The new compare function must be specified when you create the database.
714 You cannot change the ordering once the database has been created. Thus
715 you must use the same compare function every time you access the
720 =head2 Handling Duplicate Keys
722 The BTREE file type optionally allows a single key to be associated
723 with an arbitrary number of values. This option is enabled by setting
724 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
726 There are some difficulties in using the tied hash interface if you
727 want to manipulate a BTREE database with duplicate keys. Consider this
733 use vars qw($filename %h ) ;
738 # Enable duplicate records
739 $DB_BTREE->{'flags'} = R_DUP ;
741 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
742 or die "Cannot open $filename: $!\n";
744 # Add some key/value pairs to the file
745 $h{'Wall'} = 'Larry' ;
746 $h{'Wall'} = 'Brick' ; # Note the duplicate key
747 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
748 $h{'Smith'} = 'John' ;
749 $h{'mouse'} = 'mickey' ;
751 # iterate through the associative array
752 # and print each key/value pair.
754 { print "$_ -> $h{$_}\n" }
766 As you can see 3 records have been successfully created with key C<Wall>
767 - the only thing is, when they are retrieved from the database they
768 I<seem> to have the same value, namely C<Larry>. The problem is caused
769 by the way that the associative array interface works. Basically, when
770 the associative array interface is used to fetch the value associated
771 with a given key, it will only ever retrieve the first value.
773 Although it may not be immediately obvious from the code above, the
774 associative array interface can be used to write values with duplicate
775 keys, but it cannot be used to read them back from the database.
777 The way to get around this problem is to use the Berkeley DB API method
778 called C<seq>. This method allows sequential access to key/value
779 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
780 and the API in general.
782 Here is the script above rewritten using the C<seq> API method.
787 use vars qw($filename $x %h $status $key $value) ;
792 # Enable duplicate records
793 $DB_BTREE->{'flags'} = R_DUP ;
795 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
796 or die "Cannot open $filename: $!\n";
798 # Add some key/value pairs to the file
799 $h{'Wall'} = 'Larry' ;
800 $h{'Wall'} = 'Brick' ; # Note the duplicate key
801 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
802 $h{'Smith'} = 'John' ;
803 $h{'mouse'} = 'mickey' ;
805 # iterate through the btree using seq
806 # and print each key/value pair.
808 for ($status = $x->seq($key, $value, R_FIRST) ;
810 $status = $x->seq($key, $value, R_NEXT) )
811 { print "$key -> $value\n" }
824 This time we have got all the key/value pairs, including the multiple
825 values associated with the key C<Wall>.
827 =head2 The get_dup() Method
829 B<DB_File> comes with a utility method, called C<get_dup>, to assist in
830 reading duplicate values from BTREE databases. The method can take the
833 $count = $x->get_dup($key) ;
834 @list = $x->get_dup($key) ;
835 %list = $x->get_dup($key, 1) ;
837 In a scalar context the method returns the number of values associated
838 with the key, C<$key>.
840 In list context, it returns all the values which match C<$key>. Note
841 that the values will be returned in an apparently random order.
843 In list context, if the second parameter is present and evaluates
844 TRUE, the method returns an associative array. The keys of the
845 associative array correspond to the values that matched in the BTREE
846 and the values of the array are a count of the number of times that
847 particular value occurred in the BTREE.
849 So assuming the database created above, we can use C<get_dup> like
852 my $cnt = $x->get_dup("Wall") ;
853 print "Wall occurred $cnt times\n" ;
855 my %hash = $x->get_dup("Wall", 1) ;
856 print "Larry is there\n" if $hash{'Larry'} ;
857 print "There are $hash{'Brick'} Brick Walls\n" ;
859 my @list = $x->get_dup("Wall") ;
860 print "Wall => [@list]\n" ;
862 @list = $x->get_dup("Smith") ;
863 print "Smith => [@list]\n" ;
865 @list = $x->get_dup("Dog") ;
866 print "Dog => [@list]\n" ;
871 Wall occurred 3 times
873 There are 2 Brick Walls
874 Wall => [Brick Brick Larry]
878 =head2 Matching Partial Keys
880 The BTREE interface has a feature which allows partial keys to be
881 matched. This functionality is I<only> available when the C<seq> method
882 is used along with the R_CURSOR flag.
884 $x->seq($key, $value, R_CURSOR) ;
886 Here is the relevant quote from the dbopen man page where it defines
887 the use of the R_CURSOR flag with seq:
889 Note, for the DB_BTREE access method, the returned key is not
890 necessarily an exact match for the specified key. The returned key
891 is the smallest key greater than or equal to the specified key,
892 permitting partial key matches and range searches.
894 In the example script below, the C<match> sub uses this feature to find
895 and print the first matching key/value pair given a partial key.
901 use vars qw($filename $x %h $st $key $value) ;
907 my $orig_key = $key ;
908 $x->seq($key, $value, R_CURSOR) ;
909 print "$orig_key\t-> $key\t-> $value\n" ;
915 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
916 or die "Cannot open $filename: $!\n";
918 # Add some key/value pairs to the file
919 $h{'mouse'} = 'mickey' ;
920 $h{'Wall'} = 'Larry' ;
921 $h{'Walls'} = 'Brick' ;
922 $h{'Smith'} = 'John' ;
927 for ($st = $x->seq($key, $value, R_FIRST) ;
929 $st = $x->seq($key, $value, R_NEXT) )
931 { print "$key -> $value\n" }
933 print "\nPARTIAL MATCH\n" ;
957 DB_RECNO provides an interface to flat text files. Both variable and
958 fixed length records are supported.
960 In order to make RECNO more compatible with Perl the array offset for
961 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
963 As with normal Perl arrays, a RECNO array can be accessed using
964 negative indexes. The index -1 refers to the last element of the array,
965 -2 the second last, and so on. Attempting to access an element before
966 the start of the array will raise a fatal run-time error.
968 =head2 The 'bval' Option
970 The operation of the bval option warrants some discussion. Here is the
971 definition of bval from the Berkeley DB 1.85 recno manual page:
973 The delimiting byte to be used to mark the end of a
974 record for variable-length records, and the pad charac-
975 ter for fixed-length records. If no value is speci-
976 fied, newlines (``\n'') are used to mark the end of
977 variable-length records and fixed-length records are
980 The second sentence is wrong. In actual fact bval will only default to
981 C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
982 openinfo parameter is used at all, the value that happens to be in bval
983 will be used. That means you always have to specify bval when making
984 use of any of the options in the openinfo parameter. This documentation
985 error will be fixed in the next release of Berkeley DB.
987 That clarifies the situation with regards Berkeley DB itself. What
988 about B<DB_File>? Well, the behavior defined in the quote above is
989 quite useful, so B<DB_File> conforms it.
991 That means that you can specify other options (e.g. cachesize) and
992 still have bval default to C<"\n"> for variable length records, and
993 space for fixed length records.
995 =head2 A Simple Example
997 Here is a simple example that uses RECNO.
1003 tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
1004 or die "Cannot open file 'text': $!\n" ;
1006 # Add a few key/value pairs to the file
1011 # Check for existence of a key
1012 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
1014 # use a negative index
1015 print "The last element is $h[-1]\n" ;
1016 print "The 2nd last element is $h[-2]\n" ;
1020 Here is the output from the script:
1023 Element 1 Exists with value blue
1024 The last element is yellow
1025 The 2nd last element is blue
1027 =head2 Extra Methods
1029 As you can see from the example above, the tied array interface is
1030 quite limited. To make the interface more useful, a number of methods
1031 are supplied with B<DB_File> to simulate the standard array operations
1032 that are not currently implemented in Perl's tied array interface. All
1033 these methods are accessed via the object returned from the tie call.
1035 Here are the methods:
1039 =item B<$X-E<gt>push(list) ;>
1041 Pushes the elements of C<list> to the end of the array.
1043 =item B<$value = $X-E<gt>pop ;>
1045 Removes and returns the last element of the array.
1047 =item B<$X-E<gt>shift>
1049 Removes and returns the first element of the array.
1051 =item B<$X-E<gt>unshift(list) ;>
1053 Pushes the elements of C<list> to the start of the array.
1055 =item B<$X-E<gt>length>
1057 Returns the number of elements in the array.
1061 =head2 Another Example
1063 Here is a more complete example that makes use of some of the methods
1064 described above. It also makes use of the API interface directly (see
1065 L<THE API INTERFACE>).
1068 use vars qw(@h $H $file $i) ;
1076 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
1077 or die "Cannot open file $file: $!\n" ;
1079 # first create a text file to play with
1087 # Print the records in order.
1089 # The length method is needed here because evaluating a tied
1090 # array in a scalar context does not return the number of
1091 # elements in the array.
1093 print "\nORIGINAL\n" ;
1094 foreach $i (0 .. $H->length - 1) {
1095 print "$i: $h[$i]\n" ;
1098 # use the push & pop methods
1101 print "\nThe last record was [$a]\n" ;
1103 # and the shift & unshift methods
1105 $H->unshift("first") ;
1106 print "The first record was [$a]\n" ;
1108 # Use the API to add a new record after record 2.
1110 $H->put($i, "Newbie", R_IAFTER) ;
1112 # and a new record before record 1.
1114 $H->put($i, "New One", R_IBEFORE) ;
1119 # now print the records in reverse order
1120 print "\nREVERSE\n" ;
1121 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1122 { print "$i: $h[$i]\n" }
1124 # same again, but use the API functions instead
1125 print "\nREVERSE again\n" ;
1126 my ($s, $k, $v) = (0, 0, 0) ;
1127 for ($s = $H->seq($k, $v, R_LAST) ;
1129 $s = $H->seq($k, $v, R_PREV))
1130 { print "$k: $v\n" }
1135 and this is what it outputs:
1144 The last record was [four]
1145 The first record was [zero]
1169 Rather than iterating through the array, C<@h> like this:
1173 it is necessary to use either this:
1175 foreach $i (0 .. $H->length - 1)
1179 for ($a = $H->get($k, $v, R_FIRST) ;
1181 $a = $H->get($k, $v, R_NEXT) )
1185 Notice that both times the C<put> method was used the record index was
1186 specified using a variable, C<$i>, rather than the literal value
1187 itself. This is because C<put> will return the record number of the
1188 inserted line via that parameter.
1192 =head1 THE API INTERFACE
1194 As well as accessing Berkeley DB using a tied hash or array, it is also
1195 possible to make direct use of most of the API functions defined in the
1196 Berkeley DB documentation.
1198 To do this you need to store a copy of the object returned from the tie.
1200 $db = tie %hash, "DB_File", "filename" ;
1202 Once you have done that, you can access the Berkeley DB API functions
1203 as B<DB_File> methods directly like this:
1205 $db->put($key, $value, R_NOOVERWRITE) ;
1207 B<Important:> If you have saved a copy of the object returned from
1208 C<tie>, the underlying database file will I<not> be closed until both
1209 the tied variable is untied and all copies of the saved object are
1213 $db = tie %hash, "DB_File", "filename"
1214 or die "Cannot tie filename: $!" ;
1219 See L<The untie() Gotcha> for more details.
1221 All the functions defined in L<dbopen> are available except for
1222 close() and dbopen() itself. The B<DB_File> method interface to the
1223 supported functions have been implemented to mirror the way Berkeley DB
1224 works whenever possible. In particular note that:
1230 The methods return a status value. All return 0 on success.
1231 All return -1 to signify an error and set C<$!> to the exact
1232 error code. The return code 1 generally (but not always) means that the
1233 key specified did not exist in the database.
1235 Other return codes are defined. See below and in the Berkeley DB
1236 documentation for details. The Berkeley DB documentation should be used
1237 as the definitive source.
1241 Whenever a Berkeley DB function returns data via one of its parameters,
1242 the equivalent B<DB_File> method does exactly the same.
1246 If you are careful, it is possible to mix API calls with the tied
1247 hash/array interface in the same piece of code. Although only a few of
1248 the methods used to implement the tied interface currently make use of
1249 the cursor, you should always assume that the cursor has been changed
1250 any time the tied hash/array interface is used. As an example, this
1251 code will probably not do what you expect:
1253 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1254 or die "Cannot tie $filename: $!" ;
1256 # Get the first key/value pair and set the cursor
1257 $X->seq($key, $value, R_FIRST) ;
1259 # this line will modify the cursor
1260 $count = scalar keys %x ;
1262 # Get the second key/value pair.
1263 # oops, it didn't, it got the last key/value pair!
1264 $X->seq($key, $value, R_NEXT) ;
1266 The code above can be rearranged to get around the problem, like this:
1268 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1269 or die "Cannot tie $filename: $!" ;
1271 # this line will modify the cursor
1272 $count = scalar keys %x ;
1274 # Get the first key/value pair and set the cursor
1275 $X->seq($key, $value, R_FIRST) ;
1277 # Get the second key/value pair.
1279 $X->seq($key, $value, R_NEXT) ;
1283 All the constants defined in L<dbopen> for use in the flags parameters
1284 in the methods defined below are also available. Refer to the Berkeley
1285 DB documentation for the precise meaning of the flags values.
1287 Below is a list of the methods available.
1291 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1293 Given a key (C<$key>) this method reads the value associated with it
1294 from the database. The value read from the database is returned in the
1295 C<$value> parameter.
1297 If the key does not exist the method returns 1.
1299 No flags are currently defined for this method.
1301 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1303 Stores the key/value pair in the database.
1305 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1306 will have the record number of the inserted key/value pair set.
1308 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1311 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1313 Removes all key/value pairs with key C<$key> from the database.
1315 A return code of 1 means that the requested key was not in the
1318 R_CURSOR is the only valid flag at present.
1320 =item B<$status = $X-E<gt>fd ;>
1322 Returns the file descriptor for the underlying database.
1324 See L<Locking Databases> for an example of how to make use of the
1325 C<fd> method to lock your database.
1327 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1329 This interface allows sequential retrieval from the database. See
1330 L<dbopen> for full details.
1332 Both the C<$key> and C<$value> parameters will be set to the key/value
1333 pair read from the database.
1335 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1336 R_FIRST, R_LAST, R_NEXT and R_PREV.
1338 =item B<$status = $X-E<gt>sync([$flags]) ;>
1340 Flushes any cached buffers to disk.
1342 R_RECNOSYNC is the only valid flag at present.
1346 =head1 HINTS AND TIPS
1349 =head2 Locking Databases
1351 Concurrent access of a read-write database by several parties requires
1352 them all to use some kind of locking. Here's an example of Tom's that
1353 uses the I<fd> method to get the file descriptor, and then a careful
1354 open() to give something Perl will flock() for you. Run this repeatedly
1355 in the background to watch the locks granted in proper order.
1366 my($oldval, $fd, $db, %db, $value, $key);
1368 $key = shift || 'default';
1369 $value = shift || 'magic';
1373 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1374 || die "dbcreat /tmp/foo.db $!";
1376 print "$$: db fd is $fd\n";
1377 open(DB_FH, "+<&=$fd") || die "dup $!";
1380 unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
1381 print "$$: CONTENTION; can't read during write update!
1382 Waiting for read lock ($!) ....";
1383 unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
1385 print "$$: Read lock granted\n";
1387 $oldval = $db{$key};
1388 print "$$: Old value was $oldval\n";
1389 flock(DB_FH, LOCK_UN);
1391 unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
1392 print "$$: CONTENTION; must have exclusive lock!
1393 Waiting for write lock ($!) ....";
1394 unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
1397 print "$$: Write lock granted\n";
1399 $db->sync; # to flush
1402 flock(DB_FH, LOCK_UN);
1406 print "$$: Updated db to $key=$value\n";
1408 =head2 Sharing Databases With C Applications
1410 There is no technical reason why a Berkeley DB database cannot be
1411 shared by both a Perl and a C application.
1413 The vast majority of problems that are reported in this area boil down
1414 to the fact that C strings are NULL terminated, whilst Perl strings are
1417 Here is a real example. Netscape 2.0 keeps a record of the locations you
1418 visit along with the time you last visited them in a DB_HASH database.
1419 This is usually stored in the file F<~/.netscape/history.db>. The key
1420 field in the database is the location string and the value field is the
1421 time the location was last visited stored as a 4 byte binary value.
1423 If you haven't already guessed, the location string is stored with a
1424 terminating NULL. This means you need to be careful when accessing the
1427 Here is a snippet of code that is loosely based on Tom Christiansen's
1428 I<ggh> script (available from your nearest CPAN archive in
1429 F<authors/id/TOMC/scripts/nshist.gz>).
1435 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1436 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1438 $HISTORY = "$dotdir/.netscape/history.db";
1440 tie %hist_db, 'DB_File', $HISTORY
1441 or die "Cannot open $HISTORY: $!\n" ;;
1443 # Dump the complete database
1444 while ( ($href, $binary_time) = each %hist_db ) {
1446 # remove the terminating NULL
1447 $href =~ s/\x00$// ;
1449 # convert the binary time into a user friendly string
1450 $date = localtime unpack("V", $binary_time);
1451 print "$date $href\n" ;
1454 # check for the existence of a specific key
1455 # remember to add the NULL
1456 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
1457 $date = localtime unpack("V", $binary_time) ;
1458 print "Last visited mox.perl.com on $date\n" ;
1461 print "Never visited mox.perl.com\n"
1466 =head2 The untie() Gotcha
1468 If you make use of the Berkeley DB API, it is I<very> strongly
1469 recommended that you read L<perltie/The untie Gotcha>.
1471 Even if you don't currently make use of the API interface, it is still
1474 Here is an example which illustrates the problem from a B<DB_File>
1483 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
1484 or die "Cannot tie first time: $!" ;
1490 tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1491 or die "Cannot tie second time: $!" ;
1495 When run, the script will produce this error message:
1497 Cannot tie second time: Invalid argument at bad.file line 14.
1499 Although the error message above refers to the second tie() statement
1500 in the script, the source of the problem is really with the untie()
1501 statement that precedes it.
1503 Having read L<perltie> you will probably have already guessed that the
1504 error is caused by the extra copy of the tied object stored in C<$X>.
1505 If you haven't, then the problem boils down to the fact that the
1506 B<DB_File> destructor, DESTROY, will not be called until I<all>
1507 references to the tied object are destroyed. Both the tied variable,
1508 C<%x>, and C<$X> above hold a reference to the object. The call to
1509 untie() will destroy the first, but C<$X> still holds a valid
1510 reference, so the destructor will not get called and the database file
1511 F<tst.fil> will remain open. The fact that Berkeley DB then reports the
1512 attempt to open a database that is alreday open via the catch-all
1513 "Invalid argument" doesn't help.
1515 If you run the script with the C<-w> flag the error message becomes:
1517 untie attempted while 1 inner references still exist at bad.file line 12.
1518 Cannot tie second time: Invalid argument at bad.file line 14.
1520 which pinpoints the real problem. Finally the script can now be
1521 modified to fix the original problem by destroying the API object
1530 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1534 =head1 COMMON QUESTIONS
1536 =head2 Why is there Perl source in my database?
1538 If you look at the contents of a database file created by DB_File,
1539 there can sometimes be part of a Perl script included in it.
1541 This happens because Berkeley DB uses dynamic memory to allocate
1542 buffers which will subsequently be written to the database file. Being
1543 dynamic, the memory could have been used for anything before DB
1544 malloced it. As Berkeley DB doesn't clear the memory once it has been
1545 allocated, the unused portions will contain random junk. In the case
1546 where a Perl script gets written to the database, the random junk will
1547 correspond to an area of dynamic memory that happened to be used during
1548 the compilation of the script.
1550 Unless you don't like the possibility of there being part of your Perl
1551 scripts embedded in a database file, this is nothing to worry about.
1553 =head2 How do I store complex data structures with DB_File?
1555 Although B<DB_File> cannot do this directly, there is a module which
1556 can layer transparently over B<DB_File> to accomplish this feat.
1558 Check out the MLDBM module, available on CPAN in the directory
1559 F<modules/by-module/MLDBM>.
1561 =head2 What does "Invalid Argument" mean?
1563 You will get this error message when one of the parameters in the
1564 C<tie> call is wrong. Unfortunately there are quite a few parameters to
1565 get wrong, so it can be difficult to figure out which one it is.
1567 Here are a couple of possibilities:
1573 Attempting to reopen a database without closing it.
1577 Using the O_WRONLY flag.
1581 =head2 What does "Bareword 'DB_File' not allowed" mean?
1583 You will encounter this particular error message when you have the
1584 C<strict 'subs'> pragma (or the full strict pragma) in your script.
1585 Consider this script:
1590 tie %x, DB_File, "filename" ;
1592 Running it produces the error in question:
1594 Bareword "DB_File" not allowed while "strict subs" in use
1596 To get around the error, place the word C<DB_File> in either single or
1597 double quotes, like this:
1599 tie %x, "DB_File", "filename" ;
1601 Although it might seem like a real pain, it is really worth the effort
1602 of having a C<use strict> in all your scripts.
1606 Moved to the Changes file.
1610 Some older versions of Berkeley DB had problems with fixed length
1611 records using the RECNO file format. This problem has been fixed since
1612 version 1.85 of Berkeley DB.
1614 I am sure there are bugs in the code. If you do find any, or can
1615 suggest any enhancements, I would welcome your comments.
1619 B<DB_File> comes with the standard Perl source distribution. Look in
1620 the directory F<ext/DB_File>. Given the amount of time between releases
1621 of Perl the version that ships with Perl is quite likely to be out of
1622 date, so the most recent version can always be found on CPAN (see
1623 L<perlmod/CPAN> for details), in the directory
1624 F<modules/by-module/DB_File>.
1626 This version of B<DB_File> will work with either version 1.x or 2.x of
1627 Berkeley DB, but is limited to the functionality provided by version 1.
1629 The official web site for Berkeley DB is
1630 F<http://www.sleepycat.com/db>. The ftp equivalent is
1631 F<ftp.sleepycat.com:/pub>. Both versions 1 and 2 of Berkeley DB are
1634 Alternatively, Berkeley DB version 1 is available at your nearest CPAN
1635 archive in F<src/misc/db.1.85.tar.gz>.
1637 If you are running IRIX, then get Berkeley DB version 1 from
1638 F<http://reality.sgi.com/ariel>. It has the patches necessary to
1639 compile properly on IRIX 5.3.
1643 Copyright (c) 1997 Paul Marquess. All rights reserved. This program is
1644 free software; you can redistribute it and/or modify it under the same
1645 terms as Perl itself.
1647 Although B<DB_File> is covered by the Perl license, the library it
1648 makes use of, namely Berkeley DB, is not. Berkeley DB has its own
1649 copyright and its own license. Please take the time to read it.
1651 The license for Berkeley DB version 2, and how it relates to DB_File
1652 does need some extra clarification. Here are are few words taken from
1653 the Berkeley DB FAQ regarding the version 2 license:
1655 The major difference is that the license for DB 2.0, when
1656 downloaded from the net, requires that the software that
1657 uses DB 2.0 be freely redistributable.
1659 That means that if you want to use DB_File, and you have changed either
1660 the source for Berkeley DB or Perl, then the changes must be freely
1663 In the case of Perl, the term source refers to the complete source
1664 code for Perl (e.g. sv.c, toke.c, perl.h) and any external modules that
1665 you are using (e.g. DB_File, Tk).
1667 Note that any Perl scripts that you write are your property - this
1668 includes scripts that make use of DB_File. Neither the Perl license or
1669 the Berkeley DB license place any restriction on what you have to do
1672 If you are in any doubt about the license situation, contact either the
1673 Berkeley DB authors or the author of DB_File. See L<"AUTHOR"> for details.
1678 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>
1682 The DB_File interface was written by Paul Marquess
1683 E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
1684 Questions about the DB system itself may be addressed to
1685 E<lt>db@sleepycat.com<gt>.