1 # DB_File.pm -- Perl 5 interface to Berkeley DB
3 # written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
4 # last modified 12th Mar 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.
236 $arg[4] = tied %{ $arg[4] }
237 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
242 *TIEARRAY = \&TIEHASH ;
246 croak "Usage: \$db->get_dup(key [,flag])\n"
247 unless @_ == 2 or @_ == 3 ;
254 my $wantarray = wantarray ;
260 # iterate through the database until either EOF ($status == 0)
261 # or a different key is encountered ($key ne $origkey).
262 for ($status = $db->seq($key, $value, R_CURSOR()) ;
263 $status == 0 and $key eq $origkey ;
264 $status = $db->seq($key, $value, R_NEXT()) ) {
266 # save the value or count number of matches
269 { ++ $values{$value} }
271 { push (@values, $value) }
278 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
289 DB_File - Perl5 access to Berkeley DB
295 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
296 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
297 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
299 $status = $X->del($key [, $flags]) ;
300 $status = $X->put($key, $value [, $flags]) ;
301 $status = $X->get($key, $value [, $flags]) ;
302 $status = $X->seq($key, $value, $flags) ;
303 $status = $X->sync([$flags]) ;
307 $count = $X->get_dup($key) ;
308 @list = $X->get_dup($key) ;
309 %list = $X->get_dup($key, 1) ;
323 B<DB_File> is a module which allows Perl programs to make use of the
324 facilities provided by Berkeley DB. If you intend to use this
325 module you should really have a copy of the Berkeley DB manual pages at
326 hand. The interface defined here mirrors the Berkeley DB interface
329 Please note that this module will only work with version 1.x of
330 Berkeley DB. Once Berkeley DB version 2 is released, B<DB_File> will be
331 upgraded to work with it.
333 Berkeley DB is a C library which provides a consistent interface to a
334 number of database formats. B<DB_File> provides an interface to all
335 three of the database types currently supported by Berkeley DB.
343 This database type allows arbitrary key/value pairs to be stored in data
344 files. This is equivalent to the functionality provided by other
345 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
346 the files created using DB_HASH are not compatible with any of the
347 other packages mentioned.
349 A default hashing algorithm, which will be adequate for most
350 applications, is built into Berkeley DB. If you do need to use your own
351 hashing algorithm it is possible to write your own in Perl and have
352 B<DB_File> use it instead.
356 The btree format allows arbitrary key/value pairs to be stored in a
357 sorted, balanced binary tree.
359 As with the DB_HASH format, it is possible to provide a user defined
360 Perl routine to perform the comparison of keys. By default, though, the
361 keys are stored in lexical order.
365 DB_RECNO allows both fixed-length and variable-length flat text files
366 to be manipulated using the same key/value pair interface as in DB_HASH
367 and DB_BTREE. In this case the key will consist of a record (line)
372 =head2 Interface to Berkeley DB
374 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
375 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
376 allows B<DB_File> to access Berkeley DB files using either an
377 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
378 array (for the DB_RECNO file type).
380 In addition to the tie() interface, it is also possible to access most
381 of the functions provided in the Berkeley DB API directly.
382 See L<THE API INTERFACE>.
384 =head2 Opening a Berkeley DB Database File
386 Berkeley DB uses the function dbopen() to open or create a database.
387 Here is the C prototype for dbopen():
390 dbopen (const char * file, int flags, int mode,
391 DBTYPE type, const void * openinfo)
393 The parameter C<type> is an enumeration which specifies which of the 3
394 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
395 Depending on which of these is actually chosen, the final parameter,
396 I<openinfo> points to a data structure which allows tailoring of the
397 specific interface method.
399 This interface is handled slightly differently in B<DB_File>. Here is
400 an equivalent call using B<DB_File>:
402 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
404 The C<filename>, C<flags> and C<mode> parameters are the direct
405 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
406 performs the function of both the C<type> and C<openinfo> parameters in
409 In the example above $DB_HASH is actually a pre-defined reference to a
410 hash object. B<DB_File> has three of these pre-defined references.
411 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
413 The keys allowed in each of these pre-defined references is limited to
414 the names used in the equivalent C structure. So, for example, the
415 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
416 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
418 To change one of these elements, just assign to it like this:
420 $DB_HASH->{'cachesize'} = 10000 ;
422 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
423 usually adequate for most applications. If you do need to create extra
424 instances of these objects, constructors are available for each file
427 Here are examples of the constructors and the valid options available
428 for DB_HASH, DB_BTREE and DB_RECNO respectively.
430 $a = new DB_File::HASHINFO ;
438 $b = new DB_File::BTREEINFO ;
448 $c = new DB_File::RECNOINFO ;
457 The values stored in the hashes above are mostly the direct equivalent
458 of their C counterpart. Like their C counterparts, all are set to a
459 default values - that means you don't have to set I<all> of the
460 values when you only want to change one. Here is an example:
462 $a = new DB_File::HASHINFO ;
463 $a->{'cachesize'} = 12345 ;
464 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
466 A few of the options need extra discussion here. When used, the C
467 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
468 to C functions. In B<DB_File> these keys are used to store references
469 to Perl subs. Below are templates for each of the subs:
475 # return the hash value for $data
481 my ($key, $key2) = @_ ;
483 # return 0 if $key1 eq $key2
484 # -1 if $key1 lt $key2
485 # 1 if $key1 gt $key2
486 return (-1 , 0 or 1) ;
491 my ($key, $key2) = @_ ;
493 # return number of bytes of $key2 which are
494 # necessary to determine that it is greater than $key1
498 See L<Changing the BTREE sort order> for an example of using the
501 If you are using the DB_RECNO interface and you intend making use of
502 C<bval>, you should check out L<The 'bval' Option>.
504 =head2 Default Parameters
506 It is possible to omit some or all of the final 4 parameters in the
507 call to C<tie> and let them take default values. As DB_HASH is the most
508 common file format used, the call:
510 tie %A, "DB_File", "filename" ;
514 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
516 It is also possible to omit the filename parameter as well, so the
523 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
525 See L<In Memory Databases> for a discussion on the use of C<undef>
526 in place of a filename.
528 =head2 In Memory Databases
530 Berkeley DB allows the creation of in-memory databases by using NULL
531 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
532 uses C<undef> instead of NULL to provide this functionality.
536 The DB_HASH file format is probably the most commonly used of the three
537 file formats that B<DB_File> supports. It is also very straightforward
540 =head2 A Simple Example
542 This example shows how to create a database, add key/value pairs to the
543 database, delete keys/value pairs and finally how to enumerate the
544 contents of the database.
548 use vars qw( %h $k $v ) ;
550 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
551 or die "Cannot open file 'fruit': $!\n";
553 # Add a few key/value pairs to the file
554 $h{"apple"} = "red" ;
555 $h{"orange"} = "orange" ;
556 $h{"banana"} = "yellow" ;
557 $h{"tomato"} = "red" ;
559 # Check for existence of a key
560 print "Banana Exists\n\n" if $h{"banana"} ;
562 # Delete a key/value pair.
565 # print the contents of the file
566 while (($k, $v) = each %h)
567 { print "$k -> $v\n" }
579 Note that the like ordinary associative arrays, the order of the keys
580 retrieved is in an apparently random order.
584 The DB_BTREE format is useful when you want to store data in a given
585 order. By default the keys will be stored in lexical order, but as you
586 will see from the example shown in the next section, it is very easy to
587 define your own sorting function.
589 =head2 Changing the BTREE sort order
591 This script shows how to override the default sorting algorithm that
592 BTREE uses. Instead of using the normal lexical ordering, a case
593 insensitive compare function will be used.
602 my ($key1, $key2) = @_ ;
603 "\L$key1" cmp "\L$key2" ;
606 # specify the Perl sub that will do the comparison
607 $DB_BTREE->{'compare'} = \&Compare ;
609 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
610 or die "Cannot open file 'tree': $!\n" ;
612 # Add a key/value pair to the file
613 $h{'Wall'} = 'Larry' ;
614 $h{'Smith'} = 'John' ;
615 $h{'mouse'} = 'mickey' ;
616 $h{'duck'} = 'donald' ;
621 # Cycle through the keys printing them in order.
622 # Note it is not necessary to sort the keys as
623 # the btree will have kept them in order automatically.
629 Here is the output from the code above.
635 There are a few point to bear in mind if you want to change the
636 ordering in a BTREE database:
642 The new compare function must be specified when you create the database.
646 You cannot change the ordering once the database has been created. Thus
647 you must use the same compare function every time you access the
652 =head2 Handling Duplicate Keys
654 The BTREE file type optionally allows a single key to be associated
655 with an arbitrary number of values. This option is enabled by setting
656 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
658 There are some difficulties in using the tied hash interface if you
659 want to manipulate a BTREE database with duplicate keys. Consider this
665 use vars qw($filename %h ) ;
670 # Enable duplicate records
671 $DB_BTREE->{'flags'} = R_DUP ;
673 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
674 or die "Cannot open $filename: $!\n";
676 # Add some key/value pairs to the file
677 $h{'Wall'} = 'Larry' ;
678 $h{'Wall'} = 'Brick' ; # Note the duplicate key
679 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
680 $h{'Smith'} = 'John' ;
681 $h{'mouse'} = 'mickey' ;
683 # iterate through the associative array
684 # and print each key/value pair.
686 { print "$_ -> $h{$_}\n" }
698 As you can see 3 records have been successfully created with key C<Wall>
699 - the only thing is, when they are retrieved from the database they
700 I<seem> to have the same value, namely C<Larry>. The problem is caused
701 by the way that the associative array interface works. Basically, when
702 the associative array interface is used to fetch the value associated
703 with a given key, it will only ever retrieve the first value.
705 Although it may not be immediately obvious from the code above, the
706 associative array interface can be used to write values with duplicate
707 keys, but it cannot be used to read them back from the database.
709 The way to get around this problem is to use the Berkeley DB API method
710 called C<seq>. This method allows sequential access to key/value
711 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
712 and the API in general.
714 Here is the script above rewritten using the C<seq> API method.
719 use vars qw($filename $x %h $status $key $value) ;
724 # Enable duplicate records
725 $DB_BTREE->{'flags'} = R_DUP ;
727 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
728 or die "Cannot open $filename: $!\n";
730 # Add some key/value pairs to the file
731 $h{'Wall'} = 'Larry' ;
732 $h{'Wall'} = 'Brick' ; # Note the duplicate key
733 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
734 $h{'Smith'} = 'John' ;
735 $h{'mouse'} = 'mickey' ;
737 # iterate through the btree using seq
738 # and print each key/value pair.
740 for ($status = $x->seq($key, $value, R_FIRST) ;
742 $status = $x->seq($key, $value, R_NEXT) )
743 { print "$key -> $value\n" }
756 This time we have got all the key/value pairs, including the multiple
757 values associated with the key C<Wall>.
759 =head2 The get_dup() Method
761 B<DB_File> comes with a utility method, called C<get_dup>, to assist in
762 reading duplicate values from BTREE databases. The method can take the
765 $count = $x->get_dup($key) ;
766 @list = $x->get_dup($key) ;
767 %list = $x->get_dup($key, 1) ;
769 In a scalar context the method returns the number of values associated
770 with the key, C<$key>.
772 In list context, it returns all the values which match C<$key>. Note
773 that the values will be returned in an apparently random order.
775 In list context, if the second parameter is present and evaluates TRUE,
776 the method returns an associative array. The keys of the associative
777 array correspond to the the values that matched in the BTREE and the
778 values of the array are a count of the number of times that particular
779 value occurred in the BTREE.
781 So assuming the database created above, we can use C<get_dup> like
784 my $cnt = $x->get_dup("Wall") ;
785 print "Wall occurred $cnt times\n" ;
787 my %hash = $x->get_dup("Wall", 1) ;
788 print "Larry is there\n" if $hash{'Larry'} ;
789 print "There are $hash{'Brick'} Brick Walls\n" ;
791 my @list = $x->get_dup("Wall") ;
792 print "Wall => [@list]\n" ;
794 @list = $x->get_dup("Smith") ;
795 print "Smith => [@list]\n" ;
797 @list = $x->get_dup("Dog") ;
798 print "Dog => [@list]\n" ;
803 Wall occurred 3 times
805 There are 2 Brick Walls
806 Wall => [Brick Brick Larry]
810 =head2 Matching Partial Keys
812 The BTREE interface has a feature which allows partial keys to be
813 matched. This functionality is I<only> available when the C<seq> method
814 is used along with the R_CURSOR flag.
816 $x->seq($key, $value, R_CURSOR) ;
818 Here is the relevant quote from the dbopen man page where it defines
819 the use of the R_CURSOR flag with seq:
821 Note, for the DB_BTREE access method, the returned key is not
822 necessarily an exact match for the specified key. The returned key
823 is the smallest key greater than or equal to the specified key,
824 permitting partial key matches and range searches.
826 In the example script below, the C<match> sub uses this feature to find
827 and print the first matching key/value pair given a partial key.
833 use vars qw($filename $x %h $st $key $value) ;
839 my $orig_key = $key ;
840 $x->seq($key, $value, R_CURSOR) ;
841 print "$orig_key\t-> $key\t-> $value\n" ;
847 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
848 or die "Cannot open $filename: $!\n";
850 # Add some key/value pairs to the file
851 $h{'mouse'} = 'mickey' ;
852 $h{'Wall'} = 'Larry' ;
853 $h{'Walls'} = 'Brick' ;
854 $h{'Smith'} = 'John' ;
859 for ($st = $x->seq($key, $value, R_FIRST) ;
861 $st = $x->seq($key, $value, R_NEXT) )
863 { print "$key -> $value\n" }
865 print "\nPARTIAL MATCH\n" ;
889 DB_RECNO provides an interface to flat text files. Both variable and
890 fixed length records are supported.
892 In order to make RECNO more compatible with Perl the array offset for
893 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
895 As with normal Perl arrays, a RECNO array can be accessed using
896 negative indexes. The index -1 refers to the last element of the array,
897 -2 the second last, and so on. Attempting to access an element before
898 the start of the array will raise a fatal run-time error.
900 =head2 The 'bval' Option
902 The operation of the bval option warrants some discussion. Here is the
903 definition of bval from the Berkeley DB 1.85 recno manual page:
905 The delimiting byte to be used to mark the end of a
906 record for variable-length records, and the pad charac-
907 ter for fixed-length records. If no value is speci-
908 fied, newlines (``\n'') are used to mark the end of
909 variable-length records and fixed-length records are
912 The second sentence is wrong. In actual fact bval will only default to
913 C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
914 openinfo parameter is used at all, the value that happens to be in bval
915 will be used. That means you always have to specify bval when making
916 use of any of the options in the openinfo parameter. This documentation
917 error will be fixed in the next release of Berkeley DB.
919 That clarifies the situation with regards Berkeley DB itself. What
920 about B<DB_File>? Well, the behavior defined in the quote above is
921 quite useful, so B<DB_File> conforms it.
923 That means that you can specify other options (e.g. cachesize) and
924 still have bval default to C<"\n"> for variable length records, and
925 space for fixed length records.
927 =head2 A Simple Example
929 Here is a simple example that uses RECNO.
935 tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
936 or die "Cannot open file 'text': $!\n" ;
938 # Add a few key/value pairs to the file
943 # Check for existence of a key
944 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
946 # use a negative index
947 print "The last element is $h[-1]\n" ;
948 print "The 2nd last element is $h[-2]\n" ;
952 Here is the output from the script:
955 Element 1 Exists with value blue
956 The last element is yellow
957 The 2nd last element is blue
961 As you can see from the example above, the tied array interface is
962 quite limited. To make the interface more useful, a number of methods
963 are supplied with B<DB_File> to simulate the standard array operations
964 that are not currently implemented in Perl's tied array interface. All
965 these methods are accessed via the object returned from the tie call.
967 Here are the methods:
971 =item B<$X-E<gt>push(list) ;>
973 Pushes the elements of C<list> to the end of the array.
975 =item B<$value = $X-E<gt>pop ;>
977 Removes and returns the last element of the array.
979 =item B<$X-E<gt>shift>
981 Removes and returns the first element of the array.
983 =item B<$X-E<gt>unshift(list) ;>
985 Pushes the elements of C<list> to the start of the array.
987 =item B<$X-E<gt>length>
989 Returns the number of elements in the array.
993 =head2 Another Example
995 Here is a more complete example that makes use of some of the methods
996 described above. It also makes use of the API interface directly (see
997 L<THE API INTERFACE>).
1000 use vars qw(@h $H $file $i) ;
1008 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
1009 or die "Cannot open file $file: $!\n" ;
1011 # first create a text file to play with
1019 # Print the records in order.
1021 # The length method is needed here because evaluating a tied
1022 # array in a scalar context does not return the number of
1023 # elements in the array.
1025 print "\nORIGINAL\n" ;
1026 foreach $i (0 .. $H->length - 1) {
1027 print "$i: $h[$i]\n" ;
1030 # use the push & pop methods
1033 print "\nThe last record was [$a]\n" ;
1035 # and the shift & unshift methods
1037 $H->unshift("first") ;
1038 print "The first record was [$a]\n" ;
1040 # Use the API to add a new record after record 2.
1042 $H->put($i, "Newbie", R_IAFTER) ;
1044 # and a new record before record 1.
1046 $H->put($i, "New One", R_IBEFORE) ;
1051 # now print the records in reverse order
1052 print "\nREVERSE\n" ;
1053 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1054 { print "$i: $h[$i]\n" }
1056 # same again, but use the API functions instead
1057 print "\nREVERSE again\n" ;
1058 my ($s, $k, $v) = (0, 0, 0) ;
1059 for ($s = $H->seq($k, $v, R_LAST) ;
1061 $s = $H->seq($k, $v, R_PREV))
1062 { print "$k: $v\n" }
1067 and this is what it outputs:
1076 The last record was [four]
1077 The first record was [zero]
1101 Rather than iterating through the array, C<@h> like this:
1105 it is necessary to use either this:
1107 foreach $i (0 .. $H->length - 1)
1111 for ($a = $H->get($k, $v, R_FIRST) ;
1113 $a = $H->get($k, $v, R_NEXT) )
1117 Notice that both times the C<put> method was used the record index was
1118 specified using a variable, C<$i>, rather than the literal value
1119 itself. This is because C<put> will return the record number of the
1120 inserted line via that parameter.
1124 =head1 THE API INTERFACE
1126 As well as accessing Berkeley DB using a tied hash or array, it is also
1127 possible to make direct use of most of the API functions defined in the
1128 Berkeley DB documentation.
1130 To do this you need to store a copy of the object returned from the tie.
1132 $db = tie %hash, "DB_File", "filename" ;
1134 Once you have done that, you can access the Berkeley DB API functions
1135 as B<DB_File> methods directly like this:
1137 $db->put($key, $value, R_NOOVERWRITE) ;
1139 B<Important:> If you have saved a copy of the object returned from
1140 C<tie>, the underlying database file will I<not> be closed until both
1141 the tied variable is untied and all copies of the saved object are
1145 $db = tie %hash, "DB_File", "filename"
1146 or die "Cannot tie filename: $!" ;
1151 See L<The untie() Gotcha> for more details.
1153 All the functions defined in L<dbopen> are available except for
1154 close() and dbopen() itself. The B<DB_File> method interface to the
1155 supported functions have been implemented to mirror the way Berkeley DB
1156 works whenever possible. In particular note that:
1162 The methods return a status value. All return 0 on success.
1163 All return -1 to signify an error and set C<$!> to the exact
1164 error code. The return code 1 generally (but not always) means that the
1165 key specified did not exist in the database.
1167 Other return codes are defined. See below and in the Berkeley DB
1168 documentation for details. The Berkeley DB documentation should be used
1169 as the definitive source.
1173 Whenever a Berkeley DB function returns data via one of its parameters,
1174 the equivalent B<DB_File> method does exactly the same.
1178 If you are careful, it is possible to mix API calls with the tied
1179 hash/array interface in the same piece of code. Although only a few of
1180 the methods used to implement the tied interface currently make use of
1181 the cursor, you should always assume that the cursor has been changed
1182 any time the tied hash/array interface is used. As an example, this
1183 code will probably not do what you expect:
1185 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1186 or die "Cannot tie $filename: $!" ;
1188 # Get the first key/value pair and set the cursor
1189 $X->seq($key, $value, R_FIRST) ;
1191 # this line will modify the cursor
1192 $count = scalar keys %x ;
1194 # Get the second key/value pair.
1195 # oops, it didn't, it got the last key/value pair!
1196 $X->seq($key, $value, R_NEXT) ;
1198 The code above can be rearranged to get around the problem, like this:
1200 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1201 or die "Cannot tie $filename: $!" ;
1203 # this line will modify the cursor
1204 $count = scalar keys %x ;
1206 # Get the first key/value pair and set the cursor
1207 $X->seq($key, $value, R_FIRST) ;
1209 # Get the second key/value pair.
1211 $X->seq($key, $value, R_NEXT) ;
1215 All the constants defined in L<dbopen> for use in the flags parameters
1216 in the methods defined below are also available. Refer to the Berkeley
1217 DB documentation for the precise meaning of the flags values.
1219 Below is a list of the methods available.
1223 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1225 Given a key (C<$key>) this method reads the value associated with it
1226 from the database. The value read from the database is returned in the
1227 C<$value> parameter.
1229 If the key does not exist the method returns 1.
1231 No flags are currently defined for this method.
1233 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1235 Stores the key/value pair in the database.
1237 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1238 will have the record number of the inserted key/value pair set.
1240 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1243 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1245 Removes all key/value pairs with key C<$key> from the database.
1247 A return code of 1 means that the requested key was not in the
1250 R_CURSOR is the only valid flag at present.
1252 =item B<$status = $X-E<gt>fd ;>
1254 Returns the file descriptor for the underlying database.
1256 See L<Locking Databases> for an example of how to make use of the
1257 C<fd> method to lock your database.
1259 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1261 This interface allows sequential retrieval from the database. See
1262 L<dbopen> for full details.
1264 Both the C<$key> and C<$value> parameters will be set to the key/value
1265 pair read from the database.
1267 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1268 R_FIRST, R_LAST, R_NEXT and R_PREV.
1270 =item B<$status = $X-E<gt>sync([$flags]) ;>
1272 Flushes any cached buffers to disk.
1274 R_RECNOSYNC is the only valid flag at present.
1278 =head1 HINTS AND TIPS
1281 =head2 Locking Databases
1283 Concurrent access of a read-write database by several parties requires
1284 them all to use some kind of locking. Here's an example of Tom's that
1285 uses the I<fd> method to get the file descriptor, and then a careful
1286 open() to give something Perl will flock() for you. Run this repeatedly
1287 in the background to watch the locks granted in proper order.
1298 my($oldval, $fd, $db, %db, $value, $key);
1300 $key = shift || 'default';
1301 $value = shift || 'magic';
1305 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1306 || die "dbcreat /tmp/foo.db $!";
1308 print "$$: db fd is $fd\n";
1309 open(DB_FH, "+<&=$fd") || die "dup $!";
1312 unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
1313 print "$$: CONTENTION; can't read during write update!
1314 Waiting for read lock ($!) ....";
1315 unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
1317 print "$$: Read lock granted\n";
1319 $oldval = $db{$key};
1320 print "$$: Old value was $oldval\n";
1321 flock(DB_FH, LOCK_UN);
1323 unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
1324 print "$$: CONTENTION; must have exclusive lock!
1325 Waiting for write lock ($!) ....";
1326 unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
1329 print "$$: Write lock granted\n";
1331 $db->sync; # to flush
1334 flock(DB_FH, LOCK_UN);
1338 print "$$: Updated db to $key=$value\n";
1340 =head2 Sharing Databases With C Applications
1342 There is no technical reason why a Berkeley DB database cannot be
1343 shared by both a Perl and a C application.
1345 The vast majority of problems that are reported in this area boil down
1346 to the fact that C strings are NULL terminated, whilst Perl strings are
1349 Here is a real example. Netscape 2.0 keeps a record of the locations you
1350 visit along with the time you last visited them in a DB_HASH database.
1351 This is usually stored in the file F<~/.netscape/history.db>. The key
1352 field in the database is the location string and the value field is the
1353 time the location was last visited stored as a 4 byte binary value.
1355 If you haven't already guessed, the location string is stored with a
1356 terminating NULL. This means you need to be careful when accessing the
1359 Here is a snippet of code that is loosely based on Tom Christiansen's
1360 I<ggh> script (available from your nearest CPAN archive in
1361 F<authors/id/TOMC/scripts/nshist.gz>).
1367 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1368 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1370 $HISTORY = "$dotdir/.netscape/history.db";
1372 tie %hist_db, 'DB_File', $HISTORY
1373 or die "Cannot open $HISTORY: $!\n" ;;
1375 # Dump the complete database
1376 while ( ($href, $binary_time) = each %hist_db ) {
1378 # remove the terminating NULL
1379 $href =~ s/\x00$// ;
1381 # convert the binary time into a user friendly string
1382 $date = localtime unpack("V", $binary_time);
1383 print "$date $href\n" ;
1386 # check for the existence of a specific key
1387 # remember to add the NULL
1388 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
1389 $date = localtime unpack("V", $binary_time) ;
1390 print "Last visited mox.perl.com on $date\n" ;
1393 print "Never visited mox.perl.com\n"
1398 =head2 The untie() Gotcha
1400 If you make use of the Berkeley DB API, it is is I<very> strongly
1401 recommended that you read L<perltie/The untie Gotcha>.
1403 Even if you don't currently make use of the API interface, it is still
1406 Here is an example which illustrates the problem from a B<DB_File>
1415 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
1416 or die "Cannot tie first time: $!" ;
1422 tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1423 or die "Cannot tie second time: $!" ;
1427 When run, the script will produce this error message:
1429 Cannot tie second time: Invalid argument at bad.file line 14.
1431 Although the error message above refers to the second tie() statement
1432 in the script, the source of the problem is really with the untie()
1433 statement that precedes it.
1435 Having read L<perltie> you will probably have already guessed that the
1436 error is caused by the extra copy of the tied object stored in C<$X>.
1437 If you haven't, then the problem boils down to the fact that the
1438 B<DB_File> destructor, DESTROY, will not be called until I<all>
1439 references to the tied object are destroyed. Both the tied variable,
1440 C<%x>, and C<$X> above hold a reference to the object. The call to
1441 untie() will destroy the first, but C<$X> still holds a valid
1442 reference, so the destructor will not get called and the database file
1443 F<tst.fil> will remain open. The fact that Berkeley DB then reports the
1444 attempt to open a database that is alreday open via the catch-all
1445 "Invalid argument" doesn't help.
1447 If you run the script with the C<-w> flag the error message becomes:
1449 untie attempted while 1 inner references still exist at bad.file line 12.
1450 Cannot tie second time: Invalid argument at bad.file line 14.
1452 which pinpoints the real problem. Finally the script can now be
1453 modified to fix the original problem by destroying the API object
1462 $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
1466 =head1 COMMON QUESTIONS
1468 =head2 Why is there Perl source in my database?
1470 If you look at the contents of a database file created by DB_File,
1471 there can sometimes be part of a Perl script included in it.
1473 This happens because Berkeley DB uses dynamic memory to allocate
1474 buffers which will subsequently be written to the database file. Being
1475 dynamic, the memory could have been used for anything before DB
1476 malloced it. As Berkeley DB doesn't clear the memory once it has been
1477 allocated, the unused portions will contain random junk. In the case
1478 where a Perl script gets written to the database, the random junk will
1479 correspond to an area of dynamic memory that happened to be used during
1480 the compilation of the script.
1482 Unless you don't like the possibility of there being part of your Perl
1483 scripts embedded in a database file, this is nothing to worry about.
1485 =head2 How do I store complex data structures with DB_File?
1487 Although B<DB_File> cannot do this directly, there is a module which
1488 can layer transparently over B<DB_File> to accomplish this feat.
1490 Check out the MLDBM module, available on CPAN in the directory
1491 F<modules/by-module/MLDBM>.
1493 =head2 What does "Invalid Argument" mean?
1495 You will get this error message when one of the parameters in the
1496 C<tie> call is wrong. Unfortunately there are quite a few parameters to
1497 get wrong, so it can be difficult to figure out which one it is.
1499 Here are a couple of possibilities:
1505 Attempting to reopen a database without closing it.
1509 Using the O_WRONLY flag.
1513 =head2 What does "Bareword 'DB_File' not allowed" mean?
1515 You will encounter this particular error message when you have the
1516 C<strict 'subs'> pragma (or the full strict pragma) in your script.
1517 Consider this script:
1522 tie %x, DB_File, "filename" ;
1524 Running it produces the error in question:
1526 Bareword "DB_File" not allowed while "strict subs" in use
1528 To get around the error, place the word C<DB_File> in either single or
1529 double quotes, like this:
1531 tie %x, "DB_File", "filename" ;
1533 Although it might seem like a real pain, it is really worth the effort
1534 of having a C<use strict> in all your scripts.
1546 When B<DB_File> is opening a database file it no longer terminates the
1547 process if I<dbopen> returned an error. This allows file protection
1548 errors to be caught at run time. Thanks to Judith Grass
1549 E<lt>grass@cybercash.comE<gt> for spotting the bug.
1553 Added prototype support for multiple btree compare callbacks.
1557 B<DB_File> has been in use for over a year. To reflect that, the
1558 version number has been incremented to 1.0.
1560 Added complete support for multiple concurrent callbacks.
1562 Using the I<push> method on an empty list didn't work properly. This
1567 Fixed a core dump problem with SunOS.
1569 The return value from TIEHASH wasn't set to NULL when dbopen returned
1574 Merged OS/2 specific code into DB_File.xs
1576 Removed some redundant code in DB_File.xs.
1578 Documentation update.
1580 Allow negative subscripts with RECNO interface.
1582 Changed the default flags from O_RDWR to O_CREAT|O_RDWR.
1584 The example code which showed how to lock a database needed a call to
1585 C<sync> added. Without it the resultant database file was empty.
1587 Added get_dup method.
1591 Documentation update.
1593 B<DB_File> now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl
1596 The standard hash function C<exists> is now supported.
1598 Modified the behavior of get_dup. When it returns an associative
1599 array, the value is the count of the number of matching BTREE values.
1603 Minor documentation changes.
1605 Fixed a bug in hash_cb. Patches supplied by Dave Hammen,
1606 E<lt>hammen@gothamcity.jsc.nasa.govE<gt>.
1608 Fixed a bug with the constructors for DB_File::HASHINFO,
1609 DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up the
1610 constructors to make them C<-w> clean.
1612 Reworked part of the test harness to be more locale friendly.
1616 Made all scripts in the documentation C<strict> and C<-w> clean.
1618 Added logic to F<DB_File.xs> to allow the module to be built after Perl
1623 Minor namespace cleanup: Localized C<PrintBtree>.
1627 Fixed bug with RECNO, where bval wasn't defaulting to "\n".
1631 Documented operation of bval.
1635 Minor bug fix in DB_File::HASHINFO, DB_File::RECNOINFO and
1638 Changed default mode to 0666.
1642 Fixed fd method so that it still returns -1 for in-memory files when db
1647 Documented the untie gotcha.
1651 Documented the incompatibility with version 2 of Berkeley DB.
1657 Some older versions of Berkeley DB had problems with fixed length
1658 records using the RECNO file format. The newest version at the time of
1659 writing was 1.85 - this seems to have fixed the problems with RECNO.
1661 I am sure there are bugs in the code. If you do find any, or can
1662 suggest any enhancements, I would welcome your comments.
1666 B<DB_File> comes with the standard Perl source distribution. Look in
1667 the directory F<ext/DB_File>.
1669 This version of B<DB_File> will only work with version 1.x of Berkeley
1670 DB. It is I<not> yet compatible with version 2.
1672 Version 1 of Berkeley DB is available at your nearest CPAN archive (see
1673 L<perlmod/"CPAN"> for a list) in F<src/misc/db.1.85.tar.gz>, or via the
1674 host F<ftp.cs.berkeley.edu> in F</ucb/4bsd/db.tar.gz>. Alternatively,
1675 check out the Berkeley DB home page at F<http://www.bostic.com/db>. It
1676 is I<not> under the GPL.
1678 If you are running IRIX, then get Berkeley DB from
1679 F<http://reality.sgi.com/ariel>. It has the patches necessary to
1680 compile properly on IRIX 5.3.
1682 As of January 1997, version 1.86 of Berkeley DB is available from the
1683 Berkeley DB home page. Although this release does fix a number of bugs
1684 that were present in 1.85 you should be aware of the following
1685 information (taken from the Berkeley DB home page) before you consider
1688 DB version 1.86 includes a new implementation of the hash access
1689 method that fixes a variety of hashing problems found in DB version
1690 1.85. We are making it available as an interim solution until DB
1693 PLEASE NOTE: the underlying file format for the hash access method
1694 changed between version 1.85 and version 1.86, so you will have to
1695 dump and reload all of your databases to convert from version 1.85
1696 to version 1.86. If you do not absolutely require the fixes from
1697 version 1.86, we strongly urge you to wait until DB 2.0 is released
1698 before upgrading from 1.85.
1703 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>
1707 The DB_File interface was written by Paul Marquess
1708 E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
1709 Questions about the DB system itself may be addressed to Keith Bostic
1710 E<lt>bostic@cs.berkeley.eduE<gt>.