1 # DB_File.pm -- Perl 5 interface to Berkeley DB
3 # written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
4 # last modified 27th Nov 1996
7 package DB_File::HASHINFO ;
14 @DB_File::HASHINFO::ISA = qw(Tie::Hash);
44 return $self->{$key} if exists $self->{$key} ;
47 croak "${pkg}::FETCH - Unknown element '$key'" ;
57 if ( exists $self->{$key} )
59 $self->{$key} = $value ;
64 croak "${pkg}::STORE - Unknown element '$key'" ;
72 if ( exists $self->{$key} )
74 delete $self->{$key} ;
79 croak "DB_File::HASHINFO::DELETE - Unknown element '$key'" ;
87 exists $self->{$key} ;
95 croak "${pkg} does not define the method ${method}" ;
98 sub DESTROY { undef %{$_[0]} }
99 sub FIRSTKEY { my $self = shift ; $self->NotHere(ref $self, "FIRSTKEY") }
100 sub NEXTKEY { my $self = shift ; $self->NotHere(ref $self, "NEXTKEY") }
101 sub CLEAR { my $self = shift ; $self->NotHere(ref $self, "CLEAR") }
103 package DB_File::RECNOINFO ;
107 @DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
123 package DB_File::BTREEINFO ;
127 @DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
133 bless { 'flags' => 0,
148 use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO) ;
154 #typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
155 $DB_BTREE = new DB_File::BTREEINFO ;
156 $DB_HASH = new DB_File::HASHINFO ;
157 $DB_RECNO = new DB_File::RECNOINFO ;
163 @ISA = qw(Tie::Hash Exporter DynaLoader);
165 $DB_BTREE $DB_HASH $DB_RECNO
200 ($constname = $AUTOLOAD) =~ s/.*:://;
201 my $val = constant($constname, @_ ? $_[0] : 0);
203 if ($! =~ /Invalid/) {
204 $AutoLoader::AUTOLOAD = $AUTOLOAD;
205 goto &AutoLoader::AUTOLOAD;
208 my($pack,$file,$line) = caller;
209 croak "Your vendor has not defined DB macro $constname, used at $file line $line.
213 eval "sub $AUTOLOAD { $val }";
218 # import borrowed from IO::File
219 # exports Fcntl constants if available.
222 my $callpkg = caller;
223 Exporter::export $pkg, $callpkg;
226 Exporter::export 'Fcntl', $callpkg;
230 bootstrap DB_File $VERSION;
232 # Preloaded methods go here. Autoload methods go after __END__, and are
233 # processed by the autosplit program.
239 $arg[4] = tied %{ $arg[4] }
240 if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
245 *TIEARRAY = \&TIEHASH ;
249 croak "Usage: \$db->get_dup(key [,flag])\n"
250 unless @_ == 2 or @_ == 3 ;
257 my $wantarray = wantarray ;
263 # iterate through the database until either EOF ($status == 0)
264 # or a different key is encountered ($key ne $origkey).
265 for ($status = $db->seq($key, $value, R_CURSOR()) ;
266 $status == 0 and $key eq $origkey ;
267 $status = $db->seq($key, $value, R_NEXT()) ) {
269 # save the value or count number of matches
272 { ++ $values{$value} }
274 { push (@values, $value) }
281 return ($wantarray ? ($flag ? %values : @values) : $counter) ;
292 DB_File - Perl5 access to Berkeley DB
298 [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
299 [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
300 [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
302 $status = $X->del($key [, $flags]) ;
303 $status = $X->put($key, $value [, $flags]) ;
304 $status = $X->get($key, $value [, $flags]) ;
305 $status = $X->seq($key, $value, $flags) ;
306 $status = $X->sync([$flags]) ;
310 $count = $X->get_dup($key) ;
311 @list = $X->get_dup($key) ;
312 %list = $X->get_dup($key, 1) ;
326 B<DB_File> is a module which allows Perl programs to make use of the
327 facilities provided by Berkeley DB. If you intend to use this
328 module you should really have a copy of the Berkeley DB manual pages at
329 hand. The interface defined here mirrors the Berkeley DB interface
332 Berkeley DB is a C library which provides a consistent interface to a
333 number of database formats. B<DB_File> provides an interface to all
334 three of the database types currently supported by Berkeley DB.
342 This database type allows arbitrary key/value pairs to be stored in data
343 files. This is equivalent to the functionality provided by other
344 hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
345 the files created using DB_HASH are not compatible with any of the
346 other packages mentioned.
348 A default hashing algorithm, which will be adequate for most
349 applications, is built into Berkeley DB. If you do need to use your own
350 hashing algorithm it is possible to write your own in Perl and have
351 B<DB_File> use it instead.
355 The btree format allows arbitrary key/value pairs to be stored in a
356 sorted, balanced binary tree.
358 As with the DB_HASH format, it is possible to provide a user defined
359 Perl routine to perform the comparison of keys. By default, though, the
360 keys are stored in lexical order.
364 DB_RECNO allows both fixed-length and variable-length flat text files
365 to be manipulated using the same key/value pair interface as in DB_HASH
366 and DB_BTREE. In this case the key will consist of a record (line)
371 =head2 How does DB_File interface to Berkeley DB?
373 B<DB_File> allows access to Berkeley DB files using the tie() mechanism
374 in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
375 allows B<DB_File> to access Berkeley DB files using either an
376 associative array (for DB_HASH & DB_BTREE file types) or an ordinary
377 array (for the DB_RECNO file type).
379 In addition to the tie() interface, it is also possible to access most
380 of the functions provided in the Berkeley DB API directly.
381 See L<THE API INTERFACE>.
383 =head2 Opening a Berkeley DB Database File
385 Berkeley DB uses the function dbopen() to open or create a database.
386 Here is the C prototype for dbopen():
389 dbopen (const char * file, int flags, int mode,
390 DBTYPE type, const void * openinfo)
392 The parameter C<type> is an enumeration which specifies which of the 3
393 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
394 Depending on which of these is actually chosen, the final parameter,
395 I<openinfo> points to a data structure which allows tailoring of the
396 specific interface method.
398 This interface is handled slightly differently in B<DB_File>. Here is
399 an equivalent call using B<DB_File>:
401 tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
403 The C<filename>, C<flags> and C<mode> parameters are the direct
404 equivalent of their dbopen() counterparts. The final parameter $DB_HASH
405 performs the function of both the C<type> and C<openinfo> parameters in
408 In the example above $DB_HASH is actually a pre-defined reference to a
409 hash object. B<DB_File> has three of these pre-defined references.
410 Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
412 The keys allowed in each of these pre-defined references is limited to
413 the names used in the equivalent C structure. So, for example, the
414 $DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
415 C<ffactor>, C<hash>, C<lorder> and C<nelem>.
417 To change one of these elements, just assign to it like this:
419 $DB_HASH->{'cachesize'} = 10000 ;
421 The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
422 usually adequate for most applications. If you do need to create extra
423 instances of these objects, constructors are available for each file
426 Here are examples of the constructors and the valid options available
427 for DB_HASH, DB_BTREE and DB_RECNO respectively.
429 $a = new DB_File::HASHINFO ;
437 $b = new DB_File::BTREEINFO ;
447 $c = new DB_File::RECNOINFO ;
456 The values stored in the hashes above are mostly the direct equivalent
457 of their C counterpart. Like their C counterparts, all are set to a
458 default values - that means you don't have to set I<all> of the
459 values when you only want to change one. Here is an example:
461 $a = new DB_File::HASHINFO ;
462 $a->{'cachesize'} = 12345 ;
463 tie %y, 'DB_File', "filename", $flags, 0777, $a ;
465 A few of the values need extra discussion here. When used, the C
466 equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
467 to C functions. In B<DB_File> these keys are used to store references
468 to Perl subs. Below are templates for each of the subs:
474 # return the hash value for $data
480 my ($key, $key2) = @_ ;
482 # return 0 if $key1 eq $key2
483 # -1 if $key1 lt $key2
484 # 1 if $key1 gt $key2
485 return (-1 , 0 or 1) ;
490 my ($key, $key2) = @_ ;
492 # return number of bytes of $key2 which are
493 # necessary to determine that it is greater than $key1
497 See L<Changing the BTREE sort order> for an example of using the
500 =head2 Default Parameters
502 It is possible to omit some or all of the final 4 parameters in the
503 call to C<tie> and let them take default values. As DB_HASH is the most
504 common file format used, the call:
506 tie %A, "DB_File", "filename" ;
510 tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0640, $DB_HASH ;
512 It is also possible to omit the filename parameter as well, so the
519 tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0640, $DB_HASH ;
521 See L<In Memory Databases> for a discussion on the use of C<undef>
522 in place of a filename.
524 =head2 In Memory Databases
526 Berkeley DB allows the creation of in-memory databases by using NULL
527 (that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
528 uses C<undef> instead of NULL to provide this functionality.
532 The DB_HASH file format is probably the most commonly used of the three
533 file formats that B<DB_File> supports. It is also very straightforward
536 =head2 A Simple Example.
538 This example shows how to create a database, add key/value pairs to the
539 database, delete keys/value pairs and finally how to enumerate the
540 contents of the database.
544 use vars qw( %h $k $v ) ;
546 tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
547 or die "Cannot open file 'fruit': $!\n";
549 # Add a few key/value pairs to the file
550 $h{"apple"} = "red" ;
551 $h{"orange"} = "orange" ;
552 $h{"banana"} = "yellow" ;
553 $h{"tomato"} = "red" ;
555 # Check for existence of a key
556 print "Banana Exists\n\n" if $h{"banana"} ;
558 # Delete a key/value pair.
561 # print the contents of the file
562 while (($k, $v) = each %h)
563 { print "$k -> $v\n" }
575 Note that the like ordinary associative arrays, the order of the keys
576 retrieved is in an apparently random order.
580 The DB_BTREE format is useful when you want to store data in a given
581 order. By default the keys will be stored in lexical order, but as you
582 will see from the example shown in the next section, it is very easy to
583 define your own sorting function.
585 =head2 Changing the BTREE sort order
587 This script shows how to override the default sorting algorithm that
588 BTREE uses. Instead of using the normal lexical ordering, a case
589 insensitive compare function will be used.
598 my ($key1, $key2) = @_ ;
599 "\L$key1" cmp "\L$key2" ;
602 # specify the Perl sub that will do the comparison
603 $DB_BTREE->{'compare'} = \&Compare ;
605 tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
606 or die "Cannot open file 'tree': $!\n" ;
608 # Add a key/value pair to the file
609 $h{'Wall'} = 'Larry' ;
610 $h{'Smith'} = 'John' ;
611 $h{'mouse'} = 'mickey' ;
612 $h{'duck'} = 'donald' ;
617 # Cycle through the keys printing them in order.
618 # Note it is not necessary to sort the keys as
619 # the btree will have kept them in order automatically.
625 Here is the output from the code above.
631 There are a few point to bear in mind if you want to change the
632 ordering in a BTREE database:
638 The new compare function must be specified when you create the database.
642 You cannot change the ordering once the database has been created. Thus
643 you must use the same compare function every time you access the
648 =head2 Handling duplicate keys
650 The BTREE file type optionally allows a single key to be associated
651 with an arbitrary number of values. This option is enabled by setting
652 the flags element of C<$DB_BTREE> to R_DUP when creating the database.
654 There are some difficulties in using the tied hash interface if you
655 want to manipulate a BTREE database with duplicate keys. Consider this
661 use vars qw($filename %h ) ;
666 # Enable duplicate records
667 $DB_BTREE->{'flags'} = R_DUP ;
669 tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
670 or die "Cannot open $filename: $!\n";
672 # Add some key/value pairs to the file
673 $h{'Wall'} = 'Larry' ;
674 $h{'Wall'} = 'Brick' ; # Note the duplicate key
675 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
676 $h{'Smith'} = 'John' ;
677 $h{'mouse'} = 'mickey' ;
679 # iterate through the associative array
680 # and print each key/value pair.
682 { print "$_ -> $h{$_}\n" }
694 As you can see 3 records have been successfully created with key C<Wall>
695 - the only thing is, when they are retrieved from the database they
696 I<seem> to have the same value, namely C<Larry>. The problem is caused
697 by the way that the associative array interface works. Basically, when
698 the associative array interface is used to fetch the value associated
699 with a given key, it will only ever retrieve the first value.
701 Although it may not be immediately obvious from the code above, the
702 associative array interface can be used to write values with duplicate
703 keys, but it cannot be used to read them back from the database.
705 The way to get around this problem is to use the Berkeley DB API method
706 called C<seq>. This method allows sequential access to key/value
707 pairs. See L<THE API INTERFACE> for details of both the C<seq> method
708 and the API in general.
710 Here is the script above rewritten using the C<seq> API method.
715 use vars qw($filename $x %h $status $key $value) ;
720 # Enable duplicate records
721 $DB_BTREE->{'flags'} = R_DUP ;
723 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
724 or die "Cannot open $filename: $!\n";
726 # Add some key/value pairs to the file
727 $h{'Wall'} = 'Larry' ;
728 $h{'Wall'} = 'Brick' ; # Note the duplicate key
729 $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
730 $h{'Smith'} = 'John' ;
731 $h{'mouse'} = 'mickey' ;
733 # iterate through the btree using seq
734 # and print each key/value pair.
736 for ($status = $x->seq($key, $value, R_FIRST) ;
738 $status = $x->seq($key, $value, R_NEXT) )
739 { print "$key -> $value\n" }
752 This time we have got all the key/value pairs, including the multiple
753 values associated with the key C<Wall>.
755 =head2 The get_dup method.
757 B<DB_File> comes with a utility method, called C<get_dup>, to assist in
758 reading duplicate values from BTREE databases. The method can take the
761 $count = $x->get_dup($key) ;
762 @list = $x->get_dup($key) ;
763 %list = $x->get_dup($key, 1) ;
765 In a scalar context the method returns the number of values associated
766 with the key, C<$key>.
768 In list context, it returns all the values which match C<$key>. Note
769 that the values will be returned in an apparently random order.
771 In list context, if the second parameter is present and evaluates TRUE,
772 the method returns an associative array. The keys of the associative
773 array correspond to the the values that matched in the BTREE and the
774 values of the array are a count of the number of times that particular
775 value occurred in the BTREE.
777 So assuming the database created above, we can use C<get_dup> like
780 my $cnt = $x->get_dup("Wall") ;
781 print "Wall occurred $cnt times\n" ;
783 my %hash = $x->get_dup("Wall", 1) ;
784 print "Larry is there\n" if $hash{'Larry'} ;
785 print "There are $hash{'Brick'} Brick Walls\n" ;
787 my @list = $x->get_dup("Wall") ;
788 print "Wall => [@list]\n" ;
790 @list = $x->get_dup("Smith") ;
791 print "Smith => [@list]\n" ;
793 @list = $x->get_dup("Dog") ;
794 print "Dog => [@list]\n" ;
799 Wall occurred 3 times
801 There are 2 Brick Walls
802 Wall => [Brick Brick Larry]
806 =head2 Matching Partial Keys
808 The BTREE interface has a feature which allows partial keys to be
809 matched. This functionality is I<only> available when the C<seq> method
810 is used along with the R_CURSOR flag.
812 $x->seq($key, $value, R_CURSOR) ;
814 Here is the relevant quote from the dbopen man page where it defines
815 the use of the R_CURSOR flag with seq:
817 Note, for the DB_BTREE access method, the returned key is not
818 necessarily an exact match for the specified key. The returned key
819 is the smallest key greater than or equal to the specified key,
820 permitting partial key matches and range searches.
822 In the example script below, the C<match> sub uses this feature to find
823 and print the first matching key/value pair given a partial key.
829 use vars qw($filename $x %h $st $key $value) ;
835 my $orig_key = $key ;
836 $x->seq($key, $value, R_CURSOR) ;
837 print "$orig_key\t-> $key\t-> $value\n" ;
843 $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
844 or die "Cannot open $filename: $!\n";
846 # Add some key/value pairs to the file
847 $h{'mouse'} = 'mickey' ;
848 $h{'Wall'} = 'Larry' ;
849 $h{'Walls'} = 'Brick' ;
850 $h{'Smith'} = 'John' ;
855 for ($st = $x->seq($key, $value, R_FIRST) ;
857 $st = $x->seq($key, $value, R_NEXT) )
859 { print "$key -> $value\n" }
861 print "\nPARTIAL MATCH\n" ;
885 DB_RECNO provides an interface to flat text files. Both variable and
886 fixed length records are supported.
888 In order to make RECNO more compatible with Perl the array offset for
889 all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
891 As with normal Perl arrays, a RECNO array can be accessed using
892 negative indexes. The index -1 refers to the last element of the array,
893 -2 the second last, and so on. Attempting to access an element before
894 the start of the array will raise a fatal run-time error.
896 =head2 A Simple Example
898 Here is a simple example that uses RECNO.
904 tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
905 or die "Cannot open file 'text': $!\n" ;
907 # Add a few key/value pairs to the file
912 # Check for existence of a key
913 print "Element 1 Exists with value $h[1]\n" if $h[1] ;
915 # use a negative index
916 print "The last element is $h[-1]\n" ;
917 print "The 2nd last element is $h[-2]\n" ;
921 Here is the output from the script:
924 Element 1 Exists with value blue
925 The last element is yellow
926 The 2nd last element is blue
930 As you can see from the example above, the tied array interface is
931 quite limited. To make the interface more useful, a number of methods
932 are supplied with B<DB_File> to simulate the standard array operations
933 that are not currently implemented in Perl's tied array interface. All
934 these methods are accessed via the object returned from the tie call.
936 Here are the methods:
940 =item B<$X-E<gt>push(list) ;>
942 Pushes the elements of C<list> to the end of the array.
944 =item B<$value = $X-E<gt>pop ;>
946 Removes and returns the last element of the array.
948 =item B<$X-E<gt>shift>
950 Removes and returns the first element of the array.
952 =item B<$X-E<gt>unshift(list) ;>
954 Pushes the elements of C<list> to the start of the array.
956 =item B<$X-E<gt>length>
958 Returns the number of elements in the array.
962 =head2 Another Example
964 Here is a more complete example that makes use of some of the methods
965 described above. It also makes use of the API interface directly (see
966 L<THE API INTERFACE>).
969 use vars qw(@h $H $file $i) ;
977 $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
978 or die "Cannot open file $file: $!\n" ;
980 # first create a text file to play with
988 # Print the records in order.
990 # The length method is needed here because evaluating a tied
991 # array in a scalar context does not return the number of
992 # elements in the array.
994 print "\nORIGINAL\n" ;
995 foreach $i (0 .. $H->length - 1) {
996 print "$i: $h[$i]\n" ;
999 # use the push & pop methods
1002 print "\nThe last record was [$a]\n" ;
1004 # and the shift & unshift methods
1006 $H->unshift("first") ;
1007 print "The first record was [$a]\n" ;
1009 # Use the API to add a new record after record 2.
1011 $H->put($i, "Newbie", R_IAFTER) ;
1013 # and a new record before record 1.
1015 $H->put($i, "New One", R_IBEFORE) ;
1020 # now print the records in reverse order
1021 print "\nREVERSE\n" ;
1022 for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
1023 { print "$i: $h[$i]\n" }
1025 # same again, but use the API functions instead
1026 print "\nREVERSE again\n" ;
1027 my ($s, $k, $v) = (0, 0, 0) ;
1028 for ($s = $H->seq($k, $v, R_LAST) ;
1030 $s = $H->seq($k, $v, R_PREV))
1031 { print "$k: $v\n" }
1036 and this is what it outputs:
1045 The last record was [four]
1046 The first record was [zero]
1070 Rather than iterating through the array, C<@h> like this:
1074 it is necessary to use either this:
1076 foreach $i (0 .. $H->length - 1)
1080 for ($a = $H->get($k, $v, R_FIRST) ;
1082 $a = $H->get($k, $v, R_NEXT) )
1086 Notice that both times the C<put> method was used the record index was
1087 specified using a variable, C<$i>, rather than the literal value
1088 itself. This is because C<put> will return the record number of the
1089 inserted line via that parameter.
1093 =head1 THE API INTERFACE
1095 As well as accessing Berkeley DB using a tied hash or array, it is also
1096 possible to make direct use of most of the API functions defined in the
1097 Berkeley DB documentation.
1099 To do this you need to store a copy of the object returned from the tie.
1101 $db = tie %hash, "DB_File", "filename" ;
1103 Once you have done that, you can access the Berkeley DB API functions
1104 as B<DB_File> methods directly like this:
1106 $db->put($key, $value, R_NOOVERWRITE) ;
1108 B<Important:> If you have saved a copy of the object returned from
1109 C<tie>, the underlying database file will I<not> be closed until both
1110 the tied variable is untied and all copies of the saved object are
1114 $db = tie %hash, "DB_File", "filename"
1115 or die "Cannot tie filename: $!" ;
1120 All the functions defined in L<dbopen> are available except for
1121 close() and dbopen() itself. The B<DB_File> method interface to the
1122 supported functions have been implemented to mirror the way Berkeley DB
1123 works whenever possible. In particular note that:
1129 The methods return a status value. All return 0 on success.
1130 All return -1 to signify an error and set C<$!> to the exact
1131 error code. The return code 1 generally (but not always) means that the
1132 key specified did not exist in the database.
1134 Other return codes are defined. See below and in the Berkeley DB
1135 documentation for details. The Berkeley DB documentation should be used
1136 as the definitive source.
1140 Whenever a Berkeley DB function returns data via one of its parameters,
1141 the equivalent B<DB_File> method does exactly the same.
1145 If you are careful, it is possible to mix API calls with the tied
1146 hash/array interface in the same piece of code. Although only a few of
1147 the methods used to implement the tied interface currently make use of
1148 the cursor, you should always assume that the cursor has been changed
1149 any time the tied hash/array interface is used. As an example, this
1150 code will probably not do what you expect:
1152 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1153 or die "Cannot tie $filename: $!" ;
1155 # Get the first key/value pair and set the cursor
1156 $X->seq($key, $value, R_FIRST) ;
1158 # this line will modify the cursor
1159 $count = scalar keys %x ;
1161 # Get the second key/value pair.
1162 # oops, it didn't, it got the last key/value pair!
1163 $X->seq($key, $value, R_NEXT) ;
1165 The code above can be rearranged to get around the problem, like this:
1167 $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
1168 or die "Cannot tie $filename: $!" ;
1170 # this line will modify the cursor
1171 $count = scalar keys %x ;
1173 # Get the first key/value pair and set the cursor
1174 $X->seq($key, $value, R_FIRST) ;
1176 # Get the second key/value pair.
1178 $X->seq($key, $value, R_NEXT) ;
1182 All the constants defined in L<dbopen> for use in the flags parameters
1183 in the methods defined below are also available. Refer to the Berkeley
1184 DB documentation for the precise meaning of the flags values.
1186 Below is a list of the methods available.
1190 =item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
1192 Given a key (C<$key>) this method reads the value associated with it
1193 from the database. The value read from the database is returned in the
1194 C<$value> parameter.
1196 If the key does not exist the method returns 1.
1198 No flags are currently defined for this method.
1200 =item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
1202 Stores the key/value pair in the database.
1204 If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
1205 will have the record number of the inserted key/value pair set.
1207 Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
1210 =item B<$status = $X-E<gt>del($key [, $flags]) ;>
1212 Removes all key/value pairs with key C<$key> from the database.
1214 A return code of 1 means that the requested key was not in the
1217 R_CURSOR is the only valid flag at present.
1219 =item B<$status = $X-E<gt>fd ;>
1221 Returns the file descriptor for the underlying database.
1223 See L<Locking Databases> for an example of how to make use of the
1224 C<fd> method to lock your database.
1226 =item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
1228 This interface allows sequential retrieval from the database. See
1229 L<dbopen> for full details.
1231 Both the C<$key> and C<$value> parameters will be set to the key/value
1232 pair read from the database.
1234 The flags parameter is mandatory. The valid flag values are R_CURSOR,
1235 R_FIRST, R_LAST, R_NEXT and R_PREV.
1237 =item B<$status = $X-E<gt>sync([$flags]) ;>
1239 Flushes any cached buffers to disk.
1241 R_RECNOSYNC is the only valid flag at present.
1245 =head1 HINTS AND TIPS
1248 =head2 Locking Databases
1250 Concurrent access of a read-write database by several parties requires
1251 them all to use some kind of locking. Here's an example of Tom's that
1252 uses the I<fd> method to get the file descriptor, and then a careful
1253 open() to give something Perl will flock() for you. Run this repeatedly
1254 in the background to watch the locks granted in proper order.
1265 my($oldval, $fd, $db, %db, $value, $key);
1267 $key = shift || 'default';
1268 $value = shift || 'magic';
1272 $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
1273 || die "dbcreat /tmp/foo.db $!";
1275 print "$$: db fd is $fd\n";
1276 open(DB_FH, "+<&=$fd") || die "dup $!";
1279 unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
1280 print "$$: CONTENTION; can't read during write update!
1281 Waiting for read lock ($!) ....";
1282 unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
1284 print "$$: Read lock granted\n";
1286 $oldval = $db{$key};
1287 print "$$: Old value was $oldval\n";
1288 flock(DB_FH, LOCK_UN);
1290 unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
1291 print "$$: CONTENTION; must have exclusive lock!
1292 Waiting for write lock ($!) ....";
1293 unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
1296 print "$$: Write lock granted\n";
1298 $db->sync; # to flush
1301 flock(DB_FH, LOCK_UN);
1305 print "$$: Updated db to $key=$value\n";
1307 =head2 Sharing databases with C applications
1309 There is no technical reason why a Berkeley DB database cannot be
1310 shared by both a Perl and a C application.
1312 The vast majority of problems that are reported in this area boil down
1313 to the fact that C strings are NULL terminated, whilst Perl strings are
1316 Here is a real example. Netscape 2.0 keeps a record of the locations you
1317 visit along with the time you last visited them in a DB_HASH database.
1318 This is usually stored in the file F<~/.netscape/history.db>. The key
1319 field in the database is the location string and the value field is the
1320 time the location was last visited stored as a 4 byte binary value.
1322 If you haven't already guessed, the location string is stored with a
1323 terminating NULL. This means you need to be careful when accessing the
1326 Here is a snippet of code that is loosely based on Tom Christiansen's
1327 I<ggh> script (available from your nearest CPAN archive in
1328 F<authors/id/TOMC/scripts/nshist.gz>).
1334 use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
1335 $dotdir = $ENV{HOME} || $ENV{LOGNAME};
1337 $HISTORY = "$dotdir/.netscape/history.db";
1339 tie %hist_db, 'DB_File', $HISTORY
1340 or die "Cannot open $HISTORY: $!\n" ;;
1342 # Dump the complete database
1343 while ( ($href, $binary_time) = each %hist_db ) {
1345 # remove the terminating NULL
1346 $href =~ s/\x00$// ;
1348 # convert the binary time into a user friendly string
1349 $date = localtime unpack("V", $binary_time);
1350 print "$date $href\n" ;
1353 # check for the existence of a specific key
1354 # remember to add the NULL
1355 if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
1356 $date = localtime unpack("V", $binary_time) ;
1357 print "Last visited mox.perl.com on $date\n" ;
1360 print "Never visited mox.perl.com\n"
1366 =head1 COMMON QUESTIONS
1368 =head2 Why is there Perl source in my database?
1370 If you look at the contents of a database file created by DB_File,
1371 there can sometimes be part of a Perl script included in it.
1373 This happens because Berkeley DB uses dynamic memory to allocate
1374 buffers which will subsequently be written to the database file. Being
1375 dynamic, the memory could have been used for anything before DB
1376 malloced it. As Berkeley DB doesn't clear the memory once it has been
1377 allocated, the unused portions will contain random junk. In the case
1378 where a Perl script gets written to the database, the random junk will
1379 correspond to an area of dynamic memory that happened to be used during
1380 the compilation of the script.
1382 Unless you don't like the possibility of there being part of your Perl
1383 scripts embedded in a database file, this is nothing to worry about.
1385 =head2 How do I store complex data structures with DB_File?
1387 Although B<DB_File> cannot do this directly, there is a module which
1388 can layer transparently over B<DB_File> to accomplish this feat.
1390 Check out the MLDBM module, available on CPAN in the directory
1391 F<modules/by-module/MLDBM>.
1393 =head2 What does "Invalid Argument" mean?
1395 You will get this error message when one of the parameters in the
1396 C<tie> call is wrong. Unfortunately there are quite a few parameters to
1397 get wrong, so it can be difficult to figure out which one it is.
1399 Here are a couple of possibilities:
1405 Attempting to reopen a database without closing it.
1409 Using the O_WRONLY flag.
1413 =head2 What does "Bareword 'DB_File' not allowed" mean?
1415 You will encounter this particular error message when you have the
1416 C<strict 'subs'> pragma (or the full strict pragma) in your script.
1417 Consider this script:
1422 tie %x, DB_File, "filename" ;
1424 Running it produces the error in question:
1426 Bareword "DB_File" not allowed while "strict subs" in use
1428 To get around the error, place the word C<DB_File> in either single or
1429 double quotes, like this:
1431 tie %x, "DB_File", "filename" ;
1433 Although it might seem like a real pain, it is really worth the effort
1434 of having a C<use strict> in all your scripts.
1446 When B<DB_File> is opening a database file it no longer terminates the
1447 process if I<dbopen> returned an error. This allows file protection
1448 errors to be caught at run time. Thanks to Judith Grass
1449 E<lt>grass@cybercash.comE<gt> for spotting the bug.
1453 Added prototype support for multiple btree compare callbacks.
1457 B<DB_File> has been in use for over a year. To reflect that, the
1458 version number has been incremented to 1.0.
1460 Added complete support for multiple concurrent callbacks.
1462 Using the I<push> method on an empty list didn't work properly. This
1467 Fixed a core dump problem with SunOS.
1469 The return value from TIEHASH wasn't set to NULL when dbopen returned
1474 Merged OS/2 specific code into DB_File.xs
1476 Removed some redundant code in DB_File.xs.
1478 Documentation update.
1480 Allow negative subscripts with RECNO interface.
1482 Changed the default flags from O_RDWR to O_CREAT|O_RDWR.
1484 The example code which showed how to lock a database needed a call to
1485 C<sync> added. Without it the resultant database file was empty.
1487 Added get_dup method.
1491 Documentation update.
1493 B<DB_File> now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl
1496 The standard hash function C<exists> is now supported.
1498 Modified the behavior of get_dup. When it returns an associative
1499 array, the value is the count of the number of matching BTREE values.
1503 Minor documentation changes.
1505 Fixed a bug in hash_cb. Patches supplied by Dave Hammen,
1506 E<lt>hammen@gothamcity.jsc.nasa.govE<gt>.
1508 Fixed a bug with the constructors for DB_File::HASHINFO,
1509 DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up the
1510 constructors to make them C<-w> clean.
1512 Reworked part of the test harness to be more locale friendly.
1516 Made all scripts in the documentation C<strict> and C<-w> clean.
1518 Added logic to F<DB_File.xs> to allow the module to be built after Perl
1523 Minor namespace cleanup: Localized C<PrintBtree>.
1529 Some older versions of Berkeley DB had problems with fixed length
1530 records using the RECNO file format. The newest version at the time of
1531 writing was 1.85 - this seems to have fixed the problems with RECNO.
1533 I am sure there are bugs in the code. If you do find any, or can
1534 suggest any enhancements, I would welcome your comments.
1538 B<DB_File> comes with the standard Perl source distribution. Look in
1539 the directory F<ext/DB_File>.
1541 Berkeley DB is available at your nearest CPAN archive (see
1542 L<perlmod/"CPAN"> for a list) in F<src/misc/db.1.85.tar.gz>, or via the
1543 host F<ftp.cs.berkeley.edu> in F</ucb/4bsd/db.tar.gz>. Alternatively,
1544 check out the Berkeley DB home page at F<http://www.bostic.com/db>. It
1545 is I<not> under the GPL.
1547 If you are running IRIX, then get Berkeley DB from
1548 F<http://reality.sgi.com/ariel>. It has the patches necessary to
1549 compile properly on IRIX 5.3.
1553 L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>
1557 The DB_File interface was written by Paul Marquess
1558 E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
1559 Questions about the DB system itself may be addressed to Keith Bostic
1560 E<lt>bostic@cs.berkeley.eduE<gt>.