# DB_File.pm -- Perl 5 interface to Berkeley DB
#
-# written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
-# last modified 23rd June 1994
-# version 0.1
+# written by Paul Marquess (Paul.Marquess@btinternet.com)
+# last modified 4th September 1999
+# version 1.71
+#
+# Copyright (c) 1995-1999 Paul Marquess. All rights reserved.
+# This program is free software; you can redistribute it and/or
+# modify it under the same terms as Perl itself.
+
package DB_File::HASHINFO ;
+
+require 5.003 ;
+
+use strict;
use Carp;
+require Tie::Hash;
+@DB_File::HASHINFO::ISA = qw(Tie::Hash);
+
+sub new
+{
+ my $pkg = shift ;
+ my %x ;
+ tie %x, $pkg ;
+ bless \%x, $pkg ;
+}
+
sub TIEHASH
{
- bless {} ;
+ my $pkg = shift ;
+
+ bless { VALID => { map {$_, 1}
+ qw( bsize ffactor nelem cachesize hash lorder)
+ },
+ GOT => {}
+ }, $pkg ;
}
-%elements = ( 'bsize' => 0,
- 'ffactor' => 0,
- 'nelem' => 0,
- 'cachesize' => 0,
- 'hash' => 0,
- 'lorder' => 0
- ) ;
sub FETCH
{
- return $_[0]{$_[1]} if defined $elements{$_[1]} ;
+ my $self = shift ;
+ my $key = shift ;
+
+ return $self->{GOT}{$key} if exists $self->{VALID}{$key} ;
- croak "DB_File::HASHINFO::FETCH - Unknown element '$_[1]'" ;
+ my $pkg = ref $self ;
+ croak "${pkg}::FETCH - Unknown element '$key'" ;
}
sub STORE
{
- if ( defined $elements{$_[1]} )
+ my $self = shift ;
+ my $key = shift ;
+ my $value = shift ;
+
+ if ( exists $self->{VALID}{$key} )
{
- $_[0]{$_[1]} = $_[2] ;
+ $self->{GOT}{$key} = $value ;
return ;
}
- croak "DB_File::HASHINFO::STORE - Unknown element '$_[1]'" ;
+ my $pkg = ref $self ;
+ croak "${pkg}::STORE - Unknown element '$key'" ;
}
sub DELETE
{
- if ( defined $elements{$_[1]} )
+ my $self = shift ;
+ my $key = shift ;
+
+ if ( exists $self->{VALID}{$key} )
{
- delete ${$_[0]}{$_[1]} ;
+ delete $self->{GOT}{$key} ;
return ;
}
- croak "DB_File::HASHINFO::DELETE - Unknown element '$_[1]'" ;
+ my $pkg = ref $self ;
+ croak "DB_File::HASHINFO::DELETE - Unknown element '$key'" ;
}
-
-sub DESTROY {undef %{$_[0]} }
-sub FIRSTKEY { croak "DB_File::HASHINFO::FIRSTKEY is not implemented" }
-sub NEXTKEY { croak "DB_File::HASHINFO::NEXTKEY is not implemented" }
-sub EXISTS { croak "DB_File::HASHINFO::EXISTS is not implemented" }
-sub CLEAR { croak "DB_File::HASHINFO::CLEAR is not implemented" }
-
-package DB_File::BTREEINFO ;
-use Carp;
-
-sub TIEHASH
+sub EXISTS
{
- bless {} ;
-}
-
-%elements = ( 'flags' => 0,
- 'cachesize' => 0,
- 'maxkeypage' => 0,
- 'minkeypage' => 0,
- 'psize' => 0,
- 'compare' => 0,
- 'prefix' => 0,
- 'lorder' => 0
- ) ;
+ my $self = shift ;
+ my $key = shift ;
-sub FETCH
-{
- return $_[0]{$_[1]} if defined $elements{$_[1]} ;
-
- croak "DB_File::BTREEINFO::FETCH - Unknown element '$_[1]'" ;
+ exists $self->{VALID}{$key} ;
}
-
-sub STORE
+sub NotHere
{
- if ( defined $elements{$_[1]} )
- {
- $_[0]{$_[1]} = $_[2] ;
- return ;
- }
-
- croak "DB_File::BTREEINFO::STORE - Unknown element '$_[1]'" ;
-}
+ my $self = shift ;
+ my $method = shift ;
-sub DELETE
-{
- if ( defined $elements{$_[1]} )
- {
- delete ${$_[0]}{$_[1]} ;
- return ;
- }
-
- croak "DB_File::BTREEINFO::DELETE - Unknown element '$_[1]'" ;
+ croak ref($self) . " does not define the method ${method}" ;
}
-
-sub DESTROY {undef %{$_[0]} }
-sub FIRSTKEY { croak "DB_File::BTREEINFO::FIRSTKEY is not implemented" }
-sub NEXTKEY { croak "DB_File::BTREEINFO::NEXTKEY is not implemented" }
-sub EXISTS { croak "DB_File::BTREEINFO::EXISTS is not implemented" }
-sub CLEAR { croak "DB_File::BTREEINFO::CLEAR is not implemented" }
+sub FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") }
+sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") }
+sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") }
package DB_File::RECNOINFO ;
-use Carp;
+
+use strict ;
+
+@DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
sub TIEHASH
{
- bless {} ;
+ my $pkg = shift ;
+
+ bless { VALID => { map {$_, 1}
+ qw( bval cachesize psize flags lorder reclen bfname )
+ },
+ GOT => {},
+ }, $pkg ;
}
-%elements = ( 'bval' => 0,
- 'cachesize' => 0,
- 'psize' => 0,
- 'flags' => 0,
- 'lorder' => 0,
- 'reclen' => 0,
- 'bfname' => 0
- ) ;
-sub FETCH
-{
- return $_[0]{$_[1]} if defined $elements{$_[1]} ;
+package DB_File::BTREEINFO ;
- croak "DB_File::RECNOINFO::FETCH - Unknown element '$_[1]'" ;
-}
+use strict ;
+@DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
-sub STORE
+sub TIEHASH
{
- if ( defined $elements{$_[1]} )
- {
- $_[0]{$_[1]} = $_[2] ;
- return ;
- }
-
- croak "DB_File::RECNOINFO::STORE - Unknown element '$_[1]'" ;
-}
+ my $pkg = shift ;
-sub DELETE
-{
- if ( defined $elements{$_[1]} )
- {
- delete ${$_[0]}{$_[1]} ;
- return ;
- }
-
- croak "DB_File::RECNOINFO::DELETE - Unknown element '$_[1]'" ;
+ bless { VALID => { map {$_, 1}
+ qw( flags cachesize maxkeypage minkeypage psize
+ compare prefix lorder )
+ },
+ GOT => {},
+ }, $pkg ;
}
-sub DESTROY {undef %{$_[0]} }
-sub FIRSTKEY { croak "DB_File::RECNOINFO::FIRSTKEY is not implemented" }
-sub NEXTKEY { croak "DB_File::RECNOINFO::NEXTKEY is not implemented" }
-sub EXISTS { croak "DB_File::BTREEINFO::EXISTS is not implemented" }
-sub CLEAR { croak "DB_File::BTREEINFO::CLEAR is not implemented" }
+package DB_File ;
+use strict;
+use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO $db_version) ;
+use Carp;
-package DB_File ;
-use Carp;
+$VERSION = "1.71" ;
#typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
-$DB_BTREE = TIEHASH DB_File::BTREEINFO ;
-$DB_HASH = TIEHASH DB_File::HASHINFO ;
-$DB_RECNO = TIEHASH DB_File::RECNOINFO ;
+$DB_BTREE = new DB_File::BTREEINFO ;
+$DB_HASH = new DB_File::HASHINFO ;
+$DB_RECNO = new DB_File::RECNOINFO ;
-require TieHash;
+require Tie::Hash;
require Exporter;
-require AutoLoader;
-require DynaLoader;
-@ISA = (TieHash, Exporter, AutoLoader, DynaLoader);
+use AutoLoader;
+use XSLoader ();
+@ISA = qw(Tie::Hash Exporter);
@EXPORT = qw(
$DB_BTREE $DB_HASH $DB_RECNO
+
BTREEMAGIC
BTREEVERSION
DB_LOCK
R_SETCURSOR
R_SNAPSHOT
__R_UNUSED
+
);
sub AUTOLOAD {
- if (@_ > 1) {
- $AutoLoader::AUTOLOAD = $AUTOLOAD;
- goto &AutoLoader::AUTOLOAD;
- }
- local($constname);
+ my($constname);
($constname = $AUTOLOAD) =~ s/.*:://;
- $val = constant($constname, @_ ? $_[0] : 0);
+ my $val = constant($constname, @_ ? $_[0] : 0);
if ($! != 0) {
- if ($! =~ /Invalid/) {
+ if ($! =~ /Invalid/ || $!{EINVAL}) {
$AutoLoader::AUTOLOAD = $AUTOLOAD;
goto &AutoLoader::AUTOLOAD;
}
else {
- ($pack,$file,$line) = caller;
+ my($pack,$file,$line) = caller;
croak "Your vendor has not defined DB macro $constname, used at $file line $line.
";
}
goto &$AUTOLOAD;
}
-@liblist = ();
-@liblist = split ' ', $Config::Config{"DB_File_loadlibs"}
- if defined $Config::Config{"DB_File_loadlibs"};
-bootstrap DB_File @liblist;
+eval {
+ # Make all Fcntl O_XXX constants available for importing
+ require Fcntl;
+ my @O = grep /^O_/, @Fcntl::EXPORT;
+ Fcntl->import(@O); # first we import what we want to export
+ push(@EXPORT, @O);
+};
+
+## import borrowed from IO::File
+## exports Fcntl constants if available.
+#sub import {
+# my $pkg = shift;
+# my $callpkg = caller;
+# Exporter::export $pkg, $callpkg, @_;
+# eval {
+# require Fcntl;
+# Exporter::export 'Fcntl', $callpkg, '/^O_/';
+# };
+#}
+
+XSLoader::load 'DB_File', $VERSION;
# Preloaded methods go here. Autoload methods go after __END__, and are
# processed by the autosplit program.
+sub tie_hash_or_array
+{
+ my (@arg) = @_ ;
+ my $tieHASH = ( (caller(1))[3] =~ /TIEHASH/ ) ;
+
+ $arg[4] = tied %{ $arg[4] }
+ if @arg >= 5 && ref $arg[4] && $arg[4] =~ /=HASH/ && tied %{ $arg[4] } ;
+
+ # make recno in Berkeley DB version 2 work like recno in version 1.
+ if ($db_version > 1 and defined $arg[4] and $arg[4] =~ /RECNO/ and
+ $arg[1] and ! -e $arg[1]) {
+ open(FH, ">$arg[1]") or return undef ;
+ close FH ;
+ chmod $arg[3] ? $arg[3] : 0666 , $arg[1] ;
+ }
+
+ DoTie_($tieHASH, @arg) ;
+}
+
+sub TIEHASH
+{
+ tie_hash_or_array(@_) ;
+}
+
+sub TIEARRAY
+{
+ tie_hash_or_array(@_) ;
+}
+
+sub CLEAR
+{
+ my $self = shift;
+ my $key = "" ;
+ my $value = "" ;
+ my $status = $self->seq($key, $value, R_FIRST());
+ my @keys;
+
+ while ($status == 0) {
+ push @keys, $key;
+ $status = $self->seq($key, $value, R_NEXT());
+ }
+ foreach $key (reverse @keys) {
+ my $s = $self->del($key);
+ }
+}
+
+sub EXTEND { }
+
+sub STORESIZE
+{
+ my $self = shift;
+ my $length = shift ;
+ my $current_length = $self->length() ;
+
+ if ($length < $current_length) {
+ my $key ;
+ for ($key = $current_length - 1 ; $key >= $length ; -- $key)
+ { $self->del($key) }
+ }
+ elsif ($length > $current_length) {
+ $self->put($length-1, "") ;
+ }
+}
+
+sub find_dup
+{
+ croak "Usage: \$db->find_dup(key,value)\n"
+ unless @_ == 3 ;
+
+ my $db = shift ;
+ my ($origkey, $value_wanted) = @_ ;
+ my ($key, $value) = ($origkey, 0);
+ my ($status) = 0 ;
+
+ for ($status = $db->seq($key, $value, R_CURSOR() ) ;
+ $status == 0 ;
+ $status = $db->seq($key, $value, R_NEXT() ) ) {
+
+ return 0 if $key eq $origkey and $value eq $value_wanted ;
+ }
+
+ return $status ;
+}
+
+sub del_dup
+{
+ croak "Usage: \$db->del_dup(key,value)\n"
+ unless @_ == 3 ;
+
+ my $db = shift ;
+ my ($key, $value) = @_ ;
+ my ($status) = $db->find_dup($key, $value) ;
+ return $status if $status != 0 ;
+
+ $status = $db->del($key, R_CURSOR() ) ;
+ return $status ;
+}
+
+sub get_dup
+{
+ croak "Usage: \$db->get_dup(key [,flag])\n"
+ unless @_ == 2 or @_ == 3 ;
+
+ my $db = shift ;
+ my $key = shift ;
+ my $flag = shift ;
+ my $value = 0 ;
+ my $origkey = $key ;
+ my $wantarray = wantarray ;
+ my %values = () ;
+ my @values = () ;
+ my $counter = 0 ;
+ my $status = 0 ;
+
+ # iterate through the database until either EOF ($status == 0)
+ # or a different key is encountered ($key ne $origkey).
+ for ($status = $db->seq($key, $value, R_CURSOR()) ;
+ $status == 0 and $key eq $origkey ;
+ $status = $db->seq($key, $value, R_NEXT()) ) {
+
+ # save the value or count number of matches
+ if ($wantarray) {
+ if ($flag)
+ { ++ $values{$value} }
+ else
+ { push (@values, $value) }
+ }
+ else
+ { ++ $counter }
+
+ }
+
+ return ($wantarray ? ($flag ? %values : @values) : $counter) ;
+}
+
+
1;
__END__
+
+=head1 NAME
+
+DB_File - Perl5 access to Berkeley DB version 1.x
+
+=head1 SYNOPSIS
+
+ use DB_File ;
+
+ [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
+ [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
+ [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
+
+ $status = $X->del($key [, $flags]) ;
+ $status = $X->put($key, $value [, $flags]) ;
+ $status = $X->get($key, $value [, $flags]) ;
+ $status = $X->seq($key, $value, $flags) ;
+ $status = $X->sync([$flags]) ;
+ $status = $X->fd ;
+
+ # BTREE only
+ $count = $X->get_dup($key) ;
+ @list = $X->get_dup($key) ;
+ %list = $X->get_dup($key, 1) ;
+ $status = $X->find_dup($key, $value) ;
+ $status = $X->del_dup($key, $value) ;
+
+ # RECNO only
+ $a = $X->length;
+ $a = $X->pop ;
+ $X->push(list);
+ $a = $X->shift;
+ $X->unshift(list);
+
+ # DBM Filters
+ $old_filter = $db->filter_store_key ( sub { ... } ) ;
+ $old_filter = $db->filter_store_value( sub { ... } ) ;
+ $old_filter = $db->filter_fetch_key ( sub { ... } ) ;
+ $old_filter = $db->filter_fetch_value( sub { ... } ) ;
+
+ untie %hash ;
+ untie @array ;
+
+=head1 DESCRIPTION
+
+B<DB_File> is a module which allows Perl programs to make use of the
+facilities provided by Berkeley DB version 1.x (if you have a newer
+version of DB, see L<Using DB_File with Berkeley DB version 2 or 3>).
+It is assumed that you have a copy of the Berkeley DB manual pages at
+hand when reading this documentation. The interface defined here
+mirrors the Berkeley DB interface closely.
+
+Berkeley DB is a C library which provides a consistent interface to a
+number of database formats. B<DB_File> provides an interface to all
+three of the database types currently supported by Berkeley DB.
+
+The file types are:
+
+=over 5
+
+=item B<DB_HASH>
+
+This database type allows arbitrary key/value pairs to be stored in data
+files. This is equivalent to the functionality provided by other
+hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
+the files created using DB_HASH are not compatible with any of the
+other packages mentioned.
+
+A default hashing algorithm, which will be adequate for most
+applications, is built into Berkeley DB. If you do need to use your own
+hashing algorithm it is possible to write your own in Perl and have
+B<DB_File> use it instead.
+
+=item B<DB_BTREE>
+
+The btree format allows arbitrary key/value pairs to be stored in a
+sorted, balanced binary tree.
+
+As with the DB_HASH format, it is possible to provide a user defined
+Perl routine to perform the comparison of keys. By default, though, the
+keys are stored in lexical order.
+
+=item B<DB_RECNO>
+
+DB_RECNO allows both fixed-length and variable-length flat text files
+to be manipulated using the same key/value pair interface as in DB_HASH
+and DB_BTREE. In this case the key will consist of a record (line)
+number.
+
+=back
+
+=head2 Using DB_File with Berkeley DB version 2 or 3
+
+Although B<DB_File> is intended to be used with Berkeley DB version 1,
+it can also be used with version 2.or 3 In this case the interface is
+limited to the functionality provided by Berkeley DB 1.x. Anywhere the
+version 2 or 3 interface differs, B<DB_File> arranges for it to work
+like version 1. This feature allows B<DB_File> scripts that were built
+with version 1 to be migrated to version 2 or 3 without any changes.
+
+If you want to make use of the new features available in Berkeley DB
+2.x or 3.x, use the Perl module B<BerkeleyDB> instead.
+
+At the time of writing this document the B<BerkeleyDB> module is still
+alpha quality (the version number is < 1.0), and so unsuitable for use
+in any serious development work. Once its version number is >= 1.0, it
+is considered stable enough for real work.
+
+B<Note:> The database file format has changed in both Berkeley DB
+version 2 and 3. If you cannot recreate your databases, you must dump
+any existing databases with the C<db_dump185> utility that comes with
+Berkeley DB.
+Once you have rebuilt DB_File to use Berkeley DB version 2 or 3, your
+databases can be recreated using C<db_load>. Refer to the Berkeley DB
+documentation for further details.
+
+Please read L<"COPYRIGHT"> before using version 2.x or 3.x of Berkeley
+DB with DB_File.
+
+=head2 Interface to Berkeley DB
+
+B<DB_File> allows access to Berkeley DB files using the tie() mechanism
+in Perl 5 (for full details, see L<perlfunc/tie()>). This facility
+allows B<DB_File> to access Berkeley DB files using either an
+associative array (for DB_HASH & DB_BTREE file types) or an ordinary
+array (for the DB_RECNO file type).
+
+In addition to the tie() interface, it is also possible to access most
+of the functions provided in the Berkeley DB API directly.
+See L<THE API INTERFACE>.
+
+=head2 Opening a Berkeley DB Database File
+
+Berkeley DB uses the function dbopen() to open or create a database.
+Here is the C prototype for dbopen():
+
+ DB*
+ dbopen (const char * file, int flags, int mode,
+ DBTYPE type, const void * openinfo)
+
+The parameter C<type> is an enumeration which specifies which of the 3
+interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
+Depending on which of these is actually chosen, the final parameter,
+I<openinfo> points to a data structure which allows tailoring of the
+specific interface method.
+
+This interface is handled slightly differently in B<DB_File>. Here is
+an equivalent call using B<DB_File>:
+
+ tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
+
+The C<filename>, C<flags> and C<mode> parameters are the direct
+equivalent of their dbopen() counterparts. The final parameter $DB_HASH
+performs the function of both the C<type> and C<openinfo> parameters in
+dbopen().
+
+In the example above $DB_HASH is actually a pre-defined reference to a
+hash object. B<DB_File> has three of these pre-defined references.
+Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.
+
+The keys allowed in each of these pre-defined references is limited to
+the names used in the equivalent C structure. So, for example, the
+$DB_HASH reference will only allow keys called C<bsize>, C<cachesize>,
+C<ffactor>, C<hash>, C<lorder> and C<nelem>.
+
+To change one of these elements, just assign to it like this:
+
+ $DB_HASH->{'cachesize'} = 10000 ;
+
+The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
+usually adequate for most applications. If you do need to create extra
+instances of these objects, constructors are available for each file
+type.
+
+Here are examples of the constructors and the valid options available
+for DB_HASH, DB_BTREE and DB_RECNO respectively.
+
+ $a = new DB_File::HASHINFO ;
+ $a->{'bsize'} ;
+ $a->{'cachesize'} ;
+ $a->{'ffactor'};
+ $a->{'hash'} ;
+ $a->{'lorder'} ;
+ $a->{'nelem'} ;
+
+ $b = new DB_File::BTREEINFO ;
+ $b->{'flags'} ;
+ $b->{'cachesize'} ;
+ $b->{'maxkeypage'} ;
+ $b->{'minkeypage'} ;
+ $b->{'psize'} ;
+ $b->{'compare'} ;
+ $b->{'prefix'} ;
+ $b->{'lorder'} ;
+
+ $c = new DB_File::RECNOINFO ;
+ $c->{'bval'} ;
+ $c->{'cachesize'} ;
+ $c->{'psize'} ;
+ $c->{'flags'} ;
+ $c->{'lorder'} ;
+ $c->{'reclen'} ;
+ $c->{'bfname'} ;
+
+The values stored in the hashes above are mostly the direct equivalent
+of their C counterpart. Like their C counterparts, all are set to a
+default values - that means you don't have to set I<all> of the
+values when you only want to change one. Here is an example:
+
+ $a = new DB_File::HASHINFO ;
+ $a->{'cachesize'} = 12345 ;
+ tie %y, 'DB_File', "filename", $flags, 0777, $a ;
+
+A few of the options need extra discussion here. When used, the C
+equivalent of the keys C<hash>, C<compare> and C<prefix> store pointers
+to C functions. In B<DB_File> these keys are used to store references
+to Perl subs. Below are templates for each of the subs:
+
+ sub hash
+ {
+ my ($data) = @_ ;
+ ...
+ # return the hash value for $data
+ return $hash ;
+ }
+
+ sub compare
+ {
+ my ($key, $key2) = @_ ;
+ ...
+ # return 0 if $key1 eq $key2
+ # -1 if $key1 lt $key2
+ # 1 if $key1 gt $key2
+ return (-1 , 0 or 1) ;
+ }
+
+ sub prefix
+ {
+ my ($key, $key2) = @_ ;
+ ...
+ # return number of bytes of $key2 which are
+ # necessary to determine that it is greater than $key1
+ return $bytes ;
+ }
+
+See L<Changing the BTREE sort order> for an example of using the
+C<compare> template.
+
+If you are using the DB_RECNO interface and you intend making use of
+C<bval>, you should check out L<The 'bval' Option>.
+
+=head2 Default Parameters
+
+It is possible to omit some or all of the final 4 parameters in the
+call to C<tie> and let them take default values. As DB_HASH is the most
+common file format used, the call:
+
+ tie %A, "DB_File", "filename" ;
+
+is equivalent to:
+
+ tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
+
+It is also possible to omit the filename parameter as well, so the
+call:
+
+ tie %A, "DB_File" ;
+
+is equivalent to:
+
+ tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
+
+See L<In Memory Databases> for a discussion on the use of C<undef>
+in place of a filename.
+
+=head2 In Memory Databases
+
+Berkeley DB allows the creation of in-memory databases by using NULL
+(that is, a C<(char *)0> in C) in place of the filename. B<DB_File>
+uses C<undef> instead of NULL to provide this functionality.
+
+=head1 DB_HASH
+
+The DB_HASH file format is probably the most commonly used of the three
+file formats that B<DB_File> supports. It is also very straightforward
+to use.
+
+=head2 A Simple Example
+
+This example shows how to create a database, add key/value pairs to the
+database, delete keys/value pairs and finally how to enumerate the
+contents of the database.
+
+ use strict ;
+ use DB_File ;
+ use vars qw( %h $k $v ) ;
+
+ unlink "fruit" ;
+ tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
+ or die "Cannot open file 'fruit': $!\n";
+
+ # Add a few key/value pairs to the file
+ $h{"apple"} = "red" ;
+ $h{"orange"} = "orange" ;
+ $h{"banana"} = "yellow" ;
+ $h{"tomato"} = "red" ;
+
+ # Check for existence of a key
+ print "Banana Exists\n\n" if $h{"banana"} ;
+
+ # Delete a key/value pair.
+ delete $h{"apple"} ;
+
+ # print the contents of the file
+ while (($k, $v) = each %h)
+ { print "$k -> $v\n" }
+
+ untie %h ;
+
+here is the output:
+
+ Banana Exists
+
+ orange -> orange
+ tomato -> red
+ banana -> yellow
+
+Note that the like ordinary associative arrays, the order of the keys
+retrieved is in an apparently random order.
+
+=head1 DB_BTREE
+
+The DB_BTREE format is useful when you want to store data in a given
+order. By default the keys will be stored in lexical order, but as you
+will see from the example shown in the next section, it is very easy to
+define your own sorting function.
+
+=head2 Changing the BTREE sort order
+
+This script shows how to override the default sorting algorithm that
+BTREE uses. Instead of using the normal lexical ordering, a case
+insensitive compare function will be used.
+
+ use strict ;
+ use DB_File ;
+
+ my %h ;
+
+ sub Compare
+ {
+ my ($key1, $key2) = @_ ;
+ "\L$key1" cmp "\L$key2" ;
+ }
+
+ # specify the Perl sub that will do the comparison
+ $DB_BTREE->{'compare'} = \&Compare ;
+
+ unlink "tree" ;
+ tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open file 'tree': $!\n" ;
+
+ # Add a key/value pair to the file
+ $h{'Wall'} = 'Larry' ;
+ $h{'Smith'} = 'John' ;
+ $h{'mouse'} = 'mickey' ;
+ $h{'duck'} = 'donald' ;
+
+ # Delete
+ delete $h{"duck"} ;
+
+ # Cycle through the keys printing them in order.
+ # Note it is not necessary to sort the keys as
+ # the btree will have kept them in order automatically.
+ foreach (keys %h)
+ { print "$_\n" }
+
+ untie %h ;
+
+Here is the output from the code above.
+
+ mouse
+ Smith
+ Wall
+
+There are a few point to bear in mind if you want to change the
+ordering in a BTREE database:
+
+=over 5
+
+=item 1.
+
+The new compare function must be specified when you create the database.
+
+=item 2.
+
+You cannot change the ordering once the database has been created. Thus
+you must use the same compare function every time you access the
+database.
+
+=back
+
+=head2 Handling Duplicate Keys
+
+The BTREE file type optionally allows a single key to be associated
+with an arbitrary number of values. This option is enabled by setting
+the flags element of C<$DB_BTREE> to R_DUP when creating the database.
+
+There are some difficulties in using the tied hash interface if you
+want to manipulate a BTREE database with duplicate keys. Consider this
+code:
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename %h ) ;
+
+ $filename = "tree" ;
+ unlink $filename ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ # Add some key/value pairs to the file
+ $h{'Wall'} = 'Larry' ;
+ $h{'Wall'} = 'Brick' ; # Note the duplicate key
+ $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
+ $h{'Smith'} = 'John' ;
+ $h{'mouse'} = 'mickey' ;
+
+ # iterate through the associative array
+ # and print each key/value pair.
+ foreach (sort keys %h)
+ { print "$_ -> $h{$_}\n" }
+
+ untie %h ;
+
+Here is the output:
+
+ Smith -> John
+ Wall -> Larry
+ Wall -> Larry
+ Wall -> Larry
+ mouse -> mickey
+
+As you can see 3 records have been successfully created with key C<Wall>
+- the only thing is, when they are retrieved from the database they
+I<seem> to have the same value, namely C<Larry>. The problem is caused
+by the way that the associative array interface works. Basically, when
+the associative array interface is used to fetch the value associated
+with a given key, it will only ever retrieve the first value.
+
+Although it may not be immediately obvious from the code above, the
+associative array interface can be used to write values with duplicate
+keys, but it cannot be used to read them back from the database.
+
+The way to get around this problem is to use the Berkeley DB API method
+called C<seq>. This method allows sequential access to key/value
+pairs. See L<THE API INTERFACE> for details of both the C<seq> method
+and the API in general.
+
+Here is the script above rewritten using the C<seq> API method.
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename $x %h $status $key $value) ;
+
+ $filename = "tree" ;
+ unlink $filename ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ # Add some key/value pairs to the file
+ $h{'Wall'} = 'Larry' ;
+ $h{'Wall'} = 'Brick' ; # Note the duplicate key
+ $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
+ $h{'Smith'} = 'John' ;
+ $h{'mouse'} = 'mickey' ;
+
+ # iterate through the btree using seq
+ # and print each key/value pair.
+ $key = $value = 0 ;
+ for ($status = $x->seq($key, $value, R_FIRST) ;
+ $status == 0 ;
+ $status = $x->seq($key, $value, R_NEXT) )
+ { print "$key -> $value\n" }
+
+ undef $x ;
+ untie %h ;
+
+that prints:
+
+ Smith -> John
+ Wall -> Brick
+ Wall -> Brick
+ Wall -> Larry
+ mouse -> mickey
+
+This time we have got all the key/value pairs, including the multiple
+values associated with the key C<Wall>.
+
+To make life easier when dealing with duplicate keys, B<DB_File> comes with
+a few utility methods.
+
+=head2 The get_dup() Method
+
+The C<get_dup> method assists in
+reading duplicate values from BTREE databases. The method can take the
+following forms:
+
+ $count = $x->get_dup($key) ;
+ @list = $x->get_dup($key) ;
+ %list = $x->get_dup($key, 1) ;
+
+In a scalar context the method returns the number of values associated
+with the key, C<$key>.
+
+In list context, it returns all the values which match C<$key>. Note
+that the values will be returned in an apparently random order.
+
+In list context, if the second parameter is present and evaluates
+TRUE, the method returns an associative array. The keys of the
+associative array correspond to the values that matched in the BTREE
+and the values of the array are a count of the number of times that
+particular value occurred in the BTREE.
+
+So assuming the database created above, we can use C<get_dup> like
+this:
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename $x %h ) ;
+
+ $filename = "tree" ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ my $cnt = $x->get_dup("Wall") ;
+ print "Wall occurred $cnt times\n" ;
+
+ my %hash = $x->get_dup("Wall", 1) ;
+ print "Larry is there\n" if $hash{'Larry'} ;
+ print "There are $hash{'Brick'} Brick Walls\n" ;
+
+ my @list = sort $x->get_dup("Wall") ;
+ print "Wall => [@list]\n" ;
+
+ @list = $x->get_dup("Smith") ;
+ print "Smith => [@list]\n" ;
+
+ @list = $x->get_dup("Dog") ;
+ print "Dog => [@list]\n" ;
+
+
+and it will print:
+
+ Wall occurred 3 times
+ Larry is there
+ There are 2 Brick Walls
+ Wall => [Brick Brick Larry]
+ Smith => [John]
+ Dog => []
+
+=head2 The find_dup() Method
+
+ $status = $X->find_dup($key, $value) ;
+
+This method checks for the existance of a specific key/value pair. If the
+pair exists, the cursor is left pointing to the pair and the method
+returns 0. Otherwise the method returns a non-zero value.
+
+Assuming the database from the previous example:
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename $x %h $found) ;
+
+ my $filename = "tree" ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ;
+ print "Larry Wall is $found there\n" ;
+
+ $found = ( $x->find_dup("Wall", "Harry") == 0 ? "" : "not") ;
+ print "Harry Wall is $found there\n" ;
+
+ undef $x ;
+ untie %h ;
+
+prints this
+
+ Larry Wall is there
+ Harry Wall is not there
+
+
+=head2 The del_dup() Method
+
+ $status = $X->del_dup($key, $value) ;
+
+This method deletes a specific key/value pair. It returns
+0 if they exist and have been deleted successfully.
+Otherwise the method returns a non-zero value.
+
+Again assuming the existance of the C<tree> database
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename $x %h $found) ;
+
+ my $filename = "tree" ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ $x->del_dup("Wall", "Larry") ;
+
+ $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ;
+ print "Larry Wall is $found there\n" ;
+
+ undef $x ;
+ untie %h ;
+
+prints this
+
+ Larry Wall is not there
+
+=head2 Matching Partial Keys
+
+The BTREE interface has a feature which allows partial keys to be
+matched. This functionality is I<only> available when the C<seq> method
+is used along with the R_CURSOR flag.
+
+ $x->seq($key, $value, R_CURSOR) ;
+
+Here is the relevant quote from the dbopen man page where it defines
+the use of the R_CURSOR flag with seq:
+
+ Note, for the DB_BTREE access method, the returned key is not
+ necessarily an exact match for the specified key. The returned key
+ is the smallest key greater than or equal to the specified key,
+ permitting partial key matches and range searches.
+
+In the example script below, the C<match> sub uses this feature to find
+and print the first matching key/value pair given a partial key.
+
+ use strict ;
+ use DB_File ;
+ use Fcntl ;
+
+ use vars qw($filename $x %h $st $key $value) ;
+
+ sub match
+ {
+ my $key = shift ;
+ my $value = 0;
+ my $orig_key = $key ;
+ $x->seq($key, $value, R_CURSOR) ;
+ print "$orig_key\t-> $key\t-> $value\n" ;
+ }
+
+ $filename = "tree" ;
+ unlink $filename ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ or die "Cannot open $filename: $!\n";
+
+ # Add some key/value pairs to the file
+ $h{'mouse'} = 'mickey' ;
+ $h{'Wall'} = 'Larry' ;
+ $h{'Walls'} = 'Brick' ;
+ $h{'Smith'} = 'John' ;
+
+
+ $key = $value = 0 ;
+ print "IN ORDER\n" ;
+ for ($st = $x->seq($key, $value, R_FIRST) ;
+ $st == 0 ;
+ $st = $x->seq($key, $value, R_NEXT) )
+
+ { print "$key -> $value\n" }
+
+ print "\nPARTIAL MATCH\n" ;
+
+ match "Wa" ;
+ match "A" ;
+ match "a" ;
+
+ undef $x ;
+ untie %h ;
+
+Here is the output:
+
+ IN ORDER
+ Smith -> John
+ Wall -> Larry
+ Walls -> Brick
+ mouse -> mickey
+
+ PARTIAL MATCH
+ Wa -> Wall -> Larry
+ A -> Smith -> John
+ a -> mouse -> mickey
+
+=head1 DB_RECNO
+
+DB_RECNO provides an interface to flat text files. Both variable and
+fixed length records are supported.
+
+In order to make RECNO more compatible with Perl, the array offset for
+all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.
+
+As with normal Perl arrays, a RECNO array can be accessed using
+negative indexes. The index -1 refers to the last element of the array,
+-2 the second last, and so on. Attempting to access an element before
+the start of the array will raise a fatal run-time error.
+
+=head2 The 'bval' Option
+
+The operation of the bval option warrants some discussion. Here is the
+definition of bval from the Berkeley DB 1.85 recno manual page:
+
+ The delimiting byte to be used to mark the end of a
+ record for variable-length records, and the pad charac-
+ ter for fixed-length records. If no value is speci-
+ fied, newlines (``\n'') are used to mark the end of
+ variable-length records and fixed-length records are
+ padded with spaces.
+
+The second sentence is wrong. In actual fact bval will only default to
+C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
+openinfo parameter is used at all, the value that happens to be in bval
+will be used. That means you always have to specify bval when making
+use of any of the options in the openinfo parameter. This documentation
+error will be fixed in the next release of Berkeley DB.
+
+That clarifies the situation with regards Berkeley DB itself. What
+about B<DB_File>? Well, the behavior defined in the quote above is
+quite useful, so B<DB_File> conforms to it.
+
+That means that you can specify other options (e.g. cachesize) and
+still have bval default to C<"\n"> for variable length records, and
+space for fixed length records.
+
+=head2 A Simple Example
+
+Here is a simple example that uses RECNO (if you are using a version
+of Perl earlier than 5.004_57 this example won't work -- see
+L<Extra RECNO Methods> for a workaround).
+
+ use strict ;
+ use DB_File ;
+
+ my $filename = "text" ;
+ unlink $filename ;
+
+ my @h ;
+ tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_RECNO
+ or die "Cannot open file 'text': $!\n" ;
+
+ # Add a few key/value pairs to the file
+ $h[0] = "orange" ;
+ $h[1] = "blue" ;
+ $h[2] = "yellow" ;
+
+ push @h, "green", "black" ;
+
+ my $elements = scalar @h ;
+ print "The array contains $elements entries\n" ;
+
+ my $last = pop @h ;
+ print "popped $last\n" ;
+
+ unshift @h, "white" ;
+ my $first = shift @h ;
+ print "shifted $first\n" ;
+
+ # Check for existence of a key
+ print "Element 1 Exists with value $h[1]\n" if $h[1] ;
+
+ # use a negative index
+ print "The last element is $h[-1]\n" ;
+ print "The 2nd last element is $h[-2]\n" ;
+
+ untie @h ;
+
+Here is the output from the script:
+
+ The array contains 5 entries
+ popped black
+ shifted white
+ Element 1 Exists with value blue
+ The last element is green
+ The 2nd last element is yellow
+
+=head2 Extra RECNO Methods
+
+If you are using a version of Perl earlier than 5.004_57, the tied
+array interface is quite limited. In the example script above
+C<push>, C<pop>, C<shift>, C<unshift>
+or determining the array length will not work with a tied array.
+
+To make the interface more useful for older versions of Perl, a number
+of methods are supplied with B<DB_File> to simulate the missing array
+operations. All these methods are accessed via the object returned from
+the tie call.
+
+Here are the methods:
+
+=over 5
+
+=item B<$X-E<gt>push(list) ;>
+
+Pushes the elements of C<list> to the end of the array.
+
+=item B<$value = $X-E<gt>pop ;>
+
+Removes and returns the last element of the array.
+
+=item B<$X-E<gt>shift>
+
+Removes and returns the first element of the array.
+
+=item B<$X-E<gt>unshift(list) ;>
+
+Pushes the elements of C<list> to the start of the array.
+
+=item B<$X-E<gt>length>
+
+Returns the number of elements in the array.
+
+=back
+
+=head2 Another Example
+
+Here is a more complete example that makes use of some of the methods
+described above. It also makes use of the API interface directly (see
+L<THE API INTERFACE>).
+
+ use strict ;
+ use vars qw(@h $H $file $i) ;
+ use DB_File ;
+ use Fcntl ;
+
+ $file = "text" ;
+
+ unlink $file ;
+
+ $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO
+ or die "Cannot open file $file: $!\n" ;
+
+ # first create a text file to play with
+ $h[0] = "zero" ;
+ $h[1] = "one" ;
+ $h[2] = "two" ;
+ $h[3] = "three" ;
+ $h[4] = "four" ;
+
+
+ # Print the records in order.
+ #
+ # The length method is needed here because evaluating a tied
+ # array in a scalar context does not return the number of
+ # elements in the array.
+
+ print "\nORIGINAL\n" ;
+ foreach $i (0 .. $H->length - 1) {
+ print "$i: $h[$i]\n" ;
+ }
+
+ # use the push & pop methods
+ $a = $H->pop ;
+ $H->push("last") ;
+ print "\nThe last record was [$a]\n" ;
+
+ # and the shift & unshift methods
+ $a = $H->shift ;
+ $H->unshift("first") ;
+ print "The first record was [$a]\n" ;
+
+ # Use the API to add a new record after record 2.
+ $i = 2 ;
+ $H->put($i, "Newbie", R_IAFTER) ;
+
+ # and a new record before record 1.
+ $i = 1 ;
+ $H->put($i, "New One", R_IBEFORE) ;
+
+ # delete record 3
+ $H->del(3) ;
+
+ # now print the records in reverse order
+ print "\nREVERSE\n" ;
+ for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
+ { print "$i: $h[$i]\n" }
+
+ # same again, but use the API functions instead
+ print "\nREVERSE again\n" ;
+ my ($s, $k, $v) = (0, 0, 0) ;
+ for ($s = $H->seq($k, $v, R_LAST) ;
+ $s == 0 ;
+ $s = $H->seq($k, $v, R_PREV))
+ { print "$k: $v\n" }
+
+ undef $H ;
+ untie @h ;
+
+and this is what it outputs:
+
+ ORIGINAL
+ 0: zero
+ 1: one
+ 2: two
+ 3: three
+ 4: four
+
+ The last record was [four]
+ The first record was [zero]
+
+ REVERSE
+ 5: last
+ 4: three
+ 3: Newbie
+ 2: one
+ 1: New One
+ 0: first
+
+ REVERSE again
+ 5: last
+ 4: three
+ 3: Newbie
+ 2: one
+ 1: New One
+ 0: first
+
+Notes:
+
+=over 5
+
+=item 1.
+
+Rather than iterating through the array, C<@h> like this:
+
+ foreach $i (@h)
+
+it is necessary to use either this:
+
+ foreach $i (0 .. $H->length - 1)
+
+or this:
+
+ for ($a = $H->get($k, $v, R_FIRST) ;
+ $a == 0 ;
+ $a = $H->get($k, $v, R_NEXT) )
+
+=item 2.
+
+Notice that both times the C<put> method was used the record index was
+specified using a variable, C<$i>, rather than the literal value
+itself. This is because C<put> will return the record number of the
+inserted line via that parameter.
+
+=back
+
+=head1 THE API INTERFACE
+
+As well as accessing Berkeley DB using a tied hash or array, it is also
+possible to make direct use of most of the API functions defined in the
+Berkeley DB documentation.
+
+To do this you need to store a copy of the object returned from the tie.
+
+ $db = tie %hash, "DB_File", "filename" ;
+
+Once you have done that, you can access the Berkeley DB API functions
+as B<DB_File> methods directly like this:
+
+ $db->put($key, $value, R_NOOVERWRITE) ;
+
+B<Important:> If you have saved a copy of the object returned from
+C<tie>, the underlying database file will I<not> be closed until both
+the tied variable is untied and all copies of the saved object are
+destroyed.
+
+ use DB_File ;
+ $db = tie %hash, "DB_File", "filename"
+ or die "Cannot tie filename: $!" ;
+ ...
+ undef $db ;
+ untie %hash ;
+
+See L<The untie() Gotcha> for more details.
+
+All the functions defined in L<dbopen> are available except for
+close() and dbopen() itself. The B<DB_File> method interface to the
+supported functions have been implemented to mirror the way Berkeley DB
+works whenever possible. In particular note that:
+
+=over 5
+
+=item *
+
+The methods return a status value. All return 0 on success.
+All return -1 to signify an error and set C<$!> to the exact
+error code. The return code 1 generally (but not always) means that the
+key specified did not exist in the database.
+
+Other return codes are defined. See below and in the Berkeley DB
+documentation for details. The Berkeley DB documentation should be used
+as the definitive source.
+
+=item *
+
+Whenever a Berkeley DB function returns data via one of its parameters,
+the equivalent B<DB_File> method does exactly the same.
+
+=item *
+
+If you are careful, it is possible to mix API calls with the tied
+hash/array interface in the same piece of code. Although only a few of
+the methods used to implement the tied interface currently make use of
+the cursor, you should always assume that the cursor has been changed
+any time the tied hash/array interface is used. As an example, this
+code will probably not do what you expect:
+
+ $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
+ or die "Cannot tie $filename: $!" ;
+
+ # Get the first key/value pair and set the cursor
+ $X->seq($key, $value, R_FIRST) ;
+
+ # this line will modify the cursor
+ $count = scalar keys %x ;
+
+ # Get the second key/value pair.
+ # oops, it didn't, it got the last key/value pair!
+ $X->seq($key, $value, R_NEXT) ;
+
+The code above can be rearranged to get around the problem, like this:
+
+ $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
+ or die "Cannot tie $filename: $!" ;
+
+ # this line will modify the cursor
+ $count = scalar keys %x ;
+
+ # Get the first key/value pair and set the cursor
+ $X->seq($key, $value, R_FIRST) ;
+
+ # Get the second key/value pair.
+ # worked this time.
+ $X->seq($key, $value, R_NEXT) ;
+
+=back
+
+All the constants defined in L<dbopen> for use in the flags parameters
+in the methods defined below are also available. Refer to the Berkeley
+DB documentation for the precise meaning of the flags values.
+
+Below is a list of the methods available.
+
+=over 5
+
+=item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>
+
+Given a key (C<$key>) this method reads the value associated with it
+from the database. The value read from the database is returned in the
+C<$value> parameter.
+
+If the key does not exist the method returns 1.
+
+No flags are currently defined for this method.
+
+=item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>
+
+Stores the key/value pair in the database.
+
+If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
+will have the record number of the inserted key/value pair set.
+
+Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
+R_SETCURSOR.
+
+=item B<$status = $X-E<gt>del($key [, $flags]) ;>
+
+Removes all key/value pairs with key C<$key> from the database.
+
+A return code of 1 means that the requested key was not in the
+database.
+
+R_CURSOR is the only valid flag at present.
+
+=item B<$status = $X-E<gt>fd ;>
+
+Returns the file descriptor for the underlying database.
+
+See L<Locking Databases> for an example of how to make use of the
+C<fd> method to lock your database.
+
+=item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
+
+This interface allows sequential retrieval from the database. See
+L<dbopen> for full details.
+
+Both the C<$key> and C<$value> parameters will be set to the key/value
+pair read from the database.
+
+The flags parameter is mandatory. The valid flag values are R_CURSOR,
+R_FIRST, R_LAST, R_NEXT and R_PREV.
+
+=item B<$status = $X-E<gt>sync([$flags]) ;>
+
+Flushes any cached buffers to disk.
+
+R_RECNOSYNC is the only valid flag at present.
+
+=back
+
+=head1 DBM FILTERS
+
+A DBM Filter is a piece of code that is be used when you I<always>
+want to make the same transformation to all keys and/or values in a
+DBM database.
+
+There are four methods associated with DBM Filters. All work identically,
+and each is used to install (or uninstall) a single DBM Filter. Each
+expects a single parameter, namely a reference to a sub. The only
+difference between them is the place that the filter is installed.
+
+To summarise:
+
+=over 5
+
+=item B<filter_store_key>
+
+If a filter has been installed with this method, it will be invoked
+every time you write a key to a DBM database.
+
+=item B<filter_store_value>
+
+If a filter has been installed with this method, it will be invoked
+every time you write a value to a DBM database.
+
+
+=item B<filter_fetch_key>
+
+If a filter has been installed with this method, it will be invoked
+every time you read a key from a DBM database.
+
+=item B<filter_fetch_value>
+
+If a filter has been installed with this method, it will be invoked
+every time you read a value from a DBM database.
+
+=back
+
+You can use any combination of the methods, from none, to all four.
+
+All filter methods return the existing filter, if present, or C<undef>
+in not.
+
+To delete a filter pass C<undef> to it.
+
+=head2 The Filter
+
+When each filter is called by Perl, a local copy of C<$_> will contain
+the key or value to be filtered. Filtering is achieved by modifying
+the contents of C<$_>. The return code from the filter is ignored.
+
+=head2 An Example -- the NULL termination problem.
+
+Consider the following scenario. You have a DBM database
+that you need to share with a third-party C application. The C application
+assumes that I<all> keys and values are NULL terminated. Unfortunately
+when Perl writes to DBM databases it doesn't use NULL termination, so
+your Perl application will have to manage NULL termination itself. When
+you write to the database you will have to use something like this:
+
+ $hash{"$key\0"} = "$value\0" ;
+
+Similarly the NULL needs to be taken into account when you are considering
+the length of existing keys/values.
+
+It would be much better if you could ignore the NULL terminations issue
+in the main application code and have a mechanism that automatically
+added the terminating NULL to all keys and values whenever you write to
+the database and have them removed when you read from the database. As I'm
+sure you have already guessed, this is a problem that DBM Filters can
+fix very easily.
+
+ use strict ;
+ use DB_File ;
+
+ my %hash ;
+ my $filename = "/tmp/filt" ;
+ unlink $filename ;
+
+ my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
+ or die "Cannot open $filename: $!\n" ;
+
+ # Install DBM Filters
+ $db->filter_fetch_key ( sub { s/\0$// } ) ;
+ $db->filter_store_key ( sub { $_ .= "\0" } ) ;
+ $db->filter_fetch_value( sub { s/\0$// } ) ;
+ $db->filter_store_value( sub { $_ .= "\0" } ) ;
+
+ $hash{"abc"} = "def" ;
+ my $a = $hash{"ABC"} ;
+ # ...
+ undef $db ;
+ untie %hash ;
+
+Hopefully the contents of each of the filters should be
+self-explanatory. Both "fetch" filters remove the terminating NULL,
+and both "store" filters add a terminating NULL.
+
+
+=head2 Another Example -- Key is a C int.
+
+Here is another real-life example. By default, whenever Perl writes to
+a DBM database it always writes the key and value as strings. So when
+you use this:
+
+ $hash{12345} = "soemthing" ;
+
+the key 12345 will get stored in the DBM database as the 5 byte string
+"12345". If you actually want the key to be stored in the DBM database
+as a C int, you will have to use C<pack> when writing, and C<unpack>
+when reading.
+
+Here is a DBM Filter that does it:
+
+ use strict ;
+ use DB_File ;
+ my %hash ;
+ my $filename = "/tmp/filt" ;
+ unlink $filename ;
+
+
+ my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
+ or die "Cannot open $filename: $!\n" ;
+
+ $db->filter_fetch_key ( sub { $_ = unpack("i", $_) } ) ;
+ $db->filter_store_key ( sub { $_ = pack ("i", $_) } ) ;
+ $hash{123} = "def" ;
+ # ...
+ undef $db ;
+ untie %hash ;
+
+This time only two filters have been used -- we only need to manipulate
+the contents of the key, so it wasn't necessary to install any value
+filters.
+
+=head1 HINTS AND TIPS
+
+
+=head2 Locking Databases
+
+Concurrent access of a read-write database by several parties requires
+them all to use some kind of locking. Here's an example of Tom's that
+uses the I<fd> method to get the file descriptor, and then a careful
+open() to give something Perl will flock() for you. Run this repeatedly
+in the background to watch the locks granted in proper order.
+
+ use DB_File;
+
+ use strict;
+
+ sub LOCK_SH { 1 }
+ sub LOCK_EX { 2 }
+ sub LOCK_NB { 4 }
+ sub LOCK_UN { 8 }
+
+ my($oldval, $fd, $db, %db, $value, $key);
+
+ $key = shift || 'default';
+ $value = shift || 'magic';
+
+ $value .= " $$";
+
+ $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
+ || die "dbcreat /tmp/foo.db $!";
+ $fd = $db->fd;
+ print "$$: db fd is $fd\n";
+ open(DB_FH, "+<&=$fd") || die "dup $!";
+
+
+ unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {
+ print "$$: CONTENTION; can't read during write update!
+ Waiting for read lock ($!) ....";
+ unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
+ }
+ print "$$: Read lock granted\n";
+
+ $oldval = $db{$key};
+ print "$$: Old value was $oldval\n";
+ flock(DB_FH, LOCK_UN);
+
+ unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
+ print "$$: CONTENTION; must have exclusive lock!
+ Waiting for write lock ($!) ....";
+ unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
+ }
+
+ print "$$: Write lock granted\n";
+ $db{$key} = $value;
+ $db->sync; # to flush
+ sleep 10;
+
+ flock(DB_FH, LOCK_UN);
+ undef $db;
+ untie %db;
+ close(DB_FH);
+ print "$$: Updated db to $key=$value\n";
+
+=head2 Sharing Databases With C Applications
+
+There is no technical reason why a Berkeley DB database cannot be
+shared by both a Perl and a C application.
+
+The vast majority of problems that are reported in this area boil down
+to the fact that C strings are NULL terminated, whilst Perl strings are
+not. See L<DBM FILTERS> for a generic way to work around this problem.
+
+Here is a real example. Netscape 2.0 keeps a record of the locations you
+visit along with the time you last visited them in a DB_HASH database.
+This is usually stored in the file F<~/.netscape/history.db>. The key
+field in the database is the location string and the value field is the
+time the location was last visited stored as a 4 byte binary value.
+
+If you haven't already guessed, the location string is stored with a
+terminating NULL. This means you need to be careful when accessing the
+database.
+
+Here is a snippet of code that is loosely based on Tom Christiansen's
+I<ggh> script (available from your nearest CPAN archive in
+F<authors/id/TOMC/scripts/nshist.gz>).
+
+ use strict ;
+ use DB_File ;
+ use Fcntl ;
+
+ use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
+ $dotdir = $ENV{HOME} || $ENV{LOGNAME};
+
+ $HISTORY = "$dotdir/.netscape/history.db";
+
+ tie %hist_db, 'DB_File', $HISTORY
+ or die "Cannot open $HISTORY: $!\n" ;;
+
+ # Dump the complete database
+ while ( ($href, $binary_time) = each %hist_db ) {
+
+ # remove the terminating NULL
+ $href =~ s/\x00$// ;
+
+ # convert the binary time into a user friendly string
+ $date = localtime unpack("V", $binary_time);
+ print "$date $href\n" ;
+ }
+
+ # check for the existence of a specific key
+ # remember to add the NULL
+ if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
+ $date = localtime unpack("V", $binary_time) ;
+ print "Last visited mox.perl.com on $date\n" ;
+ }
+ else {
+ print "Never visited mox.perl.com\n"
+ }
+
+ untie %hist_db ;
+
+=head2 The untie() Gotcha
+
+If you make use of the Berkeley DB API, it is I<very> strongly
+recommended that you read L<perltie/The untie Gotcha>.
+
+Even if you don't currently make use of the API interface, it is still
+worth reading it.
+
+Here is an example which illustrates the problem from a B<DB_File>
+perspective:
+
+ use DB_File ;
+ use Fcntl ;
+
+ my %x ;
+ my $X ;
+
+ $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC
+ or die "Cannot tie first time: $!" ;
+
+ $x{123} = 456 ;
+
+ untie %x ;
+
+ tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
+ or die "Cannot tie second time: $!" ;
+
+ untie %x ;
+
+When run, the script will produce this error message:
+
+ Cannot tie second time: Invalid argument at bad.file line 14.
+
+Although the error message above refers to the second tie() statement
+in the script, the source of the problem is really with the untie()
+statement that precedes it.
+
+Having read L<perltie> you will probably have already guessed that the
+error is caused by the extra copy of the tied object stored in C<$X>.
+If you haven't, then the problem boils down to the fact that the
+B<DB_File> destructor, DESTROY, will not be called until I<all>
+references to the tied object are destroyed. Both the tied variable,
+C<%x>, and C<$X> above hold a reference to the object. The call to
+untie() will destroy the first, but C<$X> still holds a valid
+reference, so the destructor will not get called and the database file
+F<tst.fil> will remain open. The fact that Berkeley DB then reports the
+attempt to open a database that is alreday open via the catch-all
+"Invalid argument" doesn't help.
+
+If you run the script with the C<-w> flag the error message becomes:
+
+ untie attempted while 1 inner references still exist at bad.file line 12.
+ Cannot tie second time: Invalid argument at bad.file line 14.
+
+which pinpoints the real problem. Finally the script can now be
+modified to fix the original problem by destroying the API object
+before the untie:
+
+ ...
+ $x{123} = 456 ;
+
+ undef $X ;
+ untie %x ;
+
+ $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
+ ...
+
+
+=head1 COMMON QUESTIONS
+
+=head2 Why is there Perl source in my database?
+
+If you look at the contents of a database file created by DB_File,
+there can sometimes be part of a Perl script included in it.
+
+This happens because Berkeley DB uses dynamic memory to allocate
+buffers which will subsequently be written to the database file. Being
+dynamic, the memory could have been used for anything before DB
+malloced it. As Berkeley DB doesn't clear the memory once it has been
+allocated, the unused portions will contain random junk. In the case
+where a Perl script gets written to the database, the random junk will
+correspond to an area of dynamic memory that happened to be used during
+the compilation of the script.
+
+Unless you don't like the possibility of there being part of your Perl
+scripts embedded in a database file, this is nothing to worry about.
+
+=head2 How do I store complex data structures with DB_File?
+
+Although B<DB_File> cannot do this directly, there is a module which
+can layer transparently over B<DB_File> to accomplish this feat.
+
+Check out the MLDBM module, available on CPAN in the directory
+F<modules/by-module/MLDBM>.
+
+=head2 What does "Invalid Argument" mean?
+
+You will get this error message when one of the parameters in the
+C<tie> call is wrong. Unfortunately there are quite a few parameters to
+get wrong, so it can be difficult to figure out which one it is.
+
+Here are a couple of possibilities:
+
+=over 5
+
+=item 1.
+
+Attempting to reopen a database without closing it.
+
+=item 2.
+
+Using the O_WRONLY flag.
+
+=back
+
+=head2 What does "Bareword 'DB_File' not allowed" mean?
+
+You will encounter this particular error message when you have the
+C<strict 'subs'> pragma (or the full strict pragma) in your script.
+Consider this script:
+
+ use strict ;
+ use DB_File ;
+ use vars qw(%x) ;
+ tie %x, DB_File, "filename" ;
+
+Running it produces the error in question:
+
+ Bareword "DB_File" not allowed while "strict subs" in use
+
+To get around the error, place the word C<DB_File> in either single or
+double quotes, like this:
+
+ tie %x, "DB_File", "filename" ;
+
+Although it might seem like a real pain, it is really worth the effort
+of having a C<use strict> in all your scripts.
+
+=head1 REFERENCES
+
+Articles that are either about B<DB_File> or make use of it.
+
+=over 5
+
+=item 1.
+
+I<Full-Text Searching in Perl>, Tim Kientzle (tkientzle@ddj.com),
+Dr. Dobb's Journal, Issue 295, January 1999, pp 34-41
+
+=back
+
+=head1 HISTORY
+
+Moved to the Changes file.
+
+=head1 BUGS
+
+Some older versions of Berkeley DB had problems with fixed length
+records using the RECNO file format. This problem has been fixed since
+version 1.85 of Berkeley DB.
+
+I am sure there are bugs in the code. If you do find any, or can
+suggest any enhancements, I would welcome your comments.
+
+=head1 AVAILABILITY
+
+B<DB_File> comes with the standard Perl source distribution. Look in
+the directory F<ext/DB_File>. Given the amount of time between releases
+of Perl the version that ships with Perl is quite likely to be out of
+date, so the most recent version can always be found on CPAN (see
+L<perlmod/CPAN> for details), in the directory
+F<modules/by-module/DB_File>.
+
+This version of B<DB_File> will work with either version 1.x, 2.x or
+3.x of Berkeley DB, but is limited to the functionality provided by
+version 1.
+
+The official web site for Berkeley DB is F<http://www.sleepycat.com>.
+All versions of Berkeley DB are available there.
+
+Alternatively, Berkeley DB version 1 is available at your nearest CPAN
+archive in F<src/misc/db.1.85.tar.gz>.
+
+If you are running IRIX, then get Berkeley DB version 1 from
+F<http://reality.sgi.com/ariel>. It has the patches necessary to
+compile properly on IRIX 5.3.
+
+=head1 COPYRIGHT
+
+Copyright (c) 1995-1999 Paul Marquess. All rights reserved. This program
+is free software; you can redistribute it and/or modify it under the
+same terms as Perl itself.
+
+Although B<DB_File> is covered by the Perl license, the library it
+makes use of, namely Berkeley DB, is not. Berkeley DB has its own
+copyright and its own license. Please take the time to read it.
+
+Here are are few words taken from the Berkeley DB FAQ (at
+http://www.sleepycat.com) regarding the license:
+
+ Do I have to license DB to use it in Perl scripts?
+
+ No. The Berkeley DB license requires that software that uses
+ Berkeley DB be freely redistributable. In the case of Perl, that
+ software is Perl, and not your scripts. Any Perl scripts that you
+ write are your property, including scripts that make use of
+ Berkeley DB. Neither the Perl license nor the Berkeley DB license
+ place any restriction on what you may do with them.
+
+If you are in any doubt about the license situation, contact either the
+Berkeley DB authors or the author of DB_File. See L<"AUTHOR"> for details.
+
+
+=head1 SEE ALSO
+
+L<perl(1)>, L<dbopen(3)>, L<hash(3)>, L<recno(3)>, L<btree(3)>,
+L<dbmfilter>
+
+=head1 AUTHOR
+
+The DB_File interface was written by Paul Marquess
+E<lt>Paul.Marquess@btinternet.comE<gt>.
+Questions about the DB system itself may be addressed to
+E<lt>db@sleepycat.com<gt>.
+
+=cut
projects / p5sagit/p5-mst-13.2.git / blobdiff