# DB_File.pm -- Perl 5 interface to Berkeley DB
#
# written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
-# last modified 4th Sept 1996
-# version 1.03
+# last modified 19th November 1998
+# version 1.61
+#
+# Copyright (c) 1995-8 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;
bless \%x, $pkg ;
}
+
sub TIEHASH
{
my $pkg = shift ;
- bless { 'bsize' => undef,
- 'ffactor' => undef,
- 'nelem' => undef,
- 'cachesize' => undef,
- 'hash' => undef,
- 'lorder' => undef,
- }, $pkg ;
+ bless { VALID => { map {$_, 1}
+ qw( bsize ffactor nelem cachesize hash lorder)
+ },
+ GOT => {}
+ }, $pkg ;
}
+
sub FETCH
{
my $self = shift ;
my $key = shift ;
- return $self->{$key} if exists $self->{$key} ;
+ return $self->{GOT}{$key} if exists $self->{VALID}{$key} ;
my $pkg = ref $self ;
croak "${pkg}::FETCH - Unknown element '$key'" ;
my $key = shift ;
my $value = shift ;
- if ( exists $self->{$key} )
+ if ( exists $self->{VALID}{$key} )
{
- $self->{$key} = $value ;
+ $self->{GOT}{$key} = $value ;
return ;
}
my $self = shift ;
my $key = shift ;
- if ( exists $self->{$key} )
+ if ( exists $self->{VALID}{$key} )
{
- delete $self->{$key} ;
+ delete $self->{GOT}{$key} ;
return ;
}
my $self = shift ;
my $key = shift ;
- exists $self->{$key} ;
+ exists $self->{VALID}{$key} ;
}
sub NotHere
{
- my $pkg = shift ;
+ my $self = shift ;
my $method = shift ;
- croak "${pkg} does not define the method ${method}" ;
+ croak ref($self) . " does not define the method ${method}" ;
}
-sub DESTROY { undef %{$_[0]} }
-sub FIRSTKEY { my $self = shift ; $self->NotHere(ref $self, "FIRSTKEY") }
-sub NEXTKEY { my $self = shift ; $self->NotHere(ref $self, "NEXTKEY") }
-sub CLEAR { my $self = shift ; $self->NotHere(ref $self, "CLEAR") }
+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 ;
{
my $pkg = shift ;
- bless { 'bval' => undef,
- 'cachesize' => undef,
- 'psize' => undef,
- 'flags' => undef,
- 'lorder' => undef,
- 'reclen' => undef,
- 'bfname' => "",
- }, $pkg ;
+ bless { VALID => { map {$_, 1}
+ qw( bval cachesize psize flags lorder reclen bfname )
+ },
+ GOT => {},
+ }, $pkg ;
}
package DB_File::BTREEINFO ;
{
my $pkg = shift ;
- bless { 'flags' => undef,
- 'cachesize' => undef,
- 'maxkeypage' => undef,
- 'minkeypage' => undef,
- 'psize' => undef,
- 'compare' => undef,
- 'prefix' => undef,
- 'lorder' => undef,
- }, $pkg ;
+ bless { VALID => { map {$_, 1}
+ qw( flags cachesize maxkeypage minkeypage psize
+ compare prefix lorder )
+ },
+ GOT => {},
+ }, $pkg ;
}
package DB_File ;
use strict;
-use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO) ;
+use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO $db_version) ;
use Carp;
-$VERSION = "1.03" ;
+$VERSION = "1.61" ;
#typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
-#$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 ;
}
-# 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;
- };
-}
+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_/';
+# };
+#}
bootstrap 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 get_dup
{
croak "Usage: \$db->get_dup(key [,flag])\n"
my $counter = 0 ;
my $status = 0 ;
- # get the first value associated with the key, $key
- #$db->seq($key, $value, R_CURSOR()) ;
-
# iterate through the database until either EOF ($status == 0)
# or a different key is encountered ($key ne $origkey).
for ($status = $db->seq($key, $value, R_CURSOR()) ;
1;
__END__
-=cut
-
=head1 NAME
-DB_File - Perl5 access to Berkeley DB
+DB_File - Perl5 access to Berkeley DB version 1.x
=head1 SYNOPSIS
use DB_File ;
- use strict 'untie' ;
[$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
[$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
=head1 DESCRIPTION
B<DB_File> is a module which allows Perl programs to make use of the
-facilities provided by Berkeley DB. If you intend to use this
-module you should really have a copy of the Berkeley DB manual pages at
-hand. The interface defined here mirrors the Berkeley DB interface
-closely.
+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>). 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
=back
-=head2 How does DB_File interface to Berkeley DB?
+=head2 Using DB_File with Berkeley DB version 2
+
+Although B<DB_File> is intended to be used with Berkeley DB version 1,
+it can also be used with version 2. In this case the interface is
+limited to the functionality provided by Berkeley DB 1.x. Anywhere the
+version 2 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 without any changes.
+
+If you want to make use of the new features available in Berkeley DB
+2.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 Berkeley DB version 2.
+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 upgraded DB_File to use Berkeley DB version 2, 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 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
$a->{'cachesize'} = 12345 ;
tie %y, 'DB_File', "filename", $flags, 0777, $a ;
-A few of the values need extra discussion here. When used, the C
+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:
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
is equivalent to:
- tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0640, $DB_HASH ;
+ 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:
is equivalent to:
- tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0640, $DB_HASH ;
+ 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.
file formats that B<DB_File> supports. It is also very straightforward
to use.
-=head2 A Simple Example.
+=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 strict 'untie' ;
+ use vars qw( %h $k $v ) ;
tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
or die "Cannot open file 'fruit': $!\n";
BTREE uses. Instead of using the normal lexical ordering, a case
insensitive compare function will be used.
+ use strict ;
use DB_File ;
- use strict 'untie' ;
+
+ my %h ;
sub Compare
{
=back
-=head2 Handling duplicate keys
+=head2 Handling Duplicate Keys
The BTREE file type optionally allows a single key to be associated
with an arbitrary number of values. This option is enabled by setting
want to manipulate a BTREE database with duplicate keys. Consider this
code:
+ use strict ;
use DB_File ;
- use strict 'untie' ;
-
+
+ use vars qw($filename %h ) ;
+
$filename = "tree" ;
unlink $filename ;
Here is the script above rewritten using the C<seq> API method.
+ use strict ;
use DB_File ;
- use strict 'untie' ;
+ use vars qw($filename $x %h $status $key $value) ;
+
$filename = "tree" ;
unlink $filename ;
# 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, $value, R_NEXT) )
This time we have got all the key/value pairs, including the multiple
values associated with the key C<Wall>.
-=head2 The get_dup method.
+=head2 The get_dup() Method
B<DB_File> comes with a utility method, called C<get_dup>, to assist in
reading duplicate values from BTREE databases. The method can take the
In list context, it returns all the values which match C<$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 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.
+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:
- $cnt = $x->get_dup("Wall") ;
+ my $cnt = $x->get_dup("Wall") ;
print "Wall occurred $cnt times\n" ;
- %hash = $x->get_dup("Wall", 1) ;
+ my %hash = $x->get_dup("Wall", 1) ;
print "Larry is there\n" if $hash{'Larry'} ;
print "There are $hash{'Brick'} Brick Walls\n" ;
- @list = $x->get_dup("Wall") ;
+ my @list = $x->get_dup("Wall") ;
print "Wall => [@list]\n" ;
@list = $x->get_dup("Smith") ;
Here is the relevant quote from the dbopen man page where it defines
the use of the R_CURSOR flag with seq:
-
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
-
In the example script below, the C<match> sub uses this feature to find
and print the first matching key/value pair given a partial key.
+ use strict ;
use DB_File ;
use Fcntl ;
- use strict 'untie' ;
+
+ use vars qw($filename $x %h $st $key $value) ;
sub match
{
my $key = shift ;
- my $value ;
+ my $value = 0;
my $orig_key = $key ;
$x->seq($key, $value, R_CURSOR) ;
print "$orig_key\t-> $key\t-> $value\n" ;
$h{'Smith'} = 'John' ;
+ $key = $value = 0 ;
print "IN ORDER\n" ;
for ($st = $x->seq($key, $value, R_FIRST) ;
$st == 0 ;
-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 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.
+ use strict ;
use DB_File ;
- use strict 'untie' ;
+ my @h ;
tie @h, "DB_File", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
or die "Cannot open file 'text': $!\n" ;
=head2 Extra Methods
-As you can see from the example above, the tied array interface is
-quite limited. To make the interface more useful, a number of methods
-are supplied with B<DB_File> to simulate the standard array operations
-that are not currently implemented in Perl's tied array interface. All
-these methods are accessed via the object returned from the tie call.
+If you are using a version of Perl earlier than 5.004_57, the tied
+array interface is quite limited. The example script above will work,
+but you won't be able to use C<push>, C<pop>, C<shift>, C<unshift>
+etc. with the tied array.
+
+To make the interface more useful for older versions of Perl, a number
+of methods are supplied with B<DB_File> to simulate the missing array
+operations. All these methods are accessed via the object returned from
+the tie call.
Here are the methods:
# same again, but use the API functions instead
print "\nREVERSE again\n" ;
- my ($s, $k, $v) ;
+ my ($s, $k, $v) = (0, 0, 0) ;
for ($s = $H->seq($k, $v, R_LAST) ;
$s == 0 ;
$s = $H->seq($k, $v, R_PREV))
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. See L<The strict untie pragma> for more details.
+destroyed.
use DB_File ;
$db = tie %hash, "DB_File", "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
=head1 HINTS AND TIPS
-=head2 The strict untie pragma
-
-If you run Perl version 5.004 or later (actually any version from the
-5.003_01 development release on will suffice) and you make use of the
-Berkeley DB API, it is is I<very> strongly recommended that you always
-include the C<use strict 'untie'> pragma in any of your scripts that
-make use of B<DB_File>.
-
-Even if you don't currently make use of the API interface, it is still
-a good idea to include the pragma. It won't affect the performance of
-your script, but it will prevent problems in the future.
-
-If possible you should try to run with the full strict pragma, but that
-is another story. For further details see L<strict> and
-L<perldsc/WHY YOU SHOULD ALWAYS C<use strict>>.
-
-To illustrate the importance of including the untie pragma, here is an
-example script that fails in an unexpected place because it doesn't use
-it:
-
- use DB_File ;
- use Fcntl ;
-
- $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
- or die "Cannot tie first time: $!" ;
-
- $x{123} = 456 ;
-
- untie %x ;
-
- $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
- or die "Cannot tie second time: $!" ;
-
- untie %x ;
-
-When run the script will produce this error message:
-
- Cannot tie second time: Invalid argument at bad.file line 12.
-
-Although the error message above refers to the second tie statement in
-the script, the source of the problem is really with the untie
-statement that precedes it.
-
-To understand why there is a problem at all with the untie statement,
-consider what the tie does for a moment.
-
-Whenever the tie is executed, it creates a logical link between a Perl
-variable, the associative array C<%x> in this case, and a Berkeley DB
-database, C<tst.fil>. The logical link ensures that all operation on
-the associative array are automatically mirrored to the database file.
-
-In normal circumstances the untie is enough to break the logical link
-and also close the database. In this particular case there is another
-logical link, namely the API object returned from the tie and stored in
-C<$X>. Whenever the untie is executed in this case, only the link
-between the associative array and the database will be broken. The API
-object in C<$X> is still valid, so the database will not be closed.
-
-The end result of this is that when the second tie is executed, the
-database will be in an inconsistent state (i.e. it is still opened by
-the first tie) - thus the second tie will fail.
-
-If the C<use strict 'untie'> pragma is included in the script, like
-this:
-
- use DB_File ;
- use Fcntl ;
- use strict 'untie' ;
-
- $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
- or die "Cannot tie first time: $!" ;
-
- $x{123} = 456 ;
-
- untie %x ;
-
- $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
- or die "Cannot tie second time: $!" ;
-
-then the error message becomes:
-
- Can't untie: 1 inner references still exist at bad.file line 11.
-
-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
- ...
=head2 Locking Databases
open() to give something Perl will flock() for you. Run this repeatedly
in the background to watch the locks granted in proper order.
- use strict 'untie';
use DB_File;
use strict;
print "$$: Write lock granted\n";
$db{$key} = $value;
- $db->sync;
+ $db->sync; # to flush
sleep 10;
flock(DB_FH, LOCK_UN);
close(DB_FH);
print "$$: Updated db to $key=$value\n";
-=head2 Sharing databases with C applications
+=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.
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 strict 'untie' ;
+ use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
$dotdir = $ENV{HOME} || $ENV{LOGNAME};
$HISTORY = "$dotdir/.netscape/history.db";
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
=item 1.
-Attempting to reopen a database without closing it. See
-L<The strict untie pragma> for an example.
+Attempting to reopen a database without closing it.
=item 2.
=head1 HISTORY
-=over
-
-=item 0.1
-
-First Release.
-
-=item 0.2
-
-When B<DB_File> is opening a database file it no longer terminates the
-process if I<dbopen> returned an error. This allows file protection
-errors to be caught at run time. Thanks to Judith Grass
-E<lt>grass@cybercash.comE<gt> for spotting the bug.
-
-=item 0.3
-
-Added prototype support for multiple btree compare callbacks.
-
-=item 1.0
-
-B<DB_File> has been in use for over a year. To reflect that, the
-version number has been incremented to 1.0.
+Moved to the Changes file.
-Added complete support for multiple concurrent callbacks.
-
-Using the I<push> method on an empty list didn't work properly. This
-has been fixed.
-
-=item 1.01
-
-Fixed a core dump problem with SunOS.
-
-The return value from TIEHASH wasn't set to NULL when dbopen returned
-an error.
-
-=item 1.02
-
-Merged OS/2 specific code into DB_File.xs
-
-Removed some redundant code in DB_File.xs.
-
-Documentation update.
+=head1 BUGS
-Allow negative subscripts with RECNO interface.
+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.
-Changed the default flags from O_RDWR to O_CREAT|O_RDWR.
+I am sure there are bugs in the code. If you do find any, or can
+suggest any enhancements, I would welcome your comments.
-The example code which showed how to lock a database needed a call to
-C<sync> added. Without it the resultant database file was empty.
+=head1 AVAILABILITY
-Added get_dup method.
+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>.
-=item 1.03
+This version of B<DB_File> will work with either version 1.x or 2.x of
+Berkeley DB, but is limited to the functionality provided by version 1.
-Documentation update.
+The official web site for Berkeley DB is
+F<http://www.sleepycat.com/db>. The ftp equivalent is
+F<ftp.sleepycat.com:/pub>. Both versions 1 and 2 of Berkeley DB are
+available there.
-B<DB_File> now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl
-automatically.
+Alternatively, Berkeley DB version 1 is available at your nearest CPAN
+archive in F<src/misc/db.1.85.tar.gz>.
-The standard hash function C<exists> is now supported.
+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.
-Modified the behavior of get_dup. When it returns an associative
-array, the value is the count of the number of matching BTREE values.
+=head1 COPYRIGHT
-=head1 BUGS
+Copyright (c) 1995-8 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.
-Some older versions of Berkeley DB had problems with fixed length
-records using the RECNO file format. The newest version at the time of
-writing was 1.85 - this seems to have fixed the problems with RECNO.
+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.
-I am sure there are bugs in the code. If you do find any, or can
-suggest any enhancements, I would welcome your comments.
+Here are are few words taken from the Berkeley DB FAQ (at
+http://www.sleepycat.com) regarding the license:
-=head1 AVAILABILITY
+ Do I have to license DB to use it in Perl scripts?
-B<DB_File> comes with the standard Perl source distribution. Look in
-the directory F<ext/DB_File>.
+ 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.
-Berkeley DB is available at your nearest CPAN archive (see
-L<perlmod/"CPAN"> for a list) in F<src/misc/db.1.85.tar.gz>, or via the
-host F<ftp.cs.berkeley.edu> in F</ucb/4bsd/db.tar.gz>. It is I<not> under
-the GPL.
+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.
-If you are running IRIX, then get Berkeley DB from
-F<http://reality.sgi.com/ariel>. It has the patches necessary to
-compile properly on IRIX 5.3.
=head1 SEE ALSO
The DB_File interface was written by Paul Marquess
E<lt>pmarquess@bfsec.bt.co.ukE<gt>.
-Questions about the DB system itself may be addressed to Keith Bostic
-E<lt>bostic@cs.berkeley.eduE<gt>.
+Questions about the DB system itself may be addressed to
+E<lt>db@sleepycat.com<gt>.
=cut
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