# DB_File.pm -- Perl 5 interface to Berkeley DB
#
-# written by Paul Marquess (pmarquess@bfsec.bt.co.uk)
-# last modified 12th Mar 1997
-# version 1.12
+# written by Paul Marquess (Paul.Marquess@btinternet.com)
+# last modified 16th January 2000
+# version 1.72
#
-# Copyright (c) 1995, 1996, 1997 Paul Marquess. All rights reserved.
+# Copyright (c) 1995-2000 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.
croak ref($self) . " does not define the method ${method}" ;
}
-sub DESTROY { undef %{$_[0]} }
sub FIRSTKEY { my $self = shift ; $self->NotHere("FIRSTKEY") }
sub NEXTKEY { my $self = shift ; $self->NotHere("NEXTKEY") }
sub CLEAR { my $self = shift ; $self->NotHere("CLEAR") }
package DB_File ;
use strict;
-use vars qw($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 $use_XSLoader
+ ) ;
use Carp;
-$VERSION = "1.12" ;
+$VERSION = "1.72" ;
#typedef enum { DB_BTREE, DB_HASH, DB_RECNO } DBTYPE;
$DB_BTREE = new DB_File::BTREEINFO ;
require Tie::Hash;
require Exporter;
use AutoLoader;
-require DynaLoader;
-@ISA = qw(Tie::Hash Exporter DynaLoader);
+BEGIN {
+ $use_XSLoader = 1 ;
+ eval { require XSLoader } ;
+
+ if ($@) {
+ $use_XSLoader = 0 ;
+ require DynaLoader;
+ @ISA = qw(DynaLoader);
+ }
+}
+
+push @ISA, qw(Tie::Hash Exporter);
@EXPORT = qw(
$DB_BTREE $DB_HASH $DB_RECNO
($constname = $AUTOLOAD) =~ s/.*:://;
my $val = constant($constname, @_ ? $_[0] : 0);
if ($! != 0) {
- if ($! =~ /Invalid/) {
+ if ($! =~ /Invalid/ || $!{EINVAL}) {
$AutoLoader::AUTOLOAD = $AUTOLOAD;
goto &AutoLoader::AUTOLOAD;
}
}
-# 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_/';
- };
-}
+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);
+};
-bootstrap DB_File $VERSION;
+if ($use_XSLoader)
+ { XSLoader::load("DB_File", $VERSION)}
+else
+ { bootstrap DB_File $VERSION }
# Preloaded methods go here. Autoload methods go after __END__, and are
# processed by the autosplit program.
-sub TIEHASH
+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 %{ $arg[4] } ;
- DoTie_(@arg) ;
+ # make recno in Berkeley DB version 2 work like recno in version 1.
+ if ($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 ($status) = 0 ;
-*TIEARRAY = \&TIEHASH ;
+ for ($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
{
1;
__END__
-=cut
-
=head1 NAME
-DB_File - Perl5 access to Berkeley DB
+DB_File - Perl5 access to Berkeley DB version 1.x
=head1 SYNOPSIS
$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->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. 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.
-
-Please note that this module will only work with version 1.x of
-Berkeley DB. Once Berkeley DB version 2 is released, B<DB_File> will be
-upgraded to work with it.
+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
=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 greater, use the Perl module B<BerkeleyDB> instead.
+
+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
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>.
+C<bval>, you should check out L<The 'bval' Option>.
=head2 Default Parameters
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";
# 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': $!\n" ;
# iterate through the associative array
# and print each key/value pair.
- foreach (keys %h)
+ foreach (sort keys %h)
{ print "$_ -> $h{$_}\n" }
untie %h ;
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
-B<DB_File> comes with a utility method, called C<get_dup>, to assist in
+The C<get_dup> method assists in
reading duplicate values from BTREE databases. The method can take the
following forms:
In list context, it returns all the values which match C<$key>. Note
that the values will be returned in an apparently random order.
-In list context, if the second parameter is present and evaluates TRUE,
-the method returns an associative array. The keys of the associative
-array correspond to the the values that matched in the BTREE and the
-values of the array are a count of the number of times that particular
-value occurred in the BTREE.
+In list context, if the second parameter is present and evaluates
+TRUE, the method returns an associative array. The keys of the
+associative array correspond to the values that matched in the BTREE
+and the values of the array are a count of the number of times that
+particular value occurred in the BTREE.
So assuming the database created above, we can use C<get_dup> like
this:
+ use strict ;
+ use DB_File ;
+
+ use vars qw($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" ;
print "Larry is there\n" if $hash{'Larry'} ;
print "There are $hash{'Brick'} Brick Walls\n" ;
- my @list = $x->get_dup("Wall") ;
+ my @list = sort $x->get_dup("Wall") ;
print "Wall => [@list]\n" ;
@list = $x->get_dup("Smith") ;
Smith => [John]
Dog => []
+=head2 The find_dup() Method
+
+ $status = $X->find_dup($key, $value) ;
+
+This method checks for the existence of a specific key/value pair. If the
+pair exists, the cursor is left pointing to the pair and the method
+returns 0. Otherwise the method returns a non-zero value.
+
+Assuming the database from the previous example:
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($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, $value) ;
+
+This method deletes a specific key/value pair. It returns
+0 if they exist and have been deleted successfully.
+Otherwise the method returns a non-zero value.
+
+Again assuming the existence of the C<tree> database
+
+ use strict ;
+ use DB_File ;
+
+ use vars qw($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
$st == 0 ;
$st = $x->seq($key, $value, R_NEXT) )
- { print "$key -> $value\n" }
+ { print "$key -> $value\n" }
print "\nPARTIAL MATCH\n" ;
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
+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
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.
+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
=head2 A Simple Example
-Here is a simple example that uses RECNO.
+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", "text", O_RDWR|O_CREAT, 0640, $DB_RECNO
+ 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[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] ;
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 yellow
- The 2nd last element is blue
+ The last element is green
+ The 2nd last element is yellow
-=head2 Extra Methods
+=head2 Extra RECNO 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. 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:
undef $db ;
untie %hash ;
-See L<The untie gotcha> for more details.
+See L<The untie() Gotcha> for more details.
All the functions defined in L<dbopen> are available except for
close() and dbopen() itself. The B<DB_File> method interface to the
Returns the file descriptor for the underlying database.
-See L<Locking Databases> for an example of how to make use of the
-C<fd> method to lock your database.
+See L<Locking: The Trouble with fd> for an explanation for why you should
+not use C<fd> to lock your database.
=item B<$status = $X-E<gt>seq($key, $value, $flags) ;>
=back
-=head1 HINTS AND TIPS
+=head1 DBM FILTERS
+A DBM Filter is a piece of code that is be used when you I<always>
+want to make the same transformation to all keys and/or values in a
+DBM database.
-=head2 Locking Databases
+There are four methods associated with DBM Filters. All work identically,
+and each is used to install (or uninstall) a single DBM Filter. Each
+expects a single parameter, namely a reference to a sub. The only
+difference between them is the place that the filter is installed.
-Concurrent access of a read-write database by several parties requires
-them all to use some kind of locking. Here's an example of Tom's that
-uses the I<fd> method to get the file descriptor, and then a careful
-open() to give something Perl will flock() for you. Run this repeatedly
-in the background to watch the locks granted in proper order.
+To summarise:
- use DB_File;
+=over 5
- use strict;
+=item B<filter_store_key>
- sub LOCK_SH { 1 }
- sub LOCK_EX { 2 }
- sub LOCK_NB { 4 }
- sub LOCK_UN { 8 }
+If a filter has been installed with this method, it will be invoked
+every time you write a key to a DBM database.
- my($oldval, $fd, $db, %db, $value, $key);
+=item B<filter_store_value>
- $key = shift || 'default';
- $value = shift || 'magic';
+If a filter has been installed with this method, it will be invoked
+every time you write a value to a DBM database.
- $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 $!";
+=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.
- 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";
+=item B<filter_fetch_value>
- $oldval = $db{$key};
- print "$$: Old value was $oldval\n";
- flock(DB_FH, LOCK_UN);
+If a filter has been installed with this method, it will be invoked
+every time you read a value from a DBM database.
+
+=back
+
+You can use any combination of the methods, from none, to all four.
- unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
- print "$$: CONTENTION; must have exclusive lock!
- Waiting for write lock ($!) ....";
- unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
- }
+All filter methods return the existing filter, if present, or C<undef>
+in not.
- print "$$: Write lock granted\n";
- $db{$key} = $value;
- $db->sync; # to flush
- sleep 10;
+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: The Trouble with fd
+
+Until version 1.72 of this module, the recommended technique for locking
+B<DB_File> databases was to flock the filehandle returned from the "fd"
+function. Unfortunately this technique has been shown to be fundamentally
+flawed (Kudos to David Harris for tracking this down). Use it at your own
+peril!
+
+The locking technique went like this.
+
+ $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)
+ || die "dbcreat /tmp/foo.db $!";
+ $fd = $db->fd;
+ open(DB_FH, "+<&=$fd") || die "dup $!";
+ flock (DB_FH, LOCK_EX) || die "flock: $!";
+ ...
+ $db{"Tom"} = "Jerry" ;
+ ...
flock(DB_FH, LOCK_UN);
undef $db;
untie %db;
close(DB_FH);
- print "$$: Updated db to $key=$value\n";
+
+In simple terms, this is what happens:
+
+=over 5
+
+=item 1.
+
+Use "tie" to open the database.
+
+=item 2.
+
+Lock the database with fd & flock.
+
+=item 3.
+
+Read & Write to the database.
+
+=item 4.
+
+Unlock and close the database.
+
+=back
+
+Here is the crux of the problem. A side-effect of opening the B<DB_File>
+database in step 2 is that an initial block from the database will get
+read from disk and cached in memory.
+
+To see why this is a problem, consider what can happen when two processes,
+say "A" and "B", both want to update the same B<DB_File> database
+using the locking steps outlined above. Assume process "A" has already
+opened the database and has a write lock, but it hasn't actually updated
+the database yet (it has finished step 2, but not started step 3 yet). Now
+process "B" tries to open the same database - step 1 will succeed,
+but it will block on step 2 until process "A" releases the lock. The
+important thing to notice here is that at this point in time both
+processes will have cached identical initial blocks from the database.
+
+Now process "A" updates the database and happens to change some of the
+data held in the initial buffer. Process "A" terminates, flushing
+all cached data to disk and releasing the database lock. At this point
+the database on disk will correctly reflect the changes made by process
+"A".
+
+With the lock released, process "B" can now continue. It also updates the
+database and unfortunately it too modifies the data that was in its
+initial buffer. Once that data gets flushed to disk it will overwrite
+some/all of the changes process "A" made to the database.
+
+The result of this scenario is at best a database that doesn't contain
+what you expect. At worst the database will corrupt.
+
+The above won't happen every time competing process update the same
+B<DB_File> database, but it does illustrate why the technique should
+not be used.
+
+=head2 Safe ways to lock a database
+
+Starting with version 2.x, Berkeley DB has internal support for locking.
+The companion module to this one, B<BerkeleyDB>, provides an interface
+to this locking functionality. If you are serious about locking
+Berkeley DB databases, I strongly recommend using B<BerkeleyDB>.
+
+If using B<BerkeleyDB> isn't an option, there are a number of modules
+available on CPAN that can be used to implement locking. Each one
+implements locking differently and has different goals in mind. It is
+therefore worth knowing the difference, so that you can pick the right
+one for your application. Here are the three locking wrappers:
+
+=over 5
+
+=item B<Tie::DB_Lock>
+
+A B<DB_File> wrapper which creates copies of the database file for
+read access, so that you have a kind of a multiversioning concurrent read
+system. However, updates are still serial. Use for databases where reads
+may be lengthy and consistency problems may occur.
+
+=item B<Tie::DB_LockFile>
+
+A B<DB_File> wrapper that has the ability to lock and unlock the database
+while it is being used. Avoids the tie-before-flock problem by simply
+re-tie-ing the database when you get or drop a lock. Because of the
+flexibility in dropping and re-acquiring the lock in the middle of a
+session, this can be massaged into a system that will work with long
+updates and/or reads if the application follows the hints in the POD
+documentation.
+
+=item B<DB_File::Lock>
+
+An extremely lightweight B<DB_File> wrapper that simply flocks a lockfile
+before tie-ing the database and drops the lock after the untie. Allows
+one to use the same lockfile for multiple databases to avoid deadlock
+problems, if desired. Use for databases where updates are reads are
+quick and simple flock locking semantics are enough.
+
+=back
=head2 Sharing Databases With C Applications
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.
+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.
=head2 The untie() Gotcha
-If you make use of the Berkeley DB API, it is is I<very> strongly
+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
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
+attempt to open a database that is already open via the catch-all
"Invalid argument" doesn't help.
If you run the script with the C<-w> flag the error message becomes:
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 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.
-
-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.
-
-Allow negative subscripts with RECNO interface.
-
-Changed the default flags from O_RDWR to O_CREAT|O_RDWR.
-
-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.
-
-Added get_dup method.
-
-=item 1.03
+=head1 REFERENCES
-Documentation update.
+Articles that are either about B<DB_File> or make use of it.
-B<DB_File> now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl
-automatically.
-
-The standard hash function C<exists> is now supported.
-
-Modified the behavior of get_dup. When it returns an associative
-array, the value is the count of the number of matching BTREE values.
-
-=item 1.04
-
-Minor documentation changes.
-
-Fixed a bug in hash_cb. Patches supplied by Dave Hammen,
-E<lt>hammen@gothamcity.jsc.nasa.govE<gt>.
-
-Fixed a bug with the constructors for DB_File::HASHINFO,
-DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up the
-constructors to make them C<-w> clean.
-
-Reworked part of the test harness to be more locale friendly.
-
-=item 1.05
-
-Made all scripts in the documentation C<strict> and C<-w> clean.
-
-Added logic to F<DB_File.xs> to allow the module to be built after Perl
-is installed.
-
-=item 1.06
-
-Minor namespace cleanup: Localized C<PrintBtree>.
-
-=item 1.07
-
-Fixed bug with RECNO, where bval wasn't defaulting to "\n".
-
-=item 1.08
-
-Documented operation of bval.
-
-=item 1.09
-
-Minor bug fix in DB_File::HASHINFO, DB_File::RECNOINFO and
-DB_File::BTREEINFO.
-
-Changed default mode to 0666.
-
-=item 1.10
-
-Fixed fd method so that it still returns -1 for in-memory files when db
-1.86 is used.
+=over 5
-=item 1.11
+=item 1.
-Documented the untie gotcha.
+I<Full-Text Searching in Perl>, Tim Kientzle (tkientzle@ddj.com),
+Dr. Dobb's Journal, Issue 295, January 1999, pp 34-41
-=item 1.12
+=back
-Documented the incompatibility with version 2 of Berkeley DB.
+=head1 HISTORY
-=back
+Moved to the Changes file.
=head1 BUGS
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.
+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>.
+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 only work with version 1.x of Berkeley
-DB. It is I<not> yet compatible with version 2.
+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.
-Version 1 of 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>. Alternatively,
-check out the Berkeley DB home page at F<http://www.bostic.com/db>. It
-is I<not> under the GPL.
+The official web site for Berkeley DB is F<http://www.sleepycat.com>.
+All versions of Berkeley DB are available there.
-If you are running IRIX, then get Berkeley DB from
+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.
-As of January 1997, version 1.86 of Berkeley DB is available from the
-Berkeley DB home page. Although this release does fix a number of bugs
-that were present in 1.85 you should be aware of the following
-information (taken from the Berkeley DB home page) before you consider
-using it:
+=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
+F<http://www.sleepycat.com>) regarding the license:
+
+ Do I have to license DB to use it in Perl scripts?
- DB version 1.86 includes a new implementation of the hash access
- method that fixes a variety of hashing problems found in DB version
- 1.85. We are making it available as an interim solution until DB
- 2.0 is available.
+ 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.
- PLEASE NOTE: the underlying file format for the hash access method
- changed between version 1.85 and version 1.86, so you will have to
- dump and reload all of your databases to convert from version 1.85
- to version 1.86. If you do not absolutely require the fixes from
- version 1.86, we strongly urge you to wait until DB 2.0 is released
- before upgrading from 1.85.
+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<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>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>.
+E<lt>Paul.Marquess@btinternet.comE<gt>.
+Questions about the DB system itself may be addressed to
+E<lt>db@sleepycat.com<gt>.
=cut
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