# DB_File.pm -- Perl 5 interface to Berkeley DB
#
# written by Paul Marquess (Paul.Marquess@btinternet.com)
-# last modified 21st February 1999
-# version 1.64
+# last modified 30th July 2001
+# version 1.78
#
-# Copyright (c) 1995-9 Paul Marquess. All rights reserved.
+# Copyright (c) 1995-2001 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.
require 5.003 ;
+use warnings;
use strict;
use Carp;
require Tie::Hash;
package DB_File::RECNOINFO ;
+use warnings;
use strict ;
@DB_File::RECNOINFO::ISA = qw(DB_File::HASHINFO) ;
package DB_File::BTREEINFO ;
+use warnings;
use strict ;
@DB_File::BTREEINFO::ISA = qw(DB_File::HASHINFO) ;
package DB_File ;
+use warnings;
use strict;
-use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO $db_version) ;
+use vars qw($VERSION @ISA @EXPORT $AUTOLOAD $DB_BTREE $DB_HASH $DB_RECNO
+ $db_version $use_XSLoader
+ ) ;
use Carp;
-$VERSION = "1.64" ;
+$VERSION = "1.78" ;
#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;
}
";
}
}
- eval "sub $AUTOLOAD { $val }";
- goto &$AUTOLOAD;
+ no strict 'refs';
+ *{$AUTOLOAD} = sub { $val };
+ goto &{$AUTOLOAD};
}
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 $VERSION;
+if ($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 CLEAR
{
my $self = shift;
- my $key = "" ;
+ my $key = 0 ;
my $value = "" ;
my $status = $self->seq($key, $value, R_FIRST());
my @keys;
}
}
+
+sub SPLICE
+{
+ my $self = shift;
+ my $offset = shift;
+ if (not defined $offset) {
+ carp 'Use of uninitialized value in splice';
+ $offset = 0;
+ }
+
+ my $length = @_ ? shift : 0;
+ # Carping about definedness comes _after_ the OFFSET sanity check.
+ # This is so we get the same error messages as Perl's splice().
+ #
+
+ my @list = @_;
+
+ my $size = $self->FETCHSIZE();
+
+ # 'If OFFSET is negative then it start that far from the end of
+ # the array.'
+ #
+ if ($offset < 0) {
+ my $new_offset = $size + $offset;
+ if ($new_offset < 0) {
+ die "Modification of non-creatable array value attempted, "
+ . "subscript $offset";
+ }
+ $offset = $new_offset;
+ }
+
+ if ($offset > $size) {
+ $offset = $size;
+ }
+
+ if (not defined $length) {
+ carp 'Use of uninitialized value in splice';
+ $length = 0;
+ }
+
+ # 'If LENGTH is omitted, removes everything from OFFSET onward.'
+ if (not defined $length) {
+ $length = $size - $offset;
+ }
+
+ # 'If LENGTH is negative, leave that many elements off the end of
+ # the array.'
+ #
+ if ($length < 0) {
+ $length = $size - $offset + $length;
+
+ if ($length < 0) {
+ # The user must have specified a length bigger than the
+ # length of the array passed in. But perl's splice()
+ # doesn't catch this, it just behaves as for length=0.
+ #
+ $length = 0;
+ }
+ }
+
+ if ($length > $size - $offset) {
+ $length = $size - $offset;
+ }
+
+ # $num_elems holds the current number of elements in the database.
+ my $num_elems = $size;
+
+ # 'Removes the elements designated by OFFSET and LENGTH from an
+ # array,'...
+ #
+ my @removed = ();
+ foreach (0 .. $length - 1) {
+ my $old;
+ my $status = $self->get($offset, $old);
+ if ($status != 0) {
+ my $msg = "error from Berkeley DB on get($offset, \$old)";
+ if ($status == 1) {
+ $msg .= ' (no such element?)';
+ }
+ else {
+ $msg .= ": error status $status";
+ if (defined $! and $! ne '') {
+ $msg .= ", message $!";
+ }
+ }
+ die $msg;
+ }
+ push @removed, $old;
+
+ $status = $self->del($offset);
+ if ($status != 0) {
+ my $msg = "error from Berkeley DB on del($offset)";
+ if ($status == 1) {
+ $msg .= ' (no such element?)';
+ }
+ else {
+ $msg .= ": error status $status";
+ if (defined $! and $! ne '') {
+ $msg .= ", message $!";
+ }
+ }
+ die $msg;
+ }
+
+ -- $num_elems;
+ }
+
+ # ...'and replaces them with the elements of LIST, if any.'
+ my $pos = $offset;
+ while (defined (my $elem = shift @list)) {
+ my $old_pos = $pos;
+ my $status;
+ if ($pos >= $num_elems) {
+ $status = $self->put($pos, $elem);
+ }
+ else {
+ $status = $self->put($pos, $elem, $self->R_IBEFORE);
+ }
+
+ if ($status != 0) {
+ my $msg = "error from Berkeley DB on put($pos, $elem, ...)";
+ if ($status == 1) {
+ $msg .= ' (no such element?)';
+ }
+ else {
+ $msg .= ", error status $status";
+ if (defined $! and $! ne '') {
+ $msg .= ", message $!";
+ }
+ }
+ die $msg;
+ }
+
+ die "pos unexpectedly changed from $old_pos to $pos with R_IBEFORE"
+ if $old_pos != $pos;
+
+ ++ $pos;
+ ++ $num_elems;
+ }
+
+ if (wantarray) {
+ # 'In list context, returns the elements removed from the
+ # array.'
+ #
+ return @removed;
+ }
+ elsif (defined wantarray and not wantarray) {
+ # 'In scalar context, returns the last element removed, or
+ # undef if no elements are removed.'
+ #
+ if (@removed) {
+ my $last = pop @removed;
+ return "$last";
+ }
+ else {
+ return undef;
+ }
+ }
+ elsif (not defined wantarray) {
+ # Void context
+ }
+ else { die }
+}
+sub ::DB_File::splice { &SPLICE }
+
sub find_dup
{
croak "Usage: \$db->find_dup(key,value)\n"
=head1 SYNOPSIS
- use DB_File ;
-
+ 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 ;
$X->push(list);
$a = $X->shift;
$X->unshift(list);
+ @r = $X->splice(offset, length, elements);
+
+ # 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 ;
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>). 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.
+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
+=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. In this case the interface is
+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 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.
+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, use the Perl module B<BerkeleyDB> instead.
+2.x or greater, 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 rebuilt DB_File to use Berkeley DB version 2, your
+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 either the C<db_dump> or 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 of Berkeley DB with
-DB_File.
+Please read L<"COPYRIGHT"> before using version 2.x or 3.x of Berkeley
+DB with DB_File.
=head2 Interface to Berkeley DB
database, delete keys/value pairs and finally how to enumerate the
contents of the database.
+ use warnings ;
use strict ;
use DB_File ;
use vars qw( %h $k $v ) ;
- tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
+ unlink "fruit" ;
+ tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0666, $DB_HASH
or die "Cannot open file 'fruit': $!\n";
# Add a few key/value pairs to the file
here is the output:
Banana Exists
-
+
orange -> orange
tomato -> red
banana -> yellow
BTREE uses. Instead of using the normal lexical ordering, a case
insensitive compare function will be used.
+ use warnings ;
use strict ;
use DB_File ;
# specify the Perl sub that will do the comparison
$DB_BTREE->{'compare'} = \&Compare ;
- tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE
+ unlink "tree" ;
+ tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open file 'tree': $!\n" ;
# Add a key/value pair to the file
you must use the same compare function every time you access the
database.
+=item 3
+
+Duplicate keys are entirely defined by the comparison function.
+In the case-insensitive example above, the keys: 'KEY' and 'key'
+would be considered duplicates, and assigning to the second one
+would overwrite the first. If duplicates are allowed for (with the
+R_DUPS flag discussed below), only a single copy of duplicate keys
+is stored in the database --- so (again with example above) assigning
+three values to the keys: 'KEY', 'Key', and 'key' would leave just
+the first key: 'KEY' in the database with three values. For some
+situations this results in information loss, so care should be taken
+to provide fully qualified comparison functions when necessary.
+For example, the above comparison routine could be modified to
+additionally compare case-sensitively if two keys are equal in the
+case insensitive comparison:
+
+ sub compare {
+ my($key1, $key2) = @_;
+ lc $key1 cmp lc $key2 ||
+ $key1 cmp $key2;
+ }
+
+And now you will only have duplicates when the keys themselves
+are truly the same. (note: in versions of the db library prior to
+about November 1996, such duplicate keys were retained so it was
+possible to recover the original keys in sets of keys that
+compared as equal).
+
+
=back
=head2 Handling Duplicate Keys
want to manipulate a BTREE database with duplicate keys. Consider this
code:
+ use warnings ;
use strict ;
use DB_File ;
$filename = "tree" ;
unlink $filename ;
-
+
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
-
- tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+
+ tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open $filename: $!\n";
-
+
# Add some key/value pairs to the file
$h{'Wall'} = 'Larry' ;
$h{'Wall'} = 'Brick' ; # Note the duplicate key
# iterate through the associative array
# and print each key/value pair.
- foreach (keys %h)
+ foreach (sort keys %h)
{ print "$_ -> $h{$_}\n" }
untie %h ;
Here is the script above rewritten using the C<seq> API method.
+ use warnings ;
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
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $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 ;
$status == 0 ;
$status = $x->seq($key, $value, R_NEXT) )
{ print "$key -> $value\n" }
-
+
undef $x ;
untie %h ;
So assuming the database created above, we can use C<get_dup> like
this:
+ use warnings ;
+ use strict ;
+ use DB_File ;
+
+ use vars qw($filename $x %h ) ;
+
+ $filename = "tree" ;
+
+ # Enable duplicate records
+ $DB_BTREE->{'flags'} = R_DUP ;
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $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") ;
print "Smith => [@list]\n" ;
-
+
@list = $x->get_dup("Dog") ;
print "Dog => [@list]\n" ;
$status = $X->find_dup($key, $value) ;
-This method checks for the existance of a specific key/value pair. If the
+This method checks for the existence of a specific key/value pair. If the
pair exists, the cursor is left pointing to the pair and the method
returns 0. Otherwise the method returns a non-zero value.
Assuming the database from the previous example:
+ use warnings ;
use strict ;
use DB_File ;
-
+
use vars qw($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
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $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 $x ;
untie %h ;
prints this
- Larry Wall is there
+ Larry Wall is there
Harry Wall is not there
0 if they exist and have been deleted successfully.
Otherwise the method returns a non-zero value.
-Again assuming the existance of the C<tree> database
+Again assuming the existence of the C<tree> database
+ use warnings ;
use strict ;
use DB_File ;
-
+
use vars qw($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
+
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $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 ;
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 warnings ;
use strict ;
use DB_File ;
use Fcntl ;
$filename = "tree" ;
unlink $filename ;
- $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
+ $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $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 -> $value\n" }
-
+
+ { print "$key -> $value\n" }
+
print "\nPARTIAL MATCH\n" ;
match "Wa" ;
still have bval default to C<"\n"> for variable length records, and
space for fixed length records.
+Also note that the bval option only allows you to specify a single byte
+as a delimeter.
+
=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 warnings ;
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, 0666, $DB_RECNO
or die "Cannot open file 'text': $!\n" ;
# Add a few key/value pairs to the file
The array contains 5 entries
popped black
- unshifted white
+ shifted white
Element 1 Exists with value blue
The last element is green
The 2nd last element is yellow
Returns the number of elements in the array.
+=item B<$X-E<gt>splice(offset, length, elements);>
+
+Returns a splice of the the array.
+
=back
=head2 Another Example
described above. It also makes use of the API interface directly (see
L<THE API INTERFACE>).
+ use warnings ;
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
+ $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0666, $DB_RECNO
or die "Cannot open file $file: $!\n" ;
-
+
# first create a text file to play with
$h[0] = "zero" ;
$h[1] = "one" ;
$h[3] = "three" ;
$h[4] = "four" ;
-
+
# Print the records in order.
#
# The length method is needed here because evaluating a tied
Returns the file descriptor for the underlying database.
-See L<Locking Databases> for an example of how to make use of the
-C<fd> method to lock your database.
+See L<Locking: The Trouble with fd> for an explanation for why you should
+not use C<fd> to lock your database.
=item B<$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 "$$: Old value was $oldval\n";
- flock(DB_FH, LOCK_UN);
+If a filter has been installed with this method, it will be invoked
+every time you read a value from a DBM database.
+
+=back
- unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
- print "$$: CONTENTION; must have exclusive lock!
- Waiting for write lock ($!) ....";
- unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
- }
+You can use any combination of the methods, from none, to all four.
- print "$$: Write lock granted\n";
- $db{$key} = $value;
- $db->sync; # to flush
- sleep 10;
+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 warnings ;
+ 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 warnings ;
+ 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, 0666)
+ || 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.
I<ggh> script (available from your nearest CPAN archive in
F<authors/id/TOMC/scripts/nshist.gz>).
+ use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
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:
C<strict 'subs'> pragma (or the full strict pragma) in your script.
Consider this script:
+ use warnings ;
use strict ;
use DB_File ;
use vars qw(%x) ;
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.
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 or 2.x of
-Berkeley DB, but is limited to the functionality provided by version 1.
+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/db>. The ftp equivalent is
-F<ftp.sleepycat.com:/pub>. Both versions 1 and 2 of Berkeley DB are
-available there.
+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>.
=head1 COPYRIGHT
-Copyright (c) 1995-9 Paul Marquess. All rights reserved. This program
+Copyright (c) 1995-2001 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.
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:
+F<http://www.sleepycat.com>) regarding the license:
Do I have to license DB to use it in Perl scripts?
=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
projects / p5sagit/p5-mst-13.2.git / blobdiff