Perl Modules /DB_File.pm
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NAMEDB_File - Perl5 access to Berkeley DB version 1.x
SYNOPSISuse DB_File; [$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ; [$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ; [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ; $status = $X->del($key [, $flags]) ; $status = $X->put($key, $value [, $flags]) ; $status = $X->get($key, $value [, $flags]) ; $status = $X->seq($key, $value, $flags) ; $status = $X->sync([$flags]) ; $status = $X->fd ; # BTREE only $count = $X->get_dup($key) ; @list = $X->get_dup($key) ; %list = $X->get_dup($key, 1) ; $status = $X->find_dup($key, $value) ; $status = $X->del_dup($key, $value) ; # RECNO only $a = $X->length; $a = $X->pop ; $X->push(list); $a = $X->shift; $X->unshift(list); @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 ;
DESCRIPTIONDB_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 Using DB_File with Berkeley DB version 2 or greater). 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. DB_File provides an interface to all three of the database types currently supported by Berkeley DB. The file types are:
Using DB_File with Berkeley DB version 2 or greaterAlthough DB_File is intended to be used with Berkeley DB version 1, it can also be used with version 2, 3 or 4. In this case the interface is limited to the functionality provided by Berkeley DB 1.x. Anywhere the version 2 or greater interface differs, DB_File arranges for it to work like version 1. This feature allows DB_File scripts that were built with version 1 to be migrated to version 2 or greater 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 BerkeleyDB instead. Note: The database file format has changed multiple times in Berkeley
DB version 2, 3 and 4. If you cannot recreate your databases, you
must dump any existing databases with either the Please read COPYRIGHT before using version 2.x or greater of Berkeley DB with DB_File.
Interface to Berkeley DBDB_File allows access to Berkeley DB files using the In addition to the
Opening a Berkeley DB Database FileBerkeley DB uses the function
DB*
dbopen (const char * file, int flags, int mode,
DBTYPE type, const void * openinfo)
The parameter This interface is handled slightly differently in DB_File. Here is an equivalent call using DB_File:
tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
The In the example above $DB_HASH is actually a pre-defined reference to a hash object. DB_File has three of these pre-defined references. Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO. The keys allowed in each of these pre-defined references is limited to
the names used in the equivalent C structure. So, for example, the
$DB_HASH reference will only allow keys called To change one of these elements, just assign to it like this:
$DB_HASH->{'cachesize'} = 10000 ;
The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are usually adequate for most applications. If you do need to create extra instances of these objects, constructors are available for each file type. Here are examples of the constructors and the valid options available for DB_HASH, DB_BTREE and DB_RECNO respectively.
$a = new DB_File::HASHINFO ;
$a->{'bsize'} ;
$a->{'cachesize'} ;
$a->{'ffactor'};
$a->{'hash'} ;
$a->{'lorder'} ;
$a->{'nelem'} ;
$b = new DB_File::BTREEINFO ;
$b->{'flags'} ;
$b->{'cachesize'} ;
$b->{'maxkeypage'} ;
$b->{'minkeypage'} ;
$b->{'psize'} ;
$b->{'compare'} ;
$b->{'prefix'} ;
$b->{'lorder'} ;
$c = new DB_File::RECNOINFO ;
$c->{'bval'} ;
$c->{'cachesize'} ;
$c->{'psize'} ;
$c->{'flags'} ;
$c->{'lorder'} ;
$c->{'reclen'} ;
$c->{'bfname'} ;
The values stored in the hashes above are mostly the direct equivalent of their C counterpart. Like their C counterparts, all are set to a default values - that means you don't have to set all of the values when you only want to change one. Here is an example:
$a = new DB_File::HASHINFO ;
$a->{'cachesize'} = 12345 ;
tie %y, 'DB_File', "filename", $flags, 0777, $a ;
A few of the options need extra discussion here. When used, the C
equivalent of the keys
sub hash
{
my ($data) = @_ ;
...
# return the hash value for $data
return $hash ;
}
sub compare
{
my ($key, $key2) = @_ ;
...
# return 0 if $key1 eq $key2
# -1 if $key1 lt $key2
# 1 if $key1 gt $key2
return (-1 , 0 or 1) ;
}
sub prefix
{
my ($key, $key2) = @_ ;
...
# return number of bytes of $key2 which are
# necessary to determine that it is greater than $key1
return $bytes ;
}
See Changing the BTREE sort order for an example of using the
If you are using the DB_RECNO interface and you intend making use of
Default ParametersIt is possible to omit some or all of the final 4 parameters in the
call to
tie %A, "DB_File", "filename" ;
is equivalent to:
tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
It is also possible to omit the filename parameter as well, so the call:
tie %A, "DB_File" ;
is equivalent to:
tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
See In Memory Databases for a discussion on the use of
In Memory DatabasesBerkeley DB allows the creation of in-memory databases by using NULL
(that is, a
DB_HASHThe DB_HASH file format is probably the most commonly used of the three file formats that DB_File supports. It is also very straightforward to use.
A Simple ExampleThis 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 warnings ;
use strict ;
use DB_File ;
our (%h, $k, $v) ;
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
$h{"apple"} = "red" ;
$h{"orange"} = "orange" ;
$h{"banana"} = "yellow" ;
$h{"tomato"} = "red" ;
# Check for existence of a key
print "Banana Exists\n\n" if $h{"banana"} ;
# Delete a key/value pair.
delete $h{"apple"} ;
# print the contents of the file
while (($k, $v) = each %h)
{ print "$k -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys retrieved is in an apparently random order.
DB_BTREEThe DB_BTREE format is useful when you want to store data in a given order. By default the keys will be stored in lexical order, but as you will see from the example shown in the next section, it is very easy to define your own sorting function.
Changing the BTREE sort orderThis script shows how to override the default sorting algorithm that BTREE uses. Instead of using the normal lexical ordering, a case insensitive compare function will be used.
use warnings ;
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($key1, $key2) = @_ ;
"\L$key1" cmp "\L$key2" ;
}
# specify the Perl sub that will do the comparison
$DB_BTREE->{'compare'} = \&Compare ;
unlink "tree" ;
tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open file 'tree': $!\n" ;
# Add a key/value pair to the file
$h{'Wall'} = 'Larry' ;
$h{'Smith'} = 'John' ;
$h{'mouse'} = 'mickey' ;
$h{'duck'} = 'donald' ;
# Delete
delete $h{"duck"} ;
# Cycle through the keys printing them in order.
# Note it is not necessary to sort the keys as
# the btree will have kept them in order automatically.
foreach (keys %h)
{ print "$_\n" }
untie %h ;
Here is the output from the code above.
mouse
Smith
Wall
There are a few point to bear in mind if you want to change the ordering in a BTREE database:
Handling Duplicate KeysThe BTREE file type optionally allows a single key to be associated
with an arbitrary number of values. This option is enabled by setting
the flags element of There are some difficulties in using the tied hash interface if you want to manipulate a BTREE database with duplicate keys. Consider this code:
use warnings ;
use strict ;
use DB_File ;
our ($filename, %h) ;
$filename = "tree" ;
unlink $filename ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
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' ;
$h{'mouse'} = 'mickey' ;
# iterate through the associative array
# and print each key/value pair.
foreach (sort keys %h)
{ print "$_ -> $h{$_}\n" }
untie %h ;
Here is the output:
Smith -> John
Wall -> Larry
Wall -> Larry
Wall -> Larry
mouse -> mickey
As you can see 3 records have been successfully created with key Although it may not be immediately obvious from the code above, the associative array interface can be used to write values with duplicate keys, but it cannot be used to read them back from the database. The way to get around this problem is to use the Berkeley DB API method
called Here is the script above rewritten using the
use warnings ;
use strict ;
use DB_File ;
our ($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, 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 = $value = 0 ;
for ($status = $x->seq($key, $value, R_FIRST) ;
$status == 0 ;
$status = $x->seq($key, $value, R_NEXT) )
{ print "$key -> $value\n" }
undef $x ;
untie %h ;
that prints:
Smith -> John
Wall -> Brick
Wall -> Brick
Wall -> Larry
mouse -> mickey
This time we have got all the key/value pairs, including the multiple
values associated with the key To make life easier when dealing with duplicate keys, DB_File comes with a few utility methods.
The
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