DB_File(3) Perl Programmers Reference Guide DB_File(3)NAMEDB_File - Perl5 access to Berkeley DB version 1.x
SYNOPSIS
use DB_File ;
[$X =] tie %hash, 'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
[$X =] tie %hash, 'DB_File', $filename, $flags, $mode, $DB_BTREE ;
[$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;
$status = $X->del($key [, $flags]) ;
$status = $X->put($key, $value [, $flags]) ;
$status = $X->get($key, $value [, $flags]) ;
$status = $X->seq($key, $value, $flags) ;
$status = $X->sync([$flags]) ;
$status = $X->fd ;
# BTREE only
$count = $X->get_dup($key) ;
@list = $X->get_dup($key) ;
%list = $X->get_dup($key, 1) ;
$status = $X->find_dup($key, $value) ;
$status = $X->del_dup($key, $value) ;
# RECNO only
$a = $X->length;
$a = $X->pop ;
$X->push(list);
$a = $X->shift;
$X->unshift(list);
# DBM Filters
$old_filter = $db->filter_store_key ( sub { ... } ) ;
$old_filter = $db->filter_store_value( sub { ... } ) ;
$old_filter = $db->filter_fetch_key ( sub { ... } ) ;
$old_filter = $db->filter_fetch_value( sub { ... } ) ;
untie %hash ;
untie @array ;
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 the Using DB_File with
Berkeley DB version 2 or 3 entry elsewhere in this docu
ment). 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 inter
face closely.
Berkeley DB is a C library which provides a consistent
interface to a number of database formats. DB_File pro
vides an interface to all three of the database types cur
rently supported by Berkeley DB.
The file types are:
DB_HASH
This database type allows arbitrary key/value pairs
to be stored in data files. This is equivalent to the
functionality provided by other hashing packages like
DBM, NDBM, ODBM, GDBM, and SDBM. Remember though, the
files created using DB_HASH are not compatible with
any of the other packages mentioned.
A default hashing algorithm, which will be adequate
for most applications, is built into Berkeley DB. If
you do need to use your own hashing algorithm it is
possible to write your own in Perl and have DB_File
use it instead.
DB_BTREE
The btree format allows arbitrary key/value pairs to
be stored in a sorted, balanced binary tree.
As with the DB_HASH format, it is possible to provide
a user defined Perl routine to perform the comparison
of keys. By default, though, the keys are stored in
lexical order.
DB_RECNO
DB_RECNO allows both fixed-length and variable-length
flat text files to be manipulated using the same
key/value pair interface as in DB_HASH and DB_BTREE.
In this case the key will consist of a record (line)
number.
Using DB_File with Berkeley DB version 2 or 3
Although 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 pro
vided by Berkeley DB 1.x. Anywhere the version 2 or 3
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 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 BerkeleyDB
instead.
Note: The database file format has changed in both Berke
ley DB version 2 and 3. If you cannot recreate your
databases, you must dump any existing databases with the
"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 "db_load". Refer
to the Berkeley DB documentation for further details.
Please read the section on "COPYRIGHT" before using ver
sion 2.x or 3.x of Berkeley DB with DB_File.
Interface to Berkeley DB
DB_File allows access to Berkeley DB files using the tie()
mechanism in Perl 5 (for full details, see the tie() entry
in the perlfunc manpage). This facility allows DB_File to
access Berkeley DB files using either an associative array
(for DB_HASH & DB_BTREE file types) or an ordinary array
(for the DB_RECNO file type).
In addition to the tie() interface, it is also possible to
access most of the functions provided in the Berkeley DB
API directly. See the THE API INTERFACE entry elsewhere
in this document.
Opening a Berkeley DB Database File
Berkeley DB uses the function dbopen() to open or create a
database. Here is the C prototype for dbopen():
DB*
dbopen (const char * file, int flags, int mode,
DBTYPE type, const void * openinfo)
The parameter "type" is an enumeration which specifies
which of the 3 interface methods (DB_HASH, DB_BTREE or
DB_RECNO) is to be used. Depending on which of these is
actually chosen, the final parameter, openinfo points to a
data structure which allows tailoring of the specific
interface method.
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 "filename", "flags" and "mode" parameters are the
direct equivalent of their dbopen() counterparts. The
final parameter $DB_HASH performs the function of both the
"type" and "openinfo" parameters in dbopen().
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 struc
ture. So, for example, the $DB_HASH reference will only
allow keys called "bsize", "cachesize", "ffactor", "hash",
"lorder" and "nelem".
To change one of these elements, just assign to it like
this:
$DB_HASH->{'cachesize'} = 10000 ;
The three predefined variables $DB_HASH, $DB_BTREE and
$DB_RECNO are usually adequate for most applications. If
you do need to create extra instances of these objects,
constructors are available for each file type.
Here are examples of the constructors and the valid
options available for DB_HASH, DB_BTREE and DB_RECNO
respectively.
$a = new DB_File::HASHINFO ;
$a->{'bsize'} ;
$a->{'cachesize'} ;
$a->{'ffactor'};
$a->{'hash'} ;
$a->{'lorder'} ;
$a->{'nelem'} ;
$b = new DB_File::BTREEINFO ;
$b->{'flags'} ;
$b->{'cachesize'} ;
$b->{'maxkeypage'} ;
$b->{'minkeypage'} ;
$b->{'psize'} ;
$b->{'compare'} ;
$b->{'prefix'} ;
$b->{'lorder'} ;
$c = new DB_File::RECNOINFO ;
$c->{'bval'} ;
$c->{'cachesize'} ;
$c->{'psize'} ;
$c->{'flags'} ;
$c->{'lorder'} ;
$c->{'reclen'} ;
$c->{'bfname'} ;
The values stored in the hashes above are mostly the
direct equivalent of their C counterpart. Like their C
counterparts, all are set to a default values - that means
you don't have to set 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, $a ;
A few of the options need extra discussion here. When
used, the C equivalent of the keys "hash", "compare" and
"prefix" store pointers to C functions. In DB_File these
keys are used to store references to Perl subs. Below are
templates for each of the subs:
sub hash
{
my ($data) = @_ ;
...
# return the hash value for $data
return $hash ;
}
sub compare
{
my ($key, $key2) = @_ ;
...
# return 0 if $key1 eq $key2
# -1 if $key1 lt $key2
# 1 if $key1 gt $key2
return (-1 , 0 or 1) ;
}
sub prefix
{
my ($key, $key2) = @_ ;
...
# return number of bytes of $key2 which are
# necessary to determine that it is greater than $key1
return $bytes ;
}
See the Changing the BTREE sort order entry elsewhere in
this document for an example of using the "compare" tem
plate.
If you are using the DB_RECNO interface and you intend
making use of "bval", you should check out the The 'bval'
Option entry elsewhere in this document.
Default Parameters
It is possible to omit some or all of the final 4 parame
ters in the call to "tie" and let them take default val
ues. As DB_HASH is the most common file format used, the
call:
tie %A, "DB_File", "filename" ;
is equivalent to:
tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;
It is also possible to omit the filename parameter as
well, so the call:
tie %A, "DB_File" ;
is equivalent to:
tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
See the In Memory Databases entry elsewhere in this docu
ment for a discussion on the use of "undef" in place of a
filename.
In Memory Databases
Berkeley DB allows the creation of in-memory databases by
using NULL (that is, a "(char *)0" in C) in place of the
filename. DB_File uses "undef" instead of NULL to provide
this functionality.
DB_HASH
The 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 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 warnings ;
use strict ;
use DB_File ;
use vars qw( %h $k $v ) ;
unlink "fruit" ;
tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH
or die "Cannot open file 'fruit': $!\n";
# Add a few key/value pairs to the file
$h{"apple"} = "red" ;
$h{"orange"} = "orange" ;
$h{"banana"} = "yellow" ;
$h{"tomato"} = "red" ;
# Check for existence of a key
print "Banana Exists\n\n" if $h{"banana"} ;
# Delete a key/value pair.
delete $h{"apple"} ;
# print the contents of the file
while (($k, $v) = each %h)
{ print "$k -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order
of the keys retrieved is in an apparently random order.
DB_BTREE
The DB_BTREE format is useful when you want to store data
in a given order. By default the keys will be stored in
lexical order, but as you will see from the example shown
in the next section, it is very easy to define your own
sorting function.
Changing the BTREE sort order
This script shows how to override the default sorting
algorithm that BTREE uses. Instead of using the normal
lexical ordering, a case insensitive compare function will
be used.
use warnings ;
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($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, 0640, $DB_BTREE
or die "Cannot open file 'tree': $!\n" ;
# Add a key/value pair to the file
$h{'Wall'} = 'Larry' ;
$h{'Smith'} = 'John' ;
$h{'mouse'} = 'mickey' ;
$h{'duck'} = 'donald' ;
# Delete
delete $h{"duck"} ;
# Cycle through the keys printing them in order.
# Note it is not necessary to sort the keys as
# the btree will have kept them in order automatically.
foreach (keys %h)
{ print "$_\n" }
untie %h ;
Here is the output from the code above.
mouse
Smith
Wall
There are a few point to bear in mind if you want to
change the ordering in a BTREE database:
1. The new compare function must be specified when you
create the database.
2. You cannot change the ordering once the database has
been created. Thus you must use the same compare
function every time you access the database.
Handling Duplicate Keys
The BTREE file type optionally allows a single key to be
associated with an arbitrary number of values. This option
is enabled by setting the flags element of "$DB_BTREE" to
R_DUP when creating the database.
There are some difficulties in using the tied hash inter
face if you want to manipulate a BTREE database with
duplicate keys. Consider this code:
use warnings ;
use strict ;
use DB_File ;
use vars qw($filename %h ) ;
$filename = "tree" ;
unlink $filename ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
or die "Cannot open $filename: $!\n";
# Add some key/value pairs to the file
$h{'Wall'} = 'Larry' ;
$h{'Wall'} = 'Brick' ; # Note the duplicate key
$h{'Wall'} = 'Brick' ; # Note the duplicate key and value
$h{'Smith'} = 'John' ;
$h{'mouse'} = 'mickey' ;
# iterate through the associative array
# and print each key/value pair.
foreach (sort keys %h)
{ print "$_ -> $h{$_}\n" }
untie %h ;
Here is the output:
Smith -> John
Wall -> Larry
Wall -> Larry
Wall -> Larry
mouse -> mickey
As you can see 3 records have been successfully created
with key "Wall" - the only thing is, when they are
retrieved from the database they seem to have the same
value, namely "Larry". The problem is caused by the way
that the associative array interface works. Basically,
when the associative array interface is used to fetch the
value associated with a given key, it will only ever
retrieve the first value.
Although it may not be immediately obvious from the code
above, the associative array interface can be used to
write values with duplicate keys, but it cannot be used to
read them back from the database.
The way to get around this problem is to use the Berkeley
DB API method called "seq". This method allows sequential
access to key/value pairs. See the THE API INTERFACE entry
elsewhere in this document for details of both the "seq"
method and the API in general.
Here is the script above rewritten using the "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
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 "Wall".
To make life easier when dealing with duplicate keys,
DB_File comes with a few utility methods.
The get_dup() Method
The "get_dup" method assists in reading duplicate values
from BTREE databases. The method can take the following
forms:
$count = $x->get_dup($key) ;
@list = $x->get_dup($key) ;
%list = $x->get_dup($key, 1) ;
In a scalar context the method returns the number of val
ues associated with the key, "$key".
In list context, it returns all the values which match
"$key". Note that the values will be returned in an appar
ently random order.
In list context, if the second parameter is present and
evaluates TRUE, the method returns an associative array.
The keys of the associative array correspond to the values
that matched in the BTREE and the values of the array are
a count of the number of times that particular value
occurred in the BTREE.
So assuming the database created above, we can use
"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, 0640, $DB_BTREE
or die "Cannot open $filename: $!\n";
my $cnt = $x->get_dup("Wall") ;
print "Wall occurred $cnt times\n" ;
my %hash = $x->get_dup("Wall", 1) ;
print "Larry is there\n" if $hash{'Larry'} ;
print "There are $hash{'Brick'} Brick Walls\n" ;
my @list = sort $x->get_dup("Wall") ;
print "Wall => [@list]\n" ;
@list = $x->get_dup("Smith") ;
print "Smith => [@list]\n" ;
@list = $x->get_dup("Dog") ;
print "Dog => [@list]\n" ;
and it will print:
Wall occurred 3 times
Larry is there
There are 2 Brick Walls
Wall => [Brick Brick Larry]
Smith => [John]
Dog => []
The find_dup() Method
$status = $X->find_dup($key, $value) ;
This method checks for the existence of a specific
key/value pair. If the pair exists, the cursor is left
pointing to the pair and the method returns 0. Otherwise
the method returns a non-zero value.
Assuming the database from the previous example:
use warnings ;
use strict ;
use DB_File ;
use vars qw($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
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
The del_dup() Method
$status = $X->del_dup($key, $value) ;
This method deletes a specific key/value pair. It returns
0 if they exist and have been deleted successfully. Oth
erwise the method returns a non-zero value.
Again assuming the existence of the "tree" database
use warnings ;
use strict ;
use DB_File ;
use vars qw($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
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
Matching Partial Keys
The BTREE interface has a feature which allows partial
keys to be matched. This functionality is only available
when the "seq" method is used along with the R_CURSOR
flag.
$x->seq($key, $value, R_CURSOR) ;
Here is the relevant quote from the dbopen man page where
it defines the use of the R_CURSOR flag with seq:
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
In the example script below, the "match" sub uses this
feature to find and print the first matching key/value
pair given a partial key.
use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
use vars qw($filename $x %h $st $key $value) ;
sub match
{
my $key = shift ;
my $value = 0;
my $orig_key = $key ;
$x->seq($key, $value, R_CURSOR) ;
print "$orig_key\t-> $key\t-> $value\n" ;
}
$filename = "tree" ;
unlink $filename ;
$x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
or die "Cannot open $filename: $!\n";
# Add some key/value pairs to the file
$h{'mouse'} = 'mickey' ;
$h{'Wall'} = 'Larry' ;
$h{'Walls'} = 'Brick' ;
$h{'Smith'} = 'John' ;
$key = $value = 0 ;
print "IN ORDER\n" ;
for ($st = $x->seq($key, $value, R_FIRST) ;
$st == 0 ;
$st = $x->seq($key, $value, R_NEXT) )
{ print "$key -> $value\n" }
print "\nPARTIAL MATCH\n" ;
match "Wa" ;
match "A" ;
match "a" ;
undef $x ;
untie %h ;
Here is the output:
IN ORDER
Smith -> John
Wall -> Larry
Walls -> Brick
mouse -> mickey
PARTIAL MATCH
Wa -> Wall -> Larry
A -> Smith -> John
a -> mouse -> mickey
DB_RECNO
DB_RECNO provides an interface to flat text files. Both
variable and fixed length records are supported.
In order to make RECNO more compatible with Perl, the
array offset for all RECNO arrays begins at 0 rather than
1 as in Berkeley DB.
As with normal Perl arrays, a RECNO array can be accessed
using negative indexes. The index -1 refers to the last
element of the array, -2 the second last, and so on.
Attempting to access an element before the start of the
array will raise a fatal run-time error.
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 ""\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 doc
umentation error will be fixed in the next release of
Berkeley DB.
That clarifies the situation with regards Berkeley DB
itself. What about DB_File? Well, the behavior defined in
the quote above is quite useful, so DB_File conforms to
it.
That means that you can specify other options (e.g. cache
size) and still have bval default to ""\n"" for variable
length records, and space for fixed length records.
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 the Extra RECNO Methods entry elsewhere in
this document for a workaround).
use warnings ;
use strict ;
use DB_File ;
my $filename = "text" ;
unlink $filename ;
my @h ;
tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_RECNO
or die "Cannot open file 'text': $!\n" ;
# Add a few key/value pairs to the file
$h[0] = "orange" ;
$h[1] = "blue" ;
$h[2] = "yellow" ;
push @h, "green", "black" ;
my $elements = scalar @h ;
print "The array contains $elements entries\n" ;
my $last = pop @h ;
print "popped $last\n" ;
unshift @h, "white" ;
my $first = shift @h ;
print "shifted $first\n" ;
# Check for existence of a key
print "Element 1 Exists with value $h[1]\n" if $h[1] ;
# use a negative index
print "The last element is $h[-1]\n" ;
print "The 2nd last element is $h[-2]\n" ;
untie @h ;
Here is the output from the script:
The array contains 5 entries
popped black
shifted white
Element 1 Exists with value blue
The last element is green
The 2nd last element is yellow
Extra RECNO Methods
If you are using a version of Perl earlier than 5.004_57,
the tied array interface is quite limited. In the example
script above "push", "pop", "shift", "unshift" or deter
mining 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 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:
$X->push(list) ;
Pushes the elements of "list" to the end of the
array.
$value = $X->pop ;
Removes and returns the last element of the array.
$X->shift
Removes and returns the first element of the array.
$X->unshift(list) ;
Pushes the elements of "list" to the start of the
array.
$X->length
Returns the number of elements in the array.
Another Example
Here is a more complete example that makes use of some of
the methods described above. It also makes use of the API
interface directly (see the THE API INTERFACE entry else
where in this document).
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
or die "Cannot open file $file: $!\n" ;
# first create a text file to play with
$h[0] = "zero" ;
$h[1] = "one" ;
$h[2] = "two" ;
$h[3] = "three" ;
$h[4] = "four" ;
# Print the records in order.
#
# The length method is needed here because evaluating a tied
# array in a scalar context does not return the number of
# elements in the array.
print "\nORIGINAL\n" ;
foreach $i (0 .. $H->length - 1) {
print "$i: $h[$i]\n" ;
}
# use the push & pop methods
$a = $H->pop ;
$H->push("last") ;
print "\nThe last record was [$a]\n" ;
# and the shift & unshift methods
$a = $H->shift ;
$H->unshift("first") ;
print "The first record was [$a]\n" ;
# Use the API to add a new record after record 2.
$i = 2 ;
$H->put($i, "Newbie", R_IAFTER) ;
# and a new record before record 1.
$i = 1 ;
$H->put($i, "New One", R_IBEFORE) ;
# delete record 3
$H->del(3) ;
# now print the records in reverse order
print "\nREVERSE\n" ;
for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
{ print "$i: $h[$i]\n" }
# same again, but use the API functions instead
print "\nREVERSE again\n" ;
my ($s, $k, $v) = (0, 0, 0) ;
for ($s = $H->seq($k, $v, R_LAST) ;
$s == 0 ;
$s = $H->seq($k, $v, R_PREV))
{ print "$k: $v\n" }
undef $H ;
untie @h ;
and this is what it outputs:
ORIGINAL
0: zero
1: one
2: two
3: three
4: four
The last record was [four]
The first record was [zero]
REVERSE
5: last
4: three
3: Newbie
2: one
1: New One
0: first
REVERSE again
5: last
4: three
3: Newbie
2: one
1: New One
0: first
Notes:
1. Rather than iterating through the array, "@h" like
this:
foreach $i (@h)
it is necessary to use either this:
foreach $i (0 .. $H->length - 1)
or this:
for ($a = $H->get($k, $v, R_FIRST) ;
$a == 0 ;
$a = $H->get($k, $v, R_NEXT) )
2. Notice that both times the "put" method was used the
record index was specified using a variable, "$i",
rather than the literal value itself. This is because
"put" will return the record number of the inserted
line via that parameter.
THE API INTERFACE
As well as accessing Berkeley DB using a tied hash or
array, it is also possible to make direct use of most of
the API functions defined in the Berkeley DB documenta
tion.
To do this you need to store a copy of the object returned
from the tie.
$db = tie %hash, "DB_File", "filename" ;
Once you have done that, you can access the Berkeley DB
API functions as DB_File methods directly like this:
$db->put($key, $value, R_NOOVERWRITE) ;
Important: If you have saved a copy of the object returned
from "tie", the underlying database file will not be
closed until both the tied variable is untied and all
copies of the saved object are destroyed.
use DB_File ;
$db = tie %hash, "DB_File", "filename"
or die "Cannot tie filename: $!" ;
...
undef $db ;
untie %hash ;
See the The untie() Gotcha entry elsewhere in this docu
ment for more details.
All the functions defined in the dbopen manpage are avail
able except for close() and dbopen() itself. The DB_File
method interface to the supported functions have been
implemented to mirror the way Berkeley DB works whenever
possible. In particular note that:
The methods return a status value. All return 0 on
success. All return -1 to signify an error and set
"$!" to the exact error code. The return code 1 gen
erally (but not always) means that the key specified
did not exist in the database.
Other return codes are defined. See below and in the
Berkeley DB documentation for details. The Berkeley
DB documentation should be used as the definitive
source.
Whenever a Berkeley DB function returns data via one
of its parameters, the equivalent DB_File method does
exactly the same.
If you are careful, it is possible to mix API calls
with the tied hash/array interface in the same piece
of code. Although only a few of the methods used to
implement the tied interface currently make use of
the cursor, you should always assume that the cursor
has been changed any time the tied hash/array inter
face is used. As an example, this code will probably
not do what you expect:
$X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
or die "Cannot tie $filename: $!" ;
# Get the first key/value pair and set the cursor
$X->seq($key, $value, R_FIRST) ;
# this line will modify the cursor
$count = scalar keys %x ;
# Get the second key/value pair.
# oops, it didn't, it got the last key/value pair!
$X->seq($key, $value, R_NEXT) ;
The code above can be rearranged to get around the
problem, like this:
$X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
or die "Cannot tie $filename: $!" ;
# this line will modify the cursor
$count = scalar keys %x ;
# Get the first key/value pair and set the cursor
$X->seq($key, $value, R_FIRST) ;
# Get the second key/value pair.
# worked this time.
$X->seq($key, $value, R_NEXT) ;
All the constants defined in the dbopen manpage for use in
the flags parameters in the methods defined below are also
available. Refer to the Berkeley DB documentation for the
precise meaning of the flags values.
Below is a list of the methods available.
$status = $X->get($key, $value [, $flags]) ;
Given a key ("$key") this method reads the value
associated with it from the database. The value read
from the database is returned in the "$value" parame
ter.
If the key does not exist the method returns 1.
No flags are currently defined for this method.
$status = $X->put($key, $value [, $flags]) ;
Stores the key/value pair in the database.
If you use either the R_IAFTER or R_IBEFORE flags,
the "$key" parameter will have the record number of
the inserted key/value pair set.
Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE,
R_NOOVERWRITE and R_SETCURSOR.
$status = $X->del($key [, $flags]) ;
Removes all key/value pairs with key "$key" from the
database.
A return code of 1 means that the requested key was
not in the database.
R_CURSOR is the only valid flag at present.
$status = $X->fd ;
Returns the file descriptor for the underlying
database.
See the Locking: The Trouble with fd entry elsewhere
in this document for an explanation for why you
should not use "fd" to lock your database.
$status = $X->seq($key, $value, $flags) ;
This interface allows sequential retrieval from the
database. See the dbopen manpage for full details.
Both the "$key" and "$value" parameters will be set
to the key/value pair read from the database.
The flags parameter is mandatory. The valid flag val
ues are R_CURSOR, R_FIRST, R_LAST, R_NEXT and R_PREV.
$status = $X->sync([$flags]) ;
Flushes any cached buffers to disk.
R_RECNOSYNC is the only valid flag at present.
DBM FILTERS
A DBM Filter is a piece of code that is be used when you
always want to make the same transformation to all keys
and/or values in a DBM database.
There are four methods associated with DBM Filters. All
work identically, and each is used to install (or unin
stall) a single DBM Filter. Each expects a single parame
ter, namely a reference to a sub. The only difference
between them is the place that the filter is installed.
To summarise:
filter_store_key
If a filter has been installed with this method, it
will be invoked every time you write a key to a DBM
database.
filter_store_value
If a filter has been installed with this method, it
will be invoked every time you write a value to a DBM
database.
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.
filter_fetch_value
If a filter has been installed with this method, it
will be invoked every time you read a value from a
DBM database.
You can use any combination of the methods, from none, to
all four.
All filter methods return the existing filter, if present,
or "undef" in not.
To delete a filter pass "undef" to it.
The Filter
When each filter is called by Perl, a local copy of "$_"
will contain the key or value to be filtered. Filtering is
achieved by modifying the contents of "$_". The return
code from the filter is ignored.
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 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 some
thing 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 ter
minations 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 termi
nating NULL, and both "store" filters add a terminating
NULL.
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 "pack" when writing, and "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.
HINTS AND TIPS
Locking: The Trouble with fd
Until version 1.72 of this module, the recommended tech
nique for locking DB_File databases was to flock the file
handle 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);
In simple terms, this is what happens:
1. Use "tie" to open the database.
2. Lock the database with fd & flock.
3. Read & Write to the database.
4. Unlock and close the database.
Here is the crux of the problem. A side-effect of opening
the 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 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 modi
fies 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 DB_File database, but it does illustrate why the
technique should not be used.
Safe ways to lock a database
Starting with version 2.x, Berkeley DB has internal sup
port for locking. The companion module to this one,
BerkeleyDB, provides an interface to this locking func
tionality. If you are serious about locking Berkeley DB
databases, I strongly recommend using BerkeleyDB.
If using 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:
Tie::DB_Lock
A 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.
Tie::DB_LockFile
A 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 docu
mentation.
DB_File::Lock
An extremely lightweight 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 dead
lock problems, if desired. Use for databases where
updates are reads are quick and simple flock locking
semantics are enough.
Sharing Databases With C Applications
There is no technical reason why a Berkeley DB database
cannot be shared by both a Perl and a C application.
The vast majority of problems that are reported in this
area boil down to the fact that C strings are NULL termi
nated, whilst Perl strings are not. See the DBM FILTERS
entry elsewhere in this document for a generic way to work
around this problem.
Here is a real example. Netscape 2.0 keeps a record of the
locations you visit along with the time you last visited
them in a DB_HASH database. This is usually stored in the
file ~/.netscape/history.db. The key field in the database
is the location string and the value field is the time the
location was last visited stored as a 4 byte binary value.
If you haven't already guessed, the location string is
stored with a terminating NULL. This means you need to be
careful when accessing the database.
Here is a snippet of code that is loosely based on Tom
Christiansen's ggh script (available from your nearest
CPAN archive in authors/id/TOMC/scripts/nshist.gz).
use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
$dotdir = $ENV{HOME} || $ENV{LOGNAME};
$HISTORY = "$dotdir/.netscape/history.db";
tie %hist_db, 'DB_File', $HISTORY
or die "Cannot open $HISTORY: $!\n" ;;
# Dump the complete database
while ( ($href, $binary_time) = each %hist_db ) {
# remove the terminating NULL
$href =~ s/\x00$// ;
# convert the binary time into a user friendly string
$date = localtime unpack("V", $binary_time);
print "$date $href\n" ;
}
# check for the existence of a specific key
# remember to add the NULL
if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
$date = localtime unpack("V", $binary_time) ;
print "Last visited mox.perl.com on $date\n" ;
}
else {
print "Never visited mox.perl.com\n"
}
untie %hist_db ;
The untie() Gotcha
If you make use of the Berkeley DB API, it is very
strongly recommended that you read the The untie Gotcha
entry in the perltie manpage.
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
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: $!" ;
untie %x ;
When run, the script will produce this error message:
Cannot tie second time: Invalid argument at bad.file line 14.
Although the error message above refers to the second
tie() statement in the script, the source of the problem
is really with the untie() statement that precedes it.
Having read the perltie manpage you will probably have
already guessed that the error is caused by the extra copy
of the tied object stored in "$X". If you haven't, then
the problem boils down to the fact that the DB_File
destructor, DESTROY, will not be called until all refer
ences to the tied object are destroyed. Both the tied
variable, "%x", and "$X" above hold a reference to the
object. The call to untie() will destroy the first, but
"$X" still holds a valid reference, so the destructor will
not get called and the database file tst.fil will remain
open. The fact that Berkeley DB then reports the 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 "-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
...
COMMON QUESTIONS
Why is there Perl source in my database?
If you look at the contents of a database file created by
DB_File, there can sometimes be part of a Perl script
included in it.
This happens because Berkeley DB uses dynamic memory to
allocate buffers which will subsequently be written to the
database file. Being dynamic, the memory could have been
used for anything before DB malloced it. As Berkeley DB
doesn't clear the memory once it has been allocated, the
unused portions will contain random junk. In the case
where a Perl script gets written to the database, the ran
dom junk will correspond to an area of dynamic memory that
happened to be used during the compilation of the script.
Unless you don't like the possibility of there being part
of your Perl scripts embedded in a database file, this is
nothing to worry about.
How do I store complex data structures with DB_File?
Although DB_File cannot do this directly, there is a mod
ule which can layer transparently over DB_File to accom
plish this feat.
Check out the MLDBM module, available on CPAN in the
directory modules/by-module/MLDBM.
What does "Invalid Argument" mean?
You will get this error message when one of the parameters
in the "tie" call is wrong. Unfortunately there are quite
a few parameters to get wrong, so it can be difficult to
figure out which one it is.
Here are a couple of possibilities:
1. Attempting to reopen a database without closing it.
2. Using the O_WRONLY flag.
What does "Bareword 'DB_File' not allowed" mean?
You will encounter this particular error message when you
have the "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) ;
tie %x, DB_File, "filename" ;
Running it produces the error in question:
Bareword "DB_File" not allowed while "strict subs" in use
To get around the error, place the word "DB_File" in
either single or double quotes, like this:
tie %x, "DB_File", "filename" ;
Although it might seem like a real pain, it is really
worth the effort of having a "use strict" in all your
scripts.
REFERENCES
Articles that are either about DB_File or make use of it.
1. Full-Text Searching in Perl, Tim Kientzle (tkient
zle@ddj.com), Dr. Dobb's Journal, Issue 295, January
1999, pp 34-41
HISTORY
Moved to the Changes file.
BUGS
Some older versions of Berkeley DB had problems with fixed
length records using the RECNO file format. This problem
has been fixed since version 1.85 of Berkeley DB.
I am sure there are bugs in the code. If you do find any,
or can suggest any enhancements, I would welcome your com
ments.
AVAILABILITYDB_File comes with the standard Perl source distribution.
Look in the directory 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 the CPAN entry in
the perlmod manpage for details), in the directory mod_
ules/by-module/DB_File.
This version of DB_File will work with either version 1.x,
2.x or 3.x of Berkeley DB, but is limited to the function
ality provided by version 1.
The official web site for Berkeley DB is http://www.sleep_
ycat.com. All versions of Berkeley DB are available
there.
Alternatively, Berkeley DB version 1 is available at your
nearest CPAN archive in src/misc/db.1.85.tar.gz.
If you are running IRIX, then get Berkeley DB version 1
from http://reality.sgi.com/ariel. It has the patches nec
essary to compile properly on IRIX 5.3.
COPYRIGHT
Copyright (c) 1995-1999 Paul Marquess. All rights
reserved. This program is free software; you can redis
tribute it and/or modify it under the same terms as Perl
itself.
Although DB_File is covered by the Perl license, the
library it makes use of, namely Berkeley DB, is not.
Berkeley DB has its own copyright and its own license.
Please take the time to read it.
Here are are few words taken from the Berkeley DB FAQ (at
http://www.sleepycat.com) regarding the license:
Do I have to license DB to use it in Perl scripts?
No. The Berkeley DB license requires that software that uses
Berkeley DB be freely redistributable. In the case of Perl, that
software is Perl, and not your scripts. Any Perl scripts that you
write are your property, including scripts that make use of
Berkeley DB. Neither the Perl license nor the Berkeley DB license
place any restriction on what you may do with them.
If you are in any doubt about the license situation, con
tact either the Berkeley DB authors or the author of
DB_File. See the section on "AUTHOR" for details.
SEE ALSOperl(1), dbopen(3), hash(3), recno(3), btree(3), the dbm
filter manpage
AUTHOR
The DB_File interface was written by Paul Marquess
<Paul.Marquess@btinternet.com>. Questions about the DB
system itself may be addressed to <db@sleepycat.com<gt>.
2001-03-18 perl v5.6.1 DB_File(3)
