DB_File(3) DB_File(3)
DB_File - Perl5 access to Berkeley DB version 1.x
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) ;
# RECNO only
$a = $X->length;
$a = $X->pop ;
$X->push(list);
$a = $X->shift;
$X->unshift(list);
untie %hash ;
untie @array ;
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 the section on Using DB_File with Berkeley DB version
2). It is assumed that you have a copy of the Berkeley DB manual pages at
hand when reading this documentation. The interface defined here mirrors
the Berkeley DB interface closely.
Berkeley DB is a C library which provides a consistent interface to a
number of database formats. DB_File provides an interface to all three
of the database types currently supported by Berkeley DB.
The file types are:
DB_HASH [Toc] [Back]
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
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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 [Toc] [Back]
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 [Toc] [Back]
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
Although DB_File is intended to be used with Berkeley DB version 1, it
can also be used with version 2. In this case the interface is limited to
the functionality provided by Berkeley DB 1.x. Anywhere the version 2
interface differs, 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 without any changes.
If you want to make use of the new features available in Berkeley DB 2.x,
use the Perl module BerkeleyDB instead.
At the time of writing this document the 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.
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 db_dump185 utility that comes with Berkeley DB. Once you have
upgraded DB_File to use Berkeley DB version 2, your databases can be
recreated using db_load. Refer to the Berkeley DB documentation for
further details.
Please read the COPYRIGHT manpage before using version 2.x of Berkeley DB
with DB_File.
Interface to Berkeley DB [Toc] [Back]
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DB_File(3) DB_File(3)
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 section
on THE API INTERFACE.
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 structure. 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.
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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 $hash ;
}
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DB_File(3) DB_File(3)
sub compare
{
my ($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 section on Changing the BTREE sort order for an example of using
the compare template.
If you are using the DB_RECNO interface and you intend making use of
bval, you should check out the section on The 'bval' Option.
Default Parameters [Toc] [Back]
It is possible to omit some or all of the final 4 parameters in the call
to tie and let them take default values. 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 section on In Memory Databases for a discussion on the use of
undef in place of a filename.
In Memory Databases
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DB_File(3) DB_File(3)
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.
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 [Toc] [Back]
This example shows how to create a database, add key/value pairs to the
database, delete keys/value pairs and finally how to enumerate the
contents of the database.
use strict ;
use DB_File ;
use vars qw( %h $k $v ) ;
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 -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys
retrieved is in an apparently random order.
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DB_File(3) DB_File(3)
The DB_BTREE format is useful when you want to store data in a given
order. By default the keys will be stored in lexical order, but as you
will see from the example shown in the next section, it is very easy to
define your own sorting function.
Changing the BTREE sort order
This script shows how to override the default sorting algorithm that
BTREE uses. Instead of using the normal lexical ordering, a case
insensitive compare function will be used.
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($key1, $key2) = @_ ;
"\L$key1" cmp "\L$key2" ;
}
# specify the Perl sub that will do the comparison
$DB_BTREE->{'compare'} = \&Compare ;
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
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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 $DB_BTREE to R_DUP when creating the database.
There are some difficulties in using the tied hash interface if you want
to manipulate a BTREE database with duplicate keys. Consider this code:
use strict ;
use DB_File ;
use vars qw($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' ;
$h{'mouse'} = 'mickey' ;
# iterate through the associative array
# and print each key/value pair.
foreach (keys %h)
{ print "$_ -> $h{$_}\n" }
untie %h ;
Here is the output:
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DB_File(3) DB_File(3)
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 section on THE API INTERFACE for details of both the seq method and
the API in general.
Here is the script above rewritten using the seq API method.
use strict ;
use DB_File ;
use vars qw($filename $x %h $status $key $value) ;
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$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.
The get_dup() Method
DB_File comes with a utility method, called get_dup, to assist 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 values associated
with the key, $key.
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DB_File(3) DB_File(3)
In list context, it returns all the values which match $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 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:
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 = $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 => []
Matching Partial Keys [Toc] [Back]
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, $value, R_CURSOR) ;
Here is the relevant quote from the dbopen man page where it defines the
use of the R_CURSOR flag with seq:
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
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In the example script below, the match sub uses this feature to find and
print the first matching key/value pair given a partial key.
use strict ;
use DB_File ;
use Fcntl ;
use vars qw($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:
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DB_File(3) DB_File(3)
IN ORDER
Smith -> John
Wall -> Larry
Walls -> Brick
mouse -> mickey
PARTIAL MATCH
Wa -> Wall -> Larry
A -> Smith -> John
a -> mouse -> mickey
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 character
for fixed-length records. If no value is specified,
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 documentation 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 it.
That means that you can specify other options (e.g. cachesize) and still
have bval default to "\n" for variable length records, and space for
fixed length records.
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A Simple Example [Toc] [Back]
Here is a simple example that uses RECNO.
use strict ;
use DB_File ;
my @h ;
tie @h, "DB_File", "text", 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" ;
# 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:
Element 1 Exists with value blue
The last element is yellow
The 2nd last element is blue
Extra Methods [Toc] [Back]
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 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.
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.
$X->shift
Removes and returns the first element of the array.
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$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 [Toc] [Back]
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
section on THE API INTERFACE).
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" ;
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# 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
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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 documentation.
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.
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use DB_File ;
$db = tie %hash, "DB_File", "filename"
or die "Cannot tie filename: $!" ;
...
undef $db ;
untie %hash ;
See the section on The untie() Gotcha for more details.
All the functions defined in the dbopen manpage are available 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:
o 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 generally (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.
o Whenever a Berkeley DB function returns data via one of its
parameters, the equivalent DB_File method does exactly the same.
o 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 interface 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: $!" ;
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DB_File(3) DB_File(3)
# 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 parameter.
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 section on Locking Databases for an example of how to make
use of the fd method to lock your database.
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DB_File(3) DB_File(3)
$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 values 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.
Locking Databases
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 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.
use DB_File;
use strict;
sub LOCK_SH { 1 }
sub LOCK_EX { 2 }
sub LOCK_NB { 4 }
sub LOCK_UN { 8 }
my($oldval, $fd, $db, %db, $value, $key);
$key = shift || 'default';
$value = shift || 'magic';
$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 $!";
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";
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DB_File(3) DB_File(3)
$oldval = $db{$key};
print "$$: Old value was $oldval\n";
flock(DB_FH, LOCK_UN);
unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {
print "$$: CONTENTION; must have exclusive lock!
Waiting for write lock ($!) ....";
unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
}
print "$$: Write lock granted\n";
$db{$key} = $value;
$db->sync; # to flush
sleep 10;
flock(DB_FH, LOCK_UN);
undef $db;
untie %db;
close(DB_FH);
print "$$: Updated db to $key=$value\n";
Sharing Databases With C Applications [Toc] [Back]
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 terminated, whilst Perl strings are not.
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 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";
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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 section on The untie Gotcha 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 ;
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DB_File(3) DB_File(3)
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 references 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 alreday
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
...
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
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DB_File(3) DB_File(3)
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 random 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 module which can
layer transparently over DB_File to accomplish 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 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.
Page 24
DB_File(3) DB_File(3)
Moved to the Changes file.
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 comments.
DB_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 modules/by-
module/DB_File.
This version of 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.
The official web site for Berkeley DB is http://www.sleepycat.com/db. The
ftp equivalent is ftp.sleepycat.com:/pub. Both versions 1 and 2 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 necessary to compile
properly on IRIX 5.3.
Copyright (c) 1997 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 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.
The license for Berkeley DB version 2, and how it relates to DB_File does
need some extra clarification. Here are are few words taken from the
Berkeley DB FAQ regarding the version 2 license:
The major difference is that the license for DB 2.0, when
downloaded from the net, requires that the software that
uses DB 2.0 be freely redistributable.
That means that if you want to use DB_File, and you have changed either
the source for Berkeley DB or Perl, then the changes must be freely
Page 25
DB_File(3) DB_File(3)
available.
In the case of Perl, the term source refers to the complete source code
for Perl (e.g. sv.c, toke.c, perl.h) and any external modules that you
are using (e.g. DB_File, Tk).
Note that any Perl scripts that you write are your property - this
includes scripts that make use of DB_File. Neither the Perl license or
the Berkeley DB license place any restriction on what you have to do with
them.
If you are in any doubt about the license situation, contact either the
Berkeley DB authors or the author of DB_File. See the section on AUTHOR
for details.
the perl(1) manpage, the dbopen(3) manpage, the hash(3) manpage, the
recno(3) manpage, the btree(3) manpage
The DB_File interface was written by Paul Marquess
<[email protected]>. Questions about the DB system itself may be
addressed to <[email protected]<gt>.
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DB_File(3) DB_File(3)
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