DB_File(3p) Perl Programmers Reference Guide DB_File(3p) #
DB_File(3p) Perl Programmers Reference Guide DB_File(3p)
NNAAMMEE #
DB_File - Perl5 access to Berkeley DB version 1.x
SSYYNNOOPPSSIISS #
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);
@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 ;
DDEESSCCRRIIPPTTIIOONN #
DDBB__FFiillee 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. DDBB__FFiillee provides an interface to all three
of the database types currently supported by Berkeley DB.
The file types are:
DDBB__HHAASSHH #
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 DDBB__FFiillee use it instead.
DDBB__BBTTRREEEE #
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.
DDBB__RREECCNNOO #
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.
UUssiinngg DDBB__FFiillee wwiitthh BBeerrkkeelleeyy DDBB vveerrssiioonn 22 oorr ggrreeaatteerr Although DDBB__FFiillee 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, DDBB__FFiillee arranges for it to work like version 1. This feature allows DDBB__FFiillee 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
<https://metacpan.org/pod/BerkeleyDB> instead.
NNoottee:: 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 "db_dump" or the "db_dump185"
utility that comes with Berkeley DB. Once you have rebuilt DB_File to use
Berkeley DB version 2 or greater, your databases can be recreated using
"db_load". Refer to the Berkeley DB documentation for further details.
Please read "COPYRIGHT" before using version 2.x or greater of Berkeley
DB with DB_File.
IInntteerrffaaccee ttoo BBeerrkkeelleeyy DDBB DDBB__FFiillee allows access to Berkeley DB files using the ttiiee(()) mechanism in Perl 5 (for full details, see “ttiiee(())” in perlfunc). This facility allows DDBB__FFiillee 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 ttiiee(()) interface, it is also possible to access most of
the functions provided in the Berkeley DB API directly. See "THE API
INTERFACE". #
OOppeenniinngg aa BBeerrkkeelleeyy DDBB DDaattaabbaassee FFiillee Berkeley DB uses the function ddbbooppeenn(()) to open or create a database. Here is the C prototype for ddbbooppeenn(()):
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,
_o_p_e_n_i_n_f_o points to a data structure which allows tailoring of the
specific interface method.
This interface is handled slightly differently in DDBB__FFiillee. Here is an
equivalent call using DDBB__FFiillee:
tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
The "filename", "flags" and "mode" parameters are the direct equivalent
of their ddbbooppeenn(()) counterparts. The final parameter $DB_HASH performs the
function of both the "type" and "openinfo" parameters in ddbbooppeenn(()).
In the example above $DB_HASH is actually a pre-defined reference to a
hash object. DDBB__FFiillee 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.
Here are examples of the constructors and the valid options available for
DB_HASH, DB_BTREE and DB_RECNO respectively.
$a = DB_File::HASHINFO->new();
$a->{'bsize'} ;
$a->{'cachesize'} ;
$a->{'ffactor'};
$a->{'hash'} ;
$a->{'lorder'} ;
$a->{'nelem'} ;
$b = DB_File::BTREEINFO->new();
$b->{'flags'} ;
$b->{'cachesize'} ;
$b->{'maxkeypage'} ;
$b->{'minkeypage'} ;
$b->{'psize'} ;
$b->{'compare'} ;
$b->{'prefix'} ;
$b->{'lorder'} ;
$c = DB_File::RECNOINFO->new();
$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 _a_l_l of the values when you only
want to change one. Here is an example:
$a = DB_File::HASHINFO->new();
$a->{'cachesize'} = 12345 ;
tie %y, 'DB_File', "filename", $flags, 0777, $a ;
A few of the options need extra discussion here. When used, the C
equivalent of the keys "hash", "compare" and "prefix" store pointers to C
functions. In DDBB__FFiillee these keys are used to store references to Perl
subs. Below are templates for each of the subs:
sub hash
{
my ($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 "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 'bval' Option".
DDeeffaauulltt PPaarraammeetteerrss 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 "In Memory Databases" for a discussion on the use of "undef" in place
of a filename.
IInn MMeemmoorryy DDaattaabbaasseess Berkeley DB allows the creation of in-memory databases by using NULL (that is, a “(char *)0” in C) in place of the filename. DDBB__FFiillee uses “undef” instead of NULL to provide this functionality.
DDBB__HHAASSHH #
The DB_HASH file format is probably the most commonly used of the three
file formats that DDBB__FFiillee supports. It is also very straightforward to
use.
AA SSiimmppllee EExxaammppllee 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 ;
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 -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys
retrieved is in an apparently random order.
DDBB__BBTTRREEEE #
The DB_BTREE format is useful when you want to store data in a given
order. By default the keys will be stored in lexical order, but as you
will see from the example shown in the next section, it is very easy to
define your own sorting function.
CChhaannggiinngg tthhee BBTTRREEEE ssoorrtt oorrddeerr This script shows how to override the default sorting algorithm that BTREE uses. Instead of using the normal lexical ordering, a case insensitive compare function will be used.
use warnings ;
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($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, 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:
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.
3. Duplicate keys are entirely defined by the comparison function. In
the case-insensitive example above, the keys: 'KEY' and 'key' would
be considered duplicates, and assigning to the second one would
overwrite the first. If duplicates are allowed for (with the R_DUP
flag discussed below), only a single copy of duplicate keys is
stored in the database --- so (again with example above) assigning
three values to the keys: 'KEY', 'Key', and 'key' would leave just
the first key: 'KEY' in the database with three values. For some
situations this results in information loss, so care should be taken
to provide fully qualified comparison functions when necessary. For
example, the above comparison routine could be modified to
additionally compare case-sensitively if two keys are equal in the
case insensitive comparison:
sub compare {
my($key1, $key2) = @_;
lc $key1 cmp lc $key2 ||
$key1 cmp $key2;
}
And now you will only have duplicates when the keys themselves are
truly the same. (note: in versions of the db library prior to about
November 1996, such duplicate keys were retained so it was possible
to recover the original keys in sets of keys that compared as
equal).
HHaannddlliinngg DDuupplliiccaattee KKeeyyss 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 $DB_BTREE to R_DUP when creating the database.
There are some difficulties in using the tied hash interface if you want
to manipulate a BTREE database with duplicate keys. Consider this code:
use warnings ;
use strict ;
use DB_File ;
my ($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 "Wall" -
the only thing is, when they are retrieved from the database they _s_e_e_m 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 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 warnings ;
use strict ;
use DB_File ;
my ($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 "Wall".
To make life easier when dealing with duplicate keys, DDBB__FFiillee comes with
a few utility methods.
TThhee ggeett__dduupp(()) MMeetthhoodd 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 values associated
with the key, $key.
In list context, it returns all the values which match $key. Note that
the values will be returned in an apparently random order.
In list context, if the second parameter is present and evaluates TRUE,
the method returns an associative array. The keys of the associative
array correspond to the values that matched in the BTREE and the values
of the array are a count of the number of times that particular value
occurred in the BTREE.
So assuming the database created above, we can use "get_dup" like this:
use warnings ;
use strict ;
use DB_File ;
my ($filename, $x, %h) ;
$filename = "tree" ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
$x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open $filename: $!\n";
my $cnt = $x->get_dup("Wall") ;
print "Wall occurred $cnt times\n" ;
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 => []
TThhee ffiinndd__dduupp(()) MMeetthhoodd $status = $X->find_dup($key, $value) ;
This method checks for the existence of a specific key/value pair. If the
pair exists, the cursor is left pointing to the pair and the method
returns 0. Otherwise the method returns a non-zero value.
Assuming the database from the previous example:
use warnings ;
use strict ;
use DB_File ;
my ($filename, $x, %h, $found) ;
$filename = "tree" ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
$x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open $filename: $!\n";
$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
TThhee ddeell__dduupp(()) MMeetthhoodd $status = $X->del_dup($key, $value) ;
This method deletes a specific key/value pair. It returns 0 if they exist
and have been deleted successfully. Otherwise the method returns a non-
zero value.
Again assuming the existence of the "tree" database
use warnings ;
use strict ;
use DB_File ;
my ($filename, $x, %h, $found) ;
$filename = "tree" ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
$x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
or die "Cannot open $filename: $!\n";
$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
MMaattcchhiinngg PPaarrttiiaall KKeeyyss The BTREE interface has a feature which allows partial keys to be matched. This functionality is _o_n_l_y available when the “seq” method is used along with the R_CURSOR flag.
$x->seq($key, $value, R_CURSOR) ;
Here is the relevant quote from the dbopen man page where it defines the
use of the R_CURSOR flag with seq:
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
In the example script below, the "match" sub uses this feature to find
and print the first matching key/value pair given a partial key.
use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
my ($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, 0666, $DB_BTREE
or die "Cannot open $filename: $!\n";
# Add some key/value pairs to the file
$h{'mouse'} = 'mickey' ;
$h{'Wall'} = 'Larry' ;
$h{'Walls'} = 'Brick' ;
$h{'Smith'} = 'John' ;
$key = $value = 0 ;
print "IN ORDER\n" ;
for ($st = $x->seq($key, $value, R_FIRST) ;
$st == 0 ;
$st = $x->seq($key, $value, R_NEXT) )
{ print "$key -> $value\n" }
print "\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
DDBB__RREECCNNOO #
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.
TThhee ‘’bbvvaall’’ OOppttiioonn 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 documentation error
will be fixed in the next release of Berkeley DB.
That clarifies the situation with regards Berkeley DB itself. What about
DDBB__FFiillee? Well, the behavior defined in the quote above is quite useful,
so DDBB__FFiillee conforms to 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.
Also note that the bval option only allows you to specify a single byte
as a delimiter.
AA SSiimmppllee EExxaammppllee 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 “Extra RECNO Methods” 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, 0666, $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
EExxttrraa RREECCNNOO MMeetthhooddss 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 determining the array length will not work with a tied array.
To make the interface more useful for older versions of Perl, a number of
methods are supplied with DDBB__FFiillee to simulate the missing array
operations. All these methods are accessed via the object returned from
the tie call.
Here are the methods:
$$XX-->>ppuusshh((lliisstt)) ;;
Pushes the elements of "list" to the end of the array.
$$vvaalluuee == $$XX-->>ppoopp ;;
Removes and returns the last element of the array.
$$XX-->>sshhiifftt
Removes and returns the first element of the array.
$$XX-->>uunnsshhiifftt((lliisstt)) ;;
Pushes the elements of "list" to the start of the array.
$$XX-->>lleennggtthh
Returns the number of elements in the array.
$$XX-->>sspplliiccee((ooffffsseett,, lleennggtthh,, eelleemmeennttss));;
Returns a splice of the array.
AAnnootthheerr EExxaammppllee 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 API INTERFACE”). #
use warnings ;
use strict ;
my (@h, $H, $file, $i) ;
use DB_File ;
use Fcntl ;
$file = "text" ;
unlink $file ;
$H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0666, $DB_RECNO
or die "Cannot open file $file: $!\n" ;
# first create a text file to play with
$h[0] = "zero" ;
$h[1] = "one" ;
$h[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.
TTHHEE AAPPII IINNTTEERRFFAACCEE #
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
DDBB__FFiillee methods directly like this:
$db->put($key, $value, R_NOOVERWRITE) ;
IImmppoorrttaanntt:: If you have saved a copy of the object returned from "tie",
the underlying database file will _n_o_t 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 uunnttiiee(()) Gotcha" for more details.
All the functions defined in dbopen are available except for cclloossee(()) and
ddbbooppeenn(()) itself. The DDBB__FFiillee 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 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.
• Whenever a Berkeley DB function returns data via one of its
parameters, the equivalent DDBB__FFiillee 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 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: $!" ;
# 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 dbopen 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.
$$ssttaattuuss == $$XX-->>ggeett(($$kkeeyy,, $$vvaalluuee [[,, $$ffllaaggss]])) ;;
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.
$$ssttaattuuss == $$XX-->>ppuutt(($$kkeeyy,, $$vvaalluuee [[,, $$ffllaaggss]])) ;;
Stores the key/value pair in the database.
If you use either the R_IAFTER or R_IBEFORE flags, the $key
parameter will have the record number of the inserted key/value pair
set.
Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
R_SETCURSOR. #
$$ssttaattuuss == $$XX-->>ddeell(($$kkeeyy [[,, $$ffllaaggss]])) ;;
Removes all key/value pairs with key $key from the database.
A return code of 1 means that the requested key was not in the
database.
R_CURSOR is the only valid flag at present.
$$ssttaattuuss == $$XX-->>ffdd ;;
Returns the file descriptor for the underlying database.
See "Locking: The Trouble with fd" for an explanation for why you
should not use "fd" to lock your database.
$$ssttaattuuss == $$XX-->>sseeqq(($$kkeeyy,, $$vvaalluuee,, $$ffllaaggss)) ;;
This interface allows sequential retrieval from the database. See
dbopen 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.
$$ssttaattuuss == $$XX-->>ssyynncc(([[$$ffllaaggss]])) ;;
Flushes any cached buffers to disk.
R_RECNOSYNC is the only valid flag at present.
DDBBMM FFIILLTTEERRSS #
A DBM Filter is a piece of code that is be used when you _a_l_w_a_y_s want to
make the same transformation to all keys and/or values in a DBM database.
An example is when you need to encode your data in UTF-8 before writing
to the database and then decode the UTF-8 when reading from the database
file.
There are two ways to use a DBM Filter.
1. Using the low-level API defined below.
2. Using the DBM_Filter module. This module hides the complexity of
the API defined below and comes with a number of "canned" filters
that cover some of the common use-cases.
Use of the DBM_Filter module is recommended.
DDBBMM FFiilltteerr LLooww--lleevveell AAPPII There are four methods associated with DBM Filters. All work identically, and each is used to install (or uninstall) a single DBM Filter. Each expects a single parameter, namely a reference to a sub. The only difference between them is the place that the filter is installed.
To summarise:
ffiilltteerr__ssttoorree__kkeeyy
If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.
ffiilltteerr__ssttoorree__vvaalluuee
If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.
ffiilltteerr__ffeettcchh__kkeeyy
If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.
ffiilltteerr__ffeettcchh__vvaalluuee
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.
TThhee FFiilltteerr 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.
AAnn EExxaammppllee -–- tthhee NNUULLLL tteerrmmiinnaattiioonn pprroobblleemm.. 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 _a_l_l keys and values are NULL terminated. Unfortunately when Perl writes to DBM databases it doesn’t use NULL termination, so your Perl application will have to manage NULL termination itself. When you write to the database you will have to use something like this:
$hash{"$key\0"} = "$value\0" ;
Similarly the NULL needs to be taken into account when you are
considering the length of existing keys/values.
It would be much better if you could ignore the NULL terminations issue
in the main application code and have a mechanism that automatically
added the terminating NULL to all keys and values whenever you write to
the database and have them removed when you read from the database. As
I'm sure you have already guessed, this is a problem that DBM Filters can
fix very easily.
use warnings ;
use strict ;
use DB_File ;
my %hash ;
my $filename = "filt" ;
unlink $filename ;
my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
or die "Cannot open $filename: $!\n" ;
# Install DBM Filters
$db->filter_fetch_key ( sub { s/\0$// } ) ;
$db->filter_store_key ( sub { $_ .= "\0" } ) ;
$db->filter_fetch_value( sub { s/\0$// } ) ;
$db->filter_store_value( sub { $_ .= "\0" } ) ;
$hash{"abc"} = "def" ;
my $a = $hash{"ABC"} ;
# ...
undef $db ;
untie %hash ;
Hopefully the contents of each of the filters should be self-explanatory.
Both "fetch" filters remove the terminating NULL, and both "store"
filters add a terminating NULL.
AAnnootthheerr EExxaammppllee -–- KKeeyy iiss aa CC iinntt.. 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} = "something" ;
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 = "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.
HHIINNTTSS AANNDD TTIIPPSS #
LLoocckkiinngg:: TThhee TTrroouubbllee wwiitthh ffdd Until version 1.72 of this module, the recommended technique for locking DDBB__FFiillee databases was to flock the filehandle returned from the “fd” function. Unfortunately this technique has been shown to be fundamentally flawed (Kudos to David Harris for tracking this down). Use it at your own peril!
The locking technique went like this.
$db = tie(%db, 'DB_File', 'foo.db', O_CREAT|O_RDWR, 0644)
|| die "dbcreat 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 DDBB__FFiillee
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 DDBB__FFiillee database
using the locking steps outlined above. Assume process "A" has already
opened the database and has a write lock, but it hasn't actually updated
the database yet (it has finished step 2, but not started step 3 yet).
Now process "B" tries to open the same database - step 1 will succeed,
but it will block on step 2 until process "A" releases the lock. The
important thing to notice here is that at this point in time both
processes will have cached identical initial blocks from the database.
Now process "A" updates the database and happens to change some of the
data held in the initial buffer. Process "A" terminates, flushing all
cached data to disk and releasing the database lock. At this point the
database on disk will correctly reflect the changes made by process "A".
With the lock released, process "B" can now continue. It also updates the
database and unfortunately it too modifies the data that was in its
initial buffer. Once that data gets flushed to disk it will overwrite
some/all of the changes process "A" made to the database.
The result of this scenario is at best a database that doesn't contain
what you expect. At worst the database will corrupt.
The above won't happen every time competing process update the same
DDBB__FFiillee database, but it does illustrate why the technique should not be
used.
SSaaffee wwaayyss ttoo lloocckk aa ddaattaabbaassee Starting with version 2.x, Berkeley DB has internal support for locking. The companion module to this one, BerkeleyDB https://metacpan.org/pod/BerkeleyDB, provides an interface to this locking functionality. If you are serious about locking Berkeley DB databases, I strongly recommend using BerkeleyDB https://metacpan.org/pod/BerkeleyDB.
If using BerkeleyDB <https://metacpan.org/pod/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:
TTiiee::::DDBB__LLoocckk
A DDBB__FFiillee 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.
TTiiee::::DDBB__LLoocckkFFiillee
A DDBB__FFiillee wrapper that has the ability to lock and unlock the
database while it is being used. Avoids the tie-before-flock problem
by simply re-tie-ing the database when you get or drop a lock.
Because of the flexibility in dropping and re-acquiring the lock in
the middle of a session, this can be massaged into a system that
will work with long updates and/or reads if the application follows
the hints in the POD documentation.
DDBB__FFiillee::::LLoocckk
An extremely lightweight DDBB__FFiillee wrapper that simply flocks a
lockfile before tie-ing the database and drops the lock after the
untie. Allows one to use the same lockfile for multiple databases to
avoid deadlock problems, if desired. Use for databases where updates
are reads are quick and simple flock locking semantics are enough.
SShhaarriinngg DDaattaabbaasseess WWiitthh CC AApppplliiccaattiioonnss 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.
See "DBM FILTERS" for a generic way to work around this problem.
Here is a real example. Netscape 2.0 keeps a record of the locations you
visit along with the time you last visited them in a DB_HASH database.
This is usually stored in the file _~_/_._n_e_t_s_c_a_p_e_/_h_i_s_t_o_r_y_._d_b. 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 _g_g_h
script (available from your nearest CPAN archive in
_a_u_t_h_o_r_s_/_i_d_/_T_O_M_C_/_s_c_r_i_p_t_s_/_n_s_h_i_s_t_._g_z).
use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
my ($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 ;
TThhee uunnttiiee(()) GGoottcchhaa If you make use of the Berkeley DB API, it is _v_e_r_y strongly recommended that you read “The untie Gotcha” in perltie.
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 DDBB__FFiillee
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 ttiiee(()) statement in
the script, the source of the problem is really with the uunnttiiee(())
statement that precedes it.
Having read perltie 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 DDBB__FFiillee
destructor, DESTROY, will not be called until _a_l_l references to the tied
object are destroyed. Both the tied variable, %x, and $X above hold a
reference to the object. The call to uunnttiiee(()) will destroy the first, but
$X still holds a valid reference, so the destructor will not get called
and the database file _t_s_t_._f_i_l 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
...
CCOOMMMMOONN QQUUEESSTTIIOONNSS #
WWhhyy iiss tthheerree PPeerrll ssoouurrccee iinn mmyy ddaattaabbaassee?? 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 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.
HHooww ddoo II ssttoorree ccoommpplleexx ddaattaa ssttrruuccttuurreess wwiitthh DDBB__FFiillee?? Although DDBB__FFiillee cannot do this directly, there is a module which can layer transparently over DDBB__FFiillee to accomplish this feat.
Check out the MLDBM module, available on CPAN in the directory
_m_o_d_u_l_e_s_/_b_y_-_m_o_d_u_l_e_/_M_L_D_B_M.
WWhhaatt ddooeess “"wwiiddee cchhaarraacctteerr iinn ssuubbrroouuttiinnee eennttrryy"” mmeeaann?? You will usually get this message if you are working with UTF-8 data and want to read/write it from/to a Berkeley DB database file.
The easist way to deal with this issue is to use the pre-defined "utf8"
DDBBMM__FFiilltteerr (see DBM_Filter) that was designed to deal with this
situation.
The example below shows what you need if _b_o_t_h the key and value are
expected to be in UTF-8.
use DB_File;
use DBM_Filter;
my $db = tie %h, 'DB_File', '/tmp/try.db', O_CREAT|O_RDWR, 0666, $DB_BTREE;
$db->Filter_Key_Push('utf8');
$db->Filter_Value_Push('utf8');
my $key = "\N{LATIN SMALL LETTER A WITH ACUTE}";
my $value = "\N{LATIN SMALL LETTER E WITH ACUTE}";
$h{ $key } = $value;
WWhhaatt ddooeess “"IInnvvaalliidd AArrgguummeenntt"” mmeeaann?? 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.
WWhhaatt ddooeess “"BBaarreewwoorrdd ‘’DDBB__FFiillee’’ nnoott aalllloowweedd"” mmeeaann?? 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 ;
my %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.
RREEFFEERREENNCCEESS #
Articles that are either about DDBB__FFiillee or make use of it.
1. _F_u_l_l_-_T_e_x_t _S_e_a_r_c_h_i_n_g _i_n _P_e_r_l, Tim Kientzle (tkientzle@ddj.com), Dr.
Dobb's Journal, Issue 295, January 1999, pp 34-41
HHIISSTTOORRYY #
Moved to the Changes file.
BBUUGGSS #
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.
SSUUPPPPOORRTT #
General feedback/questions/bug reports should be sent to
<https://github.com/pmqs/DB_File/issues> (preferred) or
<https://rt.cpan.org/Public/Dist/Display.html?Name=DB_File>.
AAVVAAIILLAABBIILLIITTYY #
DDBB__FFiillee comes with the standard Perl source distribution. Look in the
directory _e_x_t_/_D_B___F_i_l_e. 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 "CPAN" in
perlmodlib for details), in the directory _m_o_d_u_l_e_s_/_b_y_-_m_o_d_u_l_e_/_D_B___F_i_l_e.
DDBB__FFiillee is designed to work with any version of Berkeley DB, but is
limited to the functionality provided by version 1. If you want to make
use of the new features available in Berkeley DB 2.x, or greater, use the
Perl module BerkeleyDB <https://metacpan.org/pod/BerkeleyDB> instead.
The official web site for Berkeley DB is
<http://www.oracle.com/technology/products/berkeley-db/db/index.html>.
All versions of Berkeley DB are available there.
Alternatively, Berkeley DB version 1 is available at your nearest CPAN
archive in _s_r_c_/_m_i_s_c_/_d_b_._1_._8_5_._t_a_r_._g_z.
CCOOPPYYRRIIGGHHTT #
Copyright (c) 1995-2022 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 DDBB__FFiillee 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 a few words taken from the Berkeley DB FAQ (at
<http://www.oracle.com/technology/products/berkeley-db/db/index.html>)
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, contact either the
Berkeley DB authors or the author of DB_File. See "AUTHOR" for details.
SSEEEE AALLSSOO #
perl, ddbbooppeenn(3), hhaasshh(3), rreeccnnoo(3), bbttrreeee(3), perldbmfilter, DBM_Filter
AAUUTTHHOORR #
The DB_File interface was written by Paul Marquess <pmqs@cpan.org>.
perl v5.36.3 2024-03-20 DB_File(3p)