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) ;
$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 ;
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 "Using DB_File with Berkeley DB version 2 or greater"). It
is assumed that you have a copy of the Berkeley DB manual pages at hand when
reading this documentation. The interface defined here mirrors the Berkeley DB
interface closely.
Berkeley DB is a C library which provides a consistent interface to a number of
database formats.
DB_File provides an interface to all three of the
database types currently supported by Berkeley DB.
The file types are:
- 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.
Although
DB_File is intended to be used with Berkeley DB version 1, it
can also be used with version 2, 3 or 4. In this case the interface is limited
to the functionality provided by Berkeley DB 1.x. Anywhere the version 2 or
greater interface differs,
DB_File arranges for it to work like version
1. This feature allows
DB_File scripts that were built with version 1
to be migrated to version 2 or greater without any changes.
If you want to make use of the new features available in Berkeley DB 2.x or
greater, use the Perl module BerkeleyDB
<
https://metacpan.org/pod/BerkeleyDB> instead.
Note: The database file format has changed multiple times in Berkeley DB
version 2, 3 and 4. If you cannot recreate your databases, you must dump any
existing databases with either the "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.
DB_File allows access to Berkeley DB files using the
tie()
mechanism in Perl 5 (for full details, see "
tie()" in
perlfunc). 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 API
INTERFACE".
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.
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
all 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, $a ;
A few of the options need extra discussion here. When used, the C equivalent of
the keys "hash", "compare" and "prefix" store
pointers to C functions. In
DB_File these keys are used to store
references to Perl subs. Below are templates for each of the subs:
sub hash
{
my ($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 "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".
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.
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.
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 -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys retrieved
is in an apparently random order.
The DB_BTREE format is useful when you want to store data in a given order. By
default the keys will be stored in lexical order, but as you will see from the
example shown in the next section, it is very easy to define your own sorting
function.
This script shows how to override the default sorting algorithm that BTREE uses.
Instead of using the normal lexical ordering, a case insensitive compare
function will be used.
use warnings ;
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($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).
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
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 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,
DB_File comes with
a few utility methods.
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 => []
$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
$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
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, $value, R_CURSOR) ;
Here is the relevant quote from the dbopen man page where it defines the use of
the R_CURSOR flag with seq:
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
In the example script below, the "match" sub uses this feature to find
and print the first matching key/value pair given a partial key.
use warnings ;
use strict ;
use DB_File ;
use Fcntl ;
my ($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
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 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
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. 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.
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
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
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.
-
$X->splice(offset, length,
elements);
- Returns a splice of the array.
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.
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.
use DB_File ;
$db = tie %hash, "DB_File", "filename"
or die "Cannot tie filename: $!" ;
...
undef $db ;
untie %hash ;
See "The
untie() Gotcha" for more details.
All the functions defined in dbopen 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:
- •
- 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 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 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.
-
$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 "Locking: The Trouble with fd" 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 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.
-
$status =
$X->sync([$flags]) ;
- Flushes any cached buffers to disk.
R_RECNOSYNC is the only valid flag at present.
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. 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.
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:
- 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.
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.
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 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.
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.
Until version 1.72 of this module, the recommended technique for locking
DB_File databases was to flock the filehandle returned from the
"fd" function. Unfortunately this technique has been shown to be
fundamentally flawed (Kudos to David Harris for tracking this down). Use it at
your own peril!
The locking technique went like this.
$db = tie(%db, 'DB_File', '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
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 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
DB_File database, but it does illustrate why the technique should not
be used.
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:
- 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 documentation.
- 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
deadlock problems, if desired. Use for databases where updates are reads
are quick and simple flock locking semantics are enough.
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
~/.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 ;
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 ;
If you make use of the Berkeley DB API, it is
very 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
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 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
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 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
...
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.
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.
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"
DBM_Filter (see DBM_Filter) that was designed to deal
with this situation.
The example below shows what you need if
both 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;
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.
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.
Articles that are either about
DB_File or make use of it.
- 1.
-
Full-Text Searching in Perl, Tim Kientzle
([email protected]), Dr. Dobb's Journal, Issue 295, January 1999, pp
34-41
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.
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>.
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 "CPAN" in
perlmodlib for details), in the directory
modules/by-module/DB_File.
DB_File 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
src/misc/db.1.85.tar.gz.
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
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 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.
perl,
dbopen(3),
hash(3),
recno(3),
btree(3),
perldbmfilter, DBM_Filter
The DB_File interface was written by Paul Marquess <
[email protected]>.