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RedHat 9 (Linux i386) - man page for db_file (redhat section 3pm)

DB_File(3pm)			 Perl Programmers Reference Guide		     DB_File(3pm)

NAME
       DB_File - Perl5 access to Berkeley DB version 1.x

SYNOPSIS
	use DB_File;

	[$X =] tie %hash,  'DB_File', [$filename, $flags, $mode, $DB_HASH] ;
	[$X =] tie %hash,  'DB_File', $filename, $flags, $mode, $DB_BTREE ;
	[$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;

	$status = $X->del($key [, $flags]) ;
	$status = $X->put($key, $value [, $flags]) ;
	$status = $X->get($key, $value [, $flags]) ;
	$status = $X->seq($key, $value, $flags) ;
	$status = $X->sync([$flags]) ;
	$status = $X->fd ;

	# BTREE only
	$count = $X->get_dup($key) ;
	@list  = $X->get_dup($key) ;
	%list  = $X->get_dup($key, 1) ;
	$status = $X->find_dup($key, $value) ;
	$status = $X->del_dup($key, $value) ;

	# RECNO only
	$a = $X->length;
	$a = $X->pop ;
	$X->push(list);
	$a = $X->shift;
	$X->unshift(list);
	@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 ;

DESCRIPTION
       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 Berke-
       ley DB version 2 or greater").  It is assumed that you have a copy of the Berkeley DB man-
       ual 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 sup-
       ported 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 com-
	    patible 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 manipu-
	    lated using the same key/value pair interface as in DB_HASH and DB_BTREE.  In this
	    case the key will consist of a record (line) number.

       Using DB_File with Berkeley DB version 2 or greater

       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 pro-
       vided by Berkeley DB 1.x. Anywhere the version 2 or greater interface differs, DB_File
       arranges for it to work like version 1. This feature allows DB_File scripts that were
       built with version 1 to be migrated to version 2 or greater without any changes.

       If you want to make use of the new features available in Berkeley DB 2.x or greater, use
       the Perl module BerkeleyDB instead.

       Note: The database file format has changed multiple times in Berkeley DB version 2, 3 and
       4. If you cannot recreate your databases, you must dump any existing databases with either
       the "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.

       Interface to Berkeley DB

       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".

       Opening a Berkeley DB Database File

       Berkeley DB uses the function dbopen() to open or create a database.  Here is the C proto-
       type 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, construc-
       tors are available for each file type.

       Here are examples of the constructors and the valid options available for DB_HASH,
       DB_BTREE and DB_RECNO respectively.

	    $a = new DB_File::HASHINFO ;
	    $a->{'bsize'} ;
	    $a->{'cachesize'} ;
	    $a->{'ffactor'};
	    $a->{'hash'} ;
	    $a->{'lorder'} ;
	    $a->{'nelem'} ;

	    $b = new DB_File::BTREEINFO ;
	    $b->{'flags'} ;
	    $b->{'cachesize'} ;
	    $b->{'maxkeypage'} ;
	    $b->{'minkeypage'} ;
	    $b->{'psize'} ;
	    $b->{'compare'} ;
	    $b->{'prefix'} ;
	    $b->{'lorder'} ;

	    $c = new DB_File::RECNOINFO ;
	    $c->{'bval'} ;
	    $c->{'cachesize'} ;
	    $c->{'psize'} ;
	    $c->{'flags'} ;
	    $c->{'lorder'} ;
	    $c->{'reclen'} ;
	    $c->{'bfname'} ;

       The values stored in the hashes above are mostly the direct equivalent of their C counter-
       part. Like their C counterparts, all are set to a default values - that means you don't
       have to set all of the values when you only want to change one. Here is an example:

	    $a = new DB_File::HASHINFO ;
	    $a->{'cachesize'} =  12345 ;
	    tie %y, 'DB_File', "filename", $flags, 0777, $a ;

       A few of the options need extra discussion here. When used, the C equivalent of the keys
       "hash", "compare" and "prefix" store pointers to C functions. In DB_File these keys are
       used to store references to Perl subs. Below are templates for each of the subs:

	   sub hash
	   {
	       my ($data) = @_ ;
	       ...
	       # return the hash value for $data
	       return $hash ;
	   }

	   sub compare
	   {
	       my ($key, $key2) = @_ ;
	       ...
	       # return  0 if $key1 eq $key2
	       #	-1 if $key1 lt $key2
	       #	 1 if $key1 gt $key2
	       return (-1 , 0 or 1) ;
	   }

	   sub prefix
	   {
	       my ($key, $key2) = @_ ;
	       ...
	       # return number of bytes of $key2 which are
	       # necessary to determine that it is greater than $key1
	       return $bytes ;
	   }

       See "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".

       Default Parameters

       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.

       In Memory Databases

       Berkeley DB allows the creation of in-memory databases by using NULL (that is, a "(char
       *)0" in C) in place of the filename.  DB_File uses "undef" instead of NULL to provide this
       functionality.

DB_HASH
       The DB_HASH file format is probably the most commonly used of the three file formats that
       DB_File supports. It is also very straightforward to use.

       A Simple Example

       This example shows how to create a database, add key/value pairs to the database, delete
       keys/value pairs and finally how to enumerate the contents of the database.

	   use warnings ;
	   use strict ;
	   use DB_File ;
	   our (%h, $k, $v) ;

	   unlink "fruit" ;
	   tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0666, $DB_HASH
	       or die "Cannot open file 'fruit': $!\n";

	   # Add a few key/value pairs to the file
	   $h{"apple"} = "red" ;
	   $h{"orange"} = "orange" ;
	   $h{"banana"} = "yellow" ;
	   $h{"tomato"} = "red" ;

	   # Check for existence of a key
	   print "Banana Exists\n\n" if $h{"banana"} ;

	   # Delete a key/value pair.
	   delete $h{"apple"} ;

	   # print the contents of the file
	   while (($k, $v) = each %h)
	     { print "$k -> $v\n" }

	   untie %h ;

       here is the output:

	   Banana Exists

	   orange -> orange
	   tomato -> red
	   banana -> yellow

       Note that the like ordinary associative arrays, the order of the keys retrieved is in an
       apparently random order.

DB_BTREE
       The DB_BTREE format is useful when you want to store data in a given order. By default the
       keys will be stored in lexical order, but as you will see from the example shown in the
       next section, it is very easy to define your own sorting function.

       Changing the BTREE sort order

       This script shows how to override the default sorting algorithm that BTREE uses. Instead
       of using the normal lexical ordering, a case insensitive compare function will be used.

	   use warnings ;
	   use strict ;
	   use DB_File ;

	   my %h ;

	   sub Compare
	   {
	       my ($key1, $key2) = @_ ;
	       "\L$key1" cmp "\L$key2" ;
	   }

	   # specify the Perl sub that will do the comparison
	   $DB_BTREE->{'compare'} = \&Compare ;

	   unlink "tree" ;
	   tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 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 data-
       base:

       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-insensi-
	    tive 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_DUPS 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 data-
	    base 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).

       Handling Duplicate Keys

       The BTREE file type optionally allows a single key to be associated with an arbitrary num-
       ber 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

       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 =>      []

       The find_dup() Method

	   $status = $X->find_dup($key, $value) ;

       This method checks for the existence of a specific key/value pair. If the pair exists, the
       cursor is left pointing to the pair and the method returns 0. Otherwise the method returns
       a non-zero value.

       Assuming the database from the previous example:

	   use warnings ;
	   use strict ;
	   use DB_File ;

	   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

       The del_dup() Method

	   $status = $X->del_dup($key, $value) ;

       This method deletes a specific key/value pair. It returns 0 if they exist and have been
       deleted successfully.  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

       Matching Partial Keys

       The BTREE interface has a feature which allows partial keys to be matched. This function-
       ality 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_CUR-
       SOR 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
       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. Attempt-
       ing to access an element before the start of the array will raise a fatal run-time error.

       The 'bval' Option

       The operation of the bval option warrants some discussion. Here is the definition of bval
       from the Berkeley DB 1.85 recno manual page:

	   The delimiting byte to be used to mark  the	end  of  a
	   record for variable-length records, and the pad charac-
	   ter for fixed-length records.  If no  value	is  speci-
	   fied,  newlines  (``\n'')  are  used to mark the end of
	   variable-length records and	fixed-length  records  are
	   padded with spaces.

       The second sentence is wrong. In actual fact bval will only default to "\n" when the open-
       info 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 delimeter.

       A Simple Example

       Here is a simple example that uses RECNO (if you are using a version of Perl earlier than
       5.004_57 this example won't work -- see "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

       Extra RECNO Methods

       If you are using a version of Perl earlier than 5.004_57, the tied array interface is
       quite limited. In the example script above "push", "pop", "shift", "unshift" or determin-
       ing 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 sup-
       plied 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 the array.

       Another Example

       Here is a more complete example that makes use of some of the methods described above. It
       also makes use of the API interface directly (see "THE 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.

THE API INTERFACE
       As well as accessing Berkeley DB using a tied hash or array, it is also possible to make
       direct use of most of the API functions defined in the Berkeley DB documentation.

       To do this you need to store a copy of the object returned from the tie.

	       $db = tie %hash, "DB_File", "filename" ;

       Once you have done that, you can access the Berkeley DB API functions as DB_File methods
       directly like this:

	       $db->put($key, $value, R_NOOVERWRITE) ;

       Important: If you have saved a copy of the object returned from "tie", the underlying
       database file will not be closed until both the tied variable is untied and all copies of
       the saved object are destroyed.

	   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:

       o    The methods return a status value. All return 0 on success.  All return -1 to signify
	    an error and set $! to the exact error code. The return code 1 generally (but not
	    always) means that the key specified did not exist in the database.

	    Other return codes are defined. See below and in the Berkeley DB documentation for
	    details. The Berkeley DB documentation should be used as the definitive source.

       o    Whenever a Berkeley DB function returns data via one of its parameters, the equiva-
	    lent DB_File method does exactly the same.

       o    If you are careful, it is possible to mix API calls with the tied hash/array inter-
	    face 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.

DBM FILTERS
       A DBM Filter is a piece of code that is be used when you always want to make the same
       transformation to all keys and/or values in a DBM database.

       There are four methods associated with DBM Filters. All work identically, and each is used
       to install (or 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.

       The Filter

       When each filter is called by Perl, a local copy of $_ will contain the key or value to be
       filtered. Filtering is achieved by modifying the contents of $_. The return code from the
       filter is ignored.

       An Example -- the NULL termination problem.

       Consider the following scenario. You have a DBM database that you need to share with a
       third-party C application. The C application assumes that all keys and values are NULL
       terminated. Unfortunately when Perl writes to DBM databases it doesn't use NULL termina-
       tion, 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 appli-
       cation code and have a mechanism that automatically added the terminating NULL to all keys
       and values whenever you write to the database and have them removed when you read from the
       database. As I'm sure you have already guessed, this is a problem that DBM Filters can fix
       very easily.

	   use warnings ;
	   use strict ;
	   use DB_File ;

	   my %hash ;
	   my $filename = "/tmp/filt" ;
	   unlink $filename ;

	   my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
	     or die "Cannot open $filename: $!\n" ;

	   # Install DBM Filters
	   $db->filter_fetch_key  ( sub { s/\0$//    } ) ;
	   $db->filter_store_key  ( sub { $_ .= "\0" } ) ;
	   $db->filter_fetch_value( sub { s/\0$//    } ) ;
	   $db->filter_store_value( sub { $_ .= "\0" } ) ;

	   $hash{"abc"} = "def" ;
	   my $a = $hash{"ABC"} ;
	   # ...
	   undef $db ;
	   untie %hash ;

       Hopefully the contents of each of the filters should be self-explanatory. Both "fetch"
       filters remove the terminating NULL, and both "store" filters add a terminating NULL.

       Another Example -- Key is a C int.

       Here is another real-life example. By default, whenever Perl writes to a DBM database it
       always writes the key and value as strings. So when you use this:

	   $hash{12345} = "soemthing" ;

       the key 12345 will get stored in the DBM database as the 5 byte string "12345". If you
       actually want the key to be stored in the DBM database as a C int, you will have to use
       "pack" when writing, and "unpack" when reading.

       Here is a DBM Filter that does it:

	   use warnings ;
	   use strict ;
	   use DB_File ;
	   my %hash ;
	   my $filename = "/tmp/filt" ;
	   unlink $filename ;

	   my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH
	     or die "Cannot open $filename: $!\n" ;

	   $db->filter_fetch_key  ( sub { $_ = unpack("i", $_) } ) ;
	   $db->filter_store_key  ( sub { $_ = pack ("i", $_) } ) ;
	   $hash{123} = "def" ;
	   # ...
	   undef $db ;
	   untie %hash ;

       This time only two filters have been used -- we only need to manipulate the contents of
       the key, so it wasn't necessary to install any value filters.

HINTS AND TIPS
       Locking: The Trouble with fd

       Until version 1.72 of this module, the recommended 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', '/tmp/foo.db', O_CREAT|O_RDWR, 0666)
	       || die "dbcreat /tmp/foo.db $!";
	   $fd = $db->fd;
	   open(DB_FH, "+<&=$fd") || die "dup $!";
	   flock (DB_FH, LOCK_EX) || die "flock: $!";
	   ...
	   $db{"Tom"} = "Jerry" ;
	   ...
	   flock(DB_FH, LOCK_UN);
	   undef $db;
	   untie %db;
	   close(DB_FH);

       In simple terms, this is what happens:

       1.   Use "tie" to open the database.

       2.   Lock the database with fd & flock.

       3.   Read & Write to the database.

       4.   Unlock and close the database.

       Here is the crux of the problem. A side-effect of opening the DB_File database in step 2
       is that an initial block from the database will get read from disk and cached in memory.

       To see why this is a problem, consider what can happen when two processes, say "A" and
       "B", both want to update the same DB_File database using the locking steps outlined above.
       Assume process "A" has already opened the database and has a write lock, but it hasn't
       actually updated the database yet (it has finished step 2, but not started step 3 yet).
       Now process "B" tries to open the same database - step 1 will succeed, but it will block
       on step 2 until process "A" releases the lock. The important thing to notice here is that
       at this point in time both processes will have cached identical initial blocks from the
       database.

       Now process "A" updates the database and happens to change some of the data held in the
       initial buffer. Process "A" terminates, flushing all cached data to disk and releasing the
       database lock. At this point the database on disk will correctly reflect the changes made
       by process "A".

       With the lock released, process "B" can now continue. It also updates the database and
       unfortunately it too 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 data-
       base.

       The result of this scenario is at best a database that doesn't contain what you expect. At
       worst the database will corrupt.

       The above won't happen every time competing process update the same DB_File database, but
       it does illustrate why the technique should not be used.

       Safe ways to lock a database

       Starting with version 2.x, Berkeley DB  has internal support for locking.  The companion
       module to this one, BerkeleyDB, provides an interface to this locking functionality. If
       you are serious about locking Berkeley DB databases, I strongly recommend using Berke-
       leyDB.

       If using BerkeleyDB isn't an option, there are a number of modules available on CPAN that
       can be used to implement locking. Each one implements locking differently and has differ-
       ent 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.

       Sharing Databases With C Applications

       There is no technical reason why a Berkeley DB database cannot be shared by both a Perl
       and a C application.

       The vast majority of problems that are reported in this area boil down to the fact that C
       strings are NULL 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 (avail-
       able 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 ;

       The untie() Gotcha

       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 refer-
       ence 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 origi-
       nal problem by destroying the API object before the untie:

	   ...
	   $x{123} = 456 ;

	   undef $X ;
	   untie %x ;

	   $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
	   ...

COMMON QUESTIONS
       Why is there Perl source in my database?

       If you look at the contents of a database file created by DB_File, there can sometimes be
       part of a Perl script included in it.

       This happens because Berkeley DB uses dynamic memory to allocate buffers which will subse-
       quently 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.

       How do I store complex data structures with DB_File?

       Although DB_File cannot do this directly, there is a module which can layer transparently
       over DB_File to accomplish this feat.

       Check out the MLDBM module, available on CPAN in the directory modules/by-module/MLDBM.

       What does "Invalid Argument" mean?

       You will get this error message when one of the parameters in the "tie" call is wrong.
       Unfortunately there are quite a few parameters to get wrong, so it can be difficult to
       figure out which one it is.

       Here are a couple of possibilities:

       1.   Attempting to reopen a database without closing it.

       2.   Using the O_WRONLY flag.

       What does "Bareword 'DB_File' not allowed" mean?

       You will encounter this particular error message when you have the "strict 'subs'" pragma
       (or the full strict pragma) in your script.  Consider this script:

	   use 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.

REFERENCES
       Articles that are either about DB_File or make use of it.

       1.   Full-Text Searching in Perl, Tim Kientzle (tkientzle@ddj.com), Dr. Dobb's Journal,
	    Issue 295, January 1999, pp 34-41

HISTORY
       Moved to the Changes file.

BUGS
       Some older versions of Berkeley DB had problems with fixed length records using the RECNO
       file format. This problem has been fixed since version 1.85 of Berkeley DB.

       I am sure there are bugs in the code. If you do find any, or can suggest any enhancements,
       I would welcome your comments.

AVAILABILITY
       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 perlmod for details), in the directory modules/by-module/DB_File.

       This version of DB_File will work with either version 1.x, 2.x or 3.x of Berkeley DB, but
       is limited to the functionality provided by version 1.

       The official web site for Berkeley DB is http://www.sleepycat.com.  All versions of Berke-
       ley DB are available there.

       Alternatively, Berkeley DB version 1 is available at your nearest CPAN archive in
       src/misc/db.1.85.tar.gz.

       If you are running IRIX, then get Berkeley DB version 1 from http://reality.sgi.com/ariel.
       It has the patches necessary to compile properly on IRIX 5.3.

COPYRIGHT
       Copyright (c) 1995-2002 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 are few words taken from the Berkeley DB FAQ (at http://www.sleepycat.com)
       regarding the license:

	   Do I have to license DB to use it in Perl scripts?

	   No. The Berkeley DB license requires that software that uses
	   Berkeley DB be freely redistributable. In the case of Perl, that
	   software is Perl, and not your scripts. Any Perl scripts that you
	   write are your property, including scripts that make use of
	   Berkeley DB. Neither the Perl license nor the Berkeley DB license
	   place any restriction on what you may do with them.

       If you are in any doubt about the license situation, contact either the Berkeley DB
       authors or the author of DB_File. See "AUTHOR" for details.

SEE ALSO
       perl(1), dbopen(3), hash(3), recno(3), btree(3), dbmfilter

AUTHOR
       The DB_File interface was written by Paul Marquess <Paul.Marquess@btinternet.com>.  Ques-
       tions about the DB system itself may be addressed to <db@sleepycat.com<gt>.

perl v5.8.0				    2002-06-01				     DB_File(3pm)


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