DB_File

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



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) ;

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

        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 the section on Using DB_File with Berkeley DB
       version 2). It is assumed that you have a copy of the Berkeley DB
       manual pages at hand when reading this documentation. The interface
       defined here mirrors the Berkeley DB interface closely.

       Berkeley DB is a C library which provides a consistent interface to a
       number of database formats.  DB_File provides an interface to all three
       of the database types currently supported by Berkeley DB.

       The file types are:

       DB_HASH
            This database type allows arbitrary key/value pairs to be stored
            in data files. This is equivalent to the functionality provided by
            other hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM.
            Remember though, the files created using DB_HASH are not
            compatible with any of the other packages mentioned.

            A default hashing algorithm, which will be adequate for most
            applications, is built into Berkeley DB. If you do need to use
            your own hashing algorithm it is possible to write your own in
            Perl and have DB_File use it instead.

       DB_BTREE
            The btree format allows arbitrary key/value pairs to be stored in
            a sorted, balanced binary tree.

            As with the DB_HASH format, it is possible to provide a user
            defined Perl routine to perform the comparison of keys. By
            default, though, the keys are stored in lexical order.

       DB_RECNO
            DB_RECNO allows both fixed-length and variable-length flat text
            files to be manipulated using the same key/value pair interface as
            in DB_HASH and DB_BTREE.  In this case the key will consist of a
            record (line) number.

       Using DB_File with Berkeley DB version 2

       Although DB_File is intended to be used with Berkeley DB version 1, it
       can also be used with version 2. In this case the interface is limited
       to the functionality provided by Berkeley DB 1.x. Anywhere the version
       2 interface differs, DB_File arranges for it to work like version 1.
       This feature allows DB_File scripts that were built with version 1 to
       be migrated to version 2 without any changes.

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

       At the time of writing this document the BerkeleyDB module is still
       alpha quality (the version number is < 1.0), and so unsuitable for use
       in any serious development work. Once its version number is >= 1.0, it
       is considered stable enough for real work.

       Note: The database file format has changed in Berkeley DB version 2.
       If you cannot recreate your databases, you must dump any existing
       databases with the db_dump185 utility that comes with Berkeley DB.
       Once you have upgraded DB_File to use Berkeley DB version 2, your
       databases can be recreated using db_load. Refer to the Berkeley DB
       documentation for further details.

       Please read the COPYRIGHT manpage before using version 2.x of Berkeley
       DB with DB_File.

       Interface to Berkeley DB

       DB_File allows access to Berkeley DB files using the tie() mechanism in
       Perl 5 (for full details, see the tie() entry in the perlfunc manpage).
       This facility allows DB_File to access Berkeley DB files using either
       an associative array (for DB_HASH & DB_BTREE file types) or an ordinary
       array (for the DB_RECNO file type).

       In addition to the tie() interface, it is also possible to access most
       of the functions provided in the Berkeley DB API directly.  See the
       section on THE API INTERFACE.

       Opening a Berkeley DB Database File

       Berkeley DB uses the function dbopen() to open or create a database.
       Here is the C prototype for dbopen():

             DB*
             dbopen (const char * file, int flags, int mode,
                     DBTYPE type, const void * openinfo)

       The parameter type is an enumeration which specifies which of the 3
       interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.
       Depending on which of these is actually chosen, the final parameter,
       openinfo points to a data structure which allows tailoring of the
       specific interface method.

       This interface is handled slightly differently in DB_File. Here is an
       equivalent call using DB_File:

               tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;

       The filename, flags and mode parameters are the direct equivalent of
       their dbopen() counterparts. The final parameter $DB_HASH performs the
       function of both the type and openinfo parameters in dbopen().

       In the example above $DB_HASH is actually a pre-defined reference to a
       hash object. DB_File has three of these pre-defined references.  Apart
       from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.

       The keys allowed in each of these pre-defined references is limited to
       the names used in the equivalent C structure. So, for example, the
       $DB_HASH reference will only allow keys called bsize, cachesize,
       ffactor, hash, lorder and nelem.

       To change one of these elements, just assign to it like this:

               $DB_HASH->{'cachesize'} = 10000 ;

       The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are
       usually adequate for most applications.  If you do need to create extra
       instances of these objects, constructors are available for each file
       type.

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

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

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

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

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

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

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

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

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

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

       See the section on Changing the BTREE sort order for an example of
       using the compare template.

       If you are using the DB_RECNO interface and you intend making use of
       bval, you should check out the section on The 'bval' Option.

       Default Parameters

       It is possible to omit some or all of the final 4 parameters in the
       call to tie and let them take default values. As DB_HASH is the most
       common file format used, the call:

           tie %A, "DB_File", "filename" ;

       is equivalent to:

           tie %A, "DB_File", "filename", O_CREAT⎪O_RDWR, 0666, $DB_HASH ;

       It is also possible to omit the filename parameter as well, so the
       call:

           tie %A, "DB_File" ;

       is equivalent to:

           tie %A, "DB_File", undef, O_CREAT⎪O_RDWR, 0666, $DB_HASH ;

       See the section on In Memory Databases for a discussion on the use of
       undef in place of a filename.

       In Memory Databases

       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 strict ;
           use DB_File ;
           use vars qw( %h $k $v ) ;

           tie %h, "DB_File", "fruit", O_RDWR⎪O_CREAT, 0640, $DB_HASH
               or die "Cannot open file 'fruit': $!\n";

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

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

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

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

           untie %h ;

       here is the output:

           Banana Exists

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

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

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 strict ;
           use DB_File ;

           my %h ;

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

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

           tie %h, "DB_File", "tree", O_RDWR⎪O_CREAT, 0640, $DB_BTREE
               or die "Cannot open file 'tree': $!\n" ;

           # Add a key/value pair to the file
           $h{'Wall'} = 'Larry' ;
           $h{'Smith'} = 'John' ;
           $h{'mouse'} = 'mickey' ;
           $h{'duck'}  = 'donald' ;

           # Delete
           delete $h{"duck"} ;

           # Cycle through the keys printing them in order.
           # Note it is not necessary to sort the keys as
           # the btree will have kept them in order automatically.
           foreach (keys %h)
             { print "$_\n" }

           untie %h ;

       Here is the output from the code above.

           mouse
           Smith
           Wall

       There are a few point to bear in mind if you want to change the
       ordering in a BTREE database:

       1.   The new compare function must be specified when you create the
            database.

       2.   You cannot change the ordering once the database has been created.
            Thus you must use the same compare function every time you access
            the database.

       Handling Duplicate Keys

       The BTREE file type optionally allows a single key to be associated
       with an arbitrary number of values. This option is enabled by setting
       the flags element of $DB_BTREE to R_DUP when creating the database.

       There are some difficulties in using the tied hash interface if you
       want to manipulate a BTREE database with duplicate keys. Consider this
       code:

           use strict ;
           use DB_File ;

           use vars qw($filename %h ) ;

           $filename = "tree" ;
           unlink $filename ;

           # Enable duplicate records
           $DB_BTREE->{'flags'} = R_DUP ;

           tie %h, "DB_File", $filename, O_RDWR⎪O_CREAT, 0640, $DB_BTREE
               or die "Cannot open $filename: $!\n";

           # Add some key/value pairs to the file
           $h{'Wall'} = 'Larry' ;
           $h{'Wall'} = 'Brick' ; # Note the duplicate key
           $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
           $h{'Smith'} = 'John' ;
           $h{'mouse'} = 'mickey' ;

           # iterate through the associative array
           # and print each key/value pair.
           foreach (keys %h)
             { print "$_  -> $h{$_}\n" }

           untie %h ;

       Here is the output:

           Smith   -> John
           Wall    -> Larry
           Wall    -> Larry
           Wall    -> Larry
           mouse   -> mickey

       As you can see 3 records have been successfully created with key Wall -
       the only thing is, when they are retrieved from the database they seem
       to have the same value, namely Larry. The problem is caused by the way
       that the associative array interface works. Basically, when the
       associative array interface is used to fetch the value associated with
       a given key, it will only ever retrieve the first value.

       Although it may not be immediately obvious from the code above, the
       associative array interface can be used to write values with duplicate
       keys, but it cannot be used to read them back from the database.

       The way to get around this problem is to use the Berkeley DB API method
       called seq.  This method allows sequential access to key/value pairs.
       See the section on THE API INTERFACE for details of both the seq method
       and the API in general.

       Here is the script above rewritten using the seq API method.

           use strict ;
           use DB_File ;

           use vars qw($filename $x %h $status $key $value) ;

           $filename = "tree" ;
           unlink $filename ;

           # Enable duplicate records
           $DB_BTREE->{'flags'} = R_DUP ;

           $x = tie %h, "DB_File", $filename, O_RDWR⎪O_CREAT, 0640, $DB_BTREE
               or die "Cannot open $filename: $!\n";

           # Add some key/value pairs to the file
           $h{'Wall'} = 'Larry' ;
           $h{'Wall'} = 'Brick' ; # Note the duplicate key
           $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
           $h{'Smith'} = 'John' ;
           $h{'mouse'} = 'mickey' ;

           # iterate through the btree using seq
           # and print each key/value pair.
           $key = $value = 0 ;
           for ($status = $x->seq($key, $value, R_FIRST) ;
                $status == 0 ;
                $status = $x->seq($key, $value, R_NEXT) )
             {  print "$key -> $value\n" }

           undef $x ;
           untie %h ;

       that prints:

           Smith   -> John
           Wall    -> Brick
           Wall    -> Brick
           Wall    -> Larry
           mouse   -> mickey

       This time we have got all the key/value pairs, including the multiple
       values associated with the key Wall.

       The get_dup() Method

       DB_File comes with a utility method, called get_dup, to assist in
       reading duplicate values from BTREE databases. The method can take the
       following forms:

           $count = $x->get_dup($key) ;
           @list  = $x->get_dup($key) ;
           %list  = $x->get_dup($key, 1) ;

       In a scalar context the method returns the number of values associated
       with the key, $key.

       In list context, it returns all the values which match $key. Note that
       the values will be returned in an apparently random order.

       In list context, if the second parameter is present and evaluates TRUE,
       the method returns an associative array. The keys of the associative
       array correspond to the values that matched in the BTREE and the values
       of the array are a count of the number of times that particular value
       occurred in the BTREE.

       So assuming the database created above, we can use get_dup like this:

           my $cnt  = $x->get_dup("Wall") ;
           print "Wall occurred $cnt times\n" ;

           my %hash = $x->get_dup("Wall", 1) ;
           print "Larry is there\n" if $hash{'Larry'} ;
           print "There are $hash{'Brick'} Brick Walls\n" ;

           my @list = $x->get_dup("Wall") ;
           print "Wall =>      [@list]\n" ;

           @list = $x->get_dup("Smith") ;
           print "Smith =>     [@list]\n" ;

           @list = $x->get_dup("Dog") ;
           print "Dog =>       [@list]\n" ;

       and it will print:

           Wall occurred 3 times
           Larry is there
           There are 2 Brick Walls
           Wall =>     [Brick Brick Larry]
           Smith =>    [John]
           Dog =>      []


       Matching Partial Keys

       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 strict ;
           use DB_File ;
           use Fcntl ;

           use vars qw($filename $x %h $st $key $value) ;

           sub match
           {
               my $key = shift ;
               my $value = 0;
               my $orig_key = $key ;
               $x->seq($key, $value, R_CURSOR) ;
               print "$orig_key\t-> $key\t-> $value\n" ;
           }

           $filename = "tree" ;
           unlink $filename ;

           $x = tie %h, "DB_File", $filename, O_RDWR⎪O_CREAT, 0640, $DB_BTREE
               or die "Cannot open $filename: $!\n";

           # Add some key/value pairs to the file
           $h{'mouse'} = 'mickey' ;
           $h{'Wall'} = 'Larry' ;
           $h{'Walls'} = 'Brick' ;
           $h{'Smith'} = 'John' ;


           $key = $value = 0 ;
           print "IN ORDER\n" ;
           for ($st = $x->seq($key, $value, R_FIRST) ;
                $st == 0 ;
                $st = $x->seq($key, $value, R_NEXT) )

             {  print "$key -> $value\n" }

           print "\nPARTIAL MATCH\n" ;

           match "Wa" ;
           match "A" ;
           match "a" ;

           undef $x ;
           untie %h ;

       Here is the output:

           IN ORDER
           Smith -> John
           Wall  -> Larry
           Walls -> Brick
           mouse -> mickey

           PARTIAL MATCH
           Wa -> Wall  -> Larry
           A  -> Smith -> John
           a  -> mouse -> mickey


DB_RECNO
       DB_RECNO provides an interface to flat text files. Both variable and
       fixed length records are supported.

       In order to make RECNO more compatible with Perl the array offset for
       all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.

       As with normal Perl arrays, a RECNO array can be accessed using
       negative indexes. The index -1 refers to the last element of the array,
       -2 the second last, and so on. Attempting to access an element before
       the start of the array will raise a fatal run-time error.

       The 'bval' Option

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

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

       The second sentence is wrong. In actual fact bval will only default to
       "\n" when the openinfo parameter in dbopen is NULL. If a non-NULL
       openinfo parameter is used at all, the value that happens to be in bval
       will be used. That means you always have to specify bval when making
       use of any of the options in the openinfo parameter. This documentation
       error will be fixed in the next release of Berkeley DB.

       That clarifies the situation with regards Berkeley DB itself. What
       about DB_File? Well, the behavior defined in the quote above is quite
       useful, so DB_File conforms it.

       That means that you can specify other options (e.g. cachesize) and
       still have bval default to "\n" for variable length records, and space
       for fixed length records.

       A Simple Example

       Here is a simple example that uses RECNO.

           use strict ;
           use DB_File ;

           my @h ;
           tie @h, "DB_File", "text", O_RDWR⎪O_CREAT, 0640, $DB_RECNO
               or die "Cannot open file 'text': $!\n" ;

           # Add a few key/value pairs to the file
           $h[0] = "orange" ;
           $h[1] = "blue" ;
           $h[2] = "yellow" ;

           # Check for existence of a key
           print "Element 1 Exists with value $h[1]\n" if $h[1] ;

           # use a negative index
           print "The last element is $h[-1]\n" ;
           print "The 2nd last element is $h[-2]\n" ;

           untie @h ;

       Here is the output from the script:

           Element 1 Exists with value blue
           The last element is yellow
           The 2nd last element is blue


       Extra Methods

       If you are using a version of Perl earlier than 5.004_57, the tied
       array interface is quite limited. The example script above will work,
       but you won't be able to use push, pop, shift, unshift etc. with the
       tied array.

       To make the interface more useful for older versions of Perl, a number
       of methods are supplied with DB_File to simulate the missing array
       operations. All these methods are accessed via the object returned from
       the tie call.

       Here are the methods:

       $X->push(list) ;
            Pushes the elements of list to the end of the array.

       $value = $X->pop ;
            Removes and returns the last element of the array.

       $X->shift
            Removes and returns the first element of the array.

       $X->unshift(list) ;
            Pushes the elements of list to the start of the array.

       $X->length
            Returns the number of elements in the array.

       Another Example

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

           use strict ;
           use vars qw(@h $H $file $i) ;
           use DB_File ;
           use Fcntl ;

           $file = "text" ;

           unlink $file ;

           $H = tie @h, "DB_File", $file, O_RDWR⎪O_CREAT, 0640, $DB_RECNO
               or die "Cannot open file $file: $!\n" ;

           # first create a text file to play with
           $h[0] = "zero" ;
           $h[1] = "one" ;
           $h[2] = "two" ;
           $h[3] = "three" ;
           $h[4] = "four" ;


           # Print the records in order.
           #
           # The length method is needed here because evaluating a tied
           # array in a scalar context does not return the number of
           # elements in the array.

           print "\nORIGINAL\n" ;
           foreach $i (0 .. $H->length - 1) {
               print "$i: $h[$i]\n" ;
           }

           # use the push & pop methods
           $a = $H->pop ;
           $H->push("last") ;
           print "\nThe last record was [$a]\n" ;

           # and the shift & unshift methods
           $a = $H->shift ;
           $H->unshift("first") ;
           print "The first record was [$a]\n" ;

           # 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 section on The untie() Gotcha for more details.

       All the functions defined in the dbopen manpage are available except
       for close() and dbopen() itself. The DB_File method interface to the
       supported functions have been implemented to mirror the way Berkeley DB
       works whenever possible. In particular note that:

       ·    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 the dbopen manpage for use in the flags
       parameters in the methods defined below are also available. Refer to
       the Berkeley DB documentation for the precise meaning of the flags
       values.

       Below is a list of the methods available.

       $status = $X->get($key, $value [, $flags]) ;
            Given a key ($key) this method reads the value associated with it
            from the database. The value read from the database is returned in
            the $value parameter.

            If the key does not exist the method returns 1.

            No flags are currently defined for this method.

       $status = $X->put($key, $value [, $flags]) ;
            Stores the key/value pair in the database.

            If you use either the R_IAFTER or R_IBEFORE flags, the $key
            parameter will have the record number of the inserted key/value
            pair set.

            Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
            R_SETCURSOR.

       $status = $X->del($key [, $flags]) ;
            Removes all key/value pairs with key $key from the database.

            A return code of 1 means that the requested key was not in the
            database.

            R_CURSOR is the only valid flag at present.

       $status = $X->fd ;
            Returns the file descriptor for the underlying database.

            See the section on Locking Databases for an example of how to make
            use of the fd method to lock your database.

       $status = $X->seq($key, $value, $flags) ;
            This interface allows sequential retrieval from the database. See
            the dbopen manpage for full details.

            Both the $key and $value parameters will be set to the key/value
            pair read from the database.

            The flags parameter is mandatory. The valid flag values are
            R_CURSOR, R_FIRST, R_LAST, R_NEXT and R_PREV.

       $status = $X->sync([$flags]) ;
            Flushes any cached buffers to disk.

            R_RECNOSYNC is the only valid flag at present.

HINTS AND TIPS
       Locking Databases

       Concurrent access of a read-write database by several parties requires
       them all to use some kind of locking.  Here's an example of Tom's that
       uses the fd method to get the file descriptor, and then a careful
       open() to give something Perl will flock() for you.  Run this
       repeatedly in the background to watch the locks granted in proper
       order.

           use DB_File;

           use strict;

           sub LOCK_SH { 1 }
           sub LOCK_EX { 2 }
           sub LOCK_NB { 4 }
           sub LOCK_UN { 8 }

           my($oldval, $fd, $db, %db, $value, $key);

           $key = shift ⎪⎪ 'default';
           $value = shift ⎪⎪ 'magic';

           $value .= " $$";

           $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT⎪O_RDWR, 0644)
                   ⎪⎪ die "dbcreat /tmp/foo.db $!";
           $fd = $db->fd;
           print "$$: db fd is $fd\n";
           open(DB_FH, "+<&=$fd") ⎪⎪ die "dup $!";

           unless (flock (DB_FH, LOCK_SH ⎪ LOCK_NB)) {
               print "$$: CONTENTION; can't read during write update!
                           Waiting for read lock ($!) ....";
               unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }
           }
           print "$$: Read lock granted\n";

           $oldval = $db{$key};
           print "$$: Old value was $oldval\n";
           flock(DB_FH, LOCK_UN);

           unless (flock (DB_FH, LOCK_EX ⎪ LOCK_NB)) {
               print "$$: CONTENTION; must have exclusive lock!
                           Waiting for write lock ($!) ....";
               unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }
           }

           print "$$: Write lock granted\n";
           $db{$key} = $value;
           $db->sync;  # to flush
           sleep 10;

           flock(DB_FH, LOCK_UN);
           undef $db;
           untie %db;
           close(DB_FH);
           print "$$: Updated db to $key=$value\n";


       Sharing Databases With C Applications

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

       The vast majority of problems that are reported in this area boil down
       to the fact that C strings are NULL terminated, whilst Perl strings are
       not.

       Here is a real example. Netscape 2.0 keeps a record of the locations
       you visit along with the time you last visited them in a DB_HASH
       database.  This is usually stored in the file ~/.netscape/history.db.
       The key field in the database is the location string and the value
       field is the time the location was last visited stored as a 4 byte
       binary value.

       If you haven't already guessed, the location string is stored with a
       terminating NULL. This means you need to be careful when accessing the
       database.

       Here is a snippet of code that is loosely based on Tom Christiansen's
       ggh script (available from your nearest CPAN archive in
       authors/id/TOMC/scripts/nshist.gz).

           use strict ;
           use DB_File ;
           use Fcntl ;

           use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;
           $dotdir = $ENV{HOME} ⎪⎪ $ENV{LOGNAME};

           $HISTORY = "$dotdir/.netscape/history.db";

           tie %hist_db, 'DB_File', $HISTORY
               or die "Cannot open $HISTORY: $!\n" ;;

           # Dump the complete database
           while ( ($href, $binary_time) = each %hist_db ) {

               # remove the terminating NULL
               $href =~ s/\x00$// ;

               # convert the binary time into a user friendly string
               $date = localtime unpack("V", $binary_time);
               print "$date $href\n" ;
           }

           # check for the existence of a specific key
           # remember to add the NULL
           if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {
               $date = localtime unpack("V", $binary_time) ;
               print "Last visited mox.perl.com on $date\n" ;
           }
           else {
               print "Never visited mox.perl.com\n"
           }

           untie %hist_db ;


       The untie() Gotcha

       If you make use of the Berkeley DB API, it is very strongly recommended
       that you read the section on The untie Gotcha in the perltie manpage.

       Even if you don't currently make use of the API interface, it is still
       worth reading it.

       Here is an example which illustrates the problem from a DB_File
       perspective:

           use DB_File ;
           use Fcntl ;

           my %x ;
           my $X ;

           $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR⎪O_TRUNC
               or die "Cannot tie first time: $!" ;

           $x{123} = 456 ;

           untie %x ;

           tie %x, 'DB_File', 'tst.fil' , O_RDWR⎪O_CREAT
               or die "Cannot tie second time: $!" ;

           untie %x ;

       When run, the script will produce this error message:

           Cannot tie second time: Invalid argument at bad.file line 14.

       Although the error message above refers to the second tie() statement
       in the script, the source of the problem is really with the untie()
       statement that precedes it.

       Having read the perltie manpage you will probably have already guessed
       that the error is caused by the extra copy of the tied object stored in
       $X.  If you haven't, then the problem boils down to the fact that the
       DB_File destructor, DESTROY, will not be called until all references to
       the tied object are destroyed. Both the tied variable, %x, and $X above
       hold a reference to the object. The call to untie() will destroy the
       first, but $X still holds a valid reference, so the destructor will not
       get called and the database file tst.fil will remain open. The fact
       that Berkeley DB then reports the attempt to open a database that is
       alreday open via the catch-all "Invalid argument" doesn't help.

       If you run the script with the -w flag the error message becomes:

           untie attempted while 1 inner references still exist at bad.file line 12.
           Cannot tie second time: Invalid argument at bad.file line 14.

       which pinpoints the real problem. Finally the script can now be
       modified to fix the original problem by destroying the API object
       before the untie:

           ...
           $x{123} = 456 ;

           undef $X ;
           untie %x ;

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


COMMON QUESTIONS
       Why is there Perl source in my database?

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

       This happens because Berkeley DB uses dynamic memory to allocate
       buffers which will subsequently be written to the database file. Being
       dynamic, the memory could have been used for anything before DB
       malloced it. As Berkeley DB doesn't clear the memory once it has been
       allocated, the unused portions will contain random junk. In the case
       where a Perl script gets written to the database, the random junk will
       correspond to an area of dynamic memory that happened to be used during
       the compilation of the script.

       Unless you don't like the possibility of there being part of your Perl
       scripts embedded in a database file, this is nothing to worry about.

       How do I store complex data structures with DB_File?

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

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

       What does ""Invalid Argument"" mean?

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

       Here are a couple of possibilities:

       1.   Attempting to reopen a database without closing it.

       2.   Using the O_WRONLY flag.

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

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

           use strict ;
           use DB_File ;
           use vars qw(%x) ;
           tie %x, DB_File, "filename" ;

       Running it produces the error in question:

           Bareword "DB_File" not allowed while "strict subs" in use

       To get around the error, place the word DB_File in either single or
       double quotes, like this:

           tie %x, "DB_File", "filename" ;

       Although it might seem like a real pain, it is really worth the effort
       of having a use strict in all your scripts.

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 the
       CPAN entry in the perlmod manpage for details), in the directory
       modules/by-module/DB_File.

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

       The official web site for Berkeley DB is http://www.sleepycat.com/db.
       The ftp equivalent is ftp.sleepycat.com:/pub. Both versions 1 and 2 of
       Berkeley DB are available there.

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

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

COPYRIGHT
       Copyright (c) 1995-8 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 the section on AUTHOR
       for details.

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

AUTHOR
       The DB_File interface was written by Paul Marquess
       <pmarquess@bfsec.bt.co.uk>.  Questions about the DB system itself may
       be addressed to <db@sleepycat.com<gt>.
































































3rd Berkeley Distribution    perl 5.005, patch 02                   DB_File(3)