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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) ;
        $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 Berkeley DB version 2 or greater").  It is
       assumed that you have a copy of the Berkeley DB manual pages at hand when reading this documentation. The
       interface defined here mirrors the Berkeley DB interface closely.

       Berkeley DB is a C library which provides a consistent interface to a number of database formats.  DB_File pro-
       vides 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 compari-
            son 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 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 provided by Berkeley DB 1.x. Anywhere the version 2
       or greater interface differs, DB_File arranges for it to work like version 1. This feature allows DB_File
       scripts that were built with version 1 to be migrated to version 2 or greater without any changes.

       If you want to make use of the new features available in Berkeley DB 2.x or greater, use the Perl module Berke-
       leyDB 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 Berke-
       ley 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 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 "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 database:

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

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

       3    Duplicate keys are entirely defined by the comparison function.  In the case-insensitive example above,
            the keys: 'KEY' and 'key' would be considered duplicates, and assigning to the second one would overwrite
            the first. If duplicates are allowed for (with the R_DUP flag discussed below), only a single copy of
            duplicate keys is stored in the database --- so (again with example above) assigning three values to the
            keys: 'KEY', 'Key', and 'key' would leave just the first key: 'KEY' in the database with three values. For
            some situations this results in information loss, so care should be taken to provide fully qualified com-
            parison functions when necessary.  For example, the above comparison routine could be modified to addi-
            tionally 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 number of values. This
       option is enabled by setting the flags element of $DB_BTREE to R_DUP when creating the database.

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

           use warnings ;
           use strict ;
           use DB_File ;

           my ($filename, %h) ;

           $filename = "tree" ;
           unlink $filename ;

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

           tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
               or die "Cannot open $filename: $!\n";

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

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

           untie %h ;

       Here is the output:

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

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

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

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

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

           use warnings ;
           use strict ;
           use DB_File ;

           my ($filename, $x, %h, $status, $key, $value) ;

           $filename = "tree" ;
           unlink $filename ;

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

           $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
               or die "Cannot open $filename: $!\n";

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

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

           undef $x ;
           untie %h ;

       that prints:

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

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

       To make life easier when dealing with duplicate keys, DB_File comes with a few utility methods.

       The get_dup() Method

       The "get_dup" method assists in reading duplicate values from BTREE databases. The method can take the follow-
       ing 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 functionality is only available
       when the "seq" method is used along with the R_CURSOR flag.

           $x->seq($key, $value, R_CURSOR) ;

       Here is the relevant quote from the dbopen man page where it defines the use of the R_CURSOR flag with seq:

           Note, for the DB_BTREE access method, the returned key is not
           necessarily an exact match for the specified key. The returned key
           is the smallest key greater than or equal to the specified key,
           permitting partial key matches and range searches.

       In the example script below, the "match" sub uses this feature to find and print the first matching key/value
       pair given a partial key.

           use warnings ;
           use strict ;
           use DB_File ;
           use Fcntl ;

           my ($filename, $x, %h, $st, $key, $value) ;

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

           $filename = "tree" ;
           unlink $filename ;

           $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
               or die "Cannot open $filename: $!\n";

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

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

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

           print "\nPARTIAL MATCH\n" ;

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

           undef $x ;
           untie %h ;

       Here is the output:

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

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

DB_RECNO
       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 parame-
       ter. This documentation error will be fixed in the next release of Berkeley DB.

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

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

       Also note that the bval option only allows you to specify a single byte as a delimiter.

       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 determining the array length will not work with a
       tied array.

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

       Here are the methods:

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

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

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

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

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

       $X->splice(offset, length, elements);
            Returns a splice of the array.

       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 possi-
       ble. In particular note that:

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

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

       ?    Whenever a Berkeley DB function returns data via one of its parameters, the equivalent DB_File method does
            exactly the same.

       ?    If you are careful, it is possible to mix API calls with the tied hash/array interface in the same piece
            of code. Although only a few of the methods used to implement the tied interface currently make use of the
            cursor, you should always assume that the cursor has been changed any time the tied hash/array interface
            is used. As an example, this code will probably not do what you expect:

                $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
                    or die "Cannot tie $filename: $!" ;

                # Get the first key/value pair and set  the cursor
                $X->seq($key, $value, R_FIRST) ;

                # this line will modify the cursor
                $count = scalar keys %x ;

                # Get the second key/value pair.
                # oops, it didn't, it got the last key/value pair!
                $X->seq($key, $value, R_NEXT) ;

            The code above can be rearranged to get around the problem, like this:

                $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
                    or die "Cannot tie $filename: $!" ;

                # this line will modify the cursor
                $count = scalar keys %x ;

                # Get the first key/value pair and set  the cursor
                $X->seq($key, $value, R_FIRST) ;

                # Get the second key/value pair.
                # worked this time.
                $X->seq($key, $value, R_NEXT) ;

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

       Below is a list of the methods available.

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

            If the key does not exist the method returns 1.

            No flags are currently defined for this method.

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

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

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

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

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

            R_CURSOR is the only valid flag at present.

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

            See "Locking: The Trouble with fd" for an explanation for why you should not use "fd" to lock your
            database.

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

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

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

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

            R_RECNOSYNC is the only valid flag at present.

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 unin-
       stall) 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 applica-
       tion. The C application assumes that all keys and values are NULL terminated. Unfortunately when Perl writes to
       DBM databases it doesn't use NULL termination, so your Perl application will have to manage NULL termination
       itself. When you write to the database you will have to use something like this:

           $hash{"$key\0"} = "$value\0" ;

       Similarly the NULL needs to be taken into account when you are considering the length of existing keys/values.

       It would be much better if you could ignore the NULL terminations issue in the main application code and have a
       mechanism that automatically added the terminating NULL to all keys and values whenever you write to the
       database and have them removed when you read from the database. As I'm sure you have already guessed, this is a
       problem that DBM Filters can fix very easily.

           use warnings ;
           use strict ;
           use DB_File ;

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

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

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

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

       Hopefully the contents of each of the filters should be self-explanatory. Both "fetch" filters remove the ter-
       minating 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} = "something" ;

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

       Here is a DBM Filter that does it:

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

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

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

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

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', 'foo.db', O_CREAT|O_RDWR, 0644)
               || die "dbcreat foo.db $!";
           $fd = $db->fd;
           open(DB_FH, "+<&=$fd") || die "dup $!";
           flock (DB_FH, LOCK_EX) || die "flock: $!";
           ...
           $db{"Tom"} = "Jerry" ;
           ...
           flock(DB_FH, LOCK_UN);
           undef $db;
           untie %db;
           close(DB_FH);

       In simple terms, this is what happens:

       1.   Use "tie" to open the database.

       2.   Lock the database with fd & flock.

       3.   Read & Write to the database.

       4.   Unlock and close the database.

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

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

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

       With the lock released, process "B" can now continue. It also updates the database and unfortunately it too
       modifies the data that was in its initial buffer. Once that data gets flushed to disk it will overwrite
       some/all of the changes process "A" made to the database.

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

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

       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 BerkeleyDB.

       If using BerkeleyDB isn't an option, there are a number of modules available on CPAN that can be used to imple-
       ment locking. Each one implements locking differently and has different goals in mind. It is therefore worth
       knowing the difference, so that you can pick the right one for your application. Here are the three locking
       wrappers:

       Tie::DB_Lock
            A DB_File wrapper which creates copies of the database file for read access, so that you have a kind of a
            multiversioning concurrent read system. However, updates are still serial. Use for databases where reads
            may be lengthy and consistency problems may occur.

       Tie::DB_LockFile
            A DB_File wrapper that has the ability to lock and unlock the database while it is being used. Avoids the
            tie-before-flock problem by simply re-tie-ing the database when you get or drop a lock.  Because of the
            flexibility in dropping and re-acquiring the lock in the middle of a session, this can be massaged into a
            system that will work with long updates and/or reads if the application follows the hints in the POD docu-
            mentation.

       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 dead-
            lock 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 ter-
       minated, whilst Perl strings are not. See "DBM FILTERS" for a generic way to work around this problem.

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

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

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

           use warnings ;
           use strict ;
           use DB_File ;
           use Fcntl ;

           my ($dotdir, $HISTORY, %hist_db, $href, $binary_time, $date) ;
           $dotdir = $ENV{HOME} || $ENV{LOGNAME};

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

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

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

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

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

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

           untie %hist_db ;

       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 reference to the object. The call to untie() will destroy the first, but $X still holds a
       valid reference, so the destructor will not get called and the database file tst.fil will remain open. The fact
       that Berkeley DB then reports the attempt to open a database that is already open via the catch-all "Invalid
       argument" doesn't help.

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

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

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

           ...
           $x{123} = 456 ;

           undef $X ;
           untie %x ;

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

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 Berke-
       ley 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 (tkientzleATddj.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 perlmodlib for details), in the directory mod-
       ules/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 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) 1995-2005 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, dbopen(3), hash(3), recno(3), btree(3), perldbmfilter

AUTHOR
       The DB_File interface was written by Paul Marquess <pmqsATcpan.org>.  Questions about the DB system itself may
       be addressed to <dbATsleepycat.com>.



perl v5.8.8                       2001-09-21                        DB_File(3)