lib:DB_File

See the current Perl documentation for lib:DB_File.

Here is our local, out-dated (pre-5.6) version:

DB_File - Perl5 access to Berkeley DB version 1.x

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

    

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). 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 COPYRIGHT 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
 tie). This facility allows DB_File to access Berkeley 
DB files using either an associative array (for 
DB_HASH & 
DB_BTREE file types) or an ordinary array (for the 
DB_RECNO file type).


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



Opening a Berkeley DB Database File

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


.

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

.

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 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 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:


.

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) )

.

Notice that both times the put method was used the record index was specified using a variable, $i, rather than the literal value itself. This is because put will return the record number of the inserted line via that parameter.




THE API INTERFACE

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


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


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


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


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


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


    use DB_File ;
    $db = tie %hash, "DB_File", "filename" 
        or die "Cannot tie filename: $!" ;
    ...
    undef $db ;
    untie %hash ;


See The untie() Gotcha for more details.


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




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 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
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.




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 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
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:


.

Attempting to reopen a database without closing it. 

.

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

perlmod 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
 AUTHOR for details.



SEE ALSO

perl(1), dbopen(3), hash(3), recno(3), btree(3) 

 



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.