NAME

DB_File - Perl5 access to Berkeley DB

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 ;
        
     untie %hash ;
     untie @array ;

DESCRIPTION

DB_File is a module which allows Perl programs to make use of the facilities provided by Berkeley DB. If you intend to use this module you should really have a copy of the Berkeley DB manual page at hand. 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/data 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/data 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.

How does 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 perlfunc/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 use most of the functions provided in the Berkeley DB API.

Differences with Berkeley DB

Berkeley DB uses the function dbopen() to open or create a database. Below 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 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 ;

RECNO

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.

In Memory Databases

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

Using the Berkeley DB Interface Directly

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

To do this you need to remember the return value from the tie. 

            $db = tie %hash, DB_File, "filename"
Once you have done that, you can access the Berkeley DB API functions directly. 

            $db->put($key, $value, R_NOOVERWRITE) ;
All the functions defined in dbx(3X) are available except for close() and dbopen() itself. The DB_File interface to these functions have been implemented to mirror the the way Berkeley DB works. In particular note that all the functions return only a status value. Whenever a Berkeley DB function returns data via one of its parameters, the DB_File equivalent does exactly the same.

All the constants defined in dbopen are also available.

Below is a list of the functions available.

get
Same as in recno except that the flags parameter is optional. Remember the value associated with the key you request is returned in the $value parameter.

put
As usual the flags parameter is optional.

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.

del
The flags parameter is optional.

fd
As in recno.

seq
The flags parameter is optional.

Both the key and value parameters will be set.

sync
The flags parameter is optional.

EXAMPLES

It is always a lot easier to understand something when you see a real example. So here are a few.

Using HASH

            use DB_File ;
            use Fcntl ;
            
            tie %h,  DB_File, "hashed", O_RDWR|O_CREAT, 0640, $DB_HASH ;
            
            # Add a key/value pair to the file
            $h{"apple"} = "orange" ;
            
            # Check for existence of a key
            print "Exists\n" if $h{"banana"} ;
            
            # Delete 
            delete $h{"apple"} ;
            
            untie %h ;

Using BTREE

Here is sample of code which used BTREE. Just to make life more interesting the default comparision function will not be used. Instead a Perl sub, Compare(), will be used to do a case insensitive comparison. 

            use DB_File ;
            use Fcntl ;
             
            sub Compare
            {
                my ($key1, $key2) = @_ ;
            
                "\L$key1" cmp "\L$key2" ;
            }
            
            $DB_BTREE->{compare} = 'Compare' ;
             
            tie %h,  DB_File, "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE ;
             
            # 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

Using RECNO

            use DB_File ;
            use Fcntl ;
            
            $DB_RECNO->{psize} = 3000 ;
            
            tie @h,  DB_File, "text", O_RDWR|O_CREAT, 0640, $DB_RECNO ;
            
            # Add a key/value pair to the file
            $h[0] = "orange" ;
            
            # Check for existence of a key
            print "Exists\n" if $h[1] ;
            
            untie @h ;

WARNINGS

If you happen find any other functions defined in the source for this module that have not been mentioned in this document -- beware. I may drop them at a moments notice.

If you cannot find any, then either you didn't look very hard or the moment has passed and I have dropped them.

BUGS

Some older versions of Berkeley DB had problems with fixed length records using the RECNO file format. The newest version at the time of writing was 1.85 - this seems to have fixed the problems with RECNO.

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

Berkeley DB is available via the hold ftp.cs.berkeley.edu in the directory /ucb/4bsd/db.tar.gz. It is not under the GPL.

SEE ALSO

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

Berkeley DB is available from F

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 Keith Bostic <bostic@cs.berkeley.edu>.