NAME

CDB_File - Perl extension for access to cdb databases


SYNOPSIS

    use CDB_File;
    $c = tie %h, 'CDB_File', 'file.cdb' or die "tie failed: $!\n";
    $fh = $c->handle;
    sysseek $fh, $c->datapos, 0 or die ...;
    sysread $fh, $x, $c->datalen;
    undef $c;
    untie %h;
    $t = new CDB_File ('t.cdb', "t.$$") or die ...;
    $t->insert('key', 'value');
    $t->finish;
    CDB_File::create %t, $file, "$file.$$";

or

    use CDB_File 'create';
    create %t, $file, "$file.$$";


DESCRIPTION

CDB_File is a module which provides a Perl interface to Dan Berstein's cdb package:

    cdb is a fast, reliable, lightweight package for creating and
    reading constant databases.

Reading from a cdb

After the tie shown above, accesses to %h will refer to the cdb file file.cdb, as described in tie in the perlfunc manpage.

Low level access to the database is provided by the three methods handle, datapos, and datalen. To use them, you must remember the CDB_File object returned by the tie call: $c in the example above. The datapos and datalen methods return the file offset position and length respectively of the most recently visited key (for example, via exists).

Beware that if you create an extra reference to the CDB_File object (like $c in the example above) you must destroy it (with undef) before calling untie on the hash. This ensures that the object's DESTROY method is called. Note that perl -w will check this for you; see the perltie manpage for further details.

Creating a cdb

A cdb file is created in three steps. First call new CDB_File ($final, $tmp), where $final is the name of the database to be created, and $tmp is the name of a temporary file which can be atomically renamed to $final. Secondly, call the insert method once for each (key, value) pair. Finally, call the finish method to complete the creation and renaming of the cdb file.

A simpler interface to cdb file creation is provided by CDB_File::create %t, $final, $tmp. This creates a cdb file named $final containing the contents of %t. As before, $tmp must name a temporary file which can be atomically renamed to $final. CDB_File::create may be imported.


EXAMPLES

These are all complete programs.

1. Convert a Berkeley DB (B-tree) database to cdb format.

    use CDB_File;
    use DB_File;
    tie %h, DB_File, $ARGV[0], O_RDONLY, undef, $DB_BTREE or
            die "$0: can't tie to $ARGV[0]: $!\n";
    CDB_File::create %h, $ARGV[1], "$ARGV[1].$$" or
            die "$0: can't create cdb: $!\n";

2. Convert a flat file to cdb format. In this example, the flat file consists of one key per line, separated by a colon from the value. Blank lines and lines beginning with # are skipped.

    use CDB_File;
    $cdb = new CDB_File("data.cdb", "data.$$") or
            die "$0: new CDB_File failed: $!\n";
    while (<>) {
            next if /^$/ or /^#/;
            chop;
            ($k, $v) = split /:/, $_, 2;
            if (defined $v) {
                    $cdb->insert($k, $v);
            } else {
                    warn "bogus line: $_\n";
            }
    }
    $cdb->finish or die "$0: CDB_File finish failed: $!\n";

3. Perl version of cdbdump.

    use CDB_File;
    tie %data, 'CDB_File', $ARGV[0] or
            die "$0: can't tie to $ARGV[0]: $!\n";
    while (($k, $v) = each %data) {
            print '+', length $k, ',', length $v, ":$k->$v\n";
    }
    print "\n";

4. For really enormous data values, you can use handle, datapos, and datalen, in combination with sysseek and sysread, to avoid reading the values into memory. Here is the script bun-x.pl, which can extract uncompressed files and directories from a bun file.

    use CDB_File;
    sub unnetstrings {
        my($netstrings) = @_;
        my @result;
        while ($netstrings =~ s/^([0-9]+)://) {
                push @result, substr($netstrings, 0, $1, '');
                $netstrings =~ s/^,//;
        }
        return @result;
    }
    
    my $chunk = 8192;
    
    sub extract {
        my($file, $t, $b) = @_;
        my $head = $$b{"H$file"};
        my ($code, $type) = $head =~ m/^([0-9]+)(.)/;
        if ($type eq "/") {
                mkdir $file, 0777;
        } elsif ($type eq "_") {
                my ($total, $now, $got, $x);
                open OUT, ">$file" or die "open for output: $!\n";
                exists $$b{"D$code"} or die "corrupt bun file\n";
                my $fh = $t->handle;
                sysseek $fh, $t->datapos, 0;
                $total = $t->datalen;
                while ($total) {
                        $now = ($total > $chunk) ? $chunk : $total;
                        $got = sysread $fh, $x, $now;
                        if (not $got) { die "read error\n"; }
                        $total -= $got;
                        print OUT $x;
                }
                close OUT;
        } else {
                print STDERR "warning: skipping unknown file type\n";
        }
    }
    die "usage\n" if @ARGV != 1;
    my (%b, $t);
    $t = tie %b, 'CDB_File', $ARGV[0] or die "tie: $!\n";
    map { extract $_, $t, \%b } unnetstrings $b{""};

5. Although a cdb file is constant, you can simulate updating it in Perl. This is an expensive operation, as you have to create a new database, and copy into it everything that's unchanged from the old database. (As compensation, the update does not affect database readers. The old database is available for them, till the moment the new one is finished.)

    use CDB_File;
    $file = 'data.cdb';
    $new = new CDB_File($file, "$file.$$") or
            die "$0: new CDB_File failed: $!\n";
    # Add the new values; remember which keys we've seen.
    while (<>) {
            chop;
            ($k, $v) = split;
            $new->insert($k, $v);
            $seen{$k} = 1;
    }
    # Add any old values that haven't been replaced.
    tie %old, 'CDB_File', $file or die "$0: can't tie to $file: $!\n";
    while (($k, $v) = each %old) {
            $new->insert($k, $v) unless $seen{$k};
    }
    $new->finish or die "$0: CDB_File finish failed: $!\n";


REPEATED KEYS

Most users can ignore this section.

A cdb file can contain repeated keys. If the insert method is called more than once with the same key during the creation of a cdb file, that key will be repeated.

Here's an example.

    $cdb = new CDB_File ("$file.cdb", "$file.$$") or die ...;
    $cdb->insert('cat', 'gato');
    $cdb->insert('cat', 'chat');
    $cdb->finish;

Normally, any attempt to access a key retrieves the first value stored under that key. This code snippet always prints gato.

    $catref = tie %catalogue, CDB_File, "$file.cdb" or die ...;
    print "$catalogue{cat}";

However, all the usual ways of iterating over a hash---keys, values, and each---do the Right Thing, even in the presence of repeated keys. This code snippet prints cat cat gato chat.

    print join(' ', keys %catalogue, values %catalogue);

And these two both print cat:gato cat:chat, although the second is more efficient.

    foreach $key (keys %catalogue) {
            print "$key:$catalogue{$key} ";
    }
    while (($key, $val) = each %catalogue) {
            print "$key:$val ";
    }

The multi_get method retrieves all the values associated with a key. It returns a reference to an array containing all the values. This code prints gato chat.

    print "@{$catref->multi_get('cat')}";

multi_get always returns an array reference. If the key was not found in the database, it will be a reference to an empty array. To test whether the key was found, you must test the array, and not the reference.

    $x = $catref->multiget($key);
    warn "$key not found\n" unless $x; # WRONG; message never printed
    warn "$key not found\n" unless @$x; # Correct


RETURN VALUES

The routines tie, new, and finish return undef if the attempted operation failed; $! contains the reason for failure.


DIAGNOSTICS

The following fatal errors may occur. (See eval in the perlfunc manpage if you want to trap them.)

Modification of a CDB_File attempted
You attempted to modify a hash tied to a CDB_File.

CDB database too large
You attempted to create a cdb file larger than 4 gigabytes.

[ Write to | Read of | Seek in ] CDB_File failed: <error string>
If error string is Protocol error, you tried to use CDB_File to access something that isn't a cdb file. Otherwise a serious OS level problem occurred, for example, you have run out of disk space.

Use CDB_File::FIRSTKEY before CDB_File::NEXTKEY
If you are using the NEXTKEY method directly (I can't think of a reason why you'd want to do this), you need to call FIRSTKEY first.


BUGS

The create() inteface could be done with TIEHASH.


SEE ALSO

cdb(3).


AUTHOR

Tim Goodwin, <tjg@star.le.ac.uk>. CDB_File began on 1997-01-08.

Now maintained by Matt Sergeant, <matt@sergeant.org>