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CDB_File(3pm) User Contributed Perl Documentation CDB_File(3pm)


CDB_File - Perl extension for access to cdb databases


    use CDB_File;
    $c = tie(%h, 'CDB_File', 'file.cdb') or die "tie failed: $!\n";
    # If accessing a utf8 stored CDB_File
    $c = tie(%h, 'CDB_File', 'file.cdb', utf8 => 1) 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 = CDB_File->new('t.cdb', "t.$$") or die ...;
    $t->insert('key', 'value');
    CDB_File::create %t, $file, "$file.$$";


    use CDB_File 'create';
    create %t, $file, "$file.$$";
    # If you want to store the data in utf8 mode.
    create %t, $file, "$file.$$", utf8 => 1;

CDB_File is a module which provides a Perl interface to Dan Bernstein'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 perlfunc.

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 perltie 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.

Alternatively, call the "insert()" method with multiple key/value pairs. This can be significantly faster because there is less crossing over the bridge from perl to C code. One simple way to do this is to pass in an entire hash, as in: "$cdbmaker->insert(%hash);".

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.

UTF8 support.

When CDB_File was created in 1997 (prior even to Perl 5.6), Perl SVs didn't really deal with UTF8. In order to properly store mixed bytes and utf8 data in the file, we would normally need to store a bit for each string which clarifies the encoding of the key / values. This would be useful since Perl hash keys are downgraded to bytes when possible so as to normalize the hash key access regardless of encoding.

The CDB_File format is used outside of Perl and so must maintain file format compatibility with those systems. As a result this module provides a utf8 mode which must be enabled at database generation and then later at read. Keys will always be stored as UTF8 strings which is the opposite of how Perl stores the strings. This approach had to be taken to assure no data corruption happened due to accidentally downgraded SVs before they are stored or on retrieval.

You can enable utf8 mode by passing "utf8 => 1" to new, tie, or create. All returned SVs while in this mode will be encoded in utf8. This feature is not available below 5.14 due to lack of Perl macro support.

NOTE: read/write of databases not stored in utf8 mode will often be incompatible with any non-ascii data.


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 /^#/;
            ($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, 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 "finish"ed.)

    use CDB_File;
    $file = 'data.cdb';
    $new = new CDB_File($file, "$file.$$") or
            die "$0: new CDB_File failed: $!\n";
    # Add the new values; remember which keys we've seen.
    while (<>) {
            ($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";


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');

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

The "fetch_all" method returns a hashref of all keys with the first value in the cdb. This is useful for quickly loading a cdb file where there is a 1:1 key mapping. In practice it proved to be about 400% faster then iterating a tied hash.

    # Slow
    my %copy = %tied_cdb;
    # Much Faster
    my $copy_hashref = $catref->fetch_all();


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


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

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


Sometimes you need to get the most performance possible out of a library. Rumour has it that perl's tie() interface is slow. In order to get around that you can use CDB_File in an object oriented fashion, rather than via tie().

  my $cdb = CDB_File->TIEHASH('/path/to/cdbfile.cdb');
  if ($cdb->EXISTS('key')) {
      print "Key is: ", $cdb->FETCH('key'), "\n";

For more information on the methods available on tied hashes see perltie.


This algorithm is described at <> It is small enough that it is included inline in the event that the internet loses the page:

A structure for constant databases

Copyright (c) 1996 D. J. Bernstein,

A cdb is an associative array: it maps strings ('keys'') to strings ('data'').

A cdb contains 256 pointers to linearly probed open hash tables. The hash tables contain pointers to (key,data) pairs. A cdb is stored in a single file on disk:

    | p0 p1 ... p255 | records | hash0 | hash1 | ... | hash255 |

Each of the 256 initial pointers states a position and a length. The position is the starting byte position of the hash table. The length is the number of slots in the hash table.

Records are stored sequentially, without special alignment. A record states a key length, a data length, the key, and the data.

Each hash table slot states a hash value and a byte position. If the byte position is 0, the slot is empty. Otherwise, the slot points to a record whose key has that hash value.

Positions, lengths, and hash values are 32-bit quantities, stored in little-endian form in 4 bytes. Thus a cdb must fit into 4 gigabytes.

A record is located as follows. Compute the hash value of the key in the record. The hash value modulo 256 is the number of a hash table. The hash value divided by 256, modulo the length of that table, is a slot number. Probe that slot, the next higher slot, and so on, until you find the record or run into an empty slot.

The cdb hash function is "h = ((h << 5) + h) ^ c", with a starting hash of 5381.


The "create()" interface could be done with "TIEHASH".




Tim Goodwin, <>. CDB_File began on 1997-01-08.

Work provided through 2008 by Matt Sergeant, <>

Now maintained by Todd Rinaldo, <>

2020-12-14 perl v5.32.0