DragonFly On-Line Manual Pages

cdb(5)                   DragonFly File Formats Manual                  cdb(5)

NAME
       cdb - Constant DataBase file format

DESCRIPTION
       A cdb database is a single file used to map `keys' to `values', having
       records of (key,value) pairs.  File consists of 3 parts: toc (table of
       contents), data and index (hash tables).

       Toc has fixed length of 2048 bytes, containing 256 pointers to hash
       tables inside index sections.  Every pointer consists of position of a
       hash table in bytes from the beginning of a file, and a size of a hash
       table in entries, both are 4-bytes (32 bits) unsigned integers in
       little-endian form.  Hash table length may have zero length, meaning
       that corresponding hash table is empty.

       Right after toc section, data section follows without any alingment.
       It consists of series of records, each is a key length, value (data)
       length, key and value.  Again, key and value length are 4-byte unsigned
       integers.  Each next record follows previous without any special
       alignment.

       After data section, index (hash tables) section follows.  It should be
       looked to in conjunction with toc section, where each of max 256 hash
       tables are defined.  Index section consists of series of hash tables,
       with starting position and length defined in toc section.  Every hash
       table is a sequence of records each holds two numbers: key's hash value
       and record position inside data section (bytes from the beginning of a
       file to first byte of key length starting data record).  If record
       position is zero, then this is an empty hash table slot, pointed to
       nowhere.

       CDB hash function is
         hv = ((hv << 5) + hv) ^ c
       for every single c byte of a key, starting with hv = 5381.

       Toc section indexed by (hv % 256), i.e. hash value modulo 256 (number
       of entries in toc section).

       In order to find a record, one should: first, compute the hash value
       (hv) of a key.  Second, look to hash table number hv modulo 256.  If it
       is empty, then there is no such key exists.  If it is not empty, then
       third, loop by slots inside that hash table, starting from slot with
       number hv divided by 256 modulo length of that table, or ((hv / 256) %
       htlen), searching for this hv in hash table.  Stop search on empty slot
       (if record position is zero) or when all slots was probed (note cyclic
       search, jumping from end to beginning of a table).  When hash value in
       question is found in hash table, look to key of corresponding record,
       comparing it with key in question.  If them of the same length and
       equals to each other, then record is found, overwise, repeat with next
       hash table slot.  Note that there may be several records with the same
       key.

SEE ALSO
       cdb(1), cdb(3).

AUTHOR
       The tinycdb package written by Michael Tokarev <mjt@corpit.ru>, based
       on ideas and shares file format with original cdb library by Dan
       Bernstein.

LICENSE
       Public domain.

                                   Apr, 2005                            cdb(5)