cdb(5)								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 begin-
       ning  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)