NAME
cdb —
format of the constant
database
DESCRIPTION
Thecdb database format provides a space-efficient
(key,value) database. The format doesn't allow updates in any convenient form.
The file overhead is around 5 bytes per key and 5 bytes per entry. Keys are
not stored and it is the responsibility of the caller to validate matches. The
index structure is based on a minimal perfect hash table, so exactly one entry
has to be checked for a match.
General Format
The header record of a cdb database
consists of the following:
struct header_cdb {
uint8_t magic[7];
uint8_t version;
uint8_t description[16];
uint32_t data_size;
uint32_t entries;
uint32_t entries_index;
uint32_t seed;
};
All fields are in Little Endian byte order.
This is followed by a description of the hash function of entries_index records. The size of each index entry is the logarithm of entries to base 256, rounded up.
The index records are followed by the start offsets of the entries, followed by data_size. The offsets are relative to the end of the offset record table and are monotonically increasing. The size of each offset record is the logarithm of data_size to base 256, rounded up.
The offset table is followed by the entries in order. No separation or padding is added.
Limitations
The cdb file format is by design intended
for a database that can be mapped into memory. The hard limit for the number
of entries and keys is 3435973836. The total size of all values must be
smaller than 4GiB.
SEE ALSO
HISTORY
Support for the cdb format first appeared
in NetBSD 6.0.
AUTHORS
The cdbr and cdbw
functions have been written by Joerg Sonnenberger
<joerg@NetBSD.org>.