.\"	$NetBSD: cdb.5,v 1.6 2015/02/08 19:09:56 wiz Exp $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Joerg Sonnenberger.
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.Dd April 27, 2010
.Dt CDB 5
.Os
.Sh NAME
.Nm cdb
.Nd format of the constant database
.Sh DESCRIPTION
The
.Nm
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.
.Ss General Format
The header record of a
.Nm
database consists of the following:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
All fields are in Little Endian byte order.
.Pp
This is followed by a description of the hash function of
.Va entries_index
records.
The size of each index entry is the logarithm of
.Va entries
to base 256, rounded up.
.Pp
The index records are followed by the start offsets of the entries,
followed by
.Va 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
.Va data_size
to base 256, rounded up.
.Pp
The offset table is followed by the entries in order.
No separation or padding is added.
.Ss Limitations
The
.Nm
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.
.Sh SEE ALSO
.Xr cdbr 3 ,
.Xr cdbw 3
.Sh HISTORY
Support for the
.Nm cdb
format first appeared in
.Nx 6.0 .
.Sh AUTHORS
The
.Nm cdbr
and
.Nm cdbw
functions have been written by
.An Joerg Sonnenberger Aq Mt joerg@NetBSD.org .