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CDB(5)			    BSD File Formats Manual			CDB(5)

NAME
     cdb — format of the constant database

DESCRIPTION
     The cdb database format provides a space-efficient (key,value) database.
     The format doesn't allow updates in any convenient form.  The file over‐
     head 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, fol‐
     lowed by data_size.  The offsets are relative to the end of the offset
     record table and are monotically 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
     cdbr(3), cdbw(3)

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⟩.

BSD				April 27, 2010				   BSD

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