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+.\" Copyright (c) 1990, 1993, 1994, 1995
+.\"	The Regents of the University of California.  All rights reserved.
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+.\"	@(#)db_hash.3	8.13 (Berkeley) 8/1/95
+.\"
+.TH DB_HASH 3 "August 1, 1995"
+.UC 7
+.SH NAME
+db_hash \- hash database access method
+.SH DESCRIPTION
+.so db.so
+.GN
+specific details of the hashing access method.
+.PP
+The hash data structure is an extensible, dynamic hashing scheme.
+Backward compatible interfaces to the functions described in
+.IR dbm (3),
+and
+.IR ndbm (3)
+are provided, however these interfaces are not compatible with
+previous file formats.
+.SH "ACCESS METHOD SPECIFIC INFORMATION"
+The hash access method specific data structure provided to
+.I db_open
+is typedef'd and named HASHINFO.
+A HASHINFO structure has at least the following fields,
+which may be initialized before calling
+.IR db_open :
+.TP 5
+u_int bsize;
+.I Bsize
+defines the hash table bucket size, and is, by default, 256 bytes.
+It may be preferable to increase the page size for disk-resident tables
+and tables with large data items.
+.TP 5
+u_int cachesize;
+A suggested maximum size, in bytes, of the memory cache.
+This value is
+.B only
+advisory, and the access method will allocate more memory rather
+than fail.
+.TP 5
+u_int ffactor;
+.I Ffactor
+indicates a desired density within the hash table.
+It is an approximation of the number of keys allowed to accumulate in any
+one bucket, determining when the hash table grows or shrinks.
+The default value is 8.
+.TP 5
+u_int32_t (*hash)(const void *, size_t);
+.I Hash
+is a user defined hash function.
+Since no hash function performs equally well on all possible data, the
+user may find that the built-in hash function does poorly on a particular
+data set.
+User specified hash functions must take two arguments (a pointer to a byte
+string and a length) and return a 32-bit quantity to be used as the hash
+value.
+.IP
+If a hash function is specified,
+.I hash_open
+will attempt to determine if the hash function specified is the same as
+the one with which the database was created, and will fail if it is not.
+.TP 5
+int lorder;
+The byte order for integers in the stored database metadata.
+The number should represent the order as an integer; for example, 
+big endian order would be the number 4,321.
+If
+.I lorder
+is 0 (no order is specified) the current host order is used.
+If the  file already exists, the specified value is ignored and the
+value specified when the tree was created is used.
+.TP 5
+u_int nelem;
+.I Nelem
+is an estimate of the final size of the hash table.
+If not set or set too low, hash tables will expand gracefully as keys
+are entered, although a slight performance degradation may be noticed.
+The default value is 1.
+.PP
+If the file already exists (and the O_TRUNC flag is not specified), the
+values specified for the parameters bsize, ffactor, lorder and nelem are
+ignored and the values specified when the tree was created are used.
+.SH "DB OPERATIONS"
+The functions returned by
+.I db_open
+for the hash access method are as described in
+.IR db_open (3).
+.SH ERRORS
+The
+.I hash
+access method functions may fail and set
+.I errno
+for any of the errors specified for the library function
+.IR db_open (3).
+.SH "SEE ALSO"
+.IR db_btree (3),
+.IR db_lock (3),
+.IR db_log (3),
+.IR db_mpool (3),
+.IR db_open (3),
+.IR db_recno (3),
+.IR db_txn (3)
+.sp
+.IR "Dynamic Hash Tables" ,
+Per-Ake Larson, Communications of the ACM, April 1988.
+.sp
+.IR "A New Hash Package for UNIX" ,
+Margo Seltzer, USENIX Proceedings, Winter 1991.