1.\" $NetBSD: hash.3,v 1.10 2003/08/07 16:42:43 agc Exp $ 2.\" 3.\" Copyright (c) 1990, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)hash.3 8.6 (Berkeley) 8/18/94 31.\" 32.Dd April 17, 2003 33.Dt HASH 3 34.Os 35.Sh NAME 36.Nm hash 37.Nd hash database access method 38.Sh SYNOPSIS 39.In sys/types.h 40.In db.h 41.Sh DESCRIPTION 42The routine 43.Fn dbopen 44is the library interface to database files. 45One of the supported file formats is hash files. 46The general description of the database access methods is in 47.Xr dbopen 3 , 48this manual page describes only the hash specific information. 49.Pp 50The hash data structure is an extensible, dynamic hashing scheme. 51.Pp 52The access method specific data structure provided to 53.Fn dbopen 54is defined in the 55.Aq Pa db.h 56include file as follows: 57.Bd -literal 58typedef struct { 59 u_int bsize; 60 u_int ffactor; 61 u_int nelem; 62 u_int cachesize; 63 u_int32_t (*hash)(const void *, size_t); 64 int lorder; 65} HASHINFO; 66.Ed 67.Pp 68The elements of this structure are as follows: 69.Bl -tag -width cachesizex 70.It Fa bsize 71.Fa bsize 72defines the hash table bucket size, and is, by default, 256 bytes. 73It may be preferable to increase the page size for disk-resident 74tables and tables with large data items. 75.It Fa ffactor 76.Fa ffactor 77indicates a desired density within the hash table. 78It is an approximation of the number of keys allowed to accumulate in 79any one bucket, determining when the hash table grows or shrinks. 80The default value is 8. 81.It Fa nelem 82.Fa nelem 83is an estimate of the final size of the hash table. 84If not set or set too low, hash tables will expand gracefully as keys 85are entered, although a slight performance degradation may be 86noticed. 87The default value is 1. 88.It Fa cachesize 89A suggested maximum size, in bytes, of the memory cache. 90This value is 91.Em only 92advisory, and the access method will allocate more memory rather 93than fail. 94.It Fa hash 95.Fa hash 96is a user defined hash function. 97Since no hash function performs equally well on all possible data, the 98user may find that the built-in hash function does poorly on a 99particular data set. 100User specified hash functions must take two arguments (a pointer to a 101byte string and a length) and return a 32-bit quantity to be used as 102the hash value. 103.It Fa lorder 104The byte order for integers in the stored database metadata. 105The number should represent the order as an integer; for example, 106big endian order would be the number 4,321. 107If 108.Fa lorder 109is 0 (no order is specified) the current host order is used. 110If the file already exists, the specified value is ignored and the 111value specified when the tree was created is used. 112.El 113.Pp 114If the file already exists (and the 115.Dv O_TRUNC 116flag is not specified), the values specified for the parameters 117.Fa bsize , 118.Fa ffactor , 119.Fa lorder , 120and 121.Fa nelem 122are ignored and the values specified when the tree was created are 123used. 124.Pp 125If a hash function is specified, 126.Fn hash_open 127will attempt to determine if the hash function specified is the same 128as the one with which the database was created, and will fail if it is 129not. 130.\".Pp 131.\"Backward compatible interfaces to the routines described in 132.\".Xr dbm 3 , 133.\"and 134.\".Xr ndbm 3 135.\"are provided, however these interfaces are not compatible with 136.\"previous file formats. 137.Sh ERRORS 138The 139.Nm 140access method routines may fail and set 141.Va errno 142for any of the errors specified for the library routine 143.Xr dbopen 3 . 144.Sh SEE ALSO 145.Xr btree 3 , 146.Xr dbopen 3 , 147.Xr mpool 3 , 148.Xr recno 3 149.Pp 150.Rs 151.%T "Dynamic Hash Tables" 152.%A Per-Ake Larson 153.%J Communications of the ACM 154.%D April 1988 155.Re 156.Rs 157.%T "A New Hash Package for UNIX" 158.%A Margo Seltzer 159.%J USENIX Proceedings 160.%D Winter 1991 161.Re 162.Sh BUGS 163Only big and little endian byte order is supported. 164