1.\" $OpenBSD: hash.3,v 1.7 1999/07/04 18:59:42 aaron Exp $ 2.\" $NetBSD: hash.3,v 1.6 1996/05/03 21:26:50 cgd Exp $ 3.\" 4.\" Copyright (c) 1997, Phillip F Knaack. All rights reserved. 5.\" 6.\" Copyright (c) 1990, 1993 7.\" The Regents of the University of California. All rights reserved. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the University of 20.\" California, Berkeley and its contributors. 21.\" 4. Neither the name of the University nor the names of its contributors 22.\" may be used to endorse or promote products derived from this software 23.\" without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 26.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 27.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 28.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 29.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 30.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 31.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 32.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 33.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 34.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 35.\" SUCH DAMAGE. 36.\" 37.\" @(#)hash.3 8.6 (Berkeley) 8/18/94 38.\" 39.Dd August 18, 1994 40.Os 41.Dt HASH 3 42.Sh NAME 43.Nm hash 44.Nd hash database access method 45.Sh SYNOPSIS 46.Fd #include <sys/types.h> 47.Fd #include <db.h> 48.Sh DESCRIPTION 49The routine 50.Fn dbopen 51is the library interface to database files. 52One of the supported file formats is hash files. 53The general description of the database access methods is in 54.Xr dbopen 3 . 55This manual page describes only the hash specific information. 56.Pp 57The hash data structure is an extensible, dynamic hashing scheme. 58.Pp 59The access method specific data structure provided to 60.Fn dbopen 61is defined in the 62.Aq Pa db.h 63include file as follows: 64.Pp 65.Bl -item -compact 66typedef struct { 67.It 68.Bl -item -compact -inset -offset indent 69.It 70u_int bsize; 71.It 72u_int ffactor; 73.It 74u_int nelem; 75.It 76u_int cachesize; 77.It 78u_int32_t (*hash)(const void *, size_t); 79.It 80int lorder; 81.El 82.It 83} HASHINFO; 84.El 85.Pp 86The elements of this structure are as follows: 87.Bl -tag -width XXXXXX -offset indent 88.It bsize 89.Fa bsize 90defines the hash table bucket size, and is, by default, 256 bytes. 91It may be preferable to increase the page size for disk-resident tables 92and tables with large data items. 93.It ffactor 94.Fa ffactor 95indicates a desired density within the hash table. 96It is an approximation of the number of keys allowed to accumulate in any 97one bucket, determining when the hash table grows or shrinks. 98The default value is 8. 99.It nelem 100.Fa nelem 101is an estimate of the final size of the hash table. 102If not set or set too low, hash tables will expand gracefully as keys 103are entered, although a slight performance degradation may be noticed. 104The default value is 1. 105.It cachesize 106A suggested maximum size, in bytes, of the memory cache. 107This value is 108.Em only 109advisory, and the access method will allocate more memory rather 110than fail. 111.It hash 112.Fa hash 113is a user defined hash function. 114Since no hash function performs equally well on all possible data, the 115user may find that the built-in hash function does poorly on a particular 116data set. 117User specified hash functions must take two arguments (a pointer to a byte 118string and a length) and return a 32-bit quantity to be used as the hash 119value. 120.It lorder 121The byte order for integers in the stored database metadata. 122The number should represent the order as an integer; for example, 123big endian order would be the number 4,321. If 124.Fa lorder 125is 0 (no order is specified) the current host order is used. 126If the file already exists, the specified value is ignored and the 127value specified when the tree was created is used. 128.El 129.Pp 130If the file already exists (and the 131.Dv O_TRUNC 132flag is not specified), the 133values specified for the parameters bsize, ffactor, lorder and nelem are 134ignored and the values specified when the tree was created are used. 135.Pp 136If a hash function is specified, 137.Fa hash_open 138will attempt to determine if the hash function specified is the same as 139the one with which the database was created, and will fail if it is not. 140.Pp 141Backward compatible interfaces to the routines described in 142.Xr dbm 3 , 143and 144.Xr ndbm 3 145are provided, although these interfaces are not compatible with 146previous file formats. 147.Sh ERRORS 148The 149.Nm 150access method routines may fail and set 151.Va errno 152for any of the errors specified for the library routine 153.Xr dbopen 3 . 154.Sh SEE ALSO 155.Xr btree 3 , 156.Xr dbopen 3 , 157.Xr mpool 3 , 158.Xr recno 3 159.Rs 160.%T "Dynamic Hash Tables" 161.%A Per-Ake Larson 162.%J Communications of the ACM 163.%D April 1988 164.Re 165.Rs 166.%T "A New Hash Package for UNIX" 167.%A Margo Seltzer 168.%J USENIX Proceedings 169.%D Winter 1991 170.Re 171.Sh BUGS 172Only big and little endian byte order is supported. 173