| 1 | .\" Copyright (c) 1990, 1993 |
| 2 | .\" The Regents of the University of California. All rights reserved. |
| 3 | .\" |
| 4 | .\" Redistribution and use in source and binary forms, with or without |
| 5 | .\" modification, are permitted provided that the following conditions |
| 6 | .\" are met: |
| 7 | .\" 1. Redistributions of source code must retain the above copyright |
| 8 | .\" notice, this list of conditions and the following disclaimer. |
| 9 | .\" 2. Redistributions in binary form must reproduce the above copyright |
| 10 | .\" notice, this list of conditions and the following disclaimer in the |
| 11 | .\" documentation and/or other materials provided with the distribution. |
| 12 | .\" 3. Neither the name of the University nor the names of its contributors |
| 13 | .\" may be used to endorse or promote products derived from this software |
| 14 | .\" without specific prior written permission. |
| 15 | .\" |
| 16 | .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND |
| 17 | .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| 18 | .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
| 19 | .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE |
| 20 | .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL |
| 21 | .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS |
| 22 | .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) |
| 23 | .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT |
| 24 | .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY |
| 25 | .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF |
| 26 | .\" SUCH DAMAGE. |
| 27 | .\" |
| 28 | .\" @(#)hash.3 8.6 (Berkeley) 8/18/94 |
| 29 | .\" $FreeBSD: src/lib/libc/db/man/hash.3,v 1.4.2.3 2003/02/23 19:45:52 trhodes Exp $ |
| 30 | .\" $DragonFly: src/lib/libc/db/man/hash.3,v 1.4 2006/05/26 19:39:36 swildner Exp $ |
| 31 | .\" |
| 32 | .Dd August 18, 1994 |
| 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 |
| 42 | The routine |
| 43 | .Fn dbopen |
| 44 | is the library interface to database files. |
| 45 | One of the supported file formats is |
| 46 | .Nm |
| 47 | files. |
| 48 | The general description of the database access methods is in |
| 49 | .Xr dbopen 3 , |
| 50 | this manual page describes only the |
| 51 | .Nm |
| 52 | specific information. |
| 53 | .Pp |
| 54 | The |
| 55 | .Nm |
| 56 | data structure is an extensible, dynamic hashing scheme. |
| 57 | .Pp |
| 58 | The access method specific data structure provided to |
| 59 | .Fn dbopen |
| 60 | is defined in the |
| 61 | .In db.h |
| 62 | include file as follows: |
| 63 | .Bd -literal |
| 64 | typedef struct { |
| 65 | u_int bsize; |
| 66 | u_int ffactor; |
| 67 | u_int nelem; |
| 68 | u_int cachesize; |
| 69 | u_int32_t (*hash)(const void *, size_t); |
| 70 | int lorder; |
| 71 | } HASHINFO; |
| 72 | .Ed |
| 73 | .Pp |
| 74 | The elements of this structure are as follows: |
| 75 | .Bl -tag -width indent |
| 76 | .It Va bsize |
| 77 | The |
| 78 | .Va bsize |
| 79 | element |
| 80 | defines the |
| 81 | .Nm |
| 82 | table bucket size, and is, by default, 256 bytes. |
| 83 | It may be preferable to increase the page size for disk-resident tables |
| 84 | and tables with large data items. |
| 85 | .It Va ffactor |
| 86 | The |
| 87 | .Va ffactor |
| 88 | element |
| 89 | indicates a desired density within the |
| 90 | .Nm |
| 91 | table. |
| 92 | It is an approximation of the number of keys allowed to accumulate in any |
| 93 | one bucket, determining when the |
| 94 | .Nm |
| 95 | table grows or shrinks. |
| 96 | The default value is 8. |
| 97 | .It Va nelem |
| 98 | The |
| 99 | .Va nelem |
| 100 | element |
| 101 | is an estimate of the final size of the |
| 102 | .Nm |
| 103 | table. |
| 104 | If not set or set too low, |
| 105 | .Nm |
| 106 | tables will expand gracefully as keys |
| 107 | are entered, although a slight performance degradation may be noticed. |
| 108 | The default value is 1. |
| 109 | .It Va cachesize |
| 110 | A suggested maximum size, in bytes, of the memory cache. |
| 111 | This value is |
| 112 | .Em only |
| 113 | advisory, and the access method will allocate more memory rather |
| 114 | than fail. |
| 115 | .It Va hash |
| 116 | The |
| 117 | .Va hash |
| 118 | element |
| 119 | is a user defined |
| 120 | .Nm |
| 121 | function. |
| 122 | Since no |
| 123 | .Nm |
| 124 | function performs equally well on all possible data, the |
| 125 | user may find that the built-in |
| 126 | .Nm |
| 127 | function does poorly on a particular |
| 128 | data set. |
| 129 | User specified |
| 130 | .Nm |
| 131 | functions must take two arguments (a pointer to a byte |
| 132 | string and a length) and return a 32-bit quantity to be used as the |
| 133 | .Nm |
| 134 | value. |
| 135 | .It Va lorder |
| 136 | The byte order for integers in the stored database metadata. |
| 137 | The number should represent the order as an integer; for example, |
| 138 | big endian order would be the number 4,321. |
| 139 | If |
| 140 | .Va lorder |
| 141 | is 0 (no order is specified) the current host order is used. |
| 142 | If the file already exists, the specified value is ignored and the |
| 143 | value specified when the tree was created is used. |
| 144 | .El |
| 145 | .Pp |
| 146 | If the file already exists (and the |
| 147 | .Dv O_TRUNC |
| 148 | flag is not specified), the |
| 149 | values specified for the |
| 150 | .Va bsize , ffactor , lorder |
| 151 | and |
| 152 | .Va nelem |
| 153 | arguments |
| 154 | are |
| 155 | ignored and the values specified when the tree was created are used. |
| 156 | .Pp |
| 157 | If a |
| 158 | .Nm |
| 159 | function is specified, |
| 160 | .Fn hash_open |
| 161 | will attempt to determine if the |
| 162 | .Nm |
| 163 | function specified is the same as |
| 164 | the one with which the database was created, and will fail if it is not. |
| 165 | .Pp |
| 166 | Backward compatible interfaces to the older |
| 167 | .Em dbm |
| 168 | and |
| 169 | .Em ndbm |
| 170 | routines are provided, however these interfaces are not compatible with |
| 171 | previous file formats. |
| 172 | .Sh ERRORS |
| 173 | The |
| 174 | .Nm |
| 175 | access method routines may fail and set |
| 176 | .Va errno |
| 177 | for any of the errors specified for the library routine |
| 178 | .Xr dbopen 3 . |
| 179 | .Sh SEE ALSO |
| 180 | .Xr btree 3 , |
| 181 | .Xr dbopen 3 , |
| 182 | .Xr mpool 3 , |
| 183 | .Xr recno 3 |
| 184 | .Rs |
| 185 | .%T "Dynamic Hash Tables" |
| 186 | .%A Per-Ake Larson |
| 187 | .%R "Communications of the ACM" |
| 188 | .%D April 1988 |
| 189 | .Re |
| 190 | .Rs |
| 191 | .%T "A New Hash Package for UNIX" |
| 192 | .%A Margo Seltzer |
| 193 | .%R "USENIX Proceedings" |
| 194 | .%D Winter 1991 |
| 195 | .Re |
| 196 | .Sh BUGS |
| 197 | Only big and little endian byte order is supported. |