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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)hash.3 8.6 (Berkeley) 8/18/94 33.\" $FreeBSD$ 34.\" 35.Dd August 18, 1994 36.Dt HASH 3 37.Os 38.Sh NAME 39.Nm hash 40.Nd "hash database access method" 41.Sh SYNOPSIS 42.In sys/types.h 43.In db.h 44.Sh DESCRIPTION 45The routine 46.Fn dbopen 47is the library interface to database files. 48One of the supported file formats is 49.Nm 50files. 51The general description of the database access methods is in 52.Xr dbopen 3 , 53this manual page describes only the 54.Nm 55specific information. 56.Pp 57The 58.Nm 59data structure is an extensible, dynamic hashing scheme. 60.Pp 61The access method specific data structure provided to 62.Fn dbopen 63is defined in the 64.Aq Pa db.h 65include file as follows: 66.Bd -literal 67typedef struct { 68 u_int bsize; 69 u_int ffactor; 70 u_int nelem; 71 u_int cachesize; 72 u_int32_t (*hash)(const void *, size_t); 73 int lorder; 74} HASHINFO; 75.Ed 76.Pp 77The elements of this structure are as follows: 78.Bl -tag -width indent 79.It Va bsize 80.Va Bsize 81defines the 82.Nm 83table bucket size, and is, by default, 256 bytes. 84It may be preferable to increase the page size for disk-resident tables 85and tables with large data items. 86.It Va ffactor 87.Va Ffactor 88indicates a desired density within the 89.Nm 90table. 91It is an approximation of the number of keys allowed to accumulate in any 92one bucket, determining when the 93.Nm 94table grows or shrinks. 95The default value is 8. 96.It Va nelem 97.Va Nelem 98is an estimate of the final size of the 99.Nm 100table. 101If not set or set too low, 102.Nm 103tables will expand gracefully as keys 104are entered, although a slight performance degradation may be noticed. 105The default value is 1. 106.It Va cachesize 107A suggested maximum size, in bytes, of the memory cache. 108This value is 109.Em only 110advisory, and the access method will allocate more memory rather 111than fail. 112.It Va hash 113.Va Hash 114is a user defined 115.Nm 116function. 117Since no 118.Nm 119function performs equally well on all possible data, the 120user may find that the built-in 121.Nm 122function does poorly on a particular 123data set. 124User specified 125.Nm 126functions must take two arguments (a pointer to a byte 127string and a length) and return a 32-bit quantity to be used as the 128.Nm 129value. 130.It Va lorder 131The byte order for integers in the stored database metadata. 132The number should represent the order as an integer; for example, 133big endian order would be the number 4,321. 134If 135.Va lorder 136is 0 (no order is specified) the current host order is used. 137If the file already exists, the specified value is ignored and the 138value specified when the tree was created is used. 139.El 140.Pp 141If the file already exists (and the 142.Dv O_TRUNC 143flag is not specified), the 144values specified for the parameters 145.Va bsize , ffactor , lorder 146and 147.Va nelem 148are 149ignored and the values specified when the tree was created are used. 150.Pp 151If a 152.Nm 153function is specified, 154.Fn hash_open 155will attempt to determine if the 156.Nm 157function specified is the same as 158the one with which the database was created, and will fail if it is not. 159.Pp 160Backward compatible interfaces to the older 161.Em dbm 162and 163.Em ndbm 164routines are provided, however these interfaces are not compatible with 165previous file formats. 166.Sh ERRORS 167The 168.Nm 169access method routines may fail and set 170.Va errno 171for any of the errors specified for the library routine 172.Xr dbopen 3 . 173.Sh SEE ALSO 174.Xr btree 3 , 175.Xr dbopen 3 , 176.Xr mpool 3 , 177.Xr recno 3 178.Rs 179.%T "Dynamic Hash Tables" 180.%A Per-Ake Larson 181.%R "Communications of the ACM" 182.%D April 1988 183.Re 184.Rs 185.%T "A New Hash Package for UNIX" 186.%A Margo Seltzer 187.%R "USENIX Proceedings" 188.%D Winter 1991 189.Re 190.Sh BUGS 191Only big and little endian byte order is supported. 192