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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 80The 81.Va bsize 82element 83defines the 84.Nm 85table bucket size, and is, by default, 256 bytes. 86It may be preferable to increase the page size for disk-resident tables 87and tables with large data items. 88.It Va ffactor 89The 90.Va ffactor 91element 92indicates a desired density within the 93.Nm 94table. 95It is an approximation of the number of keys allowed to accumulate in any 96one bucket, determining when the 97.Nm 98table grows or shrinks. 99The default value is 8. 100.It Va nelem 101The 102.Va nelem 103element 104is an estimate of the final size of the 105.Nm 106table. 107If not set or set too low, 108.Nm 109tables will expand gracefully as keys 110are entered, although a slight performance degradation may be noticed. 111The default value is 1. 112.It Va cachesize 113A suggested maximum size, in bytes, of the memory cache. 114This value is 115.Em only 116advisory, and the access method will allocate more memory rather 117than fail. 118.It Va hash 119The 120.Va hash 121element 122is a user defined 123.Nm 124function. 125Since no 126.Nm 127function performs equally well on all possible data, the 128user may find that the built-in 129.Nm 130function does poorly on a particular 131data set. 132User specified 133.Nm 134functions must take two arguments (a pointer to a byte 135string and a length) and return a 32-bit quantity to be used as the 136.Nm 137value. 138.It Va lorder 139The byte order for integers in the stored database metadata. 140The number should represent the order as an integer; for example, 141big endian order would be the number 4,321. 142If 143.Va lorder 144is 0 (no order is specified) the current host order is used. 145If the file already exists, the specified value is ignored and the 146value specified when the tree was created is used. 147.El 148.Pp 149If the file already exists (and the 150.Dv O_TRUNC 151flag is not specified), the 152values specified for the 153.Va bsize , ffactor , lorder 154and 155.Va nelem 156arguments 157are 158ignored and the values specified when the tree was created are used. 159.Pp 160If a 161.Nm 162function is specified, 163.Fn hash_open 164will attempt to determine if the 165.Nm 166function specified is the same as 167the one with which the database was created, and will fail if it is not. 168.Pp 169Backward compatible interfaces to the older 170.Em dbm 171and 172.Em ndbm 173routines are provided, however these interfaces are not compatible with 174previous file formats. 175.Sh ERRORS 176The 177.Nm 178access method routines may fail and set 179.Va errno 180for any of the errors specified for the library routine 181.Xr dbopen 3 . 182.Sh SEE ALSO 183.Xr btree 3 , 184.Xr dbopen 3 , 185.Xr mpool 3 , 186.Xr recno 3 187.Rs 188.%T "Dynamic Hash Tables" 189.%A Per-Ake Larson 190.%R "Communications of the ACM" 191.%D April 1988 192.Re 193.Rs 194.%T "A New Hash Package for UNIX" 195.%A Margo Seltzer 196.%R "USENIX Proceedings" 197.%D Winter 1991 198.Re 199.Sh BUGS 200Only big and little endian byte order is supported. 201