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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 1.3 2005/09/19 09:20:37 asmodai 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 42The routine 43.Fn dbopen 44is the library interface to database files. 45One of the supported file formats is 46.Nm 47files. 48The general description of the database access methods is in 49.Xr dbopen 3 , 50this manual page describes only the 51.Nm 52specific information. 53.Pp 54The 55.Nm 56data structure is an extensible, dynamic hashing scheme. 57.Pp 58The access method specific data structure provided to 59.Fn dbopen 60is defined in the 61.Aq Pa db.h 62include file as follows: 63.Bd -literal 64typedef 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 74The elements of this structure are as follows: 75.Bl -tag -width indent 76.It Va bsize 77The 78.Va bsize 79element 80defines the 81.Nm 82table bucket size, and is, by default, 256 bytes. 83It may be preferable to increase the page size for disk-resident tables 84and tables with large data items. 85.It Va ffactor 86The 87.Va ffactor 88element 89indicates a desired density within the 90.Nm 91table. 92It is an approximation of the number of keys allowed to accumulate in any 93one bucket, determining when the 94.Nm 95table grows or shrinks. 96The default value is 8. 97.It Va nelem 98The 99.Va nelem 100element 101is an estimate of the final size of the 102.Nm 103table. 104If not set or set too low, 105.Nm 106tables will expand gracefully as keys 107are entered, although a slight performance degradation may be noticed. 108The default value is 1. 109.It Va cachesize 110A suggested maximum size, in bytes, of the memory cache. 111This value is 112.Em only 113advisory, and the access method will allocate more memory rather 114than fail. 115.It Va hash 116The 117.Va hash 118element 119is a user defined 120.Nm 121function. 122Since no 123.Nm 124function performs equally well on all possible data, the 125user may find that the built-in 126.Nm 127function does poorly on a particular 128data set. 129User specified 130.Nm 131functions must take two arguments (a pointer to a byte 132string and a length) and return a 32-bit quantity to be used as the 133.Nm 134value. 135.It Va lorder 136The byte order for integers in the stored database metadata. 137The number should represent the order as an integer; for example, 138big endian order would be the number 4,321. 139If 140.Va lorder 141is 0 (no order is specified) the current host order is used. 142If the file already exists, the specified value is ignored and the 143value specified when the tree was created is used. 144.El 145.Pp 146If the file already exists (and the 147.Dv O_TRUNC 148flag is not specified), the 149values specified for the 150.Va bsize , ffactor , lorder 151and 152.Va nelem 153arguments 154are 155ignored and the values specified when the tree was created are used. 156.Pp 157If a 158.Nm 159function is specified, 160.Fn hash_open 161will attempt to determine if the 162.Nm 163function specified is the same as 164the one with which the database was created, and will fail if it is not. 165.Pp 166Backward compatible interfaces to the older 167.Em dbm 168and 169.Em ndbm 170routines are provided, however these interfaces are not compatible with 171previous file formats. 172.Sh ERRORS 173The 174.Nm 175access method routines may fail and set 176.Va errno 177for 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 197Only big and little endian byte order is supported. 198