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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 1.6 2007/08/18 20:48:47 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 LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In sys/types.h 42.In db.h 43.Sh DESCRIPTION 44The routine 45.Fn dbopen 46is the library interface to database files. 47One of the supported file formats is 48.Nm 49files. 50The general description of the database access methods is in 51.Xr dbopen 3 , 52this manual page describes only the 53.Nm 54specific information. 55.Pp 56The 57.Nm 58data structure is an extensible, dynamic hashing scheme. 59.Pp 60The access method specific data structure provided to 61.Fn dbopen 62is defined in the 63.In db.h 64include file as follows: 65.Bd -literal 66typedef struct { 67 u_int bsize; 68 u_int ffactor; 69 u_int nelem; 70 u_int cachesize; 71 u_int32_t (*hash)(const void *, size_t); 72 int lorder; 73} HASHINFO; 74.Ed 75.Pp 76The elements of this structure are as follows: 77.Bl -tag -width indent 78.It Va bsize 79The 80.Va bsize 81element 82defines the 83.Nm 84table bucket size, and is, by default, 256 bytes. 85It may be preferable to increase the page size for disk-resident tables 86and tables with large data items. 87.It Va ffactor 88The 89.Va ffactor 90element 91indicates a desired density within the 92.Nm 93table. 94It is an approximation of the number of keys allowed to accumulate in any 95one bucket, determining when the 96.Nm 97table grows or shrinks. 98The default value is 8. 99.It Va nelem 100The 101.Va nelem 102element 103is an estimate of the final size of the 104.Nm 105table. 106If not set or set too low, 107.Nm 108tables will expand gracefully as keys 109are entered, although a slight performance degradation may be noticed. 110The default value is 1. 111.It Va cachesize 112A suggested maximum size, in bytes, of the memory cache. 113This value is 114.Em only 115advisory, and the access method will allocate more memory rather 116than fail. 117.It Va hash 118The 119.Va hash 120element 121is a user defined 122.Nm 123function. 124Since no 125.Nm 126function performs equally well on all possible data, the 127user may find that the built-in 128.Nm 129function does poorly on a particular 130data set. 131User specified 132.Nm 133functions must take two arguments (a pointer to a byte 134string and a length) and return a 32-bit quantity to be used as the 135.Nm 136value. 137.It Va lorder 138The byte order for integers in the stored database metadata. 139The number should represent the order as an integer; for example, 140big endian order would be the number 4,321. 141If 142.Va lorder 143is 0 (no order is specified) the current host order is used. 144If the file already exists, the specified value is ignored and the 145value specified when the tree was created is used. 146.El 147.Pp 148If the file already exists (and the 149.Dv O_TRUNC 150flag is not specified), the 151values specified for the 152.Va bsize , ffactor , lorder 153and 154.Va nelem 155arguments 156are 157ignored and the values specified when the tree was created are used. 158.Pp 159If a 160.Nm 161function is specified, 162.Fn __hash_open 163will attempt to determine if the 164.Nm 165function specified is the same as 166the one with which the database was created, and will fail if it is not. 167.Pp 168Backward compatible interfaces to the older 169.Em dbm 170and 171.Em ndbm 172routines are provided, however these interfaces are not compatible with 173previous file formats. 174.Sh ERRORS 175The 176.Nm 177access method routines may fail and set 178.Va errno 179for any of the errors specified for the library routine 180.Xr dbopen 3 . 181.Sh SEE ALSO 182.Xr btree 3 , 183.Xr dbopen 3 , 184.Xr mpool 3 , 185.Xr recno 3 186.Rs 187.%T "Dynamic Hash Tables" 188.%A Per-Ake Larson 189.%R "Communications of the ACM" 190.%D April 1988 191.Re 192.Rs 193.%T "A New Hash Package for UNIX" 194.%A Margo Seltzer 195.%R "USENIX Proceedings" 196.%D Winter 1991 197.Re 198.Sh BUGS 199Only big and little endian byte order is supported. 200