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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.\" $FreeBSD$ 30.\" 31.Dd August 18, 1994 32.Dt HASH 3 33.Os 34.Sh NAME 35.Nm hash 36.Nd "hash database access method" 37.Sh SYNOPSIS 38.In sys/types.h 39.In db.h 40.Sh DESCRIPTION 41The routine 42.Fn dbopen 43is the library interface to database files. 44One of the supported file formats is 45.Nm 46files. 47The general description of the database access methods is in 48.Xr dbopen 3 , 49this manual page describes only the 50.Nm 51specific information. 52.Pp 53The 54.Nm 55data structure is an extensible, dynamic hashing scheme. 56.Pp 57The access method specific data structure provided to 58.Fn dbopen 59is defined in the 60.In db.h 61include file as follows: 62.Bd -literal 63typedef struct { 64 u_int bsize; 65 u_int ffactor; 66 u_int nelem; 67 u_int cachesize; 68 uint32_t (*hash)(const void *, size_t); 69 int lorder; 70} HASHINFO; 71.Ed 72.Pp 73The elements of this structure are as follows: 74.Bl -tag -width indent 75.It Va bsize 76The 77.Va bsize 78element 79defines the 80.Nm 81table bucket size, and is, by default, 4096 bytes. 82It may be preferable to increase the page size for disk-resident tables 83and tables with large data items. 84.It Va ffactor 85The 86.Va ffactor 87element 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 97The 98.Va nelem 99element 100is an estimate of the final size of the 101.Nm 102table. 103If not set or set too low, 104.Nm 105tables will expand gracefully as keys 106are entered, although a slight performance degradation may be noticed. 107The default value is 1. 108.It Va cachesize 109A suggested maximum size, in bytes, of the memory cache. 110This value is 111.Em only 112advisory, and the access method will allocate more memory rather 113than fail. 114.It Va hash 115The 116.Va hash 117element 118is a user defined 119.Nm 120function. 121Since no 122.Nm 123function performs equally well on all possible data, the 124user may find that the built-in 125.Nm 126function does poorly on a particular 127data set. 128User specified 129.Nm 130functions must take two arguments (a pointer to a byte 131string and a length) and return a 32-bit quantity to be used as the 132.Nm 133value. 134.It Va lorder 135The byte order for integers in the stored database metadata. 136The number should represent the order as an integer; for example, 137big endian order would be the number 4,321. 138If 139.Va lorder 140is 0 (no order is specified) the current host order is used. 141If the file already exists, the specified value is ignored and the 142value specified when the tree was created is used. 143.El 144.Pp 145If the file already exists (and the 146.Dv O_TRUNC 147flag is not specified), the 148values specified for the 149.Va bsize , ffactor , lorder 150and 151.Va nelem 152arguments 153are 154ignored and the values specified when the tree was created are used. 155.Pp 156If a 157.Nm 158function is specified, 159.Fn hash_open 160will attempt to determine if the 161.Nm 162function specified is the same as 163the one with which the database was created, and will fail if it is not. 164.Pp 165Backward compatible interfaces to the older 166.Em dbm 167and 168.Em ndbm 169routines are provided, however these interfaces are not compatible with 170previous file formats. 171.Sh ERRORS 172The 173.Nm 174access method routines may fail and set 175.Va errno 176for any of the errors specified for the library routine 177.Xr dbopen 3 . 178.Sh SEE ALSO 179.Xr btree 3 , 180.Xr dbopen 3 , 181.Xr mpool 3 , 182.Xr recno 3 183.Rs 184.%T "Dynamic Hash Tables" 185.%A Per-Ake Larson 186.%R "Communications of the ACM" 187.%D April 1988 188.Re 189.Rs 190.%T "A New Hash Package for UNIX" 191.%A Margo Seltzer 192.%R "USENIX Proceedings" 193.%D Winter 1991 194.Re 195.Sh BUGS 196Only big and little endian byte order is supported. 197