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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.Dd August 18, 1994 29.Dt HASH 3 30.Os 31.Sh NAME 32.Nm hash 33.Nd "hash database access method" 34.Sh SYNOPSIS 35.In sys/types.h 36.In db.h 37.Sh DESCRIPTION 38The routine 39.Fn dbopen 40is the library interface to database files. 41One of the supported file formats is 42.Nm 43files. 44The general description of the database access methods is in 45.Xr dbopen 3 , 46this manual page describes only the 47.Nm 48specific information. 49.Pp 50The 51.Nm 52data structure is an extensible, dynamic hashing scheme. 53.Pp 54The access method specific data structure provided to 55.Fn dbopen 56is defined in the 57.In db.h 58include file as follows: 59.Bd -literal 60typedef struct { 61 u_int bsize; 62 u_int ffactor; 63 u_int nelem; 64 u_int cachesize; 65 uint32_t (*hash)(const void *, size_t); 66 int lorder; 67} HASHINFO; 68.Ed 69.Pp 70The elements of this structure are as follows: 71.Bl -tag -width indent 72.It Va bsize 73The 74.Va bsize 75element 76defines the 77.Nm 78table bucket size, and is, by default, 4096 bytes. 79It may be preferable to increase the page size for disk-resident tables 80and tables with large data items. 81.It Va ffactor 82The 83.Va ffactor 84element 85indicates a desired density within the 86.Nm 87table. 88It is an approximation of the number of keys allowed to accumulate in any 89one bucket, determining when the 90.Nm 91table grows or shrinks. 92The default value is 8. 93.It Va nelem 94The 95.Va nelem 96element 97is an estimate of the final size of the 98.Nm 99table. 100If not set or set too low, 101.Nm 102tables will expand gracefully as keys 103are entered, although a slight performance degradation may be noticed. 104The default value is 1. 105.It Va cachesize 106A suggested maximum size, in bytes, of the memory cache. 107This value is 108.Em only 109advisory, and the access method will allocate more memory rather 110than fail. 111.It Va hash 112The 113.Va hash 114element 115is a user defined 116.Nm 117function. 118Since no 119.Nm 120function performs equally well on all possible data, the 121user may find that the built-in 122.Nm 123function does poorly on a particular 124data set. 125User specified 126.Nm 127functions must take two arguments (a pointer to a byte 128string and a length) and return a 32-bit quantity to be used as the 129.Nm 130value. 131.It Va lorder 132The byte order for integers in the stored database metadata. 133The number should represent the order as an integer; for example, 134big endian order would be the number 4,321. 135If 136.Va lorder 137is 0 (no order is specified) the current host order is used. 138If the file already exists, the specified value is ignored and the 139value specified when the tree was created is used. 140.El 141.Pp 142If the file already exists (and the 143.Dv O_TRUNC 144flag is not specified), the 145values specified for the 146.Va bsize , ffactor , lorder 147and 148.Va nelem 149arguments 150are 151ignored and the values specified when the tree was created are used. 152.Pp 153If a 154.Nm 155function is specified, 156.Fn hash_open 157will attempt to determine if the 158.Nm 159function specified is the same as 160the one with which the database was created, and will fail if it is not. 161.Pp 162Backward compatible interfaces to the older 163.Em dbm 164and 165.Em ndbm 166routines are provided, however these interfaces are not compatible with 167previous file formats. 168.Sh ERRORS 169The 170.Nm 171access method routines may fail and set 172.Va errno 173for any of the errors specified for the library routine 174.Xr dbopen 3 . 175.Sh SEE ALSO 176.Xr btree 3 , 177.Xr dbopen 3 , 178.Xr mpool 3 , 179.Xr recno 3 180.Rs 181.%T "Dynamic Hash Tables" 182.%A Per-Ake Larson 183.%R "Communications of the ACM" 184.%D April 1988 185.Re 186.Rs 187.%T "A New Hash Package for UNIX" 188.%A Margo Seltzer 189.%R "USENIX Proceedings" 190.%D Winter 1991 191.Re 192.Sh BUGS 193Only big and little endian byte order is supported. 194