xref: /dragonfly/lib/libc/db/man/hash.3 (revision 33311965) 
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28.\"	@(#)hash.3	8.6 (Berkeley) 8/18/94
29.\" $FreeBSD: head/lib/libc/db/man/hash.3 231564 2012-02-12 18:29:56Z ed $
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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 LIBRARY
38.Lb libc
39.Sh SYNOPSIS
40.In sys/types.h
41.In db.h
42.Sh DESCRIPTION
43The routine
44.Fn dbopen
45is the library interface to database files.
46One of the supported file formats is
47.Nm
48files.
49The general description of the database access methods is in
50.Xr dbopen 3 ,
51this manual page describes only the
52.Nm
53specific information.
54.Pp
55The
56.Nm
57data structure is an extensible, dynamic hashing scheme.
58.Pp
59The access method specific data structure provided to
60.Fn dbopen
61is defined in the
62.In db.h
63include file as follows:
64.Bd -literal
65typedef struct {
66	unsigned int bsize;
67	unsigned int ffactor;
68	unsigned int nelem;
69	unsigned int cachesize;
70	uint32_t (*hash)(const void *, size_t);
71	int lorder;
72} HASHINFO;
73.Ed
74.Pp
75The elements of this structure are as follows:
76.Bl -tag -width indent
77.It Va bsize
78The
79.Va bsize
80element
81defines the
82.Nm
83table bucket size, and is, by default, 4096 bytes.
84It may be preferable to increase the page size for disk-resident tables
85and tables with large data items.
86.It Va ffactor
87The
88.Va ffactor
89element
90indicates a desired density within the
91.Nm
92table.
93It is an approximation of the number of keys allowed to accumulate in any
94one bucket, determining when the
95.Nm
96table grows or shrinks.
97The default value is 8.
98.It Va nelem
99The
100.Va nelem
101element
102is an estimate of the final size of the
103.Nm
104table.
105If not set or set too low,
106.Nm
107tables will expand gracefully as keys
108are entered, although a slight performance degradation may be noticed.
109The default value is 1.
110.It Va cachesize
111A suggested maximum size, in bytes, of the memory cache.
112This value is
113.Em only
114advisory, and the access method will allocate more memory rather
115than fail.
116.It Va hash
117The
118.Va hash
119element
120is a user defined
121.Nm
122function.
123Since no
124.Nm
125function performs equally well on all possible data, the
126user may find that the built-in
127.Nm
128function does poorly on a particular
129data set.
130User specified
131.Nm
132functions must take two arguments (a pointer to a byte
133string and a length) and return a 32-bit quantity to be used as the
134.Nm
135value.
136.It Va lorder
137The byte order for integers in the stored database metadata.
138The number should represent the order as an integer; for example,
139big endian order would be the number 4,321.
140If
141.Va lorder
142is 0 (no order is specified) the current host order is used.
143If the file already exists, the specified value is ignored and the
144value specified when the tree was created is used.
145.El
146.Pp
147If the file already exists (and the
148.Dv O_TRUNC
149flag is not specified), the
150values specified for the
151.Va bsize , ffactor , lorder
152and
153.Va nelem
154arguments
155are
156ignored and the values specified when the tree was created are used.
157.Pp
158If a
159.Nm
160function is specified,
161.Fn __hash_open
162will attempt to determine if the
163.Nm
164function specified is the same as
165the one with which the database was created, and will fail if it is not.
166.Pp
167Backward compatible interfaces to the older
168.Em dbm
169and
170.Em ndbm
171routines are provided, however these interfaces are not compatible with
172previous file formats.
173.Sh ERRORS
174The
175.Nm
176access method routines may fail and set
177.Va errno
178for any of the errors specified for the library routine
179.Xr dbopen 3 .
180.Sh SEE ALSO
181.Xr btree 3 ,
182.Xr dbopen 3 ,
183.Xr mpool 3 ,
184.Xr recno 3
185.Rs
186.%T "Dynamic Hash Tables"
187.%A Per-Ake Larson
188.%R "Communications of the ACM"
189.%D April 1988
190.Re
191.Rs
192.%T "A New Hash Package for UNIX"
193.%A Margo Seltzer
194.%R "USENIX Proceedings"
195.%D Winter 1991
196.Re
197.Sh BUGS
198Only big and little endian byte order is supported.
199