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32.\" @(#)hash.3 8.6 (Berkeley) 8/18/94
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33.\" $FreeBSD: head/lib/libc/db/man/hash.3 84306 2001-10-01 16:09:29Z ru $
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| 33.\" $FreeBSD: head/lib/libc/db/man/hash.3 108087 2002-12-19 09:40:28Z ru $ |
34.\"
35.Dd August 18, 1994
36.Dt HASH 3
37.Os
38.Sh NAME
39.Nm hash
40.Nd "hash database access method"
41.Sh SYNOPSIS
42.In sys/types.h
43.In db.h
44.Sh DESCRIPTION
45The routine
46.Fn dbopen
47is the library interface to database files.
48One of the supported file formats is
49.Nm
50files.
51The general description of the database access methods is in
52.Xr dbopen 3 ,
53this manual page describes only the
54.Nm
55specific information.
56.Pp
57The
58.Nm
59data structure is an extensible, dynamic hashing scheme.
60.Pp
61The access method specific data structure provided to
62.Fn dbopen
63is defined in the
64.Aq Pa db.h
65include file as follows:
66.Bd -literal
67typedef struct {
68 u_int bsize;
69 u_int ffactor;
70 u_int nelem;
71 u_int cachesize;
72 u_int32_t (*hash)(const void *, size_t);
73 int lorder;
74} HASHINFO;
75.Ed
76.Pp
77The elements of this structure are as follows:
78.Bl -tag -width indent
79.It Va bsize
|
80.Va Bsize
|
80The
81.Va bsize
82element |
83defines the
84.Nm
85table bucket size, and is, by default, 256 bytes.
86It may be preferable to increase the page size for disk-resident tables
87and tables with large data items.
88.It Va ffactor
|
87.Va Ffactor
|
89The
90.Va ffactor
91element |
92indicates a desired density within the
93.Nm
94table.
95It is an approximation of the number of keys allowed to accumulate in any
96one bucket, determining when the
97.Nm
98table grows or shrinks.
99The default value is 8.
100.It Va nelem
|
97.Va Nelem
|
101The
102.Va nelem
103element |
104is an estimate of the final size of the
105.Nm
106table.
107If not set or set too low,
108.Nm
109tables will expand gracefully as keys
110are entered, although a slight performance degradation may be noticed.
111The default value is 1.
112.It Va cachesize
113A suggested maximum size, in bytes, of the memory cache.
114This value is
115.Em only
116advisory, and the access method will allocate more memory rather
117than fail.
118.It Va hash
|
113.Va Hash
|
119The
120.Va hash
121element |
122is a user defined
123.Nm
124function.
125Since no
126.Nm
127function performs equally well on all possible data, the
128user may find that the built-in
129.Nm
130function does poorly on a particular
131data set.
132User specified
133.Nm
134functions must take two arguments (a pointer to a byte
135string and a length) and return a 32-bit quantity to be used as the
136.Nm
137value.
138.It Va lorder
139The byte order for integers in the stored database metadata.
140The number should represent the order as an integer; for example,
141big endian order would be the number 4,321.
142If
143.Va lorder
144is 0 (no order is specified) the current host order is used.
145If the file already exists, the specified value is ignored and the
146value specified when the tree was created is used.
147.El
148.Pp
149If the file already exists (and the
150.Dv O_TRUNC
151flag is not specified), the
|
144values specified for the parameters
|
| 152values specified for the |
153.Va bsize , ffactor , lorder
154and
155.Va nelem
|
| 156arguments |
157are
158ignored and the values specified when the tree was created are used.
159.Pp
160If a
161.Nm
162function is specified,
163.Fn hash_open
164will attempt to determine if the
165.Nm
166function specified is the same as
167the one with which the database was created, and will fail if it is not.
168.Pp
169Backward compatible interfaces to the older
170.Em dbm
171and
172.Em ndbm
173routines are provided, however these interfaces are not compatible with
174previous file formats.
175.Sh ERRORS
176The
177.Nm
178access method routines may fail and set
179.Va errno
180for any of the errors specified for the library routine
181.Xr dbopen 3 .
182.Sh SEE ALSO
183.Xr btree 3 ,
184.Xr dbopen 3 ,
185.Xr mpool 3 ,
186.Xr recno 3
187.Rs
188.%T "Dynamic Hash Tables"
189.%A Per-Ake Larson
190.%R "Communications of the ACM"
191.%D April 1988
192.Re
193.Rs
194.%T "A New Hash Package for UNIX"
195.%A Margo Seltzer
196.%R "USENIX Proceedings"
197.%D Winter 1991
198.Re
199.Sh BUGS
200Only big and little endian byte order is supported.
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