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30 .\" @(#)hash.3 8.6 (Berkeley) 8/18/94
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32 .Dd September 5, 2008
33 .Dt HASH 3
34 .Os
35 .Sh NAME
36 .Nm hash
37 .Nd hash database access method
38 .Sh SYNOPSIS
39 .In sys/types.h
40 .In db.h
41 .Sh DESCRIPTION
42 The routine
43 .Fn dbopen
44 is the library interface to database files.
45 One of the supported file formats is hash files.
46 The general description of the database access methods is in
47 .Xr dbopen 3 ,
48 this manual page describes only the hash specific information.
49 .Pp
50 The hash data structure is an extensible, dynamic hashing scheme.
51 .Pp
52 The access method specific data structure provided to
53 .Fn dbopen
54 is defined in the
55 .Aq Pa db.h
56 include file as follows:
57 .Bd -literal
58 typedef struct {
59 u_int bsize;
60 u_int ffactor;
61 u_int nelem;
62 u_int cachesize;
63 uint32_t (*hash)(const void *, size_t);
64 int lorder;
65 } HASHINFO;
66 .Ed
67 .Pp
68 The elements of this structure are as follows:
69 .Bl -tag -width cachesizex
70 .It Fa bsize
71 .Fa bsize
72 defines the hash table bucket size, and defaults to 4096 for in-memory tables.
73 If
74 .Fa bsize
75 is 0 (no bucket size is specified) a bucket size is chosen based on the
76 underlying file system I/O block size.
77 It may be preferable to increase the page size for disk-resident
78 tables and tables with large data items.
79 .It Fa ffactor
80 .Fa ffactor
81 indicates a desired density within the hash table.
82 It is an approximation of the number of keys allowed to accumulate in
83 any one bucket, determining when the hash table grows or shrinks.
84 The default value is 8.
85 .It Fa nelem
86 .Fa nelem
87 is an estimate of the final size of the hash table.
88 If not set or set too low, hash tables will expand gracefully as keys
89 are entered, although a slight performance degradation may be
90 noticed.
91 The default value is 1.
92 .It Fa cachesize
93 A suggested maximum size, in bytes, of the memory cache.
94 This value is
95 .Em only
96 advisory, and the access method will allocate more memory rather
97 than fail.
98 .It Fa hash
99 .Fa hash
100 is a user defined hash function.
101 Since no hash function performs equally well on all possible data, the
102 user may find that the built-in hash function does poorly on a
103 particular data set.
104 User specified hash functions must take two arguments (a pointer to a
105 byte string and a length) and return a 32-bit quantity to be used as
106 the hash value.
107 .It Fa lorder
108 The byte order for integers in the stored database metadata.
109 The number should represent the order as an integer; for example,
110 big endian order would be the number 4,321.
111 If
112 .Fa lorder
113 is 0 (no order is specified) the current host order is used.
114 If the file already exists, the specified value is ignored and the
115 value specified when the tree was created is used.
116 .El
117 .Pp
118 If the file already exists (and the
119 .Dv O_TRUNC
120 flag is not specified), the values specified for the parameters
121 .Fa bsize ,
122 .Fa ffactor ,
123 .Fa lorder ,
124 and
125 .Fa nelem
126 are ignored and the values specified when the tree was created are
127 used.
128 .Pp
129 If a hash function is specified,
130 .Fn hash_open
131 will attempt to determine if the hash function specified is the same
132 as the one with which the database was created, and will fail if it is
133 not.
134 .\".Pp
135 .\"Backward compatible interfaces to the routines described in
136 .\".Xr dbm 3 ,
137 .\"and
138 .\".Xr ndbm 3
139 .\"are provided, however these interfaces are not compatible with
140 .\"previous file formats.
141 .Sh ERRORS
142 The
143 .Nm
144 access method routines may fail and set
145 .Va errno
146 for any of the errors specified for the library routine
147 .Xr dbopen 3 .
148 .Sh SEE ALSO
149 .Xr btree 3 ,
150 .Xr dbopen 3 ,
151 .Xr mpool 3 ,
152 .Xr recno 3
153 .Pp
154 .Rs
155 .%T "Dynamic Hash Tables"
156 .%A Per-Ake Larson
157 .%J Communications of the ACM
158 .%D April 1988
159 .Re
160 .Rs
161 .%T "A New Hash Package for UNIX"
162 .%A Margo Seltzer
163 .%J USENIX Proceedings
164 .%D Winter 1991
165 .Re
166 .Sh BUGS
167 Only big and little endian byte order is supported.